Size and structure evolution of kaolin-Al(OH)_3 floes in the electroflocculation process: A study using static light scattering

摘要

Electroflocculation (EF) is gaining recognition as an alternative process to conventional coagulation/flocculation. The electrical current applied in EF that generates the active coagulant species creates a unique chemical/physical environment in which competing redox reactions occur, primarily water electrolysis. This causes a transient rise in pH, due to cathodic formation of hydroxyl ions, which, in turn, causes a continuous shift in coagulation/flocculation mechanisms throughout the process. This highly impacts the formation of a sweep floe regime that relies on precipitation of metal hydroxide and its growth into floe. The size and structural evolution of kaolin-Al(OH)_3 floes was examined using static light scattering techniques, in aim of elucidating kinetic aspects of the process. An EF cell was operated in batch mode and comprised of two concentric electrodes -a stainless steel cathode (inner electrode) and an aluminum anode (outer electrode). The cell was run at constant current between 0.042 A and 0.22 A, and analyses performed at pre-determined time intervals. The results demonstrate that EF is able to generate a range of floes, exhibiting different growth rates and structural characteristics, depending on the conditions of operation. Growth patterns were sigmoidal and a linear correlation between growth rate and current applied was observed. The dependency of growth rate on current can be related to initial pH and aluminum dosing, with a stronger dependency apparent for initial optimal sweep floe regime. All floes exhibited a fragile nature and undergo compaction and structural fluctuations during growth. This is the first time size and structural evolution of floes formed in the EF process is reported.