Effect of microbubble ozonation advanced oxidation on contaminant removal and toxicity reduction to Daphnia magna in mining and metallurgical effluents

The microbubble ozonation effect on treatment efficiency and acute toxicity of industrial effluents was evaluated. Eight effluents were sampled from two gold mines and one Ni Cu mine in northern Canada, as well as a smelter from Abitibi-Temiscamingue region of Canada. Effluents showed contrasted salinities (from <0.05 mg/L to >50 g/L of Cl − ) and ammonia nitrogen (from <0.1 mg/L to >475 mg/L of ₃ NH ₃ − N ). Treatability tests used microbubble ozonation, in an 18 L laboratory reactor, for 150 to 200 min, with the pH adjusted to 9. Physicochemical parameters and ₃ NH ₃ − N concentrations were measured before, during, and after treatment, while toxicity on Daphnia magna was evaluated before and after the treatment. Results showed variable initial ₃ NH ₃ − N concentrations (0.06–482 mg/L) and a sharp decrease to below the detection limit (0.003 mg/L) after treatment, corresponding to removal efficiencies exceeding 99%. Before treatment, acute toxicity ranged from 0.46 to 16.2 toxic units (TU) and decreased after ozonation to 0.14–10.9 TU (−33% to −73%, depending on the effluent). Fe and Mn concentrations decreased in most of the treated effluents, suggesting the precipitation of oxyhydroxides. Conversely, Zn concentrations slightly increased in some effluents, suggesting its potential mobilization from fine particles (<0.45 μm), most likely ZnS. The microbubble ozonation had significant effect on treatment efficiency and the reduction of toxicity to D. magna in all tested effluents. Following treatment, only the highest saline effluent (Cl − > 50 g/L) showed toxicity, indicating that residual toxicity was likely driven primarily by salinity rather than oxidizable contaminants.