Harnessing graft-to polymerization for pulp reject-based flocculants: Chain-length dependent adsorption bridging drives efficient antibiotic removal

The rational design of sustainable flocculants for combating antibiotic pollution in aquatic systems requires a fundamental mechanistic understanding of structure-activity relationships. In this study, three pulp reject-based flocculants (PRBF-to, PRBF-from, and PRBF-to-from) with tailored branched-chain architectures were synthesized via distinct grafting copolymerization strategies – “graft to” and “graft from”. Comprehensive structural characterization confirmed significant differences in chain length, with PRBF-to demonstrating the most extended branched architecture. The flocculation performance was systematically evaluated for the removal of four representative antibiotics-norfloxacin (NOR), cefalexin (CFX), oxytetracycline (OTC), and sulfamethoxazole (SMX)- in both isolated systems and in the presence of humic acid (HA) and Kaolin. Jar test results demonstrated that PRBF-to achieved superior NOR removal efficiency (26.15 %±3.11 %), significantly outperforming PRBF-from and PRBF-to-from by 16 % and 17 %, respectively. Mechanistic studies highlighted chain-length-dependent behaviors: PRBF-from primarily functioned through charge neutralization, while PRBF-to leveraged extended hydrogen-bonding domains and adsorption bridging via its elongated architecture, as evidenced by XPS and interaction force analysis. In contrast, PRBF-to-from relied on sweep flocculation. The dominance of adsorption bridging in antibiotic removal highlights the critical importance of chain length optimization for enhancing flocculant-antibiotic interactions. This study not only advances lignin valorization for eco-friendly flocculant design but also establishes a chain-length engineering framework to tailor contaminant-specific removal mechanisms. These findings provide practical strategies for wastewater treatment in antibiotic-laden environments.