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5

Selectivity Considerations inModeling the Treatment of

Perchlorate UsingIon-Exchange Processes

Anthony R. Tripp and Dennis A. Clifford

University of Houston, Houston, Texas, U.S.A.

I. INTRODUCTION

Perchlorate is a singly charged negative ion used as an oxidant for solid rocket fuelby aerospace and military industries. At present in the United States, more than 30states have detected perchlorate-contaminated water due to past handling of ammo-nium perchlorate fuel. Many wells in California have already been idled because of the presence of perchlorate, which has also been detected in Lake Mead in Nevada,threatening the source of drinking water for millions of people in the south-western United States [1]. The U.S. Environmental Protection Agency (USEPA)has added perchlorate to the contaminant candidate list (CCL) to acquire

information to guide regulations controlling the concentration in drinking water.The State of California has set a provisional action level of 4 mg =L pending furtherUSEPA regulations.

Though perchlorate compounds can be very reactive, as an example hot,concentrated perchloric acid is an extremely strong oxidizing solution, in diluteconcentrations the perchlorate ion is stable and extremely nonreactive. Theseproperties greatly facilitate its transport with groundwater. Inorganic reductionof the perchlorate ion is possible under extreme reducing conditions; however,microbial reduction of perchlorate is achievable under the proper redox and water

chemistry conditions.

Copyright © 2004 by Taylor & Francis Group, LLC

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The perchlorate ion cannot be removed from solution by coprecipitation andis weakly complexed by activated carbon. Reverse osmosis and electrodialysis arecapable of removing the ion from solution, and microbial reduction is possiblefor pump-and-treat remediation of contaminated groundwater. Ion exchange hasbeen identified as the best potential treatment process for drinking waterproduction. This chapter investigates the fundamentals of using strong-base anion-exchange resins for the removal of microgram per liter levels of perchlorate fromdrinking water sources to concentrations at or below the perchlorate detection limit.

A. The Ion-Exchange Process

During the ion-exchange process a polymer-based resin with charged functionalgroups is used to remove specific ions from solution, replacing them with ions

already on the resin. For anion exchange, positively charged quaternary aminefunctional groups in the chloride form (chloride counter ions) exchange thenegatively charged chloride ion for other negatively charged anions in the feedsolution. The sequence in which the anions (e.g., nitrate, sulfate, bicarbonate,perchlorate) are exchanged depends upon the selectivity of the resin functionalgroup for that anion relative to the chloride anion. Anions with selectivity lessthan chloride emerge in the column effluent very early in the run length of a column with a gradual increase in concentration. Anions with selectivities greaterthan chloride emerge later in the run with a much sharper concentration gradient

[2–4].During removal, the ions are concentrated, in order of preference, in bands orzones in the resin column. For an n -component system, there are n possible zonesand n 1 boundaries, or wave fronts, between these zones. As these resin bound-aries move through the column, the effluent histories of the anions are developed.The order of emergence of ions from the resin is determined solely by the selectivity sequence. The species exit the column in reverse preferential order, with the less pre-ferred ions (smallest separation factor) emerging first. The most preferred species isthe last to exit the column.

As the water from these zones of enrichment leaves the column, the concen-

tration of the less preferred ion is generally higher than in the influent. Peaking con-centrations several times, those in the influent can be observed for less-preferredcomponents. The final anion to exit the column has the highest selectivity value.This anion will not experience peaking because it does not form an enrichment zonewithin the column. When the most preferred species exits the column, the columnis in equilibrium with the influent composition and no more exchange occurs. By definition, a resin is ‘‘exhausted’’ when the effluent concentration of a specifiedanion reaches a predetermined value, such as the maximum concentration level(MCL) or some fraction thereof. At this point the resin needs to be regenerated

or replaced before the influent water is processed further.


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B. Regeneration of Spent Anion-Exchange Resin

To remove the accumulated anions from the resin, a regenerant solution, typically concentrated NaCl, is passed through the column. This process is continued until

the target anions are sufficiently removed and the resin is adequately converted tothe chloride form. Complete removal of the target ion, perchlorate in this case, isnot generally practiced. Once regenerated, the resin can once again be used toremove the target anion. If the regenerant solution is passed through the columnin the same direction as the feed solution during exhaustion, the regeneration islabeled cocurrent. If the regenerant solution is passed through the column in theopposite direction, then the regeneration is labeled countercurrent. The type of regeneration process can have a significant impact upon the ion-exchange process,as will be discussed later.

The removal of the anions during regeneration is accomplished by one of twomethods. For univalent anions (e.g., perchlorate), the method is one of overwhelm-ing mass balance. A large concentration of chloride anions forces equilibriumtoward a resin in the chloride form. The amount of chloride necessary to achievethis imbalance is a function of the selectivity of the anion being removed. Thegreater the selectivity for the target anion, the greater the amount of chloriderequired.

For multivalent anions such as sulfate or arsenate, regeneration is accom-plished by providing a chloride concentration of sufficient strength to achieve a reversal of selectivity such that the chloride anion is overwhelmingly preferred,

regardless of the original selectivity the resin had for the multivalent ion. The expla-nation for this process is given in the next section and is derived directly from theequations defining selectivity.

C. Definition of Selectivity for

Ion-Exchange Reactions

An expression of a general ion-exchange reaction between ions A and B is

bR a A þ aBb Ð aR bB þ b A a ð1Þ

where R represents the resin phase and a and b the valence charges of ions A and B,respectively.

The separation factor describes the affinity of the resin for ion A comparedwith ion B and is given as

a A B ¼

y A xB

x A yB

ð2Þ


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where y i and x i indicate the fraction of component i in the resin phase or thesolution phase, respectively. These y i and x i components are defined as follows:

y A ¼ q A

QT

ð3Þ

x A ¼ C A

C T ð4Þ

xB ¼ C B

C T ð5Þ

yB ¼ qB

QT

ð6Þ

where C A and C B are the equilibrium concentrations (meq=L) of the two com-ponents in solution, C T is the total solution concentration (meq=L), q A and q Bare the equilibrium concentrations (meq=g resin) of the two components in theresin phase, and Q T is the capacity of the resin (meq=g resin). If ion A is preferred,the separation factor is larger than unity, and if B is preferred, the separationfactor is smaller than unity.

The selectivity coefficient is also used for describing the ion-exchangeequilibria of Eq. 1. This coefficient is defined as

K A B ¼

q A ð Þa C Bð Þb

C A ð Þb qBð Þa ð7Þ

where q and C are as defined for the separation factor but the values of the ionicvalences are used as exponents. Therefore, for univalent–univalent exchange, theseparation factor and the selectivity coefficient are the same. When the valencesof the counter ions are not the same, then the separation factor and the selectivity coefficient are usually quite different. For the case where component A is divalent

and B is univalent, the separation factor is related to the selectivity coefficient by

a A B ¼ K A

B

qB

C B

ð8Þ

In such cases, the selectivity coefficient remains more nearly constant whenthe total solution concentration is varied. The separation factor will vary with solu-tion concentration and in the case where component A is divalent and B is univalentwill decrease with increasing solution concentration. If the concentration change issufficient, the separation factor will change from greater than unity to less than

unity. This reversal can also be explained in terms of the Donnan potential. The


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Donnan potential attracts counter ions (anions) into the resin phase and balancestheir tendency to diffuse out into the more dilute solution phase. The force of the Donnan potential is proportional to the ionic charge. The counter ion withthe higher charge (divalent) is more strongly attracted and preferred by the resin.

As the solution ionic strength increases, the counter ion concentration differencebetween the two phases decreases, as does the strength of the Donnan potentialand the preference of the resin phase for the counter ion with the higher charge.The result is the reversal of selectivity and the rapid displacement of divalent ionsduring regeneration with concentrated ionic solutions [2,3,5].

The selectivity coefficient, K A B is not convenient to use when comparing the

preference of the resin phase for various anions because the coefficient does notyield a value readily comparable to the selectivity coefficient of other anions. Thesulfate ion is preferred relative to the chloride ion by most strong-base anion-

exchange resins, yet the selectivity coefficient for sulfate is less than 1. The nitrateion is generally preferred less than sulfate in most standard anion-exchange resins,yet the selectivity coefficient for nitrate relative to sulfate is greater than 1.

The separation factor is generally used for comparing the relative preferenceof the resin phase for different anions because its values are directly comparable.The separation factor yields an indication of the selectivity of the resin phase thatis accurate not only in sequence but also in the magnitude of that preference relativeto all the components for which the value is known. Therefore, the separation factorwill be used for reporting the selectivity of the resins used in this chapter.

D. Selectivity Theory

There are numerous references in the early ion-exchange literature discussing themeasured selectivity sequences of anion-exchange resins for a wide range of anions,including perchlorate [6–12]. Researchers at that time were attempting to deter-mine the underlying reasons for observed ionic selectivity patterns.

Advances in the understanding of how ion-exchange resins function have ledto the development of polyacrylic and polyvinylpyridine matrices and the addition

of different types of quaternary amine functional groups. How these resins will reactwith perchlorate must be determined prior to evaluating their application as ion-exchange resins for the removal of perchlorate from drinking water sources.

E. Resin Selection, Structure, and Characterization

All the ion-exchange resins used in this study were supplied by commercial vendors.These vendors (and brand names) include Bayer (Lewatit), Rhom and Haas

(Amberlite), Purolite (Purolite), Reilly Enterprises (Reillex), and Sybron (Ionac).


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These resins were selected to cover a wide range of characteristics including threepolymer matrices—polystyrene, polyacrylic, and polyvinylpyridine (Figs. 1–3,

respectively)—and eight quaternary amine functional groups—trimethyl, dimethy-lethanol, triethyl, tripropyl, methylpyridine, benzylpyridine, and dipyridine—asshown in Figs. 4 and 5. The resins were also characterized by different degrees of divinylbenzene cross-linking of the polymer matrix and resin bead structure (gel,macroporous, isoporous). Table 1 is a listing of the resins used and their respectivecharacteristics.

Figure 1 Resin matrix of a polystyrene, Type I, strong-base anion-exchange resin in thechloride form. (From Ref. 2.)

Figure 2 Resin matrix of a polyacrylic, trimethylamine, strong-base anion-exchange

resin in the chloride form. (From Ref. 5.)


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Figure 3 Resin matrix of a polyvinylpyridine, methylpyridineamine, strong-base anion-exchange resin in the chloride form. (From Ref. 13.)

Figure 4 Quaternary amine functional groups for polystyrene resins. (From Refs. 13

and 14.)


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II. RESULTS AND DISCUSSION

A. Binary Isotherm Experiments

Several basic resin characteristics were determined for these resins. The total strong-base þ weak-base anion exchange capacity for each resin was determined by the

Figure 5 Quaternary amine functional groups for polyvinylpyridine resins. (Courtesy of

D. McQuigg, personal communication, 2000.)

Table 1 Characteristics of the Strong-Base Anion Resins Used in This Study

Resin Resin name Matrix Functionality Porosity

1 Duolite A-101D STY-DVB Q-1 Isoporous2 Ionac ASB-1 STY-DVB Q-1 Microporous3 Ionac ASB-2 STY-DVB Q-2 Microporous4 Ionac SR-7 STY-DVB TPA Macroporous5 Lewatit OC-1950 STY-DVB Q-1 Microporous6 Amberlite IRA-400 STY-DVB Q-1 Microporous7 Amberlite IRA-402 STY-DVB Q-1 Microporous8 Amberlite IRA-404 STY-DVB Q-1 Microporous9 Amberlite IRA-458 Acrylic Q-1 Microporous

10 Amberlite IRA-900 STY-DVB Q-1 Macroporous11 Amberlite IRA-958 Acrylic Q-1 Macroporous12 Amberlite IRA-996 STY-DVB TEA Macroporous13 Reillex HPQ PYR-DVB Methylpyridine Macroporous14 Reillex B-1 PYR-DVB Benzylpyridine Macroporous15 Reillex DP-1 PYR-DVB Dipyridine Macroporous

STY-DVB, polystyrene-divinylbenzene polymer; PVP-DVB, polyvinylpyridine-divinylbenzene polymer;

Q-1, quaternary amine, Type I; Q-2, quaternary amine, Type II; TEA, triethylamine; TPA, tripropylamine.


