Modelling diffusion and adsorption of As species in Fe/GAC adsorbent beds

Abstract

Background: Arsenic decontamination of drinking water by adsorption is a simple and robust operation. When designing packed bed adsorbers for arsenic, the main problems are the slow diffusion kinetics of As in microporous media and the lack of simple equations for predicting the performance of the equipment. Commercial iron-doped granular activated carbon adsorbents (Fe/GAC) for groundwater arsenic abatement were studied in this work. Basic parameters for arsenate (AsV) adsorption were measured and their performance at larger scale was simulated with an approximate analytical model. Results: In the 0-300 μgAs L-1 range, the AsV adsorption isotherm on Fe/GAC was found to be approximately linear. Assuming Henry's law for adsorption and homogeneous surface diffusion with constant diffusivity for intrapellet mass transfer, an approximate model for flow and adsorption of arsenate inside packed bed adsorbers was developed, and reduced to an analytic compact solution using the quasi-lognormal distribution (Q-LND) approximation. The use of this model with fitted and reported parameters enabled the approximate simulation of industrial adsorbers and home point-of-use filters. Results show that industrial adsorbers meet the breakthrough condition with incomplete utilization of the adsorbent unless convenient process configurations are used. In point-of-use systems with short residence times intraparticle diffusion would drastically reduce the adsorbent performance. Conclusion: Assuming linear adsorption of AsV over Fe/GAC, an analytical approximate solution for flow and adsorption in packed beds can be obtained. The model seems to represent correctly the main features of industrial and home filters, however, more experimental data is necessary for scale-up purposes.