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Journal of Environmental Quality 31:2104-2110 (2002)
© 2002 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America

TECHNICAL REPORTS
Wetlands and Aquatic Processes

Evaluation of the Macroalga, Muskgrass, for the Phytoremediation of Selenium-Contaminated Agricultural Drainage Water by Microcosms

Z.-Q. Lina, M. de Souzaa, I. J. Pickeringb and N. Terry*,a

a Dep. of Plant and Microbial Biology, University of California, 111 Koshland Hall, Berkeley, CA 94720-3102
b Stanford Synchrotron Radiation Laboratory, Stanford Linear Accelerator Center, MS 69, 2575 Sand Hill Road, Menlo Park, CA 94025-7015

* Corresponding author (nterry{at}nature.berkeley.edu)

Received for publication September 10, 2001.

   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Previous field studies suggested that the macroalga, muskgrass (Chara canescens Desv. & Lois), plays an important role in the removal of selenium (Se) from agricultural drainage water. This study evaluated the efficiency of Se removal from drainage water by muskgrass-vegetated wetland microcosms, and determined the extent to which muskgrass removed Se through phytoextraction and biovolatilization. Six flow-through wetland microcosms were continuously supplied with drainage water containing an average Se concentration of 22 µg L-1 over a 24-d experimental period. The Se mass input and outflow and the rate of Se volatilization were monitored daily for each microcosm. Three microcosms containing muskgrass reduced the daily mass Se input in the inflow drainage water by 72.1%; this compared with a reduction of 50.6% of the mass Se input for three unvegetated control microcosms. Selenium accumulated in muskgrass tissues accounted for 1.9% of the total mass Se input in the microcosm, followed by 0.5% via biological volatilization. The low rates of Se volatilization from selenate-supplied muskgrass, which were 10-fold less than from selenite, were probably due to a major rate limitation in the reduction of selenate to organic forms of Se in muskgrass. This conclusion was derived from X-ray absorption spectroscopy speciation analysis, which showed that muskgrass treated with selenite contained 91% of the total Se in organic forms (selenoethers and diselenides), compared with 47% in muskgrass treated with selenate.


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
SELENIUM (Se) contamination of agricultural drainage water is one of the most serious environmental problems confronting California agriculture. Soils on the western side of the San Joaquin Valley are derived from Cretaceous shale rocks that contain high levels of Se (McNeal and Balisteri, 1989). As these soils are irrigated, Se-contaminated drainage water is collected and eventually channeled into evaporation ponds. With evaporation and continual loading of these ponds, Se and other trace elements may build up to high toxic concentrations and pose a potential threat to wildlife (Ong et al., 1997; Ohlendorf et al., 1986).

Selenium may be removed from Se-contaminated waters with vegetated flow-through constructed wetlands, the biological and physical components of which interact to provide a mechanical and biogeochemical filter (Mitsch and Gosselink, 1993). Hansen et al. (1998) reported that 89% of the Se entering the Chevron constructed wetland (Richmond, CA), via contaminated oil refinery effluent, was removed daily. It was estimated that about 10 to 30% of the total Se retained daily by the wetland may be removed through biological volatilization. A major advantage of Se volatilization is that inorganic Se is taken up by plants and metabolized into volatile forms of Se such as dimethyl selenide (DMSe) (Terry and Zayed, 1994), which is about 600 times less toxic than inorganic forms of Se (Wilber, 1980; Ganther et al., 1966; McConnell and Portman, 1952). Because Se volatilization removes Se from contaminated waters harmlessly into the atmosphere (CH2M Hill, 1988; Lin et al., 2000), biological volatilization of Se represents a novel and environment-friendly process for the bioremediation of Se-contaminated drainage water (Frankenberger and Karlson, 1989; Zayed et al., 2000).

