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TECHNICAL REPORTS
Heavy Metals in the Environment

Fate of Dimethyldiselenide in Soil

Department of Environmental Sciences, University of California, Riverside, CA 92521-0424

* Corresponding author (williamf{at}orange.ucr.edu)

Received for publication June 12, 2001.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Volatilization of dimethyldiselenide (DMDSe) is one of the most important processes for removing selenium (Se) from Se-contaminated environments. However, the fate of DMDSe in soil is not known. In this study, we monitored the changes of DMDSe in the head space of soil samples spiked with known amounts of DMDSe gas, and fractionated and speciated the resulting Se forms in soil. Dimethyldiselenide was highly dissolved in water in a closed air–water system and was highly sorbed onto soil in a closed air–soil system. Chemical and biological transformations of DMDSe in soil converted a large amount of DMDSe to nonvolatile Se compounds. Elemental Se [Se(0)] and nonvolatile organic Se were the major forms of Se transformed from spiked DMDSe. Microbial conversion of DMDSe to dimethylselenide (DMSe) in soil increased the production of DMSe. Calculation of the mass recovery showed that about 85 to 93% of the added DMDSe was recovered as Se(0), organic Se, organic material Se (OM-Se), Se(IV), and volatile organic Se in the head space in the non-autoclaved soils and 50 to 70% of the added DMDSe was recovered in the autoclaved soils. These results indicate that DMDSe is not a stable form of Se, and it may be one of the important precursors of DMSe in the soil environment.

Abbreviations: DMDSe, dimethyldiselenide • DMSe, dimethylselenide • OM-Se, organic material selenium

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
SELENIUM (Se) methylation–volatilization is an important process responsible for removing Se from Se-contaminated environments to the atmosphere. The pathway for the methylation of inorganic Se involves reduction of selenate [Se(VI)] to selenite [Se(IV)] and/or selenide [Se(-II)], followed by an assimilation process to organic Se (non-dimethylselenonium compounds), such as selenomethionine (Semet) and selenocystein (Secys). Through the binding of methyl groups (CH⁺₃), organic Se is converted to nonvolatile dimethylselenonium compounds such as methylselenomethionine and methylselenocystein. Finally, nonvolatile dimethylselenonium compounds yield dimethylselenide (DMSe) and dimethyldiselenide (DMDSe), which can volatilize into the atmosphere (Challenger, 1945; Cooke and Bruland, 1987; Zhang and Frankenberger, 2000). Karlson and Frankenberger (1988) have shown that major measurable volatile Se species in the environment are DMSe along with relatively small amount of DMDSe.

Dimethyldiselenide is one of the most important organic volatile Se species in the environment. It has been detected in air and water (Amouroux and Donard, 1996; Chau et al., 1976; Jiang et al., 1983, 1989) and measured in water, soil, sewage sludge, and algae added with different Se species in laboratory experiments (Amouroux et al., 2000; Chau et al., 1976; Fan et al., 1998; Karlson and Frankenberger, 1988; Reamer and Zoller, 1980; Zhang and Frankenberger, 2000). Vapor pressure is a very important property of volatile Se compounds. Dimethyldiselenide has a lower vapor pressure (0.38 kPa) than DMSe (32 kPa) (Karlson and Frankenberger, 1994). Therefore, DMDSe would have a relatively longer retention time in soil than DMSe where they are formed. This long retention time may affect volatilization of DMDSe.

In a recent study on the formation of dimethylselenonium compounds in soil, Zhang and Frankenberger (2000) used a flow-trap system to quantitatively determine the total amount of volatile Se produced from the reaction of selenomethionine with a moist soil, and used a closed system to qualitatively determine volatile Se species in the head space of the soil. We found that the amount of head space DMDSe was much higher than DMSe at Day 1 of the experiment, and decreased with time, along with an increase of DMSe. The disappearance of DMDSe in the head space was due to the soil serving as a sink to DMDSe. However, the fate of DMDSe in soil was not investigated in the previous work. In this study, we performed several experiments spiked with known amounts of DMDSe gas to a soil with two different moisture contents. Under these conditions, we determined the fate of DMDSe in soil by measuring the changes of volatile Se in the head space, and fractionating Se species formed from spiked DMDSe.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Dimethylselenide (DMSe, 99%) was obtained from Strem Chemical (Newburyport, MA) and DMDSe (98%) was purchased from Aldrich (Milwaukee, WI). The Se(IV) standard solution (1000 mg/L), NaBH₄, and other chemicals were purchased from Fisher Scientific (Pittsburgh, PA).

