NOTES
Concentration and Separation of Arsenic from Polluted Water by Ion Exchange Shingara S. Sandhu* Claflin College, Orangeburg, S.C. 291 15
Peter Nelson South Carolina State College, Orangeburg, S.C. 291 17
Several metal ions-Co(II), Cr(VI),Cu(II), Hg(II), M O W , Ni(II), and Sb(II1)-have been reported to interfere in the determination of arsenic in water and wastewater by the silver diethyldithiocarbamate (SDDC) method. A technique for the concentration and separation of arsenic from polluted environmental samples, containing these ions, was developed. Aqueous samples containing arsenic were digested with potassium permanganate and eluted through chromatographic columns packed with Amberlite, IRA-40 IS C.P., ion exchange resin. Arsenic retained by the resin was extracted with hydrochloric acid prior to its determination by the SDDC method. The technique facilitated the quantitative determination of arsenic below 0.005 mg L-l, even in extremely polluted waters. Five grams of resin was found satisfactory for routine analysis of polluted environmental aqueous samples. Arsenic is a suspected carcinogen and is widely distributed in the aqueous environment ( 1 ) . The U.S. Public Health Service recommends that arsenic concentration in drinking water should not exceed 0.01 mg L-1, and water with an arsenic concentration greater than 0.05 mg L-1 should be rejected for human consumption ( 2 ) .The arsenic concentration of potable water is generally less than 0.005 mg L-l, although a concentration as high as 0.1 mg L-1 has been reported (3). Several methods (4-13) are available for the determination of arsenic in aqueous samples but only the silver diethyldithiocarbamate (SDDC) (10)method is officially accepted for quantitative determination of arsenic in water and wastewater. This method consists of reducing inorganic arsenic in a water sample by an acid-zinc reaction to arsine (AsH:j),which is scrubbed through lead acetate impregnated glass wool and is absorbed by silver diethyldithiocarbamate dissolved in pyridine. The color developed by the arsine (AsH&silver diethyldithiocarbamate reaction is photometrically measured a t 535 nm. This method provides reliable data for arsenic in fairly polluted waters. However, metal ion-chromium(VI), cobalt(II), molybdenum(VI), and nickel(I1)-concentrations above 5.0 mg L-' decrease the arsenic recovery. Antimony(II1) and mercury(I1) concentrations of 0.3 and 1.5 mg L-l, respectively, significantly increase the apparent arsenic recovery (14).
The attempts made in the past (5-7) to eliminate metal ion interference in the determination of arsenic by the SDDC method met with partial success. It has also been suggested that arsenic in a small volume of water sample (15.0 mL) can be concentrated by its distillation as arsenic(II1) trichloride in hydrochloric acid ( 1 5 ) ,but this technique was not tried for the determination of arsenic concentrations generally found in surface water. Ion exchange behavior of arsenic(III), a t relatively higher concentrations (40 g L-l), on Varian anion exchange resins was evaluated for acidic as well as for alkaline aqueous samples ( 1 6 ) . 476
Environmental Science & Technology
Experimental Apparatus. The generator, absorber assembly, and the spectrophotometer used in this study have been previously described (14). Materials. The ion exchange resin, Amberlite IRA-40 IS C.P., code 3401, was purchased from Mallinckrodt Chemical Works, St. Louis, Mo. The properties of the resin and the solutions necessary for its regeneration were supplied by the manufacturer ( 17 ) . Stock solutions for arsenic(III), chromium(VI), cobalt(II), copper (II), mercury(11), molybdenum(VI), nickel(11), and antimony(III), containing 1g L-l of ionic concentration, were prepared from arsenic trioxide (As203),potassium chromate (K&r04), cobalt chloride (CoClz.6Hz0), copper nitrate (Cu(N0&-3H30), mercuric chloride (HgClZ), ammonium nickel nitrate (Ni(N03)2), molybdate ( (NH4)6M070z4-4H20), and antimony trichloride (SbC13), respectively. The intermediate solutions were prepared by diluting the stock solutions l:lO, and the working solutions containing requisite concentrations of various ions were obtained by diluting the intermediate solutions. Analytical grade reagents were used. The