This study investigates the removal of As(III) and As(V) by ferric salts coagulation under solution chemistry influences with particular emphasis on the role of zeta potential and size of the precipitates. The results indicate that As(III) removal efficiency increased with increase of solution pH, but there was almost no correlation with zeta potential of the precipitates. In contrast, the highest As(V) removal occurred in the pH range of positive zeta potentials and the removal efficiencies declined significantly as the precipitates became more negatively charged at high pH values. The relative importance of the two coagulation removal mechanisms, co-precipitation and adsorption, was found to be dependent on the solution pH. The sizes of the early-formed precipitates were in the colloidal range, with the largest size being at the isoelectric point (IEP). Precipitate sizes affected the residual Fe level at certain pH values after microfiltration and sand filtration, but showed much less effect on the arsenic removal. Electrolyte concentrations showed a noticeable effect on the precipitate size, but only a small effect on the arsenic removal. Humic acid exhibited profound influences both on zeta potential, size of the precipitates and arsenic removal. (C) 2014 Elsevier B.V. All rights reserved.