Environ. Sci. Technol. 1991, 25, 420-427
Cox, R. A.; Goldstone, A. Proceedings of the 2nd European
Symposium on the Physic0 Chemical Behavior of Atmospheric Pollutants; D. Riedel Publishing Co.: Dordrecht, Holland, 1982; pp 112-119. Japar, S. M.; Wallington, T. J.; Richert, J. F. 0.; Ball, J. C. Int. J . Chem. Kinet. 1990, 22, 1257. Wallington, T. J.; Japar, S. M. Environ. Sci. Technol., preceding article in this issue. Carter, W. P. L.; Atkinson, R. J. Atmos. Chem. 1985,3,377. Hogo, H.; Gery, M. W. User's guide for executing OZIPM-4 with CRM-IV or optional mechanism; U.S. Environmental Protection Agency Report No. EPA/600/8-88/073b; U S . EPA: Research Triangle Park, NC, 1988. Lurmann, F. W.; Carter, W. P. L.; Coyner, L. A. A surrogate
species chemical reaction mechanism f o r urban-scale air quality simulation models. Adaption of the mechanism; U.S. Environmental Protection Agency Report No.
EPA/600/3-87/014a; U.S. EPA: Research Triangle Park, NC. 1987: Vol. I. (16) Wallington, T. J.; Dagaut, P.; Liu, R.; Kurylo, M. J. Znt. J . Chem. Kinet. 1988, 20, 177. (17) Atkinson, R. Chem. Rev. 1986, 86, 69. (18) Jeffries, H. E.; Sexton, K. G.; Arnold, J. R. Validation
testing o f new mechanisms with outdoor chamber data. Analysis of VOC data for the CB4 and CAL photochemical mechanisms; U S . Environmental Protection Agency cooperative agreement No. CR-813107; U S . EPA: Research Triangle Park, NC, 1989; Vol. 2. (19) Wallington, T. J.; Kurylo, M. J. Int. J. Chem. Kinet. 1987, 19, 1015. (20) Atkinson, R., private communication, 1989.
Received f o r review June 8, 1990. Revised manuscript received October 16, 1990. Accepted October 24, 1990.
Biogeochemistry of Arsenic in Natural Waters: The Importance of Methylated Species Linda C. D. Anderson* and Kenneth W. Bruland Earth Sciences Board and Institute of Marine Sciences, University of California at Santa Cruz, Santa Cruz, California 95064
Water samples from a number of lakes and estuaries, mostly in California, showed measurable concentrations of methylated arsenic (equivalent to 1-59% of total As) with the exception of one highly alkaline lake. Neither depleted phosphate concentrations nor high dissolved salts correlated with the appearance of methylated forms of As. A temporal study of As speciation in Davis Creek Reservoir, a seasonally anoxic lake in northern California, demonstrated that dimethylarsinic acid increased sufficiently to become the dominant form of dissolved As within the surface photic zone during late summer and fall. Methylated forms decreased while arsenate increased when the lake over-turned in early December, which suggested a degradation of dimethylarsinic acid to arsenate.
Introduction Dissolved arsenic can occur in natural waters in both inorganic and organic forms. Arsenic's inorganic forms include formal oxidation states As(V), arsenate, and As(111),arsenite, with primary aqueous species at natural pHs being anionic in arsenate (H2As0,- and HAsOZ-) or neutral for arsenite (As(OH),O). The location of As on the periodic table directly below phosphorus predicts an analogous chemical behavior for arsenate and phosphate including incorporation into organic molecules. However, As has no recognized use in enzymatic systems and could potentially interfere with numerous biological mechanisms normally dependent on phosphorus ( I ) . It has been suggested that organisms have developed mechanisms to isolate and detoxify As by producing organoarsenicals (2). In addition, incorporation of As into arsonium zwitterions such as arsenobetaine and arsenocholine may serve dual purposes of detoxification and osmoregulation analogous to some sulfur compounds ( 3 ) . Identification of organoarsenicals produced during culture experiments with bacteria, fungi, and algae demonstrate that biosynthesis of methylated arsenicals is common ( 4 , 5). Dissolved As compounds measured in culture exudates include arsine (ASH,),monomethylarsonic 420
Environ. Sci. Technol., Vol. 25, No. 3, 1991
Table I. Chemical Forms of Arsenic Observed in Water Samples, Culture Exudates, or Tissue Extractions name
chem formula
Water Samples or Culture Exudates arsenate HZASO, arsenite As(OH)3 arsine ASH^ monomethylarsonate CH~ASO~OH(MMAA) dimethylarsenate (DMAA) (CH,),AsOOdimethylarsine (DMA) (CH,),AsH trimethylarsine (TMA) (CH,),As trimethylarsine oxide (CHJ3AsO (TMAO) arsenobetaine arsenocholine arsenoribosides (e.g.)
