Environ. Sci. Technol. 1987, 21, 1214-1219
Aquatic Chemistry of Selenium: Evidence of Biomethylationt Terence D. Cooke" and Kenneth W. Bruland" Institute of Marine Sciences, University of California, Santa Cruz, California 95064
The chemical species of dissolved selenium were examined in surface waters from three sites in the San Joaquin and Imperial Valleys of California. Six dissolved selenium species were identified the inorganic species selenate and selenite; nonvolatile organic selenides, including seleno amino acids and a dimethylselenonium ion; and the volatile methylated forms dimethyl selenide and dimethyl diselenide. The occurrence of methylated selenium species in the aquatic environment has important implications regarding the biogeochemical behavior of selenium in natural aqueous systems. Laboratory studies indicate that the nonvolatile dimethylselenonium ion can be transformed into volatile dimethyl selenide at neutral pH, providing a pathway for the in situ production of dimethyl selenide in natural waters. Geochemical flux calculations indicate that outgassing of dimethyl selenide may be an important removal mechanism for dissolved selenium from aqueous systems.
Introduction Selenium is highly enriched in atmospheric aerosols relative to its crustal abundance (enrichment factor relative to crust, EFAl = 1000-6000), and a large fraction of atmospheric selenium appears to exist in an unidentified vapor phase (1,2). Biomethylation of selenium followed by release of volatile dimethyl selenide to the atmosphere has been proposed as a pathway for the atmospheric enrichment of selenium (1). Because the surface ocean has recently been postulated to be a major source of vaporphase selenium to the atmosphere (3),biomethylation of selenium in surface waters may be important on a global scale. Despite the postulated importance of this process on a global basis, little data exist demonstrating that alkyl selenides are produced in surface waters of natural systems. Nevertheless, some indirect evidence supports this hypothesis. Dimethyl selenide and dimethyl diselenide have been observed emanating from plants ( 4 ) , soils (5), and laboratory cultures of aqueous suspensions of sewage sludge, soil, and lake sediment (with and without added inorganic selenium) (6, 7). Alkyl selenides have also been detected in the atmosphere just above a freshwater lake (8).
The aquatic chemistry of selenium is complicated since it can exist in four different oxidation states and as a variety of inorganic and organic compounds. Dissolved inorganic selenium can exist in natural waters as Se(-II), primarily as biselenide (HSe-); Se(O), as colloidal elemental selenium; Se(IV), as the selenite oxyanion (HSeOS-and Se032-); and Se(VI), as the selenate oxyanion (Se0,2-). Organic forms of selenium are analogous to those of sulfur and include seleno amino acids and their derivatives, methyl selenides, methyl seleninic esters, methyl selenones, and methylselenonium ions. The pathways for the biotransformation of inorganic Se to the various organic forms and the interconversion between these different molecular species of selenium are not well understood. At present, there are few analytical methods available that can isolate and quantify the broad spectrum of bio'Portions of this work were presented a t the Fall 1986 AGU/ ASLO Meeting. 1214
Environ. Sci. Technol., Vol. 21, No. 12, 1987
geochemically imporant selenium species (noted above) that may be present in natural water systems. To address this concern, we developed a procedure that enabled the measurement of both volatile alkyl selenide species and their precursors as well as other inorganic and organic forms of selenium. Although other methods have been reported for the sampling and analysis of alkyl selenides ( 6 4 , our procedure involving in-field cryogenic trapping with subsequent laboratory separation/detection via thermal-gradient gas chromatography (GC)/gas-phase atomic absorption (AA) allowed interfacing with methods used for the determination of inorganic selenium species (9). This paper reports the results obtained on the distribution of dissolved selenium species in two lakes and one river in the San Joaquin and Imperial Valleys of California, all of which receive agricultural drainage. The results provide insight into the biogeochemical cycling of selenium in natural waters.
