Environ. Sci. Technol. 1999, 33, 2934-2938
Adsorption and Degradation of Dimethyl Selenide in Soil LEI GUO, WILLIAM T. FRANKENBERGER, JR., AND WILLIAM A. JURY* Department of Environmental Sciences, University of California, Riverside, California
Dimethyl selenide (DMSe) is an organic vapor that is formed naturally from inorganic selenium species in soil. The behavior of DMSe directly regulates the geochemical cycling of selenium in the environment. In this study, the two fundamental reactions of DMSe in soil, adsorption and degradation, were investigated in batch experiments using headspace analysis. Adsorption of DMSe is negligible between the temperature of 4 and 40 °C and is insensitive to organic amendments. Degradation of DMSe, measured by a first-order rate constant, varied with soil type and organic amendment and was dependent on temperature but independent of soil moisture content over the range of 10% (by weight) to saturation. Degradation proceeded rapidly in a Hanford sandy loam, with a half-life of only 9 h. This compares to 875 h in a Losthill clay loam. Addition of compost manure or gluten (a pure protein) to the Hanford soil greatly increased the persistence of DMSe, which implies that the microorganisms consuming DMSe preferred the carbon source contained in the organic amendments. Our results show that, with respect to selenium remediation by dissipation into the atmosphere, many environmental and soil factors can be optimized to increase the probability of its diffusive transport through a soil.
Introduction Contamination of selenium (Se) in soils and sediments is a common environmental problem in many areas of the western U.S. (1-4). The presence of high concentrations of Se in these media poses a potential threat to wildlife and humans (5-7). Past research has revealed that biomethylation of Se is an important process that redistributes Se in the environment at both global and local scales (8-11). This process biologically converts nonvolatile Se species into volatile forms and, therefore, has been proposed as a means to decontaminate seleniferous soils (12-14). The major volatile species of Se that have been identified in the environment include dimethyl selenide (DMSe), dimethyl diselenide (DMDSe), methaneselenone [(CH3)2SeO2], methaneselenol (CH3SeH), and dimethyl selenenyl sulfide (CH3SeSCH3), the most important of which is DMSe (15-19). The environmental behavior of all these gaseous Se compounds, however, remains largely unknown, and very little experimental information regarding their physicochemical properties is available in the literature. Zieve and Peterson (20) were the first to investigate the interactions of DMSe with soil. They reported a high uptake of DMSe in soil and concluded that soil acts as a sink for atmospheric DMSe. * Corresponding author. Phone: (909) 787-5134; fax: (909) 7873993; e-mail:
[email protected]. 2934
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 33, NO. 17, 1999
Karlson et al. (21) later determined the vapor pressure and water solubility of DMSe and DMDSe. They found that the water solubility of DMSe is very high (2.4%) and suggested that dissolution of DMSe into the soil water film might be a critical process impeding Se volatilization. More recently, Ansede and Yoch (22) tested DMSe production and degradation in estuarine sediments amended with organo-Se species and observed that DMSe can be used as a substrate for methanogenesis. Except for these studies, no other experimental work pertaining to DMSe behavior was found in the literature. Because methylation of Se is a natural process that occurs widely in seleniferous environments, the information on the fate of gaseous Se is important for accurate assessment of the role of Se methylation in Se cycling and for the development of effective remedial measures of Se contamination in soil. In this study, we conducted laboratory experiments to characterize the adsorption and degradation of DMSe in soil under a wide range of environmental conditions. Adsorption and degradation are the two most important reactions that govern dissipation of DMSe in terrestrial environments. Therefore, the information we obtain from this study would not only complement our current understanding of the fundamental biogeochemistry of Se but also be useful for developing optimum strategies for remediation of Secontaminated soils and sediments.
Materials and Methods Materials. Two soils were used in this study: a Hanford sandy loam and a Losthill clay loam. The Hanford sandy loam was collected from the Agricultural Experimental Station at the University of California, Riverside. The soil contained 35% sand, 58% silt, 7% clay, and 0.64% total carbon. The Losthill clay loam was collected at Losthill, CA. The soil had 30% sand, 37% silt, 33% clay, and 0.64% total carbon. Both soils were low in natural Se content (0.14 mg/kg for the Hanford soil and 0.28 mg/kg for the Losthill soil). Before use, the soils were air-dried and passed through a 1-mm sieve. The Hanford sandy loam was also amended with a compost dairy manure and a purified protein (gluten) at the rate of 24 × 103 and 500 mg/kg, respectively. The analytical grade dimethyl selenide was obtained from Sigma Chemical Company. The chemical was kept at freezing temperature during storage and was brought to room temperature at least 24 h before use. Determination of the Adsorption Coefficient, Kd. The adsorption coefficient, Kd, representing the equilibrium partitioning of DMSe between the solid and liquid phases, was determined in closed systems consisting of 21-mL headspace vials sealed with a Teflon-faced septum. The experiments were carried out at three temperatures (4, 21, and 40 °C) with four replicates. The soils were autoclaved following a standard procedure (2 h at 120 °C at 15 psi). Approximately 2 g of sterilized soil was placed in each headspace vial, and then a small amount of water (0.32 mL for the Hanford soil and 0.5 mL for the Losthill soil) was added. After the soil was completely wetted, the vial was purged with N2 and closed. This procedure was conducted as an additional precaution to prevent potential degradation losses of DMSe during the equilibration time. Our preliminary experiments showed that DMSe can be rapidly degraded within a few hours in unsterilized Hanford soil and that expelling O2 with N2 can effectively inhibit this degradation. After closure, 0.1 mL of saturated DMSe (from the equilibrated headspace over a liquid DMSe) was introduced into each vial by injection, and the system was allowed to equilibrate 10.1021/es981076m CCC: $18.00
1999 American Chemical Society Published on Web 07/29/1999
TABLE 1. Adsorption Coefficient (Kd) of DMSe Measured in Hanford and Losthill Soils at Different Temperatures temp, °C soil
4
21
40
Hanford sandy loam unamended manure-amended gluten-amended Losthill clay loam
0.038 ((0.013)a 0.091 ((0.048) 0.045 ((0.003) ND
NDb ND ND ND
ND ND ND ND
a Values in parentheses reflect one standard deviation error. denotes none detected.
