A GLOBAL PROSPECTIVE ON THE OCCURRENCE OF TOXIC TRACE ELEMENTS AND RADIONUCLIDES IN PHOSPHATE FERTILIZERS

Fertilizer utilization is an essential part of globally sustainable crop production. Phosphate rocks are the primary source of phosphorous in mineral fertilizers, which is an important nutrient for plant growth. Systematic investigation of global phosphate ores from major phosphate-producing countries shows that sedimentary phosphate rocks of Mid-Miocene to Permian age have elevated levels of trace metals, such as Cd, Cr, V, and U, and that ²²⁶Ra is in secular equilibrium with ²³⁸U, inferring relatively high Ra activities. Older phosphate rocks contain lower levels of trace metals. Investigation of phosphate (P-) fertilizers and phosphogypsum byproduct generated from phosphate rocks reveals chemical fractionation in which U is selectively enriched in the P-fertilizers, while ²²⁶Ra is selectively incorporated into phosphogypsum, Consequently, the radionuclide distribution in P-fertilizers is generally outside of the ²³⁸U-²²⁶Ra secular equilibrium (²²⁶Ra/²³⁸U < 1). Due to the U>>Th in the phosphate rocks, P-fertilizers are also consistently characterized by a ²²⁸Ra/²²⁶Ra < 1. P- fertilizers commonly utilize in the U.S. have higher levels of trace metals and radionuclides relative to common soils. For example, Cd (a range of < 1 to 119 mg/kg) and U (2 to 338 mg/kg) in global P-fertilizers are up to ~100-fold higher than the average concentrations in U.S. soil (0.35 and 2.7 mg/kg, respectively). While cationic elements like Cd are expected to accumulate on topsoil in agriculture fields, oxyanions-forming elements like U, V, and Cr are likely mobilized into the underlying unsaturated zone and groundwater in unconfined aquifers. The continuous application of mineral P-fertilizers originating from Mid-Miocene to Permian age rocks generates a constant stream of toxic metals into the soils and poses potential environmental risks for the accumulation of Cd in agricultural soils and metal (U, Cr, V) contamination of associated ground water resources.