Environ. Scl. Technol. 1988, 2 0 , 149-155
(7) Hermann, M.; Chaude, 0.;Weill, N.; Bedouelle, H.; Hofnung, M. Mutat. Res. 1980, 77, 327-339. (8) Pitts, J. N.; Van Cauwenberghe, K. A,; Grosjean, D.; Schmid, J. P.;Fritz, D. R.; Belser, W. L., Jr.; Knudson, G. B.; Hynds, P. M. Science 1978,202, 515-519. (9) Shuetzle, D.; Lee, F. S.-C.;Prater, T. J.; Tejada, S. B. Znt. J . Environ. Anal. Chem. 1981, 9, 93-144. (10) Lofroth, G. Environ. Sci. Res. 1981, 22, 319-336. (11) Pelroy, R. A.; Sklarew, D. S.; Downey, S. P. Mutat. Res. 1981,90, 233-245. (12) Lockard, J. M.; Prater, J. W.; Viau, C. J.; Enoch, H. G.; Sabbarwal, P. S. Mutat. Res. 1982, 102, 221-235. (13) Sheppard, E. P.; Wells, R. A.; Georghiou, P. E. Environ. Res. 1983, 30, 427-441. (14) Guerin, M. R.; Rubin, I. B.; Rao, T. K.; Clark, B. R.; Epler, J. L. Fuel 1981, 60, 282-288.
Terry Vdokales as undergraduate research assistants is acknowledged. We are indebted to B. N. Ames for the donation of the tester strains. "3,
Registry NO. NO2,10102-44-0; SOZ, 7446-09-5; H2S, 7783-06-4; 7664-41-7; N20, 10024-97-2.
Literature Cited (1) Payne, J. F.; Martins, I.; Rahimtula, A. Science 1978,200, 329-330. (2) Guerin, M. R.; Ho, C. H.; Clark, B. R.; Epler, J. L. Znt. J . Environ. Anal. Chem. 1980,8, 217-225. (3) Bingham, E.; Trosset, R. P.; Warshawsky, D. J . Environ. Pathol. Toxicol. 1980,3, 483-563. (4) Ames, B. N.; McCann, J.; Yamasaki, E. Mutat. Res. 1975, 31, 347-364. (5) Maron, D. M.; Ames, B. N. Mutat. Res. 1983,113,173-215. (6) Rao, T. K.; Allen, B. E.; Ramey, D. W.; Epler, J. L.; Rubin, I. B.; Guerin, M. R.; Clark, B. R. Mutat. Res. 1981,85,29-39.
Received for review January 23,1985. Revised manuscript received May 10,1985. Accepted October 3,1985. Research funding through St. John's University is greatly appreciated.
Important Process Variables in Chromate Ion Exchange Arup K. Sengupta
and Dennls Cllfford
Environmental Engineering Program, University of Houston-University The effects of competing ion concentrations, pH, and ionic strength on chromate selectivity have been studied in relation to chromate ion exchange. The competing effects of sulfate and chloride anions have been explained with the aid of governing chromate ion-exchange mechanism and chromate chemistry. This study also reveals that the choice of acidic pH for a conventional chromate-exchange process is due to the selectivity reversal between HCr04- and Cr042-at the prevailing ionic strength. There is a critical acidic pH; a t pH lower than that, no increase in chromate removal capacity is observed.
Introduction In recirculating cooling water systems, 5-20 mg/L chromate is deliberately added as a corrosion inhibitor, while sulfate and chloride are normally present a t concentrations (500-4000 mg/L) several orders of magnitude higher than chromate. Despite the severe competition from sulfate and chloride, several authors (1-4) have confirmed the viability of the chromate ion-exchange process due to chromate's very high preference for commercial styrene-dinvylbenzene (STY-DVB) anion exchange resins a t acidic pH. In a binary system (i and j), the selectivity or preference of the component i is indicated by its distribution coefficient (Ai), Le., the ratio of its equivalent fraction in the exchanger phase and the aqueous phase &/xi); the higher the number is, the greater is the selectivity. yi and xi indicate equivalent fractions of component i in the exchanger phase and aqueous phase, respectively, i.e., Yi = Ci/Q (1) xi = Ci/CT (2) where Ci and Ci denote the concentrations of component i in the exchanger phase and aqueous phase, respectively. Q and CT represent the total exchange capacity of the resin and total aqueous-phase concentration, respectively. Hexavalent chromium, Cr(VI), may exist in the aqueous phase in different ionic forms with total chromate con+ Present address: Civil Engineering Department, Lehigh University, Bethlehem, PA 18015.
