Ion Exchange

Newell PA, Wrigley SP, Sehn P and Whipple SS (1996)An economic comparison of reverse osmosis and ionexchange in Europe. In: Greig JA (ed.) Ion ExchangeDevelopment and Applications, pp. 58}66. Cambridge:Royal Society of Chemistry.

Nolan J and Irving J (1984) The effect on the capacity ofstrong base anion exchange resins of the ratio of chlorideto sulphate in the feed water. In: Naden D and StreatM (eds) Ion Exchange Technology, pp. 160}168. Chi-chester: Ellis Horwood.

Anion Exchangers: Ion Exchange

W. H. Ho ll, Karlsruhe Nuclear Research Center,Karlsruhe, GermanyCopyright^ 2000 Academic Press

Introduction

Anion exchange resins consist of a polymeric matrixto which different functional groups are attached.Most weakly basic anion exchangers contain tertiaryamino groups; in a few cases primary and secondarygroups are also encountered. In many cases weaklybasic anion exchangers are not monofunctional butpossess a variety of amino groups. Strongly basicresins contain quaternary ammonium groups. Stan-dard commercially available exchangers con-tain either }N#(CH3)3 groups (type 1 resins) or}N#(CH3)2C2H4OH groups (type 2 resins). Bothweakly and strongly basic exchange resins are avail-able in gel-type or macroporous modiRcations.

Properties and Relds of application mainly dependon the dissociation properties of the functionalgroups in which dissociation plays the most impor-tant role. By means of the mass action law, dissoci-ation constants of the protonated amino andammonium groups can be estimated. The respectivenumerical values are in the range of pKa'13 forstrongly basic resins and 5}8 for weakly basic resins.Therefore, strongly basic resins are protonated overthe entire pH range but weakly basic exchangers areprotonated at pH values below 5}8, depending on thetype. As a consequence, strongly basic resins willexchange anions in both acid and alkaline solutions.In addition, these exchangers can adsorb weak acidsand even ionize very weakly dissociated acids. Weak-ly basic resins, however, can operate only in acidicmedia and are unable to convert neutral salts to therespective hydroxides (e.g. NaCl to NaOH). Further-more, they cannot normally adsorb weak acids.

The uptake of anions by resins is subject to speciRcinteractions between counterions and co-ions and thedistribution of exchangeable ions depends on theproperties of both the exchanger and the ions. Conse-quently, a favoured sorption of certain types ofanions occurs. The sequence of afRnities is giveneither qualitatively by the selectivity series or quantit-

atively by, for example, separation factors. For weak-ly basic anion exchangers the sequence of most com-mon anions in fresh water is:

OHSO24 'NO3 'Cl

Due to the dissociation properties of the functionalgroups, hydroxyl ions are strongly preferred. This isimportant for the conversion of these resins to thehydroxyl (or free base) form in the regeneration step,in which only slightly more than the stoichiometricamount of OH} bearing solutions is required.

For strongly basic anion exchangers the selectivityseries is:

SO24 'NO3 'Cl'HCO3 'OH

For these resins hydroxyl ions are the least prefer-red among the standard anions. Conversion of theresins to the hydroxyl form therefore requires com-paratively large excess amounts of sodium hydroxide.

For elimination of nitrate anions from drinkingwater the preferred sorption of sulfate ions causesconsiderable disadvantages. Extensive research dur-ing the 1980s has shown that this drawback can beovercome by introducing functional groupswhich aremore hydrophobic and bulkier. By these means, theability to adsorb sulfate ions is considerably de-creased. The so-called nitrate-selective resins whichare now commercially available contain triethyl in-stead of trimethyl groups and, therefore, exhibit a re-versed preference for nitrate and sulfate ions.

The rate of exchange depends mainly on the inter-nal interdiffusion of exchanging ions. On the com-pletely ionized strong base resins this diffusion israther quick and the overall rate of exchange mainlydepends on the particle size distribution.With weaklybasic exchangers, however, the poor dissociation andfurther speciRc interactions between functional sitesand diffusing ions considerably slow the rate of ex-change. For these resins, both the uptake of acids andconversion to the free base form by means of sodiumhydroxide strongly depend on the concentration ofthe liquid phases.

Strongly basic anion exchangers in the hydroxylform are subject to considerable degradation of

III /WATER TREATMENT /Anion Exchangers: Ion Exchange 4477

functional groups at temperatures above 403C, lead-ing to a loss of strongly basic functionality. Weaklybasic exchangers are very stable.

