Effect of iron ion configurations on Ni²⁺ removal in electrocoagulation

Electrocoagulation (EC) has been widely used to treat the heavy metal wastewater in industry. A novel process of sinusoidal alternating current electrocoagulation (SACC) is adopted to remove Ni²⁺ in wastewater in this study. The morphology of precipitates and the distribution of the main functional iron configurations were investigated. Ferron timed complex spectroscopy can identify the monomeric iron configurations [Fe(a)], oligomeric iron configurations [Fe(b)] and polymeric iron configurations [Fe(c)]. The optimal operating conditions of SACC process were determined through single-factor experiments. The maximum Ni²⁺ removal efficiency [Re(Ni²⁺)] was achieved under the conditions of pH₀=7, current density (j) = 7 A/m², electrolysis time (t) = 25 min, c₀(Ni²⁺) = 100 mg/L. At pH=7, the proportion of Fe(b) and Fe(c) in the system was 50.4 at.% and 23.1 at.%, respectively. In the SACC process, Fe(b) and Fe(c) are the main iron configurations in solution, while Fe(c) are the vast majority of the iron configurations in the direct current electrocoagulation (DCC) process. Re(Ni²⁺) is 99.56% for SACC and 98.75% for DCC under the same optimum conditions, respectively. The precipitates produced by SACC have a high proportion of Fe(b) configurations with spherical α-FeOOH and γ-FeOOH structures which contain abundant hydroxyl groups. Moreover, it is demonstrated that Fe(b) has better adsorption capacity than Fe(c) through adsorption experiments of methyl orange (MO) dye. Fe(a) configurations in the homogeneous solution had no effect on the removal of nickel.