Effects of Titanium Doping in Titanomagnetite on Neptunium Sorption and Speciation

摘要

Neptunium-237 is a radionuclide of great interest owing to its long half-life (2.14 × 10~6 years) and relative mobility as the neptunyl ion (NpO_2~+) under many surface and groundwater conditions. Reduction to tetravalent neptunium (Np~Ⅳ) effectively immobilizes the actinide in many instances due to its low solubility and strong interactions with natural minerals. One such mineral that may facilitate the reduction of neptunium is magnetite (Fe~(2+)Fe~(3+)_2O_4). Natural magnetites often contain titanium impurities which have been shown to enhance radionuclide sorption via titanium's influence on the Fe~(2+)/Fe~(3+) ratio (R) in the absence of oxidation. Here, we provide evidence that Ti-substituted magnetite reduces neptunyl species to Np~Ⅳ. Titanium-substituted magnetite nanoparticles were synthesized and reacted with NpO_2~+ under reducing conditions. Batch sorption experiments indicate that increasing Ti concentration results in higher Np sorption/reduction values at low pH. High-resolution transmission electron microscopy of the Ti-magnetite particles provides no evidence of NpO_2 nanoparticle precipitation. Additionally, X-ray absorption spectroscopy confirms the nearly exclusive presence of Np~Ⅳ on the titanomagnetite surface and provides supporting data indicating preferential binding of Np to terminal Ti-O sites as opposed to Fe-O sites.