Insights into the facet-dependent adsorption of phenylarsonic acid on hematite nanocrystals

Organic arsenics have been frequently detected in natural environments and regarded as a group of emerging contaminants. Elucidating the migration of organic arsenics requires a deep understanding of intrinsic adsorption mechanisms of organic arsenics on iron-bearing minerals, which are seldom investigated. In this study, we investigated the facet-dependent adsorption behavior of phenylarsonic acid (PhAs) on hematite nanocrystals with open circuit potential (OCP) analysis, batch adsorption experiments, adsorption isotherm curves, synchrotron-based As K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy, attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, and density functional theory (DFT) calculations. We found that organic arsenics adsorbed on two facets (e.g. {001} and {012}) of hematite through the same non-protonated inner-sphere coordination but in different molecular configurations. Surface complexation models were set up to analyze the preferred coordination geometries dependent on the facets, and the {012} facet was found to favor the PhAs adsorption more than the {001} facet. These findings clarified the dependence of organic arsenic adsorption on the hematite facets, and shed light on the environmental effects of hematite on the migration of PhAs.