2 Environmental Radionuclides: Geochemical Behavior, Tracer Applications, and Potential Health Consequences

Friday, 17 March 2023
8:00 AM - 12:00 PM
Regency Ballroom A

URANIUM- AND THORIUM-DECAY NUCLIDES IN GLOBAL PHOSPHATE ROCKS AND FERTILIZERS

Paper number: 2-2
Program: Symposia
Time: 8:05 AM – 8:25 AM

Authors

Abstract

Since mid-1990’s, the annual global phosphate rocks production rates increased to a peak of 270 million metric tons (as P2O5) in 2017 and declined to a level of 220 million tons in 2021. Consistently, global phosphate fertilizer consumption has increased up to 47 million metric tons in 2020. Previous studies have reported that sedimentary phosphate rocks and associated fertilizers contain elevated levels of uranium, which could induce soil and water contamination. Here we report new data of the uranium- and thorium-decay nuclides in phosphate rocks from major global producing countries and associate fertilizers and phosphogypsum byproducts. The data show that young (<100 Ma) sedimentary phosphate ores exhibit high concentrations of U with the expected secular equilibrium between 238U and decay progeny (226Ra/238U activity ratio ~1), and predominance of U- over Th-decay series (Th/U and 228Ra/226Ra<<1). Older sedimentary phosphate rocks from China and India show lower U concentrations and higher Th/U and 228Ra/226Ra ratios. Systematic analysis of several pairs of phosphate rock and P-fertilizer as well as phosphogypsum shows, however, selective enrichment of U over Ra (226Ra/238U<1) in P-fertilizer. In contrast, phosphogypsum is characterized by Ra enrichment over U (226Ra/238U>1). This suggests that during the production of phosphate fertilizers, U and Ra partition between fertilizers and phosphogysum. The differential distribution of the uranium- and thorium decay nuclides in the products of phosphate rocks provides a novel toolkit to delineate the impact of P-fertilizers and phosphogypsum on the environment. We hypothesize that the combined low 228Ra/226Ra and 226Ra/238U activity ratios in P-fertilizers are different from the composition of common soils (228Ra/226Ra>1 and 226Ra/238U ~1). Likewise, radionuclide contamination from residual phosphogypsum would be characterized by a distinct geochemical composition (228Ra/226Ra<1 and 226Ra/238U >1). Given the elevated levels of radionuclides in phosphate rocks (e.g., 226Ra+228Ra activity in Florida phosphate rock is 1875 Bq/kg), future studies should include Ra nuclides measurements in attempts to evaluate the impact of phosphate fertilizers and phosphogypsum utilization on the quality of agriculture soils and associated water resources.

Geological Society of America Abstracts with Programs. Vol. 55, No. 2

doi: 10.1130/abs/2023SE-384955

© Copyright 2023 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.