Mass transfer and multi-phase fluid flow in electro-coagulation: A review

In electrocoagulation (EC), the electro-generated coagulants Fe²⁺ (or Al³⁺) and OH^- transfer into the bulk solution and undergo spontaneous hydrolysis reactions to form flocs which can absorb various pollutants through surface adsorption, complexation reaction and ligand exchange. This work presents a review on recent literatures about the multi-phase fluid flow (the transport of bubbles and flocs phase) and the mass transfer of electro-generated species (coagulants, OH^-, dissolved oxygen, etc.), as well as their impact on flocs generation, pollutant removal and energy consumption. The coupled relationship among the electric field, the mass transfer and the multi-phase fluid flow could be studied by the modeling methods. The simplified model without considering mass transfer could be applied to describe the current and electric field characteristics. The model considering mass transfer and fluid flow could solve the coupled relationship among electric field, concentration field and flow field. The continuous wastewater phase forms a multi-phase flow with the subsequent generation of bubble and floc phases. The multi-phase fluid flow is proven to have a crucial effect on EC performance. This work also reviews the effect of mass transfer of electro-generated coagulants and OH^- on flocs generation and clarifies that it is not the initial value but the local pH or pH profiles has a more important impact on the floc generation and pollutant removal. This review provides insight into multi-phase fluid flow and mass transfer in EC, delivers guidelines for sustaining high treatment performance, and offers an outlook for the future development of EC.