Biosorption of Hexavalent Chromium Cr(VI) onto Ziziphus Lotus Fruits Powder: Kinetics, Equilibrium, and Thermodynamics

Abstract

Ziziphus lotus has been the subject of several researchs because of its nutritional benefits and ecological attributes. The removal of hexavalent chromium Cr(VI) from a solution using powdered Zizphus lotus fruits, for its qualities of being inexpensive as well as environmentally friendly, was investigated. The results obtained showed that at pH = 2, at 30°C, after 600 min of adsorbent/adsorbate contact, with 100 mg/L as initial concentration of Cr(VI) and a biosorbent dosage of 5 g/L, the biosorption of Cr(VI) on Zizphus lotus fruit powder (ZLFP) is at its maximum rate. The sorption process was exothermic (∆H° = -6.69 kJ/mol), and was characterized by a positive entropy values (∆S° = 46.76 J/K mol) suggesting a high affinity of the ZLFP for Cr(VI). Given that the Gibbs free energy (∆G°) is negative and decreases as temperatures increase from 293 to 323 K, the process of biosorption is both feasible and spontaneous. The Temkin model and the Langmuir model both generated excellent fits to the equilibrium data. The maximum monolayer biosorption capacity was 36.11 mg/g. The pseudo second order model was used to fit the kinetic data relating to the adsorption of Cr(VI) on the ZLFP. The FTIR spectral analysis allowed the characterization of the biochemical groups mainly involved in the sorption of Cr(VI) ions on the ZLFP, and which are: N-C, H-O, O-C, H-C, and O=C. The capacity of Ziziphus lotus fruit as an inexpensive, effective, and ecofriendly biosorbent is confirmed through this study.