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conventional procedure described by Fisher and Kunin [15]. No attempt was madeto differentiate between the weak-base and strong-base fractions of the totalcapacity. The measured capacities, specific water regain, and percent moisture of the resins tested to date are shown in Table 2, which demonstrates units of both milliequivalents per gram for calculation purposes and the more familiarmilliequivalents per milliliter used for comparison purposes.

The volume of a known mass of resin in a 0.005 M NaCl solution was usedto convert the capacity from a mass basis to a volume basis (meq=mL resin). Thespecific water regain was determined by a procedure described by Simon [16].The percent of mass lost from the resin during 105C drying was by definitionthe specific water regain.

B. Perchlorate–Chloride Binary Isotherm Procedure

For the binary isotherm procedure, a series of experiments were conducted, varying the mass of resin added to achieve equilibrium perchlorate concentrations of 5–50 mg =L. The target equilibrium concentrations for perchlorate were specifically chosen to reflect the use of ion-exchange resins in the process of removing perchlo-rate from drinking water. Total solution concentration of all anions in a treatmentprocess will be low, with 5 meq=L being a representative value. Though it wouldhave been far easier to work with the higher concentrations typical for isotherms,there was concern that the separation factors generated would not be representative

for the conditions in which they would be applied, because several referencesreported that the perchlorate separation factor was a function of the amount of perchlorate in the resin phase [10,17,18].

The calculated mass of resin was weighed to the nearest tenth of a milligramand transferred to a solution of 0.005 meq=L (500 mg =L) perchlorate and4.995 meq=L chloride. This mixture was shaken for 24 h at 100 cycles=min in a water bath maintained within 1C temperature. At the end of this period theperchlorate and chloride concentrations were measured to construct binary isotherms and calculate perchlorate separation factors.

These equilibrium data were then used to construct binary isotherms for per-

chlorate relative to chloride. For these binary isotherm figures the x axis was thefraction of perchlorate in solution at equilibrium relative to the total solution con-centration (meq=L), and the y axis was the fraction of perchlorate in the resin phaseat equilibrium relative to the total capacity of the resin (meq=g). The separationfactor can be obtained from the diagram (see Fig. 6) as the ratio of the area of rec-tangle I to that of rectangle II formed at any one point on the isotherm. In theisotherm of Fig. 6, the ratios of the areas of I and II yield a separation factorof approximately 6, with perchlorate being the preferred species. Separation factorsreported in this study were not the result of singular points but the median of

all values determined.


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Table 2 Characteristics of the Strong-Base Anion Resins Selected for the Perchlorate Isotherm Tes

Resinnumber

Resinname

Capacity (meq=g)

Waterregain

(%)

Specific waterregain (mmol

water=meq resin)

Resin unitvolume(mL=g)

1 Duolite A-101D

3.32 47 14.8 2.62

2 Ionac ASB-1 3.60 39 9.9 2.47 3 Ionac ASB-2 2.77 36 11.8 2.27 4 Ionac SR-7 2.06 46 23.5 2.7 5 Lewatit

OC-19503.62 50 15.4 3.03

6 AmberliteIRA-400

3.65 44 11.8 2.78

7 AmberliteIRA-402

3.62 50 15.3 2.96

8 AmberliteIRA-404

3.40 62 26.6 3.72

9 AmberliteIRA-458

3.78 60 21.8 3.70

10 AmberliteIRA-900

3.65 58 20.8 3.47

11 AmberliteIRA-958

3.66 69 34.3 4.87

12 AmberliteIRA-996

3.12 50 17.9 3.35

13 Reillex HPQ 4.04 56 17.6 3.41 14 Reillex B-1 3.44 50 16.0 3.06 15 Reillex DP-1 4.05 49 13.3 2.57

na ¼ not available.


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Isotherms were constructed for all the resins at equilibrium temperatures of 20, 40, and 60C using the same procedure. The results of the binary isothermexperiments for 20, 40, and 60C are presented in Figs. 7–21. These figures show

the constructed binary isotherms (fraction perchlorate in solution versus fractionperchlorate on resin) over the scale of interest for this study for each resin (compareto axis scales of Fig. 6). Note that the scales may differ from figure to figure tomaximize the detail present. By comparing the characteristics of different resins,certain observations and conclusions can be made. The characteristics studiedincluded resin polymer matrix, percentage divinylbenzene cross-linking, resinmacrostructure (porosity), and functional group.

The median values and standard deviation of the perchlorate separation factorrelative to chloride for these 15 resins are listed in Table 3 for the three temperatures

tested (20, 40, and 60

C).

Figure 6 Example of a binary isotherm and the two rectangles, I and II, formed at any point on the curve, which can be used to determine the separation factor at that specificpoint.


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Figure 7 Twenty-four hour binary isotherms for the polystyrene Type I microporousgel resin Duolite A-101D.

Figure 8 Twenty-four hour binary isotherms for the polystyrene Type I microporous

gel resin Ionac ASB-1.


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Figure 9 Twenty-four hour binary isotherms for the polystyrene Type II microporousgel resin Ionac ASB-2.

Figure 10 Twenty-four hour binary isotherms for the polystyrene, tripropylamine,

macroporous resin, Ionac SR-7.


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Figure 11 Twenty-four hour binary isotherms for the polystyrene Type I microporousgel resin Lewatit OC-1950.


gel resin Amberlite IRA-400.


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Figure 13 Twenty-four hour binary isotherms for the polystyrene Type I microporousgel resin Amberlite IRA-402.


gel resin Amberlite IRA-404.


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Figure 15 Twenty-four hour binary isotherms for the polyacrylic Type I microporousgel resin Amberlite IRA-458.

Figure 16 Twenty-four hour binary isotherms for the polystyrene Type I macroporous

resin Amberlite IRA-900.


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Figure 17 Twenty-four hour binary isotherms for the polyacrylic Type I macroporousresin Amberlite IRA-958.

Figure 18 Twenty-four hour binary isotherms for the polystyrene triethylamine macro-

porous resin Amberlite IRA-996.


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Figure 19 Twenty-four hour binary isotherms for the polyvinylpyridine methylpyridineamine macroporous resin Reillex HPQ.

Figure 20 Twenty-four hour binary isotherms for the polyvinylpyridine benzylpyridine

amine macroporous resin Reillex B-1.


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1. Effect of Resin Matrix on Selectivity

The effect of the resin matrix upon the perchlorate separation factor was deter-mined by comparing resins with similar functional groups but differing polymermatrices. Figure 22 shows the binary isotherms for three such resins—AmberliteIRA-458 (polyacrylic), Ionac ASB-1 (polystyrene), and Reillex HPQ (polyvinyl-pyridine). The greater the slope of the isotherms, the greater were the perchlorate–chloride separation factor and the relative affinity exhibited for perchlorate. Clearly,the polyacrylic resin, with a hydrophilic structure, exhibited the lowest perchlorateselectivity.

These results are supported by papers that mention the effect the resin matrix

has upon selectivity. These indicate that the separation factor of sulfate relative tonitrate was significantly greater for acrylic, epoxy, and phenolic matrices than forstyrene [19,20]. As resins become more hydrophilic (polar), the sulfate selectivity increases [21]. Similar results were reported by Jackson and Bolto [22] forpolyacrylic and polystyrene resins.

The selectivity of polyacrylic resins for chlorophenols was significantly lessthan that of polystyrene resins, with the difference increasing as the number of chlorine substituents increased [23]. Compared with polyacrylic resins, polystyreneresins also exhibit a greater affinity for humic and fulvic acids present in natural

organic matter (NOM) of drinking water supplies [24]. The NOM is a mixed

Figure 21 Twenty-four hour binary isotherms for the polyvinylpyridine dipyridinemacroporous resin Reillex DP-1.


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Table 3 Median Separation Factors from the Binary Isotherm Experiments for the Changes with Temperature

Resinnumber

Median seMatrix Functionality Porosity 20C s

11 Acrylic Q-1 Microporous 5.5 0.513 Acrylic Q-1 Macroporous 10.4 1.010 STY-DVB Q-1 Microporous 81 13

7 STY-DVB Q-1 Microporous 85 13 9 STY-DVB Q-1 Microporous 94 10

5 STY-DVB Q-2 Microporous 110 17 8 STY-DVB Q-1 Microporous 125 12 1 STY-DVB Q-1 Isoporous 134 9.0

12 STY-DVB Q-1 Macroporous 145 35 4 STY-DVB Q-1 Microporous 170 18 3 STY-DVB Q-1 Macroporous 191 37

15 PYR-DVB Methylpyridine Macroporous 230 61 17 PYR-DVB Dipyridine Macroporous 258 52

2 STY-DVB Q-1 Macroporous 373 72 16 PYR-DVB Benzylpyridine Macroporous 632 199 14 STY-DVB TEA Macroporous 800 238

6 STY-DVB TPA Macroporous 1300 623


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assemblage of organic compounds of varying size and structure, with various car-boxyl and phenolic hydroxyl functional groups with differing pK a values that aregenerally charged species at pH 5 or above. The breakthrough curves of NOMand sulfate showed several alternating stages of peaking for the NOM and thesulfate. The fraction of NOM with selectivity lower than that of sulfate peakedbefore sulfate breakthrough, whereas NOM with higher affinity than sulfate causedsulfate peaking. For the treatment of water with significant amounts of NOM,a polyacrylic resin was recommended to decrease fouling and aid in regeneration.

Many papers indirectly recognize the contribution that the polymer matrix has upon selectivity when discussing the environment of the resin phase and itseffect upon ionic hydration, structural breaking of water, or hydrophobic refuges[25–28].

2. Effect of Percent Cross-Linking andPorosity on Selectivity

The discussions of percent cross-linking and porosity are combined because they are

physically related. The difference between microporous and macroporous resins is

Figure 22 Comparison of perchlorate–chloride binary isotherms for three different resinpolymer matrices: polyacrylic, polystyrene, and polyvinylpyridine. The median perchlorate–chloride separation factors are given in parentheses.


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due not only to the process by which the physical structure of the macroporous resinis manipulated to introduce porosity but also to the chemical structure of the resin[29]. As part of the procedure of creating porosity in a resin, the percentage of divi-nylbenzene cross-linking can be increased to provide a more rigid structure. In thiswork, experiments with resins of varying porosity showed that as divinylbenzenecross-linking increased, the perchlorate–chloride separation factor also increased(Fig. 23). Figure 24 demonstrates that the perchlorate separation factor washigher for macroporous resins than for microporous resins. It was not possible todetermine if this increase was due to the physical structure (increased porosity) of the resin or to the increased percentage of cross-linking required to physically holdthe porous structure together. The higher cross-linking provides a more rigid struc-ture that is resistant to expansion relative to the less cross-linked resin. This resis-tance to swelling makes it more difficult for ions with large hydration shells to

enter the resin without losing part of that shell. Likewise, the highly cross-linkedenvironment of lower hydration was hydrophobic, which was attractive to ionssuch as perchlorate [9,10,30,31].

Figure 23 Comparison of perchlorate–chloride binary isotherms for resins with 8% and

4% divinylbenzene cross-linking. Perchlorate–chloride separation factors are in parentheses.


