Characteristics of boron adsorption on strong-base anion-exchange resin

~Inn. Nucl. Energy, Vol. 20, No. 7, pp. 455M62, 1993 Printed in Great Britain. All rights reserved

0306-4549/93 $6.00+0.00 Copyright © 1993Pergamon Press Ltd

CHARACTERISTICS OF BORON ADSORPTION ON STRONG-BASE ANION-EXCHANGE RESIN JUNG WON NA Chemical Process Department, Korea Atomic Energy Research Institute, P.O. Box 7, Daeduk-Danji, Taejon, Korea KUN JAI LEE Department of Nuclear Engineering, Korea Advanced Institute of Science and Technology, 373-1, Kusong-dong, Yusong-gu, Taejon, Korea (Received 19 October 1992)

Abstract--Tbe adsorption characteristics of boric acid o'n a strong-base anion-exchange resin, Amberlite IRN-78LC in OH- form, were investigated at 10, 30, 45 and 60°C in concentrations of boron up to 1500 ppm. The expression for the adsorption of boric acid is proposed as q = q°~(KLC/(1 + K L C ) + K c C + K o ) ,

where q and q~ are the quantity of boron adsorbed on the resin and the ion exchange capacity of the resin, respectively, and C is the concentration of boron in solution. The KL introduced represents a Langmuirtype adsorption of boric acid in a low concentration of boron. Kc can be interpreted as a so-called polymerization effect representing that borate ions with less boron atoms in the resin turn into polyborate ions with more boron atoms as the boron concentration in solution increases. In addition, K0 represents the initial adsorption that takes place abruptly in the extremely low concentration of boron. 1. INTRODUCTION Operation of the BTRS (boron thermal regeneration system) in a pressurized water reactor is based on the ion-exchange characteristic of boron that the amount of boron adsorbed on the ion-exchange resin depends on temperature. Namely, borate ions are adsorbed on the resin mostly in the form of polyborate at low temperature, whereas at high temperature they are adsorbed on the resin mainly in the form of monoborate. This principle enables the concentration of boron in the primary coolant to be controlled. Although, the BTRS plays an important role in controlling the boron concentration in the coolant as well as in reducing the volume of liquid waste, the system has not been in effective operation yet because of the lack of detailed information regarding the adsorption characteristics of boron. When boric acid dissolves in water, it is ionized into borate ions. It has been reported that there may exist several borate ion species: monoborate, diborate, triborate, tetraborate and pentaborate ions in solution (Maya, 1976; Mesmer et al., 1972a; Ingri, 1963). It is also known that there is no species satisfying the condition a/b > 1 in the expression Bb(OH)~-~a, where a and b denote the ionic valence and the number of boron atoms, respectively (Mesmer et al., 1972b). In general, only monoborate ions exist for boron concentration <0.025 M, triborate ions are present for boron concentration >0.025 M and several poly-

borate ions (the average number of boron atoms per ion is about 3.2) exist at higher concentration, > 0.1 M (Ingri, 1957; 1962; Stetten, 1951). For the adsorption of boric acid on an ion-exchange resin, the quantity adsorbed increases as the concentration of boric acid increases or as the pH decreases (Everest and Popiel, 1956). Tomizawa (1979, 1981) investigated the effects of temperature on the adsorption of boron on a strong-base anionexchange resin D I A I O N SAN-1 and reported that the quantity of boron adsorbed decreased as the temperature increased. Tomizawa thought that boron was adsorbed on the resin in the forms of monoborate, tetraborate and pentaborate ions. He suggested further that, while the monoborate ion is dominant at high temperatures, the tetraborate and pentaborate ions (monovalent and divalent) are dominant at low temperatures. Tomizawa (1983) concluded that the adsorption of boron is not affected in the pH range 4.5-8.0, but it is decreased in the pH range >8.0 under a given concentration. Peteka (1980) confirmed Tomizawa's report, but suggested that the most predominant polyborate ion was triborate (monovalent and divalent). Assuming that monoborate and triborate ions mainly exist in the solution phase, when the boron concentration is not high, Cohen (1980) and Qixia (1986) reported the equation for the adsorption of boric acid on an ion-exchange resin, based on the

