Environ. Sci. Technol. 2000, 34, 776-783
Formation of Dimethylselenonium Compounds in Soil YIQIANG ZHANG AND WILLIAM T. FRANKENBERGER, JR.* Department of Environmental Sciences, University of California, Riverside, California 92521-0424
Selenium (Se) methylation/volatilization from Se-contaminated environments has been extensively studied over the last 15 years, but the methylation pathway of Se still remains largely unknown. We developed a sequential purgeand-trap system to collect volatile dimethylselenide (DMSe) and dimethyldiselenide (DMDSe) and to extract dimethylselenonium compounds. These dimethylselenonium compounds included dissolved DMSe and DMDSe, dimethylselenoxide (DMSeO), methylselenomethionine (MSemet), and dimethylselenoniopropionate (DMSeP). We monitored volatile DMSe and DMDSe and soluble dimethylselenonium compounds in a soil spiked with selenomethionine (Semet) at levels ranging from 16 to 80 µg of Se/g. In a moist soil spiked with Semet (80 µg of Se/ g), concentrations of volatile DMSe and DMDSe were much higher than those of dissolved DMSe and DMDSe, DMSeO, MSemet, and DMSeP, although all of these compounds followed a similar trend in concentration over the course of the experiment. Concentration of dissolved DMSe and DMDSe, DMSeO, MSemet, and DMSeP increased with soil moisture and Semet concentration. After a 15day experiment with a moist soil, about 65% of the spiked Semet was transformed to volatile DMSe and DMDSe, 26% was still in soluble forms of Se, and only 9% accumulated in the soil. This study suggests that Semet can undergo a methylation pathway to DMSe (Semet f MSemet f DMSeP f DMSe), and oxidation of dissolved DMSe to DMSeO may occur in soil.
Introduction Selenium methylation/volatilization is one of the most important processes of the biogeochemical cycling of Se in aqueous and terrestrial environments. Dimethylselenide (DMSe) is the major volatile Se species emitted to the air (1-4). Several methylation pathways from inorganic selenate (Se[VI]) and selenite (Se[IV]) to DMSe have been proposed (5-9). Some intermediates and/or precursors of methylation to DMSe are dimethylselenone (DMSeO2), methanselenol (MSel), selenomethionine (Semet), and methylselenomethionine (MSemet) (Table 1), but the methylation pathway of Se in the environment still remains largely unknown. Biological methylation of inorganic and organic sulfur (S) to dimethyl sulfide (DMS) has been extensively studied in aqueous environments (10). Dimethylsulfoniopropionate (DMSP) is the most important precursor of DMS (10). DMSP is a common osmolyte in marine organisms. It was detected in higher plants, macroalgae, phytoplankton, and cyano* Corresponding author phone: (909)787-3405; fax: (909)787-2954; e-mail:
[email protected]. 776
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bacteria and was measured in a salt marsh sediment and marine micromat as well as in oceanic and coastal waters. On the other hand, not all DMS is emitted to the atmosphere. Some DMS in aqueous systems can be oxidized to dimethyl sulfoxide (DMSO) (11), which was detected in marine waters and freshwaters (12). On the basis of the similarities between S and Se chemistry and biochemistry, dimethylselenoniopropionate (DMSeP) can exist as a precursor of DMSe (13) and dimethylselenoxide (DMSeO) can exist as a main oxidation product of DMSe in aqueous and terrestrial environments (14). In a recent study, Ansede et al. (15) reported that DMSeP existed in a salt marsh plant. Selenomethionine, which has been found in soils, sediment, plants, and algae, is one of the important Se amino acids in the aqueous and terrestrial environments (16-20). Semet is highly soluble in water. Doran and Alexander (21), Frankenberger and Karlson (22), and Martens and Suarez (23) all found that a significant amount of spiked Semet in soil was transformed to DMSe emitted to the air. Therefore, Semet is an important intermediate of Se methylation to DMSe. Soluble dimethylselenonium compounds in environmental samples can be measured based on their abilities to release DMSe in different chemical solutions. DMSeO can be reduced to DMSe in a reducing solution such as sodium borohydride (NaBH4) (6, 14, 24). In an alkaline solution, DMSeP can be cleaved into DMSe and acrylate at room temperature (13, 15), and at high temperatures (105-110 °C), MSemet can be converted to DMSe (8, 15). In this study, we developed a sequential purge-and-trap method to separate dimethylselenonium compounds such as volatile DMSe and DMDSe, dissolved DMSe and DMDSe, DMSeO, MSemet, and DMSeP and used an alkaline hydrogen peroxide (H2O2) solution to trap released DMSe and DMDSe. With this method, we determined the concentrations of soluble dimethylselenonium compounds in a soil treated with Semet.
Materials and Methods Chemicals. Seleno-DL-methionine and sodium selenate (Na2SeO4) were purchased from Sigma (St. Louis, MO), and DMSe (99%) and DMDSe (98%) were purchased from Strem Chemical. DMSeO was provided by Dr. Howard E. Ganther (University of Wisconsin). DMSeO2 was provided by Dr. Robert S. Dungan (University of California at Riverside). Acrylic acid was purchased from Aldrich Chemical Co. (Milwaukee, MI). Se[IV] standard solution (1000 µg/mL), NaBH4, and other chemicals were purchased from Fisher Scientific (Pittsburgh, PA). DMSeS was synthesized by the reaction of DMDS and DMDSe in an acidic condition with the presence of zinc (25). DMSeP was synthesized by a method similar to the one used by Ansede and Yoch (13), in which 0.8 mL of acrylic acid was added to 2 mL of DMSe in a 25-mL glass tube, followed by purging with HCl gas through the mixture for about 2 min. After the mixture turned into a heavy viscous solution, 10 mL of absolute ethanol was added into the tube to wash the solution. After that, a small vacuum was used to dry the heavy viscous solution until a white precipitate formed. Part of the white precipitate was dissolved in the deionized water, followed by purging with pure N2 for 1 h to remove unreacted DMSe. The final DMSeP solution was stored in a freezer at -20 °C. This solution was determined to be about 97% pure by the method described below. Experiments. The soil used in this study was collected from the Tulare Basin, CA, in 1997. The soil had the following properties: pH 7.50; soluble Se, 0.011 µg/g (selenite 16 h) 2) 1-2 M HCl + 4 mL of 5% NaBH4 (0.5 h) 1) 1-2 M HCl + 4 mL of 5% NaBH4 (0.5 h) 2) 3-4 M NaOH (>16 h) 1) 3-4 M NaOH (>16 h) 2) 1-2 M HCl + 4 mL of 5% NaBH4 (0.5 h) 1) 1-2 M HCl + 4 mL of 5% NaBH4 (0.5 h) 2) 3-4 M NaOH (>16 h) 3-4 M NaOH (> heating at boiling water, 2 h)
recovery of DMSeO (%) 94.7 ( 2.31a 1.80 ( 0.317 96.0 ( 3.08 94.2 ( 1.27
recovery of DMSeP (%)
recovery of MSemet (%)
BDLb 95.6 ( 3.10 94.9 ( 3.42 98.0 ( 1.49
30.4 ( 4.42
Below the detection limit.
FIGURE 5. Procedure to extract dimethylselenonium compounds in a deionized water spiked with dimethylselenonium compounds and a soil water extract. solution, recovery of DMSe released from DMSeO was below the detection limit. When this basic solution was adjusted to 1 M HCl with 6 M HCl, followed by addition of 4 mL of 1 M NaBH4, recovery of DMSeO was 95%. It should be noted that Se[IV] can release H2Se gas during a reaction with NaBH4 in an acid solution, which can be partially trapped in an alkaline H2O2 solution (data not shown). Therefore, Se[IV] 780
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must be removed from the soil solution with an anionexchange resin column before extraction of DMSeO (14). When the Se [IV] concentration in soil solution is extremely low (100 °C). However, DMSeO in a NaOH solution at a high temperature was unstable (Table 2). About 30% of DMSeO was lost upon 2-h heating in a boiling water bath. Therefore, if DMSeO is present, it must be removed from the soil solution before the extraction of MSemet. Figure 5 provides the sequential procedure developed to extract soluble dimethylselenonium compounds (dissolved DMSe and DMDSe, DMSeO, MSemet, and DMSeP) formed from spiked Semet in soil. Two methylated Se compounds (MSecys and TMSe) can also release volatile Se compounds at high temperatures (8, 15). Therefore, we performed a test in which a soil water extract was exposed to the basic solution at 21 and 97 °C. We did not detect DMDSe, which can be released from MSecys in a basic solution at high temperatures (8), and only detected DMSe (data not shown). Also, we tested whether trimethylselenonium ion exists in the soil water extract by using a cation-exchange resin (29). We did not detect TMSe. Therefore, in this study, the dimethylselenonium compounds most likely in the soil water extract were DMSeP and MSemet, which released DMSe during the reaction with a basic solution at room (21 °C) and high (97 °C) temperatures, respectively. Formation of Dimethyselenonium Compounds in Soil. Bottino et al. (30) proposed a methylation pathway of Se from selenite to DMSe and DMDSe and found that MSemet is a precursor of DMSe and DMDSe. Hanson and Gage (31) found that DMSP in a plant originates from methionine via a series of processes that involves methylation of methionine, followed by deamination and decarboxylation to DMSPaldehyde, then via oxidation to DMSP. In this study, we detected significant amounts of MSemet and DMSeP and two volatile Se compounds in soil treated with Semet (Figures 6-8). Based on the similarity between S and Se and Bottino’s methylation pathway of Se, the methylation pathway from Semet to DMSe and DMDSe in soil is proposed as follows:
Formation of MSemet and DMSeP in a moist (33%) soil spiked with Semet at reaction time intervals is shown in Figure 6A. At day 1 of the experiment, concentrations of MSemet and DMSeP formed were 0.12 and 0.53 µg/g, respectively,
FIGURE 6. Formation of dimethylselenonium compounds in a moist (33%) soil treated with Semet (80 µg of Se/g). Error bars show (1 SD. (A) Relationship between reaction time and formation of dimethylselenonium compounds. (B) Relationship between reaction time and volatilization rate of DMSe and DMDSe. (C) Relationship between reaction time, total soluble Se, and cumulated production of total volatilized DMSe and DMDSe. and then increased to 0.59 and 0.82 µg/g with time. At day 5-7, concentrations of MSemet and DMSeP decreased with time. At the final day (day 15) of the experiment, concentrations of MSemet and DMSeP declined to 0.23 and 0.36 µg/g. In contrast, the amount of DMSe and DMDSe formed in the soil was about 2-5 times higher than the sum of MSemet and DMSeP, although they all showed a similar trend of production over the course of the experiment (Figure 6A,B). Concentration of total soluble Se in the soil decreased from 80 µg of Se/g at starting time of the experiment to 21 µg of Se/g at the end of the experiment (Figure 6C). In contrast, the amount of the cumulative production of total volatilized DMSe and DMDSe increased from 4.1 to 51.7 µg/g over the course of the experiment. From day 1 to day 15, the sum of the amount of soluble Se and the amount of the cumulative production of total volatilized DMSe and DMDSe was very close, ranging from 70.4 to 74.9 µg/g. At the final day of the experiment, about 65% of spiked Semet was transformed to volatile DMSe and DMDSe, 26% was still in soluble forms of VOL. 34, NO. 5, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 7. Formation of dimethylselenonium compounds in a soil treated with Semet (80 µg of Se/g) and with different moisture regimes. Error bars show (1 SD. (A) Relationship between soil moisture and formation of dimethylselenonium compounds at day 3. (B) Relationship between reaction time and volatilization rate of DMSe and DMDSe.
