H. LEVIN,’ W. J. DIAMOND,? and B. J. BROWN Whirlpool Corp., St. Joseph, Mich.
Influence of Ionic Strength on Ion Exchange In dilute ion exchange systems, such as those used for nuclear decontamination, ion removal depends on ionic strength in a w a y contradictory to the l a w of mass action
E
A u Y RESULTS iN nonequilibrium ion exchange experiments appeared to contradict the applicability of the law of inass action to dilute solutions. To exjilore this anomaly further, ion exchange equilibrium experiments were undertaken \rith dilute solutions of selected electrolytes in irhich ion concentrations, as low as 10-10 mole per liter, were employed. T h e investigation described here shoired that among the reactions considered, ion removal in dilute solution depends upon ionic strength, ji: in a way contradictor), to the laiv of mass action. Removing a low concentration of a n ion from \rater by ion exchange can be carried out most effectively if is adjusted to an optimum value. This principle, significant for engineering ion exchanqe processes because it affects size and efficiency of column operation. ]vi11 apply readily to decontamination problems in nuclear engineering: maintenance of high purity \rater in reactor systems; reinoval of cesium-1 37 and srrontium-30 from dilute \caste solutions resulting. from chemical reprocessins of nuclear fuels; decontamination of laboratory, hospital, and 1aundr)- waste liquids; and separation of traces of reactor-bred uranium and transuranium isotopes from dilute aqueous systems. To illustrate the principle. consider that strontium-00 is present in a body of water a t mole per liter (10 times its maximum permissible concentration), and that this Lvater must be used for drinking purposes. I n decontaminating
Present address, Marquardt Aircraft Corp., Van Nuys, Calif. * Present address; Brunswick Balke Collender Co., Muskegon, Mich.
0
e REACTION B
W
I
REACTlON A
2l
0
REACTION C
V REACTION D X
REACTiON E
0 REACTION F
-2
+ 0
I
Figure 1.
C-; REACTION G 1
I
I
i -
2 3 4 5 LOG OF IONIC S T R E N G T H ( ~ J J )
6
pK vs. pp for equilibria, Sr++ and monovalent resins VOL. 51, NO. 3
0
MARCH 1959
313
Y
REACTION J Y 3NaR+3NaStYR3
'++
/
+
\
8
REACTION A
6 SI'
't 2 N a R +
Y
cz
- 4 Y
b/
LL
0 c3
s I
2
REACTION I Srt'+CaR2 \
-Catt+SrR2
0
Y
Na't
HR+
H++NaR
-2 -LOG OF IONIC S T R E N G T H ( P J J ) Figure 2.
p K vs. pp for cation exchange reactions
the Ivater by cation exchange, efficiency of removal per unit volume of resin may be increased by factors as high as 1000, provided ,LA is adjusted to 2.5 X mole per liter (Figure 4). Actual increase in efficiency depends on the difference between initial ionic strength of the solution and optimum ionic strength. Thus, by adjustment of ,LA, a safer water supply may be obtained. Experimental
iMaterials. All glassware was cleaned with chromic acid and rinsed with deionized water until the water showed a resistance of at least 2 X 106 ohms per cc. Conductivity was measured by an L and N No. 4866 bridge and a dip-type conductivity cell. IR-120 (Rohm & Haas Co.) was used as a sodium resinate of 30 to 40 mesh, and Nalcite SBR No. 8 (National illuminate Co.), as a resin chloride of 20 to 30 mesh. Electrolyte solutions were prepared by diluting concentrates. Resin was converted to the appropriate cation or anion form by adding a converting electrolyte. A
