Variations in NOM during floc aging: Effect of typical Al-based coagulants and different particle sizes

Most studies on the interaction between coagulation and NOM (natural organic matter) currently focus on pollutant removal and coagulant species distribution, while studies on floc aging are lacking. Investigation onto the effects of floc aging could guide further processes that utilize flocs, such as densadeg sludge recirculation, floc predeposition for ultrafiltration, sludge condensation, and other traditional sludge reflux processes. In this study, flocs generated by Al₁₃ and AlCl₃ in microparticle- and nanoparticle-containing water were investigated, and the effect of floc aging on NOM was quantified based on several organic matter characterization techniques. Flocs absorb and release organics during aging. The flocs generated from micro-SiO₂ have a significant absorbing effect for LWM-N (low-molecular-weight neutral substances) and protein-like substances, while the absorption of NOM by flocs generated from nano-SiO₂ is insignificant. HS (humic substances) with high aromaticity are released during floc aging. From the molecular perspective, the molecules released during floc aging are those with higher double bond equivalents and higher aromaticity, while the absorbed molecules are those with lower double bond equivalents and lower aromaticity. 2D-COS (two-dimensional correlation spectroscopy) demonstrated that the flocs generated by Al₁₃ and AlCl₃ had the same organic release patterns but different intensities, while the flocs generated in the micro-SiO₂ and nano-SiO₂ systems had different organics release patterns. Abundant aluminum hydrolysates with low polymerization and amorphous Al(OH)₃ would be produced from AlCl₃ during the coagulation process and then undergo hydroxyl‑bridging reaction and crystallization during floc aging, thus releasing more HS with high aromaticity into the supernatant; in comparison, prehydrolyzed Al₁₃ produces a more stable floc and releases less HS during aging. The flocs produced by nano-SiO₂ and Al-based coagulants release higher aromaticity HS into the water than those produced by micro-SiO₂, which may be related to the formation of more highly polymerized degree hydrolysates and nanocrystalline Al(OH)₃ in the nano-SiO₂ system. The flocs generated in water with micro-SiO₂ may contain a large amount of Al-OH and have a loose structure, thus further absorbing NOM, such as protein-like substances and LWM-N. In contrast, the flocs generated from nano-SiO₂ possess abundant adsorbed water and a denser structure; thus, organic matter cannot be absorbed stably.