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Behaviour of Ion Exchange Resins andCorrosion Inhibitors in Dilute ChemicalDecontaminationSankaralingarm VELMURUGAN a , Valil Sreedharan SATHYASEELAN a ,Therthala Veedu PADMAKUMARI a , Sevilimedu Veeravalli NARASIMHANa & Pratap Kumar MATHUR aa Water and Steam Chemistry Laboratory , Indira Gandhi Centre forAtomic Research Campus , Kalpakkam , TamilNadu , 603102 , INDIAPublished online: 15 Mar 2012.

To cite this article: Sankaralingarm VELMURUGAN , Valil Sreedharan SATHYASEELAN , Therthala VeeduPADMAKUMARI , Sevilimedu Veeravalli NARASIMHAN & Pratap Kumar MATHUR (1991) Behaviour of IonExchange Resins and Corrosion Inhibitors in Dilute Chemical Decontamination, Journal of NuclearScience and Technology, 28:6, 517-529, DOI: 10.1080/18811248.1991.9731391

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Journal of NucLEAR SciENCE and TEcHNOLOGY, 28(6), pp. 517-529 (June 1991).

Behaviour of Ion Exchange Resins and Corrosion Inhibitors in Dilute

Chemical Decontamination

Sankaralingam VELMURUGAN, Valil Sreedharan SATHYASEELAN, Therthala Veedu PADMAKUMARI, Sevilimedu Veeravalli NARASIMHAN

and Pratap Kumar MATHUR

Water and Steam Chemistry Laboratory, Indira Gandhi Centre for Atomic Research Campus*

Received September 14, 1990

Ethylene diamine tetra acetic acid (EDT A) is a versatile complexing agent and is being employed in decontamination formulations. The dilute chemical decontamination (DCD) process employs ion exchange resins for regeneration of complexants, collection of metal ions/active isotopes and for removal of the decontaminating chemicals. In this work the interactions of EDT A on cation and anion exchange resins have been studied. The pickup of EDT A on cation exchange resin is by ion exchange mechanism and not possibly by precipitation at the low pH existing in ion exchange resin matrix. A mathematical relationship has been worked out to calculate the amount of EDT A adsorbed per unit volume of the cation exchange resin at a given pH. In addition, the behaviour of DTPA, HEEDTA and NTA on cation exchange resin has been evaluated. The chromatographic behaviour of oH- form of strong base anion exchange resin for a formulation containing EDT A, oxalic acid and citric acid has been reported and its relevance to the decontamination process has been discussed.

Even though the corrosion rate of DCD is very low on most of the materials of construc­tion, influence of time, temperature and the composition of the formulation on carbon steel is studied. Different classes of inhibitors were evaluated for reducing carbon steel corrosion.

KEYWORDS: decontamination, ion exchange, EDT A, re•in•, pH value, complexoneB, regeneration, •electivitg coe/licientB, li••ion product•, ion exchange chromatography, corro•ion, inhibitor•

517

I. INTRODUCTION

Dilute chemical decontamination process (DCD) is accepted world over as a suitable process for the decontamination of nuclear coolant systems since periodic decontamination with not-too-high decontamination factors (DF) are considered adequate to keep system con­tamination at low level. In addition, problems due to specific corrosion attacks are not common in this process. The process involves addition of decontaminating formulation chemicals to the primary coolant at a con­centration of about 0.1%. The solution is circulated through the system and a portion

of it is bled through an ion exchange circuit for removal of metal ions/active isotopes and for regeneration of formulation chemicals<1>. After achieving sufficient level of decontami­nation, the solution is passed through mixed bed to collect all the remaining metal ions and formulation chemicals and the coolant is brought back to the operating water chem­istry specification limits. The advantages of this process are (1) low concentration of chemicals only are used and hence base metal corrosion is less, (2) only solid active waste is generated in the form of ion exchange resins which dispenses the need for elaborate * Kalpakkam, Tamil Nadu-603102, INDIA.

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liquid active waste processing, (3) the time required for this process is only a few days, (4) in case of heavywater systems, down­grading of D20 is avoided.

Various combinations of organic acids, redox reagents and complexants are being employed as constituents of dilute chemical decontaminating formulations<2l, e.g. citric acid, oxalic acid (Citrox); ethylene diamine tetra acetic acid (EDT A), citric acid, oxalic acid (CANDECON); alkaline permanganate, oxalic acid, citric acid (AP Citrox); low oxi­dation state metal ions (LOMI)<'l etc. In the development and evaluation of the decontami­nation formulation, the emphasis is placed on the interaction of formulation chemicals and its metal complexes on the ion exchange resin used for regeneration of formulation chemicals which is essentially cation exchange resin in H+ form and/or formulation equilibrated anion exchange resin<•J. In this context, the present work throws light on the ion exchange behaviour of EDT A, its complexes and the complexones which are structurally related to EDTA.