The soil used in this study was collected from the Tulare Basin of California. The soil had the following properties (Zhang et al., 1999a): pH 7.50; total Se, 0.269 mg/kg; total C, 13.1 g/kg; total organic C, 6.55 g/kg; total N, 1.07 g/kg; and texture, 17.5% clay, 10% silt, and 72.5% sand. The soil was air-dried and sieved (<1 mm) prior to experiments.

Solubility of Dimethyldiselenide in Autoclaved Deionized Water
This experiment was performed in triplicates in laboratory microcosms to assess DMDSe solubility. In this experiment, 0.5 to 10 mL of autoclaved deionized water was placed in a 20.85-mL head space vial with a teflon-faced septum, followed by injecting 5600 ng of Se (DMDSe). Karlson and Frankenberger (1994) found that DMDSe is not a stable form of Se in water. Therefore, the samples were slightly shaken for only 2 h, and then stood in the dark for another 22 h at 21°C. Concentration of DMDSe in the head space of the samples was determined by gas chromatography (GC). In a similar test on DMDSe solubility in deionized water, we found that about 95% of soluble DMDSe can be removed from the deionized water after purging with pure N₂ (80–100 mL/min) for 2 h (data not shown), showing that DMDSe had little change in the deionized water during the one-day experiment. Therefore, DMDSe in the autoclaved deionized water was determined as the difference between the amount of DMDSe in the head space of the deionized water and the amount of DMDSe added.

Sorption of Dimethyldiselenide in Autoclaved Soil
This experiment was performed in triplicates in laboratory microcosms to assess DMDSe sorption in an autoclaved soil. In this experiment, 0.5 to 10 g of autoclaved moist soil (10% moisture) was placed in a 20.85-mL head space vial with a teflon-faced septum, followed by injecting 5600 ng of Se (DMDSe). The samples were slightly shaken for 2 h, and then stood in the dark for another 22 h at 21°C. Concentration of DMDSe in the head space of the soil samples was determined by gas chromatography. Sorbed DMDSe in the autoclaved soil was determined as the difference between the amount of DMDSe in the head space of the soil samples and the amount of DMDSe added.

Dimethyldiselenide Transformation in Soil
This experiment was performed in triplicates in laboratory microcosms to assess DMDSe transformation in soil. In this experiment, 1 g of soil (autoclaved vs. nonsterile) was placed in a 20.85-mL head space vial with a teflon-faced septum, followed by adding a known amount of autoclaved deionized water adjusted to 10 and 100% moisture contents. Following this step, 1660 ng of Se (DMDSe) was injected in the vials. The experiments using nonsterile soil samples without adding DMDSe were run as controls. The vials stood in the dark for 1 to 6 d. Volatile Se species in the head space of the samples were analyzed by gas chromatography. In the final day (Day 6) of the experiment, accumulated Se in the soil samples from the added DMDSe and the original Se in the controlled soil samples was fractionated and speciated using a modified sequential fractionation and speciation method described below.

