Modified vermiculite as adsorbent of hexavalent chromium in aqueous solution

The aim of this study was to investigate the efficiency of removing Cr6+ from aqueous solutions using two exfoliated vermiculite: (1) heated abruptly at 1000◦C and (2) irradiated with microwave radiation. The effects investigated were contact time, adsorbate concentration and initial Cr6+ concentration. The adsorption with both exfoliated vermiculites was well described by the DKR isotherm, indicative of a cooperative process and with the pseudo second order kinetic model. The Kd value for the two exfoliated vermiculites was similar, 0.2 × 1010 µg/Kg. The maximum adsorption capacity of Cr6+ with thermo-exfoliated vermiculite, 2.81 mol/g, was much higher than with microwave irradiated vermiculite, 0.001 mol/g; both values were obtained with 0.5 g of vermiculite in contact with distilled water enriched with 1 ppm of Cr6+ for 24 h. Factors such as ion chemistry, the solution pH and ionic strength, influence the values of capacity, adsorption energy and initial adsorption rate values of the exfoliated vermiculite. In addition, these values depended on the exfoliation process, being the adsorption capacity highest with abrupt heating of vermiculite, while the adsorption energy and rate values showed just a slight increase with microwave irradiation. This aspect is important to select the most suitable vermiculite modification treatment to use it as an adsorbent.