Published in J. Environ. Qual. 33:559-564 (2004).
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TECHNICAL REPORT
Heavy Metals in the Environment
Fate of Colloidal-Particulate Elemental Selenium in Aquatic Systems
Yiqiang Zhanga,
Zahir A. Zahirb and
William T. Frankenberger, Jr.*,a
a Department of Environmental Sciences, University of California, Riverside, CA 92521
b Institute of Soil & Environmental Sciences, University of Agriculture, Faisalabad, Pakistan
* Corresponding author (william.frankenberger{at}ucr.edu).
Received for publication May 4, 2003.
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ABSTRACT
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Bacterial reduction of selenate [Se(VI)] to elemental Se [Se(0)] is considered an effective bioremediation technique to remove selenium (Se) from agricultural drainage water. However, the fate of the newly formed Se(0) in aquatic systems is not known when it flows out of the treatment system. A set of laboratory experiments was conducted to determine the fate of the colloidal-particulate Se(0) in a water column and in a water–sediment system. Results showed that the newly formed colloidal-particulate Se(0) followed two removal pathways in aquatic systems: (i) flocculation–sedimentation to the bottom of the water and (ii) oxidation to selenite [Se(IV)] and Se(VI). During 58 d of the experiments, 51% of the added Se(0) was precipitated to the bottom of the water and 47% was oxidized to Se(IV) in the water column. In the water–sediment system, Se(IV) in the water accounted for 21 to 25% of the added Se(0). Adsorption of Se(IV) to the bottom sediment resulted in a relatively low amount of Se(IV) in the water. This study indicates that the newly formed Se(0) may be an available form of Se for uptake by organisms if it flows to aquatic systems from a treatment site. Therefore, an effective bioremediation system for removing Se from drainage water must reduce Se(VI) to Se(0) and remove Se(0) directly from the drainage water.
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INTRODUCTION
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AN AQUATIC SYSTEM is a complex aggregate of water, sediment, and various types of organic materials. Selenium (Se) in this system exists in four different oxidation states (–II, 0, IV, and VI) and a variety of organic compounds. The bioavailability of Se is largely dependent on the speciation of Se (Besser et al., 1993; Lemly et al., 1993; Wang and Lovell, 1997). Selenate [Se(VI)], selenite [Se(IV)], and organic forms of Se are available Se forms to aquatic organisms (Besser et al., 1993; Lemly et al., 1993; Wang and Lovell, 1997). Elemental Se [Se(0)] has been commonly considered as an unavailable form of Se because of its insolubility. However, Se(0) is one of the largest pools of Se in aquatic systems (George et al., 1996; Weres et al., 1989), accounting for about 30 to 60% of total Se in sediment (Gao et al., 2000; Velinsky and Cutter, 1991; Weres et al., 1989; Zhang and Moore, 1996). Therefore, changes in its stability could affect its availability to aquatic organisms.
Recent studies have shown that Se(0) can contribute toxic levels of Se to aquatic organisms through two pathways (Dowdle and Oremland, 1998; Luoma et al., 1992; Schlekat et al., 2000). Particulate Se(0) in sediment may be available directly to aquatic organisms. Luoma et al. (1992) and Schlekat et al. (2000) found that a significant amount of added Se(0) in sediment was assimilated by the bivalves Potamocorbula amurensis and Macoma balthica. Elemental Se can also be available indirectly to aquatic organisms through its oxidation to Se(IV) and Se(VI) (Dowdle and Oremland, 1998; Schlekat et al., 2000). These studies have provided valuable information regarding the bioavailability of particulate Se(0) in aquatic systems.
Formation of Se(0) in aquatic systems generally involves a two-step reduction process from Se(VI) to Se(IV), and then to colloidal Se(0). If this process occurs in the sediment, Se(0) can directly attach on the surface of the sediment when it is formed. If this reduction process occurs in the water, newly formed colloidal Se(0) may remain in the water column for a long period of time due to its small size. The subsequent colloidal aggregation–flocculation or attachment to inorganic and organic particles in water can lead to settling and removal of Se(0) from the water column to the bottom sediment. In a pilot-scale Se bioremediation system in the San Joaquin Valley, California, Cantafio et al. (1996) found that bacterial Se(VI) reduction to Se(0) proceeded rapidly in a series of four columns. About 98% of the Se(VI) and Se(IV) in agricultural drainage water was reduced. However, flocculation and precipitation of the newly formed Se(0) in the columns proceeded very slowly. Some of the Se(0) flowed out of the columns. At Days 184 and 186, Se(0) accounted for 91 to 96% of the total Se in the outflow water. This physicochemical character of the slow flocculation and precipitation of newly formed colloidal Se(0) in aquatic systems can make Se(0) directly available to organisms living in the water, and indirectly available to organisms by its oxidation to Se(IV) and Se(VI). Therefore, a detailed study on the fate of colloidal-particulate Se(0) is needed to understand the bioavailability of Se(0) in aquatic systems.