455

456

JUNG WON NA and KUN JAI LEE

mass-action law. Then the total quantity of botron adsorbed was calculated from the quantity of monoborate and triborate ions in solution, which deviated from the experimental observation by about 14% (Cohen, 1980). However, for the operation of the BTRS, more accurate data are required for the adsorption equilibria, which raises the need for a precise adsorption isotherm. This study aims to estimate the adsorption behavior rather precisely, assuming the existence of four possible species of borate ion in the solution equilibria. In order to determine the adsorption of boric acid, it is necessary to know the amounts of the different borate ions in solution with respect to the concentration of boron, the temperature and the pH. This study estimated the composition of the polyborate ions in the aqueous phase as well as the average number of boron atoms of polyborate ions in relation to the boron concentration and temperature. Furthermore, through experiments and theoretical analysis, an isotherm expression was proposed for the adsorption of boron on an anion-exchange resin (Amberlite IRN-78LC) at 10, 30, 45 and 60°C, in the boron concentration range 0-1500 ppm, which covers the normal operating conditions of the BTRS.

The ionizations for borate ions and the corresponding equilibrium constants can be expressed as follows :

[B-] HB+OH- ¢>S-,

K,~ -- [HB][OH-] ;

2HB+OH-~,B~,

K2, - [HB]2[OH_],

(4)

3HB+OH-~=>B3,

[BjI K3, - [HB]3[OH_],

(5)

4HB+2OH-c~B42-,

[B~-] K42-IHBl,[OH_12.

(6)

[B;]

(3)

and

The ionization constants of the borate ions are calculated using the equations of Mesmer et al. (1972b), which are useful under the condition of variable temperature. The ionic strength, L which is required for determining the equilibrium constants, is calculated as follows : I = 0.5([B-] + [B2] + [B3] +4[B 2-] + [ O H - ] + [H+]).

(7)

The ion product of water, which is a function of the temperature, is expressed as

2. THEORY

H20~=>[H+]+[OH-],

2.1. Composition o f borate ions Since boric acid dissolves in water to form various borate ions, the composition of borate ions should be calculated to understand the solution behavior which depends on a number of factors, including the concentration of boric acid, the temperature and the pH. For calculating the composition, three types of equations are needed : a mass balance equation ; an electroneutrality condition; and relations for ionic equilibria. Assuming that boric acid is ionized into B(OH)4, B2(OH)7, B3(OH)~-0 and B4(OH),24 (Mesmer et al., 1972b), these will be hereafter abbreviated as B , B2, B3, and B{,-, the mass balance equation for boron is given by BT=[HB]+[B

]+2[B~-]+3[B3]+4[B42-],

Kw=[H+][OH-].

(8)

The value of Kw was obtained from the literature (Marshall and Franck, 1981; Keenan and Keys, 1936). Combining equations (1)-(8), the mole fractions of borate ions were calculated as functions of the boron concentration, temperature and pH. 2.2. Boron adsorption At low boron concentration, <100 ppm, only monoborate ions are assumed to exist. The monoborate ion is exchanged with a counter ion (i.e. O H ion) in the anion resin : ROH+B-~RB+OH-,

KR~

RB[OH-]

ROH[B-]'

(9)

(1)

where BT and [HB] represent the total concentration of boron and the concentration of undissociated boric acid, respectively. Since in solution the quantity of positive charge should be the same as that of negative charge, from the electroneutrality point of view, the following equation holds :

[H+] = [B-]+[B~-]+[By]+2[B42-]+[OH-]. (2)

where R represents a functional group of the resin. From equations (3) and (9), the adsorption of boric acid is RB = KR,ROH[B-]/[OH-] = KR,KtI(E-- RB)[HB].