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3. Effect of Functional Group onPerchlorate Selectivity

Polystyrene resins with similar matrices but three different functional groups—trimethyl- (Amberlite IRA-400), triethyl- (Amberlite IRA-996), and tripropyl-amine (Ionac SR-7)—were used to determine the effect of the functional groupon perchlorate selectivity. As shown in Fig. 25, the larger the functional group,the greater the perchlorate selectivity. For resins with triethyl and tripropyl func-tional groups, the perchlorate–chloride binary isotherms and the separation factors

are listed as ‘‘less than’’ values because the solutions were not at equilibrium at theend of the 24 h period. Instead, a lower limit was listed as an apparent separationfactor. It was assumed that mass transfer limitations were responsible for the long equilibrium period. As the functional group increased in size, so did the masstransfer limitations, thereby requiring more time for the isotherm reaction toachieve equilibrium. If given enough time, the reaction would achieve equilibriumand a more accurate and constant separation factor would be obtained. Numerousstudies have demonstrated that as the alkyl group increases in size, the selectivity relative to chloride increases for monovalent anions and decreases for divalent anions

[20,22,32–36]. There are several theories to explain this observed phenomenon,

Figure 24 Comparison of perchlorate–chloride binary isotherms for polyacrylic resinswith microporous and macroporous structures. Perchlorate separation factors are inparentheses.


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such as the polarity of the functional groups, reduced electrostatic ion-pairing,enhanced water-structure ion pairing, and stearic hindrance. For nitrate–sulfateselectivity, this change in selectivity can be large enough to reverse the affinity sequence so that nitrate is more preferred [20,22,37].

4. Kinetics of Perchlorate Uptake

All of the binary isotherm experiments were run assuming that a period of 24 h wassufficient to achieve equilibrium. To determine if this time was sufficient, the

experiments were repeated with reaction times of 3 days and 7 days in an attemptto determine the effect that kinetics may have upon the perchlorate removalprocess. For those resins exhibiting kinetic limitations during the 1, 3, and 7 day tests, the long-term equilibrium experiments were conducted at 40 and 60C todetermine the effect of temperature upon kinetics.

All resins were tested for reaction periods of 24, 72, and 168 h todetermine the minimum time necessary to achieve equilibrium. Of these resins, fivedisplayed significant variations in the apparent perchlorate–chloride separation fac-tors between the 24, 72, and 168 h reaction periods. These five resins were then

tested for 24, 72, and 168h reaction periods at 40 and 60

C temperatures to

Figure 25 Twenty-four hour perchlorate–chloride binary isotherms at 20C comparing three polystyrene resins with varying alkyl chain lengths of the quaternary amine functionalgroup. The perchlorate separation factors are in parentheses.


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determine the effect on the equilibrium of the perchlorate–chloride exchangereaction. The SR-7 and B-1 resins (Figs. 26 and 27) exhibited apparent separationfactors that had not stabilized within the 7 day equilibrium period at 20C. Whenthe temperature was increased to 40C, both resins stabilized in 7 days. At 60C, allthe resins achieved a stable perchlorate separation factor within an equilibrationperiod of 24 h.

The three processes controlling the rate of ion exchange are film diffusion,particle diffusion, and reaction at the exchange site. Of these three, reaction atthe exchange site is not generally considered rate-limiting for normal ion-exchangeapplications [2]. These kinetics tests were not designed, or intended, to determinethe relative importance of the components controlling kinetics, but certain generalobservations can be made.

The five resins that did not reach equilibrium after 24 h (as evidenced by the

change in the apparent separation factor) were the polyvinylpyridine resins (HPQ,DP-1, B-1) and the nitrate-selective resins (IRA-996, SR-7). These also are theresins with the highest perchlorate separation factors. The resin with the highestseparation factor also had the poorest kinetics. The kinetics for all six wereimproved significantly by increasing the equilibrium temperature. Although thiseffect was expected [38–41], it was not specific to either film or particle diffusion.

Figure 26 Perchlorate–chloride separation factor for the polystyrene tripropylamine

resin Ionac SR-7 at 20, 40, and 60

C for 1, 3, and 7 day equilibrium periods.


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Film diffusion cannot be totally ignored, although in the binary isothermexperiments the effect upon kinetics should have been similar for all the resinstested. Therefore, it was probable that particle diffusion was the rate-limiting stepfor perchlorate exchange, and it increases with increasing perchlorate selectivity.Further research with the specific objective of determining diffusion rates for resinswith large perchlorate selectivity are needed to ascertain the relative importance of film and particle diffusion.

B. Thermodynamics of Perchlorate–ChlorideExchange

With a few assumptions and generalizations, the thermodynamic values of enthalpy (DH ), free energy (DG ), and entropy (T DS ) for the exchange of chloride forperchlorate can be estimated. The exchange reaction can be depicted as

RCl þ ClO4

, RClO4

þ Cl ð9Þ

where the overbar represents the resin phase and the positively charged functional

group of a strong-base anion-exchange resin. The empirical equilibrium constant

Figure 27 Perchlorate–chloride separation factors for the polyvinylpyridine benzylpyri-dine amine resin Reillex B-1 at 20, 40, and 60C for 1, 3, and 7 day equilibrium periods.


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K for the reaction in Eq. (9) can be expressed as

K ClO

4

Cl ¼C C zCl

ClO4

C zClO

4

Cl

C C

zClO4

Cl C zCl

ClO4

ð10Þ

where the overbar represents the molar concentration of the component in the resinphase and the lack of an overbar represents the molar concentration in solution.This is also referred to as the selectivity coefficient. For univalent–univalentexchange, the selectivity coefficient and the separation factor have the samevalue.

The overall free energy change of the reaction, DG , is related to theequilibrium constant by the relationship

DG ¼ RT ln K ð11Þ

where R is the universal gas constant and T is the temperature in kelvins.The standard enthalpy change, DH , can be obtained from the van’t Hoff

equation

d lnK

dT ¼

DH

RT 2 ð12Þ

lnK ¼

DH

R 1

T

þ constant ð13Þ

provided that the temperature change is relatively small. The plot of ln K versus1=T should result in a straight line with a positive slope for exothermic reactionsand a negative slope for endothermic reactions [42]. The combined van’t Hoff plots for perchlorate–chloride ion exchange of the 15 resins tested are shown inFig. 28. The slopes of the lines from plot to plot were fairly constant for most of the resins studied but not for the polyacrylic resins. The slopes of the lines forthe polyacrylic resins are much lower.

Once the overall free energy and enthalpic change have been obtained, thestandard entropic contribution T DS can be obtained from the relationship

T DS ¼ DH DG ð14Þ

with T being the temperature in kelvins.These values were calculated for the 15 resins for the temperature range 293–

333 K (20–60C). Table 4 lists the values for DH , DG , and T DS (all kJ=mol)for the 15 resins.

With the exception of the two polyacrylic resins (IRA-458 and IRA-958), all

the resins were exothermic, as indicated by the positive slope of the van’t Hoff plots


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[43]. This is consistent with the observation that all these resins showed a decreasein the separation factor with increase in temperature. The enthalpic contributionwas similar for all resins, which is consistent with the similar temperature effectof a 30% decrease in selectivity for each 20C increase in temperature.

Helfferich [2] notes that for most ion-exchange reactions, temperature haslittle effect upon the final equilibrium condition but does affect the rate of equili-brium. However, there are examples where selectivity changes with temperaturehave been noted and used in ion-exchange processes.

Bailly and Tondeur [44] used the difference in selectivity of calcium and

potassium between 4C and 60C to fractionate the solution in a countercurrentmoving-bed process. Muraviev et al. [45] used the differences in the extent to whichthe separation factors of calcium and magnesium change, relative to sodium, withtemperature to propose a method for the concentration and separation of mag-nesium from seawater. As the temperature increased, the separation factors forcalcium and magnesium increased relative to sodium but to a lesser extent for mag-nesium. By alternating between 10C and 80C, this difference was used over fourcycles to concentrate magnesium to a level 4 times that of the initial concentration.

Both Dobrevsky and Konova [46] and Peterka [47] proposed the use of

temperature to regulate the ion-exchange resin concentration of boric acid in the

Figure 28 The combined van’t Hoff plot, ln K vs. 1=T , for perchlorate–chlorideexchange for the 15 resins tested over the 20–60C range.


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coolant of pressurized water reactors. As the temperature increased, the concentra-tion of boric acid in the resin phase decreased, resulting in a solution-phaseconcentration increase.

Jackson and Bolto [22] reported a decrease in the separation factor of nitraterelative to sulfate and to a lesser degree, chloride. This change was used to try toimprove the efficiency of the regeneration process using NaCl solutions. Only a tri-butyl resin exhibited any benefit from increased temperature of regeneration, withan almost two-third increase in efficiency reported.

There are precedents for the use of temperature as a major component of theion-exchange process. What may be unique for perchlorate treatment was the

relative magnitude of change in selectivity with temperature. The question of whether that change can be exploited and incorporated into a treatment process willbe explored in a later section.

C. Computer Prediction of Column Performance

After experimentally determining the perchlorate separation factors and resin capa-cities, computer simulations were used to predict the multicomponent column per-formance of resins as a function of various factors. To assess the validity of the

models, laboratory-scale experiments were performed to compare data from actual

Table 4 Thermodynamic Information for Resins Over the TemperatureRange of 20–60C

Resin

number Matrix Functionality Porosity

DG

(kJ=mol)

DH

(kJ=mol)

T DS

(kJ=mol)

1 STY-DVB Q-1 Isoporous 10.8 19.6 8.92 STY-DVB Q-1 Microporous 11.3 19.1 7.83 STY-DVB Q-2 Microporous 10.3 19.9 9.64 STY-DVB TPA Macroporous 18.0 13.4 þ4.55 STY-DVB Q-1 Microporous 10.4 15.8 5.46 STY-DVB Q-1 Microporous 10.9 14.2 3.37 STY-DVB Q-1 Microporous 10.6 16.0 5.38 STY-DVB Q-1 Microporous 10.1 21.1 11.09 Acrylic Q-1 Microporous 4.5 0.1 þ4.6

10 STY-DVB Q-1 Macroporous 11.9 10.4 þ1.511 Acrylic Q-1 Macroporous 5.7 5.0 þ0.712 STY-DVB TEA Macroporous 16.0 15.3 þ0.713 PYR-DVB Methylpyridine Macroporous 12.3 14.2 1.914 PYR-DVB Benzylpyridine Macroporous 14.9 13.6 þ1.215 PYR-DVB Dipyridine Macroporous 12.8 17.8 4.9


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column exhaustions and regenerations to computer predictions. Toward that end,the elution of perchlorate from polyacrylic and polystyrene resins using various saltconcentrations, temperatures, and contact times was investigated. Anothervalidation experiment used a laboratory-prepared solution similar to an actualperchlorate-contaminated groundwater for the exhaustion of two resins with highperchlorate–chloride separation factors. These results were compared withcomputer predictions to evaluate the ability of these computer programs to predictthe performance of these resins.

D. Influence of Salt Concentration, Temperature,

and Contact Time on Perchlorate Elution

Tests were conducted to determine the influence of salt concentration, flow rate,and temperature on the elution of perchlorate from ion-exchange resins. An equili-brium multicomponent computer program was used to calculate the amount of perchlorate in the resin phase that would be removed during treatment of a groundwater (perchlorate 100 mg =L) by a resin with a given perchlorate separationfactor. By a batch procedure, this amount of perchlorate was loaded onto an ion-exchange resin to simulate an exhausted resin. This resin was then divided intoaliquots that were placed into mini columns and subjected to various elutionexperiments to determine the effect of salt concentration, contact time, andtemperature upon the elution process.

1. Effect of Salt Concentration on Regenerationof Polyacrylic Resin

Portions of a polyacrylic resin, loaded with approximately 0.7% perchlorate (7 meqperchlorate=L resin), were eluted with NaCl solutions of varying concentration(0.5, 1.0 N) at differing empty-bed contact times (1, 7, 10, and 30 min) todetermine the effect of chloride concentration and contact time on perchlorateremoval. The resin perchlorate loading of 0.7% was comparable to what would

be achieved from the exhaustion of a polyacrylic resin with a typical groundwatercontaining 100 mg =L of perchlorate. Figure 29 shows the effect of increasing thechloride concentration from 0.5 to 1.0 N on elution of perchlorate. Comparedstrictly on the basis of equivalents of chloride supplied, the 0.5 N chloride solutionwas slightly more effective than the 1.0 N solution. Of course, the 0.5 N solutionrequired approximately twice as much regenerant volume as 1.0 N NaCl. Theseresults are consistent with the elution of a monovalent anion (ClO4

) b y a monovalent anion (Cl).

