Enhanced etching terminal wastewater treatment and H production by in-situ deposited heavy metals on carbon dots/g-CN photocathode microbial electrolysis cells.

Sustainable and cost-effective semiconducting photocathodes of microbial electrolysis cells (MECs) are attractively promising for efficient treatment of actual industrial wastewaters containing complex recalcitrant organics and multiple heavy metals. Herein carbon dots/graphitic carbon nitride (CDs/g-CN) photocathodes were employed to achieve efficient treatment of actual etching terminal wastewater (ETW) with simultaneous H production in MECs, allowing the effluent meeting national discharge standards (GB39731-2020). The progressively in-situ deposited heavy metals on the CDs/g-CN photocathodes, formed as metal oxides/CDs/g-CN after simple calcinations, further enhanced the ETW treatment (recalcitrant organics mineralization: 42.2 mg/L/h vs. 35.5 mg/L/h; heavy metal removal: Cu(II): 9.9 mg/L/h vs. 7.4 mg/L/h, Ni(II): 4.7 mg/L/h vs. 3.5 mg/L/h, Zn(II): 0.7 mg/L/h vs. 0.5 mg/L/h) and H production (0.1138 m/m/d vs. 0.0662 m/m/d). The importation of heavy metals, formed as metal oxides/CDs/g-CN altered the proportion of reactive oxidative species and thus promoted mineralization of recalcitrant organics, besides offering additional electrochemical removal of heavy metals with simultaneous more H production. This study demonstrates a new feasible protocol for achieving efficient ETW treatment, and gives a comprehensive appreciation of the effect of in-situ deposited heavy metals on the CDs/g-CN photocathodes, which has a profound effect on subsequent ETW treatment with simultaneous H production.