3 14
concentrated solution of the electrolyte (500 ml.), added to 700 ml. of the resin, was agitated for 15 minutes and then decanted. This was repeated 20 times. Moisture content of the various resins was determined by the Association of Official Agricultural Chemists official method, Ca2A-45: 19.50. Presence of resin caused a substantial increase in ionic strength of solutions, apparently because unexchanged ions diffused out of interstices of resin. Ionic strength of solutions could be reduced and maintained more nearly constant by using resin agitated in a continuous floiv of deionized Lvater for a minimum of 7 2 hours. Therefore, washed resins were used unless otherwise indicated. Salts of radioisotopes, sodium-22, strontium-89 and -90, yttrium-91, iodine131: and phosphorus-32 Lvere obtained from Oak Ridge National Laboratory. Except for phosphorus-32, they were carrier-free. Radio-purity was ascertained by a radiation absorption technique (5). In using strontium-90 as a tracer, it
INDUSTRIAL AND ENGINEERING CHEMISTRY
was found that its ratio to its daughter product, yttrium-90, was altered by the exchange phenomena. To determine strontium-90 content at equilibrium, decay curves ivere plotted for each sample, and each sample was counted daily for a total period of 183 hours (3 half lives of yttrium-90). The other radioisotopes presented no difficulties in counting. An Atomic Instrument Co. scintillation spectrometer (Model 51 3) and a Kuclear Instrument Co. proportional counter were used to,count gamma and beta activities, respectively. T h e resin was weighed into 8-ounce polyethylene bottles. After measuring conductivity and obtaining samples for radiometric and other analyses, the solutions were added to the resin. Bottles containing cation exchange resin and solution were placed in a constant ternperature water bath (26.7 i 0.5' C.) and agitated for 2 hours to assure equilibrium. Because anion exchange equilibrium was more slowly attained, the bottles containing anion exchange resin were placed in a n Atlas Launderometer a t 37.8 & 0.5" C. and rotated at 14 r.p.m. for 24 to 48 hours. Samples of solution for conductimetric and radiometric analysis were taken simultaneously. I n ion exchange reactions-e.g., .A f BR @ B AR-concentration of ion A a t equilibrium \vas always determined by radiometric methods. I n solutions stronger than O.OIAM, concentration of ion B was either calculated stoichiometrically from the radiometric analyses or determined by titration. I n solutions weaker than 0.01.if: advantage was taken of the finding that at equilibrium. the electrolyte present was almost entirely B, displaced from and diffusing out of the resin. Thus a t less than O.OIM, conductivity measurements could be converted into concentration of B and ionic strength, Lvith only a slight correction for A remaining at equilibrium. For solutions bveaker than 1.94 X l o - > 3.58 x 10-5 1.25 x 10-5
