ATR-FTIR studies on the kinetics of dimethylarsinic acid (DMA) surface complexation with iron(oxyhydr)oxides

Dimethylarsinic acid (DMA) is an organoarsenical compound that, along with monomethylarsonic acid, poses a health and an environmental risk, and a challenge to the energy industry. Little is known about the surface chemistry of DMA at the molecular level with materials relevant to geochemical environments and industrial sectors. The surface chemistry of phosphorus and arsenic compounds in their organic and inorganic forms is of great interest due to its role in controlling their transport, bioaccessibility and speciation. This thesis reports the first in-situ and surface-sensitive rapid kinetic studies on the adsorption and desorption of DMA to and from Fe-(oxyhydr)oxide films hematite and goethite at pH 7 and I=0.01 M KCl using attenuated total reflection Fourier transform infrared spectrometry (ATR-FTIR). The adsorption of phosphate in the presence and absence of DMA and arsenate was also carried out. Values for the apparent rates of adsorption and desorption of DMA were extracted from experimental data as a function of spectral components, flow rate of the aqueous phase, film thickness of hematite, and using chloride and hydrogen phosphate as desorbing agents.;The DMA adsorption kinetic data showed fast and slow rates, consistent with the formation of more than one type of adsorbed DMA complex. Apparent pseudo adsorption and pseudo 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 chloride solutions, and were found to be higher by 1- 2 orders of magnitude than those using chloride.;The phosphate adsorption kinetic data showed adsorption rate constants of phosphate on Fe-(oxyhydr)oxide films increase in this order: arsenate-covered < DMA-covered ≤ freshly-prepared. Also, desorption rate constants of DMA complexes were 7 times higher than arsenate using phosphate as a desorbing agent. When these results are combined with earlier work on the thermodynamics, kinetics, and structure of surface complexes, this data suggests that, within minutes to 1 hr of surface interaction, increasing organic substitution on arsenate increases the proportion of relatively weakly-bonded complexes (ie. monodentate and outer-sphere).