AmericanChemicalSociety.com

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 refining processes. Our goal is to investigate fundamental properties of DMA interaction with metal (oxyhydr)oxides surfaces relevant to the petroleum industry. This surface phenomena is of interest because organoarsenicals can 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 pH and ionic strength in H2O and D2O. Spectroscopic signature of adsorbed DMA was analyzed for surface structure and binding mechanism. Desorption behavior due to chloride and phosphate was also studied a function of time from the decrease in the absorbance of apparent spectral features. Quantum mechanical calculations 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. Adsorption of DMA on iron (oxyhydr)oxides occurs mainly through ligand exchange mechanism. Results indicate the simultaneous formation of inner- and outer-sphere complexes with distinct spectral components. A number of ligand exchange reactions were formulated. These reactions will be useful for the construction of surface complexation models. Desorption experiments have shown that both phosphate and chloride anions are capable of desorbing DMA. Spectral components assigned to strongly-bonded inner-sphere complexes showed slower kinetics than those having major contributions from weakly-bonded outersphere complexes. Our results indicate that under neutral to acidic conditions with relatively high iron and aluminium 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 of each complex experimentally by using the model derived from the adsorption mechanism. Quantum chemical calculations on the ligand exchange reactions mentioned above will also be performed to estimate binding thermodynamics of these clusters. Carefully designed kinetics experiments will be conducted to derive relative desorption rate constants from spectral data.