For the elution experiments, pump flow rates were set to maintain a

superficial linear velocity (SLV) of at least 3 cm=min for all experiments to prevent


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channeling of the eluent. This SLV was 50% greater than the minimum SLV of 2.0 cm=min recommended by resin manufactures for the regeneration of anionresin [48]. Owing to the length of the small chromatography columns (0.7 cmi.d., 15 cm length) used in the typical minicolumn experiment, a 3 cm=minSLV represented an empty-bed contact time (EBCT) of no more than 5 min.For industrial regenerations of ion-exchange resins, a typical EBCT is on the orderof 30 min. To obtain more typical EBCTs with small volumes of resin, longerlengths and smaller diameters were required. Flexible Nalgene tubing with smaller

internal diameters [6.4 mm (1=4 in.) and 3.2 mm (1=8 in.)] was used, increasing theEBCT while maintaining the SLV. With tubing lengths of 1 m (6.4 mmi.d. ¼ 32cm3; 3.2 mm i.d. ¼ 8 cm3), EBCTs of 30 min could be attained with SLVsof 3 cm=min or greater. By varying the diameter and length of thecolumns, the effect of contact time could be investigated while maintaining a constant superficial linear velocity.

The effect of solution contact time upon perchlorate elution is shown inFig. 30. An EBCT of 1 min proved too fast for efficient elution of perchlorate.For an EBCT of 7 min or greater, there was no difference in the ability of a 1 N

chloride solution to elute perchlorate from the resin.

Figure 29 The elution of perchlorate from a polyacrylic resin using solutions with twodifferent chloride concentrations and 7 min EBCT values. The total regeneration contacttime for 10 equiv Cl=equiv resin was 84 min for 1.0 N and 168 min for 0.5 N NaCl.


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For these experiments, a column length of 1 m was used. To maintain a mini-mum superficial linear velocity of 9 cm=min, flexible PVC tubing of differing dia-meters was used. To verify that the small column diameters used (3.2 and 6.4 mmi.d.) did not produce serious wall effects during regeneration, two columns with thesame length but different diameters were regenerated. Results indicated (Fig. 31)that with similar EBCT and SLV values there was no difference in using tubing with 3.2 mm i.d. (1=8 in.) compared with tubing of 6.4 mm i.d. (1=4 in.). Thus,

wall effects did not appear to be significant.

2. Comparison of Regeneration of Polyacrylicand Polystyrene Resins

To compare the regenerability of low- and high-selectivity resins (polyacrylic andpolystyrene, respectively), perchlorate-loaded resin samples were eluted with 1 NNaCl. Figure 32 compares the elution of polyacrylic and polystyrene resins at23C using 1 N NaCl. Clearly, the polystyrene resin was far more difficult to

regenerate. The polyacrylic resin required only 9 equiv Cl=equiv resin (40 lb

Figure 30 The effect of varying contact time upon the elution of perchlorate from a

polyacrylic resin using a 1 N chloride solution.


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Figure 31 Effect of column diameter on perchlorate elution from polyacrylic resin with1 N NaCl using 1 m lengths of tubing with inside diameters of 3.2 mm and 6.4 mm. EBCTand SLV were 10 min and 9 cm=min, respectively, for both columns.

Figure 32 Comparison of elution of perchlorate from polyacrylic and polystyrene resins

with 1 N NaCl solution.


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NaCl=ft3) to achieve 90% elution of perchlorate, whereas 90% perchlorate elutionfrom polystyrene resin required 135 equiv Cl=equiv resin (690 lb NaCl=ft3).

3. Temperature Effect on Polystyrene Regeneration

To determine the effect of temperature upon the elution process, the whole elutionsystem (pump, column, tubing, and reservoir) was placed inside an oven. With thetubing coiled into three loops, the 1 m length was easier to manage and smallenough to place inside an oven to allow for experiments at different temperatures.

A small access hole allowed the effluent from the column to be collected into anexternal fraction collector. This ensured that there were no temperature changesat any point or time within the system during the elution process.

Figure 33 compares the results of elution of the polystyrene resin with 1 Nchloride solutions at 23, 45, and 60C. For these tests a polystyrene resin (Ionac

ASB-2) was loaded with approximately 0.7% perchlorate (10 meq ClO4=L resin)

for elution. This level of perchlorate loading is representative of completeexhaustion of a polystyrene resin with groundwater contaminated with perchlorate

Figure 33 Effect of temperature on elution of perchlorate from a polystyrene resin using 1 N chloride solutions. Perchlorate–chloride separation factors, a values, are given for each

temperature.


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at concentrations of 50–100mg =L. The complete removal of perchlorate fromthis resin with a 1 N chloride solution at 23C required over 250 BV (180 equiv Cl=equiv resin, 900 lb NaCl=ft3) of solution. A significant portion of this wasdue to tailing at the end of the elution. The removal process became more efficientas the temperature of the 1 N chloride solution increased. This was due to a low-ered perchlorate chloride separation factor and decreased tailing due to better masstransfer characteristics at the elevated temperatures (45 and 60C). Integration of the area under the elution curves indicated essentially complete regeneration at alltemperatures.

4. EMCT Elution Curves

After experimental determination of the perchlorate elution curves, the equili-

brium multicomponent theory (EMCT) program developed at the University of Houston [49,50], was used to predict the theoretical shapes of the curves, giventhe perchlorate loading and the known separation factors at temperatures of 23,45, and 60C. It assumes instantaneous equilibrium, knowledge of feedwatercomposition, homogeneous presaturation, constant separation factors, and constant

Figure 34 EMCT-generated elution curves compared with the actual elution curves of a

polystyrene resin using 1 N chloride solutions at temperatures of 23, 45, and 60

C.


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resin capacity. The EMCT Windows program predicts effluent concentrationhistories (effluent concentration versus time or BV) and resin bed profiles (resinconcentration versus bed depth). It is based on an analytical solution to the setof coupled partial differential equations describing the columnar ion-exchangeprocess [51]. Figure 34 is an overlay of the EMCT predictions with the actual elu-tion curves. The program, which was based on equilibrium multicomponent chro-matography theory, was fairly accurate in predicting the trend of effluentperchlorate concentration and the break of the elution curve but was not designedto predict the amount of tailing at the end of the curve. This tailing resulted frommass transfer limitations that are not incorporated into the EMCT program,because it assumes instantaneous equilibrium, i.e., no mass transfer limitations.The mass transfer coefficients can be obtained only from actual experimentalconditions. The EMCT prediction more closely matched the actual data as the

temperature was increased, and the mass transfer limitations decreased.

III. COMPUTER MODELING

A. Computer Programs to Predict

Column Performance

1. Complete Exhaustion=Regeneration

Using the EMCT program and a Big Dalton water composition (a perchlorate-

contaminated groundwater; see Table 5), the run lengths to complete perchlorateexhaustion for the 15 resins were computed. Known values of the separation factorsfor the other main components in solution (chloride, bicarbonate, nitrate, and sul-fate) were used when available, and estimates based upon expected performancewere used when these values were not available. The run length to perchloratebreakthrough was plotted relative to the perchlorate separation factor (Fig. 35).There was a general linear increase in run length with increase in separation factor.

Table 5 Chemical Composition of Big Dalton Water

ComponentConcentration

(mg =L)Concentration

(meq=L)

Perchlorate 0.05 0.0005Chloride 30 0.8Sulfate 50 1.04Nitrate N 6 0.4Bicarbonate 122 2

Note : The Big Dalton well is a perchlorate-contaminated well in the San Gabriel

Basin in Baldwin Park, CA.


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The program predicted the volume of a 1 N chloride solution required tocompletely regenerate the exhausted resin so that an estimate of the efficiency of the exhaustion=regeneration process could be obtained. In this case, efficiency was defined as the ratio of the bed volumes to perchlorate breakthrough relativeto the bed volumes of regenerant applied. Figure 36 shows the number of bedvolumes of regenerant theoretically required as a function of the separationfactor. The resins with the highest separation factors required the greatest amountof regenerant. Examination of the efficiency (ratio of bed volumes to break-through=to bed volumes of 1 N regenerant) relative to the separation factor showsthat an increasing separation factor did not translate into progressively increasing

efficiency (Fig. 37). In fact, the resin with the lowest perchlorate separation factor(a¼ 5.5) exhibited the highest efficiency, but the resin with the next smallestseparation factor (a¼ 10.5) exhibited an efficiency similar to those of the next10 resins. The low separation factor resins are polyacrylic with a general selectivity sequence of SO4

2 >ClO4 >NO3

>Cl >HCO3 for exhaustion and

ClO4 >NO3

>Cl > SO42- >HCO3

for regeneration. Because perchlorate(a minor ion in solution) was less preferred than sulfate (a major ion in solution),the run length to perchlorate breakthrough was controlled by sulfate breakthrough.Therefore, for a resin with this selectivity sequence, the run length to perchlorate

breakthrough will be similar regardless of the perchlorate separation factor. But

Figure 35 The EMCT estimated run length to perchlorate breakthrough as a functionof perchlorate separation factor for the 15 resins tested using a Big Dalton watercomposition and a perchlorate concentration of 50 mg =L.


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Figure 36 The EMCT-calculated bed volumes of 1 N chloride needed to completely regenerate a resin as a function of perchlorate separation factor for the 15 resins testedusing a Big Dalton water composition and a perchlorate concentration of 50 mg =L.

Figure 37 The efficiency of complete exhaustion=regeneration for the 15 resins testedusing Big Dalton water with a 50mg =L perchlorate concentration and a 1 N chloride regen-

eration solution.


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for regeneration, the perchlorate separation factor controls the amount of regener-ant, the larger the separation factor the greater the amount of regenerant and thelower the efficiency.

For the polystyrene and polyvinylpyridine resins, perchlorate selectivity wasgreater than sulfate selectivity; therefore, the breakthrough of sulfate does not con-trol perchlorate breakthrough. Ten of these resins with separation factor values inthe 80–380 range exhibited similar efficiencies in the range of 70–80 BV productwater=BV 1 N regenerant. Finally, the three resins with the highest separation factorvalues, 650–1300, exhibited similar efficiencies in the 110–115 BV productwater=BV 1 N regenerant range. This increase in efficiency is due to the preferenceof these resins for nitrate relative to sulfate. The decreased sulfate separation factordiminished the effect of sulfate upon the run length of the resin. This selectivity switch has no effect upon the amount of regenerant required to remove the perchlo-

rate and results in a net increase in efficiency relative to resins with sulfate selectivity greater than nitrate selectivity. Thus, it appeared that the polyacrylic resin with thelowest perchlorate–chloride affinity produced the highest theoretical efficiency in terms of bed volumes of product relative to bed volumes of 1 N regenerant.This efficiency was approximately double that of the conventional polystyrene andpolyvinylpyridine resins. This fact makes it difficult to conceive of a conventionalperchlorate ion-exchange process using nonpolyacrylic resins, unless elevatedtemperature is used for regeneration.

2. Effect of Regenerant Temperature Upon EfficiencyBased on the observed decrease in perchlorate affinity for all nonpolyacrylic resins,increased regeneration efficiency can be realized by increasing the temperature of the regenerant solutions when regenerating nonpolyacrylic resins. As thetemperature increases, the perchlorate separation factor decreases, and smallervolumes of regenerant are required to remove the perchlorate from the resin. Thelarger the increase in temperature, the greater the increase in efficiency. This canbe seen by plotting the efficiency as a function of the equilibrium temperatures(20, 40, and 60C) tested (Fig. 38). At 20C, the polyacrylic resin was the most

efficient, as discussed earlier (Fig. 37). As the regeneration temperature increased,the efficiency of the polystyrene and polyvinylpyridine resins increased so that at40C they were similar in efficiency to the polyacrylic resin (IRA-458) and at60C they were much more efficient. Because the perchlorate separation factordid not change with temperature for the polyacrylic resin, no benefit results fromincreasing the regeneration temperature.

