Comparative performance of green rusts generated in Fe0-electrocoagulation for Cd2+ removal from high salinity wastewater: Mechanisms and optimization.

The treatment of wastewater containing high concentration of inorganic salts has always been one of the focuses of environmental researchers. In this work, the effect of Cl- and SO42- on the removal of Cd2+ from wastewater using Fe0-electrocoagulation (Fe0-EC) were investigated by evaluating the transformation of Fe mineral. The experimental results indicated that the removal of Cd2+ from wastewater was depended on the property of Fe minerals. The generation of sulfate green rust (GRSO4) produced in the presence of SO42- showed stronger adsorption than the chloride green rust (GRCl) for Cd2+, and GRSO4 was obtained even in the mixture Cl- and SO42- solutions, because Fe(II)-Fe(III) GRs (layered double hydroxides, LDHs) showed stronger affinity for divalent SO42- than monovalent Cl-. High concentration of inorganic anions in wastewater resulted in the negative charged Fe flocs. High concentration of Cl- promoted the oxidation of Fe(II) to Fe(III) by chlorine-containing oxidants, and increased the proportion of Fe(III)/Fe(II) in Fe flocs, secondary Fe mineral magnetite (Fe3O4) was formed because of the increase of pH. Therefore, the presence of GRSO4 intermediate increased the Cd2+ removal by adsorption (coagulation and coprecipitation), and then the generated GRSO4 were gradually transformed into lepidocrocite (γ-FeOOH) by oxygen from air. Finally, the parameter optimization were conducted by adjusting the ratio of Cl- and SO42- (RC:S), current density (j), initial pH (pHi), initial Cd2+ concentration (C0), and temperature (T0). The removal efficiency of Cd2+ reached 99.5% after 10 min Fe0-EC under the optimal parameters: RC:S = 25:50 mmoL/mmol, j = 6 mA/cm2, pHi = 7-9, and T0 = 40 °C.