Characteristics and mechanisms of aluminum salts on arsenate removal by coagulation: Significance of aluminum speciation distribution and transformation

It is well known that the hydrolyzed aluminum species of Al-based coagulants play a critical role in coagulation process. So far, however, little information has been collected on arsenate removal from the perspectives of aluminum speciation distribution and transformation. In this study, three different Al-based coagulants have been applied to investigate the distribution and transformation of hydrolyzed aluminum species and their coagulation behaviors for arsenate removal. High levels of in situ Al₁₃ were produced from AlCl₃ under weakly acidic conditions. Lab-prefabricated polyaluminum chloride PACl₁ was almost entirely preformed Al₁₃ under acidic conditions while large polymeric aluminum species Al₁₄ and Al₁₅ became dominated under alkaline conditions. While for commercial polyaluminum chloride PACl₂, large polymeric aluminum species were the dominated in the pH range of 4–9. Results showed that the best coagulation effect occurred at pH 6 for three coagulants with arsenate removal were all above 95%. The distribution and transformation of Al species were variant in different Al-based coagulants, which results in difference of mechanisms on arsenate removal. AlCl₃ had synergistic effects on arsenate removal based on charge neutralization of in situ Al₁₃ and sweep flocculation of Al(OH)₃. Charge neutralization of preformed Al₁₃ generated by PACl₁ and the bridging adsorption of Al₁₃ aggregates have a key role to play in arsenate removal. The sweep flocculation by Al(OH)₃ during PACl₂ coagulation process was proved to be the main function for arsenate removal. This study provides a new insight into the interaction and mechanism between arsenate and various aluminum species of Al-based coagulants.