Applications

Water Demineralization

The most important application of anion exchangersin water treatment has been in demineralization pro-cesses. Demineralization consists of the subsequentcation exchange for hydrogen ions as the Rrst stepand either adsorption of acids by a weakly basicexchanger or real anion exchange for hydroxyl ionson a strongly basic exchanger as the second step.As a result, demineralized water is produced: this maybe used for various purposes, such as boiler feedwater.

The efSuent from the preceding cation exchangestep consists of a mixture of strong mineral acidsand carbonic acid; this is equivalent to the cationsoriginally dissolved. Furthermore it contains silicaand dissolved organic compounds, neither of whichare retained by the cation exchangers. If no silicaremoval is needed, weakly basic anion exchangersin their free base form are used to remove strongmineral acids. This process develops as the uptakeof acids:

R3N#(HCl, H2SO4) R3NH##(Cl, SO24 ) [1]

(Parantheses are used to express the stoichiometry ofthe exchange; R3 denotes the matrix of the weaklybasic exchanger.) In technical Rlter columns theuptake of sulfuric and hydrochloric acid results inthe development of zones with different predominantloadings: Rrstly, a sulfate-rich zone close to theinlet; secondly, a chloride zone; and Rnally, a zonein which the resin is still in the free base form.During the Rlter run the sulfate and chloride zonesbecome larger and the boundaries between the zonesare shifted towards the column outlet. Consequently,chloride ions are the Rrst to appear in the columnefSuent.

Theoretically, the strong acids should be com-pletely eliminated. In practical installations, however,the leakage of sodium from the strongly acidic cationexchanger cannot be avoided and it is in the range of2}50 g L1, depending on the level of regeneration.By this means, the feed solution for the anion ex-changer contains some neutral salt which cannot beeliminated. Consequently, an equivalent amount ofchloride ions remains in the efSuent. Apart from thissodium leakage-caused slip, the presence of chloride

ions may also be due to the hydrolysis of the hydro-chloride form of weak base resins:

R3NH#Cl R3N#HCl [2]

Weak acids like carbonic acid are only adsorbed toa small extent and hydrogen carbonate ions are rap-idly replaced by anions of strong acids. Carbonic acidor CO2 is therefore eliminated either by physicaldegassing or by means of a strongly basic exchanger.Since the capacity of weakly basic resin increases withincreasing total ionic concentration (to which carbon-ic acid contributes), degassing of CO2 is often placedafter the weakly basic exchanger Rlter.

Regeneration of weakly basic anion exchangers isusually carried out by means of sodium hydroxide,although other chemicals like Ca(OH)2 or am-monium hydroxide have also been proposed. Toavoid the precipitation of calcium sulfate, the regen-erant solutions of anion exchangers generally requirecalcium-free water. Since the strong acids are easilyneutralized, regeneration is efRcient. The operatingcapacity depends on the composition of the rawwater and the resulting loading of the anion ex-changer. If equal amounts of caustic are applied in theregeneration step, the effective capacity decreases byabout 10% if the raw water contains exclusivelysulfate instead of only chloride ions.

Unlike with weakly basic resins, the removal ofacids by a strongly basic resin develops as a neutral-ization reaction:

R4N#OH#(HCl, H2SO4)

R4N#(Cl, SO24 )#H2O [3]

(R4 denotes the matrix of the strong base resin.)Because of the high pH value in the resin phase,strongly basic exchangers can ionize and adsorb ionsfrom carbonic acid and even from silica:

R4N#OH#H2CO3 R4N#HCO3 #H2O [4]

R4N#OH#H4SiO4 R4N#H3SiO4 #H2O[5]

For boiler feed water, for example, the removal ofboth carbonic acid and silica is of great interest.Usually, most of the carbonic acid is removed byphysically degassing. By this means a large part of thecapacity of the strongly basic anion exchanger issaved and the resin has to eliminate only the remain-ing traces of carbonic acid. The removal of silica isfar more complicated than shown in eqn [5] above.

4478 III /WATER TREATMENT /Anion Exchangers: Ion Exchange

Figure 1 Interdependence of sodium and silica leakage.

During the accumulation in the resin phase, silicastarts to polymerize. This is favoured by long cyclesas well as by high concentrations in the feed water.Regeneration using NaOH may, therefore, becomea dissolution rather than a true ion exchange process.Warm solutions (approximately 503C) are consider-ably more effective than solutions at ambient temper-ature. In addition, regeneration may be adverselyinSuenced by the mode of regeneration and the pos-sible displacement of hydrogen carbonate ions. Theleakage of silica is closely related to the leakage ofsodium in the preceding cation exchanger (Figure 1).

