Preparation and characterization of a cost-effective red mud/polyaluminum chloride composite coagulant for enhanced phosphate removal from aqueous solutions

Composite coagulant has raised great concerns due to its cost-effectiveness and outstanding coagulation performance. A novel composite red mud (RM)/polyaluminum chloride (PACl) coagulant (RMPACl) was prepared for effective removal of phosphate from aqueous solution. The RM obtained from alumina industrial waste was employed as an alternative alkali to adjust the basicity of the semi-product of polyaluminum chloride (ACl). The RMPACl was characterized by pH titration, Al(III) species distribution, TEM, XRD, FTIR, phosphate removal performance as well as flocs properties analysis. The results showed that RM can significantly enhance the basicity of ACl. The TEM and FTIR results indicated that RMPACl was composed of polymeric aluminum species and residual RM particles. In addition, the phosphate removal experiments demonstrated that the optimum dosage of RMPACl was 147.5 mg/l for the decontamination of phosphate with an initial concentration of 5 mg/l, which is much smaller than that of PACl. RMPACl with relatively smaller and porous flocs exhibited a faster settling time than PACl, because the additional RM played an important role in the phosphate removal. More importantly, the RMPACl performed better than PACl in the treatment of the synthetic and natural phosphate contaminated water. It is anticipated that RMPACl can be proposed as a cost-effective coagulant for the effective removal of phosphate from natural water.