Adsorptive removal of cadmium by natural red earth: Equilibrium and kinetic studies

Natural red earth (NRE), an iron-coated sand found in the north western part of Sri Lanka, was used to examine the retention behaviour of cadmium, a heavy metal postulated as a factor of chronic kidney disease in Sri Lanka. Adsorption studies were conducted as a function of pH, ionic strength, initial Cd loading and time. The Cd adsorption increased from 6% to 99% with the pH increase from 4 to 8.5. The maximum adsorption was reached at pH>7.5. Cadmium adsorption was not changed over 100-fold variations of NaNO₃, providing evidence for the dominance of an inner-sphere bonding mechanism for both 10-fold variation of initial Cd concentrations. Surface complexation modelling suggests a monodentate bonding mechanism. Isotherm data were fairly fitted to a two-site Langmuir isotherm model and sorption maximums of 9.11×10^-6 and 3.89×10⁷molg^-1 were obtained for two surface sites. The kinetic study reveals that Cd uptake by NRE is so fast that the equilibrium was reached within 15 min and ∼ 1h for 4.44 and 44.4μM initial Cd concentrations, respectively, and the chemisorption was the dominant mechanism over intra-particle diffusion. The study indicates the potential of NRE as a material for decontaminating environmental water polluted with Cd.