Development of a radiochemical sequential procedure for the quantification of Th- and U-decay series elements in mining residues

This article discusses a new complementary method for gamma spectrometry for the assessment of natural radioactivity in mining residues. The proposed analytical strategy allows us to determine if secular equilibrium is achieved in the sample with an HF-free dissolution process and a robust sequential radiochemical procedure for U, Th, Ra, Pb and Po isotopes. Various mineralization strategies were investigated on certified mining residues to completely dissolve both refractory (Th, U) and volatile (Po) species, either via open vessel or microwave-assisted acid digestion with nitric and hydrochloric acids followed by an alkaline fusion on the undissolved solids. Then 5 naturally occurring radioelements (Th, U, Ra, Pb and Po) present in the digested sample were separated with 3 stacked selective resins (TRU, Sr resin and HRa), from which they were individually eluted. The single loading step, composed of HCl/HNO3, was optimized to ensure selective retention onto the resins without the need to alter the loading matrix by varying the pH or adding salt, acid, or solvents. The 5 eluted solutions containing individual fractions of the desired elements could then be analyzed by either ICP-MS/MS or alpha spectrometry. More than 92% of each analyte were recovered in certified reference materials with the proposed procedure (dissolution and extraction).