The purpose of this study was to determine the fate of newly formed colloidal-particulate Se(0) in a water column and a water–sediment system.
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MATERIALS AND METHODS
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Sediment samples used in this study were collected from Stewart Lake, UT and Tulare Lake, CA. Air-dried rice (Oryza sativa L.) straw was obtained from the Broadview Water District, CA. Artificial drainage water was prepared in the laboratory with a salinity (electrical conductivity, EC) of 10.4 dS m–1 and a pH of 8.1. This artificial drainage water contained the major elements present in agricultural drainage water, including: SO2–4 (5000 mg L–1), Cl– (1500 mg L–1), HCO–3 (300 mg L–1), Ca2+ (550 mg L–1), Mg2+ (300 mg L–1), and Na+ (2285 mg L–1). Before mixing the chemical solutions, each chemical (NaCl, Na2SO4, NaHCO3, CaCl2·2H2O, and MgSO4) was separately dissolved in deionized water and autoclaved (0.124 MPa [18 psi] at 121°C) for 20 min. Selenate was added to the drainage water at a concentration of 10 mg L–1 after passing through a sterile 0.2-µm membrane.
Preparation of Red Colloidal-Particulate Elemental Selenium
Colloidal-particulate Se(0) was produced from bacterial reduction of Se(VI) in artificial drainage water in the presence of rice straw (Zhang and Frankenberger, 2003a). In this experiment, 4 L of drainage water containing 10 mg L–1 of Se(VI) was added to a 4-L flask and 20 g of rice straw was added to the water. The flask was incubated in the laboratory at room temperature (21°C) for 6 d when almost all of the Se(VI) was reduced to Se(0) [Se(VI) plus Se(IV) was less than 50 µg L–1]. After an analysis of the red particles in the flask with transmission electron microscopy (TEM) coupled with X-ray energy dispersive spectroscopy (EDS) (Losi and Frankenberger, 1997), the red particles were found to be Se(0) with a size of about 0.1 µm (Fig. 1) . The separation of particulate Se(0) and colloidal Se(0) in the water samples was based on the size of newly formed Se(0). In this study, Se(0) with a size of >0.4 µm was defined as particulate Se(0) and Se(0) with a size of 
0.4 µm was defined as colloidal Se(0). The drainage water (3.6 L) containing colloidal-particulate Se(0) was then passed through a 5-µm filter to an 8-L flask. The drainage water was diluted by half with 3.6 L deionized water. After mixing, the water contained 1860 µg L–1 of colloidal Se(0), 557 µg L–1 of particulate Se(0), 11.2 µg L–1 of Se(VI), 11.3 µg L–1 of Se(IV), and 73.5 µg L–1 of organic Se, and was used immediately for the experiments on the fate of colloidal-particulate Se(0) in aquatic systems.
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Fig. 1. Transmission electron micrograph (A) and energy dispersive spectrum (B) of colloidal Se(0) particles formed by reduction of Se(VI) in drainage water in the presence of rice straw. Arrow shows Se(0) particle.
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Batch Experiments
Batch experiments were conducted to determine the fate of newly formed colloidal-particulate Se(0) in a water column and in a water–sediment system. In this experiment, the sediment sample (Stewart Lake and Tulare Lake) was added to the bottom of a 1-L glass cylinder (44 cm in length with a 10-cm i.d.) to a thickness of 5 cm, followed by the addition of water containing the high amount of colloidal-particulate Se(0). The experiment without the sediment (water column) was set up as a control for the effect of sediment on the fate of the newly formed Se(0) in aquatic systems. Depth of the water column was 42 cm in the water column and 37 cm in the water–sediment system. The experiment was run in duplicates with a plastic cover on the top of the cylinder at room temperature (21°C). The cover had 20 holes, so that air could easily diffuse into the water. Water samples at a depth of 5 cm below the surface of the water were collected at 2- to 3-d intervals for 37 d, and then for a 1- to 2-wk interval for Se species analysis. At Day 37, water samples at a depth of 30 cm below the surface of the water were also collected for Se species analysis. Water samples were immediately treated after collection by filtration to separate particulate Se(0), and centrifugation to remove Se(0) (Fig. 2)
. All of the samples for determining total Se, total soluble Se plus colloidal Se(0), total soluble Se, Se(IV), and Se(IV) plus organic Se were placed into a freezer and analyzed within 7 d. Upon completion of the experiments, water was removed from the cylinders, and the top 2 to 3 cm of sediment was collected for Se speciation analysis.