0013-936X/86/0920-0149$01.50/0
Park, Houston, Texas 77004
centration and pH dictating which particular chromate species will predominate. For convenience in discussion, we will describe the total chromate species as Cr(V1) or chromate, while each individual species will be represented by its chemical formula. The following are the important equilibrium reactions for different Cr(V1) species (5, 6): H2Cr04F? Hf + HCr04- log K (25 "C) -0.8 (3)
+ Cr042- log K (25 "C) -6.5 2HCr04- F? Cr2072-+ H 2 0 log K (25 "C) 1.52 HCr207-e Hf + Cr2072- log K (25 "C) 0.07 HCr04- e H+
(4)
(5) (6)
Reaction 5 does not contain any H+ terms, and therefore, in a certain pH range (2-5) this reaction is independent of pH and depends only on total Cr(V1) concentration. This may be regarded as a dimerization reaction for HCr04- a t acidic pH. Since the distribution of chromate species is dependent on both pH and total Cr(V1) concentration, a predominance diagram (Figure 1) has been drawn using both pH and total Cr(V1) as variables. The solid lines on the diagram separate the areas in which the indicated species predominate. Two horizontal dashed lines on the predominance diagram indicate the range of Cr(V1) concentration (5-20 mg/L) normally encountered in cooling water. HCr04- and Cr02- are the predominant species in this total Cr(V1) concentration range, but the relative distribution of each varies with pH. One questionable aspect of many previous studies, including that of Arden and Giddings (7)) in relation to chromate ion exchange a t acidic pH lies in considering Cr2072-as the only counterion in ion-exchange reactions as cited below: strongly basic (R4N+)2S042+ Cr20T2-s (R4N+)zCr2072-+ S042- (7) weakly basic (R3NH+)zS042+ Cr2072-e (R3NH+)2Crz072-+ (8)
From the predominance diagram, however, it may be noted that HCr04- is practically the only Cr(V1) species
0 1986 American Chemical Society
Environ. Sci. Technol., Vol. 20, No. 2, 1986
149
2
1 -
cr20:-
present in the aqueous phase. Sengupta (8)confirmed that HCr04 is by far the most predominant Cr(VI) species both in the aqueous and exchanger phase. Dichromate, Cr2O,", is, however, present in the exchanger phase, and the presence of Cr20T2-in the exchanger phase causes early, gradual Cr(V1) breakthrough during fixed-bed column runs. Binary chromate/sulfate and chromate/chloride isotherms a t acidic pH have inflection points due to the presence of Cr2072-in the exchanger phase. Nevertheless, chromate selectivity with respect to the competing species is dictated by HCr04-. The governing chromate ion-exchange mechanism (9) and some other unique characteristics of chromate ion exchange (10) have been presented elsewhere.
Experimental Details Resins. Altogether seven different resins were used for this study. However, for brevity and precision, the experimental data for the following two styrene-divinylbenzene (STY-DVB) resins are presented here: IRA800 (STY-DVB matrix, macroporous, strongly basic) and IRA-94 (STY-DVB matrix, macroporous, weakly basic). These two types of ion-exchange resins, &dentally, offer maximum chromate removal capacity h the chromate-exchange process. Although these resins were from one manufacturer (Rohm and Haas Co.), no endorsement is implied, and similar resins may be available from other manufacturers. After screening to remove bigger particles, the average particle size used was 500 f 50 pM. The resins were conditioned following the standard procedure of cyclic exhaustion with 2 N hydrochloric or sulfuric acid and regenerations with 2 N sodium hydroxide. Finally, the resins were converted into air-dried S042-and C1- forms. Chromate Isotherms. Chromate/sulfate and chromate/chloride isotherms (23 f 2 "C) were determined for different resins a t acidic pH (4.0). Isotherm data were generated by a batch equilibration technique where a weighed amount of resin (chloride or sulfate form) was gently agitated for 4-6 h with a fixed volume of solution containing sulfate and Cr(V1) or chloride and Cr(V1) of known initial composition. At the end of the equilibration, solution composition was determined again to calculate Cr(V1) uptake. Equlibrium was achieved in 4 h. Column Runs. Column runs were carried out at room temperature (23 f 2 "C) using plexiglass columns and constant-flow, positive-displacement pumps. Since the study was aimed at investigating the equilibrium behavior, the superficial linear phase velocity (SLV) was kept lower than is normally used in commercial practice. For all the column runs, the SLV and particle Reynolds number, (Re),, were the same or very close, their values being 2.15 m/h and 0.295, respectively. Unless otherwise stated, the effluent pH in all the column runs was within *0.2 unit 150 Environ. Scl. Technol., Vol. 20, No. 2, 1986
