Selenium Nanowire Formation by Reacting Selenate with Magnetite

Abstract

The mobility of 79 Se, a fission product of 235 U and longlived radioisotope, is an important parameter in the safety assessment of radioactive nuclear waste disposal systems. Nonradioactive selenium is also an important contaminant of drainage waters from black shale mountains and coal mines. Highly mobile and soluble in its high oxidation states, selenate (Se(VI)O 4 2− ) and selenite (Se(IV)O 3 2−) oxyanions can interact with magnetite, a mineral present in anoxic natural environments and in steel corrosion products, thereby being reduced and consequently immobilized by forming low-solubility solids. Here, we investigated the sorption and reduction capacity of synthetic nanomagnetite toward Se(VI) at neutral and acidic pH, under reducing, oxygen-free conditions. The additional presence of Fe(II) aq , released during magnetite dissolution at pH 5, has an effect on the reduction kinetics. X-ray absorption spectroscopy analyses revealed that, at pH 5, trigonal gray Se(0) formed and that sorbed Se(IV) complexes remained on the nanoparticle surface during longer reaction times. The Se(0) nanowires grew during the reaction, which points to a complex transport mechanism of reduced species or to active reduction sites at the tip of the Se(0) nanowires. The concomitant uptake of aqueous Fe(II) and Se(VI) ions is interpreted as a consequence of small pH oscillations that result from the Se(VI) reduction, leading to a re-adsorption of aqueous Fe(II) onto the magnetite, renewing its reducing capacity. This effect is not observed at pH 7, where we observed only the formation of Se(0) with slow kinetics due to the formation of an oxidized maghemite layer. This indicates that the presence of aqueous Fe(II) may be an important factor to be considered when examining the environmental reactivity of magnetite.

Revisiting Selenium Interactions with Pyrite: From Adsorption to Coprecipitation

“…Furthermore, integrating selenium within the pyrite stoichiometric structure may emerge as a more effective treatment avenue. This approach has the potential to minimize the mobility of precipitated, reduced Selenium species, as demonstrated by Poulain et al in 2022 . Considering the low reactivity of Se­(VI) with pyrite, a promising strategy for future studies to enhance removal involves incorporating zero-valent iron.…”

“…This approach has the potential to minimize the mobility of precipitated, reduced Selenium species, as demonstrated by Poulain et al in 2022. 64 Considering the low reactivity of Se(VI) with pyrite, a promising strategy for future studies to enhance removal involves incorporating zero-valent iron. Zero-valent iron appears to facilitate the conversion of Se(VI) into Se(IV) and Se(0), thereby enhancing selenium reduction processes.…”

Mössbauer spectrometry insights into the redox reactivity of Fe-bearing phases in the environment

“…Magnetite nanoparticles have been shown to reduce U VI , Se VI , Se IV , Cr VI , Pu IV , Sb V , and As V . In the former two cases, UO 2 (s) or Se(0) nanowire reduced species are formed, and these nanowires are much longer than the nanoparticles themselves [141][142][143] pointing out to the importance and transfer of intermediate reduced species. Se IV and U V outer-sphere Magnetite surface complex species are observed, and the second electron transfer may occur at the nanowire growth site-or tip end-after co-adsorption of Magnetite released Fe 2+ .…”

“…The Magnetite oxidation, i.e., the formation of Magnetite particle containing some proportion of Maghemite was followed by Mössbauer spectrometry. At pH values larger than 7, the quasi-isomorphic oxidative transformation of Magnetite to Maghemite is slow in the case of Se VI or U VI as oxidant, these slow kinetics attributed to the formation of an oxidized Maghemite layer [141,142,149]. In the case of Cr VI , increased Cr concentrations lead to a maximum limiting Cr reduction capacity by Magnetite.…”

Near Quantitative Removal of Selenate and Sulfate Anions from Wastewaters by Cocrystallization with Chelating Hydrogen-Bonding Guanidinium Ligands

“…Ion exchange resins have proven effective for selenate or selenite removal; however, in the presence of competing anions, such as sulfate, their separation efficiency is much lower . Inorganic and microbial approaches utilize reductive methods to produce elemental selenium , or iron selenides, but more work is needed to elucidate the factors that control the formation and selectivity of various reduced Se phases (e.g., ferroselite, dzharkenite, Se-substituted pyrite). Adsorption methods are more effective for selenite separation than for selenate, and may leach the oxyanion back into solution upon changes in the surrounding environment .…”