Deciphering the flocculation mechanism of cellulose - modified polymers for antibiotic and co-pollutant removal: Structural determinants from structure-activity relationships.

The widespread use of antibiotics has led to their persistent accumulation in aquatic environments. Although flocculation is cost-effective, traditional flocculants exhibit limited removal efficiency for antibiotics. To address this problem, a novel cellulose-based flocculant (CE-DMC) was synthesized from paper pulp rejects. The methacryloxyethyltrimethyl ammonium chloride (DMC) monomer was successfully grafted onto the cellulose backbone with a grafting rate of 64 %, endowing CE-DMC with high positive charge density (+71.1 mV at pH 6) and a branched architecture. Flocculation experiments demonstrated the superior performance of CE-DMC in removing four representative antibiotics. Notably distinct removal efficiencies were achieved: ofloxacin exhibited the highest removal (52.6 %), followed by sulfamethoxazole (36.4 %), norfloxacin (30.9 %), and amoxicillin (29.8 %), reflecting differential interaction mechanisms with the adsorbent. The flocculation mechanism of CE-DMC could be concluded as follows: (1) charge neutralization via quaternary ammonium groups of DMC chains, (2) n-π* electron donor-acceptor interactions and π-π* conjugated interactions mediated by benzene rings and furan rings of cellulose skeleton, and (3) hydrogen bonding force provided by the hydroxyl group from the cellulose skeleton and the DMC chains. Notably, coexisting humic acid (HA) and kaolin enhanced antibiotic removal efficiency by forming large composite pollutants via n-π* electron donor-acceptor interactions, hydrogen bonding forces, and π-π* conjugated interactions. CE-DMC synergistically removed antibiotics and co-pollutants, with its flocculation mechanism elucidated through structure-activity relationships and structural determinants. This work advances the rational design of natural polymer-based flocculants for multi-mechanistic contaminant removal.