Heavy Metal Mobility and Environmental Risk in Phosphate Industry Sludge (Metlaoui, Tunisia): Implications for Sustainable Waste Management

Abstract The Gafsa-Metlaoui basin is responsible for over 90% of Tunisia’s phosphate production. It yields considerable sludge waste enriched with potentially toxic elements (Zn, Cr, Cu, Ni, and Cd). Despite well-documented total metal concentrations, insights into their precise geochemical fractionation and environmental mobility under arid conditions are notably lacking. In this study of the speciation, mobility, and ecological risk of these potentially toxic elements (PTEs), the Tessier sequential extraction procedure was applied to sludge samples from the region’s drainage network. Critically, this investigation presents the first comprehensive Tessier-based assessment of Metlaoui phosphate sludge and, uniquely, directly links PTE speciation to site-specific acidification processes induced by phosphogypsum leachates, a causal relationship previously overlooked. The selected sampling sites span major drainage sectors with differentiated industrial input signatures, ensuring robust spatial representation. Results indicated that although PTEs are predominantly associated with the stable residual fraction (44%), a substantial proportion (56%) is bound to more labile phases (exchangeable, carbonate, Fe/Mn oxides, and organic matter). Critically, the risk assessment code (RAC) identified Cd (43%) as posing a high risk and Ni (29%) a moderate risk due to their association with the most bioavailable fractions. The mobility index (MI) confirmed that Ni (50%) and Cd (45%) are highly mobile and susceptible to leaching. A key novelty of this research lies in the explicit demonstration, via principal component analysis (PCA), that pH is the master variable dictating PTE speciation. The strong negative correlations observed suggest that acidification (the acidity of phosphogypsum leachates) could significantly promote increased metal mobilization, a factor traditionally recognized as a source of contamination but which has never previously been quantified as the primary factor influencing the in situ mobility of PTE within the Metlaoui phosphate sludge deposits. By linking solid-phase speciation to site-specific geochemical triggers (e.g., acidification and redox changes), this study moves beyond total concentration surveys to provide a process-based risk assessment. The findings underscore the urgent need for engineered sludge management strategies, such as alkaline stabilization or phytoremediation, to mitigate the imminent threats to groundwater and ecosystem health in this vulnerable region, where shallow water tables and seasonal flooding create favorable conditions for contaminant transport.