Potentially Toxic Elements in Phosphate Processing: A Comparative Assessment of Solid Wastes and Effluents from Beneficiation and Fertilizer Production in Southwest Tunisia

This study investigates the environmental and health impacts of potentially toxic elements (PTEs) in solid and liquid wastes from phosphate beneficiation and fertilizer production in the Mdhilla area, Gafsa Basin, Tunisia. Solid wastes, including phosphate tailings (PTs) and phosphogypsum (PG), and associated industrial effluents from phosphate beneficiation (PBE) and fertilizer production (PFE), were characterized using physicochemical analysis, ICP-MS, SEM-EDX, and ion chromatography. Single and integrated pollution indices, along with conservative human health risk assessments, were applied to evaluate cumulative contamination and potential risks. PT exhibited near-neutral pH (7.64) and high PTE enrichment (Zn 350 mg kg^-1, Cr 329 mg kg^-1, Cd 38.8 mg kg^-1), whereas PG was strongly acidic (pH 3.13) and comparatively depleted in metals, reflecting process-dependent partitioning. Despite neutral pH, PBEs contained markedly higher metal concentrations than PFEs, with Fe (163 mg L^-1), Cr (3.09 mg L^-1), Cd (0.49 mg L^-1), and Pb (0.71 mg L^-1) exceeding discharge limits. Pollution indices indicated severe to extreme contamination, with PBE showing an exceptionally high contamination degree (C_deg = 63,659) compared to PFE (C_deg = 12,815), and elevated Toxic Element Pollution Index (PTEPI) values confirmed stronger cumulative pollution in PBE. Potential ecological risk indices (PERI > 600) revealed very high ecological risk for both effluents, primarily driven by Cd, Co, and Tl. Although dermal contact may represent the most frequent exposure route, risk assessment results indicated that accidental oral ingestion is the dominant pathway contributing to both non-carcinogenic and carcinogenic risks, with children being particularly vulnerable. Non-carcinogenic risk thresholds (HQ > 1) were exceeded for PBE, while total carcinogenic risks approached or exceeded regulatory limits (10^-6-10^-4), mainly due to Cd and Cr. Overall, phosphate beneficiation was the primary source of cumulative metal loading and associated ecological and health risks, while fertilizer production partially reduced contamination. These findings underscore the urgent need for improved management and mitigation of phosphate-processing wastes in industrial regions.