Experimental Section Sampling Sites. Three natural aqueous systems in California were selected for the examination of dissolved selenium species. These systems are environments in which we believed the presence of methylated selenium species might be detected. System 1,the Kesterson National Wildlife Refuge (Kesterson Reservoir), is a marsh in the western San Joaquin Valley. During this study, the Kesterson Reservoir received agricultural drainage waters containing extremely high concentrations of dissolved selenium (ca. 4 pM) from the San Luis Drain (SLD). The elevated selenium in the agricultural drainage water originates from weathering of selenium-rich marine shales in the surrounding hills, which resulted in the deposition of high levels of selenate in the soils along the Panoche Fan formation located in the western edge of the San Joaquin Valley. The SLD water is delivered to ponds 1 and 2 of the Kesterson Reservoir. The water then flows sequentially through a series of 12 evaporation ponds. System 2, five sites along the San Joaquin River (Figure l),was chosen because this river is a route for the removal of excess agricultural drainage waters from the San Joaquin Valley to the Sacramento-San Joaquin River Delta and San Francisco Bay. System 3, the Salton Sea in the Imperial Valley of southern California, is a saline lake that also receives agricultural drainage water. However, due to the absence of major outcrops of selenium-rich marine shales in the surrounding hills, the irrigated soils have lower selenium concentrations. Surface water was collected from the Kesterson Reservoir during two visits (February 1985 and May 1986),while oxic groundwater was sampled in June 1986. Groundwater was also sampled from another site in the western San Joaquin Valley near Mendota. Surface waters were sampled from the five sites along the San Joaquin River and from three depths in the Salton Sea. Sampling Techniques. Procedures were developed for sampling and storage of volatile methylated selenium compounds (methyl selenides) and dimethylselenonium ions (DMSe+-R) that minimized loss and interconversion of these reactive compounds (Figure 2). The procedures involved in-field purging and cryogenic trapping of volatile compounds on glass columns packed with silanized glass
0013-936X/87/0921-1214$01,50/0
0 1987 American Chemical Society
i00601
lY 0 0 4 0
f
l 1, I'
I - 000 0
" -
1
0
O 1
2
L 3
5
4
\
O'
6
7
8
9
1
8
PY
:+ ::,,
liSJ0
Flgure 1. San Joaquin River sampling site locations.
Flgure 3. pH dependence of the production of DMSe from DMSe+-R during purging. DMSeC-R was isolated from surface water samples collected from Kesterson Reservoir on M a y 13, 1986, on Dowex AG50 cation-exchange resin (see text).
I
SAMPLE pH : . 5 (HCI)
"\ U wge/trap
selenides
pH=2 (HCI)
valve
out
4N
HC'l Se0:~
Alk yk Selenium oris . .-
NaBH4
Figure 2. Apparatus for sampling alkyl selenium compounds.
beads and immersed in liquid nitrogen. Initial studies using 10 ng of diethyl selenide as an internal standard to assess the efficiency and stability of the in-field trapping technique demonstrated >95% recovery after storage of the traps for up to 3 days in liquid nitrogen. Water samples were collected in a Plexiglas vessel taking great care to minimize turbulence. Water samples were immediately filtered through a 0.3-vM polycarbonate membrane filter (Nucleopore) with a Teflon filter sandwich. Several aliquots were drawn with use of a six-way Telfon valve. Sample aliquots were acidified to pH 2 (Baker HC1) to minimize conversion of DMSe+-R to dimethyl selenide (DMSe) during stripping of volatile selenides (Figure 3). For the collection of nonvolatile methylated selenium compounds, aliquots that had been previously stripped of volatile methyl selenides were acidified to 4 M with HC1 (Baker) and reacted with 4 mL of 3% NaBH, over 7 min to produce volatile selenide derivatives, which were then
BH
(1
~
,
Flgure 4. Analytical procedure for selenium speciation determinations.
stripped and trapped. For the less reactive species (e.g., selenate and selenite), samples were stored refrigerated (after acidification to pH 1.5 with HCl) in acid-cleaned, silanized, borosilicate glass bottles with Teflon-lined caps until analysis (