surface waters, from the Edisto River and Caw Caw Swamp, Orangeburg, S.C., the Congree River, Columbia, S.C., and the Savannah River a t U S . Rt. 1and Ga. Rt. 18,Georgia, were also collected. Procedure. Amberlite resin (2-5 g) was packed in 10-mL burets, conditioned with 100.0 mL of 9.0 M hydrochloric acid, and washed with 250.0 mL of deionized water. The synthetic water samples containing Cr(VI), Co(II), Cu(II), Mo(VI), Ni(II), Sb(III), and Hg(I1) ions a t interfering levels and not less than 0.005 mg L-l of arsenic(II1) were digested using potassium permanganate ( 1 4 ) .The excess of potassium permanganate was destroyed by quantitative addition of hydroxylamine hydrochloride. The digested aqueous samples were eluted through the anion exchange chromatographic columns. The arsenic retained by the resin was leached with 80.0 mL of 9.0 M hydrochloric acid, followed by sufficient water to yield 100.0 mL of effluent. The leaching of arsenic with hydrochloric acid regenerated the resin. The flow of eluting solutions was maintained a t 0.75 mL per min in each case. An aliquot of effluent, containing not less than 1.0 pg of arsenic, was transferred to the arsine generator and the volume diluted to 57.0 mL with water and hydrochloric acid (if needed to maintain the acid concentration of approximately 1.66 M). The arsenic in the effluent was determined according to the procedure described previously (14). Two liters of natural water, in triplicate, was digested ( 1 4 ) and eluted through the chromatographic columns packed with 5.0 g of Amberlite resin for the determination of ambient arsenic. The arsenic in the elute was determined with the SDDC method (IO). The natural water samples were spiked with 18.0 mg L-' of interfering ions (3.0 mg L-' each of Co(II),Cr(V1). Cu(II), Hg(II), Ni(II), and Sb(II1)) and digested with potassium permanganate. The ambient arsenic was determined in the
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Table I. Arsenic Recovery by Ion Exchange from Synthetic Water Samples lnlerlerlng Ions added
recovery, a
SD,
oh
Oh
Resin Used, 2.0 g 0.005d 0.010= 0.020e
88.6 92.0 96.7
5.8 5.0 4.6
0.005~~
87.0
6.0
0.005d
88.3
5.8
0.005d
88.4
5.8
86.8 67.8
6.3 7.8
89.0
7.5
theor concn, mg L-1
0 0 0 individually 10.0 Sb(lll) 1.o
Hg(ll) 5.0 collectively 9.0 (1.5 each) 18.0 (3.0 each) 18.0 (3.0 each)
0.005d 0.005 Resin Used, 5.0 g 0.005d
Mean of 12 determinations; readings corrected for reagent arsenic based on label values. Ions used: Co(ll), Cr(VI), Cu(ll), Mo(VI), Ni(1l). Data are the average for 30 determinations, 6 for each ion. Ions used: Co(ll), Cr(V1). Cu(ll). Hg(il), Ni(ll), and Sb(lll). Mean of ten determinations. Two-liter water sample, containing 0.005 my L-' of arsenic, was eluted. * One-liter water sample was eluted. a
digested samples to evaluate the effect of interfering ions. The natural water samples were also spiked with 18.0 mg L-l of interfering ions (3.0 mg L-l each of Co(II), Cr(VI), Cu(II), Hg(II), Ni(II), and Sb(II1)) and 0.005 mg L-l of arsenic(II1) for the study of the effect of interfering ions on the recovery of added arsenic to the natural water, as well as the interaction of added arsenic(II1) and ambient arsenic.
Results a n d Discussion The Amberlite, IRA-40 IS C.P., code 304, is a quaternary ammonium polystyrene resin and is strongly basic in nature with -N(CH&+Cl- as the functional group. I t is effective not only in separating anions from cations, but can also separate various anions from each other (17). Potassium Permanganate used for the digestion and release of organically bonded arsenic in aqueous samples also oxidizes the various lower valence forms of arsenic to arsenic(V). The excess of potassium permanganate is destroyed by quantitative addition of hydroxylamine hydrochloride, which also reduces the hexavalent form of chromium to its trivalent form as is evidenced by the discharge of solution color. An excess of hydroxylamine hydrochloride should be carefully avoided to eliminate the potential reduction of arsenate (As04"). I t is evident from the redox potential of various reactions ex-
pected in the system (18) that arsenate ( A s O ~ ~ reduction -) probably will not occur as long as a micro amount of hexavalent chromium or permanganate is still present in the solution.