When EDT A was being used as a constit­uent of a dilute chemical decontamination formulation, it was observed<5l that the free EDT A got removed in the H+ form of cation exchange resin during the reagent regener­ation stage of the process. Detailed studies which were carried out to understand the phenomenon of adsorption of EDT A on cation exchange resin indicated that the above observation could be explained by simple ion exchange equilibria.

The EDT A forms complexes of varying sta­bility with various metal ions like Fe2 +, Fe'+, Cu2+, Ni2+, Co2+ etc. The stability of these com­plexes depends on the charge on the metal ion and the stabilization energy arising out of the electronic rearrangement in d shell when the ligands approach the metal ion. The medium in which the complex exists, also plays an important role in determining the stability. As a general rule with increasing H+ ion concentration in the medium the stability decreases. When the formulation chemicals react with corrosion product oxides containing

]. Nucl. Sci. Techno!.,

activated and fission product isotopes, they form complexes which are negatively charged. While passing through H+ form of cation exchange resin, the complexes which are weak, breakup and the metal ion is picked up by the resin thereby releasing the H+ ion and the complexant

Mx++EDTN--- [M-EDTA]-•u, ( 1)

R-SO,H+[M-EDTA]-Hx--

(R-SO,)xM +EDT A<-+ xH+, ( 2 )

thereby regenerating the reagent and main­taining constant H+ ion concentration. If the complex is strong, then it simply passes through the resin without transferring the metal ion and hence in this case reagent regeneration is not possible with cation ex­change resin. However these complexes could be picked up either on formulation equilibrated anion exchange resin or on oH- form of anion exchange resin. An attempt has been made to classify the various activated corrosion product and fission product isotopes under the above two categories viz. isotopes which are adsorbed on cation exchange resin and which are adsorbed on anion exchange resins.

When a formulation containing EDT A, oxalic acid and citric acid is passed through a oH- form of anion exchange resin, initially all the components are taken up followed by ion chromatographic separation of the com­ponents. In addition, the measured capacities of anion exchange resin for various compo­nents of the formulation have been discussed.

Corrosion rates of various primary heat transport (PHT) system construction materials in dilute chemical decontaminating formu­lations are low but on carbon steel the corro­sion rate is still significant. Under simulated test conditions the rate is about 0.3-0.5 p.m/ h<•l. It has a bearing in deciding the quantum of ion exchange resin requirement for the decontamination process. In the present study carbon steel corrosion as a function of temper­ature has been evaluated. Different inhibitors have been studied for assessing their effective­ness in inhibiting carbon steel corrosion.

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Vol. 28, No. 6 (June 1991)

ll. EXPERIMENTAL

1. Chemicals and Equipments Only AR/GR grade chemicals were used in

the experiments and analysis. Shimadzu UV-240 spectrophotometer was used for measure­ment of optical density and Radiometer pH meter was used for measurement of pH and potential after due calibration. Canberra high purity germanium detector coupled with ORTEC multichannel analyser was used for r-spectro­metric analysis.

2. Analytical Procedures Estimation of individual components in a

mixture containing EDT A, oxalic acid and citric acid :

(1) EDT A Estimation by [Co( ill) EDT A] Complex Formation<7l

A suitable volume of EDT A containing solution was mixed with 5 ml of 0.01 M Co(N08) 2 solution, 2 ml of 20% NaN02, 2 ml of 6% HCl and 2 ml of 30% H20 2 • The mixture was boiled for a few minutes then cooled and made upto 50 ml. The Co( ill) EDT A complex thus formed was measured at 535 nm. Inde­pendent recovery experiments showed that there was no interference from citric acid and oxalic acid. However if iron was present as Fe(ill) EDTA, only the free EDTA responded to this analysis. If the EDT A solution itself contained excess HCl, it was boiled just to dryness to expel HCl, the residue dissolved in water and the reagents added to develop the colour.

(2) Estimation of Oxalic Acid by Gravimetry

Oxalic acid was estimated gravimetrically< B)

by precipitating as calcium oxalate using cal­cium chloride and neutralizing the solution with ammonia.

(3) Estimation of Citric Acid The concentration of citric acid was esti­

mated by determining the total acidity by titrating against NaOH and then subtracting the value for EDT A and citric acid. In the case of EDT A only 3 out of 4 acidic groups were assumed to be neutralized at this end point.

(4) Estimation of Complexones by Copper Ion Selective Electrode

519

Other complexones (HEEDT A, NT A and DTPA) were estimated by potentiometric titra­tion using copper ion selective electrode(9) and saturated calomel as the reference elec­trode. Equal volumes (50 ml) of acetic acid/ sodium acetate buffer of pH 5.0 (total acetate

cone. 0.1 M) was mixed with 50 ml of the test solution containing complexones. The titrant used was 5 mM CuS04 solution. In all these analyses, where HCl was used for adjusting the acidity, the solution was evaporated just to dryness and the residue dissolved in water before the titration. Care was taken to avoid overheating which may result in decomposi­tion of analyte. This procedure was also used for EDT A estimation to countercheck the other method.

