Comparative performance of green rusts generated in Fe⁰-electrocoagulation for Cd²⁺ removal from high salinity...

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 SO₄^2- 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 SO₄^2- showed stronger adsorption than the chloride green rust (GRCl) for Cd²⁺, and GRSO₄ was obtained even in the mixture Cl^- and SO₄^2- solutions, because Fe(II)-Fe(III) GRs (layered double hydroxides, LDHs) showed stronger affinity for divalent SO₄^2- 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 SO₄^2- (R_C:S), current density (j), initial pH (pH_i), initial Cd²⁺ concentration (C₀), and temperature (T₀). The removal efficiency of Cd²⁺ reached 99.5% after 10 min Fe⁰-EC under the optimal parameters: R_C:S = 25:50 mmoL/mmol, j = 6 mA/cm², pH_i = 7-9, and T₀ = 40 °C.