Deciphering the roles of organic matter types and hydraulic conditions on the crystal transformation and release behavior of organic matter during floc aging

There have been numerous studies on floc formation between coagulants and natural organic matter (NOM). However, research on floc aging remains relatively scarce. This study systematically investigated the influence of NOM types and hydraulic conditions on the morphology, structure and stability of flocs during the aging process. Results indicated that flocs underwent hydroxyl-bridging reactions and crystallization during aging, leading to a decrease in floc activity. Humic acid (HA) exhibited a significant release phenomenon during aging, because its adsorption and complexation with aluminum could promote the aluminum dissolution-precipitation process. In contrast, bovine serum albumin (BSA) remained relatively stable during aging, which was attributed to the steric hindrance effect and hydrogen bonding interactions. Based on the two dimensional correlation spectroscopy analysis, high-intensity rapid agitation could inhibit hydroxyl-bridging reactions by reducing the number of η-OH 2 active sites on floc surfaces. Low-intensity slow agitation was conductive to floc formation. Compared with Al-HA flocs, Al-BSA flocs possessed higher shear resistance due to sweep flocculation. Consequently, the hydraulic parameters of the coagulation process need to be adjusted according to the specific water quality characteristics. Specifically, for water matrices with high loads of HA-like substances, it is recommended to either shorten the hydraulic retention time or implement the hydraulic conditions defined for the S-20 group. This study provided new insights into the interaction between NOM and coagulants during aging, which laid a theoretical foundation for the optimization of technical parameters in drinking water treatment and the reuse of flocs.