Efficient removal of polystyrene microplastics using a novel clay-biochar composite: An insight into effect of pH, DOM and interfering ions

The widespread accumulation of microplastics (MPs) in aquatic environments highlights the urgent need for effective remediation strategies. Biochar-based adsorption has emerged as a sustainable and cost-efficient approach. In this study, a novel montmorillonite-biochar composite (MBC) was synthesized from bamboo biomass to enhance polystyrene microplastics (PS-MPs) removal efficiency. The synthesized adsorbents were characterized using scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), Brunauer–Emmett–Teller (BET) analysis, X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). These characterization techniques confirmed the successful loading of montmorillonite onto the biochar surface. The results showed that MBC exhibited a markedly higher removal efficiency (90 %) compared to pristine biochar (BC), which achieved 78 % under the same experimental conditions. SEM analysis revealed three key immobilization mechanisms—’Stuck,’ ‘Trapped,’ and ‘Entangled’—affecting PS-MPs retention on the adsorbent surface. Dissolved organic matter (humic acid), pH, and Ca 2+ concentration significantly impacted the adsorption capacity, with optimal adsorption occurring at pH 6. The maximum adsorption capacities of BC and MBC were 45.78 and 55.66 mg/g, respectively. The adsorption mechanisms of BC were mainly governed by van der Waals forces and hydrophobic interactions, whereas MBC additionally benefited from electrostatic and π–π interactions. The adsorption kinetics fitted well with pseudo-first-order and pseudo-second-order models (R 2  > 0.94), and isotherm fitting revealed compatibility with Langmuir and Freundlich models (R 2  > 0.93). These results demonstrate that MBC is a promising material for efficient MPs removal and support the development of low-cost, environmentally friendly water treatment technologies suitable for mitigating plastic pollution in aquatic ecosystems.