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TECHNICAL REPORTS
Bioremediation and Biodegradation

Characterization of Selenate Removal from Drainage Water Using Rice Straw

Department of Environmental Sciences, Univ. of California, Riverside, CA 92521-0424

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

Received for publication April 21, 2002.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Removal of selenium (Se) from agricultural drainage water is very important for protecting wildlife in wetland systems. We conducted a series of experiments on selenite [Se(IV)] adsorption and selenate [Se(VI)] reduction to determine Se removal from drainage water amended with 1000 µg/L of Se(VI) or Se(IV) and 5 g of rice (Oryza sativa L.) straw. Under sterile conditions, the added Se(IV) was not adsorbed to the rice straw within 2 d of the experiment and the added Se(VI) was not reduced within 14 d. In contrast, added Se(VI) in a nonsterile rice-straw solution was reduced rapidly, from 930 µg/L at Day 3 to 20 µg/L at Day 5, with an increase in unprecipitated elemental Se [Se(0)] and total Se(0). In the last several days of the experiments, unprecipitated Se(0) was the major Se form in the rice-straw solution, with a small amount of organic Se(-II). This study showed that Se removal from drainage water in the presence of rice straw involves a two-step process. The first is the microbial reduction of Se(VI) to Se(IV) and then to colloidal Se(0). The second is flocculation and precipitation of colloidal Se(0) to the bottom of the experimental flasks and the surface of rice straw.

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
SELENIUM (SE) contamination of wetlands in California results from agricultural drainage water containing Se at levels that range from 140 to 1400 µg/L (mostly as selenate [Se(VI)]) (Fujii and Deverel, 1989; Sylvester, 1990; Weres et al., 1989). Bioaccumulation of Se in wetlands creates serious hazards to fish and waterfowl (Ohlendorf, 1989; Presser and Ohlendorf, 1987). In an effort to minimize environmental effects of Se, the State of California Water Resources Control Board (1987) has recommended an interim maximum mean monthly Se concentration of 2 to 5 µg/L in rivers and wetlands receiving agricultural drainage water. It is very important for scientists and wetland managers to find ways to remove Se from drainage water before it reaches the wetlands.

The biogeochemistry of Se in the aquatic system is quite complicated, because Se can exist in four oxidation states (-II, 0, IV, and VI). The major features affecting the movement and solubility of Se are associated with changes in those species (Cooke and Bruland, 1987; Jayaweera and Biggar, 1996; Masscheleyn et al., 1990; Masscheleyn and Patrick, 1993). Selenate is the most oxidized form of Se, and is highly soluble in water (Elrashidi et al., 1987; Mikkelsen et al., 1989b). Selenite [Se(IV)] is easily adsorbed to sediment and soil constituents due to its high affinity with sorption sites (Balistrieri and Chao, 1990; Bar-Yosef and Meek, 1987; Frize and Hall, 1988; Neal et al., 1987a,b). Under reducing conditions, elemental Se [Se(0)] and selenide [Se(-II)] are thermodynamically stable forms (Elrashidi et al., 1987). Because of the insolubility character of Se(0) in aquatic systems, reduction of Se(VI) to Se(0) is considered a useful technique for bioremediation.

Microbial reduction of Se (VI) to Se(0) is one of the most important biogeochemical processes in aquatic systems, and has been observed in wetlands throughout the western United States and in many laboratory studies (Cantafio et al., 1996; Lortie et al., 1992; Losi and Frankenberger, 1997; Oremland et al., 1989; Zhang and Moore, 1996, 1997). A sequence reduction pathway of Se(VI) involves reduction of Se(VI) to Se(IV) and then to Se(0). In aquatic systems, Se oxyanions [Se(VI) and Se(IV)] can serve as electron acceptors for microbial respiration. Many bacteria have been found to be capable of reducing Se(VI) to Se(0), including Sulfurodpirillum barnesii, Enterobacter cloacae, Pseudomonas stutzeri, Wolinella succinogenes, and Thauera selenatis (Cantafio et al., 1996; Francisco et al., 1992; Lortie et al., 1992; Losi and Frankenberger, 1997; Oremland et al., 1999). For efficient microbial reduction of Se(VI) to Se(0), chemical nutrients such as acetate, lactate, and glucose are needed as sources of carbon, energy, and electrons (Cantafio et al., 1996; Francisco et al., 1992; Lortie et al., 1992; Losi and Frankenberger, 1997; Oremland et al., 1999). The high cost of these chemicals make them impractical for remediating Se(VI)-contaminated drainage water under field conditions (Cantafio et al., 1996). Therefore, there is a need for alternative, economical organic sources to provide electron donors, carbon sources, and nutrients for microorganisms to reduce Se(VI) to Se(0), and also serve as attachment sites for microorganisms and Se(0).