(10)

Where E, E = ROH + RB, denotes the total ion-exchange capacity of the resin. Arranging equation (10), RB reads

Boron adsorption on anion-exchange resin Table 2. Specificationsof the reactor for adsorption

Table 1. Characteristics of Amberlite IRN-78LC resin Ionic form Degree of cross linking True density (wet) Void fraction Particle size Effectivesize Uniformity coefficient Moisture content pH range Max. operating temperature Salt splitting capacity

OH 6% DVB 1.1 g/cm3 32.7% 16-50 mesh 0.55 mm 1.35 50% 0-14 60°C 1.497 mequiv/ml

EKR l K~ I [HB] RB = 1 + K R I K I , [HB]"

BS 1+

K,, [OH]

(12)

Substituting equation (12) into equation (11), RB -

EKR1K11BS 1 + K j l [ O H - ] + K R I K I IBS"

(13)

In equation (13), since the equilibrium constant of the m o n o b o r a t e ion p K , , is about 9.25 and pH < 7, the value of the K~ I[OH-] is sufficiently small (<< l) and negligible. Thus, equation (13) can be reduced to the usual form of Langmuir adsorption about BS : EKRIKIIBS RB - 1 +KRIKI IBS"

Content

Specification

Inside diameter Height Volume No. of baffle Motor (DC) Agitation speed Heater Temp. control Pressure of cooling water Material Power gupply Model Manufacturer

132 mm 200 mm 2600 ml 3 EA 40 W 50-1500 rpm 30+ 80 W (2 EA) 0~60°C 0.8-1.2 kg/cm2 Pyrex glass, SUS 316 AC 110V,60 Hz SY-250 Korea Fermentor Co.

(1 l)

Referring to BS as the total concentration of boron in solution, then from equation (1) : [Ha] -

457

(14)

Co.) connected to the reactor. Table 2 lists the reactor specifications and Fig. 1 shows it schematically. The operating temperature of the B T R S is 10°C for dilution and 60°C for boration. The concentration of boron in the coolant decreases from about 1500 ppm at the beginning of the reactor fuel cycle to about 20 ppm at the end of the cycle. Accounting for these operating conditions of the BTRS, the experiments were carried out at 10, 30, 45 and 60°C in the boron concentration range 0-1500 ppm. The resin was pretreated via the following procedures. It was fully swelled, converted into O H form by 2 N N a O H solution and then rinsed thor-

sampling port

4

~

thermometer

In other words, at low boron concentration there exist only monoborate ions and the adsorption of boron on the resin can be quantified with a Langmuir equation. 3. EXPERIMENTAL

The boric acid (Aldrich Co.) employed in this study was 99.99%. All other chemicals used were of G R grade. Water, distilled and deionized through a N A N O P u r e purification system (Barnstead 18.5 Mf~), was used as solvent. Amberlite I R N - 7 8 L C (Lot No. 16082), which is a gel-type, strong-base anion-exchange resin manufactured by R o h m & Haas Co., was used as an anionexchanger throughout the experiments ; its characteristics are listed in Table 1. A temperature-adjustable fermentor of 2600 ml capacity (SY-250, Korea F e r m e n t o r Co.) was used as the batch reactor for the adsorption reaction of boric acid. The temperature of the reactor was controlled with an E N D O C A L RTE-210 thermostat ( N E S L A B

J_ /--132mm 3 baffles

~

200 m m

I I

_l

. magnetic bar

Fig. 1. Schematic illustration of the reactor for adsorption.

