Sustainable design of biomass-derived biochar–clay composites for selective micropollutant removal via thermoplastic waste valorization

Abstract

The pervasive presence of micropollutants poses a serious threat to aquatic ecosystems and human health, demanding efficient and sustainable treatment strategies. In this study, circular-economy-driven composite membranes were fabricated by valorising waste polyvinyl chloride (PVC) and reinforcing it with biochar derived from tea waste and kaolinitic siliceous clay as multifunctional fillers for the simultaneous removal of microplastics and dyes. The biochar exhibited a mesoporous structure with a BET surface area of 13.97 m² g⁻¹ and average pore diameter of 2.78 nm, providing abundant adsorption sites and continuous transport pathways that promoted an efficient micropollutant rejection of 85% for the biochar composite membrane. The kaolinite-modified membrane has a maximum micropollutant rejection of 93.0%, while the dual-filler membrane demonstrated a mixed-contaminant removal of 82% with a pure water flux of 13.4 L m⁻² h⁻¹. Surface characterisation showed an increase in roughness from 33.05 nm (M0) to 129.26 nm (M3), enhancing adsorption and antifouling properties. Adsorption behaviour followed the Freundlich isotherm and pseudo-second-order kinetics, indicating heterogeneous chemisorption-dominated dye uptake. The dual filler membrane further demonstrated improved antifouling performance, with enhanced flux recovery and reduced irreversible fouling during repeated filtration cycles. Overall, this study demonstrates a cost-effective and sustainable circular-economy-based approach by valorising thermoplastic waste into composite membranes for efficient simultaneous removal of microplastics and dyes.