One-step synthesis of magnetic biochar via co-pyrolysis of walnut shells and Fe-rich mine tails for adsorption capacity improvement of polystyrene sulfonate microplastics: Role of microplastic size

Microplastics made of polystyrene sulfonate microplastics (PSMPs) are highly mobile in aquatic ecosystems and can consequently lead to undesirable health effects in humans. Herein, the adsorption capacity of PSMPs was comprehensively analyzed using untreated biochar made from ground walnut shells (WSB) and iron (Fe) engineered WSB acquired from Fe-mining waste (Fe-WSB), to explore the changes in the adsorption potential and mechanisms by the co-pyrolysis of walnut shells and Fe-rich mine tailings. The adsorption of PSMPs for Fe-WSB (adsorption capacity (Q_e) = 0.77–6.75 mg/g) was greater than that for WSB (Q_e = 0.27–0.79 mg/g), particularly at lower pH levels, indicating that Fe integration and electrostatic interaction between Fe-WSB and PSMPs significantly affected the adsorption of PSMPs. The R² values for adsorption kinetics and isotherms highlighted that chemisorption plays a fundamental role in PSMP adsorption using WSB and Fe-WSB in liquid solutions. Further, thermodynamic assessments indicated that PSMP₂₁₀ (210 Da), PSMP_{10 K} (10,000 Da), and PSMP_{32 K} (32,000 Da) were adsorbed exothermically, with the adsorption efficiency decreasing as van der Waals forces became weaker at high temperatures. The results of X-ray photoelectron spectroscopy, which was conducted on WSB and Fe-WSB both before and after the adsorption of PSMPs, supported the notion that an enhancement in the primary adsorption mechanism (electrostatic interactions, pore-filling effects, π-π and H-bond interactions), following the integration of Fe-oxides onto the WSB surfaces, improved the adsorption of PSMPs in aqueous environments.