Environ. Sci. Technol. 1999, 33, 3415-3420
Effect of Soil Moisture on Dimethylselenide Transport and Transformation to Nonvolatile Selenium YI QIANG ZHANG AND WILLIAM T. FRANKENBERGER, JR.* Department of Environmental Sciences, University of California, Riverside, California 92521-0424 JOHNNIE N. MOORE Department of Geology, University of Montana, Missoula, Montana 59812-1019
Volatilization of dimethylselenide (DMSe) to the atmosphere has been extensively studied, but little is known on its fate in soil environments. This study examined the transport and transformation of DMSe in soil. The transport of DMSe in soil columns was related to the soil moisture content. About 87-96% of the total injected DMSe in airdried soil was volatilized to the air, whereas in a moist and flooded soil, only 20-88% and 4.3-16.8% were emitted to the air, respectively. After incubation of 40 days, about 13.1-38.8% of the total DMSe was converted to nonvolatile Se in moist soils (35% moisture). This transformation increased with increasing soil moisture content and was also much higher in soil amended with MnO2 in which about 95% of the total DMSe was converted to nonvolatile Se. Oxidized dimethylated Se (ODMSe), which included dimethyl selenoxide (DMSeO) and/or dimethyl selenone (DMSeO2), was the dominant form of nonvolatile Se, accounting for 43.5-53% in the moist soil and 8090% in the flooded soils. The percentage of other Se species in the experimental soils followed a general descending order: Se[0, -II] > Se[IV] > organic Se > Se[VI]. This study showed that accumulation and transformation of DMSe may be important processes responsible for a low volatilization rate of DMSe in flooded soils and sediments.
Introduction Selenium (Se) methylation followed by the transport of volatile Se species from aquatic and terrestrial environments to the atmosphere is an important process of Se biogeochemical cycling. Dimethylselenide (DMSe) is the major volatile Se species emitted to the air (1-4). In the last 10 years, volatilization of DMSe to the air has been extensively studied since the discovery of Se poisoning of waterfowl in Kesterson Reservoir, CA (5), but its fate in aquatic and terrestrial environments is still unknown. Challenger (6) and Reamer and Zoller (7) each proposed a similar pathway for the methylation of inorganic Se, which first involves a reduction step of selenate (Se[VI]) to selenite (Se[IV]), followed by the binding of a methyl group (CH3+) with Se[IV] to form methaneselenonic acid (CH3SeO3H). Then * Corresponding author phone: (909)787-3405; fax: (909)787-2954; e-mail:
[email protected]. 10.1021/es981136o CCC: $18.00 Published on Web 08/27/1999
1999 American Chemical Society
through ionization and reduction, methaneselenonic acid converts to methaneselenic acid (CH3SeO2-), followed by the binding of a second methyl group to form DMSeO2, which is directly reduced to DMSe. Doran (8) suggested that the methylation pathway from inorganic Se[IV] to DMSe goes through a series of intermediates such as elemental Se (Se[0]), selenide (HSeX), and methanselenol (CH3SeH). Cooke and Bruland (9) also proposed that the assimilation, reduction, and methylation processes leading from inorganic Se[VI] and Se[IV] to DMSe pass through two intermediates, selenomethionine (CH3Se(CH2)2CHNH2COOH) and methyl selenomethionine ((CH3)2Se(CH2)2CHNH2COOH). In a recent study on Se volatilization from a filamentous cyanophytedominant mat, Fan et al. (10) found that the precursor of DMSe was methyl selenomethionine. Zhang and Chasteen (11) reported that added DMSeO2 to a microbial culture was rapidly transferred to DMSe. Therefore, DMSeO2, methanselenol, and methyl selenomethionine may be important intermediates for the complete transformation of inorganic Se to DMSe. On the other hand, Wang and Burau (12) reported that dissolved DMSe was rapidly oxidized to DMSeO by the presence of MnO2 in suspension. Oremland and Zehr (13) found that DMSe in an anoxic sediment is rapidly demethylated to both CH4 and CO2 by methanogenic and sulfaterespiring bacteria. Therefore, a reverse pathway of Se methylation may exist in the environment. Dimethylselenide has an approximate vapor pressure of 32 kPa and a solubility of 0.0244 g/g in water (14). Therefore, the stability of DMSe followed by its emission into the air is affected by the water content of the matrix where DMSe is formed. Under dry soil conditions, higher DMSe emission rates would be expected since there is little water to dissolve DMSe. In aquatic systems and moist terrestrial environments (such as moist soil), DMSe may be partially dissolved into water or trapped in the water-soil/sediment phase before its emission into the air. If oxidants such as manganese oxides, which can oxidize DMSe to nonvolatile Se (12), are present in water or in the interface between water and soil/sediment, dissolved DMSe may directly react with these oxidants, resulting in a transformation of DMSe to ODMSe. Therefore, information about the effect of soil moisture on DMSe transport and transformation to nonvolatile Se is extremely important in understanding the fate of DMSe in aquatic and terrestrial environments. In this study, we designed two laboratory microcosm experiments to examine the fate of DMSe in soil. The first determined the transport of DMSe in soil columns under different moisture regimes, and the second determined the transformation of accumulated DMSe in soil.