3.08 3.08 3.06 5.82 1.08 3.75
X 10-2 X 10-b
X 10-1 X 10-1 X 10-1 X 10-l
1.82 7.30 2.78 5.86 1.49 5.71
3 . 3 7 x 10-1 1 . 3 9 X 10-l 1 . 1 5 X 10-2 1.27 10-3 2 . 1 8 x 10-4 1 . 0 6 x 10-4
4.48 1.39 1.15 1.27 2.18 1.06
X
2.94 X 10-1 1.08 X lo-' 6.06 x 10-3 3.54 x 10-6 6.56 x 10-9 2.08 X
1.55 X 10-1 7 . 9 9 x 10-2 7.99 x 10-2 7 . 9 9 x lo-' 7 . 9 9 x 10-2 7.99 x 10-2
5.05 5.87 5.69 1.61 1.96 3.59
X lo-" X 10-5
x
6.71 X 10-2 8.10 x 10-3 8 . 7 8 x 10-4 1.41 x 10-4 7.48 X 10-5 7.25 10-5
x
7.25 X 10-2 8.159 X 7.79 x 10-4 1.41 x 10-4 7.48 X 10-5 7.25 X 10-5
1 . 7 8 X 10' 1.88 X 10' 1.89 X l o L 1.06 X 10' 4.62 2.45
18.6 165 1703 6029 4936 2698
6 - 6 1 x 10-3 1.29 x 10-4 1.27 X 2.57 X 3.12 x 10-9 6 . 7 7 X 10-10
3.27 X 4.73 x 10-3 4.64 x 10-4 7.4 10-5 5.4 x 10-5 4.51 x 10-5
1.05 X lo-' 1.43 X 1.39 x 10-3 2.22 x 10-4 1 - 6 2 x 10-4 1.35 x 10-4
2.47 2.0 1.85 7.1 3.5 6.24
x x x x x x
14.2 75 764 3769 3 109 1432
7.46 3.85 3.85 3.85 3.85 3.85
X 10 1 X 10-I X 10-1 X 10-1
X 10-i X 10-1
X X X X X X
INDUSTRIAL AND ENGINEERING CHEMISTRY
10-2 lo-? 10-2 10-2
X 10-6 X lo-' X lo-' x 10-5
X lo-' X 10-8 X 10-9 10-1'1
x
x
X lo-' X 10-3 X
x x
10-4 10-5
X 10-1
X 10-l X 10-2 X 10-3 x 10-4
103 10' 105 105 105 104
IONIC STRENGTH Table I. Equilibrium Ion Exchange Reaction Data (Continued) Initial, Moie/L. Equilibrium, Mole/ L . Iniiic Strength, Reactant Exchanged Reactant Resin ion ion ion P
Reaction 1 0 ; 3 NaI
11; NaI
+ Rap01
+ ROH
1 2 ; Na2HPOI f 3 RCl
13; Na3POr f 2 RC1
X
X lo-?
x
lo-? lo-? 10-2 10-2 10-2
5.01 8.06 8.75 1.63 1.87 2.64
9 . 3 7 x 10-2 9 - 6 7 x 10-3 9.67 x 10-4 9 . 6 7 X 10-j 9.67 X 10-6 9.67 x 10-7 4.84 X
1 . 5 5 x 10-1 7.99 X 10-2 7.99 x 10-2 7.99 x 10-2 7.99 x 10-2 7.99 x 10-2 7.99 x 10-2
2.83 1.28 1.48 1.22 3.28 1.77 6.20
8.81 X 10-3 9.37 x 10-4 9.67 x 10-5 9 . 6 7 X 10-6 9.67 X l o - '
2.76 1.47 7.56 7.56 7.56
X 10-1 X 10-1 X lo-' X lo-? X
2.02 x 1.22 x 8.13 x 9.12 x 1.53 X
8.81 X 9.37 x 10-4 9.67 X 10-j 9.67 X 10-6 9.67 X lo-'
2.76 1.47 7.56 7.56 7.56
X 10-1 X lo-'
1.75 X 10-4 9 - 8 6 x 10-7 4.40 X 2.36 X 4.44 x 10-9
9.29 9.67 9.67 9.67 9.67 9.67
4.89 2.52 2.52 2.52 2.52 2.52
x x
10-3 10-4 X 10-5 X X lo-'
X X X X X
X lo-? X lo-? X
X X X X X
10-3 10-5 lo-' 10-8 10-n 10-lo
3.23 2.44 1.93 1.99 6.1 4.62
X lo-' X 10-2 x 10-3 x 10-4 X 10-5
x x x
X 10-5 X lo-'
9.37 8.85 8.59 9.67 1.76 1.67 4.41
1.72 X 10-2 1.74 x 10-3 1 - 9 4 x 10-5 5.94 x 1 0 - 5 4.96 x 10-5
1.78 1.74 1.94 5.94 4.96
2.59 X 10-2 x 10-7 3.02 X 10-8 4.19 X 10-8 2.73 X
2.695 X 10-2 2.87 x 10-3 3.02 X 10-4 4.19 X 10-6 2.73 x 10-5
9.56 X 7 . 8 x 10-3c 6 . 3 x 10-4c 1 . 1 3 X 10-hc 5.16 X 10-be 4 . 3 x 1o-jc
x
10-3 X 10-j X lo-' x 10-9
3.8 8.85 8.59 9.67 1.76 1.67 4.41
X X 10-0 x 10-3
10-4 10-6
10-3 10-9
Reinoval K
X 10-2 x 10-3 x 10-4 X 10-j
X 10-5
2.87
X 10-5
2.22 x 4.33 x 4.99 x 1.40 x 2.16 x 2.31 X
Factor 105 10" 1013 103 10-3 lo-'
18.5 120 1106 5932 5165 3665
10-2 10-3 10-4 X 10-j x 10-5 X 10-5 x 10-4
4.55 x 101 9.51 X 10' 7.10 x 10' 9.59 6.49 X lo-' 1.14 X 10-1 4.3 x 10-1
33 755 6551 7898 2949 547 7232
X
5.27 X 1.63 X 5.94 x 10-4 4.94 x 10-5 2.05 X 10-6
43.5 765 1190 1060 630
4.18 x 10-3 1.62 X 8.05 X lo-' 3.97 x 10-10 5.82 X 10-11
50 950 2195 410 2 18
