Hydraulic retention time (HRT) extension and nitrate addition mitigate Se(VI) inhibition and enhance selenium removal in constructed wetlands: Potential role of nitrate and Fe/Mn pumps.

Selenate (Se(VI)) and nitrate as co-contamination pose significant threats to both environment and human health. This study intends to examine the effects of the addition of Se(VI) on nitrogen removal in iron/manganese ore and wood based two-stage vertical flow constructed wetlands (B-CWs and C-CWs), as well as the feasibility of simultaneous removal. The results indicated that low Se(VI) concentration (0.5 mg/L) caused acute inhibition for nitrogen removal. Increasing nitrate concentration and prolonging HRT resulted in simultaneous and efficient removal of Se(VI) and nitrogen in B-CWs and C-CWs, with C-CWs having a higher total selenium removal. However, nitrogen removal was inhibited when further increasing Se(VI) concentration (1 mg/L) in C-CWs, but Se(VI) removal remained efficient. High concentrations of Se(VI) (5 mg/L) had a long-term inhibition for Se(VI) and nitrogen removal, which was not recoverable. Se(0) was the main reduction end product, accompanied by some Se(IV) production. High abundance of denitrifying/Se(VI)-reducing bacteria coupled with iron/manganese cycling pumps promoted efficient Se(VI) bioreduction.