Synchronous ozonation promoted the generation of freshly-formed hydrolyzed metal coagulant species: An inspiring approach for organic matter removal with low-coagulability

The removal of low-coagulability organic matter remains a major challenge for conventional coagulation (CC) process. Enhancing the formation of highly active polymeric coagulant species such as Al₁₃ is a possible strategy for improving the removal efficiency of low-coagulability organic matter. In this work, it was found that synchronous ozonation promoted the generation of freshly-formed polymeric hydrolyzed Al species during coagulation process at pH 5 and 7 through hydroxyl bridges (μ-OH) derived from surface hydroxyl groups and surface protonated hydroxyl groups. The freshly-formed polymeric hydrolyzed Al species generation during synchronous ozonation was unveiled through density functional theory (DFT) calculation. Therefore, the hybrid ozonation-coagulation (HOC) process was established, in which ozonation and coagulation occurred synchronously in a single reactor. Using 2,4-dichlorophenoxyaceticacid (2,4-D) as a typical low-coagulability organic, the HOC process achieved nearly 60 % dissolved organic carbon (DOC) removal efficiency, which significantly outperformed both the pre-ozonation coagulation (POC) and CC processes. Surface hydroxyl groups and surface protonated hydroxyl groups formed during Al hydrolysis can also serve as active sites to catalyze ozone to enhance •OH generation, which can improve the transformation of 2,4-D to oxidized intermediates. Freshly-formed polymeric hydrolyzed Al species exhibited a stronger ability to capture oxidized 2,4-D intermediates such as 2,4-dichlorophenol, 4,6-dichlororesorcin, 2-chlorohydroquinone, facilitating the separation performance due to enhanced floc formation. Stronger binding ability of freshly-formed hydrolyzed Al species with 2,4-D intermediates via hydrogen bonding and complexation was also proved by DFT calculation in molecular scale. As consequence, freshly-formed polymeric hydrolyzed Al species generated during the HOC process enhanced the capture of low-coagulability organic intermediates, which finally improved the low-coagulability organic matter removal.