Removal of trace levels of arsenic and selenium from aqueous solutions by calcined and uncalcined layered double hydroxides (LDH)

The adsorption of As and Se from dilute aqueous solutions, representing model power-plant effluent streams, by calcined and uncalcined layered double hydroxide (LDH) adsorbents has been investigated. The results indicate that As(V) has a greater adsorption capacity than Se(IV) for both calcined and uncalcined LDHs. An mth-order kinetic rate equation describes the adsorption kinetics of As and Se on these adsorbents. The adsorption isotherms obey the Freundlich model. The adsorption capacities of As(V) and Se(IV) on the calcined LDH are higher than those on the uncalcined LDH. The starting solution pH does not significantly influence adsorption of As and Se on calcined LDH, as long as it is higher than 4; As and Se adsorption on uncalcined LDH is, however, more sensitive to variations in the initial pH. Studies of the effect of the metal oxidation state on adsorption reveal that As(III) is more difficult to remove than As(V) by both the calcined and uncalcined LDHs. However, the adsorption of Se(IV) and Se(VI) is relatively similar on both calcined and uncalcined LDHs. Competing ions have a greater effect on Se(IV) uptake than on As(V) uptake. Desorption of As(V) and Se(IV) from the LDH depends on the type of ion species and their concentration in the desorbing solutions.