Electrochemical <i>In Situ</i> Production\nof Magnetite for the Removal of Se from Wastewater

Anthropogenic activities have dramatically increased\nthe level\nof selenium (Se) discharge. We propose a simple yet highly efficient\nsystem based on iron electrocoagulation (Fe-EC) for simultaneous magnetite\nproduction and Se removal. The <i>in situ</i>-generated\nreductants [Fe(II)] and adsorbents (magnetite) led to Se removal (201\nmg L<sup>–1</sup> h<sup>–1</sup>) significantly outperforming <i>ex situ</i> magnetite adsorption (29.2 mg L<sup>–1</sup> h<sup>–1</sup>). Selenite was rapidly adsorbed by iron (hydr)oxides\nand reduced to less toxic Se<sup>0</sup>, alleviating the risk of\nSe re-release from Se-laden solids. Se can be efficiently removed\nby 4 min Fe electrolysis at 0.11 A followed by an 11 min stirring,\naffording 84% removal efficiency under oxic conditions and 90% under\nanoxic conditions. Se removal continued during settling, reaching\n>99% after 24 h. Likewise, selenate could also be efficiently removed.\nIn addition, the inhibitory effects of coexisting sulfate and bicarbonate\nwere eliminated by adjusting the pH, current intensity, and dissolved\noxygen concentration. Moreover, the practicality was verified in two\ntypes of simulated wastewater, managing Se removal from 600 to ≤3\nμg L<sup>–1</sup> at an energy consumption of 0.66–1\nkWh m<sup>–3</sup>. Meanwhile, the produced particles were\nblack solids with magnetic properties, which can be easily harvested\nfrom treated water by magnetic separation, overcoming the drawbacks\nof conventional (electrochemical) coagulation. This study provides\nnew insights for developing a robust Fe-EC system for Se-laden wastewater\ntreatment.