458

JUNG WON NA a n d KUN JAI LEE

oughly with deionized water. After pretreatment of the resin, 1000 ml boric acid solution of a given concentration was put into the batch reactor. The reactor was maintained at the desired temperature to within +0.5°C. Then, a weighed amount (8-16 g) of the pretreated resin was put into the reactor. The system was then stirred at 700 rpm for about 2 h. Finally, after the adsorption equilibrium had been reached, the solution was sampled and analyzed to determine the concentration of boron. For sampling, a special pipette with a wire mesh at its tip was used to prevent resin grains from being sucked in the pipette. The amount of boron adsorbed on the resin was estimated from the solution-phase concentration of boron at equilibrium. Since boric acid is weakly dissociated, it cannot be titrated directly with NaOH, a strong base. For this

1.0

,

0.8-

'

I

FIB

0.6

reason, an excess ofmannitol (polyalcohol) was added to the sample solution to convert the boric acid into an organic acidic complex, a monobasic acid. The complex was then titrated with 0.1 N NaOH standard solution using a DL25 titrator (Mettler Co.). 4. RESULTS AND DISCUSSION

4.1. Composition of borate ions The results calculated employing equations (1)-(8) are shown in Fig. 2. It can be seen from Fig. 2, that the boric acid hardly dissociates at low pH, begins to dissociate into monoborate ions as the pH rises and dissociates completely into monoborate ions at pH > 12. Furthermore, Fig. 2 shows that, while for 1000 ppm there exist monoborate and triborate ions at about pH 7 of the coolant, for 100 ppm there exist

1.0

'

B"

'

~ 7

..... 9 pH

'

HB

'

B-

3

0.2

a,-

/

_\

a-

,

~' 11

0.0

'

13

1 o

~

'

~

-""

'

7

9 pH

11

13

7

9 pH

11

13

x

•~ 0.8 _

I

o

o

5

'

0.8

oc }

0.0

,

.-

0.6

0.6

oron:

0.4 !

0.4

0.2

~ 0.2

0.0

'

7

9 pH

11

0.0

5

Fig. 2. Composition of polyborate ions (HB, H3BO3; B-, B(OH)~ ; B~, B2(OH)7 ; B3, B 3 ( O H ) t~}; B42- ; B4(OH)24).

Boron adsorption on anion-exchange resin 0.10

I

R 0.05

0.10

IB.]/""-~/

Boron : I000 ppm Temp. : I0 "C

459

Bon.l,,m B-/

t

/ / " / / / / /

Temp. : 60 "C

/ /

B3-

0.05

B3 ° B 4-

.

0.00

0.00 6

7

8

9

6

7

pH

BI/

0.I0

9

8

9

0.10

Boron : 1001ppm Temp. : 60 "C

t 0

8 pH

B" 1 /

0.05

o.o5

O

/

132

/

B 0.00

0.00 6

7

8

9

6

7

pH

pH

Fig. 3. Composition of polyborate ions in the pH range 6-9. Abbreviations as in Fig. 1.

few other ion species besides monoborate ions at the same pH. In other words, the mole fraction of polyborate ions with more boron atoms increases with the concentration of boric acid. As for the effects of temperature on the formation of borate ions, it was shown that more polyborate ions exist at 10°C than at 60°C, i.e. the lower the temperature is the more polyborate ions are generated. This trend is shown more clearly in Fig. 3, which is an enlarged version of Fig. 2 for pH 6-9. For the composition of borate ions around pH 7 at 1000 ppm, it was shown that triborate ions are dominant at 10°C, while monoborate ions dominate at 60°C. Figures 4 and 5 show how the quantities of polyborate ions and O H - ions vary with the concentration of boric acid. As shown in these figures, diborate and tetraborate ions are generated in relatively small quantities, compared with monoborate and triborate ions.

The average number of boron atoms per 1 equiv borate ions generated from the dissociation of boric acid is plotted in Fig. 6 as a function of boron concentration. From Fig. 6, it was found that the average number is 1.0 for boron concentrations of up to 100 ppm and about 2.6 at 10°C and about 2.2 at 60°C at 1500 ppm, i.e. polyborate ions with a relatively large number of boron atoms are generated at low temperatures. 4.2. Boron adsorption

Figure 7 shows the adsorption isotherms of the boric acid on the IRN 78-LC resin with boron concentration at 10, 30, 45 and 60°C. As shown in Fig. 7, the amount of boric acid adsorbed on the resin tends to increase with temperature and the concentration of boric acid. In Fig. 7, the isotherms intercept the y-axis at zero

