Mechanistic Study on the Continuous Flow Electrocoagulation of Silica Nanoparticles from Polishing Wastewater

This study investigates the removal mechanisms involved in the monopolar, continuous-flow electrocoagulation of a polishing wastewater containing negatively charged silica particles. Using iron as the anodes, the mechanisms were inferred from the experimental observations of the changes in pH, zeta-potential, and turbidity as a function of reaction time, as well as from the sludge characterization. Two types of distinct mechanisms were observed, one involving particle destabilization by oppositely charged ferrous ions and the other involving coprecipitation or enmeshment of silica particles with the iron hydroxides. The former mechanism was apparently responsible for the formation of "surface flocs", whereas the latter one was responsible for the "sediment sludge". The sludge collected separately from the two “sinks” was distinctly different in both physical appearance and settling velocity. Furthermore, based on the sheer quantity of the surface flocs and the sediment sludge, the two mechanisms involved in the electrocoagulation process were equally important to the silica removal.