+ + + +
+
---
+
Mn04- 8H+ 5e Mn2+ 4H20 ( E o= 1.51 V) Cr2072- 14Hf 6e 2Cr3+ 7 H 2 0 ( E o= 1.33 V) Cr042- 8H+ 3e Cr3+ 4H20 ( E o = 1.19 V) A ~ 0 4 ~ -2H+ 2e AsO& HzO ( E o= 0.56 V) 2NH30H+ N2 4H+ 2H20 2e ( E o= 1.87 V) N2 + H20 N20 2H+ e ( E o= 1.77 V)
+
+ + +
--
+ + + + + + +
The hydrogen ion concentration of the solution eluting through the anion exchange column is about 3.20 M ( 1 4 ) . Consequently, H3As04 is expected to predominate in solution (18). The results on the recovery of arsenic concentrated by the ion exchange technique before its determination by the SDDC method (10)are given in Table I. The recovery of arsenic a t a 0.005 mg L-l level is about 88%, which improves with increasing arsenic levels in synthetic water samples. The recovery of arsenic in the presence of interfering ions added to the demineralized water containing 0.005 mg L-l of arsenic(II1) is also shown in this table. The concentration of each metal ion used in this study is higher than the level where it has been found to interfere in the SDDC method (14). The results on the recovery of arsenic appear to be quantitative to certain levels of interfering ions beyond which the arsenic recovery decreases. It appears that 2.0 g of resin is not enough for the quantitative recovery of arsenic from aqueous solutions containing 9.0 mg L-l or more of interfering ions. The decrease is speculated to be due to the competitive adsorption of other anions for available sites on the anion exchange resin. This problem was alleviated by using successively larger amounts of resin. I t was found that 5.0 g of resin per chromatographic column provided the optimum recovery for 0.005 mg L-' of arsenic from the synthetic water samples that also contained 18.0 mg L-l of interfering ions. The ion exchange resin system was applied for the elimination of interfering ions as well as for the concentration of arsenic from natural surface water prior to its determination by the SDDC method. The results for such an analysis are given in Table 11. The arsenic in surface waters varies from 0.0017 mg L-l for the Edisto River to 0.214 mg L-' in the Caw Caw Swamp. The amount of ambient arsenic determined, after the addition of 18.0 mg L-l of pollutants (3.0 mg L-l each of Co(II),Cr(VI), Cu(II), Hg(II), Ni(II), and Sb(III)),did not vary significantly from the one determined prior to the addition of these interfering ions. This information suggests that the Amberlite resin is effective in eliminating the interfering ions from the aqueous environmental samples. The
Table II. Analysis of Environmental Samples a samples
arsenic SD, %
Edisto River
0.0017 6.7
Caw Caw Swamp
Congree River
(A) Ambient Arsenic, mg L-' 0.2146 0.0136 5.5 7.7
Savannah River at U.S. Rt. 1 Ga. Rt. 18
0.0128 7.4
0.01 13 6.2
(B) Ambient Arsenic,b mg L-', after the Samples Were Spiked with 18.0 mg L-' of Interfering IonsC arsenic 0.0018 0.2137 SD, % 6.9 6.3 (C) Arsenic RecoveredbsdfromSamples That Were Spiked with 18.0 mg L-' of Interfering and 0.005 mg L-' of Arsenic arsenic recovered, % 84.0 80.0 86.0 78.0 SD, % 6.1 5.6 4.1 3.5 a Two-liter water samples were used for each determination. Mean of nine determinations, Ions added: 3 mg L-' each of Co(ll), Cr(VI), Cu(ll), Hg(li), Ni(ll), and Sb(ll1). Arsenic recovered is the difference between the total and the ambient arsenic (Part A).
Volume 13, Number 4, April 1979
477
recovery of 0.005 mg L-l of arsenic(II1) added t o the environmental samples which also contained 18.0 mg L-l of interfering ions appears satisfactory. The information presented here suggests that arsenic in water samples can be successfully concentrated and isolated from the interfering metal ions by the use of the anion exchange technique before its determination by the standard method (SDDC). The amount of resin used for this process depends on the extent of pollutants expected in wastewater. However, 5.0 g of the Amberlite resin appears to be sufficient for the routine elimination of interfering ions as well as for the concentration of arsenic from the environmental samples. The maximum desirable limit of arsenic in drinking water is 0.01 mg L-1 and the maximum safe limit is 0.05 mg L-I; the minimum detectable limit of the SDDC method is 0.5 wg ( 1 4 ) . Consequently, the ion exchange technique can be effectively applied to facilitate the concentration of arsenic from drinking water, even below the desirable limit, before its determination by the SDDC method (IO).
