Genesis of pure Se(0) nano- and micro-structures in wastewater with nanoscale zero-valent iron (nZVI)†
Formation of pure elemental selenium has important implications in the separation, purification and recovery of selenium, a trace but vital and increasingly important industrial material. nZVI can quickly reduce both Se(VI) and Se(IV) to Se(0). Herein, we report highly efficient and rapid separation of selenium from wastewater and the discovery of distinctive nano- and micro-scale structures of pure selenium via the reduction of Se(IV) to Se(0) with nanoscale zero-valent iron (nZVI). Specifically, spherical and needle shaped Se(0) nanoparticles were observed under ambient conditions. Aqueous Se(IV) is first adsorbed to the nZVI surface, diffuses across the oxide layer and subsequently reduced to Se(0). The formed Se(0) particles were characterized using spherical aberration corrected scanning transmission electron microscopy (Cs-STEM) combined with energy dispersive spectroscopy (EDS), electron energy loss spectroscopy (EELS), scanning electron microscopy (SEM) and X-ray diffraction (XRD). XRD revealed that the Se(0) nanoparticles were pure hexagonal phase selenium crystals. EELS analysis provided additional and independent evidence on the Se(IV) reduction to Se(0). Selenium has long been discharged to the environment due to low efficiency of conventional technologies, and the discovery of pure Se(0) solids by nZVI reduction offers an innovative route for recovery of this valuable resource from wastewater.