Rice straw is an important source of carbon and nutrients. In a study on the decomposition of rice straw, Villegas-Pangga et al. (2000) reported that rice straw contained a large amount of carbon, ranging from 30 to 35%, and nutrients such as K, P, N, S, Na, and Mg. After soaking in water, rice straw releases carbon and nutrients in solution (Jenkins et al., 1996) that can be used by microorganisms for carbon, energy, and electrons. In flooded soil grown with rice, Mikkelsen et al. (1989a) found that added Se(VI) was completely reduced during 10 weeks of experiments. Therefore, rice straw can provide carbon and nutrients, and may also be a good carrier of Se(VI)-reducing bacteria.

In this study, rice straw was used to remove Se(VI) from drainage water through Se(VI) reduction to Se(0). The reduction process was characterized in a series of batch experiments.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Sodium selenate (Na₂SeO₄) was purchased from Sigma (St. Louis, MO). The Se(IV) standard solution (1000 mg/L), sodium borohydride (NaBH₄), sodium bicarbonate (NaHCO₃), sodium sulfate (Na₂SO₄), sodium chloride (NaCl), calcium chloride (CaCl₂), magnesium sulfate (MgSO₄), and other chemicals were purchased from Fisher Scientific (Pittsburgh, PA).

Air-dried rice straw was obtained from the Broadview Water District, California and was used without post-treatment. The rice straw contained 0.412 mg/kg of total Se and 0.346 mg/kg of soluble Se. To keep the same salt concentrations in a series of batch experiments, artificial drainage water with a salinity [electrical conductivity (EC)] of 10.4 dS/m and a pH of 8.1 was used in this study. This artificial drainage water contained the major elements present in agricultural drainage water: SO^2-₄ (5000 mg/L), Cl^- (1500 mg/L), HCO^-₃ (300 mg/L), Ca²⁺ (550 mg/L), Mg²⁺ (300 mg/L) and Na⁺ (2285 mg/L). The trace elements normally found in drainage water were not included, except for Se, which was added in specific amounts and forms for each experiment. Before mixing the chemical solutions, each chemical (NaCl, Na₂SO₄, NaHCO₃, CaCl₂, and MgSO₄) was separately dissolved in deionized water and autoclaved (124 200 Pa [18 psi] at 121°C) for 20 min. The Se standard solutions [Se(VI) = 10 000 mg/L and Se(IV) = 1000 mg/L] were passed through a sterile 0.2-µm membrane before addition to the drainage water.

Batch Experiments
A series of batch experiments was conducted in the laboratory to determine Se removal from drainage water in the presence of rice straw (Fig. 1) . All experiments were run in triplicate at room temperature (21 ± 1°C). Because of the heterogeneous character of the untreated rice straw, the first batch of experiments tested for the effects of different parts of the rice straw on Se removal from drainage water (Fig. 1, E1). The rice straw was separated into three parts: lower stem, upper stem and leaf, and panicle. In the experiments, 5 g of rice straw was placed in each 500-mL flask, followed by 450 mL of the drainage water containing 1000 µg/L of Se(VI). Each flask was capped with a plastic foam plug to avoid buildup of the gases produced in the flasks. Water samples were collected at 0.3- to 1-d intervals for total soluble Se analysis, until the soluble Se concentration showed little change with time.

View larger version (39K): [in this window] [in a new window]   Fig. 1. A flow diagram characterizing Se(VI) removal from drainage water.

In the third batch of experiments we tested for Se(VI) reduction to Se(0) in drainage water in the presence of rice straw. In these experiments (Fig. 1, E3), 5 g of rice straw or 5 g of sterile rice straw were placed in 500-mL flasks, followed by 450 mL of the drainage water containing 1000 µg/L of Se(VI). Each flask was capped with a plastic foam plug. Water samples in the nonsterile rice straw experiments were collected for Se speciation at 0.3- to 2-d intervals for 14 d. Redox potential and pH of the rice-straw solution were measured daily in other batches of the experiments. The experiments with sterile rice straw were set up as controls. Water samples in the control experiments were collected for total soluble Se analysis at 1- to 2-d intervals. On the final day of the experiment, water samples were collected for Se species analysis.

pH and Eh measurement
A 720A pH/ISE meter (Thermo Orion, Beverly, MA) was used to measure pH and redox potential (Eh) in the experimental flasks. The pH was measured with an Accumet pH combination electrode (Fisher Scientific, Pittsburgh, PA). The redox potential was measured with an Accumet combination platinum electrode (Ag/AgCl). The measured potential (Eh_measured) was converted to potential in the rice straw solution (Eh_actual) relative to a standard H electrode as follows (Jayaweera and Biggar, 1996): Eh_actual = Eh_measured + 224.4 mV. The Eh electrode was tested by immersion in pH 4 and 7 buffer solutions saturated with quinhydrone (Q₂H₂) before each day's measurement.