of the influent pH, and all column runs were conducted a t 23 f 2 OC. The column runs were lengthy (1-20 days), and fluctuations in the influent flow rate were less than f5% relative standard deviation. Ion-exchange resins were either in the sulfate or chloride form prior to starting the column operation. Following each run, the columns were completely regenerated with a greater than 5-fold excess of 4% NaCl or 1% NaOH or both. Mass balance checks comparing chromium in the spent regenerant to chromium removed in the exhaustion cycle were within f5%. Other experimental details have been provided by Sengupta (8). Chromium was analyzed by GFAAS with a Perkin-Elmer Model 372 atomic absorption spectrophotometer with a graphite furnace accessory after the necessary dilutions had been made. Sulfate and chloride were analyzed with a Model 16 Dionex ion chromatograph with 250-mm separator columns and the standard anion eluent. The FGAAS total chromium analyses were verified by use of the colorimetric diphenylcarbazide method for chromium (11). All the chromium in the aqueous phase was found to be in the hexavalent form. For relatively long column runs at acidic pH, some hexavalent chromium was reduced to trivalent chromium inside the ion-exchange resins. The trivalent chromium formed inside the resin was easily eluted out with 1% H2S04following regeneration. Mass balance checks showed that less than 2% of the total chromium removed by the ion-exchange resins was converted to trivalent chromium (8). Selectivity Reversal Experiment. The objective of this experiment was to confirm the possible selectivity reversal between the two Cr(V1) species, namely, monovalent HCr04- and divalent CrO:-, at different total aqueous-phase electrolyte concentrations. Since both of them are Cr(V1) species and they coexist at a very narrow pH range, this particular experimental setup was necessitated. Conditioned resins (IRA-900)in chloride or sulfate forms were added to a beaker containing 50-100 mg/L Cr(V1) solution at pH 6.0f 1. HCr04- and Cr042-are practically the only Cr(V1) species a t this pH and comprise the total Cr(V1) concentration. The beaker was covered by a rubber stopper, and nitrogen was slowly bubbled through the apparatus to prevent ambient C02 from entering the system. Provisions were made to add either acid or alkali in order to keep the pH of the solution constant. The following forms the basis for this experiment: as the resin beads are added to the beaker, HCr04- and Cr0:- will compete with each other for ion-exchange sites in the resin beads. Depending on the relative preference between HCr04- and CrO:-, pH will increase or decrease, and acid or base is to be added to keep the pH constant. By observing whether acid or base needs to be added, relative preference of the two Cr(V1) species may be known. Relative distribution of HCr04- and Cr042-in the aqueous phase is constant during the experiment because pH does not change. This allows for the determination of HCr04- and Cr042- selectivities under experimental conditions.
Results and Discussion In the chromate ion-exchange process, sulfate and chloride are, for all practical purposes, the major competing anions in cooling water systems. Binary isotherm tests were carried out a t different concentrations of the competing sulfate and chloride anions (2000 mg/L, 4000 mg/L); they are shown in Figures 2 and 3 for a strongly basic quaternary styrene-divinylbenzene resin (IRA-900). The noteworthy aspects of the isotherms are as follows: (a) for chromate/sulfate isotherms, an increase in com-
360
1
A E:
u.i)
~
AVS,
Resin: I R A - 9 0 0 (STY - DVB, SEA)
Resin: p H - 4 IRA 0
- 900SSA
0.6
0.8
280
6
tI.
0.4 -
@
4000 m g l L Sulfate
200
L
>-"
0.3 -
120
-
0.2
0 2000 mg/L Sulfate 0
O.l 0
t ;3/
0
40
4000 mg/L Sulfate p H = 4.0
0.4
0.2
YC I
Figure 4. Chromate selectivity vs. resin compositlon at different competing sulfate concentrations.
4.0
2.0
6.0
8.0
320 I
c r ( v i ) in Solution (mg/L)
IRA - 900 (Sty-DVB. SBA)
Figure 2. Chromate/sulfate Isotherms at different aqueous-phase sulfate concentrations. 0.7I
I
Resin IRA -
1.0
/
900
n
I
1
0.4 pH
0.3
120
0
40
I
0.2
0.4
0.2 0
0
2000 m g / L Chloride 4000 m g / L Chloride
I
2.0
40
60
8 0
0.8
0
Figure 5. Chromate selectivity vs. resin composition at different competing chloride concentraflons.
pH=40
0' 0
0.6 YC,
10 0
Cr(VI) in Solution (mg/L)
Figure 3. Chromate/chlorMe isotherms at different aqueous-phase chloride concentratlons.
