Comparison analysis of cationic-polyacrylamide as flocculant aid in titanium salt coagulants

The combination of organic and inorganic coagulants is considered to be an effective strategy for Humic Acid (HA) removal. In this study, the performance of Titanium-based coagulants aided by Cationic-Polyacrylamide (CPAM) to HA molecules was compared. It was found that CPAM had a higher promotion effect in the TXC system. The reason for why CPAM was more suitable for the TXC system could be explained by analyzing Zeta potential, FTIR, and XPS. By comparing the coagulation mechanism of TiCl4 (TC) and Poly-Titanium Chloride (PTC), we found that complexation was the main mechanism of TC and PTC in acidic and neutral environments. The C-O-Ti Bond was formed by complexing the Ti-OH with the C-OH and -COOH functional groups of HA molecules. Sweeping was the most important removal mechanism of TXC in the neutral and alkaline environment, and CPAM bridged floc through hydrogen bond force for enhancing the sweeping effect. In addition, the role of CPAM in TXC system was further demonstrated by Zeta potential analysis. Both TC and PTC were positively charged in the measured pH range, so the removal of Humic acid was limited by increasing zeta potential. In both neutral and alkaline environments, TXC kept a negative surface charge and CPAM significantly enhanced the adsorption of HA on the TXC floc surface. This study provided clues for the interaction mechanism between titanium salt coagulants and HA, and it also provided a reference for the application of TXC coagulant combined with CPAM flocculant for the removal of HA molecules.