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Fig. 2. A procedure to determine Se species in water samples. Particulate Se(0) = total Se – total soluble Se plus colloidal Se(0); Colloidal Se(0) = total soluble Se plus colloidal Se(0) – total soluble Se; Se(VI) = total soluble Se – Se(IV) plus organic Se; and organic Se = Se(IV) plus organic Se – Se(IV).
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Selenium Species Analysis
Figure 2 shows our procedure for determination of Se species in the water samples as described by the method developed by Zhang et al. (1999) and Zhang and Frankenberger (2003a). The Se species included total Se, total soluble Se, particulate Se(0), colloidal Se(0), Se(VI), Se(IV), and organic Se.
Selenium speciation in the experimental sediment samples was determined using a parallel extraction procedure (Zhang and Frankenberger, 2003b). After collecting the sediment, samples were immediately extracted with deionized water, 0.1 M NaOH, and 30% of H2O2 and 6 M HCl. Deionized water was used to extract water-soluble Se(VI), Se(IV), and organic Se in the sediment samples; 0.1 M NaOH was used to extract Se(VI), Se(IV), and 0.1 M NaOH soluble organic Se in the sediment samples; and 30% of H2O2 and 6 M HCl were used to digest total Se. Before extraction, the wet sediment was placed into a plastic bag and mixed by hand. Then, the mixed sample was put into a 60-mL beaker and mixed again with a spatula. For the deionized water and 0.1 M NaOH extraction, 1.7 to 2 g of the wet samples were placed in 40-mL Teflon centrifuge tubes, followed by 30 mL of the extractant. The centrifuge tubes were tightly capped and placed horizontally in a gyrotory shaker and shaken for 20 h. Then, the tubes were centrifuged at 17300 x g (relative centrifugal force) for 20 min. The supernatant from each tube was passed through a 0.2-µm membrane filter (Fisher Scientific, Hampton, NH) into a 40-mL glass vial. For the H2O2–HCl sediment digestion, 1 to 1.2 g sediment was used. The moisture content of wetland sediment was determined on an air-dry basis (21°C). A detailed extraction procedure used in this study can be found in the work of Zhang and Frankenberger (2003b). Determination of Se species in the deionized water and 0.1 M NaOH extracts in the sediment was the same as the determination of Se species in water samples after Se(0) was removed from the samples (Fig. 2). Adsorbed Se(IV) in the 0.1 M NaOH extract was estimated by the difference between Se(IV) in the experimental sediment samples and in the original sediment after soluble Se(IV) was removed from 0.1 M NaOH extractable Se(IV).
Selenium concentrations in prepared solutions (water samples and different sediment extracts) were analyzed by hydride generation atomic absorption spectrometry (HGAAS) (Zhang and Frankenberger, 2003a; Zhang et al., 1999). The pH and redox potential in the water were measured using a 720A pH/ISE meter (Thermo Orion, Beverly, MA) (Jayaweera and Biggar, 1996; Zhang and Frankenberger, 2003a).
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RESULTS AND DISCUSSION
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Elemental Se is a stable form of Se in aquatic systems under a reducing condition (Elrashidi et al., 1987). However, it can become unstable when a reducing environment is transited to an oxidizing environment. In a study on Se(VI) reduction to Se(0) in a water–sediment system, Tokunaga et al. (1996) found that 60% of the newly formed Se(0) in sediment during an experiment was reoxidized to Se(IV) and Se(VI) at Day 2 after collection. In a flooded sediment system at pH 7, Masscheleyn and Patrick (1993) reported that the boundary between Se(VI) and Se(IV) is at an Eh of about 250 to 285 mV, and between Se(IV) and Se(0), at an Eh of about –10 to –40 mV. In this study, the redox potential in the water column and water–sediment system increased during the experiments (Fig. 3)
from 265 to 410 mV with a pH range from 7.3 to 8.3. In such an environment, the newly formed Se(0) was unstable in the water column and its fate was controlled by flocculation–sedimentation and oxidation to Se(IV) and Se(VI).
