Anal. Chem. 1994,66, 3247-3252
Speciation of Arsenic in Natural Waters by Solvent Extraction and Hydride Generation Atomic Absorption Spectrometry Hlroshl Hasegawa,'*t Yoshlkl Sohrln,* Masakazu Matsul,* Masashl Hojo,t and Munetsugu Kawashlmal Department of Chemistry, Faculty of Science, Kochi University, Akebono-cho, Kochi 780, Japan, Institute for Chemical Research, Kyoto University, Uji, Kyoto 6 I 1, Japan, and Faculty of Liberal Arts and Education, Shiga University, Otsu, Shiga 520, Japan
A new approach is describedfor the speciation of arsenic species including trivalent methylarsenicals in natural waters. Arsenious acid [As(III)I,monomethylarsonousacid[MMAA(III)], and dimethylarsinous acid [DMAA(III)] are separated from pentavalent species by solvent extraction using diethylammonium diethyldithiocarbamate (DDDC) and determined by hydride generation atomic absorptionspectrometry (HG-AAS) after cold trapping and chromatographic separation. The detection Limits for the trivalent species are about 13-17 pM. The sum of concentrations of the trivalent and pentavalent species are determined directly by HG-AAS in aliquots of the same samples. This is the first report of trivalent methylarsenicals being found and measured in natural waters.
Speciation of organic forms of elements is important in understanding their interaction with the biota, mobilization, adsorption, and transport of the elements in aquatic systems. Although rigorous characterization of the organic species is ordinarily difficult, it is possible on a-bonded organometallic compounds, which have a relatively low molecular weight and contain kinetically inert metal-carbon bonds. Arsenic is one of the elements intensively studied because of its toxicological interest, and its organic forms are dominated by a-bonded organometallic species. Arsenic-containing ribofuranosides are ubiquitous in algae,2Jand arsenobetaine is the predominant form in marine animak4v5 Arsenic-containing ribofuranosides and arsenobetaine, however, probably do not make up the bulk of the arsenic budget in natural waters, since the product excreted by algae and aquatic animal culture appear to be limited to the inorganic and methylated species.lV6 In natural waters, monomethylarsonic acid [ MMAA(V)] and dimethylarsinic acid [DMAA(V)] have been determined.7-9 Arsenic is pentavalent in the above-mentioned species. On the other hand, inorganic trivalent arsenic [As(OH)s; As(III)] is also distributed in the hydrosphere, whereas it is thermodynami+ Kochi University. 8 Kyoto University. I Shiga University.
(1) Cullen, W. R.; Reimer, K. J. Chem. Reu. 1989, 89, 713. (2) Edmonds, J. S.;Franccsconi, K. A. Nature 1981, 289, 602. (3) Edmonds, J. S.;Francesconi, K. A. J. Chem. SOC.Perkin Tram. 1 1983,2375. (4) Edmonds, J. S.;Francesconi, K. A.; Cannon, J. R.; Raston, C. L.; Skelton, B. W.; White, A. H. Tetrahedron Lett. 1977, 18, 1543. ( 5 ) Edmonds, J. S.;Fran-ni, K. A. Experientia 1987, 43, 5 5 3 . (6) Craig, P. J. In Organometallic Compounds in the Enuironment: Principles and Reactions; Craig. P. J., Ed.; Longman: New York, 1985; pp 1-58. (7) Andreae, M. 0. Limnol. Oceanogr. 1979,24,440. ( 8 ) Braman, R. S.;Foreback, C. C. Science (Washington, D. C.)1973,182,1247. (9) Sanders, J. G. Mar. Chem. 1985, 17, 329.