The submerged macroalga, muskgrass, is an effective plant species for the phytoremediation of metal-contaminated wastewater. This conclusion originated from the following observations. (i) Muskgrass accumulates extremely high concentrations of metal elements in its tissues. Our previous research at the Allegheny Power Service constructed wetland in Springdale, PA, showed that muskgrass was able to extract iron (Fe), manganese (Mn), and nickel (Ni) from coal-ash leachate at concentrations that were 10-fold greater than the concentrations observed in vascular plants such as cattail (Typha latifolia L.) (e.g., 23 900 mg Mn kg-1 and 300 mg Ni kg-1 in muskgrass, compared with 2000 mg Mn kg-1 and 16 mg Ni kg-1 in cattail) (Ye et al., 2001). (ii) Muskgrass produces a large biomass under field conditions, and, therefore, has a great surface area contacting the wastewater. For example, biomasses of 349 g dry weight per m2 for C. fibrosa (Herrera-Silveira, 1994) and 600 g dry weight per m2 for C. hornemannii (Carneiro et al., 1994) have been recorded. Such levels of biomass production observed for Chara species are comparable with the aboveground biomass of baltic rush (Juncus balticus Willd.) or cordgrass (Spartina alterniflora Loisel.) growing in the Corcoran constructed wetland, which is currently being used for the remediation of Se-contaminated agricultural drainage water (Terry, 1998). (iii) Muskgrass was found to grow throughout the year in the Allegheny Power Service constructed wetlands, including the winter months when most vascular wetland plants become senescent in the field (Ye et al., 2001). Therefore, the addition of muskgrass might substantially enhance the phytoremediation efficiency of contaminants by constructed wetlands in the winter season.

Muskgrass has great potential to be an important candidate for the phytoremediation of Se-contaminated agricultural drainage water in the San Joaquin Valley of central California. As a native species, muskgrass dominated the aquatic community in the deeper waters at Kesterson Reservoir while the system was run as a drainage storage facility (Horne, 1991). An early 0.4-ha mesocosm study conducted at the Kesterson Reservoir by Horne (1991) showed that the soluble Se concentration in the water column dominated by muskgrass changed from 65 to <10 µg L-1 in the mesocosm over a 6-mo study period. Horne (2000) concluded that a mixture of muskgrass and its associated microbial community of bacteria, microalgae, fungi, protozoans, and rotifers was able to immobilize high concentrations of Se, thereby removing Se from the water body into the sediments at the Kesterson Reservoir. It is not clear, however, what proportion of the mass Se removed from the water column was accumulated in muskgrass tissues, and what percentage of the Se was volatilized from muskgrass into the atmosphere. We hypothesized that muskgrass might have a great capacity to volatilize Se and that biological volatilization is responsible for the substantial decline of Se concentrations in young muskgrass tissues, from 51 to 15 mg kg-1, over a 6-mo study period (Horne, 1991). This hypothesis was based on the fact that muskgrass species produce a strong odor of sulfurated compounds (Langangen, 1993), and that sulfate and selenate have a similar assimilation pathway in higher plants (Terry et al., 2000), suggesting that muskgrass might also have great capability to volatilize Se. Thus, the specific objectives of this study were to (i) determine the Se phytoextraction potential of muskgrass (i.e., accumulation of Se in tissues), (ii) determine the extent to which Se was attenuated through biological volatilization by muskgrass microcosms, and (iii) evaluate whether the addition of muskgrass to wetland microcosms substantially improves their efficiency in removing Se from drainage waters. In addition, physiological experiments on Se speciation and the kinetics of Se accumulation and volatilization were also conducted to obtain insight into the factors that control phytoextraction and volatilization of Se by muskgrass.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Flow-Through Wetland Microcosms
Flow-through wetland microcosms were used to simulate constructed wetlands for the remediation of Se-laden agricultural drainage water. Six microcosms were constructed, each with dimensions of 0.76-m long, 0.76-m wide, and 0.2-m deep. The microcosms had a surface area of 0.58 m2 and a volume of 0.12 m3. A Plexiglas barrier was placed vertically down the center of each microcosm to maximize the path length of the water flow through the microcosm. The opening for outflow water was positioned at 18 cm above the bottom. Each microcosm was filled with 95 kg of soil from the Corcoran constructed wetland location (15% water content), which formed a final 8-cm depth of sediment and a standing water depth of approximately 10 cm in the microcosm. The 8-cm sediment depth conformed with our other field study at the Corcoran wetland, showing that Se was mainly accumulated in the top 5 cm of sediments during the first year of receiving Se-laden drainage water (Lin and Terry, 2000). The experimental soil contained a background total Se concentration of approximately 0.3 mg kg-1, mainly in the form of selenate (Presser, 1994).

Three microcosms were randomly selected to receive 1.5 kg fresh muskgrass, and the other three microcosms without muskgrass were used as controls. The muskgrass was collected from the Allegheny Power Service constructed wetland, and was propagated for one month in plastic trays containing a mixture of the original wetland sediments and distilled water. The microcosms were maintained in a temperature-controlled greenhouse on the University of California, Berkeley campus. This flow-through microcosm experiment was terminated 24 d after inception when the rate of Se volatilization from the microcosms began to decrease with time and the accumulation of Se in muskgrass tissues did not increase with time in the drainage water.

Drainage Water and Water Flow Management
Agricultural drainage water was collected from the inlet of the Corcoran constructed wetland that was built in 1996 to treat Se-contaminated agricultural tile-drainage water from the Tulare Lake Drainage District (TLDD, approximately 375 km away from Berkeley) in central California (Terry, 1998). The drainage water was transported with plastic containers, which were stored at a temperature of 4°C in the greenhouse. The original drainage water had a pH of 8.5, an electrical conductivity of 9 dS m-1, a sulfate (SO42-) concentration of 868 ± 39 mg L-1, and a total Se concentration of approximately 11 to 15 µg L-1 (92% selenate; see Gao et al., 2000). The drainage water collected in fall 1999 had a relatively low Se concentration compared with the average Se concentration (approximately 20 µg L -1) in the inflow drainage water observed in the previous study years (Terry, 1998; Lin and Terry, 2000), so additional Se (Na2SeO4) was spiked into the original agricultural drainage water. Because a large quantity of drainage water was required, several Se-spiked water reservoirs were prepared, and the Se concentration in each reservoir was determined accordingly. The final Se concentration in inflow drainage water used in this experiment was approximately 20 µg L -1.

Each microcosm was initially filled with 65 L drainage water (25 µg Se L -1) before the surface flow-through was started. The Se concentration in the standing water was approximately 20 µg L -1 2 d after the initial fill-up. Drainage water was supplied continuously to the inlet of each microcosm by a peristaltic pump. The inlet flow rate was monitored daily throughout the experimental period to ensure that a steady flow rate of about 4 L per day was achieved. Water from the outlet of each microcosm was collected in separate containers. The average daily inflow and outflow rates of drainage water to the microcosms were 4.0 ± 0.2 and 3.3 ± 0.3 L d-1, respectively. The drainage water had an average residence time of approximately 12 d in the muskgrass microcosms, which was calculated from T = V{epsilon}/Q (Kadlec, 1989), where V is the water volume above sediment in the microcosm (approximately 58 L), {epsilon} is the porosity correction for water volume occupied by submerged muskgrass tissue, (approximately 0.85), and Q is the average of inflow and outflow rates (approximately 4 L d-1).

Measurements of Selenium Volatilization
Volatile Se was collected with an open-flow sampling chamber system (Lin et al., 1999). The dimensions of the Plexiglas collection chamber were 0.71 m long, 0.71 m wide and 0.76 m high; the chamber provided an internal volume of 0.38 m3 and enclosed a bottom area of 0.5 m2. The sampling chamber was fitted to the top of each microcosm. Each chamber was attached to three 500-mL gas-washing bottles that were connected in series, and each contained 200 mL of the alkaline-peroxide trap solution (0.05 M NaOH and 30% H2O2 at a volume ratio of 4:1). The trap solution was changed daily and analyzed for the total Se to calculate the flux (µg Se m-2 d-1) of volatile Se from each microcosm.

The water temperature inside the sampling chambers was 23.5 ± 2.7°C during the study period. Each sample of volatile Se was continuously collected for 24 h at an airflow rate of 0.36 m-3 h-1. The Se in the trap solution was oxidized to Se(VI) in 15% H2O2 and then reduced to Se(IV) in 6 M HCl at 91°C for 30 min (Brimmer et al., 1987), prior to Se analysis by atomic absorption spectrophotometry (Varian [Mulgrave, VIC, Australia] SpectrAA 220 FS) with a hydride generator (Varian VGA-77) (Ward and Gray, 1996).

This chamber sampling technique was calibrated with a dimethyl selenide standard for optimal sampling conditions, including sampling duration, airflow rate, and trapping capacity of the alkaline-peroxide extraction solution. Since the sampling chamber did not distinguish between the Se volatilized by muskgrass and the Se volatilized by microbes, the rate of Se volatilization determined in the microcosm is therefore referred to as biological volatilization.

Water, Sediment, and Plant Sampling
Inlet and outlet water samples were collected daily from each microcosm. Four samples of the surface water overlaying the sediments were also collected from each microcosm to determine the total Se mass remaining in the standing water at the end of the experiment. The pH and electrical conductivity in the inlet and outlet water samples collected daily from each microcosm were measured immediately after collection with a Corning Checkmate modular system (Fisher Scientific, Pittsburgh, PA). Water samples were stored at 4°C until they were analyzed for total Se. Four sediment core samples were collected with an AMS (American Falls, ID) sediment recovery probe with replaceable 5- x 15-cm butyrate liners from each microcosm at the beginning and at the end of the experiment, respectively. The sediment core was then divided into 0- to 4- and 4- to 8-cm layers. The top sediment contained little muskgrass detritus on the top surface. The muskgrass was completely removed from the microcosms at the end of the experiment and immediately rinsed with distilled deionized water. The fresh weight of muskgrass from each microcosm was recorded, and samples were dried in an enforced ventilation drying room at 50°C for two days to a constant weight (Carlson et al., 1991), after which the dry biomass was measured. The propagated muskgrass samples were analyzed prior to the experiment to determine the background value of Se in muskgrass tissues.

Kinetics of Selenium Accumulation in Muskgrass and Volatilization from Selenite versus Selenate
Muskgrass (12 g fresh) was immersed in 300 mL of 30 µg Se L -1 agricultural drainage water (the same water used in microcosms) in plastic containers in the greenhouse. Five of the 40 treated muskgrass containers (i.e., five replicates of each sampling interval) were randomly selected every 24 h after the addition of muskgrass in the drainage water. The muskgrass tissues were rinsed with deionized distilled water prior to drying in an oven at 50°C. The Se concentration in the rinsing solution and the volume of the rinsing solution were measured to determine the amount of Se adsorbed onto the surface of muskgrass. The change in biomass production of muskgrass was monitored daily for an 8-d study period.

Selenium volatilization was measured with alkaline-peroxide traps as described above. Muskgrass were submerged in 200 mL of 1/8-strength Hoagland solution in Magenta (plastic) boxes (7 x 7 x 9 cm; Sigma, St. Louis, MO), and treated with 1.58 µg Se L -1 Na2SeO4 or Na2SeO3 for 5 d in the greenhouse. Muskgrass Magenta boxes were put in gas-tight acrylic volatilization chambers (approximately 3 L in volume). A continuous airflow (1.5 L min-1) was passed through the chamber by applying suction at the outlet, while the incoming air was bubbled into the Se-treated muskgrass and hydroponic solution. All connections between the chamber and the glass trap were made with Teflon tubing. The alkaline peroxide solution used to trap volatile Se was changed every 24 h and kept at 4°C until analysis.

Total Selenium Analysis
The total Se concentrations in the water, sediment, plant samples, and the volatilization trap solutions were measured by atomic absorption spectrophotometry. The sediment samples were air-dried and then passed through a 2-mm sieve to remove gravel and coarse debris (Carlson et al., 1991). Samples were ground and wet-digested for total Se analysis. Surface water and finely ground sediment samples were digested with HNO3 and H2O2, and total Se concentrations were determined following USEPA Methods 3050B (USEPA, 1996) and 7742 (USEPA, 1994). The dried plant tissues were ground to pass a 0.4-mm mesh screen with a Wiley stainless steel mill in preparation for chemical analysis. The total Se in plant tissues was analyzed according to the method reported by Bañuelos and Pflaum (1990). Standard reference materials SRM-2709 and SRM-1567a (National Institute of Standards and Technology) were used as internal quality controls for analyses of Se in soil and plant samples.

Selenium Speciation by X-Ray Absorption Spectroscopy
Muskgrass plants were treated with 1 mg Se L-1 selenate (Na2SeO4) or selenite (Na2SeO3) for 5 d in 500-mL plastic boxes. The samples were rinsed with deionized water, frozen in liquid nitrogen, ground to a fine texture, and then stored at -80°C for analysis of the chemical forms of Se in muskgrass tissues with X-ray absorption spectroscopy. Frozen plant specimens were carefully packed into 2-mm-path-length lucite sample holders with mylar tape for windows and kept under liquid nitrogen (Lee et al., 2001).

X-ray absorption spectroscopy analysis was conducted at beam line 4-3 of the Stanford Synchrotron Radiation Laboratory (SSRL). A Si(220) double crystal monochromator was used with an upstream vertical aperture of 1 mm, and harmonic rejection was achieved by detuning one crystal by 50%. The source electron energy was 3.0 GeV with a current ranging from 60 to 100 mA. Samples were positioned at a 45° angle to the X-ray beam and were maintained at 15 K in a flowing liquid helium cryostat. X-ray absorption spectra were collected by monitoring the Se K{alpha} fluorescence with a Canberra (Meriden, CT) 13-element Ge detector, in a series of replicate scans. Spectra were also collected for standard materials of Se, and energy was calibrated by using the spectrum of hexagonal Se(0), the first energy inflection of which is assumed to be 12658.0 eV. Data were collected and analyzed with the program suites XAS-Collect (George, 2000) and EXAFSPAK (Stanford Synchrotron Radiation Laboratory, 2002), respectively.

Quantitative analysis was performed with an edge-fitting method (Pickering et al., 1995), in which the normalized edge spectrum of a sample containing unknown Se species is fit to a linear combination of the spectra of standard Se compounds by using a least-squares minimization procedure. The fractional contribution of a standard spectrum to the fit is equivalent to the fractional abundance of Se in that chemical species in a sample. In the present work, spectra were fit to 10 mM solutions of selenate, selenite, selenomethionine (SeMet), and selenocysteine (RSeSeR), chosen to be representative of potential Se species present (Pickering et al., 1999).

Statistical Analysis
Statistical analyses were performed with the statistical analysis system (SAS). Multiple comparisons among different means were conducted by PROC GLM with the Duncan option, and correlation analyses were performed by PROC CORR (SAS Institute, 1988).


   RESULTS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Removal of Selenium in Drainage Water by Muskgrass Microcosms
The mass of Se removed daily by each microcosm was monitored as the difference between the daily masses of Se entering and leaving the microcosm. The daily mass removal rates (%) for the microcosms became stabilized about 12 d after the introduction of drainage water into the microcosms, a time period roughly equal to the average time it would take to replace the initial drainage water in the microcosm (i.e., 12-d residence time) (Fig. 1) . The efficiency of Se removal increased more rapidly with time in the muskgrass microcosm than in the unvegetated microcosm, and the daily Se removal efficiency became constant with time after the 12th day. Averaging the efficiency of mass Se removal over the last 12 d of the experimental period, 72.1% of the mass Se was removed from the inflow drainage water by the muskgrass microcosm daily; this compared with 50.6% by the unvegetated control microcosms. Over the same time period, the microcosms also significantly (P < 0.05) reduced the Se concentration in the drainage water, by 39.8% in the muskgrass microcosm and 28.9% in the unvegetated microcosm (Table 1).



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  		Fig. 1. Percentage of Se mass removed daily from the Se input by the wetland microcosms during the study period. Data shown are means of three replicates.

 

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  		Table 1. Selenium concentrations in the inflow and outflow waters during the last 12 d of the study period (after the treatment system was stabilized).{dagger}

 
In addition to removing Se from the drainage water, our electrical conductivity results showed that the muskgrass microcosms removed other dissolved salts in the drainage water as well. After passing through the muskgrass microcosms, the outlet water electrical conductivity was significantly (P < 0.05) lower than that from the unvegetated microcosms, ranging from 8.6 ± 0.1 to 9.8 ± 0.4 dS m-1, compared with an average of 9.2 dS m-1 in the original experimental drainage water. This might be partially due to the precipitation of calcium carbonate on the surface of muskgrass (Langangen, 1993). No significant (P < 0.05) changes in water pH were observed from the microcosms with or without muskgrass (data not shown).

Selenium Volatilization by Muskgrass
Overall, the muskgrass microcosms had a 2.7-fold higher rate of volatile Se production than the unvegetated microcosms, with average rates of 1.9 ± 0.1 µg Se m-2 d-1 with muskgrass and 0.7 ± 0.2 µg Se m-2 d-1 without muskgrass (Fig. 2) . With muskgrass, Se volatilization increased with time during the first 2 wk, reaching a highest rate of 3.0 ± 2.5 µg Se m-2 d-1 during the study period. The total Se mass removed through volatilization during the 24-d study period was 0.02 mg Se per muskgrass microcosm and 0.01 mg per unvegetated microcosm.



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  		Fig. 2. Changes of Se volatilization with time from the wetland microcosms over the 24-d experimental period. Data shown are means and standard deviation (n = 3).

 
Selenium Accumulation in Muskgrass
The total amount of mass Se accumulated in muskgrass tissues was 0.073 ± 0.013 mg Se per microcosm (Table 2). The average concentration of Se in muskgrass tissues was 1.00 ± 0.1 mg kg-1. To further confirm this finding with microcosms and to explore physiologically the kinetics of Se uptake and accumulation in muskgrass, an additional pot experiment was conducted with muskgrass grown in drainage water containing 30 µg Se L -1 over an 8-d period (Fig. 3) . The results showed that apparent accumulation of Se by muskgrass increased by about twofold within the first 24 h, then remained the same for the remainder of the experimental period. The precipitation of Se on muskgrass surfaces was about 3 ± 0.6 µg Se g-1 dry muskgrass biomass, which is about twofold greater than the total accumulation of Se in muskgrass tissue.


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  		Table 2. Accumulation of Se in muskgrass grown in the wetland microcosm during the study period.{dagger}

 


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  		Fig. 3. Kinetics of Se accumulation in muskgrass tissues. Data shown are means and standard deviation (n = 5).

 
Selenium Speciation in Muskgrass Tissues
Like many other plant species, the low rates of Se volatilization by muskgrass supplied with selenate-contaminated drainage water may be limited by the poor reduction of selenate to organic Se forms in muskgrass tissues. This hypothesis was tested with X-ray absorption spectroscopy for the Se speciation in the tissues of muskgrass supplied with selenate or selenite (Fig. 4) , and by measuring rates of Se volatilization by muskgrass supplied with selenate or selenite (Fig. 5) . The X-ray absorption spectroscopy results showed that selenate-supplied muskgrass contained about 47% of Se in organic forms (selenoethers and diselenides), compared with 91% of Se in organic forms when muskgrass was supplied with selenite. Since the rates of Se volatilization from selenite were approximately 10-fold greater than from selenate (Fig. 5), one might conclude that the reduction of selenate is rate-limiting Se volatilization in muskgrass.



