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Title: Spectral characterization and surface complexation modeling of low molecular weight organics on hematite nanoparticles: Role of electrolytes in the binding mechanism

item SITUM, ARTHUR - Wilfrid Laurier University
item RAHMAN, MOHAMMAD - Wilfrid Laurier University
item Goldberg, Sabine
item AL-ABADLEH, HIND - Wilfrid Laurier University

Submitted to: Environmental Science: Nano
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/5/2016
Publication Date: 7/6/2016
Citation: Situm, A., Rahman, M.A., Goldberg, S.R., Al-Abadleh, H.A. 2016. Spectral characterization and surface complexation modeling of low molecular weight organics on hematite nanoparticles: Role of electrolytes in the binding mechanism. Environmental Science: Nano. 3:910-926. doi:10.1039/c6en00132g..

Interpretive Summary: Hematite nanoparticles have unique reactivity with organic matter and are ubiquitous in environmental systems. Understanding of their surface chemistry and their reaction with low molecular weight organic molecules can lead to better prediction of environmental fate under biologically relevant conditions. A better understanding of the adsorption behavior of organic species is necessary. Adsorption of citric acid, oxalic acid, and pyrocatechol by iron oxide mineral, hematite, nanoparticles was investigated under changing conditions of solution ion concentration and solution pH. The adsorption behavior of citric acid, oxalic acid, and pyrocatechol was evaluated and described using a chemical surface complexation model. Spectral analysis provided molecular-level details for the development of thermodynamic models based on the binding mechanisms. Our results will benefit scientists who are developing models of organic species and nanoparticle movement in soils. The results can be used to improve predictions of organic species behavior in soils and thus aid action and regulatory agencies in the management of soils which contain elevated concentrations of organic species and nanoparticles. An improved understanding of the organic species-hematite nanoparticle interface can lead to better predictions of environmental fate reactivity and aid in design of medical devices and drugs.

Technical Abstract: Given the ubiquity of organic-metal oxide interfaces in environmental and medical systems, it is incumbent to obtain mechanistic details at the molecular level from experimental procedures that mimic real systems and conditions. We report herein the adsorption pH envelopes (range 9-5) and isotherms at pH 7 using attenuated total internal reflectance Fourier transform infrared spectroscopy (ATR-FTIR) for citrate, oxalate and pyrocatechol on hematite nanoparticles. Experimental data were coupled with the application of triple layer surface complexation models derived for each organic compound based on the interpretation of spectral data. Results indicate that the structure of the organic species affects the type and relative amounts of surface complexes formed with consequences on surface charge as shown from ionic strength-dependent studies: citrate forms a mix of protonated monodentate inner-sphere with one negative charge and deprotonated outer-sphere with net two negative charges; oxalate forms mostly doubly deprotonated outer-sphere with inaccessible neighboring site with contributions from deprotonated inner-sphere, and pyrocatechol forms mostly bidentate inner-sphere complexes. Ion-pairing effects between uncomplexed carboxylate groups and the electrolyte used to adjust the ionic strength caused an overall enhancement in the amount adsorbed of citrate and oxalate. Also, compared to adsorbed oxalate, hematite nanoparticles retain more adsorbed citrate and pyrocatechol when flowing chloride as a negatively-charged weak desorbing agent at pH 7. These results have implications on the overall surface chemistry of hematite nanoparticles in the presence of organic matter, particular those containing carboxylate and phenolate functional groups.