Enhanced residual water quality in the dewatering process of dredged sediment slurry via flocculation: Intrinsic mechanisms of dissolved organic matter removal using cationic flocculants.

Cationic flocculants are extensively applied to enhance dredged sediment dewaterability, yet residual water quality-particularly dissolved organic matter (DOM) removal-remains underexplored. This study systematically compares polydiallyl dimethyl ammonium chloride (PDDA) and cationic polyacrylamide (CPAM) for DOM removal during sediment dewatering, employing synthetic (silica-HA/BSA systems) and real dredged slurries. In synthetic systems, PDDA achieved peak removal efficiencies of 94.8 % for humic acid (HA) and 98.4 % for bovine serum albumin (BSA), surpassing CPAM by 10 % and 30 % respectively. Real-slurry tests demonstrated PDDA's superior performance, removing 68.0 % humic substances and 49.5 % proteins versus CPAM's 24.1 % and 10.0 %. PDDA treatment reduced residual water COD from 71.2 mg/L to 7.6 mg/L and turbidity from 1618 NTU to 4.8 NTU, confirming its dual efficacy in dewatering and water purification. Mechanistic analysis revealed PDDA's enhanced performance stems from robust electrostatic interactions between its cationic N groups and HA's anionic moieties (COO, O). For BSA, PDDA's compact structure facilitated intermolecular salt bridge formation with minimal steric hindrance. Conversely, CPAM primarily engaged HA through weaker n-π and hydrogen bonding, while its bulky polymer chains sterically hindered BSA binding. This work elucidates the DOM removal mechanisms of cationic flocculants in sediment dewatering and establishes PDDA as a high-efficiency alternative for simultaneous sludge conditioning and water quality improvement. The findings advance sustainable dredging practices by addressing both solid-liquid separation and secondary pollution control.