A green approach for remediation of hexavalent chromium [Cr (VI)] from tannery effluent using rice hull biochar and assessing its impact on germination of Cicer arietinum (Chickpea)

The leather industry in India contributes significantly to water pollution through the discharge of hexavalent chromium [Cr (VI)]. Conventional Cr (VI) removal techniques are costly and environmentally unsustainable. This study presents an eco-friendly approach of Cr (VI) remediation from tannery wastewater using rice hull biochar (RHB). This study demonstrates the efficacy of RHB as a sustainable biosorbent for hexavalent chromium [Cr(VI)] removal from tannery effluent. Under optimized conditions—pH 4, biosorbent dosage of 0.8 g/L, and contact time of 120 min, RHB achieved 91% Cr (VI) removal efficiency with a maximum adsorption capacity of 10.54 mg/g. Characterization studies revealed that RHB has a specific surface area of 81.80 m²/g, an average pore size of 3.32 nm, and a Type IV isotherm with H4 hysteresis loop, confirming its mixed microporous-mesoporous structure suitable for heavy metal adsorption. FTIR analysis identified key functional groups (hydroxyl, amine, and carbonyl) responsible for chromium binding, while SEM-EDS confirmed the heterogeneous porous morphology. Kinetic analysis demonstrated that the adsorption process follows pseudo-second-order kinetics (R² > 0.98), indicating chemisorption as the dominant mechanism. Intraparticle diffusion modelling revealed a two-stage adsorption process involving boundary layer diffusion and intraparticle diffusion. Isotherm studies showed excellent fits to both Langmuir (R² = 0.95) and Freundlich (R² = 0.96) models, with Freundlich parameters (Kₗ = 4.040, 1/n = 1.133) suggesting favourable multilayer adsorption. The study evaluated the phytotoxicity of treated effluent through Cicer arietinum seed germination trials. Seed germination trials demonstrated that treated effluent at a 1:4 dilution ratio (20% effluent, 80% biochar residue) significantly enhanced root length, shoot length, leaf number, nodule formation, and fresh/dry biomass compared to controls. It confirmed the detoxification of chromium and suitability for agricultural reuse. These findings establish RHB as a cost-effective, eco-friendly biosorbent that transforms agricultural waste into a valuable resource for industrial wastewater treatment, supporting circular economy principles and Sustainable Development Goals through water recycling and agricultural applications.