Modeling One-Dimensional Transport of Arsenate Through Iron Oxide-Coated Sands

The fate and transport of metals in groundwater are commonly controlled to a significant extent by sorption reactions occurring at aqueous/solid interfaces. Traditionally, isotherms have been preferred in the groundwater literature to model sorption, since the incorporation of these models into groundwater transport equations is relatively straightforward. Alternatively, surface complexation models (SCMs) present a more mechanistic approach to sorption modeling. Due to their relative complexity and parameter requirements, however, their application in groundwater transport models has been more limited. In this work, the influence of sorption model selection on the behavior of transport predictions is explored using two alternative numerical models. Results are compared for predictions of arsenate transport experiments in columns packed with iron oxide-coated sands. The first modeling approach incorporates experimentally determined batch isotherms, that describe sorption of arsenate under constant pH conditions, in a mass-conservative finite element transport code developed for this study. In the second approach, the hydrogeochemical transport code PHREEQC-2 is employed to simulate transport of arsenate using a SCM sub-model. This SCM is based upon a self-consistent set of surface complexation parameters, developed from available titration data for arsenate sorption onto the iron oxide surfaces. Model predictions are compared with physical measurements of arsenate breakthrough under varying pH conditions. Preliminary results illustrate the promise of the SCM modeling approach, suggesting that independent batch sorption measurements can be used, within the framework of the SCM, to produce a more versatile transport model, capable of accounting accurately for temporal and spatial variations in geochemical conditions.