Isotherm, Kinetic, and Thermodynamic Insights on Effective Sequestration of Cr(VI) by using De-oiled Karanja Seed Biochar

Abstract

This study investigates the potential of non-edible waste-derived Karanja seed (Pongamia pinnata) biochar (KSB) as a biosorbent for Cr(VI) removal from synthetic effluents. Characterization of KSB before and after Cr(VI) biosorption using thermogravimetric analy­sis, scanning electron microscopy/energy dispersive X-ray spectroscopy, X-ray diffraction, and Fourier-transform infrared spectroscopy confirmed that the adsorption process primarily occurs through chemisorption, ion exchange, and/or complexation mechanisms. The effects of several process parameters, including pH, contact time, initial Cr(VI) concentration, KSB dosage, and temperature, on Cr(VI) biosorption efficiency were systematically examined through batch experiments. The results indicated an optimum biosorption efficiency of 91% under the following conditions: initial Cr(VI) concentration of 20 mg L⁻¹, contact time of 60 min, pH 2, KSB loading of 0.01 g, and a temperature of 303 K. Adsorption equilibrium data were analyzed using the Freundlich and Langmuir isotherm models, with the Langmuir model yielding a maximum biosorption capacity of 164 mg g⁻¹, suggesting monolayer biosorption. Kinetic analysis demonstrated that the pseudo-second-order model provided the best fit, indicating that chemisorption governed Cr(VI) uptake on KSB, involving three distinct intra-particle diffusion stages. Thermodynamic parameters (ΔH⁰, ΔG⁰, and ΔS⁰) were evaluated, confirming that the biosorption process was spontaneous, exothermic, and thermodynamically feasible. Additionally, the biosorption-desorption performance of KSB for Cr(VI) was assessed through cyclic experiments, highlighting its regeneration potential for practical applications. Finally, the results revealed that KSB is an efficient and cheap biosorbent for the sequestration of Cr (VI) from a synthetic medium.