Kinetic ATR-FTIR studies on phosphate adsorption on iron (oxyhydr)oxides in the absence and presence of surface arsenic: molecular-level insights into the ligand exchange mechanism.

The surface chemistry of phosphorus and arsenic compounds in their organic and inorganic forms is of great interest to the scientific and industrial communities due to its role in controlling their transport, bioaccessibility and speciation. We report herein in situ and surface-sensitive rapid kinetic studies on the adsorption of phosphate to Fe (oxyhydr)oxides in the presence and absence of dimethylarsinic acid (DMA) and arsenate. These studies were conducted at pH 7 using ATR-FTIR in the flow mode, which were complemented with detailed kinetic analysis of the desorption behavior of DMA and arsenate over the same range of aqueous [phosphate]. Values for apparent rates of adsorption and desorption were extracted from the time dependence of given spectral components characteristic of surface phosphate, DMA, or arsenate. Results show that pseudo adsorption rate constants of phosphate on Fe (oxyhydr)oxide films increase in this order: arsenate-covered < DMA-covered ≤ freshly prepared. Also, pseudo desorption rate constants of DMA complexes are 7-12 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, they suggest that, during initial times of surface interactions, increasing organic substitution on arsenate increases the proportion of relatively weakly bonded complexes (monodentate and outer-sphere).