Cd2+ adsorption on alkaline-pretreated diatomaceous earth: equilibrium and thermodynamic studies

Naturally occurring diatomaceous earth was modified by alkaline pretreatment, and its effectiveness for Cd2+ removal from contaminated water was investigated. Batch experiments were carried out to determine Cd2+ adsorption capacity and the efficiency of the sorption process under different experimental conditions. Experimental data showed good fitting to Langmuir and Freundlich isotherms models. The Cd2+ maximum adsorption capacity was 0.058 mmol g−1 for raw diatomite and increased to 0.195 mmol g−1 for alkaline-pretreated diatomite with efficiency higher than 96% (diatomite dose 2.5 g L−1, pH 6). Adsorption of Cd2+ to alkaline-pretreated diatomite increased as the temperature increased. Thermodynamic parameters were calculated to evaluate the feasibility of the adsorption process at different temperatures. The adsorption process was spontaneous and endothermic. The interaction between Cd2+ ions and diatomite surface was weak enough to be considered as physical sorption, confirmed by the low value of activation energy.