Binary sacrificial anodes using transition metals (M + Fe, M = Ni, Cu, Zn) for electrocoagulation of boron and recovery of magnetic spinel oxides

Electrocoagulation (EC) using metals Fe, Cu, Ni, Zn, and Al as sacrificial anodes was performed for boron removal. Boron (B) was efficiently swept by the EC sludge that could be recovered as spinel oxides when combined the two anodes, Cu + Fe, Ni + Fe, Zn + Fe, and Ni + Al, and Al cathode. Effects of EC conditions, such as applied current, pH, initial concentration, on the faradaic efficiency, energy consumption, and B removal were investigated. A consecutive kinetic model was proposed to evaluate the rates of coagulation and metal dissolution as affected by the metal type and current density. The coagulant generated during EC was magnetic ferrites, including CuFe 2 O 4 (20.23 emu g −1 ), NiFe 2 O 4 (51 emu g −1 ), and ZnFe 2 O 4 (6.54 emu g −1 ) (emu = electromagnetic unit), depending on the type of two anodes. The dissolution potential (i.e., open circuit potential) of metals influenced the sludge formation under constant current, while the electrostatic interactions between the sludge and B were the determining factor in the EC efficacy. EC was optimized at pH where the positively charged coagulants tended to trap boron oxyanions (B(OH) 4 − ). Among combined metals, EC sludge of Ni + Fe and Ni + Al had relatively high coagulation capacity due to the high pH pzc, which obtained higher than 90% removal ([B] 0  = 100 mg L −1 ) at 12.5 mA cm −2 in one hour.