Evaluation of the nanoplastics removal by using starch-based coagulants: Roles of the chain architecture and hydrophobicity of the coagulant

Four starch-based coagulants with similar charge densities but different chain architectures and hydrophilicities were designed and fabricated. These are a linear and hydrophilic coagulant, St-CTA (Starch-3-chloro-2-hydroxypropyl triethyl ammonium chloride); a graft and hydrophilic coagulant, CS-AM-DMC (cationic starch-graft-poly[2-methacryloyloxyethyl trimethyl ammonium chloride and acrylamide]); and two graft and partially hydrophobic coagulants, CS-AM-DML (cationic starch-graft-poly[methacrylic acid 2-(benzyldimethylaminio) ethyl chloride and acrylamide]) and CS-AM-DMR (cationic starch-graft-poly[2-(methacryloyloxy)-N,N-dimethyl-N-alkyl ammonium chloride and acrylamide]) with aromatic and n-octane groups respectively. These starch-based coagulants in conjunction with polysilicic acid were employed in a current major environmental challenge which is to efficiently remove nano-sized plastics (NPs) from water. NPs of polystyrenes, polymethyl methacrylates and polyvinyl chlorides were removed from different water sources under various conditions of pH and salinity. The effects of the chain architectures and hydrophilicities of the starch-based coagulants on the NPs removal have been investigated in detail. According to experimental evidence, three graft starch-based coagulants exhibited higher NPs removal efficiencies and better flocs properties than the linear St-CTA. This was attributed to the superiority of the graft chain architecture in the graft starch-based coagulants causing higher charge neutralization efficiencies in coagulative removal of NPs. In the three graft starch-based coagulants, CS-AM-DMR and CS-AM-DML with partially hydrophobic structures were more effectively to remove three NPs than CS-AM-DMC, and produced large, compact, durable and rapidly regrown flocs, due to the synergistic effects of charge neutralization and hydrophobic association. The interaction energies between NPs and the coagulants obtained from the extended Derjaguin-Landau-Verwey-Overbeek theory further confirmed that CS-AM-DMR and CS-AM-DML could achieve faster coagulation and thus higher efficiency in NPs removal. In addition, the NPs removal efficiency increased in acidic and highly saline solutions. These results obtained are of importance in guiding the design and selection of suitable polymeric coagulants for efficiently treating target contaminants in water.