Arsenic Speciation in Vesuvianite: Insights from Combined EPR, XPS, and Synchrotron XAS Analyses

Abstract Rock-forming silicate minerals such as feldspars, garnets, and vesuvianite can contain elevated arsenic (As) contents and, therefore, are potentially important contributors to As contamination via chemical weathering and other processes. This study investigated a suite of 16 vesuvianite samples by combining inductively coupled plasma mass spectrometry (ICP-MS), electron paramagnetic resonance (EPR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and synchrotron X-ray absorption spectroscopy (XAS). ICP-MS analyses show that the vesuvianite samples contain 1.11 to 2041 ppm As, with the majority (69%) having ≥29 ppm As. The EPR spectra measured before and after gamma-ray irradiation reveal Fe3+, Mn2+, and VO2+ centers but do not have any arsenic-related oxyradicals. Microbeam As 3d XPS and bulk As K-edge XAS analyses both suggest mixed As3+ and As5+ oxidation states in vesuvianite. The As5+/ΣAs values (where ΣAs = As3+ + As5+) of vesuvianite calculated from As 3d XPS and As K-edge XAS correlate positively with the Fe3+/ΣFe values from Fe 2p XPS data, suggesting an oxygen fugacity effect. Fittings of As K-edge extended X-ray absorption fine structure (EXAFS) data of an As3+ dominant sample show that this species resides mainly at the T1 site in vesuvianite. The occurrences of mixed As3+ and As5+ species in vesuvianite add to the complexity in the uptake and release mechanisms of this heavy element.