x
x x x
10-3 10-4 10-5 10-5
C a + +initially present in inoles/l. a S a - initially present in moles/l. and descending order, 3.88 X 10-1, 10-2, 10-3,10-4, 1 0 - 6 , 1 0 - 6 , and 10-6. PI&---). and de-centling order, 2.23 x 10-1,x 10. c As total moles of phosphate present at equilibrium (HzPO4- HPO4--
+
++ +
+
Reaction H NaH R F? H + NaR Reaction I S r + + CaR? i, C a t + SrR2 Reaction .4 SrT+ 2 S a R Fi 2 N a f SrRl Reaction J Y + + + 3 NaR 3 Na+ YR ?
+
*
+ +
1700 -
-
+
-
-
and for anion exchange reactions:
* + + so,3 I- + RJPOJ 3 RI + PO,' I - + ROH Ft R I + OHHP04- + 2 RC1 ?=! RzHPO, + 2 CIPO,' f 3 RC1 + RjPOc + 3 c1-
I300 -
+
Reaction 8 I RC1 R I ClReaction 9 2 I R2SOa F? 2 RI f Reaction 10 Reaction 11 Reaction 12 Reaction 1 3
+
-
F?
Data for these reactions are given in Table I . Previousl!- it has been pointed out (2-J): that systems of ion exchange can be treated as single-phase condensed systems and that single phase equilibrium constants: independent of ionic strength, may be applied. Such treatment Lvould derive from consideration of resin as a pol>-elecrrolyte:influencing: in much the same manner as a true e l e c t r o l p , properties of the solution in its vicinity. Ho\\el-er, in the dilute regions, parameter I; strongly depends on ionic strength fFigures 2 and 3). Contrary to expectation, 1 ; does not approach the value of a single phase equilibrium constant as the solution becomes more dilute. Therefore. a single phase equilibrium constant does not express the free energy of the ion exchange reaction in dilute solution, even \vhen properly considering
Tr
-
0
t-
y
LL
1
a
900-
-
>
I
I
\
\
I
0
H W a
500 -
-
1000
I
2
- LOG OF Figure 4.
3 4 IONIC STRENGTH ( p p )
5
6
Removal factor vs. ionic strength for cations VOL. 51, NO. 3
MARCH 1 9 5 9
317
of at least monovalent anions is influenced more by affinity of ion to solution than by affinity of ion to resin. I t appears also from the foregoing experiments that unequally effective removal of cations and anions from a single solution by a mixed bed ion exchanger will usually occur. For example. using a mixture of N a R and RC1 to remove YII from the solution, Y + - + \vi11 be removed most effectively at a @ o f 5 X 1 0 - 2 a n d I - a t a p o f 1 . 4 X lW4.
8,OOC
7,000
6,000
5,000 REACTION 8
Summary
Lz
0 k
A REACTION 9
2 4,000 LL
1
> U 0
0
I
E
REACTION 10
3,000 A REACTION I I 2,ooc 0
I,OOC
,
REACTION 12
0 REACTION 13
In ion exchange s)stems. dilute and concentrated solutions behave differently. Single phase equilibrium relationships and the law of mass action appear applicable in relatively concentrated ion exchange svstems. I n dilute solutions. the abi1it)- of the resin to exchange specific ions at equilibrium is a function of ionic strength. maximum removal factor occurs at a specific ionic strength in every instance. Ionic strength can be adjusted to provide maximum removal of ions at least under equilibrium conditions. Acknowledgment
0
1
2
3
T h e authors \\.ish to express their appreciation to associates at the Whirlpool Corp. for advice and interest in this work and to Louise Shultz and Donald Wood for technical assistance.