peting sulfate ion concentration from 2000 to 4000 mg/L has only minor effects on chromate removal capacity; (b) for chromate/chloride isotherms, however, an increase in chloride concentration is accompanied by appreciable reduction in the chromate removal capacity. Similar kinds of isotherms were also obtained for other resins (8),namely weakly basic styrene-divinylbenzene anion-exchange resins (IRA-94) and strongly basic acrylic resins (IRA-458); observations a and b were found to be equally applicable for the isotherms of these resins. In order to enhance the prominence of the effects of the competing ion concentrations, chromate selectivity (Ycr/ xcr) vs. resin composition (ycr) has been plotted for these binary isotherms in Figures 4 and 5. Selectivity (or separation factor) of any counterion for a given ion-exchange equilibrium depends on the resin composition (12). In Figures 4 and 5, the equivalent exchanger-phase Cr(V1) fraction (ycr)has been plotted as the abscissa so that, a t identical resin composition, the effects of the competing ion conentration on chromate selectivity can be compared. Figures 4 and 5 demonstrate the following two points: (a) Chromate selectivity, regardless of the competing ion concentrations or type, goes through a maxima, Le., the plots show the occurrence of peaks. Such peaks correspond to the inflection points in the isotherms, and the inflection points are caused by the presence of Cr2072-in the exchanger phase along with predominant HCr04-. This behaivor can be explained with the governing chromate ion-exchange mechanism and has been discussed in detail elsewhere (IO).
(b) At any particular ycr value, i.e., resin composition, chromate selectivity (Ycr/xcr)is higher at increased sulfate concentration while it remains practically unchanged for an increase in chloride concentration. The general chromate ion-exchange equilibrium for chromate/sulfate exchange has been derived as given below (9) [*Cr(VI)] = [*RHCr04] 2[*R2Cr207]=
+
+ Asterisks and brackets denote the resin phase and molar concentrations, respectively. The above equation indicates both HCr04- and Cr207" are present in the exchanger phase, and their exchanger-phase concentrations may be expressed in terms of total aqueous-phase chromate concentrations, Cr(VI), exchanger-phase activity coefficients, fi, monovalent activity coefficient, yl,and various equilibrium constants. The subscripts HCr, Cr2, and S represent HCr04-, Cr20T2-,and sulfate, respectively. Cr2OY2is present in the exchanger phase and causes early, gradual breakthrough during column runs, but HCr04- is the prime counterion, and chromate selectivity or chromate removal capacity is dictated by HCr04- (8). Since chromate selectivity is of greater importance in this study, in order to avoid complexity, HCr04- will be considered as the sole exchanging Cr(VI) species. In that case, [*Cr(VI)] = [*RHCr04] =
[
]
K'fs[ *R2S04] [so42-l
1'2
[Cr(VI)]
fHc,yl
Environ. Sci. Technoi., Voi. 20, No. 2, 1986
(10) 151
Now, as shown in Figure 4, a theoretical comparison of chromate selectivities a t two different concentrations of the competing sulfate can be made with the aid of eq 10. Such a comparison is best made only a t identical resin compositions, i.e., same ycr values, because they would otherwise affect the selectivity. Again, at identical resin compositions the exchanger-phaseactivity coefficients,f H C r and fs, may be considered constants if we neglect shrinking of the resin due to the change in aqueous-phase sulfate concentration from 2000 to 4000 mg/L. For the aqueous phase, the Debye-Huckel approximation may be applied a t the prevailing ionic strength, I , to determine the monovalent ion activity coefficient, y1 -log y1 = 0.51112 (11) Upon dividing both sides of eq 10 by total exchange capacity, Q, and total aqueous phase concentration, CT, and performing necessary algebraic manipulations, eq 10 becomes
120 Resin I R A - 9 4 (STY - DVB. WBA)
- A q u e o u s P h a s e C r ( V I ) Conc. loo
ao L
0
1000
= (constant)C~o~5(100~5p’z) (17)
(1 - YC~)O.’ It may be noted from eq 17 that, a t identical resin
composition (same yCr),an increase in Cs is accompanied by a decrease in xcr; Le., an increase in Cs causes an increase in chromate selectivity or the chromate distribution coefficient (ycr/xcr).Equation 17 is thus qualitatively in agreement with the experimental data shown in Figure 4 and includes the effects of both concentration of the competing sulfate ion and associated nonideality effect (aqueous-phase activity coefficient) due to the change in electrolyte concentration. The above effect is similar to the “selectivity reversal phenomenon” (13) in heterovalent ion exchange, i.e., counterions of lower valences become more selective compared to counterions of higher valences as the aqueousphase electrolyte concentration is increased. For a weakly basic anion resin (IRA-94), chromate distribution coefficients were experimentally determined a t a constant aqueous-phase Cr(V1) concentration for various sulfate concentrations. Figure 6 shows a consistent increase in chromate selectivity with an increase in sulfate concentration. Equation 17 may further be used to predict the quantitative effects of the competing sulfate concentration on chromate selectivity. At Cs = 2000 mg/L sulfate and corresponding ionic strength, the values of the constants were determined for the experimental data, and chromate distribution coefficients were subsequently determined a t a sulfate concentration of 4000 mg/L. Figure 7 shows the 152
Environ. Sci. Technol., Vol. 20, No. 2, 1986
3000
4000
Sulfate Concentration (rng/L)
I
I0
(13) Y s = 1 - Ycr xs = 1- XCr (14) and total exchange capacity, Q, may be regarded as a constant. Since chromate concentration (5-20 mg/L) is very small compared to competing sulfate concentration, CT is practically equal to sulfate concentration, Cs, i.e., 1- X C r 1.0 (15) CT CS (16) Equation 12 then becomes
2000
Figure 6. Effect of competing sulfate concentration on chromate selectivity. 500
Again,
= 17.0mglL
p H = 4 0
400
l 00
Predicted Experimental Data at 4000 m g l L Sulfate
0.2o
0.4
Resin: I R A - 900 (STY - DVB, S B A )
o0.6
0.8 r
Y CI
Figure 7. Comparison of experlmentally determined chromate selectivities with the predicted ones at 4000 mg/L sulfate concentration.