Tissue Culture Extracts (CH,),As+CH,COOH (CHjj,As+(CH2),0H
YY
(CHd2h-CHz
arsenophospholipids (e.g.)
OH
OCHpCHOHCHpR
OH
CHp-
I I
0
CH
I
CH-O-P-O-CH,CH~~S+(CH& ~
b~~~~~
~
acid (MMAA), dimethylarsinic acid (DMAA), dimethylarsine (DMA), trimethylarsine oxide (TMAO), and trimethylamine (TMA) (Table I). At natural pHs, MMAA and DMAA occur as anionic monomethylarsenate (CH3As020H-) and dimethylarsinate ((CH,),AsOO-j, respectively. Confusion exists in the literature over whether the formal oxidation state of As in TMAO, DMAA, and MMAA is As(II1) or As(V). A recent review by Cullen and Reimer (5) states that these compounds all contain As(V). In addition, a large diversity of more complex organic compounds has been measured in tissue extractions, including arsenobetaine, arsenocholine, arsenoribosides, and arsenophospholipids (Table I). The importance of biotic transformations of metal(1oid)s is becoming increasingly recognized. Natural occurrences
0013-936X/91/0925-0420$02.50/0
0 1991 American Chemical Society
of dissolved organometals and organometalloids have been identified for selenium, arsenic, antimony, germanium, tin, lead, and mercury (13-17). These forms would not be predicted by thermodynamic equilibrium models. Potential biotic impacts, such as toxicity or biolimitation, cannot be assessed without first determining the form of the metal(1oid). Processes that promote methylation of As are still poorly understood. No detailed studies exist of methylated forms of As in freshwaters and only a few studies have detected methylated forms in oceanic and estuarine waters (6-10). Andreae (6, 7) suggested that methylation occurs when phosphate concentrations are sufficiently low (or the ratio of arsenate/phosphate is close to 1)that organisms require detoxification mechanisms to exclude the chemically similar arsenate from interfering with phosphate requirements. However, investigations by Howard et al. (10, 11) in estuarine waters found no correlation between As methylation and phosphate concentrations. These investigations hypothesized that methylation occurred only in oceanic or estuarine waters. However, although somewhat limited, other studies have shown evidence of methylated forms in freshwaters (6, 12). Biotic transformations of As may depend on the organism’s strategy of dealing with nutrients, with organisms in continuously nutrient-depleted environments having better strategies for discriminating against arsenate than organisms in environments with large episodic inputs of nutrients (9). Because different organisms produce different organoarsenic forms (5), the biota present may determine the presence or absence of specific dissolved organoarsenicals. In this study, we identified and quantified the different forms of As in a variety of water bodies to assess the relative importance of methylated forms to the aqueous chemistry of As. We chose our sample sites in order to investigate whether organoarsenicals are found in fresh water as well as saline environments (11),and whether their production is linked to phosphate-depleted waters (6, 7). In addition, we undertook a detailed temporal study of the biogeochemical cycling of As in a seasonally anoxic lake. This allowed us to examine the chemical effects of seasonal changes in the biology of the photic zone and the redox cycle in the hypolimnion on the chemical speciation of As.