(5) Determination of EDT A Solubility In 150 ml stoppered bottles 100 ml of

demineralized water was taken and excess EDT A added in each bottle and their pH adjusted accurately in the range 0 to 3 by adding NaOH/HCl and the solution thermost­ated at 32°C overnight and the filtrate was analysed for EDT A.

(6) Determination of EDT A Pickup on Ion Exchanger (IX)

Experimental procedure for LX batch experiments :

The 5 ml of the strong acid cation exchange resin (lndion FF 225, Ion exchange, India) was equilibrated for 12 h in a 500 ml closed glass container with the 100 mg/ l of EDT A solu­tion whose pH was suitably adjusted with HCI. The supernatant was analysed for EDT A and the amount adsorbed was calculated by difference. Similarly 400 mg/l of NT A, HE EDT A and DTP A and a suitable volume of resin were used for their IX retention studies.

Experimental procedure for column experi­ments:

The 25 ml of strong acid cation ex­change resin or strong base anion exchange resin (lndion-FF -IP, Ion exchange, India) were packed in a glass column of 25 mm I. D. and a flow rate of 25 ml/min was used for elution

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studies. Aliquots of 500 ml each were collected for estimation of EDT A and other components.

(7) Glass Loop Experiment A 1 l glass loop consisting of a reaction

vessel, peristaltic pump, IX columns and cooler was used for the decontamination experiment. The details can be found elsewhere< 10

). An active Monel-400 coupon which was exposed in a primary heat transport system bypass autoclave of a PHWR was kept in the reaction vessel containing 400 mg/ l of EDT A and the temperature was maintained at 85'C. The solution was circulated for 6 h through the ion exchange column containing cation ex­change resin, and then the EDT A equilibrated anion exchange resin was kept in series and the solution was circulated for about 2 h. Finally the solution was drained after con­necting a oH- form of resin in series to the other two columns. The solution flow rate was maintained at 23 ml/min. The top layers of the resin were sampled and 7 -spectrometri­cally analysed.

Corrosion rate measurements were done by hanging 2 polished carbon steel coupons (ASTM 106 Grade B) in a similar loop with only cation exchange resin in the column. The resin was equilibrated with the formulation before connecting to the loop, so as to avoid any removal of EDT A from the formulation. The coupon was exposed for a period of 5 h. at the temperature of 85'C and for estimating the corrosion rate the weight loss of the coupon was measured after cleaning it with water and acetone.

m. 0BSERV ATIONS AND

DISCUSSIONS

When EDT A or any of its analogues is one of the constituents of the formulation complication arises in the process of reagent regeneration. Free EDT A itself gets picked up on the cation exchange resin, which gets released to the solution subsequent to the loading of the cation exchange resin with metal ions<"J. This phenomenon was studied in detail in the case of EDT A which in view of its strong complexing capacity and its compatibility with metal systems is an uni-

]. Nucl. Sci. Techno/.,

versally accepted decontaminant. The EDT A and the structurally related com­

plexones have both carboxylic and amino groups as coordinating ligands*. The carboxylic acid group could dissociate to give protons and the amino group could take up a proton because of its basic nature. Thus EDT A could exist as seven different ionic species which are in equilibrium with each other. The contribution of each species at a given temperature is a function of H+ ion concent­ration. In the region at pH values below 3.0 which is of relevance to the dilute chemical decontamination, only four species exist at significant concentration and are given in Fig. 1. Thus the net charge on the molecule is in the range --1 to +2 when the pH of the medium is <3 whereas at pH values more than 3, the EDT A molecule assumes net negative charge whose magnitude increases with increasing pH and going upto -4. Similar trend is exhibited by hydroxy ethyl ethylene diamine tri acetic acid (HEEDTA),

nitrilotriacetic acid (NT A) and diethylene tri­amine penta acetic acid (DTP A). The chemical structure of these compounds are given in Fig. 1. All these complexones interact with the cation exchange resin through the proto­nated amino group when the net charge on the molecule is ::2: + 1.

1. Solubility of EDT A The saturation solubility values when

plotted against pH gave a parabolic curve (Fig. 2) exhibiting a minimum at a pH of about 1.6. The solubility value at this mini­mum is 140±10 mg/l at 32'C. Dependence of pH of solubility could be explained by considering the dominating species of EDT A in this region. At the minimum of solubilty the dominant species is neutral EDT A. In the still lower pH regions monopositive and dipositive ions dominate while in the higher pH regions negatively charged EDT A species dominates (Fig. 3). It is known that in a medium of high dielectric constant the charged species enhance solvation and thus the solu­bility.

* Ligand refers to a complexing molecule.

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Vol. 28, No. 6 (June 1991) 521

NTA

HEEDTA

DTPA

HOOC-CI-fe H• H/• CH1 -COOH /'N-CH -CH -N

HOOC-CHe 1 1 'cH1 -COO H

HOOC-Cf-tt H• H:CHa -coo-'N-CH -CH -N

HOOC-Cf-tt/ I • 'cHI-CCCH

-oac-c~-~e H. H~CH1 -ccc-)N-CH -CH -N

HOOC-CHW I I 'cH.-CCCH EDTA E

Fig. 1 Structural formula of complexones

2500 ero_~_s_o,_uo_"_"v_._mo_" __________ Eo_~_s_o_lu_ou_H~v._m~g/ISOO Fraction of EDTA

400

300

1000 200

500 100

oL---L---L---~--~--~---L--~o 0 0.5 1.5 2 2.5 3 3.5

pH

Fig. 2 Solubility vs. pH diagram for EDT A (Temp.: 32'C)

1,-------------------------------.

0.8

0.5 1.5 2 2.5 3 3.5 4

pH

- (H6Y)2• -+- (H5Y) • --¥-- (H4 Y) -a- (H3Y)-

Fig. 3 EDT A species distribution as a function of pH ( Y = (C10N2H120s) 4-)

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2. Estimation of EDT A Pickup on Cation Exchange Resin at Various pH Values

The amount of EDT A picked up per unit volume of the cation exchange resin as a function of pH in batch experiment showed a maximum at pH 1.6 (Fig. 4), which occurred at the same pH as that of the minimum solubility. Considering this fact alone, one would be tempted to conclude that it is the solubility criterion that governs the EDT A adsorption process. It is specifically for this reason that the concentration of EDT A was kept at 100 mg/l, well below the minimum solubility value, thus there was no chance of precipitation at any pH value. This trend in variation of EDT A adsorption with pH could however be explained by considering the ion exchange equilibria involved in the process. This equilibria is controlled not only by the protonated ligands but also by H+. Hence 100% loading is not realizable.

Total EDTA loaded, mM/1 IX 14r-------------------------------,

12

10

8

6

4

2

o~--~--~----L---~--~----L---~

0 0.5 1.5 2 2.5 pH

observed -+- Ill ted

[EDTA]=lOO mg/l

3

Fig. 4 Variation of EDT A adsorption on cation lX as a function of pH (By batch experiment)

3.5

For the sake of simplicity, the monoposi­tive EDT A is abbreviated as E+ and dipositive EDT A as E2+. The ion exchange reaction

]. Nucl. Sci. Techno/.,

could be represented as

R-H+E+ -- R-E+H+,

2R-H+E2 + ~ R2-E+2H+.

( 3)

(4)

The equivalent ionic fraction selctivity co­efficient can be defined as given below<12),

For monopositive ion

(XE+)(XH+)

(XH+)(XE+) ,

For dipositive ion

Kb= (XE2+)(XH+)2

(XH+Y(XE2+),

where XH+=mH+I (mH++mE++2mE2+) XE+=mE+I (mH++mE++2mE2+) XE2+=2mE+I (mH++mE++2mE2+) The bar represents resin phase. m=moles/1.

( 5)

( 6)

These equations can be simplified after considering the assumption that XH+ is close to unity since concentrations in moles of E+, E2+ are much less than H+ in both resin and solution phases.

Hence

Similarly

XE+=KaXE+

XE2+=K~E2+

Summing up the amount of EDT A adsorbed on the resin in the two ionic forms, the total moles of EDT A adsorbed per litre of the resin

=(KamE++KbmE2+)fliH+/mH+. ( 7)

Let the fraction of E+ and E2+ in the total EDT A be y and z, then their concentrations in solution are y[EDT A] and z[EDT A] respectively. Hence the total moles of EDT A adsorbed per litre of the resin is expressed as

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Vol. 28, No. 6 (June 1991)

(YKa+zKb)[EDTA](ffiH+/mH+). (8)

It is clear from Eq. ( 8 ) that the amount of EDT A adsorbed per unit volume of the resin depends on y and z, the fractions of EDT A molecules which exist in the mono and diposi­tive forms respectively in solution. These fractions were obtained from the EDT A species distribution curve which in turn was generated from the dissociation constant values. The equilibria involved are given below03l:

HsY-- H2Y"-+H+,

H.Y•- --~ HY3 -+H+,

HY3 - -~ y~-+H+'

H,Y+H+- H.Y+,

K,=l.OxiO-•

K.=2.16xlo-s

K3 =6.92x10-7

K,=5.50xlo-ll

K =1.26x10-'

K'=2.51 xlo-•,

where H,Y represents undissociated EDTA. The fractions of monopositive and dipositive EDT A species were estimated from the formula, using the above K values :

k[H+]" y= DENOM Fraction of H5Y+, ( 9)

[H+]" z= DENOM Fraction of H6 y•+, (10)

where DENOM= [WJ6+k'[WJ"+k' k[W]' +k' kk1[W]3 +k' kk1k2[W]2

+k'kk1kzk3 [H+J+k'kk 1k2k 3k 4•

Similarly H,Y and H3Y- were computed using a programme and Fig. 3 was generated. The experimentally determined quantities of EDT A adsorbed on cation exchange resin at different pH values in a batch experiment were fitted into Eq. ( 8) for calculating Ka and Kb. The results of curve fitting is shown in (Fig. 4). The calculated values are

Ka=l.93,

It is more appropriate to countercheck these results in column studies as it simulates the decontamination process. Adsorption experi­ments of EDT A at different values of pH were carried out in column where the EDT A was

523

passed till the influent and effluent cone. (10 mg/l) are equal. It was found that the experimentally observed loading of the bed could be predicted by computation using Eq. ( 8 ) with the Ka and Kb obtained in the batch process (Fig. 5).

