Valorization of waste biomass for biochar production and arsenic removal: A comparative assessment

The objectives of the study was to assess the impact of different waste feedstocks: crop straw (wheat), kitchen waste, leaf litter (sal), invasive plant (Lantana), fruit peel (orange), and dried fruit waste (walnut), on the physico-chemical properties of biochar (produced at 500 °C) and their influence on As(III) adsorption at varying As(III) concentrations (0.5–14 mg L⁻¹), reaction time (15–240 min), reaction temperature (5–45 °C), solution pH (3–12), and adsorbent dose (1–15%). Biochars were produced with yields of 19.3–29.3%, and contained 41.37–78.08% of fixed carbon and 5.3–40.5% of ash content, which could be crucial during remediation. The biochars were alkaline and porous (up to 0.093 cm³ g⁻¹) with high surface area (up to 148.05 m² g⁻¹), surface functionality (like –OH, =C[dbnd]O, =C[dbnd]C =, and –CH), and mineral content (calcite, graphite, quartz, and sylvine). Optimum As(III) adsorption capacity was observed as 1.8 mg g⁻¹ for wheat straw-based biochar, 2.5 mg g⁻¹ for kitchen waste-based biochar, 2.7 mg g⁻¹ for leaf litter-based biochar, 3.9 mg g⁻¹ for Lantana camara-based biochar, 6.5 mg g⁻¹ for orange peel-based biochar, and 2.8 mg g⁻¹ for walnut shell-based biochar. Emergence/disappearance of peaks in FTIR spectra and XRD diffractograms suggested participation of surface functional groups and mineral components of biochars in As(III) removal via complexation, electrostatic attraction, and hydrogen bonding. While Freundlich isotherm model was found to fit more suitably among various isotherm models (Langmuir, Freundlich, Dubinin-Radushkevich, and Temkin) indicative of multilayer As(III) adsorption, pseuso-second order kinetic model fitted better among the kinetic models (pseudo-first and pseudo-second order) denoting dominance of chemisorptive mechanisms. The study observed participation of different mechanisms in As(III) removal and suggests conversion of biomass wastes into biochars for sustainable removal of As(III) from dynamic hydro-geosphere systems.