predicted chromate distribution coefficients as opposed to the actual experimental data. A fairly good agreement is observed between the predicted chromate selectivities and the experimental data, but the predicted selectivities were consistently somewhat lower than the experimental ones. The following is likely to be the reason for lower values of the predicted selectivity: With an increase in sulfate concentration, ion-exchange resins shrink to some extent due to the increased osmotic pressure of the aqueous phase; i.e., resins are slightly less hydrated at 4000 mg/L sulfate than a t 2000 mg/L. Between HCr04- and Sod2-, the former has one charge while the latter has two. Bichromate ion (HCr04-), therefore, has a weaker electrostatic interaction with polar water molecules, i.e., HCr04- is less hydrated than sulfate. The result is HCr0; prefers less hydrated exchanger phase to sulfate. In other words, as the sulfate concentration increases, chromate selectivity slightly increases due to the decrease in hydration of the ion-exchange resin. Qualitatively, this is similar to the effect due to increased crosslinking; with an increase in degree of crosslinking, resins get less hydrated and prefer relatively less hydrated counterions (14). Equation 17 does not account for such effects because no volume change of the resin is assumed. However, the magnitude of this effect is much less pronounced compared to the competing effects from increased sulfate concentration. From a practical veiwpoint, the nature of the effects of sulfate concentration on chromate selectivity bears special significance. In open, recirculating cooling water systems, sulfuric acid is normally added to the make-up water to
Table I. Ionic Strength Effect on Selectivities of HCr04- and CrO2parameter
values/description
pH of experiment initial Cr(V1) concn., mg/L electrolyte (Na2S04or NaC1) concn. resin" acid/alkali addition calculated HCrOc selectivity,
6.5 50.0 nil
6.5 50.0
6.5 50.0 1000 mg/L chloride
750 mg/L sulfate
IRA-900 (STY-DVB) in chloride form IRA-900 in chloride form alkali addition required acid addition required 1.2 0.86
Yncr/Xncr
calculated Cr02- selectivity, YCrOa/XCr04
remarks
IRA-900 in sulfate form acid addition required 1.25
1.12
0.88
(2102- preferred over HCr04-
HCr0; preferred over CrO2- HCr0; preferred over Cr02-
0.8
OFor resins with acrylic matrix, selectivity reversal occurred at higher ionic strength. The above experiments were carried out also at pH 5.0; identical trend in selectivity reversal was observed. 320
HOURS
R e s i n I R A - 900 (STY - D V B , SEA)
96
144
192 240
288
336 384
200
6 240
5
>-o
432 480 528
SLV = 2 15 mlhr
v i
2000 m g / L Sulfate 2000 mg1L Chloride 5 0 m g t L Cr(VI)
20
200
160
0
0
0
Experimental D a t a a t 4000 m g / L C h l o r i d e
0.4
0.6
0.0
1.0
520 1040 1 5 6 0 2 0 8 0 2 6 0 0 3 1 2 0 3 6 4 0 4 1 6 0 4680 5200
Figure 9. Acidic vs. alkaline pH comparison between the chromate removal capacities for an IRA-900 resin with sulfate and chloride as competing species.
