Adsorption of Cr (VI) Onto Low-Cost Adsorbent Developed from Acacia Nilotica Leaf Activated with Phosphoric Acid: Kinetic, Equilibrium Isotherm and Thermodynamic Studies

A carbonaceous adsorbent prepared from an indigenous waste, by acid treatment was tested for its efficiency in removing metal ions. This adsorbent was found to be effective and economically attractive. Batch mode experiments were conducted to study the effects of pH, contact time, adsorbent dose, initial concentration of adsorbate on the adsorption of Cr(VI). Adsorption equilibrium was obtained in 90min for 50 to 200 mg/L of Cr(VI) concentrations. Cr(VI) adsorption was significantly dependent on solution pH, and the optimum adsorption was observed at pH 2. Equilibrium isotherm data were analyzed by the Langmuir, Freundlich, D-R Temkin, Harkin-Jura and Frenkel Halsey models. Results indicated that adsorption process obeys Freundlich isotherm model than other models with the R2 value of 0.9698 for Chromium (VI), which indicates favorable adsorption of Cr(VI) in single and multiple system onto Acacia nilotica leaf carbon. Pseudo-first-order, pseudo-second-order, Elovich and intraparticle diffusion models were used to analyze the kinetic data obtained at different initial Cr(VI) concentrations. The adsorption kinetic data were described very well by the pseudo-second-order model. The adsorption capacity of Cr(VI) was found to be 96%. The values of thermodynamic parameters like (ΔG°), (ΔH°) and (ΔS°) indicated that adsorption was spontaneous and endothermic in nature. In column studies, the decrease of the flow rate resulted in the increase of the removal efficiency. Further biosorption trials with adsorbents showed a considerable reduction in metal uptake capability of adsorbents in binary and ternary systems as compared to singly metal system. It was concluded that the interactions between metals in mixture may be of importance as the antagonistic interactions contribute negatively to the accumulation of heavy metals in waste water discharges. That is of great concern in the design of adsorption system in real wastewater treatment plant. The eco-friendly adsorbent PAN is expected to be an environmentally and economically feasible adsorbent for the removal of Cr(VI) from aqueous solution and real waste water.