Removal of selenate from wastewater using a bioelectrochemical reactor: The importance of measuring selenide and the role of competing anions

Removal of selenate (SeO₄^2-) from selenate-contaminated wastewater is challenging due to the commonly co-existing and competing anions of sulfate (SO₄^2-) and nitrate (NO₃^-). This study investigates SeO₄²⁻ reduction to elemental selenium (Se⁰) in a cathode-based bioelectrochemical (BEC) reactor and a conventional biofilm reactor (i.e., an upflow anaerobic reactor). The simulated wastewater contained SeO₄²⁻ at a typical concentration of 5 mg Se/L, SO₄²⁻ at a typical concentration of 1000 mg S/L, and NO₃⁻ at concentrations that varied from 0 to 10 mg N/L. The impact of sulfate on the BEC reactor was much lower than that on the conventional reactor: The selenium removal, defined as (selenate in influent – dissolved selenium in effluent)/selenate in influent, was 99 % in the BEC reactor versus 69 % in the conventional biofilm reactor. The lower selenium removal in the conventional reactor was mainly due to the >10 times higher reduction of sulfate, which directly caused competition between sulfate and selenate for the common resources such as electrons. The more reduction of sulfate in the conventional reactor further led to 45 times higher production of selenide. Selenide is usually assumed to be minimal and therefore not measured in the literature. This simplification may significantly overestimate selenium removal when the influent sulfate concentration is very high. NO₃^- in the influent of the BEC reactor promoted selenium removal when it was less than 5.0 mg N/L but inhibited selenate removal when it was more than 7.5 mg N/L. This was supported by the microbial community analysis and intermediate (nitrite) analysis.