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 Fe 2+ and selenite, suggesting selenium immobilized on pyrite through electrodeposition was in the form of both Se 0 and FeSe 2 ; while Se 0 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.
Petroleum coke (petcoke) ash fusibility is closely related to the ash-related fouling and slagging, which have significant effects on its clean and efficient utilization. Sodium (Na) element in petcoke ash is considered to induce ash fouling and slagging. In this paper, we investigate the effect of Na2O on the petcoke ash fusibility from the perspectives of atmosphere, Na2O content, and temperature. The crystalline minerals and surface morphologies of high-temperature ashes were determined by X-ray diffraction and scanning electronic microscopy, respectively. Thermodynamic software FactSage was applied to calculate the ash melting process. The results show that the ash fusion temperature (AFT) of petcoke ash exhibits a continuous decline with the addition of Na2O at both oxidizing and reducing atmospheres, which is ascribed to the mineral transformation behaviors of high-temperature ashes. Under oxidizing atmosphere, the low-melting Na-bearing albite (NaAlSi3O8) formed at high-temperature ash with the addition of Na2O decreases the AFT, and the decomposition of high-melting anorthite (CaAl2Si2O8) and quartz (SiO2) further leads to the decline of AFT. Under reducing atmosphere, another low-melting Na-bearing nepheline (NaAlSiO4) is found in high-temperature ash. Moreover, mullite and anorthite disappear with increasing Na2O content, which can both contribute to the progressive decline of AFT. The high-temperature ash samples with high Na2O content have a higher melting extent at oxidizing and reducing atmospheres. These ash particles present a more smooth surface and a denser layer structure.
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