The in-situ and ex-situ adsorption of iron flocs generated by electrocoagulation: Application for nickel, fluoride and methyl orange removal

Electrocoagulation (EC) is considered to be a highly efficient and environment-friendly water treatment technology. The adsorption of flocs generated in-situ contributes to the pollutant removal. The iron flocs with various structures (Green rusts (GRs), Fe3O4, FeOOH) are generated during in-situ EC process with Fe electrode. However, there is a lack of researches on the impact of flocs structures on pollutant adsorption. As is well known, the unsaturated adsorbent flocs are usually scraped out of the EC reactor as solid waste. It has therefore become necessary to study the adsorption characteristics of the flocs both during and after the EC process. In this work, the in-situ (direct treatment of wastewater by EC) and ex-situ (removal of pollutants in wastewater by dosing flocs produced by EC process) adsorption were designed to investigate the flocs adsorption during and after EC process, respectively. Flocs with different structure was generated by precise control of EC process for in-situ and ex-situ adsorption. The pollutant removal by flocs during in-situ adsorption was found to be higher than that of ex-situ adsorption. Meanwhile, GRs outperformed Fe3O4 and FeOOH, especially during ex-situ adsorption. The ageing resistance of GRs appeared to be superior to that of Fe3O4 and FeOOH. The structural transformation of flocs during ex-situ adsorption showed that the evolution from GRs to Fe3O4 and FeOOH resulted in the change of morphology and adsorption capacity.