Selenium nanowire formation by reaction of selenate with magnetite

The mobility of 79Se, a long half-life radioisotope and fission product of 235U, and contaminant of drainage waters from black shale mountains and from coal mines, is an important parameter in the safety assessment of radioactive nuclear waste disposal systems. Highly mobile and soluble in its high oxidation states (Se(VI)O42-, Se(IV)O32-), selenium oxyanions can interact with magnetite, a mineral present in anoxic natural environments and in steel corrosion products, and be precipitated by reduction, and thus immobilized. Here, the sorption and reduction capacity of synthetic nanomagnetite towards Se(VI) was investigated at neutral and acidic pH, under reducing, oxygen free conditions. The additional presence of Fe(II)aq, released during magnetite dissolution at pH 5, is shown to have an effect on the reduction kinetics. XANES analyses revealed that, at pH 5, trigonal gray Se(0) formed, and that outer-sphere Se(IV) complexes existed at the nanoparticle surface at 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, indicating that the presence of aqueous Fe(II) may be an important factor to be considered when examining the environmental reactivity of magnetite.