Enhanced Method for Molybdenum Separation and Isotopic Determination in Geological Samples and Uranium-Rich Materials

Abstract:

Molybdenum (Mo) shares analogous geochemical properties with uranium. Mo is
present as a minor or a trace element in uranium ores under two main oxidation states: +IV
and +VI. Mo has seven stable isotopes (92, 94, 95, 96, 97, 98 and 100). In natural systems,
Mo and Mo isotopes were shown to fractionate during redox reactions. Because Mo
represents an impurity difficult to separate in the nuclear fuel cycle, it has the potential to be
used as an indicator of the origins of uranium concentrates, in the framework of nuclear
forensics.
This work focuses on developing an enhanced separation method for Mo from a
uranium-rich matrix (uranium ore, uranium concentrate) in order to analyze the mass
fractionation induced by processes typical of the nuclear fuel cycle. Purification of Mo for
isotope ratio measurements is performed with a three-step separation on ion-exchange resins,
with yields between 45 and 82%. Matrix and isobaric interferences (Zr, Ru) were reduced in
geological and uranium standards, such as U/Mo ≤ 2*10^-4, Zr/Mo ≤ 1*10^-3, Ru/Mo ≤ 6*10^-4
and Fe/Mo ≤ 4*10^-3. Mo isotopic compositions were measured on a Neptune Plus MC-ICPMS
equipped with Jet cones, for a concentration of 30 ng/ml. The achieved sensitivity is
~1200-1800 V/ppm with interferences below 10 mV and an overall reproducibility of 0.02 ‰
on the δ⁹⁸Mo values. A double spike, with ⁹⁷Mo and ¹⁰⁰Mo, was added to the samples before
the purification. It allows for correction of the chemical and instrumental mass fractionations,
without requiring a quantitative yield. For igneous rocks, δ⁹⁸Mo values range between -0.55
and -0.03 ‰, relative to the NIST-SRM 3134 molybdenum standard. Fractionation among
uranium ore concentrates is higher, with δ⁹⁸Mo ranging between 0.02 and -2.84 ‰.