Enhancing floc growth by oxidative and reductive modification of organic matter structure

Coagulation is a critical process in drinking water treatment for removing contaminants, but its efficiency is challenged by complex natural organic matter (NOM) and emerging micropollutants. Conventional strategies (e.g., enhanced coagulation and novel coagulants) overlook the role of NOM characteristics in floc formation. Existing studies lack systematic evaluation of how chemical preconditioning (oxidation/reduction) modifies NOM functional groups and influences floc dynamics, particularly regarding molecular-scale mechanisms and water matrix effects. This study comparatively evaluates hypochlorite (NaClO), hydrogen peroxide (H₂O₂), and sodium borohydride (NaBH₄) pretreatments for NOM modification and their impacts on floc growth during coagulation. It was found that while NaClO (0.2-4 mg/L) reduced molecular weight and increased hydrophilicity, it consistently enhanced floc size (up to 44 %) by hydroxylating aromatic NOM. H₂O₂ exhibited a biphasic effect, and floc size improved at low doses (1 mg/L) but declined at high doses (>4 mg/L) due to excessive carboxyl group formation, inducing electrostatic repulsion. NaBH₄ (50-100 mg/L) promoted floc growth by reducing carbonyl to hydroxyl groups, though high doses (>100 mg/L) reduced efficacy due to borate interference. This work establishes molecular-level mechanisms for NOM-coagulant interactions, highlights the essential role of hydroxyl groups on floc formation, and provides a scientific basis for optimizing pretreatment-coagulation sequences tailored to specific water quality.