3. Effect of Feedwater Composition

The background ions in the feedwater can have a significant impact upon efficiency

of the perchlorate removal process. As the concentrations of the major components


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increase, the run length to perchlorate breakthrough will decrease. This is a general

statement to which there are certain exceptions. For example, an increasing sulfateconcentration will reach a point of maximum effect, after which the effect willdiminish due to the selectivity reversal effect, which results in a decreasing sulfate separation factor as the ionic strength of the feedwater increases due to theincreased sulfate concentration.

Figures 39 and 40 depict the effect of increasing concentrations of sulfate andnitrate, respectively, upon the perchlorate run length for a perchlorate feed concen-tration of 50 mg =L in Big Dalton type water. This effect was shown for the threegeneral types of resins studied—polyacrylic, polystyrene, and nitrate-selective resinsrepresenting low, medium, and high perchlorate selectivity. The run length was

depicted as a percentage of the maximum run length value (lowest componentconcentration) relative to the concentration of the variable component (sulfateor nitrate).

The EMCT model demonstrated that as the sulfate concentration increasedfrom 1 to 250mg =L (0.02–5.0 meq=L), the perchlorate run length decreased by approximately 88%, 95%, and 64% for the polyacrylic, polystyrene, and nitrate-selective resins, respectively (Fig. 39). The smaller decrease for the nitrate-selectiveresin was due to the resin having a preference for nitrate relative to sulfate. There-fore, changes in sulfate concentration will have a smaller effect upon the run length

to perchlorate breakthrough than changes in nitrate concentration.

Figure 38 Efficiency change (BV product water=BV 1 N regenerant) with temperaturefor the 15 resins tested. Exhaustion with Big Dalton water, and regeneration with 1 Nchloride solution.


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Figure 39 Calculated (EMCT program) effect of increasing sulfate concentration uponrun length to perchlorate breakthrough for a polyacrylic, polystyrene, and nitrate-selectiveresin.

Figure 40 Effect of increasing nitrate concentration upon run length to perchlorate

breakthrough for a polyacrylic, polystyrene, and nitrate-selective resin.


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The EMCT program predicted that the volume of regenerant needed toremove perchlorate from the resin would remain the same regardless of the amountof perchlorate, sulfate, or nitrate initially on the resin. Therefore, the change in per-chlorate run length as a result of increasing sulfate in the raw water indicated a dras-tic change of efficiency (BV product water=BV 1 N regenerant) for the polyacrylicand polystyrene resins compared with the nitrate-selective resin.

Figure 40 illustrates the effect of increasing nitrate concentration (0.1–20 mg N per liter) upon perchlorate run length. The y axis is the percent ratio of the bed volumes to perchlorate breakthrough to the maximum perchlorate runlength (lowest nitrate concentration) for a particular resin. The greatest change inrun length with nitrate concentration was with the nitrate-selective resins, with a 76% decrease in perchlorate run length compared with 26% and 36% decreasesfor the polystyrene and polyacrylic resins, respectively. The nitrate-selective resin

showed the greatest change because nitrate was the most preferred major speciesfor this class of resin. Nitrate is not the most preferred species for the polyacrylicand polystyrene resins, and changes in nitrate concentration will not have as greatan effect on run length.

4. Partial Exhaustion=Regenerationof Polyacrylic Gel Resin

The previous discussion on regeneration dealt with complete exhaustion=regeneration, i.e., 100% removal of perchlorate from the spent resin. Completeregeneration is seldom practical for hard-to-remove species such as perchlorate.More often partial exhaustion=regeneration is employed with a view to conserving regenerant and making the process more efficient. To investigate the partial exhaus-tion and partial regeneration of these resins, the IX WINDOWS PRO (IXPRO)v2.0 program was used. The EMCT Windows program could not be used forthe simulation of partial exhaustions=partial regeneration because it assumes a homogeneous bed for exhaustion and regeneration. This assumption is violatedafter the first partial regeneration step. The IXPRO program, by Cathedral Peak

Software, is based on the same assumptions as EMCT Windows except thatnonhomogeneous presaturation is allowed throughout the bed [52]. Also, incontrast to EMCT Windows, which uses the analytical solutions of Helfferichand Klein [51], IXPRO uses algorithms that predict the equilibrium concentrationsof all components in a succession of plates, with the sum of the platesrepresenting a column. The run length for this program is set by the programmerand is independent of the condition of any solution component. Therefore, thisprogram can be used to model partial exhaustion or regeneration for numerousexhaustion=regeneration cycles. Both programs require the input of separation

factors relative to chloride, resin capacity and the composition of the influent


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and regenerant solutions. The IXPRO software also requires the number of bedvolumes of run length for exhaustion and regeneration.

The first aspect of the partial exhaustion=regeneration process investigatedwas the mode of regeneration, whether flow was cocurrent or countercurrent. Fig-ures 41 and 42 are the computer-generated effluent concentrations of perchloratefor various numbers of exhaustion=regeneration cycles for co- and countercurrentregeneration, respectively. The exhaustion=regeneration cycle consisted of a runlength of 600 BV followed by 5 BV of 1 N chloride (4.2 equiv Cl=equiv resin,19 lb NaCl=ft3) regenerant. The cocurrent regeneration (Fig. 41) produced exces-sive perchlorate leakage after the first cycle. Cycles 5–30 showed immediate, early leakage approaching 16 mg =L. In this case, there would be no useful ion exchangeafter cycle 1, assuming a 4 mg =L maximum effluent perchlorate concentration.Countercurrent regeneration (Fig. 42), on the other hand, produced < 2mg =L

perchlorate leakage for all 30 cycles.Figures 43 and 44 show the calculated distribution of perchlorate within the

resin phase (by plate segment) after exhaustion, but before regeneration, for co- andcountercurrent regeneration of a polyacrylic resin. The perchlorate was concentrated

Figure 41 IXPRO computer-generated perchlorate effluent curves for successive partialexhaustion, partial regeneration cycles for the polyacrylic gel resin. Big Dalton water, 50 mg =Lperchlorate, run length 600 BV, cocurrent regeneration with 5 BV of 1 N chloride

(19 lb=ft3

).


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Figure 43 IXPRO computer-generated distribution (over 10 plate segments) of perchlo-rate in the polyacrylic resin following 600 BV exhaustion and before cocurrent regeneration

with 5 BV of 1 N chloride (19 lb=ft3

).

Figure 42 IXPRO computer-generated perchlorate effluent curves for successive partialexhaustion, partial regeneration cycles of polyacrylic gel resin. Big Dalton water, 50 mg =Lperchlorate, run length 600 BV, countercurrent regeneration with 5 BV of 1 N chloride(19lb=ft3).


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in a zone near the effluent end of the column (right side), where it had been drivenby the sulfate wave front of the sulfate-rich zone. If the run continued, this perchlo-rate would be driven from the column into the effluent at concentrations severaltimes that of the influent. This peaking is typical of anions with separation factorsgreater than that of chloride but less than that of the most preferred major species,sulfate in this case.

Figures 45 and 46 illustrate the result of regeneration for co- and counter-current regeneration, respectively, by showing the distribution of perchloratewithin the resin after regeneration with 5 BV of 1 N chloride (4.2 equiv Cl=

equiv resin, 19 lb NaCl=ft3]. The cocurrent regeneration continues to push theperchlorate toward the outlet of the column, but the regenerant solution hastraveled the length of the column and has picked up sulfate, nitrate, and bicarbo-nate. This makes the removal of perchlorate less efficient than it could be with a pure chloride solution. As a result, not all of the perchlorate was removed andwhat was left was near the exit. When the exhaustion cycle was started, this per-chlorate exited the column, appearing as high initial leakage and lower leakagethroughout the run (Fig. 41).

For countercurrent regeneration, the regenerant solution did not travel the

length of the column before encountering perchlorate and, as a result, was more

Figure 44 IXPRO computer-generated distribution (over 10 plate segments) of per-chlorate in the polyacrylic resin following 600 BV exhaustion and before countercurrentregeneration with 5 BV of 1 N chloride (19 lb=ft3).


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Figure 45 IXPRO computer-generated distribution (over 10 plate segments) of perchlo-rate in the polyacrylic resin following 600 BV exhaustion and cocurrent regeneration with5 BV of 1 N chloride (19 lb=ft3).

Figure 46 IXPRO computer-generated distribution (over 10 plate segments) of perchlo-rate in the polyacrylic resin following 600 BV exhaustion and countercurrent regeneration

with 5 BV of 1 N chloride (19 lb=ft3

).


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efficient at removing perchlorate from near the exit of the column. The perchloratewas moved back up the column but was not completely removed, leaving a residualamount at the beginning of the column. This placement of perchlorate does notcontribute to early leakage of perchlorate but will contribute to the earlier break-through of perchlorate as indicated by a shortened run length relative to a virginexhaustion (Fig. 42).

This difference in regeneration results in the observed difference in effluentperchlorate concentration. The computer-simulated cocurrent regeneration resultedin increased perchlorate leakage with successive exhaustion=regeneration cycles. Thesimulated countercurrent regeneration resulted in a slight decrease in run lengththat stabilized after five exhaustion=regeneration cycles with no leakage of perchlorate during exhaustion.

To determine the minimum amount of regenerant necessary during counter-

current regeneration, the regenerant volume was varied from 3 BV to 6 BV of 1 N chloride (11–22 lb NaCl=ft3), keeping the run length constant at 600 BV.Figures 47–50 show the effluent concentration of perchlorate for various exhaus-tion=regeneration cycles for four different regeneration levels in the 3–6 BV range. As the regeneration level decreased, the run length to perchlorate leakage

Figure 47 IXPRO computer-generated perchlorate effluent curves for successive partialexhaustion, partial regeneration cycles for a polyacrylic gel resin. 3 BV of 1 N chloride

(2.5 equiv Cl

=equiv resin, 11 lb NaCl=ft3

).


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Figure 49 IXPRO computer-generated perchlorate effluent curves for successive partialexhaustion, partial regeneration cycles for a polyacrylic gel resin. 5 BV of 1 N chloride

(4.2 equiv Cl

=equiv resin, 19 lb NaCl=ft3

).

Figure 48 IXPRO computer-generated perchlorate effluent curves for successive partialexhaustion, partial regeneration cycles for a polyacrylic gel resin. 4 BV of 1 N chloride(3.3 equiv Cl=equiv resin, 15 lb NaCl=ft3).


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decreased, until at 3 BV of regeneration (11 lb NaCl=ft3), perchlorate leakagestarted immediately after 10 exhaustion=regeneration cycles.

5. Partial Exhaustion=Regeneration of Polystyrene

Resin A typical polystyrene resin with a perchlorate separation factor of 110 (characteristicrange of Type I and II resins) was investigated. A run length of 600 BV withBig Dalton water and 50 mg =L perchlorate was used with 5 BV of 1 N regenerantfor comparison to the polyacrylic resin. Because it failed in experimental tests,cocurrent regeneration was not investigated for the polystyrene resin.

Figures 51–53 present the effluent concentrations of perchlorate for a runlength of 600 BV and a regeneration of 5 BV of 1 N chloride (19 lb NaCl=ft3)at regeneration temperatures of 20, 40, and 60C, respectively. As can be seen,

all three kept perchlorate effluent concentrations well below 1 mg =L for over 100

Figure 50 IXPRO computer-generated perchlorate effluent curves for successive partialexhaustion, partial regeneration cycles for a polyacrylic gel resin. 6 BV of 1 N chloride(5 equiv Cl=equiv resin, 22 lb NaCl=ft3).


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exhaustion=regeneration cycles. But the effluent concentration is deceiving. To pre-dict what would happen with succeeding cycles required looking at the resin phasedistribution of perchlorate.