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Fig. 3. Changes in redox potentials (Eh), pH, and concentrations of Se species at 5 cm below the surface of the water in a water column and a water–sediment system during 58 d of the experiments.
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Removal of Se(0) from water is partially caused by its flocculation, followed by its sedimentation to the bottom of the system (Fig. 3). In the first 9 d of the experiments, particulate Se(0) (560–992 µg L–1) was relatively higher than that (557 µg L–1) at the beginning of the experiments upon a decrease in the concentration of colloidal Se(0) in the water column and water–sediment system. During this period of the time, total soluble Se, Se(IV), and Se(VI) were stable, ranging from 68.5 to 95.9, 7.3 to 46.8, and 0 to 14.5 µg L–1, respectively. Red precipitates were observed at the bottom of the cylinder in the water column and on the surface of the bottom sediment in the water–sediment system. As the size of the particulate Se(0) was larger than colloidal Se(0), its relatively rapid sedimentation resulted in the settling of Se(0) to the bottom of the systems.
Removal of Se(0) from water is also caused by its oxidation to Se(IV) in the water column and water–sediment system (Fig. 3). In the first 5 d, the concentration of Se(IV) was very low, ranging from 7.3 to 11.3 µg L–1, revealing that oxidation of Se(0) to Se(IV) did not occur. During the rest of the experiment, Se(IV) increased rapidly in the water column with an average rate of 22.3 µg L–1 d–1, from 12.2 at Day 7 to 1151 µg L–1 at Day 58. In the water–sediment system, Se(IV) in the first 5 d ranged from 7.65 to 12.7 µg L–1 in the water. Then Se(IV) increased from 12.7 µg L–1 at Day 5 to 401 µg L–1 in the water–UT sediment system at Day 23 with an average rate of 19.5 µg L–1 d–1, and from 10.7 µg L–1 at Day 5 to 397 µg L–1 in the water–CA sediment system at Day 23 with an average rate of 19.3 µg L–1 d–1. During the rest of the experiment, Se(IV) in the water column increased slightly with an average rate of 3.06 and 6 µg L–1 d–1, respectively, in the water–UT sediment and water–CA sediment systems. On the final day of the experiment, Se(IV) was the dominant form of Se in the water with a small amount of Se(VI) and organic Se. Upon 58 d of the experiment, 47% of the added Se(0) in the water column was oxidized to Se(IV) and about 3% of the added Se(0) was further oxidized to Se(VI). In the water–sediment system, Se(IV) and Se(VI) in the water accounted for 21 to 25% and 2% of the added Se(0), respectively. These results reveal that the newly formed Se(0) formed in a reducing environment is not a stable form of Se(0) in an oxic aquatic system. Elemental Se in the water can be easily oxidized to Se(IV) which can be further oxidized to Se(VI).
Elemental Se can also be oxidized to Se(IV) at the bottom of aquatic systems (Fig. 3). At Day 37 when almost all of the colloidal Se(0) and particulate Se(0) were removed from the water (Table 1), which was indicated with very low concentrations of the colloidal Se(0) and particulate Se(0) in the water at 5 and 30 cm below the surface of the water, Se(IV) still increased rapidly with time in the water column from 790 to 1151 µg L–1 at Day 58. Selenite also increased slightly in the water–sediment system. These results suggest that oxidation of Se(0) to Se(IV) also occurred at the bottom of the water column and on the surface of the sediment in the water–sediment system.
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Table 1. Concentrations of Se species in water samples collected from the sites in Utah and California at 5 cm (shallow site) and 30 cm (deep site) below the surface of the water at Day 37.