5
4
PP
Figure 5.
Removal factor vs. ionic strength for anions
activity coefficients, water transfer, and electrolyte uptake (7). Figures 4 and 5, plots of removal factor us. p ~ reveal , that in all the reactions which were investigated
+
+
A BR AR B, considered as either cation or anion exchange.
[(B)VLI(RF
-
1)
1. A maximum removal factor occurs in each instance. 2. Ionic strength is critical in determining maximum removal of a n ion species. 3. .4t ionic strength greater than pmsx-i.e., p for the maximum removal factor-the law of mass action approximately obtains. T h e greater the ionic strength, the smaller the removal factor. 4. At ionic strengths less than pmaa, the smaller the ionic strength, the smaller the removal factor.
+ 2BR e AR? + 2B, R F l ' s / V ~ . for A + 3BR A R I + 3B, RF V S / V L . K / w 3; for A + BR2 Ft AR2 f B, R F E V S / V L . 3K/p; etc.
Also in solution so dilute that the initial concentration of reactant ion is small compared to the moles of resin (5), the removal factor (RF) at equilibrium is independent of the initial concentration of reactant ion except as it influences ionic strength, and ( 8 ) the concentration of reactant ion a t equilibrium is always inversely proportional to the moles of pure resin per liter of solution initially present. Deduction (5) is well illustrated by Reactions B and C, where the reactant ion concentration was maintained roughly constant and @ was varied. Both deductions resulted from the following treatment as illustrated by
From Figures 1, 2, and 3. in the very dilute region, K can be considered as a function of pn. l'alues for n, obtained from these figures, are higher than the exponents of p given in the RF relationships listed previously; hence, the decrease of RF with decreasing p in the very dilute region. Since, in the dilute region, a relationship K = kp" can be obtained (Figure l ) , ionic strength can predict removal factor for a particular ion exchange reaction at equilibrium. From Figure 5, pmar for optimum removal of I- occurred at 1.4 X even though four chemically different anion exchange resins were used. Perhaps this is an indication that removal
3 18
(mBR)
-
Js' (Ao N Vs; and B 1.1. Then R F V s / V L K,'p. Similarly it can be shown that
But RF
mBA
>> 1 ;
=
24)
for A
K/P2;
$
INDUSTRIAL AND ENGINEERING CHEMISTRY
E
Literature Cited
(11 Bauman, E. W., hrgersinger, W. J., Jr., J . Am. Chem. Soc. 78, 1130 (1956). (2) Duncan, J. F., A u s t ~ ~ l Ji .~ Chem. n 8, 1 (1955).
(3) Duncan, J. F., Proc. Roy. Soc. (London) A214, 344 11952). (4) Gluekauf, E., Zbid., A214, 207 (1952). (5) Harley, J. H., Hallden, N., Nucleonics 13, No. 1, 32 (1955). (6) Richman, D., Henry, T. C., J. Phys. Chem. 60, 237 (1956). (7) Shubert, J. S., J . Phys. G3 Colloid Chem. 5 2 , 340 (1948). (8) Shubert, J . C., Conn, E., Nucleonics 4, 2 (1948). RECEIVED for review February 4, 1957 ACCEPTED September 2, 1958
Correction Construction Digest 1958 In the "Construction Digest 1958"
[IND. ENG. CHEhf. 51, 6 5 A (January 1959)] subitems e and 1 under item KO. 197 should be deleted. These plants are actually properties of Canadian Liquid Air Co., Ltd.: and are given under item
No.198. Subitem j under irem No. 197 should read 40 million pounds of polyethylene and not 40 thousand tons.
