ATR-FTIR and Flow Microcalorimetry Studies on the Initial Binding Kinetics of Arsenicals at the Organic-Hematite Interface.

The environmental fate of arsenic compounds depends on their surface interactions with geosorbents that include clays, metal oxides, and natural organic matter (NOM). While a number of batch studies reported that NOM inhibits the uptake of arsenicals, it remains unclear how different classes of organic functional groups affect their binding mechanisms. We report herein the adsorption kinetics of arsenate and dimethylarsinic acid (DMA) with hematite nanoparticles pre-exposed to three types of low molecular weight organics: citrate, oxalate, and pyrocatechol as representatives to the majority of reactive organic functional groups in NOM. These studies were conducted using attenuated total internal reflection Fourier transform infrared spectroscopy (ATR-FTIR) and flow microcalorimetry at pH 7 with an emphasis on the role that electrolytes (KCl, NaCl, and KBr) play in the adsorption process. Results show that (1) negatively charged carboxylate versus hydrophobic phenyl groups influence amounts and initial rates of arsenicals adsorption on hematite nanoparticles to varying degrees depending on the type of complexes they form, (2) the type of electrolytes affects initial adsorption rate of DMA to a greater extent than arsenate when oxalate is present on the surface, and (3) the extent of organics retention by hematite nanoparticles is influenced by the type of the desorbing agent.