Cr(VI) uptake by a composite of processed diatomite with MCM-41: Isotherm, kinetic and thermodynamic studies

Silica was dissolved from a processed diatomite (PD) at 110 degrees C/48 h to prepare and coat MCM-41 on the PD surface developing a composite of (PD/MCM-41). The new composite was characterized by different techniques (XRD, SEM, EDX and TEM) and used as an adsorbent for the first time in hexavalent chromium Cr(VI) uptake. The adsorption experiments were carried out as a function of solution pH, contact time, initial Cr(VI) concentration, temperature and PD/MCM-4ldose. The highest removal efficiency (93.4%) of Cr(VI) occurred at pH 2.0 through electrostatic interaction. The majority of Cr(VI) was removed within 5 min of contact time and adsorption equilibrium was attained after 30 min. For determining the behavior of Cr(VI) uptake, Langmuir, Freundlich and Dubinin-Radushkevich (D-R) equations were applied to the experimental data. Freundlich isotherm with a correlation coefficient, R-2 = 0.996 described the adsorption data well. The chemical nature of Cr (VI) uptake was confirmed by the mean free energy (E=8.765 kJ/mol) calculated from the D-R model. The pseudo-second-order model fitted (R-2 = 0.9999) the sorption kinetics well. With large concentrations of chloride, phosphate, sulphate and fluoride in solutions, the synthetic composite exhibited high selectivity for Cr(VI) at pH 2.0. Thermodynamic parameters (Delta H, Delta G and Delta S) indicated that Cr(VI) sorption is spontaneous and endothermic.