Integrated Chitosan-based coagulation and microbubble pre-treatment for improved microplastic fibre removal from water

Microplastic fibres (MPFs) dominate treatment plant influents but are difficult to remove due to their morphology, composition and low density, limiting the effectiveness of coagulation flocculation pre-treatment. Integrating coagulation-flocculation with microbubble introduction offers a sustainable route to improve MPF removal, yet conventional inorganic coagulants often require high dosages, show strong pH dependence, and pose environmental risks. Chitosan is a biodegradable and non-toxic green alternative, but its application is limited due to its low solubility and pH sensitivity. In this study, two amphoteric derivatives of Chitosan have been successfully developed: CMC-CTA, produced by modifying carboxymethyl Chitosan (CMC) with 3-chloro-2-hydroxypropyl trimethyl ammonium chloride (CTA); and CMC-g-PDMC, obtained by grafting poly[(2-methacryloyloxyethyl) trimethyl ammonium chloride] (PDMC) onto the CMC backbone. MPF removal was evaluated in a deionised (DI) water matrix containing sodium dodecyl benzene sulphonate (SDBS) surfactant using a tailored turbidity measurement approach. To broaden the investigation, additional matrices were explored, including humic acid (HA) alone and HA combined with MPF (HA + MPF) at varying HA concentrations. In the DI water + SDBS matrix, the highest MPF removal was achieved using 1 mg/L CMC-CTA at pH 7. Zeta potential measurements indicated that charge patching occurred during the coagulation process, where microbubble attachment to MPF flocs appeared to facilitate the removal mechanism. The same system was also effective in reducing over 50% of the turbidity caused by 20 mg/L HA in both the HA-only and HA + MPF matrices. The outcomes provide great insights into the hybrid pre-treatment approaches targeting MPFs' removal.