The interaction of organic matter with the interfaces of active soil components such as iron oxides is ubiquitous within soil environments. The presence of organics at these interfaces may have implications for other soil constituents whose mobility is controlled by their ability to bind to active soil components. Most of the studies performed to date which look at these interactions are bulk/batch studies performed ex-situ. Attenuated total internal reflectance Fourier transform infrared spectroscopy (ATR-FTIR) was utilized within this work to study interactions between select model organics (citrate, oxalate and pyrocatechol) and iron−(oxyhydr)oxides, as well as their effect on the surface chemistry of organic and inorganic arsenicals. Using baseline-corrected peak height measurements, model organics were reacted with iron−(oxyhydr)oxide nanoparticles as a function of pH and concentration to generate pH envelope and adsorption isotherm curves. The Langmuir adsorption model was applied to adsorption isotherm curves to obtain Keq constants for model organics on the surface of hematite nanoparticles. Kinetic adsorption experiments were performed for model organics on hematite nanoparticles over a range of ionic strength conditions, with results showing a positive correlation between ionic strength and observed initial adsorption rates (robs1), obtained from the Langmuir adsorption model, of citrate and oxalate. Experiments on the kinetic desorption of model organics from the surface of hematite nanoparticles were also carried out to obtain initial rates of model organic desorption (k’des1) using the Langmuir desorption model; 1 mM of arsenate, 1 mM of dimethylarsinic acid (DMA), and a range of chlorine concentrations were utilized as the desorbing agents. These results show that arsenate is an effective desorbing agent for all three of the model organics. Conversely, the effect of DMA and chloride as desorbing agents varied, with citrate being moderately sensitive, oxalate being very sensitive, and pyrocatechol being insensitive. 7 Arsenate and DMA adsorption kinetic experiments on hematite nanoparticles, which were either reacted or unreacted with model organics, were performed to obtain robs1. These experiments were plotted as a function of aqueous arsenic concentration, analyzed using the Langmuir adsorption model to obtain pseudo- first order adsorption rates (kads1). The results of these experiments show that the presence of surface oxalate on hematite nanoparticles has an enhancing effect on the initial rates of arsenate and DMA adsorption, when compared to unreacted hematite. Conversely, the presence of surface pyrocatechol was shown to have an inhibiting effect on the initial rate of adsorption of arsenate to hematite nanoparticles. Results shown herein, along with aqueous phase comparisons of iron organic standards, provided information that has culminated in proposed surface complexes of the studied model organics on the surface of hematite nanoparticles under environmentally-relevant conditions. It is proposed that: oxalate forms a combination of outer-sphere and inner-sphere binuclear monodentate complexes, citrate forms an inner-sphere monodentate complex with either singly- or doubly-deprotonated carboxylic groups, and pyrocatechol forms a mixture of inner-sphere binuclear bidentate and monodentate surface complexes. The significance of this work is in providing a better fundamental understanding of the effect that different organic functional groups have on the binding of arsenicals to geosorbents, and thus the mobility of arsenicals within environments where organics are prevalent
