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), NO(3)(-) (100-500 mg/l), and SO(4)(2-) (0-5000 mg/l) on the removal of Se(VI) from drainage water with 1000 microg/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 SO(4)(2-) (1000-5000 mg/l), low amounts of NO(3)(-) (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.