Selenium Species Analysis
Figure 1 shows our procedure for determination of Se species in the water samples. The soluble Se speciation was determined with a method developed by Zhang et al. (1999a) with a slight modification. Because of relatively high amounts of dissolved organic material in the water samples, this modification included the adjustment of solution pH to 10.3 instead of 10 and the addition of 0.4 to 0.5 mL 0.2 M sodium persulfate (K₂S₂O₈) to each sample when the sum of Se(IV) and organic Se(-II) was determined. In brief, Se(IV) in the water samples was determined in a pH 7 buffer solution. The sum of Se(IV) and organic Se(-II) was determined when the organic Se(-II) in the water samples was oxidized to Se(IV) by K₂S₂O₈, which was indicated by precipitation of Mn oxides formed from the oxidation of added Mn²⁺. The organic Se(-II) concentration was calculated as the difference between Se in this water sample and Se(IV) concentration determined in another subsample. Total soluble Se in the water samples was determined by oxidizing all Se to Se(VI) by K₂S₂O₈, followed by reduction to Se(IV) in 6 M HCl. The Se(VI) concentration was then calculated as the difference between total soluble Se concentration and Se(IV), and organic Se(-II) concentration determined in another subsample.

In a previous test of Se(VI) reduction to Se(0) using synthetic drainage water containing 5 mg/L of Se(VI), we found that red Se(0) formed in the rice straw solution and gradually precipitated to the bottom of the flasks and on rice-straw surfaces. After collecting water samples containing unprecipitated Se(0), we used 0.2 M K₂S₂O₈ or 30% H₂O₂ to oxidize the unprecipitated Se(0) in the samples and analyzed for total Se [sum of total soluble Se and unprecipitated Se(0)] using the same method as for total soluble Se, described above. The unprecipitated Se(0) is defined as the Se(0) that remains in drainage water before its precipitation to the bottom of the experimental flasks and to the surface of rice straw. The same results were obtained by using K₂S₂O₈ or H₂O₂ as an oxidant, which indicated that unprecipitated Se(0) could be oxidized easily. Therefore, total Se in the water samples was determined with the same procedure as for total soluble Se. The Se concentrations in all prepared solutions were analyzed by hydride generation atomic absorption spectrometry (HGAAS) (Zhang et al., 1999a,b).

	RESULTS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
The removal of Se from drainage water using different parts of the rice straw is presented in Fig. 2 . The three parts (panicle, upper stem and leaf, and lower stem) were equally effective in removing Se from the drainage water. During the first 3 d, the concentration of total soluble Se remained relatively stable, at about 950 to 1020 µg/L, and then decreased rapidly to about 50 µg/L during Days 3 to 5. Similar results from the different parts of rice straw showed that heterogeneous rice straw could be used in removal of Se from drainage water.

View larger version (18K): [in this window] [in a new window]   Fig. 2. Changes in the concentration of total soluble Se in the drainage water during 10 d of the experiments with different parts (lower stem, upper stem and leaf, and panicle) of nonsterile rice straw. Batch 1 experiments.

View larger version (20K): [in this window] [in a new window]   Fig. 3. Total soluble Se in the sterile rice straw solution during 14 d of incubation, along with Se species at Day 14. Batch 3 experiments.

View larger version (33K): [in this window] [in a new window]   Fig. 4. Changes in the concentration of Se species, pH, and redox potential (Eh) of drainage water during 14 d of incubation with nonsterile rice straw. Selenium concentrations are given for the rice straw solution with the exception of total Se(0), which was determined as the difference between added Se(VI) and total soluble Se in the entire water and rice-straw system. Symbols , , and • show triplicate experiments. Batch 3 experiments.

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Adsorption of Se in aquatic systems is a very important process for removing Se from agricultural drainage water. The adsorption behavior of Se relies on its oxidation states, various adsorbents, and solution compositions (Balistrieri and Chao, 1990; Bar-Yosef and Meek, 1987; Frize and Hall, 1988; Neal et al., 1987a,b). Selenite has a stronger affinity for sorption sites than Se(VI) and thus is readily adsorbed to sediments and soil constituents such as Fe-, Mn-, and Al-oxyhydroxides, which are the major adsorbents for removing soluble Se from water (Balistrieri and Chao, 1987, 1990; Glasauer et al., 1995; Kuan et al., 1998). In this study, there was little change to Se(IV) in the drainage water during 2 d of the adsorption experiments, revealing that the removal of Se from the rice straw solution was not caused by Se(IV) adsorption.

