Influence of Silica Coatings on Magnetite-Catalyzed Selenium Reduction

Abstract

The reactivity of iron (II/III) oxide surfaces may be influenced by their interaction with silica, which is ubiquitous in aquatic systems. Understanding the structure-reactivity relationships of Si-coated mineral surfaces is necessary to describe the complex surface behavior of nanoscale iron oxides. Here we use Si-adsorption isotherms and FTIR spectroscopy to analyze the sorption and polymerization of silica on slightly oxidized magnetite nanoparticles (15% maghemite and 85% magnetite, i.e. ~2 maghemite surface layers), showing that Si adsorption follows a Langmuir isotherm up to 2 mM dissolved Si, where surface polymerization occurs.Furthermore, the effects of silica surface coatings on the redox-catalytic ability of magnetite are analyzed using selenium as molecular probe. The results show that for partially oxidized *

Ex and In Situ Reactivity and Sorption of Selenium in Opalinus Clay in the Presence of a Selenium Reducing Microbial Community

“…Geological disposal of HLW and ILW in deep clay formations will ultimately lead to the dissolution and slow leaching of soluble radionuclides such as 79 Se, a redox sensitive radionuclide, which is expected to leach at least partially as Se oxyanions. Although part of the leached selenium could sorb on corrosion products of the steel canisters [7], the remainder will reach the surrounding clay host rock. The reducing environment in this clay host rock and the possible microbial presence might however change its speciation and thus also its transport behavior.…”

“…Due to the oxidizing conditions during spent fuel reprocessing, radiolysis in the waste and/or due to the presence of large amounts of nitrate in nitrate-bearing bituminized ILW, Se is expected to be released-at least in part-as Se oxyanions [3,6]. Although part of these oxyanions could sorb onto corrosion products present on the steel canisters or steel rebars [7], the remainder is expected to reach the clay host rock surrounding the waste repository.…”

Selenium Nanowire Formation by Reacting Selenate with Magnetite

“…The concentration of dry magnetite in four 100 mL batches was fixed at 10 g/L in 0.1 mM NaCl background electrolyte. Due to stronger selenium adsorption on magnetite in acidic conditions, pH 5 and 7 were selected for comparison. The acidity of the initial suspensions was adjusted during 96 h by adding drops of 0.1 M HCl or NaOH stock solutions, until the pH was not drifting from the desired value by more than 0.2 units within 24 h. After this equilibration step, aliquots of Se­(VI) stock solution were added to obtain a total target concentration of 8.6 mM (details in Table S1).…”

“…Among all the cementitious materials in engineered barriers and all the naturally occurring minerals in the surrounding geologic formations, only a few have the ability to uptake selenium oxyanions via either adsorption processes or reductive precipitation. These are, e.g., layered double hydroxides such as the hydrotalcite phases in cements (AFm), and the redox-active Fe­(II)-bearing phases present as the result of corrosion of the steel containers or naturally in minerals such as pyrite, mackinawite, magnetite, or Fe­(II)-bearing clay minerals. − Abiotic reduction of soluble selenium species by Fe­(II)-bearing materials (potential steel corrosion products) has been observed for several minerals such as green rust, − magnetite, ,, mackinawite and siderite, pyrite, , troilite, and zero-valent iron . The mixed-valence Fe­(II)/(III) oxide magnetite (Fe 3 O 4 ) has showed redox reactivity toward selenium species. ,, Magnetite nanoparticles have been shown to reduce Se­(IV) and Se­(VI) to elemental selenium and iron selenides, , even in the presence of oxidized layers or maghemite and coatings. , The product of these redox processes is a partially oxidized magnetite, i.e., a magnetite particle containing some proportion of maghemitean Fe­(III) mineral isostructural to magnetite, a phase that is also able to adsorb selenium oxyanions. , …”

“…For example, is electron transfer possible when the complex formed is outer-sphere? Whereas selenite is prone to form inner-sphere complexes at the magnetite–water interface, ,− selenate oxyanions adsorb forming outer-sphere complexes. , A recent study has shown that electron transfer can still take place, with a selenite intermediate formed, and the whole reaction showing slow reduction kinetics indicative of a redox process with high kinetic barriers . Fe­(II) ions are also known to be released from magnetite to solution at acidic pH, acting as a reactive species that can re-adsorb and donate electrons, thereby reducing adsorbed ions such as arsenate. − Are these processes active in the presence of selenium ions as well?…”