Coagulation of humic acid and kaolin at alkaline pH: Complex mechanisms and effect of fluctuating organics and turbidity

Coagulation was performed on synthetic test waters prepared with different concentrations of humic acid (HA) and kaolin to simulate organics and turbidity found in natural surface waters. The alkalinity of the test waters was high (180-205 mg/L) and high basicity PACl was used to coagulate the impurities as it is best suited for high alkalinity waters. Separate removal mechanisms of HA and kaolin has been dealt in detail by many researchers but in this paper the simultaneous removal mechanisms are stressed upon. Synthetic test waters were chosen instead of real surface waters to incorporate the effects of fluctuating organics and turbidity to have better insights into the removal mechanisms. Jar test was used to evaluate the removal efficiency of TOC, DOC, UV254 and turbidity. Zeta potential, particle/floc size, pH and alkalinity were also evaluated before and after the coagulation to investigate the charge neutralisation capacity of PACl at alkaline pH. The pH at optimum doses varied from 6.5 to 8 suggesting removal dominantly by sweep coagulation. The complete charge neutralisation was not possible which was indicated by the negative zeta potential even at optimum doses as some residual TOC (0.5-2 mg/L) and DOC (0.4-1.4 mg/L) remained in the treated water. The residual turbidity at optimum doses was < 1 NTU. The complex removal mechanisms of HA and kaolin for these test waters were further elucidated by using three adsorption models: Langmuir, Freundlich and BET model. The BET adsorption model showed highest R-2 values suggesting removal by multilayer adsorption.