Regeneration of strongly basic resins is generallycarried out by means of NaOH at concentrationsbetween 1 and 5%. As with weakly basic exchangers,the resulting operating capacity depends on the com-position of the feed water, the relative amount ofNaOH, temperature and, to a certain extent, the rateof Rltration during regeneration. The relative quanti-ty of NaOH required is considerably higher than forweak base resins and lies between 150 and 250% ofthe stoichiometric amount. For the slightly weakerbasic resins of type 2, the operating capacities arehigher than for type 1 resins.

There can therefore be savings in chemicals andcost if the anion exchange step is split into two stages.In the Rrst stage a weakly basic resin is applied for theelimination of strong acids. After physical degassingto remove carbon dioxide, the strongly basic ex-changer is exclusively used to eliminate the remainderof the carbonic acid and silica. Both resins are regen-erated together: the NaOH solution Rrst passesthrough the strongly basic resin which requires a con-centrated regenerant without impurities. The efSuent

then passes the weakly basic resin. Due to the strongpreference for hydroxyl ions, considerable levels ofimpurities can be tolerated.

Demineralization of water by subsequent orsimultaneous cation/anion exchange has beenrealized in countless installations for both freshand waste water treatment. It has become extremelyimportant for boiler feed water and for ultrapurewater for the electronics and pharmaceutical indus-tries. Typical conditions to be met are conductiv-ities below 0.2 S cm1 and silica concentrations(5 mg L1.

Partial Demineralization of Drinking Water

In Europe numerous water supplies distribute drink-ing water with elevated total hardness caused by thepresence of sulfate and calcium ions. Treatment ofsuch water, therefore, requires the elimination of sul-fate ions parallel to diminishing hardness. In suchapplications complete demineralization is not re-quired. Consequently, considerable leakage of the ionexchangers can be tolerated. The condition of simul-taneously diminishing the concentrations of alkalineearth and sulfate ions is met by the carbon dioxideregenerated ion exchange (CARIX) process. Thisprocess uses a mixed bed consisting of a weakly acidicresin in the free acid form and a strongly basic resin inthe hydrogen carbonate form. In contact with cal-cium- and sulfate-bearing raw water, both kinds ofions are replaced by carbonic acid:

RCCOOH RCCOO(Ca2#)

#(Ca2#, SO24 )R4N#HCO3 R4N#(SO24 )

#H2CO3 [6]

The advantage of this process is its reversibility: forregeneration, carbon dioxide is dissolved in untreatedraw water under a pressure of typically 0.4}0.5 MPa.The carbonic acid solution is pumped across themixed bed and simultaneously regenerates bothresins. Although carbon dioxide is applied in largeexcess, this does not contribute to the salt content ofthe waste water which exclusively contains the ionsremoved during the service cycle. Carbonic acid isa weakly effective regenerant for both resins andgenerates an effective capacity of+50% of the totalcapacity on the cation exchanger and only 20% onthe anion exchanger. As a consequence, completedemineralization is not possible and considerableleakage is observed. The possible throughput betweentwo regenerations is small. Within certain limits the


Figure 2 Concentration histories of chloride, nitrate and sulfate in the Carix process. Feed alkalinity (HCO3 ): 6.5 mmol L1. Circles,Cl; triangles, NO3 ; diamonds, SO24 ; squares, HCO3 .

Table 1 Partial demineralization by means of the CARIXprocess

Parameter Raw water Product water

Total hardness 5.4 mmol L1 2.3 mmol L1Hydrogen carbonate 6.6 mmol L1 3.4 mmol L1Sulfate 1.6 mmol L1 0.35 mmol L1Nitrate 0.6 mmol L1 0.45 mmol L1Chloride 1.5 mmol L1 1.3 mmol L1Conductivity 930 S cm1 470 S cm1pH Value 7.30 7.80

CARIX process can be adapted to the objectiveof treatment by adjusting the volume ratioVanion : Vcation of the two resins in the range of 3 : 1to 1 : 3. With respect to both the removal of sulfateand regeneration by means of carbonic acid, type2 and acrylic anion exchangers show the bestresults.