Amt.of EDTA ads .• mM/1 IX 30.---------~------------------~

+ 25

20

15

10

5

OL---~---L--~----L---~--~--_J

0 0.5 1.5 2 2.5 3

pH

Oba. In column ax pt. -+-computed using oq.(B)

[EDT A]= 100 mg/ l

Fig. 5 Comparison of EDT A adsorption observed in column experiment and those calculated by Eq. ( 8)

3.5

Equation ( 8 ) also indicates that the amount of EDT A adsorbed is proportional to its con­centration in solution at a constant pH value. The results of experiments carried out by varying the concentration of EDT A at .a pH of 2.2 are presented in Fig. 6 along with values calculated by Eq. ( 8 ). The good corre­spondence between the experimentally observed and calculated values confirms that the adsorption is proportional to the EDT A con­centration. This also reveals that the EDT A which are already adsorbed in the cation exchange resin need not only be released by loading with metal ion, it could be released partially by the change in the concentration of EDT A during the course of decontamination process.

Adsorption studies by equilibration method were also carried out for NT A, HEEDT A and DTPA. Nitrile triacetic acid (NT A) contains

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Amt adsorbed. mM/1 rx lOr------------------------------,

60

50

40

30

20

10

0~------------------------------~ 0 100 200 300 400 500

EDTA Cone., mg/1

-Observed -+-Computed

Fig. 6 Effect of concentration on adsorption of EDT A over cation I\ (pH 2.2)

one tertiary amine group which can be protonated<"1• The equilibrium involved here is

H,N+H+- H,N+, KN=1.5849x1o-', (11)

where H,N is the undissociated NT A molecule. The other proton dissociation constants are

]. Nucl. Sci. Techno!.,

kN,=1.9498xlQ-•; kN•=3.3884x1o-• and kNa= 1. 9498 X 10-'•. The protonated NT A replaces H+ in the cation exchange resin. The ion exchange equilibrium for this reaction is given below:

R-H+H,N+ --~ H++R-H,N. (12)

The equivalent ionic fraction selectivity coef­ficient can be given as

/\H N+=J~IJ'cN+J[,rH+J ' [Xw][XH,N+]

(13)

where XH,N+ refers to equivalent ionic fraction of H,N+=mH,N/(mH,N+mH+) and XH+ refers to equivalent ionic fraction of H+=mH+I (mH,N++mH+) and bar indicates the ionic fraction in the resin phase. Considering the relative amount of H,N+ ion at any pH value when compared with H+ ion concentration maintained in the experiment, XH+ can be approximated to unity. Also, as the amount of NT A loading on the ion exchanger is less than 1% it can be approximated that X H+ = 1. These approximations and the conversion to concentrations in moles gives

[fllH 4N+][IIllJ":_]_

[lllH4N+][nlH+J , (14)

[H+]' F=[H+J'Tk[H+J'+kk~!rH+J'+k-kN-,k~.[H+]+kkN,kN,kN,,

where F is the fraction of NTA species existing as H,N+ and [NT A] is the total NT A concentration. Analysis of the experimental data gave a KNTA value of 0.715±0.04. The trend of NT A adsorption per unit volume vs. pl-1 curve (Fig. 7) was very similar to that of EDT A curve. Similarly expriments were also carried out for HEEDT A and DTP A whose adsorption were not very much different from that of EDT A (Figs. 8 and 9). However the amount of adsorption per unit volume of the cation exchange resin is higher for both HEEDT A and DTP A when compared with NT A and EDT A. This indicates that their selectivity coefficients are higher than EDTA and NT A. Attempts were not made to esti­mate the coefficients. The pH range over

which maximum adsorption occurred increased in the order NTA<EDTA<HEEDTA<DTPA. This is explained by the existence of one positively charged species for NT A ( + 1), two species for EDT A and HEEDT A ( + 1 and +2)

and three species for DTPA ( + 1, +2, and +3). In addition, the ratio of K, of various proton­ated species will have a role in deciding the pH range over which significant adsorption occurs.

The above observations validate the ion exchange mechanism and the mathematical relationship proposed for EDT A. The order of selectivity which could be gauged from the total amounts of adsorption per unit volume of the resin could be given as NT A< EDTA<DTPA<-~HEEDTA (Fig. 10).

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Amt.of NTA ads., mM/1 IX lOr-------~~~~---------------.

