Probing the In Situ Redox Behavior of Selenium on a Pyrite Surface by Scanning Electrochemical Microscopy

The migration of released selenium because of human activities can cause many environmental issues. The immobilization of soluble selenium (i.e., selenite and selenate) on natural pyrite mineral is believed to be effective for selenium management. In this work, the redox behavior of immobilized selenium on pyrite has been investigated using in situ scanning electrochemical microscopy (SECM). The immobilization of selenium on pyrite (Se-pyrite) was realized through both electrodeposition and adsorption in selenite solution. The released species from Se-pyrite surfaces at various potentials were probed using the SECM tip. The electrodeposited Se-pyrite releases Fe²⁺ and selenite, suggesting selenium immobilized on pyrite through electrodeposition was in the form of both Se⁰ and FeSe₂; while Se⁰ was the major form on the adsorbed Se-pyrite, and was associated with the sulfur sites instead of iron sites. The maximum amount of released selenium from Se-pyrite was found to be ca. 0.6 V, although the initial potential for selenium release was as low as around the open circuit potential. The selenium reduction current distribution was achieved by SECM imaging suggesting an inhomogeneous adsorption of selenium on pyrite. Because of the change of surface species, the immobilization of selenium resulted in a decrease of surface conductivity of pyrite, which was analyzed using SECM tip approach curves and conductive atomic force microscopy. The surface conductivity followed the trend of pyrite > electrodeposited Se-pyrite > adsorbed Se-pyrite. This work provides a new approach for the in situ investigation of selenium immobilization on and release from mineral surfaces (e.g., pyrite), which can be readily applied to similar systems regarding various environmental issues.