Radionuclides and the uranium isotope fingerprint of globally produced phosphate rocks, mineral fertilizers, and phosphogypsum waste and its potential effect on the environment

Abstract

The occurrence of uranium (U), thorium (Th), and radium (Ra) in phosphate rocks (PRs) leads to enrichment of these radionuclides in produced fertilizers and phosphogypsum waste byproducts. This study presents a global dataset of U- and Th- series radionuclides (²³⁸U, ²³²Th, ²²⁶Ra, and ²²⁸Ra) and uranium isotope composition (δ²³⁸U) in PRs, fertilizers, and phosphogypsum. Results reveal systematic variations in U and Ra content and δ²³⁸U signatures based on geological age and phosphate rock type (igneous vs sedimentary). Sedimentary PRs are in ²³⁸U-²²⁶Ra secular equilibrium, with younger rocks (Miocene-Permian) exhibiting higher ²²⁶Ra activities (up to 1837 Bq/kg) than older rocks (≤241 Bq/kg). Fertilizers tend to be concentrated in ²³⁸U and depleted in ²²⁶Ra relative to source rocks, while phosphogypsum retains ²²⁶Ra. In a survey of P- and NPK-fertilizers (i.e., fertilizers that contain a mix of nitrogen, phosphorus, and potassium), we find that fertilizers preserve the U isotope signature of the source phosphate rocks, with δ²³⁸U ranging between -0.28 ‰ and -0.15 ‰ for young PRs and -0.70 ‰ and -0.34 ‰ for older PRs, which are different from the average continental crust (-0.29 ‰). Field data from an agricultural research station in North Carolina demonstrate that, despite high U and Ra input rates from P-fertilizer application, surface sandy soils retain little of these radionuclides. Fertilization did not significantly alter soil δ²³⁸U and the U and Ra levels were low, below environmental safety thresholds. These findings highlight the potential of fertilizer-derived radionuclide contamination in adjacent or underlying water resources and offers the δ²³⁸U fingerprints of fertilizers as a potential geochemical tracer for identifying fertilizer-derived U contamination in the environment.

Keywords: Fertilizer; Isotopes; Phosphate; Phosphogypsum; Radium; Soil; Uranium.