So far, the process has been realized in three full-scale plants in water works in Germany. Each of theseplants consists of one or two sets of three Rlters whichoperate in a merry-go-round mode. The totalthroughput for each Rlter between two regenerationsis 35}50 bed volumes. After half of the total through-put, a second Rlter starts its service cycle. After thefull throughput, the Rlter is regenerated and waits forthe next service cycle while the third one is started.The product water is the mixed efSuent of the twooperating Rlters. Typically, about 10% of the rawwater is needed for regeneration. From the wastewater, 90% of the unspent carbon dioxide is re-covered. Thus, the consumption of CO2 amounts to0.4}0.45 kg m3.

Figure 2 shows a typical breakthrough history forstandard anions. Average product water concentra-tions are listed in Table 1.

Nitrate Removal

For many drinking water supplies the presence ofnitrate in ground and surface water at elevated con-centrations has become a serious problem because ofits health effects. As the simplest possibility, nitratecan be eliminated fromwater by means of strong baseanion exchangers in the chloride form:

R4N#Cl#(NO3 , SO24 , HCO23 )

R4N#(NO3 , SO

24 , HCO23 )#Cl [7]

Using conventional strong-base anion exchangers,the main difRculty of this method arises from the


Figure 3 Operating capacity of a type 2 strongly basic anion exchanger depending on the concentrations of nitrate and sulfate in theraw water. Continuous line, 0.36 mmol L1; dashed line, 0.72 mmol L1; dotted and dashed line, 2.1 mmol L1. Resin: Lewatit M600.

preferred uptake of sulfate ions. Consequently,the nitrate uptake capacity strongly depends on thesulfate/nitrate concentration ratio in the rawwater. As an example, for a type 2 resin the nitrateuptake capacity decreases by 60% if the sulfateconcentration in the feed increases from 50 to220 mg L1.

During the uptake of sulfate, nitrate and hydrogencarbonate by a chloride-loaded conventional anionexchanger, loading zones develop in the Rlter column:a sulfate-rich zone, a nitrate-rich section and a mixedchloride/hydrogen carbonate zone followed by theresin in its original chloride form.

Due to the development of the zones and the dis-placement of less preferred anions, continuouschange occurs in the efSuent composition. When thehydrogen carbonate-rich zone has reached the outletthere is still removal of nitrate species, but an increasein the concentration of hydrogen carbonate and a cor-responding increase in the pH in the efSuent occur.The column run has to be stopped when the nitrate-rich zone reaches the Rlter outlet.

Regeneration is carried out using brine solutions of2}10% NaCl:

R4N#(SO24 , NO3 )#NaCl

R4N#Cl#(Na2SO4, NaNO3) [8]

The total quantity of NaCl used depends on thetolerable nitrate leakage in the service cycle. Toachieve efSuent concentrations 42 mg L1, the re-quired amount is 5200 g L1 resin. With smallerquantities the leakage becomes larger.

The operating capacity of conventional anion ex-changers depends on both the nitrate and sulfateconcentrations of the feed water. Figure 3 shows therespective interdependence for a commercially avail-able type 2 resin.

The objective of treatment normally does not con-sist of the complete elimination of nitrate but only ofits reduction below the maximum permitted concen-tration. A tolerable nitrate concentration in the prod-uct water can therefore be achieved by two differentmethods: the Rrst uses an exchanger which has beenregenerated with a large amount of NaCl. Since theleakage of the column is small, only part of the waterhas to be treated and can be blended with a by-pass ofuntreated water.

This kind of nitrate removal process was Rrst real-ized in 1975 in Long Island in a pseudo-continuousinstallation. This plant consisted of a closed-loop tubeincluding sections for back-washing, regeneration


Table 2 Data of operation of Ecodenit plant

Parameter Feed waterconcentration(mg L1 )

Product waterconcentration(mg L1 )

Waste waterconcentration(g L1 )

Nitrate 70 25 25Sulfate 50 10 10Hydrogencarbonate

65 55 55

Chloride 50 120 120

and rinsing of the exhausted resin. After exhaustionthe resin material in the contacting section is pulsedinto the back-wash part, whereas regenerated and rin-sed resin material enters the contacting section. Theplant was designed for a maximum throughput of277 m3 h1. The nitrate concentration was diminishedfrom 100 to 43 mg L1 as the tolerable maximum.

In the second possibility the amount of NaCl re-quired during regeneration is smaller. As a conse-quence, the regeneration is less efRcient and thenitrate leakage becomes larger. Thus, all the waterhas to be treated. This principle is shown in theEcodenit process developed in France. This processoperates with NaCl quantities of 4100 g L1 ofresin and uses co-Sow regeneration. The Rrst full-scale plant for the treatment of 160 m3 h1 is inservice in northern France. This plant consists ofthree Rlters operating in a merry-go-round mode.Each Rlter contains 8 m3 of a strongly basic resin.Results of the operation of the full-scale plant aresummarized in Table 2.

