Sorption kinetics of As(V) with iron-oxide-coated cement-a new adsorbent and its application in the removal of arsenic from real-life groundwater samples.

In the search of alternative adsorbents for the removal of arsenic, a new adsorbent (iron-oxide coated cement) was examined under various conditions to evaluate its suitability in removing As(V). A removal of more than 99% was obtained within 2 h for an initial As(V) concentration of 1 mg/L. Kinetic studies showed a removal which became somewhat constant after 2 h. Thorough investigations to understand the mechanism of solute adsorption onto the new sorbent with the help of four kinetic models, viz. approximately first-order reversible kinetics model, pseudo-first-order kinetic model, second-order kinetic model, and pseudo-second-order kinetic model showed that the sorption kinetics is consistent with the pseudo-second-order model from which it can be inferred that the mechanism of adsorption is chemical interaction or chemisorption. The equilibrium data followed the Langmuir isotherm at low concentrations of As(V) (<or=4 mg/L), but followed the Freundlich isotherm over a wide range of concentrations (0.5-10 mg/L) which can be due to solid solution precipitation. Studies on the effect of pH on As(V) removal revealed somewhat constant ( approximately 95%) As(V) removal over a wide pH range of 4-10. The efficiency of this adsorbent was also checked for the removal of arsenic from three real ground water samples containing both As(III) and As(V) with total arsenic in the range of 0.33-1.2 mg/L. The total arsenic removal was 71%, 87% and 75% for the samples containing 0.33, 0.88 and 1.2 mg/L total arsenic, respectively at the same adsorbent dose of 3 g/L. However, increasing the adsorbent dose will enhance removal capacity. Investigations of these real samples with the four kinetic models revealed most reliable fit with the pseudo-second order model. However, good fit was also obtained with the other three models. Values of the diffusion coefficients calculated from the first order kinetics data, indicated that, film diffusion is the rate limiting step for each of the three real life groundwater samples.