Analytical study, environmental risk assessment, and toxicity-based bioassays of effluents from phosphate fertilizer industry: a case study in Gafsa mining basin (SW Tunisia)

The phosphate fertilizer industry (PFI) in M’dhilla-Gafsa, Tunisia, discharges untreated effluents, creating environmental concerns due to unknown composition and toxic effects, leading to potential ecological and human health risks. This study characterized wastewater from three sampling points (SP1, SP2, SP3), and revealed their high acidity pH (2–3) and salinity (32.5–96.23 g/L). BOD₅/COD ratios indicated the high recalcitrance (0.01–0.1) of the effluents, reflecting their low biodegradability and persistence in the environment. Excessive levels of phosphorus (206–2094 mg/L), fluoride (154–1071 mg/L), and sulfates (835.5–9266 mg/L) were detected by ionic chromatography. ICP-MS analysis highlighted for the first time the potentially toxic elements (PTEs) content in Tunisian PFI effluent, such us Cd (0.31–0.44 mg/L), Al (7.1–27.8 mg/L), Mn (1.9–3.5 mg/L), B (15.9–25.2 mg/L), Fe (15.7–28.7 mg/L), Ni (0.37–0.48 mg/L), and Cr (1.1–2.12 mg/L). These levels exceeded the permissible national and international limits for industrial discharges, as well as those of the World Health Organization (WHO) for drinking water, indicating serious potential environmental risks. Moreover, phytotoxicity tests on Medicago sativa, Pisum sativum, and Zea mays showed decreased amylolytic and proteolytic activities during germination. Blood toxicity assays indicated significant hemolytic effects. Indeed, 50% of SP1, SP2, and SP3 wastewaters caused hemolysis percentages equal to 52.04%, 57%, and 66.77%, respectively, implying therefore an acute toxicity of the effluents. Additionally, HEK-293 cells mortality was higher after exposure to effluents, with IC₅₀ of 12.06%, 24.23%, and 17.68% for SP1, SP2, and SP3, respectively. These findings emphasized the potential risks posed by PFI effluents to the surrounding ecosystem and the agricultural sector, leading to the contamination of the food chain. This alarming threat imposes stricter wastewater treatment measures and regulatory enforcement in the PFI sector.