Master of Science (MSc)
Faculty of Science
Hind A. Al-Abadleh
Little is known about the surface chemistry of organoarsenic compounds with iron-containing materials commonly found in geosorbents and arsenic removal technologies. In this thesis, these organoarsenicals include aliphatic and aromatic arsenicals that historically have been used as herbicides, pesticides and in the poultry feed industry, respectively. Attenuated total internal reflectance Fourier transform infrared spectroscopy has been used to study the surface interactions of organoarsenicals at the molecular level under various environmentally-relevant conditions. Kinetic and thermodynamic parameters extracted from these molecular level experiments provide trends that are not observed in any published bulk experiments. Apparent initial rates of adsorption and desorption for these arsenicals were extracted from experimental data as a function of spectral components. Initial first order pseudo-adsorption rate constants () for arsenate (iAs), monomethylarsonic acid (MMA), p-arsanilic acid (pAsA) and phenylarsonic acid (PhAs) were quantified and show that arsenate and monosubstituted organoarsenicals adsorb with similar rate on Fe-(oxyhydr)oxide. Hydrogen phosphate was used as a desorbing agent due to its ubiquitous presence in arsenic contaminated aquatic environments. The desorption of monosubstituted organoarsenicals from hematite surfaces takes place faster than iAs(ads). The initial desorption of these organoarsenicals with an overall non-unity order suggests the existence of more than one type of surface complex. Adsorption kinetics for aqueous hydrogen phosphate was also investigated on surfaces in the presence and absence of surface arsenic, and the values of follow the order: Fresh hematite > pAsA/hematite ≥ PhAs/hematite > MMA(V)/hematite ≥ iAs(V)/hematite. From this study, it is observed that the amount of weakly bonded pAsA(ads) and PhAs(ads) surface species are larger than those for MMA(ads) and iAs(ads) at neutral pH.
Moreover, thermodynamic parameters were obtained for the adsorption of iAs(V), MMA(V) and DMA(V) on iron (oxyhydr)oxide surfaces as a function of concentration (0.001 to 2 mM) and temperature (5 to 50 °C). Values of the free energy of adsorption (∆G°) were extracted from fits using the Langmuir adsorption model and found to be negative within the experimental temperatures range. The entropy (∆S°) and enthalpy (∆H°) of adsorption were also extracted from the linear least-squares regression analysis of van’t Hoff plots constructed from the experimental data. The values of ∆S° are positive for all the arsenicals undertaken in this study. The values of ∆H° suggest that adsorption of iAs(ads) and MMA(ads) are endothermic processes whereas those for DMA(ads) are exothermic. The positive ∆S° along with the positive ∆H° suggest that the ligand exchange reactions are entropy driven.
In order to gain further insight into the mechanism of binding, pH(D) edge experiments were carried out for the adsorption of iAs(V) and the monosubstituted organoarsenicals, MMA(V) and PhAs, on hematite nanoparticles at room temperature. The arsenicals show stronger adsorption on iron (oxyhydr)oxide with slight shifting of the spectral features to higher wavenumbers in relatively acidic pH(D). The infrared signatures obtained from the pD edge experiments provide clear spectral information for the surface species. Spectra show that the extent of adsorption increases with decreasing pH(D). Arsenate forms mainly bidentate surface complex. In the case of MMA(ads) and PhAs(ads), surface complexes are most likely to be inner-sphere monodentate in acidic pD with longer adsorption time.
Sabur, Md Abdus, "SURFACE COMPLEXATION OF MONOSUBSTITUTED ORGANOARSENICALS ON HEMATITE: ATR-FTIR INVESTIGATIONS" (2014). Theses and Dissertations (Comprehensive). 1638.