Phosphogypsum Doses and Subsoil Contamination: A Study on the Mobility of Rare Earth Elements, Uranium, and Thorium

Abstract

Soil conditioners are commonly used by producers aiming to enhance agricultural productivity, and among these products, phosphogypsum has gained increasing attention. As an agricultural amendment, phosphogypsum contains rare earth elements (REE), uranium (U), and thorium (Th), raising environmental concerns regarding its long‐term use. This study evaluated the vertical distribution of REE, U, and Th in the soil profile of an Oxisol under increasing phosphogypsum application rates. The experimental design included plots with four replicates, with phosphogypsum applied in 2003 in a single operation along the planting row. Application rates were 0 (control), 9.4, 16, and 24 t ha −1 . Soil samples were collected from seven depth intervals (0–5, 5–10, 10–20, 20–40, 40–60, 60–100, and 100–200 cm). Following acid digestion, element concentrations were determined using ICP‐MS. The results revealed a significant effect ( p < 0.01) of phosphogypsum application on the mobility and redistribution of elements throughout the soil profile. The input of REE, U, and Th from phosphogypsum in soils can reach 3.62 g kg −1 and is mainly composed of light rare earth elements (LREE). Elevated contents of REE, U, and Th were observed, particularly in the uppermost layer. Despite considerable losses of most elements relative to the total applied, elements such as cerium, scandium, uranium, and thorium exhibited notable enrichment at various depths, especially at the surface. The application of high phosphogypsum doses can alter the vertical distribution of REE, U, and Th, potentially promoting their downward migration beyond 200 cm, which raises concerns about soil contamination with radionuclides and the risk of groundwater contamination. Background values of REE, U and Th must be stated in order to evaluate the anthropogenic contamination due to phosphogypsum application.