Distinct coagulation mechanism and model between alum and high Al13-PACl

The conventional alum and polyaluminum chloride (PACl) with high content of Al13 were selected to study the interaction between coagulants and particles in suspension. Particulate silica microspheres were used for the investigation with emphasis on the surface adsorption behavior at a constant pH of 6.5. Residual turbidity (RT), zeta potential and fractional surface coverage (θ) were studied as functions of aluminum dosage (C). Differences in the coagulation behavior between alum and PACl were illustrated graphically as log C–RT and log C–θ, respectively. It was demonstrated that some distinct mechanisms existed between alum and PACl in their adsorption and coagulation with silica particles owing to their different speciation. The results suggested that, to accomplish destabilization and aggregation, alum interacted with silica particles mainly via monomers and Al(OH)3(am) flocs, while PACl interacted via the Al13 polycations. As a result, charge-neutralization, electrostatic patch coagulation and sweep-flocculation, bridge-aggregation may be included during the coagulation process. The main mechanisms for alum were charge-neutralization/sweep-flocculation while electrostatic patch coagulation and bridge-aggregation may be both involved for PACl besides charge-neutralization. Langmuir and Freundlich isotherm models were used to describe the adsorption process of PACl and alum on silica particles, respectively. Monolayer adsorption was observed for PACl with θ < 0.5 whereas multilayer adsorption occurred for alum with θ value reaching above 1. It was concluded that different adsorption mechanisms occurred for PACl and alum. The predominant driving force in the adsorption process of PACl was of electrostatic origin, whereas for alum was mainly through chemical precipitation.