Figures 54–56 show the perchlorate resin profile, i.e., the distribution of per-chlorate in the resin bed after exhaustion but before regeneration for temperatures

of 20, 40, and 60C, respectively. For the 20C regeneration, there was a minimalamount of perchlorate removed from the resin during regeneration. Instead of being removed, the perchlorate was pushed back up the column by the countercurrentregeneration, forming a zone of enrichment or wave of perchlorate with concentra-tions in the wave higher than would be predicted based upon equilibrium with theinfluent water. The resin-phase perchlorate concentration exceeded the concentra-tion predicted for complete exhaustion of the resin. The equilibrium resin concen-tration would be the point on Fig. 54 where the influent water entered before theperchlorate wave. This same process happened for the 40C regeneration (Fig. 55),

but a much smaller wave formed due to the larger amount of perchlorate removed

Figure 51 IXPRO computer-generated effluent concentration histories of perchloratefor polystyrene resin (a¼ 110) with 600 BV run length, Big Dalton water, 50 mg =L per-chlorate, and 5 BV of 1 N chloride (4.2 equiv Cl=equiv resin, 19 lb NaCl=ft3) regenerant,countercurrent at 20C.


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during regeneration. The 20 and 40C systems would eventually fail, with effluentconcentrations of perchlorate greater than the influent concentrations, in spite of the fact perchlorate is the most favored species. The 60C regeneration (Fig. 56)was even more efficient at removing perchlorate, with perchlorate concentrationsbelow equilibrium at the start of the column. At 60 C the system appeared to

be very near steady state, where there was no further increase in the amount of perchlorate on the resin.

Because the effluent concentrations of the 600 BV run length with 5 BV of regenerant were so low, the run length and regeneration volume were doubled toexamine whether a longer run length with the same ratio of run length to regenera-tion volume could still be effective. If this were successful, then the operation wouldbecome more efficient, with fewer regenerations producing the same volume of product water.

Figures 57–59 show the IXPRO computer-generated effluent concentration

of perchlorate for a run length of 1200 BV and regenerant of 10 BV of 1 N

Figure 52 IXPRO computer-generated effluent concentration histories of perchloratefor polystyrene resin (a¼ 110) with 600 BV run length, Big Dalton water, 50 mg =L perchlo-rate, and 5 BV of 1 N chloride (4.2 equiv Cl=equiv resin, 19 lb NaCl=ft3) regenerant,countercurrent at 40C.


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chloride (37 lb NaCl=ft3), Big Dalton water and 50 mg =L perchlorate at 20, 40,and 60C, respectively. There was considerably more leakage of perchlorate forall three temperatures (note scale change from Figs. 54–56) compared with the600 BV exhaustion, 5 BV regeneration examples. Once again, looking at the

distribution and concentration of perchlorate within the resin bed helpedexplain these results.

Figures 60–62 show the distribution of perchlorate within the resin bedsfollowing 20, 40, and 60C regenerations, respectively. Compared with the samefigures for the 600 BV exhaustion, 5 BV regeneration runs, there was a shift inthe perchlorate distribution. The perchlorate advanced further down the resin dur-ing the 1200 BV exhaustion and was not pushed back sufficiently by regeneration.This resulted in a buildup of perchlorate nearer the effluent end compared with the600 BV exhaustion, 5 BV regeneration runs, and resulted in larger leakage and

earlier breakthrough, even for the 60

C regeneration process.

Figure 53 IXPRO computer-generated effluent concentration histories of perchloratefor polystyrene resin (a¼ 110) with 600 BV run length, Big Dalton water, 50 mg =L perchlo-rate, and 5 BV of 1N chloride (4.2equiv Cl=equiv resin, 19 lb NaCl=ft3) regenerant,countercurrent at 60C.


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6. Simulation of a Nitrate and PerchlorateRemoval Process

Many of the source waters contaminated with perchlorate are also contaminated

with nitrate to the extent that the nitrate has to be removed along with the per-chlorate. Therefore, a typical partial regeneration ion-exchange process for nitratetreatment [53,54] was applied to a Big Dalton type water to evaluate the removalefficiency. The process consisted of a 370 BV run length followed by regenerationwith 2 BV of 1 N NaCl solution (8 lb NaCl=ft3) as described by Clifford et al.[55] and Lauch and Guter [53]. Regeneration was cocurrent, followed by reclassifi-cation of the bed. Partial regeneration allowed a predetermined amount of leakage of nitrate during the succeeding exhaustion run. Because the MCL for nitrate (10 mg Nper liter) is rather large, a significant amount of leakage could be tolerated. By

reclassifying (mixing) the resin bed after regeneration, the nitrate was distributed

Figure 54 IXPRO computer-generated distribution of perchlorate in the resin phase forpolystyrene resin (a¼ 110) with 600 BV run length, Big Dalton water, 50 mg =L perchlorate,before countercurrent regeneration with 5 BV of 1 N chloride (4.2 equiv Cl=equiv resin,19 lb NaCl=ft3) at 20C.


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evenly throughout the bed, which minimized the large amount of leakage of nitratethat would typically be expected following partial cocurrent regeneration.

Figure 63 shows the IXPRO–generated effluent history of a nitrate treatment

process after 30 exhaustion=regeneration cycles. There is initially high perchlorateleakage following cocurrent partial regeneration with reclassification. Similar effluenthistories have been published for nitrate leakage in this same process [56]. Figure 63also compares the perchlorate effluent concentration histories for polyacrylic gel andpolystyrene resins. There was an excessive amount of leakage from both resins due tothe buildup of perchlorate and the redistribution by reclassification. The polyacrylicresin initially exceeded 23 mg =L before decreasing to a level just below 18 mg =Lfor the remainder of the run. The polystyrene resin showed lower leakage than thepolyacrylic resin, even though the concentration of perchlorate in the resin phase

was 10 times that of the polyacrylic resin. The polystyrene resin was able to maintain



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a lower leakage with larger loading due to the larger perchlorate separation factor.This advantage will not continue, because the perchlorate concentration was stillincreasing with each successive exhaustion=regeneration cycle. Approximately 20%

of the perchlorate removed by the resin during exhaustion remained on the resinafter regeneration. The system was not approaching steady state and would continueto retain perchlorate and exhibit increasing concentrations in the effluent. The sys-tem will eventually reach a steady state where the perchlorate removed by the resinduring exhaustion will subsequently be removed from the resin during regeneration.Leakage during the exhaustion run will be stable at a much higher perchlorate con-centration. Because the leakage at 30 cycles was already significant, the higher steady-state leakage would render this process not feasible. Even raising the regenerationtemperature to 60C would not make this treatment process feasible. For a 60C

regeneration, the leakage during the exhaustion run was less, but still considerable



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Figure 58 IXPRO computer-generated effluent concentration of perchlorate for poly-styrene resin (a¼ 110), run length of 1200 BV, Big Dalton water, 50 mg =L perchlorate,and countercurrent regeneration of 10 BV of 1 N chloride (7.7 equiv Cl=equiv resin,

37 lb NaCl=ft3

) at 40

C.

Figure 57 IXPRO computer-generated effluent concentration of perchlorate for poly-styrene resin (a¼ 110), run length of 1200 BV, Big Dalton water, 50 mg =L perchlorate,and countercurrent regeneration of 10 BV of 1 N chloride (7.7 equiv Cl=equiv resin,37 lb NaCl=ft3) at 20C.


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(7 mg =L after 30 cycles), and there was still 13% perchlorate loading on the resin foreach cycle.

The effluent perchlorate leakage improved significantly with countercurrentregeneration and no reclassification (Fig. 64). The leakage was very low for the poly-acrylic and polystyrene resins. However, comparing the mass balance of perchlorate

entering and exiting the system, the behavior was found to be very different forthe two resins. The perchlorate entering the system during exhaustion was eitherremoved onto the resin or allowed to leak into the effluent stream. During regen-eration, the perchlorate on the resin was either removed from the resin into thespent regenerant or retained by the resin. A comparison of the mass of perchlorateentering the system to the mass exiting the system (ignoring any possible destruc-tion) shows three possibilities:

1. The mass entering is less than the mass exiting, in which case perchlorateis being dumped by the resin. This could happen if the composition of

the influent water changes.

Figure 59 IXPRO computer-generated effluent concentration of perchlorate for poly-

styrene resin (a¼ 110), run length of 1200 BV, Big Dalton water, 50 mg =L perchlorate,and countercurrent regeneration of 10 BV of 1 N chloride (7.7 equiv Cl=equiv resin,37 lb NaCl=ft3) at 60C.


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Figure 61 IXPRO computer-generated distribution of perchlorate in the resin phase forpolystyrene resin (a¼ 70) with 1200 BV run length, Big Dalton water, 50 mg =L perchlorate,before countercurrent regeneration with 10 BV of 1 N chloride (7.7 equiv Cl=equiv resin,

37 lb NaCl=ft3

) at 40

C.

Figure 60 IXPRO computer-generated distribution of perchlorate in the resin phase forpolystyrene resin (a¼ 110) with 1200 BV run length, Big Dalton water, 50mg =L perchlo-rate, before countercurrent regeneration with 10 BV of 1 N chloride (7.7 equiv Cl=equiv resin, 37 lb NaCl=ft3) at 20C.


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Figure 63 IXPRO computer-generated perchlorate effluent concentration history forpolyacrylic gel (a¼ 5.5) and polystyrene (a¼ 110) resins after 30 exhaustion=regenerationcycles with 370 BV, Big Dalton water, 50 mg =L perchlorate, 2 BV of 1 N chloride regenerant

(7 lb NaCl=ft3

) cocurrent with resin reclassification following regeneration, 20

C.



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2. The masses entering and exiting are equal; i.e., the system is at steady state.3. The mass entering is greater than the mass exiting; i.e., the perchlorate

mass is increasing on the resin.

The polyacrylic resin appeared to have reached steady state, whereas thepolystyrene resin was not anywhere near steady state. Almost 50% of the perchlorate in

the feedwater was retained on the resin after regeneration at cycle 30. Therefore, theresin-phase concentration would continue to increase for the polystyrene resin. Thisbuildupin the resin phase would eventually lead to increased concentration in the effluent.

B. Highly Perchlorate-Selective Resins

1. Bench-Scale Experiments

To determine if the predicted run lengths for the highly perchlorate-selective resinswere accurate, the two resins with the highest perchlorate separation factors were run

to perchlorate exhaustion. The two resins selected were Sybron SR-7 and Purolite

Figure 64 IXPRO computer-generated perchlorate effluent concentration history for

polyacrylic gel and polystyrene resins after 30 exhaustion=regeneration cycles with 370BV, Big Dalton water, 50 mg =L perchlorate, 2 BV of 1 N chloride regenerant (7 lb NaCl=ft3)cocurrent without resin reclassification following regeneration, 20C.


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A-530. A synthetic groundwater (Big Dalton composition) with a deliberately high perchlorate concentration of 500 mg =L was used at an EBCT of 1 min andan SLV of 9 cm=min. Because run lengths of 100,000 BV were expected, the timefor exhaustion was estimated to be approximately 70 days. The high perchlorateconcentration and rapid flow rate were used to shorten the time required to exhaustthe resins. Figure 65 shows the perchlorate effluent curves for these two resins, andFig. 66 shows the breakthrough curves for sulfate and nitrate for both resins. Asexpected for these nitrate-selective resins, sulfate breaks through well before nitrate.

After 1000 BV, the effluent water quality did not change except for the gradually

increasing perchlorate concentration. The only difference between the water entering and that exiting the column is the removal of a trace amount of perchlorate. Eventhough perchlorate is extremely favored by the resin, at complete exhaustion itoccupied no more than 5% of the functional sites of the resin due to low influentconcentrations.

2. Computer Model Confirmation

The A-530 resin was developed by Oak Ridge National Laboratory (ORNL) for thetreatment of groundwater to remove radioactive pertechnetate ions near the

Paducah Gaseous Diffusion Plant site, Paducah, Kentucky [57,58]. This

Figure 65 Perchlorate effluent history for the exhaustion of highly perchlorate-selectiveresins SR-7 and A-530 with simulated Big Dalton water containing 500 mg =L perchlorate.EBCT of 1 min, with SLV of 9 cm=min.


