Arsenate and arsenite removal by FeCl3: Effects of pH, As/Fe ratio, initial As concentration and co-existing solutes

Abstract

Coagulation is one of the most commonly used technologies for arsenic removal from water and wastewater. Jar tests were carried out to determine the influence of pH, initial As/Fe molar ratio, equilibrium As concentration and co-occurring solutes on the crossover pH, where As(V) removal was equivalent to As(III) by FeCl3 coagulation. As(V) was more effectively removed than As(III) at lower pH values in FeCl3 coagulation process but the opposite was true at higher pH level. Increasing As/Fe ratio from 0.12 to 0.50 progressively lowered the crossover pH from 8.5 to 7.4. The arsenic isotherms revealed that As(V) removal was favored at low pH and low equilibrium concentration yet the opposite was true for As(III). The co-existing solutes had different influences on the crossover pH, depending on their ability to compete for sorption sites and to hinder or facilitate the aggregation of ferric hydroxide flocs. The presence of sulfate broadened the pH range where As(III) removal was better than As(V) removal. However, As(V) removal was always superior to As(III) removal in the presence of Ca2+ and no crossover pH was observed. The results from this study revealed that arsenic speciation, arsenic loading, and pH should be considered when predicting and managing arsenic removal by coagulation.