460

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,

,

,

,

WON

NA

and

KUN

JAI

3

[

I

'

'

'

LEE

'

'

'

' ''"1

'

'

'

' ''"1

'

1 0 "I

1o-2 1o-3 el

f

f

......

i0 -4 10.5

B"

:lO:C

/

B3

.......... -=-:~ .... ...-:.:-.: .-.. :.-..'.--.. .. . . . . . . ; .... : . . . . . . . . . . . . . . . . . . . .

xo-~ 10- 7

0

10-8 -5-,¢,.: ..... 10-9 10-1° .. 10-11 f 10-12 / o10"13 10-14 10-15 , , 0

B2-

B4

1

,

,

I

,

,

,

,

I

,

,

,

,

500 1000 Boron concentration (ppm)

1500

10

100 Boron concentration (ppm)

1000

Fig. 4. Molar concentration of each polyborate ion at 10°C. Abbreviations as in Fig. 1.

Fig. 6. Average number of boron atoms per 1 equiv, polyborate ion in solution at 10 and 60°C.

boron concentration. This seems to violate the physical view of the general adsorption phenomena. However, when a small amount of boric acid was added initially, boric acid could not be detected in solution because the concentration is within the resolution of the autotitrator. The boric acid was almost completely adsorbed on the resin at extremely low equilibrium concentration, and the quantity adsorbed increased very quickly with boron concentrations of up to 200

ppm and increased constantly thereafter with boron concentrations up to 1500 ppm. Figure 8 shows the average number of boron atoms involved in a polyborate ion in the resin phase, indicating that the average number of boron atoms exceeds unity. The Langrnuir equation can be applied for adsorption by the ion-exchange (Helfferich, 1962) ; however, the prediction employing this equation was not in good agreement with the experiments, as shown in Fig. 9. The reason for the discrepancy between prediction and experiments is presumably that the average number of boron atoms in a polyborate ion increases with the concentration of boric acid, as was shown in Fig. 8. In this study, taking this characteristic, that the adsorption of boron on the resin varies with boron concentration, into account, the following isotherm expression for the adsorption of boric acid was proposed :

10o

'

'

10-2 ,~ 10-3 ,~, 104 10.5 10-6 i

'

'

I

FIB

10- I

.

.

.

I

.

'

'

'

'

B

. . . . . . . "_. . . . . . . . . . . . "'J2 ".'= ".=". . . . . _~.B~:.~..:. ~..:.~'" . . . . . . . . . . . . . . . .

10.7 10-s - 7 10-9 10-10

: .......

.:,,P~"

i

10-u 10-12 10-13 10-14 lff is

q = q~(KLC/(I+KLC)+KcC+Ko),

B2

B 4-

--

.....

.° f

I

0

I

I

I

[

I

I

I

t

i

I

500 1000 Boron concentration (ppm)

I

I

I

1500

Fig. 5. Molar concentration of each polyborate ion at 60°C. Abbreviations as in Fig. 1.

(15)

where q* is the adsorption capacity of the resin for monoborate ions in units of g-boron/l-resin. The first term on the right-hand side of equation (15) reflects that at low boron concentration, monoborate ions are adsorbed on the resin with the Langmuir isotherm, as indicated in equation (14). The second term is introduced to consider that the monoborate ions adsorbed on the resin are replaced by polyborate ions, such as triborate ions, as the boron concentration increases. Finally, the third term represents the rapid adsorption of boric acid.

461

Boron adsorption on anion-exchange resin

.•40 30

" ! 20

~I0

A :60"C

0

,

,

,

0

,

I

~

~

,

~

I

,

500 1000 Boron cone. in solution (ppm)

,

,

,

I

1500

Fig. 7. Adsorption isotherms of boric acid on an IRN-78LC resin.

