Microscale investigation into the geochemistry of arsenic, selenium, and iron in soil developed in pyritic shale materials

In this study, we report on the distribution and mineralogy of micron-sized mineral aggregates formed in the top horizon of an acid sulfate soil. The distribution and oxidation state of arsenic (As) and selenium (Se) were also determined. The soil used in this study was formed from pyritic shale parent materials on the east side of the California Coast Range. Synchrotron-based X-ray fluorescence microprobe (μ-XRF) was used to generate elemental distribution maps of soil thin sections. Using the elemental distribution maps and optical micrographs, distinct mineral aggregates of iron oxide and iron sulfate were identified throughout the top horizon of the soil. These aggregates range in size from 10 to 100 μm in diameter and can be found only a few micrometers apart. The As and Se concentrations in the iron oxide aggregates were 5–10 times the concentrations in the iron sulfate aggregates and the weathered shale matrix. This suggests that the As and Se become preferentially associated with iron oxides during the weathering process. Using a focused micron-sized beam, Fe, As, and Se X-ray absorption spectroscopy (XAS) data were collected from the sub-millimeter soil aggregates. The micro-extended X-ray absorption fine structure (μ-EXAFS) spectrum collected from the iron oxide aggregate revealed that its mineralogy was a combination of ferrihydrite (>50%) and goethite. The μ-EXAFS spectra from the iron sulfate region suggest that these aggregates contain jarosite. Using micro-X-ray absorption near edge spectroscopy (μ-XANES), oxidation states of the As and Se were determined. Arsenic was present in the iron oxide aggregate as As(V). Selenium was present in the soil as both Se(IV) and Se(VI), with a higher percentage of Se(VI) in the jarosite aggregate than the iron oxide aggregate. These results provide direct evidence of the distribution, oxidation states, and speciation of As and Se in the solid phase of an unaltered native soil. Information on the weathering and geochemistry of the pyritic materials, and the associated arsenic and selenium is useful for predicting the pedogenic processes of acid sulfate soils and the long-term fate of newly exposed pyritic materials (e.g., mine tailings and drained wetlands).