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  		Fig. 4. Selenium K{alpha} near-edge X-ray adsorption spectra of muskgrass supplied with selenate or selenite (A), compared with the four selenium standards (B). SeMet, selenomethionine; RSeSeR, selenocysteine.

 


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  		Fig. 5. Selenium volatilization by muskgrass treated with 1.58 µg L-1 of selenate or selenite during a 5-d study. Data shown are means and standard deviation (n = 5).

 

   DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSIONS
 REFERENCES
 
The average rate of Se volatilization from the muskgrass microcosms was 1.9 ± 0.1 µg Se m-2 d-1, which is approximately 2- to 15-fold lower than the average rate from other wetland plant species, including cattail, saltmarsh bulrush (Scirpus robustus Pursh), baltic rush, cordgrass, and rabbitfoot grass [Polypogon monspeliensis (L.) Desf.], in the Corcoran wetland treating the same agricultural drainage water (Lin and Terry, 2000). Earlier work in our laboratory has shown that the reduction of selenate to organic forms of Se is an important rate-limiting step in the assimilation of selenate for volatilization by plants (Terry et al., 2000). Although biological volatilization of Se is generally influenced interactively by various environmental and biological factors, such as temperature, organic carbon, sulfate content, and the chemical forms of Se (Terry and Zayed, 1994; Frankenberger and Karlson, 1994; Zayed et al., 1998), the Se speciation results by X-ray absorption spectroscopy indicated that the low rates of Se volatilization from muskgrass supplied with selenate-contaminated drainage water was mainly limited by the poor reduction of selenate to organic Se forms in muskgrass. For example, compared with rabbitfoot grass species that volatilized Se greatly (32 ± 12 µg Se m-2) in the Corcoran wetland, muskgrass contained a smaller proportion of organic forms of Se (47%) than rabbitfoot grass (approximately 77%) (Lin and Terry, 2000).

Muskgrass supplied with drainage water containing 20 µg Se L-1 had a Se concentration of 1.0 mg kg-1. This concentration of Se in muskgrass tissues was comparable with those observed in shoot tissues of cattail, saltgrass [Distichlis spicata (L.) Greene], or baltic rush that had been growing for one year in the Corcoran wetland receiving 22 µg Se L-1 drainage water (Terry, 1998). In an earlier field study, Horne (1991) reported that Se concentrations in muskgrass tissues decreased from 65 to 5 mg kg-1 while the Se concentrations in the water column changed from 60 to approximately 10 µg L-1. The bioconcentration factor of Se by muskgrass (i.e., the ratio of Se concentration in muskgrass tissue to the Se concentration in drainage water) was approximately 70 from this microcosm experiment and approximately 370 to 1000 obtained by Horne (1991) in the Kesterson Reservoir. These bioconcentration factors of Se by muskgrass are substantially lower than the values observed previously for Fe, Mn, and Ni (>10 000) by muskgrass in the Allegheny Power Service constructed wetland (Ye et al., 2001). Thus, available results from both laboratory and field studies indicated that muskgrass is not a Se hyperaccumulator in agricultural drainage water. With respect to the hyperaccumulation of other metal elements by muskgrass in the Allegheny Power Service wetland (Ye et al., 2001), it may have been due to the association of the metals (as cations) with the iron plaque that forms on the submerged surface of muskgrass, a mechanism that is not likely to result in great accumulation of metalloid Se that is present as an oxyanion.