8

6

4

2

0~~~----~----~----~--~~--~ 0 0.5 1.5 2

pH

[NTA]=400 mg/l

2.5 3

Fig. 7 Variation of NTA adsorption with pH

Amt.ol DTPA ads .• mM/1 IX 120r---------~~~~------------~

100

eo

20

OL----L----L----L ____ L_ __ _L __ ~

0 0.5 1.5 2

pH [DTPA]=400 mg/l

2.5 3

Fig. 9 Variation of DTPA adsorption with pH

Table 1 gives the magnitude of adsorption at the adsorption maxima.

525

Amt.ol HEEDTA ads .• mM/1 IX 160.-------------------------------,

140

120

100

80

oL---~----~--~----~----L---~

0 0.5 1.5 2

pH

[HEEDTA]=400mg/l

Fig.!_S Variation of HEEDTA adsorption with pH

2.5 3

Effluent Cone .. mg/1 500.---------~--------------------,

Volume passed. I

-EDTA -+-HEEDTA "*'DTPA -&-NTA

Fig. 10 Variation of effluent concentration of EDTA, NTA, HEEDTA, DTPA on cation IX (Influent cone.: 400 mg/l)

3. Interaction of EDT A on Anion Exchange Resin

50

When the formulation containing EDT A (0.04%), oxalic acid (0.03%), and citric acid

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Table 1

Complexone

NTA EDTA DTPA HEEDTA

pH of maximum adsorption and amount of adsorption on cation exchange resin of complexones (As per batch experiment)

pH of max. adsorption

1.3 1.7 1.5 2.0

mM of complexone adsorbed per

lit. of IX

9.2 11.6

100 145

(0.03%) was passed through a OH- form of anion exchange resin, initially all the com­ponents are picked up, subsequently citric acid is eluted out by EDT A and oxalic acid followed by EDT A by oxalic acid (Fig. 11).

mmoles/100 miiX

50

40

30

20

10

Ol_~~--~~~~~~ ~ o o m w w ~ oo

Volume passed, I

-a- EDTA (cum) """*""OX (cum) -+- CA (cum)

Effluent composition as a function of volume of formulation passed. (for a volume of 25 ml. IX)

Fig, 11 Cumulative loading of formulation com­ponents on strong base anion exchanger with volume of formulation (Inset)

Thus the chemical composition of the resin bed is different from that of the formulation composition. This behaviour is akin to the chromatographic behaviour of the anion exchange resin. Thus the regeneration behaviour of equilibrated anion exchange resin will depend upon the stage of equilibration. The above organic acids are initially complete­ly neutralized as a OH- form of resin is a

]. Nucl. Sci. Techno!.,

strong base. When all the OH- is replaced, the pH in the resin matrix gradually comes down. As a consequence, the counter ions associate partly with the H+ ions of the penetrating acid. This association decreases the charge of the original counter ions, the extent of association depends on the pK value of the acids and the pK value of the anion in the resin< 12

l. Assuming Hn Y as the polybasic acid

As the incoming solution contains a mixture of organic acids of differing pK values, the charge and abundance of the species which are predominant at the medium pH determines the selectivity. At pH 2.3 the charge and abundance of the different species of the formulation constituents are given in Table 2.

Table 2 Charge and abundance of various ionic species of oxalic acid, citric acid and EDT A at pH 2.3

Compound Species Charge Abundance (%)

Oxalic acid H.ox 0 7.7 HOX -1 91.1 ox -2 1.2

Citric acid H,CA 0 87.6 H2CA -1 12.4

EDTA H4Y 0 24.6 H,Y -1 49. 1 H2Y -2 21.1

Thus in citric acid, at pH of interest the main species does not have any charge, whereas in oxalic acid more than 90% of the molecules exist with a charge of -1 and in EDT A both -1 and -2 charged species exist together. Hence citric acid is eluted by oxalic acid and EDT A.

Complete neutralization and subsequent association of the counter ion with the H+ ion makes available sites for additional adsorption. This explains the higher ion exchange capacity observed in the case of these organic acids. The observed capacities measured in meq. of completely neutralized form of anions per ml of the resin determined in separate batch experiment are given below:

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EDT A Citric acid Oxalic acid

1.7 1.6 1.4

where its true capacity is 1.0 meq/ml. 4. Studies on Pickup of Various

Corrosion Product and Fission Product Metal Ions on Ion Exchange Resins

During the process of decontamination the organic acids/complexants solubilize the corro­sion product oxides by the reactions,

Fe30,+8H++2e-~ 3Fe2++4H20, (16)

Fes0,+8H+ ~ 2Fe3++Fe2 ++4H20. (17)

Of the two reactions the former is said to predominate the latter<1''· The electrons are given by reducing agents like oxalic acid or Fe in the base metal corrosion process. The released metal ions are complexed by the complexing agents present in the formulation. The complexant having the highest stability constant for the metal ion concerned are preferred over the others. In the decontami­nation formulation studied viz. EDT A, oxalic acid and citric acid at a cone. of 0.04%, 0.03%, 0.03% w /v respectively, the EDT A complexes are preferentially formed over the others as long as the concentration of the metal ions does not exceed EDT A concentration. When [Fe(IT)-EDTA] 2