0003-2700/94/03663247$04.50/0 0 1994 American Chemical Society
cally much more unstable than arsenic acid [AsO(OH)3;As(V)] under aerobic conditions.lb12 In this context, we noted that trivalent methylarsenicals, monomethylarsonous acid [MMAA(III)] and dimethylarsinous acid [DMAA(III)], have been missed out of analysis of natural waters. MMAA(II1) and DMAA(II1) are produced through the reduction of MMAA(V) and DMAA(V) by hydrogen sulfide and exist for a considerable time even under aerobic conditions. The trivalent methylarsenicals are probable intermediates in the biosynthesis of organoarsenicals, where the methylation of As(V) proceeds through alternating reduction and oxidative methyl transfer.I3.l4 In addition, MMAA(II1) and DMAA(II1) are more toxic than As(III).lS Arsenic speciation including MMAA(II1) and DMAA(II1) is therefore meaningful for arsenic biogeochemistry. The trivalent methylarsenicals cannot be determined separately from the pentavalent species with hydride generation followed by atomic absorption spectrometry (HGAAS),16J7which is commonly used for thespeciationof arsenic in natural waters. In the conventional method, although As(111) is separated from As(V) by selective reduction to arsine, MMAA(II1) and DMAA(II1) are not separated from MMAA(V) and DMAA(V), respectively, since they are simultaneously reduced to monomethylarsine and dimethylarsine. The other problem is the change in the composition of arsenic species during sample storage, because the trivalent methylarsenicals are more subject to oxidation than As(III).~* Peterson and Carpenter have reported that even an As(III)/ As(V) ratio was not stable for longer than 1-3 days after collection.1° It is necessary, therefore, to separate the trivalent from the pentavalent species as soon as possible after sample collection. This paper describes a new speciation method for As(V),
MMAA(V),DMAA(V),As(III),MMAA(III),andDMAA(111). The trivalent species were separated and concentrated (10) Peterson, M. L.; Carpenter, R . Mar. Chem. 1983, I.?, 295. (11) Cherry, J. A.; Shaikh, A. U.; Tallman, D. E.; Nicholson, R. V. J. Hydrol. 1979, 43, 373. (12) Crecelius, E. A.; Bloom, N. S.;Cowan, C. E.; Jenne, E. A. Speciation of Selenium and Arsenic in Natural Waters andSediments: ArsenicSpeciation; Electric Power Research Institute, 1986; Vol. 2, EA-4641, Project 2020-2. (13) Cullen, W. R.; Frocse, C. L.; Lui, A.; McBride, B. C.; Patmore, D. J.; Reimer, M. J. Organomet. Chem. 1977,139,61. (14) Challenger, F. Chem. Rev. 1945, 36, 315. (15 ) Horiguchi, H. Kougoitodoku-Kikenbutsu ; Sankyosyuppan: Tokyo, 1970; p 361. (16) Braman, R. S.;Johnson, D. L.; Foreback, C. C. Anal. Chem. 1977,49,621 (17) Andreae, M.0. Anal. Chem. 1977, 49,820. (18) Cullen, W. R. Adu. Orgonomet. Chem. 1966, 4, 145.
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by solvent extraction with diethylammonium diethyldithiocarbamate (DDDC) prior to determination with HG-AAS. The sum of the concentrationsof the trivalent and pentavalent species was determined in an other aliquot of the same sample, so that the concentration of the pentavalent species was obtained as the difference. Sincethe change in the composition of arsenic species during the storage is removed, this method provided more accurate values for As(II1) than the conventional HG-AAS technique. Some results of arsenic speciation in natural waters by this method are described, which show the first distribution of MMAA(II1) and DMAA(II1) in aquatic systems. EXPERIMENTAL SECTION Reagents. Standard solutions of le3 M MMAA(II1) and DMAA( 111) were made by dissolving the corresponding bromides (Alfa) in 0.1 M sodium hydroxide under a nitrogen atmosphere. MMAA( 111) and DMAA( 111) were dissolved as hydroxides by alkaline hydrolysis. The standard solutions were stored in sealed glass tubes to avoid oxidation. A standard solution of 1t2 M arsenobetaine was prepared by dissolving appropriate amounts of (CH3)3As+CH2C02- (Tri Chemical Laboratory Inc.) in distilled water. For other arsenic stock solutions ( l e 2 M), the sodium salts [ C H ~ A S O ~was N~~ prepared by Quick's method,19and NaAsO2, Na2HAsO4, and (CH3)2As02Na were obtained from Nacalai Tesque] were dissolved in distilled water. They were diluted to the required concentrationsdaily before use. The organoarsenicalsolutions were standardized by the HG-AAS method after decomposition to As(V). The standards were stable at least 6 months. A 3% NaBH4 solution was prepared by dissolving a sodium borohydride tablet (Nacalai Tesque) in l e 2 M sodium hydroxide just before use. Super-high-purity grade sodium hydroxide (Merck) and 35% hydrochloric acid (arsenic free:
