Pollution Source or Remediation Material? Environmental Behavior, Risks, and Safe Utilization of Phosphogypsum in Agricultural Soils

Phosphogypsum (PG) is a bulk industrial solid waste generated during wet-process phosphoric acid production. Its agricultural use may improve specific soil properties but may also increase the risk of agricultural non-point-source pollution. Differences in phosphate-rock source, production process, degree of washing, stockpiling history, and treatment method result in substantial variation among PG materials in mineral composition and in the concentrations, modes of occurrence, and release potential of P, F, soluble salts, potentially toxic elements, and naturally occurring radionuclides. This review combines a structured literature search with a qualitative synthesis of risk evidence and uses agricultural soils, the root zone, and field boundaries as the principal assessment domains. It examines the release, transformation, immobilization, crop uptake, leaching below the root zone, and export through runoff, erosion, and drainage of PG-associated constituents. It also evaluates the mechanisms, pollutant partitioning, agronomic effects, and non-target risks of hazard-reduction and functionalization technologies. Available evidence indicates that the agronomic effects and environmental risks of PG cannot be assessed solely on the basis of total constituent concentrations, Ca and S supply, the results of a single leaching test, or short-term crop responses. They also depend on the releasable pollutant load, cumulative application rate, soil chemical and hydrological conditions, crop exposure, and long-term stability. Washing, leaching, separation, stabilization, thermal treatment, and biological treatment can reduce the concentrations or releasability of some pollutants in the treated solid but may also transfer pollutants to wastewater, sludge, residues, or gaseous streams. Following source traceability, risk screening, and treatment where necessary, PG may be used to ameliorate sodic soils, regulate nutrient availability, or stabilize some pollutants through ion exchange, adsorption, precipitation, surface complexation, and organic–mineral binding. Its effectiveness and associated risks depend on material properties, soil conditions, target pollutants, and application methods. Accordingly, this review proposes a conceptual framework for the safe utilization of PG that integrates source traceability, material classification, selective hazard-reduction treatment, functionalization design, soil–crop matching, tiered verification, and long-term monitoring. The framework identifies assessment priorities and management considerations for specific materials and application scenarios.