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.

The removal of As(V) by conditioned, calcined layered double hydroxide (LDH) adsorbents is investigated in continuous-flow, packed-bed columns, in order to study the effect of important operating parameters, such as the influent As concentration, the pH, the adsorbent particle size, and the flow rate. Earlier bed saturation and breakthrough were observed at higher flow rates and influent concentrations. On the other hand, a decrease in the adsorbent particle size and the influent pH resulted in an increase in the number of bed volumes at breakthrough. A column model which accounts for external, liquid-film mass transport and for diffusion and adsorption in the adsorbent particles is utilized. Two different adsorption models are employed, which were shown previously to be capable of predicting the As(V) uptake by LDH adsorbents. They are a conventional homogeneous surface diffusion model and a bidisperse pore model, the latter viewing the LDH particles as assemblages of microparticles and taking into account bulk diffusion in the intraparticle pore space, and surface diffusion within the microparticles themselves. Both models are found capable of predicting the flowcolumn experimental results.

Analysis of effluents from various power plants indicates that, in addition to arsenic (As), these effluents also contain such ions as fluorides, nitrates, chlorides, carbonates, sulfates, and phosphates. Adsorption is considered the method of choice for the treatment of waste streams containing As. Of concern, therefore, is the impact of such competing anions on the capacity for As adsorption of the various materials utilized. In this work, we systematically investigated the effects of such ions on the As removal capacity of layered double hydroxides (LDHs), a class of materials investigated by our group and others as effective adsorbents for As and other heavy metals. The adsorption of As in the presence of each of these competing ions on LDH follows the extended Sips isotherm. It is concluded from our study that, of all of these ions, fluorides and nitrates have the least effect on As adsorption, followed by chlorides, carbonates, sulfates, and phosphates. Phosphates, in particular, have a very strong impact.

This paper describes a study of CO 2 diffusion and adsorption in layered double-hydroxide (LDH) adsorbents using a gravimetric method. Four different particle size fractions, namely, 53-75, 75-90, 90-180, and 180-300 µm, were used in the study. The LDH materials were calcined in situ prior to the experiments, which resulted in significantly higher CO 2 adsorption capacities than previously reported. The CO 2 adsorption was shown to follow a Sips-type isotherm, while the adsorption capacity at saturation was found to be independent of the particle size. Two models, the homogeneous surface diffusion model (HSDM) and the bidisperse pore model (BPM), were used to fit the experimental data and to estimate the diffusivities. The main assumption made in the BPM is that the adsorbent particle is an agglomerate of a number of equalsized, single-crystal microparticles, and before a CO 2 molecule is adsorbed in the microparticles, it has to diffuse through the intercrystalline porous region. Both models perform well in fitting the experimental data. However, the HSDM yields diffusivities that are a strong function of the particle size, whereas the diffusivities estimated using the BPM are almost independent of the particle size.