3

'

'

I I

'

'

I

'

'

'

'

~

'

'

'

'

-loy

2

0

500 1000 Boron cone. in solution (ppm)

1500

Fig. 8. Average number of boron atoms per one functional group in resin at I0 and 60°C.

T a b l e 3. C a l c u l a t e d v a l u e s o f t h e c o n s t a n t s f o r e q u i l i b r i u m a d s o r p t i o n of boric acid Temp. (°C)

q'~

KL

Kc

Ko

10 30 45 60

1 6 .1 9 1 6 .1 9 16.19 16.19

1 0 . 1 7 0 x 10 - 3 6.693×10 3 5.436×10 3 3.089×10 3

5.661 x 10 - 4 3.919×10 4 3.191×10 4 2.727×10 4

0.5873 0.5573 0.5373 0.5105

Figure 9 shows the comparison of the experimental results with computed values at 10°C. As shown in this figure, a simple Langmuir model agrees with the experimental data initially, however it deviates as the concentration increases. If the constant term in equation (15) is neglected, the estimated behavior also deviates from the experimental value. The proposed model shows good agreement with the experimental values, which is useful for estimation of BTRS behavior. The constants involved in equation (15) at 10, 30, 45 and 60°C by fitting the experimental data were determined by the method of least squares. The results are listed in Table 3. As shown in Figs 7 and 9, suggested isotherm, equation (15), agrees well with the experimental data.

5. C O N C L U S I O N S

In this study, an experimental and theoretical analysis on the adsorption of boric acid on a strong-base anion-exchange resin, Amberlite IRN-78LC in O H form, at 10, 30, 45 and 60°C in concentrations of boron up to 1500 ppm was conducted. It was confirmed that polyborate ions with more boron atoms in solution dominate with decreasing temperature and increasing boron concentration. An equilibrium model was proposed to account for the excess over the average amount of adsorbate in the resin, which might be evidence for the existence of polyborate adsorbed on the resin.

462

JUNG WON NA and KUN JAI LEE 40

.

.

.

.

i

.

.

~30

.

.

i

s "'~"

t

'

'

'

>"

i

--

~"

20

~

-: Proposed model . . . . : Langmuir combined with linear function ...... : Langmuir

I0'

0

t

0

~

~

i

I

~

~

~

~

I

~

500 1000 Boron cone. in solution (ppm)

~

~

~

[

1500

Fig. 9. Comparison of the experimental results with the model prediction at 10°C.

REFERENCES

Cohen P. (1980) Water Coolant Technology of Power Reactors. American Nuclear Society, Washington, DC. Everet D. A. and Popiel W. J. (1956) J. Chem. Soc. 3183. Helfferich F. (1962) lon Exchange, Chap. 5. McGraw-Hill, New York. Ingri N. (1963) Acta. Chem. Scand. 17, 573. Ingri N. (1963) Acta. Chem. Scand. 17, 581. Ingri N. (1957) Acta. Chem. Scand. 11, 1034. Ingri N. (1962) Acta. Chem. Scand. 16, 439. Keenan J. F. and Keys F. G. (1936) Thermodynamic Properties of Steam. Wiley, New York. Marshall W. L. and Franck E. U. (1981) J. Phys. Chem. Ref. Data I0, 2.

Maya L. (1976) Inorg. Chem. 15, 2179. Mesmer R. E., Baes C. F. Jr and Sweeton F. H. (1972a) In Proc. 32nd Int. Water Conf., pp. 55-62, Pittsburgh, PA. Mesmer R. E., Baes C. F. Jr and Sweeton F. H. (1972b) Inorg. Chem. I1, 537. Peteka F. (1980) J. Chromat. 201, 359. Qixia Z. (1986) Water Chemistry for Nuclear Reactor Systems 4, pp. 333-336. BNES, London. Stetten D. Jr (1951) Analyt. Chem. 23, 1179. Tomizawa T. (1981) Denki Kagaku 49, 339. Tomizawa T. (1979) Denki Kagaku 47, 602. Tomizawa T. (1983) Denki Kagaku 51,477.