FIGURE 8. Formation of dimethylselenonium compounds in a moist (33%) soil treated with Semet ranging from 16 to 80 µg of Se/g. Error bars show (1 SD. (A) Relationship between Semet concentrations and formation of dimethylselenonium compounds at day 3. (B) Relationship between reaction time and volatilization rate of DMSe and DMDSe. Se, and only 9% accumulated in the soil. These results may indicate that the transformation process from Semet to DMSe and DMDSe mainly occurs in soil solution and/or at the interface of the soil and the soil solution due to the high solubility of Semet. Soil moisture positively affected the formation of MSemet and DMSeP in a soil spiked with Semet (80 µg of Se/g) (Figure 7). In the 15% moisture soil, concentrations of MSemet and DMSeP were 0.34 and 0.93 µg/g, respectively. In contrast, these values were 0.71 and 1.53 µg/g in the 100% saturated soil. The volatilization rate of DMSe and DMDSe was higher in the 15% moisture soil than in 100% saturated soil. This may be caused by the relatively high diffusion rate of DMSe and DMDSe in the 15% moisture soil because of higher porosity in which DMSe and DMDSe can pass through. During the 3-day experiment, the amount of the cumulative production of total volatilized DMSe and DMDSe from soil was 16.98, 16.72, 13.3, and 11.1 µg/g from a 15, 33, 50, and 100% moisture regime, respectively. Formation of MSemet and DMSeP was also related to the concentration of Semet in a moist (33%) soil (Figure 8). Concentrations of MSemet and DMSeP produced were about 3 times higher in a treated 80 µg of Semet-Se/g soil than in a 16 µg of Semet-Se/g soil. During the 3-day experiment, the amounts of the cumulative production of total volatilized DMSe and DMDSe were 4.87, 9.92, and 16.5 µg/g from a 16, 32, and 80 µg of Semet-Se/g soil, respectively. These results indicated that MSemet and DMSeP did not accumulate with time in soil. MSemet and DMSeP were not 782
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the stable forms of organic Se in soil. When formed in the soil, MSemet and DMSeP were readily converted to DMSe and DMDSe through a transformation process proposed above. Semet also was unstable in the soil. Martens and Suarez (23) reported that 40-80% of spiked Semet ranging from 5 to 50 µg of Se/g in California soils was transferred to volatile Se during a 7-day experiment. In this study, we found that about 65% of spiked Semet was transferred to DMSe and DMDSe during 15 days of the experiment. This rapid loss of Semet to DMSe and DMDSe suggests that the transfer of Semet to DMSe and DMDSe is not a limiting factor in the pathway of Se methylation in soil environments. Therefore, to enhance removal of Se from contaminated soil by Se methylation/volatilization, accelerating microbial assimilation of inorganic Se to Semet in soil is needed. Not all DMSe formed was volatilized to the atmosphere. DMSe has a solubility of 24.4 mg/g in water (28) and can be partially trapped in water. When the retention time of DMSe in soil solution is long enough, oxidants in soil such as manganese oxides can act on trapped DMSe, which results in the formation of oxidized methylated Se (14). In this study, we detected trapped DMSe and DMSeO in soil solution. Concentrations of dissolved DMSe and DMSeO increased with increasing soil moisture and Semet concentration. Concentrations of DMSeO decreased with dissolved DMSe. As with MSemet and DMSeP, dissolved DMSe and DMSeO did not accumulate in the soil with time. The decrease in concentrations of dissolved DMSe and DMSeO indicated that these compounds were not stable in this aerobic soil system.
Dissolved DMSe can be either oxidized to DMSeO or volatilized to the atmosphere. DMSeO can be re-reduced back to DMSe, which can emit to the air (data not shown) or may be further mineralized to inorganic Se. Ansede et al. (15) detected MSemet and DMSeP in a salt marsh plant. In a recent study on Se volatilization from a filamentous cyanophyte-dominant mat, Fan et al. (8) found that one possible precursor of DMSe was MSemet. Cooke and Bruland (6) also found MSemet and oxidized methylated Se in an aqueous system. Masscheleyn et al. (24) extracted oxidized methylated Se from a sediment. In this study, we detected DMSeO, MSemet, and DMSeP in soil upon treatment with Semet. All these findings suggest that DMSeO, MSemet, and DMSeP may exist in various environments. Because of their instability and low concentration, DMSeO, MSemet, and DMSeP in environmental samples may not be detected by commonly used methods for extracting different Se species. For example, longer shaking time (24 h) during extraction of Se species or extraction with NaOH solution may cause significant loss of these dimethylselenonium compounds. Therefore, to prevent the loss of dimethylselenonium compounds and to better understand the biogeochemical methylation pathway of Se in natural environments, a procedure including a short-time treatment of environmental samples and a direct measurement of these dimethylselenonium compounds should be developed in the future.
Acknowledgments We thank Dr. Howard E. Ganther and Robert S. Dungan for providing DMSeO and DMSeO2. 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.
