Design and coagulation mechanism of a new functional composite coagulant in removing humic acid

The deep removal of organic pollutants is a challenging task for coagulation technology in the drinking water and wastewater treatment plants to satisfy the rising water standards. To obtain high removal efficiency for organic matter (OM), a novel coagulant, Fe-PAA, was developed based on ferric salts and polyacrylic acid (PAA) in this study, which contained long flexible carbon chain and positive coordination sites. The humic acid (HA) was employed as a representative OM target with wide molecular weight range in the coagulation tests. The coagulant Fe-PAA with Fe3+/COOH ratio of 1:1 exhibited the promising coagulation performance meeting with the initial design principle. Compared to ferric chloride, Fe-PAA-1:1 exhibited overall advantages on HA removal, formation of large and compact flocs, and control of residual Fe3+. As a macromolecular coagulant, Fe-PAA-1:1 demonstrated the synergetic advantages of iron hydrolysis components and flexible long-chain polymers. The charge neutralization and bridging effects dominated coagulation process under acidic condition, while complexation and bridging effects were the main driving force of Fe-PAA-1:1 under neutral and alkaline conditions for HA removal. Overall, this study provides revelatory information for the development of a new kind of highly efficient and cost-effective coagulant to satisfy the demand of current water treatment.