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resin has a high affinity for large, poorly hydrated ions such as pertechnetate(TcO4

) and perchlorate. The resin has a polystyrene matrix with a mixture of trihexyl- and triethylamine functional groups, and its capacity is 0.9 meq=L.Unfortunately, attempts to determine the perchlorate separation factor of A-530resin were unsuccessful due to the nonhomogeneous nature of the resin bead

mixture. Owing to variations among resin beads, perchlorate–chloride separationfactors ranging over two orders of magnitude (30–3000) were obtained from similarbinary isotherm tests performed on the other resins in this study. The nonhomo-geneity of this resin was verified through density separation tests. The resin wasseparated into three different colors, densities, and sizes of beads using salt solutionsof varying density. Possibly, the varying colors and sizes have different ratios of thefunctional groups (triethyl and trihexyl), resulting in large variations in selectivity.

By comparing the run length of the A-530 resin to that of the SR-7 resin, itwas obvious that the macroproperties of the resin bed yielded high perchlorate

selectivity. Because the usual binary isotherm tests could not be used for the

Figure 66 Nitrate and sulfate effluent history for the exhaustion of SR-7 and A-530resins with simulated Big Dalton water containing 500 mg =L perchlorate. EBCT of 1 min,with SLV of 9 cm=min.


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determination of a precise separation factor, the IXPRO program was used to

obtain a value for the perchlorate separation factor by fitting computer-generatedcurves to the actual data. The average separation factor obtained by breakthroughcurve fitting was then used for further modeling purposes.

Figure 67 shows the actual perchlorate breakthrough curve (solid dots) super-imposed upon the computer-generated breakthrough curves for three differentvalues of the perchlorate separation factor. A perchlorate separation factor valueof 1000, with 10 plate segments, provided a good fit to the actual data. Thus, a separation factor of 1000 was used along with a perchlorate concentration of 50 mg =L in Fig. 68 to compare the computer-generated curve to a breakthroughcurve from an ORNL report [56]. In that report, the developmental prototype resin

(which was scaled up by Purolite to produce the A-530 resin) was exhausted with a simulated groundwater with perchlorate concentrations of 50 mg =L. Figure 68 is anoverlay of the actual curve and the computer-generated curve for a water that wasdifferent from the Big Dalton water used in Fig. 67. The water used in the ORNLtests contained 7 mg =L chloride, 61 mg =L nitrate, 15 mg =L sulfate, and 0.05 mg =Lperchlorate and was fed to a column with EBCT values of < 0.3 min. An exactmatch would not be expected because the EBCT was extremely short and the resinused was a prototype that would have a composition different from the industrially produced A-530 resin; however, the match was reasonably good given these con-

straints. The separation factor of 1000 for the A-530 resin was therefore used to

Figure 67 Comparison of IXPRO computer-generated exhaustion curves using perchlorate separation factors of 900, 1000, and 1100 against the actual breakthrough curvefor A-530 resin.


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model the removal of perchlorate from simulated Big Dalton water with a moretypical 50 mg =L perchlorate concentration.

A problem with such long run lengths is the potential for fouling. Fouling canresult from the accumulation of particles blocking the openings of the resin bed andcausing short-circuiting, high head loss, or complete blockage. Fouling can alsooccur on the resin bead surface or, with macroporous resins, within the resin bead

itself from organics and trace anions present in the water.One trace anion of particular concern is uranium, which often occurs in low

micrograms per liter concentrations in groundwater of the western United States.The uranyl tricarbonate ion [UO2(CO3) 3

4] is extremely favored by the resin,even more than perchlorate, which could result in concentrations in the resin bedafter 20,000 BV that would classify the resin as low-level radioactive waste.

Because the four-valent uranyl carbonate anion undergoes extreme selectivity reversal in brine, a periodic countercurrent low-level regeneration with 2 BV of 1 Nregenerant would effectively remove the uranium and also allow for backflushing

to clear potential particulate fouling. Partial low-level regeneration every 10,000

Figure 68 Comparison of IXPRO computer-generated breakthrough curve to the actual

breakthrough curves for three runs with the ORNL experimental resin prototype of the A-530 resin [57].


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BV was modeled using the IXPRO program, and the perchlorate breakthroughcurve was compared with results of a run with no regeneration (Fig. 69). As canbe seen, this low-level regeneration to prevent uranium buildup has the addedbenefit of extending the perchlorate run length, because some perchlorate isremoved during regeneration.

IV. PROCESS RECOMMENDATIONS

Combining the results of the experiments and computer modeling suggests threeoptions for the treatment to remove perchlorate from drinking water.

Option 1. The polyacrylic gel resin presents the most obvious option for waterwith sulfate concentrations of 100 mg =L or lower. This resin is the most efficientat room temperature due to low perchlorate selectivity. Countercurrent regenera-tion with 20 lb NaCl=ft3 resin should be sufficient to prevent significant leakage.If nitrate peaking is a potential problem, then run length should be to nitratebreakthrough. Because perchlorate has a lower selectivity than sulfate, the concen-

tration of sulfate in the feedwater will dictate the run length. Sulfate concentrations

Figure 69 IXPRO comparison of exhaustion of A-530 resin with and without low-levelregeneration every 10,000 BV to remove accumulated uranium.


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above 100 mg =L will result in significantly shortened run lengths and may necessitate optional treatment processes 2 or 3. Polyacrylic macroporous resinsshould be avoided because increased perchlorate selectivity coupled to decreasedcapacity will result in reduced efficiency.

Option 2. Neither polystyrene nor polyvinylpyridine resins is a realistic choicefor room temperature regeneration because of their poor regeneration efficiencies.The advantage of these resins is that their regeneration efficiencies are greater atelevated regeneration temperatures and there is no perchlorate peaking, becauseperchlorate is the most preferred species. Operated to nitrate breakthrough withregeneration temperatures of 50–60C, the efficiency of these resins can exceed thatof the polyacrylic resins. Regeneration temperatures above 60C are not indicatedfor the polystyrene resins owing to Hoffman degradation of the functional group

[59]. The polyvinylpyridine resins are not limited by this degradation processand could theoretically be operated to temperatures of 200C, although this wouldrequire some type of pressure containment vessel.

The reason for seriously considering elevated temperature regeneration is thatthermocatalytic devices capable of destroying both perchlorate and nitrate in brinesolutions are now being used for regeneration. These operate at elevated tempera-tures and pressures. Because the regenerant solution has to be heated at some point,the possibility presents itself to heat the regenerant before passing it through theresin, thus providing the option of added efficiency. With the perchlorate and

nitrate destroyed, the regenerant can be recycled and used many times before being discarded, further increasing efficiency. Users of nitrate treatment systems withnitrate destruction and brine recycling have reported the possibility of at least40 exhaustion=regeneration cycles before the resin must be discarded [56]. Thisdoes require supplementing the regenerant with chloride to maintain sufficientchloride concentrations because the chloride is removed from the brine during theregeneration process. This type of system is most efficient at making use of allthe chloride present while minimizing disposal volume.

One potential problem with the use of conventional polystyrene resins,especially Type II resins, is the potential formation of N -nitrosodimethylamine

(NDMA) within the resin from chemical reactions with the quaternary amine func-tional group [60]. California currently has a provisional action limit of 20 ng =Lfor NDMA in drinking water. Therefore, even small amounts produced from theion-exchange resin could be a potential problem.

Option 3. The third treatment option involves the use of resins with very highperchlorate selectivity. The resin is run to exhaustion and replaced or processedoff-location. There is no regeneration and therefore no salt requirement or tankageor disposal problems. This would be a viable option for a surface water source.

Surface waters tend to have relatively low concentrations of perchlorate, resulting


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in longer run lengths, and potential contamination from uranyl carbonates wouldbe much less problematic. In this case the ion-exchange resin should not requireregeneration to remove uranium, so salt is not required and does not require dis-posal. Backflushing would be recommended to prevent plugging of the resin by particulates. One point of caution with this system: When perchlorate hasbuilt up to the point where leakage occurs, the column can act as a source of per-chlorate if the concentration in the feed significantly decreases. At this point, tomaintain equilibrium with the feed, the resin could actually dump perchlorate intosolution. The perchlorate concentration in surface waters generally varies with theseason. A finishing column would be required to prevent potential dumping of perchlorate.

V. SUMMARY AND CONCLUSIONS

A. Perchlorate–Chloride Separation Factors

The value of the perchlorate–chloride separation factors ranged over 2.5 ordersof magnitude, from a low of 5.5 to a high greater than 1000 for the 15 resinstested.

B. Effects of Matrix, Functional Group,and Cross-Linking

The polyacrylic resins exhibited relatively low perchlorate separation factors. Incontrast, the polystyrene and polyvinylpyridine resins exhibited very high separa-tion factors, with similar values for both types of resins. Within matrix types,the resin with the largest, most hydrophobic functional group had the highestseparation factor. For the polystyrene matrix, the resin with the highest separa-tion factor contained the tripropylamine functional group. For the polyvinylpyr-idine matrix, the resin with the benzylpyridine functional group had the highest

separation factor.For resins with the same matrix and functional group, increasing the percent

cross-linking resulted in higher perchlorate separation factors. Highly-crosslinkedresins with a macroporous structure had higher separation factors relative to similarresins with a gel structure.

As the size of the functional group increased, the perchlorate selectivity increased and capacity decreased because fewer functional groups could be fit intoany predetermined volume. This decrease in capacity could have an effect upon a perchlorate treatment process, because for any given separation factor, the smaller

the capacity, the shorter the run length to perchlorate exhaustion.


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C. Regeneration as a Function of Temperature,

Time, and Chloride Concentration

For polystyrene and polyvinylpyridine resins, an increase in temperature resulted

in a decrease in perchlorate selectivity. The decrease was approximately thesame for all the polystyrene and polyvinylpyridine resins tested and resulted in a 30% decrease in selectivity for each 20C increase in temperature. Therefore,regeneration at higher temperatures was more efficient, and less chloride wasrequired to remove the perchlorate. The reverse of this would be expected forexhaustion at a lower temperature (< 20–25C), resulting in a longer run lengthto perchlorate exhaustion. The separation factor of the polyacrylic resins did notchange significantly with temperature, and therefore regeneration at elevatedtemperatures or exhaustion at a lower temperature would not be more efficient.

The resins with the highest perchlorate separation factor also had the poorestmass transfer kinetics. The tripropyl resin had the highest measured separation fac-tor (> 1000) and the worst kinetics, with equilibration requiring 7 days at 20C. Asthe temperature increased, the kinetics improved to the extent that at 60C all theresins achieved equilibrium within 24 h. This improvement in kinetics alsoresulted in increased efficiency of regeneration, as evidenced by the much sharperperchlorate elution curve at higher temperature with significantly smaller tailing of perchlorate at the end of the elution.

Based upon the equivalents of chloride supplied per equivalent resin capacity,the 0.5 N NaCl regenerant solution was slightly more efficient than the 1.0 N solu-

tion. However, based upon volume, the 0.5 N solution required almost twice asmuch as the 1.0 N solution.

D. Experimental Validation of EMCT Models to

Predict Run Length and Regeneration

When kinetic limitations were taken into account, the EMCT computer modelsgave a good engineering estimate of the run length and regeneration of perchloratefor a polyacrylic gel resin (a¼ 5.5) and a polystyrene resin (a¼ 110). Compared

with the bench-scale tests, the effects of temperature (23, 45, and 60C) uponregeneration were also reasonably well predicted for the polystyrene resin.

E. EMCT Models for Efficiency of Complete

Exhaustion=Regeneration

The efficiency (BV product water=BV 1 N regenerant) for all the resins was pre-dicted using the EMCT program at 20, 40, and 60C. Because their perchlorateseparation factors did not change appreciably with temperature, the efficiency of

the polyacrylic resins was not affected by temperature. For the polyacrylic resins,


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the efficiency of the gel resin was almost twice that of the macroporous resin. Thiswas due to the larger perchlorate separation factor (a¼ 10) for the macroporousresin compared with the gel polyacrylic resin (a¼ 5.5) and a decreased capacity (0.5 meq=mL) for the macroporous resin compared with the gel polyacrylic resin(1.25 meq=mL). The larger separation factor did not increase the volume of pro-duct water, but it did increase the volume of regenerant, resulting in a decreasein efficiency. The smaller capacity of the macroporous polyacrylic resin resulted inshorter run lengths to perchlorate breakthrough, again resulting in a decrease inefficiency.

