Submitted to: Journal of Colloid and Interface Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/15/2011
Publication Date: 2/24/2011
Citation: Mitchell, W., Goldberg, S.R., Al-Abadleh, H.A. 2011. In-situ ATR-FTIR and surface complexation modeling studies on the adsorption of dimethylarsenic acid and p-arsanilic acid on iron-(oxyhydr)oxides. Journal of Colloid and Interface Science. 358:534-540. Interpretive Summary: Arsenic is a trace element that is toxic to animals at elevated concentrations. Toxic concentrations of organic arsenic compounds can occur in agricultural soils and irrigation waters through the application of herbicides and contaminated poultry manure from feed additives. A better understanding of the adsorption behavior of these organic species is necessary. Adsorption of dimethylarsenic acid and p-arsanilic acid by two iron oxide minerals was investigated under changing conditions of solution pH and solution ion concentration. The adsorption behavior of dimethylarsenic acid and p-arsanilic acid was evaluated and described using a chemical surface complexation model. Our results will benefit scientists who are developing models of organic arsenic movement in soils. The results can be used to improve predictions of organic arsenic behavior in soils and thus aid action and regulatory agencies in the management of soils which contain elevated concentrations of organic arsenic species.
Technical Abstract: Arsenic is an element that exists naturally in many rocks and minerals around the world. It also accumulates in petroleum, shale, oil sands and coal deposits as a result of biogeochemical processes, and it has been found in fly ash from the combustion of solid biofuels. Arsenic compounds in their organic and inorganic forms pose both a health and an environmental risk, and continue to be a challenge to the energy industry. The environmental fate and removal technologies of arsenic compounds are controlled to a large extent by their surface interactions with inorganic and organic adsorbents. We report herein thermodynamic binding constants, Kbinding, from applying the triple layer surface complexation model to adsorption isotherm and pH-envelope data of dimethylarsenic acid, DMA, and p-arsanilic acid, p-AsA, on hematite and goethite. Ligand exchange reactions were constructed based on the interpretation of ATR-FTIR spectra of DMA and p-AsA surface complexes. Surface coverage of adsorbates was quantified in-situ from the spectral component at 840 cm-1. Best fit to the DMA adsorption data was obtained using outer-sphere complex formation, whereas for p-AsA, best fit was obtained using two monodentate inner-sphere surface complexes. The significance of the results is discussed in relation to improving modeling tools used by environmental regulators and the energy sector for optimum control of arsenic content in fuels.