Coagulation performance and mechanism analysis of humic acid by using covalently bonded coagulants: effect of pH and matching mechanism of humic acid functional groups

Conventional inorganic coagulants (Al, Fe) and Al/Fe-based covalently bonded flocculants (CAFMs) had different hydrolysis species at different pHs, which subsequently led to differences in their binding sites and complexation ability with humic acid (HA). Studying the binding sites and interactions between CAFMs, AlCl₃ (Al), and FeCl₃ (Fe) hydrolysates and HA molecules is critical to understanding the coagulation mechanism. The results found that CAFM _0.6, Al, and AlCl₃ combined FeCl₃ (Al/Fe) removed more than 90% of HA at pH 6, and CAFMs showed higher HA removal rate than that of Al, Fe, and Al/Fe under the same reaction conditions. The flocs of CAFMs contained abundant -NH₂/OH as well as the large particle size, compact structure, and excellent settling performance. The hydrolyzed species of Al and Fe were predominantly Al_b and Fe_b at pH 6, but the hydrolyzed species of CAFMs were primarily (Al + Fe)_c. Moreover, the hydrolyzed species of Al and Al/Fe were found to complex with HA functional groups such as -COOH, C = O, C-H/C–C, C = C, and C–OH to form ligand bonds, while the hydrolyzed species (Al + Fe)_c of CAFMs could deeply interact with HA functional groups including C-O, -COOH, C = O, C-H/C–C, C = C, and C–OH by the adsorption and sweeping.