Ycr
Flgure 8. Comparison of experimentally determlned chromate selectivities with the predicted ones at 4000 mg/L chloride concentration.
avoid concentration of scale-forming bicarbonate and carbonate ions. An increase in acid dosing rate always leads to increased sulfate concentration in cooling water. Such an increase in sulfate concentration, as noted, has only an insignificant effect on chromate removal capacity of the ion-exchange resins. Similarly for the chromate/chloride system, it may be shown after making necessary simplifications as before that YCr/XCr -- constant
'
BED VOLUMES
I
0.2
0
U
(18)
(1- YCJ Chromate selectivity is thus unaffected by competing chloride concentration as already noted from the experimental data. Figure 8 shows the predicted chromate selectivities a t 4000 mg/L chloride and the actual experimental data; an excellent agreement is observed. Since both HCr04- and C1- are electrostatically identical (unit charge), any effect due to shrinking of the resin with increased electrolyte concentration is insignificant in this case.
Selectivity Reversal and Critical Aqueous-Phase pH. For a typical cooling water, aqueous-phase pH governs the relative distribution of Cr(V1) species. Barring ionic strength effect, HCr04- will predominate a t acidic pH (less than 6.5) while CrOd2-will be the most predominant species a t alkaline pH. Since HCr04- occupies only half the number of ion-exchange sites per chromium atom compared to Cr042-,acidic pH operation is seemingly attractive and has been a universal choice for the chromats-exchange process (1-4). Figure 9 shows tremendous difference in chromate removal capacity between acidic and alkaline pH under otherwise similar operating conditions. Authors of this paper, however, postulated that the greater number of chromium atoms per exchange site
-
at acidic pH as suggested (1,2,15)is unlikely to make such a big difference in chromate removal capacity in favor of acidic pH. Since the selectivity reversal (13) between monovalent HCr04- and divalent Cr0:- a t the prevailing cooling water ionic strength will also make the acidic pH operation more chromate selective, the selectivity reversal experiment was carried out to confirm such a possibility. HCr04- and Cr042- stay together in a very narrow pH range (5.0-7.5), and one is formed a t the expense of the other; this experimental setup was tailored particularly for this system. The results of the experiment are tabulated in Table I. By observing the change in the aqueous-phase pH alone, the relative selectivity between HCr04- and Cr042-may be known. The three possibilities are presented below. (a) Acid or Alkali Addition Not Required. pH remains the same. Resin has equal preference for HCr04- and CrOp2-, i.e., their distribution coefficients are equal. (b) Alkali Addition Required. Ion-exchange resin prefers Cr042-to HCrOZ-; Cr042-is, therefore, more selectively removed from the aqueous phase in preference to HCr04-. pH will tend to drop because HCr04- in the aqueous phase will release protons as shown below in order to maintain the equilibrium between HCr04- and Cr042-: HCr04- H+ Cr042(19)
- + -
(c)Acid Addition Required. Ion-exchange resin prefers HCr04- to Cr042-,and here the following reaction is operative to maintain the equilibrium Cr042-+ H 2 0 HCr04- + OH(20) Table I shows that, in the first case when the aqueous-phase electrolyte concentration is very low, divalent CrOt-, as expected, is preferred over monovalent HCr04-; Le., alkali addition is needed. However, as the aqueousphase electrolyte concentration is increased by adding Environ. Sci. Technol., Voi. 20, NO. 2, 1986
153
0.7-
C r ( V I ) = i 7 . 5 mg/L
Resin, IRA-94(STY-DVB,
o0 0.1
3.0
WBA)
.
4.0
5.0
7.0 2 6.0
PH Figure 10. Effects of pH changes on relative Cr(V1) uptake of the resin at constant aqueous-phase chromate concentration.
