Selenium Reduction by a Defined Co-Culture in Batch Reactors

Selenium is a contaminant of concern due to its toxicity effect on aquatic lives. EPA regulates its concentrations at 1.5 μg/L in lentic freshwater and 3.1 μg/L in lotic water. High concentrations of selenium in wastewater discharged from agricultural activities and industries including metal refining, ore mining, fossil fuel combustion, and glass manufacturing can bring substantial risks to the local ecological system. Previous study on selenium reduction by pure cultures of bacteria showed that the second stage, from Se(IV) to Se(0), was the rate-limiting step and a significant level of Se(IV) may accumulate, especially at high levels of initial Se(VI). In order to eliminate the more toxic Se(IV) accumulation and accelerate Se(IV) reduction, a Se(IV)-reducing strain Pantoea vagans EWB32213-2 was isolated and co-cultured with a Se(VI)-reducing strain Shigella fergusonii TB42616. Batch experiments with this co-culture were performed with 10 and 1100 mg/L Se(VI) supplemented with 700 mg/L glucose and showed that Se(IV) reduction was accelerated as evidenced by the faster and darker appearance of the red color compared with the single culture. An optimal temperature of 30 °C and a pH of 8 for selenium reduction were observed at a culture composition of 1:1. The result shows that the efficiency of selenium reduction can be significantly enhanced with increasing initial concentrations of Se(VI) using the co-culture, and around 75.53% of Se(IV) accumulation was observed to be eliminated after 6 days at an initial Se(VI) concentration of 1100 mg/L.