Retention and release of hexavalent and trivalent chromium by chitosan, olive stone activated carbon, and their blend

Shrimp shells and waste of olive stones were used as feedstock for the preparation of chitosan and activated carbon. The adsorption of Crⱽᴵ and Crᴵᴵᴵ species in aqueous solution by the materials prepared and their blend were studied by using the well-known kinetic and isotherm models, Fourier transform infrared spectroscopy and scanning electron microscope. It was demonstrated that the rates of adsorption were controlled by diffusion inside particles and throughout the liquid film, and adsorption occurred spontaneously (− 26 < ∆G° < − 15 kJ/mol) in the range of 298–333 K, except for that involving Crᴵᴵᴵ species and activated carbon. The maximum amounts of Cr species retained by the composite (146 mg of Crⱽᴵ/g and 33 mg of Crᴵᴵᴵ/g at 298 K) were three times greater than those of the basic constituents. Adsorption was markedly affected by temperature and pH, and Crⱽᴵ species were substantially desorbed in acid mediums, particularly in acetic acid solution. The recovery of Crᴵᴵᴵ species varied according to the adsorbent and the solution used. The immobilization of Crⱽᴵ species (HCrO₄⁻ and CrO₄²⁻) and Crᴵᴵᴵ species (Cr(OH)₂⁺ and Cr₃(OH)₄⁵⁺) by chitosan was accomplished by means of amine moieties and hydroxyls of D-glucosamine units of the biopolymer. The adsorption of Crⱽᴵ species on activated carbon involved π electrons of aromatic rings as well as oxygenated sites (C–OH, C=O, C–O–C). In such a condition, Crⱽᴵ was partially reduced into Crᴵᴵᴵ. For the composite, the amino functional groups of chitosan and hydroxyls of both constituents were implicated in the linkage of the biopolymer and activated carbon, and the C–O–H and C–O–C functional groups of chitosan were involved in the retention of Crⱽᴵ species. For Crᴵᴵᴵ species, adsorption occurred preferentially on hydroxyls of the components, and consequently, the chains of the biopolymer recovered some flexibility.