highly ionized salts such as NaCl and NaZSO4,the selectivity is reversed; i.e., monovalent HCr04- is preferred over divalent Cr042-. Table I also shows the distribution coefficients for HCr04- and Cr042-; the distribution coefficients greater than unity indicate a resin's preference for the species over the other and vice versa. The experimental data provide sufficient evidence that, a t a chloride concentration as low as 1000 mg/L, selectivity reversal between HCr04- and Cr042- takes place for a styrenedivinylbenzene resin (IRA-900), which incidentally, is the most commoil for the chromate-exchange process. It may be justifiably concluded that use of acidic pH, contrary to previous worker's notions, stems primarily from the ionexchange resin's higher preference for HCr04- over Cr04za t the prevailing ionic strength normally encountered in cooling water systems. The preference of HCr04- is, however, supplemented due to its greater number of Cr atoms per unit charge compared to CrOd2-. Equilibrium tests were again carried out separately with constant background concentrations of 2000 mg/L sulfate and 2000 h g / L chloride, but at different pH ranging from 3.5 to 6.0. Aqueous-phase Cr(V1) concentrations were kept constant for all the tests. Results are shown in Figure 10 for a styrene-divinylbenzene weakly basic anion resin (IRA-94). The pK, value for this resin is higher than 7.0 (17,18); the total exchange capacity of the resin in the pH range of 3.5-6.0 was found to be constant in this experiment within + 5 % relative standard deviation. Figure 10 shows that equivalent resin-phase Cr(V1) fractions bebw p H 5.0 are practically constant for both the equilibrum tests. This observation suggests that reducing pH increws Cr(V1) uptake only up to a certain pH by forming more HCr04- a t the expense of Cr042-. Once this critical pH value is reached, further reduction in pH will not increase the Cr(V1) uptake. This critical pH is also a function of the ionic strength and, hence, not a unique number. Assuming that critical pH (pH,) occurs where HCr04- constitutes 90% of the total Cr(V1) present, [HCr04-] = 9.0 [Cr042-] again
where braces denote activity, while y stands for aque154
Environ. Sci. Technol., Vol. 20, No. 2, 1986
ous-phase activity coefficient. For the ionic strength in our range of interest (up to 0.5), the activity coefficients may be considered to be dependent only on the charge of the ionic species in question. For divalent CrO?-, yCrO4 = yz, and for monovalent HCr04-9 YHCr = 71. By application of Davies equation for activity coefficient (16), it may be shown that (8) 7 2 = y14 (23) By application of this identity and use of equation 21, critical pH (pH,) may be determined from eq 22 as given below: pH, = pKa + 3 log 71 - log 9 (24) ~ Now, pK, = 6.5, so pH, = 5.54 3 log y1 (25) For a given ionic strength, the monovalent ion activity coefficient, yl,may be determined by using the Davies equation or Debye-Huckel limiting law, and hence, critical pH for chromate removal can be found. Equation 25, however, does not account for the difference in hydrogen ion activity between the aqueous phase and resin phase. For anion-exchange resins, pH inside the resin is higher than the aqueous phase due to the exclusion of H+ from the resin phase according to the Donnan membrane principle. The actual critical value of pH is, therefore, somewhat less than that predicted by eq 25. For 2000 mg/L sulfate and 2000 mg/L chloride, the values of pH, calculated from eq 25 are 5.3 and 5.2, respectively. It may be seen that these predicted values are in good agreement with experimental results shown in Figure 10 and hence may be used in practice. From a practical viewpoint, the knowledge of critical pH is important because optimum level of acid dosing may be well-controlled with prior knowledge of critical pH. Secondly, hexavalent chromium is an oxidizing agent and has the potential to oxidize commercial organic base synthetic anion-exchange resins. The oxidation potential of Cr(V1) increases with decrease in pH, and hence, resins are more susceptible to oxidative attack under strongly acidic conditions. Knowledge of critical pH is particularly important in this regard; use of lower-than-criticalpH during column operations increases the oxidative action of Cr(V1) without any additional increase in chromate removal capacity.
+
Conclusions Some specific aspects of the ion-exchange process for chromate recovery from cooling water have been reported. For chromate/sulfate equilibrium, an increase in sulfate concentration is accompanied by an increase in chromate selectivity; an increase in chloride concentration, however, has practically no effect on chromate selectivity. Consideration of HCr04- as the prime counterion in the chromate-exchange process may well-explain the above effects. The equilibrium chromate ion-exchange model developed in this study may quantify the competing effects of sulfate and chloride ions on chromate removal capacity of the anion exchangers. The model accommodates the nonideality effect of the aqueous-phase ionic strength on chromate-exchange equilibria. Higher chromate removal capacity a t acidic pH as opposed to alkaline pH is primarily due to the selectivity reversal between HCrO, and Cr0f at the prevailing ionic strength. Experimental verifications of such selectivity reversal have been provided in this study. In the acidic range, there is a critical pH below which no increase in chromate removal capacity is observed. This critical pH
Environ. Sci. Technol. 1988, 20, 155-160
for a given system may be well-predicted based on the knowledge of chromate dissociation equilibria and the ionic strength of the cooling water.
Glossary molar concentration of species i in the aqueous [il phase molar concentration of species i in the exchanger El phase activity of species i in the aqueous phase, mequiv/L activity of species i in the exchanger phase, mequiv/g or mequiv/mL activity coefficient of species i in the aqueous phase Yi activity coefficient of species i in exchanger phase fi concentration of species i in aqueous phase, meCi quiv/L or mg/L concentration of species i in exchanger phase, ci mequiv/g or mequiv/mL K thermodynamic equilbrium constant total liquid-phase concentration, mequiv/L resin exchange capacity, mequiv/mL or mequiv/g equivalent fraction of species i in the liquid phase xi equivalent fraction of species i in the exchanger Yi phase separation factor of i with respect to j, dimension“ij less distribution coefficient of species i Xi t time, s or h I ionic strength, mol/L Cr(V1) total hexavalent chromium concentration, mg/L
1
Abbreviations *R asterisk denotes exchanger phase SBA strongly basic anion resin WBA weakly basic anion resin BV bed volumes SLV superficial linear velocity (Re) particle Reynolds number EB8T empty bed contact time
Registry No. IRA-900, 9050-97-9; IRA-94, 39409-19-3.
