Factors affecting removal of selenate in agricultural drainage water utilizing rice straw

Microbial reduction of selenate [Se(VI)] to elemental selenium [Se(0)] is a useful technique for removing Se from agricultural drainage water. A series of batch experiments were conducted in the laboratory to determine the effects of pH (5–10), NO3− (100–500 mg/l), and SO42− (0–5 000 mg/l) on the removal of Se(VI) from drainage water with 1000 μg/l of Se(VI) and different amounts (1–4 g) of rice straw. Results showed that rice straw was very effective in creating a reducing environment (Eh=−205 to −355 mV) in the first 3 days of the pH-effect experiments. The optimum conditions for rapid Se(VI) removal from drainage water were a pH range of 6–9, high amounts of SO42− (1 000–5 000 mg/l), low amounts of NO3− (100 mg/l) and high amounts of rice straw (3–4 g). Under these conditions, it took 5–7 days to reduce 93–95% of the added Se(VI) to Se(0). This study indicates that rice straw may be an inexpensive reducing agent to remediate Se(VI)-dominant San Joaquin Valley drainage water in the field.