Experimental Procedure Sampling Sites. We collected surface samples from a number of lakes and rivers, mostly in California, for As speciation and phosphate concentration determinations. Sample sites represent drainage areas affected by agricultural impacts, sites with known elevated total As concentrations, and basins expected to be unaffected by anthropogenic inputs. We chose Davis Creek Reservoir, a seasonally anoxic lake, for a detailed seasonal study of depth profiles (7188, 9/88, 10/88, 12/88, and 2/89) to observe both the seasonal variation in As speciation and the influence of varying redox chemistry on As forms in the hypolimnion. Sampling Procedures. Surface water survey samples were collected just below the surface with a polycarbonate sampling chamber. Davis Creek Reservoir water samples were collected from different depths with a 7-L, tracemetal-clean GoFlo (Niskin) that had been acid-cleaned for trace-metal work. Filtered samples were processed immediately under nitrogen pressure through a Teflon filter sandwich with a 0.4-gm pore size polycarbonate membrane filter (Nuclepore). Filtered and unfiltered aliquots were placed in acid-cleaned, low-density linear polyethylene bottles. Samples for Fe and Mn analysis were acidified
to pH < 2 with redistilled G. Frederick Smith tracemetal-clean nitric acid; those for As and nutrient determinations were not acidified. Samples for large-volume gas purging were transferred under nitrogen pressure directly from the GoFlo sampler for a 2-L, silanized glass, gas-purging vessel. For As analyses, arsenate and arsenite were reduced to gaseous arsine, and MMAA and DMAA were converted to corresponding gaseous methylarsines. All forms were separated and determined by a procedure modified from Andreae (18). This analytical procedure concentrates the various arsines from a given sample volume. Detection limits were lowered as required by increasing the volume of the sample purged. The standard sample chamber was designated to hold 50 mL of solution with detection limits of 0.06 nM for ASH, (3 times standard deviation of the blank), and 0.1 nM for MMA and DMA (3 times standard deviation of a 0.5 nM standard). The precision on five replicate determinations was less than 5% although variability increased to 25% for DMA measurements lower than 1 nM ( f l u relative standard deviation). This analytical procedure may not detect certain organic forms of As such as arsenobetaine or arsenocholine (19). However, for the purpose of this study, the sum of the inorganic and the various methylarsenicals will be designated as total As. Arsenite was trapped in the field and the column stored in liquid nitrogen because we found significant discrepancies between field-trapped samples and water samples frozen in liquid nitrogen, thawed, and analyzed in the laboratory. Comparisons of in-field-trapped methylated forms and trapping from refrigerated, unacidified samples analyzed within 4 days showed no measurable differences. Therefore, methylated forms were generally brought back to the laboratory to analyze. We added DMAA spikes to deep and shallow unfiltered water samples to qualitatively assess DMAA degradation rates for the October, December, and February samples from Davis Creek Reservoir (7.9, 2.2, and 5.5 nM, respectively). Samples were transferred from the GoFlo sampler immediately to silanized oxygen bottles and then spiked. Sulfide was measured in the field with a specific ion probe (silver-sulfide electrode, Orion Model 94-16). Iron and manganese were analyzed by direct injection into a graphite furnace atomic absorption spectrometer (Perkin-Elmer 5000, P E HGA500 with autosampler), using manufacturer’s recommended conditions. Precision was always less than 5% and usually around 2% ( f l u relative standard deviation). Phosphate was analyzed by complexation with molybdate and visible light spectrometry (20)*
Results Regional Survey. Our survey of total As concentrations in lake waters showed a very wide range of concentrations (Table 11, Figure 1). Total As concentrations in rivers were well within the range measured by other investigators (6). Two estuarine concentrations of total As are similar to estuarine measurements by others (10, 11, 21-23) and to concentrations observed in the Southern California Bight (18-20 nM) and the Pacific Ocean (24 nM) (7). All the lake samples had measurable amounts of methylated As species except Mono Lake (Table 11, Figure 1). The lack of detected methylated forms in Mono Lake may be due to the extremely high inorganic As concentration, which could overwhelm the analysis of small amounts of organic forms. In Pyramid Lake, where the methylated forms account for 1% of the total As, the DMAA con-
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Environ. Sci. Technol., Vol. 25, No. 3, 1991 421
Table 11. Arsenic Forms and Phosphate Concentrations in a Survey of Surface Waters of Natural Water Bodies As forms, nM location California Lake Tahoe Salton Sea Elkhorn Slough Suisun Bay Lake Berryessa Lake Ruth Davis Creek Reservoir Mono Lake Pyramid Lake Lake Ontario California rivers Sacramento Russian Davis Creek Salinas
date
As,
As,llorg
5130188 1018186 6110188 5130188 10/86 10186 10/26/89 5/30/88 5/30/88 8/18/81
15 170 19 23 8.9 160 24 230000 1300 6.9
15 130 15 20 8.0 150 9.0 230000 1300 4.2
25 8.9 34 99
25 8.9 34 99
5/29/88 5/31/88 3/88 6/10/88
phosphate,
MMAA
DMAA