Literature Cited Yamamoto,D.; Koichi, Y.; Osamu, A. “proceedings,Cooling Tower Institute Annual Meeting”; Houston, TX, 1975. Kunin, R. “Amber Hi-Lites No. 151”;Rohm and Haas Co.: Philadelphia, May 1976. Newman, J.; Reed, L. “Proceedings,Water-1979”;AIChE, 1980; Vol. 197, no. 76. Richardson, E.; Stobbe, E.; Bernstein,S. Enuiron. Sc. and Technol. 1968,2, 1006. Butler, J. N. “IonicEquilibrium”;Addison-Wesley: New York, 1967. Tong, J. Y.; King, E. L. J.Am. Chem. SOC.1953,75,6180. Arden, T. V.; Giddings, M. J. Appl. Chem. 1961,11,229. Sengupta, A. K., Ph.D. Dissertation, University of Houston-University Park, Houston, TX, 1984. Sengupta, A. K,; Clifford,D. Ind. Eng. Chem. Fundam., in press. Sengupta, A. K.; Clifford, D. Reactive Polymers, Ion Exchangers, Sorbents, 1985, in press. APHA-AWAWA-WPCF“Standard Methods for the Examination of Water and Wastewater”;Washington, D.C., 1980.
Reichenberg, D.; McCauley, D. J. J. Chem. SOC.1955, 2741-2749.
Helfferich,F. “Ion Exchange”;Xerox University Microfilms: Ann Arbor, MI, 1961. Myers, G. E.; Boyd, G. E. J . Phys. Chem. 1956,60, 521. Miller, W. S. “Ion Exchange for Pollution Control”;CRC: Boca Raton, FL, 1978; Vol. 1, p 191. Stumm, W.; Morgan, J. “Aquatic Chemistry”;Wiley: New York, 1981. Rohm and Haas Co. “Amber-Hi-Lites”;1978; no. 159. Clifford, D.; Weber, Walter Reactive Polymers, Ion Exchangers, Sorbents 1983, I , 77. Received for review April 5, 1985. Accepted August 13, 1985.
Comparison of Methods for the Concentration of Suspended Sediment in River Water for Subsequent Chemical Analysis Arthur J. Horowltz US. Geological Survey, 6481-H Peachtree Industrial Blvd., Doraville, Georgia 30340
rn Centrifugation, settling/centrifugation, and backflushfiltration procedures have been tested for the concentration of suspended sediment from water for subsequent tracemetal analysis. Either of the first two procedures is comparable with in-line filtration and can be carried out precisely, accurately, and with a facility that makes the procedures amenable to large-scale sampling and analysis programs. There is less potential for post-sampling alteration of suspended sediment-associated metal concentrations with the centrifugation procedure because sample stabilization is accomplished more rapidly than with settling/centrifugation. Sample preservation can be achieved by chilling. Suspended sediment associated metal levels can best be determined by direct analysis but can also be estimated from the difference between a set of unfiltered-digested and filtered subsamples. However, when suspended sediment concentrations ( a 5 0 mg/L) or trace-metal levels are low, the direct analysis approach makes quantitation more accurate and precise and can be accomdished with simder analvtical mocedures. Introduction Suspended sediment plays an extremely important role in the transport and geochemical cycling of trace metals
in aquatic systems (1-9). In addition, the sampling and subsequent chemical analysis of this material have been used to locate ore deposits, to identify long-term trends in water quality, and to identify sources of anthropogenic pollution (2,5-12). The classical procedure for collecting and concentrating suspended sediments entails in-line filtration using preweighed 0.45-pm membrane filters; usually two are loaded with the upper one serving to collect the sample and the lower one acting as a procedural and analytical blank. This labor-intensive and costly method is probably the most commonly used procedure for the concentration of suspended sediments for subsequent chemical analysis and is usually the method of choice for studies where sample numbers are limited (e.g., 13). However, if large-scale studies are undertaken, with numerous sampling sites or high sampling frequencies or both, such as the U.S. Geological Survey’s (USGS) National Stream Quality Accounting Network (NASQAN), where some 500 sites are sampled 4-6 times a year (14)) the analytical costs and manpower demands of the in-line fiitration procedure can become prohibitive. It is estimated that it would require a t least two full-time employees just to weigh, load, and supply users with in-line filter holders. Also, extra work and care is needed to unload the sedi-
Not subject to U.S. Copyright. Publlshed 1986 by the American Chemical Society
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