Surface interactions of aromatic organoarsenical compounds with hematite nanoparticles using ATR-FTIR: kinetic...

Aromatic organoarsenicals p-arsanilic acid (pAsA) and roxarsone (ROX) are used as feed additives in developing countries that allow the use of arsenic-containing compounds in their poultry industry. These compounds are introduced to the environment through the application of contaminated poultry litter. Little is known about the surface chemistry of these organoarsenicals on the molecular level with reactive components in soils. We report herein the first in situ and surface-sensitive rapid kinetic studies on the adsorption and desorption of pAsA to/from hematite nanoparticles at pH 7 using ATR-FTIR. Values for the apparent initial rates of adsorption and desorption were extracted from experimental data as a function of spectral components. Hydrogen phosphate was used as a desorbing agent due to its ubiquitous presence in litter, and its adsorption kinetics was investigated on surfaces with and without surface arsenic. Initial first-order pseudo-adsorption rate constant for pAsA was lower by a factor of 1.6 than that of iAs(V), suggesting an average behavior for the formation of quantitatively more weakly bonded monodentate or hydrogen-bonded complexes for the former relative to strongly bonded bidentate surface complexes for the latter under our experimental conditions. Initial first-order pseudo-adsorption rate constants for hydrogen phosphate decrease in this order: fresh hematite > pAsA/hematite ≈ phenylarsonic acid (PhAs)/hematite > iAs/hematite by factors 1.5 and 3 relative to fresh films, respectively. Initial desorption kinetics of aromatic organoarsenicals due to flowing hydrogen phosphate proceed with a nonunity overall order, suggesting a complex mechanism, which is consistent with the existence of more than one type of surface complexes. The impact of our studies on the environmental fate and transport of aromatic organoarsenicals in geochemical environments and their overall surface chemistry with iron (oxyhyr)oxides is discussed.