-, and [Fe(III)-EDTAr com­plexes are passed through a H+ form of cation exchange resin, the iron from [Fe( IT )-EDT A]2 -

is picked up by cation exchange resin, in turn releasing EDT A by the following reaction

2R-SOsH+Hz[Fe( IT )-EDTA]

~(R-SO,,)zFe+ EDT A (18)

whereas H[Fe(III)-EDT A] simply passes through the resin without getting broken up. This has been proved by polarographic meas­urements as reported elsewhere< 10'. The [Fe(III)-EDT A]- complex however could be removed from the solution by preequilibrated anion exchange resin. Other corrosion product metal ions like Ni2 +, Cu+, Cu2 +, Mn2 +, Co2 + and the fission product isotopes viz. Ce, Ru, Zr, Nb, Pr, Eu, Cs existing in their stable oxidation states could also form complexes

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with EDT A in varying degrees of stability. The strength of the complex determines whether the complex could be broken up in the cation exchange resin or not, which in turn decides whether the reagent can be regenerated or not.

A Monel-400 coupon which was exposed to PHT system by-pass autoclave in a PHWR contained the following : 141Ce, 144Ce, 154Eu, JssEu, JosRu, Jo'Ru, 9'Zr and 9'Nb as fission product isotopes and 58Co, 6°Co, and 125Sb as activated corrosion product isotopes. The coupon was decontaminated in the glass loop using EDT A alone as the formulation and the activities picked up on the various resins are given below :

H+ form of cation IX;

EDT A form of anion IX ;

OH- form of anion IX;

1osRu, ussb, soco.

However, some amount of 95Zr, 95Nb, 103Ru and 10'Ru were observed on cation exchange resin. This indicates that in a fission product dominated contamination, cation exchange resin alone is insufficient in the reagent regeneration stage of decontamination, as it will not efficiently remove all the activities, which may result in redeposition of activities< 10'. Hence, we need the equilibrated anion exchange resin to augment the efficiency of activity removal.

The decontamination factors obtained on Monel in this experiment for the various isotopes are given below. The total duration of decontamination is 10 h.

3.6 3.5 6.3 4.8 3.6 95Zr ssco 'oco 3.6 3.8 5.9 2.1 2.1

The unusual difference in the decontamination factor observed between 144Ce and 141Ce has been explained in an earlier work(l0'.

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5. Compatibility Studies on Carbon Steel with Formulations Containing EDTA

Evaluation of inhibitors: Various inhib­itors were added to the formulation containing oxalic acid (0.03%), citric acid (0.03%) and EDTA (0.04%) and tested for their efficiency in inhibiting carbon steel corrosion (Table 3).

The inhibition efficiency is defined as

Ieff=(Ro-R,)/Ro,

where R0 c::> Corrosion rate in absence of inhibitor

R 1 c::> Corrosion rate in presence of inhibitor.

Inhibitors are listed in Table 3. Remarkable reduction in corrosion rate was observed when diethyl thiourea was used as an inhibitor whereas 3-hexyn-1-ol and 5-hexyn-1-ol showed marginal reduction in corrosion rates.

Table 3 Inhibition efficiency of various corrosion inhibitors

Inhibitor

Without inhibitor 1, 2, 3-benzotriazole 4-methyl thiophenol 3-hexyn-1-o/ 5-hexyn-1-o/ 5-aminotetrazole N, N'-diethyl thiourea

Corrosion rate Inhibitor (mg/cm2/h) efficiency(%)

2. 7 0 2. 0 25.9 2.6 3. 7 1.37 50.7 1. 35 50 1. 4 48. 2 0.2 92.6

Formulation composition: Oxalic acid (0.03%), Citric

]. Nucl. Sci. Techno!.,

been observed that on virgin base metal surface the corrosion loss is proportional to the duration of exposure, provided there is no significant loss of the reagent by thermal or radiation decomposition. This is expected in this process, as the formulation chemicals are continuously regenerated which maintains a constant reagent and metal ion concentration during the process. In all these experiments rather high volume to surface ratio was maintained (about 40) that explains why the corrosion rates are higher than those observed in system decontaminations.

The corrosivity of the formulation also depends much on the temperature as given in Fig. 12.

3 Corrosion rate. mg/cm2/h

2.5

2

1.5

acid (0.03%), EDTA (0.04%); Temperature 85'C. 0.5

In the formulation oxalic acid was replaced by Ascorbic acid or Formic acid to study the effect of changing the reductant and then there was no significant change in the corrosion rate (Table 4). This indicates that changing the reductant alone in the formulation does not change the corrosion rate. It has also

Table 4 Corrosion rate on carbon steel with different reducing agents

Formulation

EDT A (0.04%), Citric acid (0.03%) and Oxalic acid (0.03%)

EDT A (0.04%), Citric acid (0.03%) and Formic acid (0.03%)

EDTA (0.04%), Citric acid (0.03%) and Ascorbic acid (0.03%)

Corrosion rate (mg/cm2/h)

2.7

2.5

2.7

0~----~----~--~L---~----~ 320 330

Fig. 12

340 350 360

Temp. K

Variation of corrosion rate with temperature in for­mulation containing oxalic acid (0.03%), citric acid (0.03%) and EDT A (0.04%)

370

Further studies are in progress to investi­gate the thiourea based compounds having ioni­zable groups, a criterion to be fulfilled for use as a constituent of a dilute chemical decon­taminant otherwise removal from the coolant will not be possible.