Sequential Extraction of Selenium Species
The fractionation and speciation of Se in the soil samples was determined by sequentially extracting the soils with 0.1 M NaOH, 0.1 M NaS₂O₃, and 30% H₂O₂. Sodium hydroxide (0.1 M) was thought to extract Se(VI), Se(IV), dissolved volatile Se species, and other organic Se associated with humic substances (Gao et al., 2000; Zawislanski and Zavarin, 1996); 1 M NaS₂O₃ was used to target elemental Se [Se(0)] (Gao et al., 2000; Velinsky and Cutter, 1990; Zhang and Moore, 1996) and 30% H₂O₂ extracted Se(-II) bound to organic material (OM-Se) that was not extracted with the 0.1 M NaOH. In brief, the experimental samples were transferred to 40-mL Teflon centrifuge tubes with 20 mL of 0.1 M NaOH and purged with pure N₂ (80–100 mL/min) for 2 h to remove volatile Se gases. The centrifuge tube was tightly capped and placed horizontally on a gyrotory shaker overnight. Then, each sample was centrifuged at 17300 x g (relative centrifugal force) for 20 min. The supernatant from each tube was transferred to a 40-mL glass vial. The sample was then rinsed and resuspended once with 10 mL of 0.1 M NaOH and centrifuged again. The final combined supernatant of the soil sample was passed through a 0.45-µm Supor membrane filter (Gelman Sciences, Ann Arbor, MI) into another glass vial. Following 0.1 M NaOH extraction, 20 mL of 1 M Na₂SO₃ (pH 7) was added to the residue and mixed well by shaking overnight. The same procedure was followed as for the 0.1 M NaOH extraction described above. After the Na₂SO₃ extraction, 1 mL of 30% H₂O₂ was added to the residue. After effervescence stopped, an additional 1 mL of H₂O₂ was added. This procedure was repeated several times. Then, 5 to 10 mL of H₂O₂ were added to the tube. The sample was heated in a hot water bath at 60 to 70°C until the sediment residue in the tube had settled to the bottom of the tube. Several drops of 1 M NaOH were added to the tube to facilitate the decomposition of the surplus H₂O₂. Generally, this extraction lasted for 6 to 8 h. After the extraction, the sample was centrifuged and filtered.

Selenium Species Analysis
In a previous test, we found that several organic Se compounds such as soluble DMDSe, DMSe, and dimethylselenoxide (DMSeO) have a signal during measurement of Se(IV) by hydride generation atomic absorption spectrometry (HGAAS). In studies on the reaction of volatile Se gas with soil, it is very likely that dissolved volatile Se and other dimethylselenonium compounds (e.g., DMSeO) can exist in the samples. Therefore, these compounds have to be removed or separated from the samples before the Se measurement. In this study, a purge-separation technique was used. After extraction, the 0.1 M NaOH extracts were purged with pure N₂ (80–100 mL/min) for 3 h to remove soluble volatile Se (DMDSe and DMSe). Following this step, an ion exchange chromatography–hydride generation atomic absorption spectrometry (IEC–HGAAS) technique (Zhang and Frankenberger, 2001) was used to separate inorganic Se(IV) and Se(VI) with some organic Se compounds such as DMSeO. In brief, an anion exchange resin (Dowex 1-10X; Dow Chemical Company, Midland, MI) column was used to separate non–amino acid Se, Se-amino acids and Se(IV), and Se(VI). Non–amino acid Se was eluted from the column by deionized water. Then, Se-amino acids and Se(IV) were eluted by 0.2 M HCl, and Se(VI) was eluted by 0.5 M HCl.

Measurable Se(IV) in the 0.2 HCl eluate was directly determined in a pH 7 buffer solution (Zhang et al., 1999b). At this stage, we do not know whether other organic Se compounds formed from the reaction of DMDSe with soil, which have a signal during the measurement of Se(IV), existed in the 0.2 M HCl eluate, although soluble DMDSe, DMSe, and DMSeO were removed by purge and ion exchange resin. Therefore, we used the term measurable Se(IV) instead of Se(IV). The Se(VI) in the 0.5 M HCl eluate was determined after a reduction of Se(VI) to Se(IV) in 6 M HCl. Total Se in the 0.1 M NaOH extract was determined by oxidizing all Se to Se(VI) by potassium persulfate, followed by reduction to Se(IV) in 6 M HCl. The organic Se concentration in the 0.1 M NaOH extract was calculated as the difference between total Se and the sum of Se(IV) and Se(VI). For determination of total Se in the Na₂SO₃ and H₂O₂ extracts, Na₂SO₃ was oxidized to sulfate by H₂O₂, and then surplus H₂O₂ was decomposed to water under a basic condition in a hot water bath (Zhang et al., 1999a). Selenium concentrations in all prepared solutions were determined by HGAAS (Zhang et al., 1999b,c). The detection limit was 0.5 µg Se(IV)/L in solution.