Literature Cited (1) Chau, Y. K.; Wong, P. T. S.; Silverberg, B. A.; Luxon, P. L.; Bengert, G. A. Science 1976, 192, 1130. (2) Doran, J. W.; Alexander, M. Soil Sci. Soc. Am. J. 1976, 40, 687. (3) Karlson, U.; Frankenberger, W. T., Jr. Soil Sci. Soc. Am. J. 1988, 52, 678. (4) Thompson-Eagle, E. T.; Frankenberger, W. T., Jr.; Karlson, U. Appl. Environ. Microbiol. 1989, 55, 1406. (5) Challenger, F. Chem. Rev. 1945, 36, 315. (6) Cooke, T. D.; Bruland, K. W. Environ. Sci. Technol. 1987, 21, 1214. (7) Doran, J. W.; Alexander, M. Adv. Microb. Ecol. 1982, 6, 1.
(8) Fan, T. W.-M.; Higashi, R. M.; Lane, A. N. Environ. Sci. Technol. 1998, 32, 3185. (9) Reamer, D. C.; Zoller, W. H. Science 1980, 208, 500. (10) Kiene, R. P.; Visscher, P. T.; Keller, M. D.; Kirst, G. O. Biological and environmental chemistry of DMSP and related sulfonium compounds; Plenum Press: New York and London, 1996. (11) Taylor, B. F.; Kiene, R. P. In Biogenic sulfur in the environment; Saltzman, E. S., Cooper, W. J., Eds.; ACS Symposium Series 393; American Chemical Society: Washington, DC, 1989; p 202. (12) Lee, P. A.; De Mora, S. J. In Biological and environmental chemistry of DMSP and related sulfonium compounds; Kiene, R. P., Visscher, P. T., Keller, M. D., Kirst, G. O., Eds.; Plenum Press: New York and London, 1996; p 391. (13) Ansede, J. H.; Yoch, D. C. FEMS Microbiol. Ecol. 1997, 23, 315. (14) Zhang, Y. Q.; Frankenberger, W. T., Jr.; Moore, J. N. Environ. Sci. Technol. 1999, 33, 3415. (15) Ansede, J. H.; Pellechia, P. J.; Yoch, D. C. Environ. Sci. Technol. 1999, 33, 2064. (16) Abrams, M. M.; Burau, R. G. Commun. Soil Sci. Plant Anal. 1989, 20, 221. (17) Fan, T. W.-M.; Lane, A. N.; Higashi, R. M. Environ. Sci. Technol. 1997, 31, 569. (18) Guo, X.; Wu, L. Environ. Toxicol. Chem. 1997, 16, 491. (19) Rael, R. M.; Frankenberger, W. T., Jr. Soil Biol. Biochem. 1995, 27, 241. (20) Sharmasarkar, S.; Vance, G. F. Environ. Geol. 1997, 29, 202. (21) Doran, J. W.; Alexander, M. Appl. Environ. Microbiol. 1977, 33, 31. (22) Frankenberger, W. T., Jr.; Karlson, U. Soil Sci. Soc. Am. J. 1989, 53, 1435. (23) Martens, D. A.; Suarez, D. L. Soil Sci. Soc. Am. J. 1997, 61, 1685. (24) Masscheleyn, P. H.; Delaune, R. D.; Patrick, W. H. J. Spectrosc. Lett. 1991, 24, 307. (25) Chasteen, T. G. Appl. Organomet. Chem. 1993, 7, 335. (26) Zhang, Y. Q.; Moore, J. N.; Frankenberger, W. T., Jr. Sci. Total Environ. 1999, 229, 183. (27) Zhang, Y. Q.; Moore, J. N.; Frankenberger, W. T., Jr. Environ. Sci. Technol. 1999, 33, 1652. (28) Karlson, U.; Frankenberger, W. T., Jr. J. Chem. Eng. Data 1994, 39, 608. (29) Yamada, H.; Miyamura, T.; Yasuda, A.; Hattori, T.; Yonebayashi, K. Soil Sci. Plant Nutr. 1994, 40, 49. (30) Bottino, N. R.; Banks, C. H.; Irgolic, K. J.; Micks, P.; Wheeler, A. E.; Ralph, A. Phytochemistry 1984, 23, 2445. (31) Hanson, A. D.; Gage, D. A. In Biological and environmental chemistry of DMSP and related sulfonium compounds; Kiene, R. P., Visscher, P. T., Keller, M. D., Kirst, G. O., Eds.; Plenum Press: New York and London, 1996; p 75.
Received for review August 16, 1999. Revised manuscript received December 6, 1999. Accepted December 8, 1999. ES990958Y
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