Laboratory and modelling studies of the sorption of arsenic on goethite
An understanding of arsenic interactions with mineral surfaces in aqueous environments is important if arsenic transport in groundwater is to be predicted. This study contributes to the understanding of arsenic (As(V)) adsorption onto goethite (alpha-FeOOH) by linking equilibrium batch experiments and dynamic column experiments to the use of atomistic modelling. In this way the mechanisms of arsenic adsorption onto goethite surfaces ore explored. The equilibrium batch experiments illustrate the declining affinity of As(V) as a negatively charged oxy-anion to sorption on goethite, with increasing solution pH. These experiments also demonstrate that non-linear, Freundlich or Langmuir adsorption is favoured. Partition coefficients obtained are consistent with previously published values. Column experiments using goethite distributed in quartz grains under a range of conditions of flow velocity, influent arsenic concentration and goethite mass concentration, being more representative of natural conditions of groundwater flow, have provided new observations on As(V) adsorption. The onedimensional solute transport code BIO1D has been used to simulate the advection and sorption processes and to fit partition coefficients to the experimental breakthrough curves. Linear and Freundlich isotherms both provide a close representation of the experimental data. The results demonstrate that the partition coefficients under dynamic flow conditions are up to three orders of magnitude less than those derived from the equilibrium batch experiments. Also, the partition coefficients show an inverse relationship with flow velocity, indicating a kinetic effect. Experimental variation of the column redox conditions has enabled the determination of a pseudo-partition coefficient for sorption under anaerobic conditions, which is up to a quarter of that derived under aerobic conditions. Atomistic modelling shows that the (011) and (111) surfaces of goethite ore preferential for binding As(V) as AsO(OH)3. The model simulations also indicate that monodentate-mononuclear complexes are favoured over bidentate-binucleor formations. The results show a preferential orientation for the adsorbed As(V) molecule, with the hydroxyl groups lying parallel and the oxygen anion perpendicular to the goethite surface. The preferred complexation mechanisms and the relative energetics calculated using the atomistic models are good indicators of the non-linear adsorption observed in the experimental work.