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, X-ray diffraction, and scanning electron microscopy. The pH value in natural oyster shell increased from 9 to 12.7 following calcination above 750 °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 µm and 50-100 µm 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 (25.25-32.36 mg/g) that was associated with calcination temperatures of up to 650 °C for 2 h, and a second region that contributed to high Cd sorption from 750 °C, with the maximum sorption capacity reaching a value of 1666.67 mg/g at 900 °C. The high Cd-removal capacity of oyster shell-derived adsorbents above 750 °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.

Behavior of cadmium and lead contained in wood during the carbonization process.

The behavior of heavy metals in wood during its carbonization process was examined. Cadmium in wood samples was found to be volatile when the samples were carbonized at 600 degrees C or higher, which demonstrated that removal of cadmium was feasible. Meanwhile, lead was found to be barely volatile even if the wood samples were carbonized at 1,000 degrees C or higher, which demonstrated that lead was difficult to remove and recover. The possibility of removing/recovering lead contained in wood by energization was then examined. By examining the concentration of sulfuric acid used as an electrolyte as well as load voltage, approximately 10% of lead was found to be recoverable.