Both principles are combined in a third modiRca-tion. Such a plant went into service in 1983 in Cali-fornia. The plant was to treat 115 m3 h1 blendedwith 45 m3 h1 raw water. It consists of three col-umns which are also operated in a merry-go-roundmode. For each Rlter the throughput between tworegenerations amounts to 250 bed volumes. Thenitrate concentration is decreased from 71 to11.5 mg L1 in the mixed efSuent of the service Rltersand to 27}36.5 mg L1 in the blended product water.Since 1983, numerous nitrate elimination plants havecome into service: most apply nitrate-selective ex-changers.

Chromate Removal

Chromic acid and chromates are widely used in manyapplications. Anion exchange offers an ideal oppor-tunity for the removal and recovery of chromates.Both strongly and weakly base resins can be applied.

In aqueous solutions chromate ions exist in differ-ent ionic forms. The speciationmainly depends on thepH value. In the acidic region (1(pH(6.5) and for

dilute solutions, HCrO4 is the predominant species.Above pH 6.5 mostly CrO24 is found.With respect toion exchange processes it is important that dimeriz-ation occurs at elevated concentrations:

2HCrO4 Cr2O27 #H2O [9]

The respective pH conditions may exist in theanion exchanger phase. Therefore, the resins can beconsidered to be at least partly loaded with Cr2O

27species.

Chromate species are strongly preferred over chlor-ide and sulfate ions under normal conditions. How-ever, at acidic pH this preference vanishes for bothweakly and strongly basic anion exchangers whenchromate is the trace component. Consequently,Rxed-bed experiments using standard acidic chro-mate-bearing waste waters exhibit a gradual increasein the efSuent concentration. An increase in the con-centration of competing sulfate ions yields only a neg-ligible decrease in chromate capacity. In contrast tothis, an increase in the chloride concentration leads toa considerable decrease in chromate capacity fora given liquid-phase concentration. For weakly basicresins the chromate capacity decreases with increas-ing pH because of the deprotonation of the functionalgroups. Below pH 5 the capacity is constant anddepends only on the background composition of thesolution.

Using strongly basic exchangers chromate can beremoved by the following exchange processes:

R4N#OH#(CrO24 ) R4N#(CrO24 )#(OH)[10]

R4N#Cl#(Cr2O27 ) R4N#(Cr2O27 )#(Cl)

[11]

Weak base anion exchangers are applied, forexample in the sulfate form:

R3N#(SO24 )#CrO24 R3N#(CrO24 )#SO24

[12]

R3N#(SO24 )#Cr2O27 R3N#(Cr2O27 )#SO24[13]

AcidiRcation of the feed solution leads to theformation of hydrogen dichromate species, whichdoubles the capacity of the resins, e.g.:

R3N#(Cr2O27 )#Cr2O27 #H2SO4

R3N#(HCr2O7 )2#SO24 [14]


Figure 4 Elimination of Cr(VI) from cooling tower blowdown. Effluent concentration histories of loading (circles) and polishing(squares) columns. Feed water composition: Cl(500 mg L1), SO24 (200 mg L1), Cr(VI) (10 mg L1), HCO3 (100 mg L1),Ca2##Mg2# (5.5 mmol L1), Na# (250 mg L1). Resin: Amberlite IRA 94.

After exhaustion, treatment with NaOH will con-vert hydrogen dichromates to chromates and, sub-sequently, yield half of the chromate loading onelution. Removal of the remainder of the chromatefrom the strongly basic exchanger requires concen-trated sodium sulfate solutions:

R4N#(CrO24 )#Na2SO4

R4N#(SO24 )#Na2CrO4 [15]

In contrast to strongly basic exchangers, weaklybasic ones can be completely regenerated by means ofNaOH. However, before the following service cycle,the resins have to be reconverted to the sulfuric acidform:

R3N#(CrO24 )#2NaOH R3N#Na2CrO4 [16]

R3N#H2SO4 R3N#(SO24 ) [17]

Chromate species elimination from cooling towerblowdown has been achieved by application ofa styrene weakly basic resin in a merry-go-roundsystem with three columns. In the plant, column 1serves as the crude loading column which eliminatesmost of the chromate. The relatively low efSuentconcentrations are further decreased by the second(polisher) column. The third column is regen-erated/conditioned or waits for service. When themaximum tolerable efSuent concentration of thepolisher column is exceeded, the sequence is switched

so that the polisher column now acts as the loadingcolumn and the freshly regenerated Rlter is appliedfor polishing. The previous loading column is regen-erated and conditioned. A typical example is given inFigure 4.

