Cd removal by direct and positive single pulse current electrocoagulation: Operating conditions and energy consumption

Due to its toxicity, cadmium (Cd) is a well-known pollutant in aquatic environments. Although direct current electrocoagulation (DC-EC) has shown great potential in industrial applications, the presence of a passive film on the electrode impedes the Cd removal efficiency and increases energy consumption during long-term operation. This issue can be resolved using positive single pulse current EC (PSPC-EC). In this study, two kinds of EC, i.e., DC-EC and PSPC-EC, were applied to remove Cd from aqueous solutions, where the optimal operating conditions, energy consumption, and Cd removal mechanisms were explored using the two EC kinds. The optimum operating conditions of the DC-EC were a pH of 8, an initial Cd concentration of 28 mg/L, a current density of 40 A/m2, and a plate spacing of 15 mm. Based on the optimum operating conditions of the DC-EC, the optimum average current density, frequency, and duty ratio were 16 A/m2, 2000 Hz, and 40% in the PSPC-EC process, respectively. Also, the removal rates of the DC-EC and PSPC-EC reached 99.74% and 99.92% with energy consumption values of 0.714 and 0.400 kWh/m3, respectively. In addition, it was indicated that the energy consumption of the PSPC-EC was 44% less than that of the DC-EC. Through the study of the flocs, it was found that the flocs produced by the PSPC-EC exhibited a larger surface for the Cd adsorption and formed larger particles in comparison with the DC-EC. In addition, the green rust-Cl (GRCl) produced by the PSPC-EC had a better Cd removal property, which removed the Cd by adsorption, coagulation, and Cd hydroxide precipitation. The results described herein provide a basis for the treatment of heavy metal-containing wastewater using the PSPC-EC method, and they can be used in future industrial applications. [Display omitted] •Optimal conditions of DC-EC and PSPC-EC for Cd removal were determined.•The energy consumption of PSPC-EC was 44% less than that of DC-EC.•Flocs produced by PSPC-EC have the better spread performance than that of DC-EC.•The GRCl produced by PSPC-EC had the better Cd removal property than that of DC-EC.•Cd and Fe hydroxide formed Cd–Fe hydroxide co-precipitate to realize Cd removal.