The polystyrene and polyvinylpyridine resins were grouped into two effi-ciency regions at 20C. The three resins with the highest perchlorate separation fac-tors had efficiency ratios of 110 BV product water=BV regenerant, while the rest of the resins had ratios around 80. As the temperature of regeneration increased to

40C, the efficiency increased to 150 for the three highest perchlorate-selectiveresins and to 110 for the remaining polystyrene and polyvinylpyridine resins. At60C, the efficiency of the three resins with the highest perchlorate selectivity ran-ged from 200 to 260, and the efficiency of the remaining polystyrene and poly-vinylpyridine resins ranged from 130 to 200.

F. EMCT Modeling of the Effect of Nitrate

and Sulfate Concentration

Increasing the concentration of any anionic component without decreasing theothers will decrease the perchlorate run length to exhaustion. The magnitude of the decrease will depend on the value of the resin affinity for that ion relative tochloride and the magnitude of the increase in concentration. Nitrate and sulfatehave the highest separation factors among the significant background ionsincluding sulfate, nitrate, chloride, and bicarbonate. Increasing the concentrationof nitrate would have a greater negative impact on the perchlorate run lengthfor the nitrate-selective resins compared to the conventional sulfate-selective resins.Increasing the sulfate concentration would have a greater impact on the per-

chlorate removal performance of the sulfate-selective resins (polystyrenes types I andII, polyacrylic) compared with the nitrate-selective resins.

G. IXPRO Modeling of Partial

Exhaustion=Regeneration

The models indicated that the partial exhaustion=regeneration of the polyacrylicgel resin was possible using counterflow regeneration with 600 BV run length and5 BV of 1 N NaCl (19 lb NaCl=ft3, 4.2 equiv Cl=equiv resin). The mass balance

of perchlorate reached steady state after five exhaustion=regeneration cycles.


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The models indicated that at 20C the polystyrene resin (a¼ 110) could notbe used with partial exhaustion=regeneration using a 600 BV run length and 5 BV of 1 N NaCl regeneration. This was due to the buildup of perchlorate on the resin,which resulted in excessive leakage of perchlorate. Increased temperature resultedin improved performance with smaller leakage. At 60C, the mass balance of perchlorate appeared to reach steady state for 600 BV run length and counter-current regeneration with 5 BV of 1 N NaCl. Doubling the run length and theregeneration amount was not as successful.

H. Bench Scale Tests of Highly

Perchlorate-Selective Resins

The tests of the two resins with very high perchlorate selectivity indicated that run

lengths of 60,000 BV or more are possible. Difficulties encountered with plugging of the resin bed indicated that operating a system for long time periods may requirespecial attention to prevent fouling of the resin.

I. Treatment Process Recommendations

Three treatment processes are proposed for the removal of perchlorate from drink-ing water sources to concentrations below 4 mg =L. The first uses polyacrylic gelresins with low perchlorate selectivity. In the presence of sufficient concentrations

of nitrate, the exhaustion should be run to nitrate breakthrough. Regenerationshould be countercurrent with 5 BV of 1 N NaCl (19 lb NaCl=ft3).

The second involves polystyrene and polyvinylpyridine resins and regenera-tion temperatures of 50–60C. The resin is generally run to nitrate breakthroughand regenerated at 60C for increased perchlorate removal efficiency. The perchlo-rate and nitrate in the spent regenerant are then thermochemically or microbially reduced, and the regenerant solution is reused after chloride supplementation.

The third procedure uses highly perchlorate-selective resins. The resin is runto exhaustion and then replaced. This process requires no on-site regeneration. The

spent resin is either discarded or regenerated off-site. Care must be taken thaturanium (in the form of uranyl carbonate) is not concentrated on the resin to ex-cessive levels, and periodic backflushing is recommended to prevent plugging of the resin bed. If uranyl carbonate concentrations are excessive, then periodic low-level regenerations (2 BV of 1 N NaCl, 7 lb NaCl=ft3) are recommended.

REFERENCES

1. Urbansky, E.T. In Perchlorate in the Environment ; Urbansky, E.T., Ed.; Kluwer=

Plenum: New York, 2000.


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2. Helfferich, F. Ion Exchange ; McGraw-Hill: New York, 1962.3. Small, H. Ion Chromatography ; Plenum Press: New York, 1990.4. Clifford, D.A. Ind. Eng. Chem. Fundam. 1982, 21, 141–153.5. Harland, C.E. Ion Exchange: Theory and Practice ; Royal Soc Chem: Cambridge, UK,

1994.6. Kunin, R.; McGarvey, F.X. Ind. Eng. Chem. 1949, 41 (6), 1265–1268.7. Wheaton, R.M.; Bauman, W.C. Ind. Eng. Chem. 1951, 43 (7), 1088–1093.8. Gregor, H.P.; Belle, J.; Marcus, R.A. J. Am. Chem. Soc. 1954, 76 , 1984–1987.9. Gregor, H.P.; Belle, J.; Marcus, R.A. J. Am. Chem. Soc. 1955, 77 , 2713–2719.

10. Aveston, J.D.; Everest, A.; Wells, R.A. J. Chem. Soc. 1958, 231–239.11. Chu, B. Factors governing the selectivity of anion exchangers. PhD Dissertation,

Cornell Univ, Ithaca, NY, 1959.12. Freeman, D.H. J. Chem. Phys. 1961, 35 (1), 189–191.13. Dorfner, L. In Ion Exchangers ; Dorfner, K., Ed.; Walter de Gruyter: Berlin, 1991.

14. Li, P.; SenGupta, A.K. React. Polym. 2000, 44 , 273–287.15. Fisher, S.; Kunin, R. Anal. Chem. 1955, 27 (7), 1191–1194.16. Simon, G.P. Ion Exchange Training Manual ; Van Nostrand Reinhold: New York,

1991.17. Gerin, M.; Fresco, J. Can. J. Chem. 1981, 59 (12), 1705–1710.18. Inczedy, J. J. Chromat. 1974, 102 , 41–45.19. Clifford, D.A.; Weber, W.J., Jr. Nitrate Removal from Water Supplies by Ion

Exchange: EPA-600=2-78-052. US Environmental Protection Agency Office of Research and Development, Cincinnati, OH, 1978.

20. Hradil, J.; Kralova, E.; Benes, M.J. React. Polym. 1997, 33 , 263–273.

21. Clifford, D.A.; Weber, W.J., Jr. React. Polym. 1983, 1, 77–89.22. Jackson, M.B.; Bolto, B.A. React. Polym. 1990, 12 , 277–290.23. Li, P.; SenGupta, A.K. Environ. Sci. Technol. 1998, 32 , 3756–3766.24. Symons, J.M.; Fu, PL-K; Kim, PH-S. In Ion Exchange Technology: Advances in Pollution

Control ; SenGupta, A.K., Ed.; Technomic: Lancaster, PA, 1995.25. Chu, B.; Whitney, D.C.; Diamond, R.M. J. Inorg. Nucl. Chem. 1962, 24 , 1405–1415.26. Diamond, R.M. J. Phys. Chem. 1963, 67 (12), 2513–2517.27. Huque, E.M. J. Chem. Educ. 1989, 66 (7), 581–585.28. Subramonian, S.; Clifford, D.A. J. Solution Chem. 1989, 18 (6), 529–543.29. Sherrington, D.C. Chem. Commun. 1998, 21, 2275–2286.30. Diamond, R.M.; Whitney, D.C. In Ion Exchange: A Series of Advances ; Marinsky, J.A.,

Ed.; Marcel Dekker: New York, 1966; 1 pp.31. Reichenberg, D. In Ion Exchange: A Series of Advances ; Marinsky, J.A., Ed.; Marcel

Dekker: New York, 1966; 1 pp.32. Barron, R.E.; Fritz, J.S. J. Chromatogr. 1984, 284 , 13–25.33. Barron, R.E.; Fritz, J.S. . J. Chromatogr. 1984, 316 , 201–210.34. Vlacil, F.; Vins, I. J. Chromatogr. 1987, 391, 133–144.35. Slingsby, R.W.; Pohl, C.A. J. Chromatogr. 1988, 458 , 241–253.36. Okada, T. J. Chromatogr. A 1997, 758 , 19–28.37. Subramonian, S.; Clifford, D.A. React. Polym. 1988, 9 , 195–209.38. Liberti, L.; Passino, R. In Ion Exchange and Solvent Extraction. A Series of Advances ;

Marinsky, J.A., Marcus, Y., Eds.; Marcel Dekker: New York, 1985; 9 pp.


7/25/2019 DK1300_ch05.pdf


39. Bolto, B.A.; Weiss, D.E. In Ion Exchange and Solvent Extraction. A Series of Advances ;Marinsky, J.A., Marcus, Y., Eds.; Marcel Dekker: New York, 1977; 7 pp.

40. Borst, C.L.; Grzegorczyk, D.S.; Strand, S.J.; Carta, G. React. Polym. 1997, 32 , 25–41.41. Fisher, S. React. Polym. 1997, 35 , 23–28.

42. Maity, N.; Payne, G.F.; Ernest, M.V., Jr. Albright, R.L. React. Polym. 1992, 17 ,273–287.

43. Saad, M.A. Thermodynamics for Engineers ; Prentice-Hall: Englewood Cliffs, NJ, 1966.44. Bailly, M.; Tondeur, D. J. Chromatogr. 1980, 201, 343–357.45. Muraviev, D.; Noguerol, J.; Valiente, M. React. Polym. 1996, 28 , 111–126.46. Dobrevsky, I.D.; Konova, G. React. Polym. 1988, 7 , 273–276.47. Peterka, F. J. Chromatogr. 1980, 201, 359–370.48. Sybron Chemicals, Inc. Engineering Data for ASB-2 Strong Base Type II Anion

Exchange Resin. Sybron Chemicals, Inc., Birmingham, NJ, 1989.49. Horng, L.L. Reaction mechanisms and chromatographic behavior of polyprotic acid

anions in multicomponent ion exchange. PhD Dissertation, Univ Houston, Houston,TX, 1983.

50. Tirupanangadu, M.S. A Visual Basic Application for Multicomponent Chromatogra-phy in Ion Exchange Columns. Master’s thesis. Univ Houston, Houston, TX, 1996.

51. Helfferich, F.; Klein, G. Multicomponent Chromatography: Theory of Interference ; MarcelDekker: New York, 1970.

52. Guter, G.A. In Ion Exchange Technology: Advances in Pollution Control ; SenGupta, K.,Ed.; Technomic: Lancaster, PA, 1995.

53. Lauch, R.P.; Guter, G.A. . J. AWWA 1986, 78 (5), 83–88.54. Liang, S.; Mann, M.A.; Guter, G.A. J. AWWA 1999, 91 (2), 79–91.

55. Clifford, D.A.; Lin, C.C.; Horng, L.L.; Boegel, J.V. Nitrate Removal from Drinking Water in Glendale, Arizona. PB 87-129 284= AS NTIS, Springfield, VA, 1987, 138 pp.56. Liu, X.; Clifford, D.A. J. AWWA 1996, 88 , 88–99.57. Gu, B.; Brown, G.M.; Alexandratos, S.D.; Ober, R.; Patel, V. Selective Anion

Exchange Resins for the Removal of Perchlorate (ClO4) from Groundwater. Envir-

onm Sci Div Publ No. 4863. Oak Ridge National Laboratory, Oak Ridge, TN.ORNL=TM-13753. 1999.

58. Bonnesen, P.V.; Brown, G.M.; Alexandratos, S.D. Environ. Sci. Technol. 2000, 34 (17), 3761–3766.

59. Baumann, E.W. J. Chem. Eng. Data 1960, 5 (3), 376–382.60. Najm, I.; Trussell, R.R. J AWWA 2001, 93 (2), 92–99.

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