The study of the effect of mass transfer of pollutants and flocs on continuous electrocoagulation processes

Abstract

In electrocoagulation (EC) process, the pollutants are mainly removed by the in-situ generated adsorptive flocs which have large surface areas and are beneficial for a rapid adsorption and pollutant trapping. The influence of multi-phase fluid flow (continuous fluid phase, the dispersed bubbles and flocs phase) on mass transfer of pollutants and flocs was systematically studied by the retention time distribution (RTD) analysis and mathematical modeling. The influence of the aerated and in-situ electro-generated bubbles on pollutants and flocs transport was investigated. The electro-generated bubbles have much smaller size and are easier to be adsorbed by the flocs. Thus, the electro-generated bubbles could advance the retention time (RT) of flocs obviously. The aerated bubbles which have larger size than electro-generated bubbles could increase the turbulence of the fluid flow. It causes the back mixture of pollutants and flocs. Thus, both electro-generated and aerated bubbles could shorten the RT of pollutants and flocs through different mechanisms, resulting in reduced removal efficiency of EC. The transport characteristics of pollutants and flocs in three typical structure reactors were studied. The vertical tubular-like EC has shorter RT than horizontal tubular-like EC. However, the former reactor has higher pollutant removal than the latter one. The reason is that the vertical tubular-like EC has uniform flocs distribution. This reveals that besides the RT of flocs, the distribution characteristics of flocs in typical structure reactors also contribute to the pollutant adsorption. The transport characteristics of flocs with different structures in EC process was studied by RTD method and was proven to differ greatly. Compared with Fe3O4 and FeOOH, green rusts (GRs) has stronger adsorption ability for the bubbles. Thus, although the GRs has shorter retention time (RT), the GRs has the highest pollutant removal. The reason is that Fe3O4 and FeOOH are easy to aggregate, however GRs has better dispersibility. GRs have the highest removal efficiency for pollutants. In EC process, the parameters should be optimized to generate flocs which is mainly composed of GRs to achieve the highest pollutant removal efficiency.