Adsorption of arsenic on conditioned layered double hydroxides: Column experiments and modeling

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 flow-column experimental results.