Halides as potential signatures for geolocation of uranium phosphate rocks for nuclear forensic applications

An analytical approach for halides determination in uranium-bearing phosphate rocks, based on halides extraction from natural calcium phosphate by a metathesis reaction, followed by ion chromatography with conductivity detection analysis, is presented in this work. Chloride extraction is performed by mixing samples with sulfuric acid, causing an interchange between phosphate and sulfate ions towards calcium ions in the solid phase (metathesis). Addition of small amounts of calcium hydroxide improves the chromatographic determination by eliminating artifacts located in proximity to chloride retention time, and reducing phosphate peaks area. Limit of quantitation (LOQ) for chloride and bromide were found to be 25 mg/Kg and 50 mg/Kg, respectively. The metathesis reaction is proven by ED-XRF analysis of the precipitate produced during this reaction, showing the interchange between sulfate and phosphate ions. Chloride determination based on this approach was performed on several uranium-bearing phosphate rock samples from different sources. Since geolocation is one of the primary techniques needed for attribution, in a nuclear forensic investigation, this methodology may contribute to find new signatures, e.g. halides, in natural phosphate rocks deposits, to support that. The Cl⁻ content in 11 phosphate rock samples was determined after applying metathesis, and found to be in the range of 30–792 mg/Kg, supporting its relevance as nuclear forensic signature. The Br⁻ content was below the detection limit in all the samples.