Materials and Methods Three soils used in this study were collected from Tulare Lake (TL), Losthill (LH), and Five Points (FP) upland areas in California in 1997. The soils were air-dried and sieved ( Se[IV] > Se[VI]. Elemental Se and organically bound Se was the highest in soil amended with MnO2.
Discussion Accumulation and transformation of DMSe may be important processes responsible for reducing DMSe volatilization from the soil to the atmosphere. DMSe has a high vapor pressure (32 kPa) and is therefore easily subject to volatilization (14). On the other hand, DMSe has a high solubility (24.4 mg/g) in water (14), so it can be easily accumulated in water. Karlson et al. (14) reported that, at an equilibrium of DMSe in an aqueous-air system, the water concentration of DMSe was 17 times greater than air concentration. Therefore, whether DMSe formed in the soil environment can finally escape to the atmosphere is partially dependent on the water content present. In the present study, we found that 83.2-95.7% of added DMSe was accumulated in flooded soils, and 1279.6% was accumulated in the moist soils; while only 4.213.3% remained in air-dried soils. This observation agrees with a field study on Se volatilization from wet sediments and open water areas in a wetland in which the Se volatilization rate from wet sediments was 4-5 times higher than that from open water areas, although Se concentrations in both environments were similar (21). In a recent laboratory
study on Se volatilization, Zhang and Moore (22) found that the volatilization rate significantly decreased when a wet sediment was flooded to a water-sediment system. Transformation of DMSe occurs in soil environments. This is evident by our observation that a large portion of ODMSe and other Se species such as Se[IV] were formed in flooded and moist soils after reaction with DMSe. The degree of transformation of DMSe was clearly affected by soil moisture. In this study, higher soil moisture contents increase the extent of DMSe reaction with soil reactants, resulting in the formation of much higher amounts of nonvolatile Se such as ODMSe in a flooded soil than in moist soil. Therefore, accumulation of DMSe followed by its transformation may be important processes responsible for a low volatilization rate of DMSe in flooded soils and sediments. Manganese oxides can enhance the transformation of DMSe in soils. Wang and Burau (12) found that dissolved DMSe was rapidly oxidized to DMSeO by the presence of MnO2 in suspension. In the present study, we found that after reaction of DMSe, the FP soil, which had relatively high MnO2, contained a higher concentration of ODMSe than the other two soils that had lower MnO2 (Table 2). In comparison to a moist soil (35% moisture) (Table 5), the soil amended with MnO2 contained much higher concentrations of Se species in water extracts: 2.5-4 times higher in organic Se and ODMSe and more than 30 times higher in Se[IV] and Se[VI]. In NaOH and NaOCl extracts, the concentrations of Se species in MnO2-amended soil were also higher than that in water amended soil. Evidence from our experiments showed that the accumulation and transformation of DMSe exist in soils, especially in flooded soils, and that ODMSe is the major product of transformation of trapped DMSe. However, it is still not clear whether transformation of DMSe reversibly follows only one of the methylation pathways proposed by Challenge (6), Reamer and Zoller (7), Doran (8), and Cooke and Bruland (9) because we did not identify all Se species present. Therefore, it is very important to identify all Se species formed by transformation of DMSe.
Acknowledgments We thank Rob Dungan and David Herman for their helpful discussion. We especially thank Dr. Howard E. Ganther for providing DMSeO. This research was funded by the UC Salinity and Drainage Program and in part by the Department of the Interior’s National Irrigation Water Quality Program. VOL. 33, NO. 19, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Received for review November 4, 1998. Revised manuscript received July 9, 1999. Accepted July 14, 1999. ES981136O