IV. CONCLUSION

(1) The species of EDT A which have a net

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charge of -t-1 or +2 interacts with the cation exchange resin by an ion exchange mechanism. The mathematical relation­ship obtained by considering the ion ex­change equilibria could be used for esti­mating the amount of EDT A pickup on cation exchange resin. Concentration of EDT A in solution and the pH decide the amount of adsorption. The experimental observations made on NT A, DTP A and HEEDT A also support the findings on EDTA.

(2) When formulation containing oxalic acid, citric acid and EDT A equilibrated with the anion exchange resin, only oxalic acid and EDT A remains in the resin phase.

(3) Fission product isotopes are not effici­ently collected by cation exchange resin, whereas preequilibrated anion exchange resin augments the efficiency during the regeneration stage of the decontamination process.

(4) Thiourea could reduce the carbon steel corrosion rate significantly.

ACKNOWLEDGEMENT

The authors are grateful to Dr. P.N. Moorthy, Head, Applied Chemistry Division, BARC for his support and encouragement to this work. The authors are also thankful to Dr. K. S. Krishna Rao, Station chemist, MAPS for providing active Monel coupons.

Additionally, they thank the Atomic Energy Society of Japan for their kind con­sideration shown in publishing this report.

---REFERENCES---

(1) PETTIT, P.]., LESURF, J.E., STEWART, W.B., STICKERT, R. ]., VAUGHAN, S. B.: Decontami­nation of the dougles point reactor by the Can Decon process, Mater. Performance, 19(1J, 34-38 (1980).

(2) Decontamination methods as related to decom­missioning of nuclear facilities, NEA Experts Report by OECD/NEA (1981).

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(3) SwAN, T., SEGAL, M.G., WILLIAMS, W.J., PicK, M. E. : LOMI decontamination reagents and related preoxidation processes, EPRI 5522 M, (1987).

(4) ANSTINE, L. D., BLOMGREN, J. C., PETTIT, P. ]. : Evaluation of a dilute chemical decontamination process for boiling water reactors, Proc. BNES Conf. Water Chemistry of Nuclear Reactor Systems- IT, (1980).

(5) KRASZNAI, ]. : Pickering NGS heat transport system decontamination using the CAN-DECON process, Proc. ANS Topical Mtg. Decontami­nation and Decommisioning of Nuclear Facilities, Sun Valley, Idaho, p. 2-55-69 (1979).

(6) MuRRAY, A. P., EcKHARDT, D. A., WEISBERG, S. L. : Dilute chemical decontamination process for pressurized and boiling water reactor appli­cations, Nucl. Techno!., 71,482-496 (1985).

(7) KAISER, K. L. E. : Determination and differenti­ation of EDTA and NTA in fresh water, Water Res., 7, 1465-1473 (1973).

(8) VoGEL, A. I.: "Quantitative Inorganic Analysis", (3rd ed.), p. 578 (1961), Longmans.

(9) SMITH, M.]., MANAHAN, S. E. : Copper deter­mination in water by standard addition poten­tiometry, Anal. Chem., 45(6J, 565-568 (1973).

M VELMURUGAN, S., NARASIMHAN, s. v., MATHUR, P. K., VENKATESWARLU, K. S.: Evaluation of a dilute chemical decontaminant for pressurized heavy water reactors, To be published in Nucl. Techno!., June 1991.

M MoNTFORD, B., LAcY, C. S. : The effect of chemistry, choice of materials and decontami­nation on radiation exposure of employees in Ontario Hydro's CANDU reactors, Proc. ] AI F Int. Conf. on Water Chemistry in Nuclear Power Plants, Apr. 19-22, Vol. 2, (1988).

(1~ HELFFERICH, F.: "Ion Exchange", p. 154 (1962), McGraw-Hill Book.

(1~ WILKINsoN, G., GILLARD, R. D., McCLEVERTY, ]. A. (Ed.) : "Comprehensive Co-ordination Chemistry", Vol. 2, p. 780 (1987), Pergamon Press.

M ANDEREGG, G. : Critical survey of stability constants of NT A complexes, Pure Appl. Chem., 54(12], 2693-2758 (1982).

M SMEE, J. L.: Dissolution characteristics of metal oxides in water cooled "reactors, "Decontamina­tion and Decommisioning of Nuclear Facilities", (Ed. M. M. OsTERHOUT), pp. 281-292 (1980), Plenum.

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