Composite adsorbent of Terminalia catappa fruit shell-derived activated carbon and chitosan for enhanced removal of chromium(VI): kinetics, thermodynamics, equilibrium and adsorption mechanism

BACKGROUND: Chromium, which is widely employed in many industries, has a high toxicity. In this research, activated carbon derived from Terminalia catappa fruit shells was combined with chitosan (TCAC-CS) to enhance adsorption capability of hexavalent chromium [Cr(VI)] in wastewater. RESULTS: Different Cr(VI) adsorption parameters were investigated. Two isotherm and two kinetic models were employed to understand the adsorption mechanism. The thermodynamic characteristics of Cr(VI) adsorption, namely the Gibbs free energy change (ΔG), enthalpy change (ΔH) and entropy change (ΔS), also were computed. The best adsorption capability was achieved at pH 3, adsorbent dose 2.5 g L⁻¹, temperature 35 °C and initial Cr(VI) concentration 50 mg L⁻¹. The Cr(VI) adsorption capacity reached 184.45 mg g⁻¹, as fitted by the Langmuir isotherm, which is superior when compared to most biomass-based carbonaceous adsorbents. The negative ΔG value shows the spontaneous nature of the Cr(VI) adsorption process on the TCAS-CS. Additionally, Fourier transform infrared characterization confirmed that the incorporation of chitosan to activated carbon resulted in chemical adsorption of Cr(VI) via chelating instead of sole physical adsorption via hydrogen bonding, which greatly improved the adsorption ability of the materials. CONCLUSION: TCAC-CS demonstrated potential utilization as an efficient, low-cost and environmentally friendly adsorbent in the removal of Cr(VI) from wastewater.