Electrochemical In Situ Production of Magnetite for the Removal of Se from Wastewater

Anthropogenic activities have dramatically increased the level of selenium (Se) discharge. We propose a simple yet highly efficient system based on iron electrocoagulation (Fe-EC) for simultaneous magnetite production and Se removal. The in situ-generated reductants [Fe(II)] and adsorbents (magnetite) led to Se removal (201 mg L^–1 h^–1) significantly outperforming ex situ magnetite adsorption (29.2 mg L^–1 h^–1). Selenite was rapidly adsorbed by iron (hydr)oxides and reduced to less toxic Se⁰, alleviating the risk of Se re-release from Se-laden solids. Se can be efficiently removed by 4 min Fe electrolysis at 0.11 A followed by an 11 min stirring, affording 84% removal efficiency under oxic conditions and 90% under anoxic conditions. Se removal continued during settling, reaching >99% after 24 h. Likewise, selenate could also be efficiently removed. In addition, the inhibitory effects of coexisting sulfate and bicarbonate were eliminated by adjusting the pH, current intensity, and dissolved oxygen concentration. Moreover, the practicality was verified in two types of simulated wastewater, managing Se removal from 600 to ≤3 μg L^–1 at an energy consumption of 0.66–1 kWh m^–3. Meanwhile, the produced particles were black solids with magnetic properties, which can be easily harvested from treated water by magnetic separation, overcoming the drawbacks of conventional (electrochemical) coagulation. This study provides new insights for developing a robust Fe-EC system for Se-laden wastewater treatment.