Hind A. Al-Abadleh, PhD, Wilfrid Laurier University
The surface chemistry of phosphorus and arsenic compounds in their organic and inorganic forms is of great interest to the scientific and industrial communities due to its role in controlling their transport, bioaccessibility and speciation. Our goal is to investigate fundamental properties of surface interactions of these oxyanions with metal (oxyhydr)oxide surfaces relevant to the petroleum industry. This surface phenomena is of interest because organoarsenicals bind to surface sites through their inorganic moieties.
We utilized the surface-sensitive technique attenuated total internal reflectance Fourier transform infrared spectroscopy (ATR-FTIR). Experiments were conducted as a function of time, flow rate of aqueous phase and film mass on the ATR crystal. Spectroscopic signature of adsorbed DMA, arsenate and phosphate were analyzed for the kinetics of adsorption and desorption. Quantum mechanical calculations of the thermodynamics of binding using DFT/B3LYP method were also performed on cluster models to aid in the interpretation of experimental data.
Results show that pseudo adsorption rate constants of phosphate on Fe-(oxyhydr)oxide films increase in this order: arsenate-covered < DMA-covered ≤ freshly-prepared. Also, pseudo desorption rate constants of DMA complexes are 7-12 times higher than arsenate using phosphate as a desorbing agent.
Published surface sensitive x-ray and infrared spectroscopic work suggested that DMA simultaneously forms inner- and outer-sphere complexes with iron-(oxyhydr)oxides. Computational work on the complexation of arsenicals with various surfaces of environmental and industrial interest provides useful information that aids in the interpretation of experimental spectroscopic data as well as predictions of thermodynamic favorability of surface interactions. We reported Gibbs free energies of adsorption and desorption, DGads and DGdes for various ligand exchange reactions between hydrated complexes of DMA, arsenate, phosphate and Fe-(oxyhydr)oxide clusters calculated using density functional theory (DFT) at the B3LYP/6-311+G(d,p) level. Calculated As-(O,Fe) bond distances and stretching frequencies of As-O bonds are also reported for comparison with experimental spectroscopic data. Our results indicate that the formation of both inner- and outer-sphere DMA complexes is thermodynamically favorable, with the former having a more negative DGads. Values of DGdes indicate that desorption favorability of DMA complexes increases in this order: bidentate < mondentate < outersphere.
When these results are combined with earlier work on the thermodynamics, kinetics, and structure of surface complexes, this data suggest that –during initial times of surface interactions- increasing organic substitution on arsenate increases the proportion of relatively weakly-bonded complexes (monodentate and outer-sphere). Hence, under neutral conditions with relatively high Fe and P conditions, DMA becomes mobilized, and readily bioaccessible for uptake and recycling to other forms of arsenic. In technologies aimed at lowering the arsenic content in organic-rich fuels or industrial waste water, introducing Fe-(oxyhdr)oxides in a form that maximizes contact with the contaminated media would be an efficient procedure. However, careful analysis has to be done to the type of stable species that co-exist with arsenic compounds, particularly those such as phosphorous that have the same or higher affinities to compete for sites on the Fe-containing removal media.
Next, we will quantify the binding strength and kinetics of DMA to surface covered with organic acids. Quantum chemical calculations on these ligand exchange reactions will also be performed to estimate binding thermodynamics of these clusters.