b
ND
for 72 h before the headspace concentration was determined by gas chromatography (GC). The value of Kd was calculated by relating the headspace concentration (Cg) to the total injected mass (m) based on the following equation:
Kd )
w (K m - KhνgCg - νlCg) Cg h
(1)
where Kh is the dimensionless Henry’s law constant, w is the soil weight, vg is the volume of headspace, and vl is the volume of water. The values of Kh determined in the separate experiments were 0.032, 0.052, and 0.135 at 4, 21, and 40 °C, respectively. Degradation. The degradation experiments consisted of a treatment matrix of three soil moisture contents and three temperatures. The experiments were performed in duplicate using a similar headspace method as described for Kd determination. Each headspace vial contained 2 g of soil and an appropriate volume of deionized distilled water. The injected volume of DMSe was 0.2 mL. The vials were purged with O2 before closure to ensure that the unnaturally closed system employed would not cause an anaerobic condition, because our preliminary experiments showed that lack of O2 can significantly limit degradation of DMSe. The O2 content in the vials was monitored throughout the experiments and was found to be 50% by volume during the experimental period. Our preliminary experiments showed that the effect of a higher O2 level on DMSe degradation is much smaller compared to that of anaerobic conditions. At specific times, 0.2 mL of the headspace volume was withdrawn and injected into a GC for DMSe determination. The total concentration in the reaction vial was derived from the headspace concentration based on the rearranged form of eq 1. Instrumental Conditions and Analytical Precision. The GC (Varian 1400) was equipped with a flame ionization detector and a stainless steel column (10 m × 2.2 mm inner diameter with 10% Carbowax 1000 on chromosorb W-AW, mesh 60/80). The operational parameters were as follows: oven temperature 60 °C, injection port and detector temperature 80 °C, carrier gas He 30 cm3/min, H2 gas 33 cm3/ min, air 400 cm3/min. Standards were prepared by analyzing blank vials spiked with various volumes of saturated DMSe. The peak area was linear with the concentration of DMSe within the range of this study. The precision of the GC measurements was calculated at 4.4% based on the relative standard deviation of the standard samples, and the detection limit was 0.8 µg/L.
Results and Discussion Adsorption Coefficient, Kd. The adsorption coefficient of DMSe was determined indirectly by measuring the concentration of DMSe in the headspace and assuming equilibrium partitioning among phases. The values of Kd measured for various soil treatments are given in Table 1. Adsorption of DMSe onto the soil solid phase was negligible. No adsorption was detected for the Losthill soil at all three temperatures,
and measurable uptake of DMSe was only detected at 4 °C in the Hanford soil. At this temperature, the adsorption coefficient was very low, ranging from 0.038 to 0.091 mL/g. Since adsorption is an exothermic process, higher adsorption may occur when the temperature is further lowered (for example, to below freezing). Zieve and Peterson (20) reported high adsorption of DMSe by soil. However, their experiments were not designed to differentiate the processes of sorption to solid surfaces, dissolution into water, or degradation and, thus, may overestimate adsorption. It appears that adsorption of DMSe was increased in the presence of the organic amendment compared to in its absence, although an F-test based on four replicates failed to show a significant trend at P ) 0.05. The near-zero adsorption of DMSe, combined with its high water solubility (21), suggests that volumetric soil water content is a critical factor controlling the mobility of DMSe in soil. The partition of DMSe into the water phase impedes vapor-phase transport in unsaturated soils both by providing a storage reservoir for the compound and by reducing the diffusion coefficient in the soil. A low water content thereby enhances the diffusive transport of DMSe into the atmosphere. However, in extremely dry soils, the transport may increase due to the direct exposure of soil solid surfaces to DMSe vapor that can dramatically increase adsorption as found for many other organic vapors (23-25). Degradation. We studied degradation of DMSe at three moisture contents, three temperatures, and with or without organic amendments. Table 2 summarizes the first-order degradation rate constants obtained under these conditions. Comparisons are made below in order to identify the effects of each individual factor. Although the decay of DMSe in some cases cannot be strictly characterized by a first-order reaction, as suggested by some of the poor nonlinear coefficients of determination (ranging from 0.532 to 0.992), the first-order degradation rate constant is still useful for providing a relative index of the overall degradation rate for comparison purposes. Aerobic vs Anaerobic Degradation. Degradation of DMSe is a process that might be affected by many factors. We first investigated the degradation of DMSe in different oxic conditions by purging the headspace of the reaction vials with O2, N2, or air, respectively. The decay of DMSe under these conditions is presented in Figure 1. The Hanford soil consumed DMSe very rapidly in both O2- and air-purged vials, with
