Investigating the generation efficiency, growth and structure of flocs produced by electrocoagulation

Abstract

Electrocoagulation (EC) is a water treatment technology that releases metal cations into solution from sacrificial anodes. Compared to chemical coagulation (CC), EC’s advantages (e.g., solid metal electrodes, reduced alkalinity consumption) make it potentially better suited for use in emergency situations or remote areas. However, EC is not commonly used, suffering from a lack of standardized design and operational procedures, and a dearth of research regarding its functional mechanisms. This thesis addresses this research gap, focusing on the generation efficiency, growth and structure of EC flocs. It was found that using a stainless steel cathode rather than an aluminum cathode resulted in faster and more efficient iron generation; this was attributed to the difference in the rate of hydrogen evolution at the cathode surface. The EC system also produced more iron per unit power when operated at lower voltages, suggesting that it is more efficient to operate using more electrodes at low power, rather than vice versa. Regarding the growth of iron precipitate EC flocs, changes in particle size were reflected by changes in scattering exponent. Flocs initially spanned a broad size range and formed loose, open structures; these initial aggregates then broke and formed into more compact structures. While operating at higher current densities resulted in larger and faster stabilizing flocs, comparing plots of scattering exponent against time revealed that the described structural progression was otherwise unaffected. Flocs were also more compact when formed from CC rather than EC, in low rather than high salt solution, and at pH 8.3 rather than pH 6.0. Flocs formed in low salt and at pH 8.3 were more stable in solution than their counterparts, likely requiring more collisions to form and producing denser structures. Transmission electron microscopy revealed that CC and EC flocs were structurally distinct, possibly affecting the scattering exponent. Flocs were also larger when produced via CC and in low salt solution; because these conditions resulted in denser flocs, they were likely less prone to breakage. Because all research was conducted at the bench-scale using synthetic solutions, further testing is required before inferences can be made regarding full-scale EC performance.