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.

This paper describes a study of arsenic(V) adsorption on appropriately conditioned calcined layered double hydroxide (LDH) adsorbents. Conditioning the adsorbent significantly reduced the dissolution previously observed with uncalcined and calcined LDH. The adsorption rates and isotherms have been investigated in batch experiments. As(V) adsorption is shown to follow a Sips-type adsorption isotherm. The As(V) adsorption rate on conditioned LDH increases with decreasing adsorbent particle size; the adsorption capacity of As(V) on conditioned LDH, on the other hand, is independent of the particle size. When a homogeneous surface diffusion model was used to fit the experimental kinetic data, the estimated diffusivity values were shown to depend on the particle size. On the other hand, when a bidisperse pore model, which considers the adsorbent particles to be agglomerates of microparticles with a porous region present between them, is applied to the same data, it predicts an intracrystalline diffusivity, which is fairly invariable with the particle size.