Microbial reduction of Se(VI) to Se(0) is one of the most important biogeochemical processes in aquatic systems (Cantafio et al., 1996; Lortie et al., 1992; Losi and Frankenberger, 1997; Oremland et al., 1989; Zhang and Moore, 1996, 1997). During the 14 d of the experiments, Se(VI) was not reduced in the sterile rice-straw solution, indicating that Se removal from drainage water in the presence of rice straw is a microbially mediated reduction process. This removal pathway involves a two-step process. The first is the microbial reduction of Se(VI) to Se(IV) and then to colloid Se(0), and the second is the colloidal Se(0) flocculation and precipitation to the bottom of the experimental flasks and to the surface of the rice straw. The rapid sequence reduction from Se(VI) to Se(0) may be caused partly by specific organic compounds released from the rice straw. In previous work on Se(VI) reduction, Stolz and Oremland (1999) reported that the free energies for Se(VI) reduction to Se(IV) and Se(IV) reduction to Se(0), coupled to H₂ oxidation, are -15.53 and -8.93 kcal/mol, respectively. When acetate and lactate are used as the electron donors, Se(VI) reduction to Se(IV) is energetically favorable, yielding -172 and -343kJ/mol, respectively. Therefore, the addition of an efficient organic material such as rice straw may enhance Se removal from drainage water through Se(VI) reduction to Se(0).

Rapid sequence reduction of Se(VI) to Se(0) can also be explained by the redox potential of the rice straw solution. Masscheleyn and Patrick (1993) reported that a sequence reduction from Se(VI) to Se(IV) and then to Se(0) at a given pH was related to the redox potential. In a flooded sediment system at pH 7, Se(VI) reduced to Se(IV) at a redox potential of 250 to 285 mV, and Se(IV) reduced to Se(0) at redox potentials of -10 to -40 mV (Masscheleyn and Patrick, 1993). In this study, the redox potential in the rice straw solution dropped rapidly to levels that were much lower than -40 mV. In this reduced environment, unprecipitated Se(0) was the major Se form in the rice straw solution over the last several days of the experiment with a small amount of organic Se(-II). Selenite instantly reduced to Se(0) when it was formed from Se(VI) reduction, as evident by the short time appearance of Se(IV) at low concentrations in the rice straw solution.

Rapid sequence reduction of Se(VI) to Se(0) was accompanied by the slow removal of Se(0) from the drainage water. This slow removal process may be related to the low precipitation rate of colloidal Se(0) due to its small size, ranging from 0.2 to 0.4 µm (Barton et al., 1994). In a pilot-scale Se bioremediation system in the San Joaquin Valley (California), Cantafio et al. (1996) found that microbial Se(VI) reduction to Se(0) proceeded rapidly in a series of four columns. About 98% of Se(VI) and Se(IV) in agricultural drainage water was reduced. However, removal of the Se(0) formed in the columns proceeded very slowly. A significant amount of formed Se(0) flowed out of the columns. Elemental Se accounted for 91 to 96% of total Se in the outflow water at Days 184 and 186. This indicated that if colloidal Se(0) is not flocculated to form larger particulate Se(0) that can precipitate, or is not attached to the surface of materials used for drainage water treatment, it may reside in the water for a long time. Therefore, enhancement of the flocculation and precipitation and attachment of Se(0) to materials is needed to remove Se rapidly from drainage water.

Organic Se(-II) was another product of the removal of Se(VI) from drainage water. This study showed that about 5% of added Se(VI) was transformed to organic Se(-II) in the rice straw solution. Although organic Se(-II) can transform into volatile organic Se compounds or be mineralized to inorganic Se species, its fate in the rice straw solution was not determined.

Irrigation in the western San Joaquin Valley has introduced high-Se drainage water to nearby wetlands, which creates serious hazards for wetland waterfowl (Ohlendorf, 1989; Presser and Ohlendorf, 1987). To protect wetland wildlife, Se must be removed from drainage water before it reaches the wetlands. Several remediation methods such as adsorption, microbial reduction, and volatilization techniques have been proposed to remove Se from drainage water (Cantafio et al., 1996; Lee, 1989). However, none of these technologies has been used full scale in the field because of high costs. This study shows that rice straw may be an excellent organic source for removing Se from drainage water in the field. A flow-through system with a flocculation unit is being built in our laboratory to determine whether rice straw can be used in the field to remove Se(VI) from drainage water economically.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
This study showed that in the presence of rice straw, about 95% of the added Se(VI) was reduced to Se(0) and about 5% of the added Se(VI) was transformed to organic Se(-II) in drainage water. About 91% of the formed Se(0) was precipitated to the bottom of the experimental flasks and to the surface of rice straw during 14 d of the experiments. These results indicate that rice straw may be useful in removing Se from drainage water in the field because it is an excellent carrier of Se(VI)-reducing bacteria, it provides carbon and nutrients, and its cost is low in the western San Joaquin Valley, California.

	ACKNOWLEDGMENTS

 
We thank Dr. Zahir A. Zahir for reviewing the manuscript. This research was funded by the UC Salinity and Drainage Program.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 

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