Hind A. Al-Abadleh, PhD
, Wilfrid Laurier University
Dimethylarsinic Acid (DMA) belongs to an important class of methylated organoarsenical compounds that exist in fossil fuels and biomass as impurities of biogeochemical origins. They lower the purity of fuel and poison catalysts used in refinery processes. Our goal is to investigate fundamental properties of DMA interaction 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) to investigate the surface interactions of DMA with goethite and hematite particles. 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 was analyzed for the kinetics of adsorption and desorption. Desorption kinetics due to flowing chloride and phosphate were studied from the decrease in the absorbance of apparent spectral features assigned to surface DMA. Quantum mechanical calculations of the thermodynamics of binding using DFT/B3LYP method were also performed on DMA bound to cluster models of iron (oxyhydr)oxides to aid in the interpretation of experimental data. The adsorption kinetic spectral data show fast and slow rates, consistent with the formation of more than one type of adsorbed DMA. Apparent adsorption and desorption rate constants were extracted from the dependency of the initial adsorption rates on [DMA(aq)]. Desorption rate constants were also extracted from desorption experiments using hydrogen phosphate, and were found to be higher by 1-2 orders of magnitude than those using chloride. In light of the complex ligand exchange reaction mechanism of DMA desorption by phosphate species at pH 7, apparent desorption rate constants were found to depend on [hydrogen phosphate] with an order of 0.3. 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, ÆGads for various ligand exchange reactions between hydrated complexes of DMA and Fe-(oxyhydr)oxide clusters calculated using density functional theory (DFT) at the B3LYP/6-311+G(d,p) level. Calculations using arsenate were also performed for comparison. Calculated As-(O,Fe) bond distances and stretching frequencies of As-O bonds are also reported for comparison with experimental spectroscopic data. Gibbs free energies of desorption, ÆGdes, due to reactions with phosphorous species at pH 7 were also calculated. Our results indicate that the formation of both inner- and outer-sphere DMA complexes is thermodynamically favorable, with the former having a more negative ÆGads. Values of ÆGdes indicate that desorption favorability of DMA complexes increases in this order: bidentate < mondentate < outersphere. Our results indicate that under neutral to acidic conditions with relatively high iron content and low phosphorus conditions, weak outer-sphere complexes become mobile, and transport of colloidal or nano-size particles with strongly-bonded DMA could become an important transport mechanism. Under high phosphorus conditions, DMA becomes mobalized and readily transportable. Technologies aimed at removing DMA could be designed to lower the arsenic content of organic-rich fuels. For example, the As-content could be reduced by washing fuels with slurries of iron-(oxyhdr)oxides instead of water alone, and contaminated particles could be collected, recycled or compressed into pellets.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.