Acknouledgment The authors are grateful to Dr. H. V. Manning, President, Dr. B. L. Gore, Dean, and Dr. N. Smith, Department Chairman, all of Claflin College, for their encouragement in undertaking this work. Literature Cited (1) Lee, H . K. D., “Metallic Contamination and Human Health”, Academic Press, New York, N.Y., 1972. (2) United States Department of Health Education and Welfare, Public Health Service, “Drinking Water Standards”, Public Health
Service Publication No. 956, U S . Government Printing Office, Washington, D.C., 1972. ( 3 ) Subcommittee on Air and Water Pollution of the Committee on Public Works. US.Senate. “Water Pollution”, Part 4, U S . Senate, 91st Congress, 2nd Session, U S . Government Printing Office, Washington, D.C., 1970. (4) Caldwell. J. C.. Lishka. R.. McFarren. E., J Am. Water Works Assoc., 65,’731-5 (1973): (5) Chaney, A. L., Harold, J. M., Ind. Eng. Chem. Anal. Ed., 12,691-3 (1940). (6) Kolthof, I. M., Elias, A., Ind. Eng. Chem. Anal. Ed., 12, 177-9 ( 1940). ( 7 ) Liederman, D., Brown, F. E., Milmer, 0. I., Anal Chem., 30, 1543-6 (1958). ( 8 ) Powers, G. W., Jr., Martin, R. L., Piehl, F. ,J,, Griffin, J. M., Anal. Chem., 31,1589-93 (1959). (9)Ballinzer. D. C.. Lishka. R. J., Gales, M. E., J Am. Water Works Assoc., 54,1424-8 (1962). (10) American Public Health Association, American Water Works Association, and Water Pollution Control Federation, “Standard Methods for the Examination of Water and Waste Water”, 13th ed., American Public Health Association, Washington, D.C., 1971. (11) T a m , K. C., Enuiron. Sci. Technol., 8, 734-6 (1974). ( 1 2 ) Kopp, J. F., Anal. Chem., 45, 1786-7 (1973). (13) Sandhu, S. S., Analyst, 101,856-61 (1976). (14) Sandhu, S. S., Nelson, P., Anal. Chem., 50,322-5 (1978). (15) Farkas, E. J., Griesbach, R. C., Schachter, D., Hutton, M., EnL’iron.Sci. Technol., 6, 1116-7 (1972). (16) Balint-Ambro, J., J . Chrornatogr., 120,457-60 (1974). (17) Kunin, R., Percival, R. W., Kneip, T. J . , Dean, W. K., “Experiments in Ion Exchange”, 2nd ed., Mallinckrodt Chemical Works, St. Louis, Mo., 1965. (18) Kleinberg, J., Argersinger, W. J., Jr., Griswold, E., “Inorganic Chemistry”, D.C. Heath, Boston, Mass., 1960.
Receiced for reuieu March 27, 1978. Accepted Nocember 13, 1978. This iuork tuas supported by EPA Grant N o . R801164OlO.
CORRESPONDENCE
SIR: In the 1976 volume of ES&T, Edgington and Robbins ( I ) have modeled a relationship between the lead concentration in Lake Michigan sediments and the annual emission of lead into the atmosphere using the following assumptions: (a) The published data for the emission of lead from the combustion of coal prior to 1960 underestimates the actual emission by a factor h . The total atmospheric emission rate is taken to be:
where J a c ( t )and Jag(t)are the data for the emission due to the combustion of coal and gasoline, respectively. (b) The sedimentation rate a t each point in the lake does not change with time and can be derived from 210Pbmeasurement by assuming a constant initial concentration of unsupported (excess) 210Pba t each stage in sediment accumulation (2). (c) The total flux of anthropogenic lead to the lake waters is a constant proportion of the annual atmospheric emission. (d) The spatial pattern of the flux of lead into the sediments does not change with time. (e) The outflow of lead from the lake is negligible. Hence, all the lead entering the lake waters is transferred to the sediments. Under these assumptions, Edgington and Robbins derive a relationship between the lead concentration in the sediments and the atmospheric emissions involving two undetermined parameters. These parameters were varied to give a leastsquares fit of the data to the model. Edgington and Robbins observe that certain cores have a more uniform lead concentration in the upper portions of the 478
Environmental Science & Technology
core, and attribute this to sediment mixing. They derive a modification to their model assuming homogeneity in the zone of mixing. In evaluating the above assumptions and procedures the following points may be noted. (a) Where a model has so many unverified assumptions, the validity of attempting to correct data, in this case the lead emission values due to the combustion of coal, is questionable. (b) Alternative calculation of the *IOPbchronology assuming a constant net rate of supply of unsupported 210Pbto the sediment surface (c.r.s. model, see ref 3 and 4 ) provides ample evidence for considerable variation in the sedimentation rate through time. (c) I t is unnecessary, and as we shall see, incorrect, to assume a constant ratio between the atmospheric lead emissions rate and the lead flux rate to the sediments. The zloPb age/ depth curves allow direct calculation of the relation between them. The total lead flux rate F ( t ) to the sediments of age t and depth x in a dated core is calculated by( means of the formula: where r ( t ) is the sedimentation rate a t time t and c ( x ) is the total lead concentration at depth x. Figure 1 compares graphs of total lead fluxhime derived from Edgington and Robbins chronology and from our c.r.s. based calculations for cores 17, 31, and 105. Using the mean sedimentation rate derived from the thickness of the Waukegan member, Edgington and Robbins calculate a mean “natural” (Le., preindustrial) lead flux of 0.16 pg/(cm2.year). By contrast, the lead fluxhime curves obtained using their
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