Speciation matching mechanisms between orthophosphate and aluminum species during advanced P removal process

Aluminum (Al) salts are widely used as coagulants to remove phosphorus (P) in water treatment. However, the relationship between P and Al species and the underlying coagulation mechanisms is rarely studied. Currently, water eutrophication is a serious issue, and therefore advanced P removal is extremely necessary. Herein, the orthophosphate removal behavior of Al coagulants with various species distributions was investigated. The results showed that AlCl₃·6H₂O (AC) had a more pronounced P removal efficiency than polyaluminum chloride (PACl). Medium (Al_b or Al₁₃) and high polymeric species (Al_c) played a more significant role in removing P than monomeric species (Al_a). During coagulation, adsorption onto flocs was the dominant P removal mechanism, which could be categorized as multilayer adsorption. Although the adsorption kinetics showed that physical adsorption best described the adsorption mechanism for AC and PACl, it is worth noting that chemical adsorption also occurred during P removal by AC because of the formation of the AlPO₄ precipitate. This could be because of the strong complex adsorption between the in situ Al₁₃ species and P. Based on the excellent P removal performance, we believe these findings will have a large potential for application in advanced P removal in water treatment.