Utilizing nano zero-valent iron impregnated biochar for removal of hexavalent chromium from water: An assessment through Box-Behnken optimization, kinetics, and isotherm studies

The current study focuses on synthesizing an ecofriendly biochar-supported nano zero-valent iron (nZVI) and evaluating its efficacy in removing hexavalent chromium [Cr(VI)] from aquatic systems under different experimental conditions. The study identifies the optimal conditions that have a substantial impact on Cr(VI) removal efficiency under different experimental sets including adsorbent dose (0.5 g L⁻¹ to 2 g L⁻¹), Cr(VI) concentration (20 mg L⁻¹ to 100 mg L⁻¹) and pH (2–8). The interactive impacts of important variables like pH, initial chromium concentration, composite doses, temperature, and time was assessed through response surface methodology (RSM) based on box-behnken design (BBD). The nZVI–biochar composite demonstrates superior performance compared to nZVI and biochar individually and highest removal efficiency was observed at pH 2. The second-order polynomial model for biosorption validated through ANOVA, have a high coefficient of determination (R² ≥ 0.98). Temkin and Sips models fit well with equilibrium data, suggesting multi-molecular layer adsorption, while pseudo-second order rate kinetics indicated chemisorption as the rate-limiting step. Furthermore, energy dispersive spectroscopy (EDS) confirmed the presence of Cr(VI) on the adsorbent surface, and fourier transform infrared spectroscopy (FTIR) indicated the contribution of functional groups in the binding mechanism.