Mechanism of cadmium biosorption from aqueous solutions using calcined oyster shells

The physicochemical properties of oyster shell-derived adsorbents prepared by calcination at different temperatures were characterized by elemental analysis, specific surface area, particle size distribution, Xray diffraction, and scanning electron microscopy. The pH value in natural oyster shell increased from 9 to 12.7 following calcination above 750 degrees C. All of the oyster shell-derived adsorbents exhibited a BET surface area that ranged from 1.8 to 64.6 m(2)/g. Clearly, the proportion of particles within the ranges 25 -50 mu m and 50-100 mu m increased after calcination, regardless of calcination temperature. The adsorption equilibrium and kinetics of cadmium (Cd) were investigated, and the mechanisms of sorption discussed. Experimental equilibrium data were fitted to a Langmuir adsorption isotherm model. Most Cd adsorption occurred during the initial hours of contact time, and a pseudo-second-order kinetic model best fitted the adsorption process. Cd sorption profiles indicated an initial, low Cd sorption region (25.25-3236 mg/g) that was associated with calcination temperatures of up to 650 degrees C for 2 h, and a second region that contributed to high Cd sorption from 750 degrees C, with the maximum sorption capacity reaching a value of 1666.67 mg/g at 900 degrees C. The high Cd-removal capacity of oyster shell-derived adsorbents above 750 degrees C is attributed to their enhanced specific surface area, their material porosity, the bulk precipitation of Cd hydroxide and otavite on shell fragments, the formation of ettringite as a secondary precipitate, and ion exchange via Ca ions. This study highlights the effectiveness of calcined oyster shells in Cd removal from highly contaminated water and wastewater. (C) 2014 Elsevier Ltd. All rights reserved.