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Title: Temperature-dependent infrared and calorimetric studies on arsenicals adsorption from solution to hematite nanoparticles

item SABUR, M - Wilfrid Laurier University
item Goldberg, Sabine
item GALE, ADRIAN - Georgia State University
item KABENGI, NADINE - Georgia State University
item AL-ABADLEH, HIND - Wilfrid Laurier University

Submitted to: Langmuir
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/19/2015
Publication Date: 2/27/2015
Citation: Sabur, M.A., Goldberg, S.R., Gale, A., Kabengi, N., Al-Abadleh, H.A. 2015. Temperature-dependent infrared and calorimetric studies on arsenicals adsorption from solution to hematite nanoparticles. Langmuir. 31(9):2749-2760.

Interpretive Summary: Arsenic is a trace element that is toxic to animals at elevated concentrations. Toxic concentrations of 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 arsenic species is necessary. Adsorption of arsenate, monomethylarsonic acid, and dimethylarsenic acid by the iron oxide mineral, hematite, was investigated under changing conditions of solution ion concentration and temperature. The adsorption behavior of arsenate, monomethylarsonic acid, and dimethylarsenic 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: To address the lack of systematic and surface sensitive studies on the adsorption energetics of arsenic compounds on metal (oxyhydr)oxides, we conducted temperature-dependent ATR-FTIR studies for the adsorption of arsenate, monomethylarsonic acid, and dimethylarsinic acid on hematite nanoparticles at pH 7. Spectra were collected as a function of concentration and temperature in the range 5-50 °C (278-323 K). Adsorption isotherms were constructed from spectral features assigned to surface arsenic. Values of Keq, adsorption enthalpy, and entropy were extracted from fitting the Langmuir model to the data and from custom-built triple-layer surface complexation models derived from our understanding of the adsorption mechanism of each arsenical. These spectroscopic and modeling results were complemented with flow-through calorimetric measurements of molar heats of adsorption. Endothermic adsorption processes were predicted from the application of mathematical models with a net positive change in adsorption entropy. However, experimentally measured heats of adsorption were exothermic for all three arsenicals studied herein, with arsenate releasing 1.6-1.9 times more heat than methylated arsenicals. These results highlight the role of hydration thermodynamics on the adsorption of arsenicals, and are consistent with the spectral interpretation of type of surface complexes each arsenical form in that arsenate is mostly dominated by bidentate, MMA by a mixture of mono- and bidentate, and DMA by mostly outer sphere.