Volatile Organic Selenium Species Analysis
Identification of volatile Se compounds in the head space of the samples was performed based on the retention time of two standard volatile Se compounds (DMSe and DMDSe). A Hewlett–Packard (Palo Alto, CA) Model 5890 gas chromatograph connected to a Hewlett–Packard Series II integrator was used in this study. The operational conditions were as follows: stainless steel column (10 m long and 2.2 mm i.d.); liquid phase, 10% Carbowax 1000; solid support, chrom W-AW; particle size, 0.18 to 0.24 mm (mesh 60/80); column temperature, 45°C; injector and detector temperature, 105°C; carrier gas, He, 30 mL/min; H₂, 33 mL/min; and air, 320 mL/min. The detection limit was 0.5 µg Se/L for DMSe and 2 µg Se/L for DMDSe. Calibration of DMSe and DMDSe was performed by using a saturated standard Se gas after a series of dilutions. The total amount of the saturated DMSe and DMDSe gases was determined by alkali H₂O₂ trapping method and HGAAS measurements as described by Zhang and Frankenberger (2000).

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Selenium volatilization can permanently remove Se from Se-contaminated sites. Dimethyldiselenide is one of the most important organic volatile Se species in the environment (Amouroux and Donard, 1996; Amouroux et al., 2000; Chau et al., 1976; Fan et al., 1998; Jiang et al., 1983, 1989; Karlson and Frankenberger, 1988; Reamer and Zoller, 1980; Zhang and Frankenberger, 2000). Dimethyldiselenide has a much lower vapor pressure (0.38 kPa) than DMSe (32 kPa) (Karlson and Frankenberger, 1994). This characteristic reveals that DMDSe has a relatively longer retention time in soil, where it is formed. This long residence time leads to a series of reactions of DMDSe affected by its solubility, sorption, and transformations.

Solubility of volatile organic Se species in water is one of the most important properties of Se in the environment. Under an equilibrium condition, the concentration of DMSe in water is 17 times higher than DMSe in air (Karlson and Frankenberger, 1994). In this study, we found that a large amount of added DMDSe was dissolved in the autoclaved deionized water (Fig. 1) . After 24 h of incubation of DMDSe, the concentration of DMDSe in the autoclaved deionized water was much higher than that in the head space of the water sample, and increased with the volume ratios of the head space to the water. In the vial added with 0.5 mL of the autoclaved deionized water, the concentration of DMDSe in the water phase reached 3200 µg/L and the concentration of DMDSe in the head space was very low, about 115 µg/L. This partitioning of DMDSe in the water phase and the head space showed that DMDSe is highly dissolved in water.

View larger version (23K): [in this window] [in a new window]   Fig. 1. Solubility of dimethyldiselenide (DMDSe) in autoclaved deionized water after a 24-h incubation of DMDSe in an air–water system.

View larger version (22K): [in this window] [in a new window]   Fig. 2. Sorption of dimethyldiselenide (DMDSe) in an autoclaved soil after a 24-h incubation of DMDSe in an air–soil system.

View larger version (23K): [in this window] [in a new window]   Fig. 3. Formation of dimethylselenide (DMSe) along with a decrease of spiked dimethyldiselenide (DMDSe) measured in the head space samples. Dimethyldiselenide symbols (top figure): , autoclaved soil (10% moisture); , autoclaved soil (100% moisture); •, non-autoclaved soil (10% moisture); , non-autoclaved soil (100% moisture).

View larger version (33K): [in this window] [in a new window]   Fig. 4. Speciation of Se in the autoclaved and non-autoclaved soils. Black bars show the control soil without adding dimethyldiselenide (DMDSe). White bars show the soil spiked with DMDSe.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
The results from this study indicate that DMDSe is not stable in the soil environment. It can be dissolved in water, sorbed onto soil, and transformed into nonvolatile Se. In the surface layer of soil, DMDSe can be relatively easily emitted to the atmosphere. In the deep layer, a significant amount of DMDSe produced from soil can be demethylated to nonvolatile Se compounds. Conversion of DMDSe to DMSe can increase the production of DMSe in soil, suggesting that DMDSe may be one of the important precursors of DMSe in environment.

	ACKNOWLEDGMENTS

 
We thank Dr. Lei Guo for helpful discussion. This research was funded by the UC Salinity and Drainage Program.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 

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