Composite coagulants of polyaluminum chloride (PAC) and natural polyelectrolytes have attracted considerable attention in recent years due to their outstanding properties in drinking water treatment. However, the selection of polyelectrolytes grafted into PAC and aluminum forms distribution in the coagulant composite are under continuous evaluation and update. This study evaluated the effectiveness of new composite coagulants based on polyaluminum chloride (PAC) and sodium alginate (SA) to remove the colloidal matter from dam water. Optimization of the basicity (OH to Al ratios) and the sodium alginate ratio were performed by experimental design to control aluminum speciation in the composites coagulants. The Al-Ferron timed spectrophotometric method, density Functional Theory (DFT), and FTIR analysis were performed to explore PAC and SA's interaction mechanism. The monitoring of aluminum speciation in the composite coagulant PAC-SA revealed that the basicity and SA ratios in the PAC-SA affected the distribution of aluminum forms (mononuclear Ala, medium polymeric Alb, colloidal, and highly polymeric Alc). The theoretical analysis identified the medium and high aluminum polymer species as the most sensitive species to react with SA. The coagulation performance revealed that the increasing percentage of the SA, and the prevalence of Alb and Alc species over Ala species in the PAC-SA, are beneficial for turbidity and oxidable matter removal. Incorporating biopolymer (SA) into the PAC reduced the concentration of inorganic coagulant consumed and increased the attraction efficiency of suspended pollutants compared to PAC. At an AlCl3 concentration of 10 mg.L-1, the incorporation of PAC (66.6% of basicity) into 20% of SA removed 99.9% of turbidity and 78.67% of oxidizable matter. This study provided new insights into the intramolecular interaction between PAC and SA and its influence on aluminum's speciation in the PAC-SA to increase surface water treatment efficiency.
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