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Abstract
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 3.89×10−7 mol g−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 ∼ 1 h for 4.44 and 44.4 μM 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.
Acknowledgements
We wish to acknowledge the Monbukagakusho Scholarship for providing financial support. Thanks go to Associate Prof. Hidekazu Tanaka and Mr Lakmal Jayaratna for the assistance given in FTIR studies. Ms Anushka Rajapakshe and Mr Chanaka Kapila Kumara are also acknowledged for the assistance given in data modelling. The authors would like to acknowledge Prof. C.B. Dissanayake for his support in improving the manuscript.