Removal of Organic Substances

Natural organic substances normally have an anionicnature at pH values above neutral. Apart from theirelimination in a suitable pretreatment step, they canbe adsorbed by both weakly and strongly basic anionexchangers. Uptake of organic substances is due toboth ionic and van der Waals forces. They may pres-ent a problem for anion exchangers because of a pos-sibly irreversible adsorption by styrene-based gel-typeresins.

Both types of exchangers, therefore, can act asscavengers for the removal of organic compoundsprior to further ion exchange steps. In general, strong-ly basic exchangers exhibit a better elimination ofhumic substances. For demineralization of industrialwater they are applied in the chloride form as scaven-ger units at the head of the deionization train. Thebasic problems associated with the application ofanion exchangers as scavengers in demineralizationplants are the additional exchanger unit and the ex-change of the organics and hydrogen carbonate ionsfor chloride. Thus, the efSuent composition may beless favourable for the application of weakly acidicexchange resins.

Depending on the total concentration of organicmatter in the feed water and to the desired level in theefSuent, different resin types are recommended. Thecombined application of weakly and strongly basic


resins has been successfully applied in the Stratabedconcept, in which two layers of the respective resins,both of the polystyrene type, are used. The applica-tion of this concept requires the use of exchangeresins with a particle size distribution which allowsthe maintenance of the stratiRed bed in each column.Since regeneration is carried out using warm NaOHsolution, precipitation of silica is avoided. Favourableconditions are met: waters of poor alkalinity andchloride and sulfate are the major part of the anionspresent. Acrylic anion exchangers allow a reversibleuptake of humic acids. Although the capacity ofacrylic resins for organics is poorer, they are highlyeffective. They can be used in all applications exceptin condensate polishing with extremely high Sowrates. Under these conditions the elastic properties ofthese resins exclude their use.

At low levels of organic matter in the feed water theapplication of macroporous weakly basic resins intheir free base form in the de-ionization train allowsan efRcient removal. Furthermore, these resins areless susceptible to irreversible fouling.

In drinking water treatment, macroporous styrene-based strong base resins were employed in Hanoverin Germany as a single treatment step for the removalof organic matter from ground water. Dissolved or-ganic carbon was to be reduced from 6.5 to about3 mg L1. After 5000 bed volumes, the resin wasregenerated by means of an alkaline NaCl solutionwhich could be reused seven times. The plant wasshut down in 1995.

Apart from the elimination of natural organicmatter, anion exchangers may also be applied to theremoval of organics from different types of wastewater. Weakly basic resins have been used for theremoval of phenol. Sorption is carried out at ratherlow Sow rates of 2}8 BV h1. NaOH or organicsolvents are used for regeneration. Macroporousweakly basic resins in the sulfuric acid form are ap-plied for decolorization of kraft bleach liquors. Afterexhaustion they are regenerated using NaOH andconditioned by means of sulfuric acid.

See also: I/Ion Exchange. III /Water Treatment: Over-view: Ion Exchange. Resins as Biosorbents: IonExchange.

Further Reading

Bayer AG (1974) Lewatit-Lewasorb Manual. Leverkusen:Bayer.

Bolto BA and Pawlowski L (1987) Wastewater Treatmentby Ion Exchange. London: E. & F. Spon.

Dorfner K (1991) Ion Exchangers. Berlin: Walter deGruyter.

Harland CE (1994) Ion Exchange, Theory and Practice.Bath: Bath Press.

Kunin R (1996) Amber-hi-lights. Littleton, CO: TallOaks.

Mitsubishi Chemical. Diaion Manual of Ion ExchangeResins and Synthetic Adsorbents. Mitsubishi Chemical.

SenGupta AK (ed.) (1995) Ion Exchange Technology.Lancaster and Basel: Technomic.

WATER-SOLUBLE VITAMINS: THIN- LAYER(PLANAR) CHROMATOGRAPHY

See III /VITAMINS/WATER-SOLUBLE: THIN-LAYER (PLANAR) CHROMATOGRAPHY

WAXES: SUPERCRITICAL FLUIDCHROMATOGRAPHY

See III /OILS, FATS AND WAXES: SUPERCRITICAL FLUID CHROMATOGRAPHY


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Ion Exchange