Adsorption of Cu2+ and Cr6+ in Aqueous Solution by a Thermally Modified Biosorbent Based on Cotton Cakes

摘要

This work focuses on the retention capacity of Cu~(2+) and Cr~(6+) ions in water by adsorption on a thermally modified biosorbent based on cotton cakes. The solutions treated are synthetic copper nitrate solutions rich in Cu~(2+) ions and potassium dichromate rich in Cr~(6+) ions in distilled water. The adsorption tests were carried out by mixing, with stirring (250 rpm), 0.02 g of the adsorbent for the solution containing the Cu~(2+) ions and 0.04 g for that containing the Cr~(6+) ions with a volume of 50 mL of these solutions. Kinetic studies have shown that the maximum adsorption is obtained after 20 minutes. In addition, the pH of the medium influences the adsorption process and the yield is improved with increasing pH. Amounts of copper and chromium adsorbed increase with increasing initial concentrations of these ions up to 40 mg/L. Similarly, adsorbed amounts increase with increasing mass of the adsorbent. On the other hand, these adsorbed quantities decrease with increasing temperature. The best adopted kinetic models are pseudo-second-order, Intraparticular and Evlovich. The adsorption isotherms conform to the Langmuir and Dubinin-Redushkovic models. The values of thermodynamic parameters such as free enthalpy (ΔH° KJ/K.mol), entropy (ΔS° KJ/K.mol) and isosteric adsorption heat (ΔG° KJ/K. mol) at temperatures of 293; 303; 313 and 323K, all negative, show that we have to do with an exothermic adsorption, physical, ordered and spontaneous. The electrostatic attraction between the Cu~(2+) and Cr~(6+) ions and the adsorbent for the pH values above the pH of the point of zero charges shows that we have to do with a chemical adsorption.