Local environment of arsenic in sulfide minerals: insights from high-resolution X-ray spectroscopies, and first-principles calculations at the As K-edge

Abstract

Whereas electronic properties of As-bearing compounds find various industrial applications in smartphone and photovoltaic devices, or in superconductor materials, arsenic is a toxic element present in a variety of contaminated surface environments. In natural media, the knowledge of its chemical speciation and oxidation state is crucial to apprehend its mobility, bioavailability and toxicity towards the ecosystems. To that purpose, chemical selective experimental methods, such as X-ray absorption spectroscopy (XAS) and related techniques are powerful tools. Here, a selection of As species in the As–Fe–S system presenting various oxidation states and local structures are thoroughly discriminated using resonant inelastic X-ray scattering (RIXS), high-energy fluorescence detection (HERFD-XANES), and first-principles X-ray absorption near-edge structure (XANES) calculations, highlighting the interest of going beyond the usual fingerprint analysis of standard experimental XANES spectra. The position of maximum fluorescence on the RIXS plane is proved to be an advanced fingerprint of the As local environment. HERFD-XANES spectra are found to significantly improve the spectral resolution and to reduce the signal background at the As K-edge, but have to be interpreted carefully as they present different shifts in energy compared to their standard XANES counterparts. First-principles As K-edge XANES calculations are performed for reference compounds with arsenic oxidation states ranging from −I to +III (arsenopyrite (FeAsS), löllingite (FeAs₂), realgar (AsS) and orpiment (As₂S₃)). Comparison with experimental spectra shows that the oxidation state of As can be retrieved from the position of the inflection point of the edge. XANES calculations are also carried out for As incorporated as an impurity in pyrite either at the S tetrahedral or Fe octahedral sites, allowing us to draw conclusions on the oxidation state of As in this mineral ubiquitous in the environment.