Graft copolymerization synthesis of chitosan-polyferric sulfate composite coagulant to improve biogas slurry treatment toward effective irrigation

Abstract

Biogas slurry from anaerobic digestion of organic wastes can be a potential biofertilizer for agricultural irrigation, which however, is challenged by suspended solids and contaminants. Thus, this study synthesized a composite coagulant and optimized its performance to advance biogas slurry treatment. A natural-synthetic polymer, chitosan (CTS), was modified by 2-methacryloxyethyltrimethyl ammonium chloride (DMC) via graft copolymerization and then combined with polyferric sulfate (PFS) to formulate the composite CTS-g(DMC)-PFS coagulant. Results show that CTS-g(DMC)-PFS exhibited stronger electrical neutralization and adsorption bridging to destabilize and aggregate colloidal particles, thus, exhibiting higher removal of suspended solids, heavy metals, and antibiotics over individual and pristine coagulants. Graft copolymerization of CTS with DMC at the mass ratio of 1:9 maximized its water solubility. Further blending this mixture with PFS at the mass ratio of 1:2 effectively improved the coagulation of biogas slurry, particularly for the removal of antibiotics and heavy metals (e.g. enrofloxacin and Cu). Moreover, CTS-g(DMC)-PFS produced dense and compact flocs for effective sedimentation. Detailed characterization attributed such improvement to the hydrolysis of cationic quaternary ammonium groups on grafted monomers and further coordinative effects between CTS-g(DMC) and Fe to enhance molecular chains and positive charges in CTS-g(DMC)-PFS to facilitate particle aggregation, contaminant adsorption, and then floc sedimentation.