Coagulation mechanism of polyaluminum manganese chloride in alkaline conditions: Role of manganese in changing hydrolysates and coordination structure

The development of highly efficient coagulants with broad pH adaptability remains a critical challenge in water treatment. In this study, a new type of polyaluminum chloride-manganese chloride composite coagulant (PAMCs) was synthesized, and the effect of manganese on changing the types of hydrolysis products and enhancing coagulation performance was systematically studied. A series of PAMCs with varying Al/Mn molar ratios and basicity ( B  = [OH - ]/[M 3+ ]) were prepared, and their interactions with humic acid (HA) were evaluated. Results demonstrated that PAMC 2 (Al/Mn = 2:1, B  = 2) exhibited superior HA removal efficiency under weakly alkaline conditions (pH 8-9), while maintaining stability over a broader pH range (pH 4-11). Mechanistic studies revealed that manganese incorporation primarily modified the speciation of hydrolysis products and the coordination structure of traditional flocs, favoring colloidal Al/Mn species rich in reactive hydroxyl groups (-OH) with stronger adsorption-complexation capabilities. Spectroscopic analyses confirmed that PAMC 2 preferentially interacted with the -COOH and -OH functional groups of HA to form stable HA-metal complexes. Hydrolysis product characterization further indicated that Mn addition promoted the formation of colloidal aggregates, enhancing sweep flocculation and ligand exchange under alkaline conditions. Furthermore, the promising application potential of PAMCs was evaluated in treating industrial dye wastewater. However, it must be acknowledged that the residual manganese concentration in treated water exceeded the WHO drinking water standard (0.1 mg/L), which currently limited its applicability in potable water treatment. Nevertheless, this study provided a new cost-effective strategy for another important issue (efficient treatment of neutral/alkaline industrial wastewater) and underscores the role of Mn in tailoring hydrolysis reactivity and broadening pH adaptability.