The average total Se mass input over the study period was 3.75 ± 0.05 mg per microcosm (vegetated or unvegetated), which includes the 1.63 mg Se in the initial fill as well as the mass Se added through the inflow drainage water (2.15 ± 0.05 mg Se per microcosm). Of the total mass Se input (3.75 ± 0.05 mg) in each microcosm, 0.5% of the Se was removed by volatilization and 1.9% of the Se was accumulated in muskgrass tissues. Although the accumulation of mass Se in muskgrass tissues accounted for a small proportion of the total mass Se input, the addition of muskgrass to the flow-through wetland microcosms enhanced the daily Se mass removal rate, from 50.6% by microcosms without muskgrass to 72.1% by the microcosms with muskgrass. The Se mass removal rates by the muskgrass microcosms (72.1%) were comparable with those observed at the Corcoran wetland where approximately 75% of the Se loaded into the flow-through constructed wetland cells was removed from the Se-laden agricultural drainage water (Lin and Terry, 2000).

The muskgrass microcosms were more efficient at removing Se mass from agricultural drainage water, compared with the microcosms without muskgrass. This finding accentuated the important roles of the macroalga muskgrass in Se removal by wetland microcosms. The addition of muskgrass provided an effective structural filter for the sedimentation of Se-bearing particles, and ensured a circuitous path for water flowing through the wetland microcosm, due to the reduction of both water velocity and turbulence by vegetation, which are ideal for the settling of suspended sediments (Fennessy and Mitsch, 1989). Particularly, muskgrass also provided habitat and energy sources to maintain a flourishing microbial population in the microcosm (Skousen et al. 1994). Previous studies indicated that the dissimilatory reduction of soluble selenate to insoluble elemental Se performed by microbes is the major process for the removal of selenate from water columns to sediments (Tokunaga et al., 1996). In the present study, the concentration of Se in the standing water of muskgrass microcosms at the end of the experiment was 15 ± 1 µg L-1, which was lower than the concentration of 18 ± 1 µg L-1 for the microcosms without muskgrass. No attempt was made for a Se mass balance calculation in this study because the increase of Se concentration in sediment was not statistically (P < 0.05) significant, changing from 0.31 ± 0.11 (background) to 0.34 ± 0.13 mg kg-1 (treatment) in the muskgrass microcosms. This was almost certainly because of the relatively small amount of mass Se received by the microcosm sediment over the experiment period.


   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
REFERENCES
 
The addition of muskgrass into the wetland microcosm significantly enhanced the efficiency of Se removal. The muskgrass-vegetated microcosm removed 72.1% of the mass Se from selenate-contaminated agricultural drainage water flowing into the microcosm daily. However, high rates of Se removal were not due to phytoextraction and volatilization of Se by muskgrass, but probably due to the stimulation of microbial reduction of selenate and structural filtration of Se-bound particles from water columns to sediments. Over a 24-d study period, the accumulation of Se in muskgrass tissues accounted for 1.9% of the total mass Se input to the microcosms, and biological volatilization removed approximately 0.5% of the total mass Se input. Like many other plant species, Se volatilization from muskgrass is limited by the reduction of selenate to selenite or organic forms of Se in plant tissues. Selenium volatilization by muskgrass was 10-fold more from selenite than from selenate. X-ray absorption spectroscopy speciation analysis showed that 91% of the Se in tissues of muskgrass supplied with selenite was in the form of organic Se, compared with 47% in tissues of muskgrass supplied with selenate.


   ACKNOWLEDGMENTS
 
Authors gratefully acknowledge funding support from University of California Water Research Center (98-4). We thank Dr. P. Silva at the University Herbarium for identifying the muskgrass species. The authors thank the Stanford Synchrotron Radiation Laboratory (SSRL) for beam-time awarded to N. Terry. SSRL is funded by the Department of Energy, Offices of Basic Energy Sciences and Biological and Environmental Research; the National Institutes of Health, National Center for Research Resources, Biomedical Technology Program; and the National Institute of General Medical Sciences.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Z.-Q. Lin, present address: Department of Biological Sciences and the Environmental Sciences Program, Southern Illinois University, Edwardsville, IL 62026-1651.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 





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