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There was a lag time of oxidation of Se(0) to Se(IV) and Se(VI) in the water column and the water–sediment system although Eh values in the water at Day 2 reached a level (265–275 mV) (Fig. 3) in which Se(0) should be oxidized to Se(IV) (Masscheleyn and Patrick, 1993). The mechanism causing this lag time is not known. One explanation might be related to the relatively high soluble organic material in the water (yellow-brown color), which was produced from the rice straw during the generation of the newly formed Se(0) used in this study. The oxidation of the easily oxidizable organic material can consume oxidants, such as dissolved O2, thus delaying the oxidation of Se(0) to Se(IV). The other explanation might be related to the large amount of Se(0) in the water. The oxidation of Se(0) is greater than that of Se(IV) to Se(VI) based on their redox potentials described above. Consumption of the oxidants (e.g., dissolved O2) by oxidation of Se(0) to Se(IV) retarded the oxidation of Se(IV) to Se(VI). Only in the final week of the experiment did oxidation of Se(IV) to Se(VI) occur when almost all of Se(0) was removed from the water by its sedimentation and oxidation to Se(IV).
Lower amounts of Se(IV) in the water–sediment system than in the water column were partially caused by the adsorption of Se(IV) to the bottom sediment in the water–sediment system (Tables 1 and 2). In aquatic systems, Se(IV) has a stronger affinity to sorption sites of sediment than Se(VI) (Balistrieri and Chao, 1987, 1990; Glasauer et al., 1995; Kuan et al., 1998). Therefore, Se(IV) can be adsorbed to the bottom sediment when it is formed from the oxidation of Se(0). During the first 19 d of the experiment, Se(IV) concentrations (349 µg L–1) in the water column were similar to 351 µg L–1 in the water–UT sediment system and 364 µg L–1 in the water–CA sediment system, and then increased much greater in the water column than the water–sediment system during the rest of the experiment. Analysis of Se species in the bottom sediment showed that Se(IV) was 7.75 to 8.71 mg kg–1 in the UT sediment and 2.95 to 3.68 mg kg–1 in the CA sediment after the experiment, which was much higher than in the original sediment before the experiment (Table 2). After removing soluble Se(IV) from the sediment, Se(IV) was 3.2 to 4.2 mg kg–1 higher in the UT sediment and 2.6 to 2.8 mg kg–1 higher in the CA sediment than the original sediments. In a recent study, Dowdle and Oremland (1998) reported that soluble Se(VI) and Se(IV) formed from the oxidation of spiked Se(0) occurred in soil slurries under an end-over-end rotation condition and a large amount of Se(IV) was bound to soil particles. These results indicate that a significant amount of Se(IV) formed from the oxidation of Se(0) can be adsorbed to the soil and sediments.
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CONCLUSIONS
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The results from this work show that the fate of the newly formed Se(0) can follow two removal pathways within a water column when it enters aquatic systems. The first is flocculation–sedimentation to the bottom of the water column and the second is oxidation to Se(IV) and Se(VI). During 58 d of the experiments, 51% of the added Se(0) was sedimentated to the bottom of the water and 47% was oxidized to Se(IV) in the water column. In the water–sediment system, Se(IV) in the water accounted for 21 to 25% of the added Se(0). Because of the difficulty to preparing colloidal-particulate Se(0) for studying pure abiotic processes to determine the fate of newly formed colloidal-particulate Se(0) in aquatic systems without changing the characteristics of the newly formed colloidal-particulate Se(0), pure abiotic oxidation of Se(0) to Se(IV) was not studied in this work. Therefore, oxidation of the newly formed Se(0) to Se(IV) in the water column and in the water–sediment system may include both abiotic and microbial processes. This study suggests that Se(0) might not exist in natural surface water where Se(VI) is the major form of Se because colloidal-particulate Se(0) had been removed from the water before oxidation of Se(IV) to Se(VI) occurred. It might also be possible that colloidal Se(0) could be partially reoxidized to Se(IV) in the water column due to its small size and long residence time in the water if colloidal Se(0) moved into the water column from the anoxic bottom sediment by disturbance. This study also suggests that a bioreactor effective only in reducing Se(VI) to Se(0) is not enough to remove Se available to aquatic organisms if the outflow water contains Se(0) from the treatment system and is allowed to enter aquatic systems. The complete removal of Se(VI) and Se(IV) from Se-contaminated agricultural drainage water needs a two-step remediation system (Barton et al., 1994): (i) a bioreactor to reduce Se(VI) and Se(IV) to colloidal Se(0) by using Se-reducing bacteria and (ii) a flocculation separator to remove colloidal Se(0) from the water by using a flocculant.
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ACKNOWLEDGMENTS
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We thank Carla Scheidlinger in the Broadview Water District for providing rice straw samples. This research was funded by the UC Salinity and Drainage Program.
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ABSTRACT
INTRODUCTION
