Ultrasound-Assisted Catalytic Ozonation-Based Treatment of Rare-Earth Processing Effluent Containing Ammonium Thiocyanate (NH₄SCN)

The production of commercially important rare-earth elements in significant proportions also leads to the problem of industrial effluents containing metal oxides, cyanides, etc. generated from mining, mineral processing, and metallurgical operations. In the current work, ultrasound (US)-based methods are used for treatment of commercially obtained effluent from rare-earth metal processing industry containing ammonium thiocyanate (NH₄SCN) as a major target contaminant. The effect of operating parameters as pH (actual values as 1, 3, 7, 9, 12), H₂O₂ loading (800 to 1400 ppm), O₃ flow rate (0.5 to 2 L/min), and catalyst type (TiO₂ or MnO₂ at fixed loading of 1 g/L) on the efficacy of treatment was studied. The treatment of effluent using the approaches of US and ozone (flow rate as 0.5 L/min) alone under the constant conditions of 120 W and 70% duty cycle showed no chemical oxidation demand (COD) reduction; however, it was significantly improved on combination giving 54.83% reduction. Under optimized conditions, the approach of US + O₃ at flow rate of 0.5 L/min and pH of 12 combined with H₂O₂ (1200 ppm) and catalyst TiO₂ (1g/L) gave still higher COD reduction as 64.28% and 69.45%, respectively. The degradation of the contaminants using the US-based approaches followed a first-order kinetic mechanism, and a comparison of different approaches has been presented based on rate constants. Overall, it has been demonstrated that combined treatments based on US + O₃ in the presence of catalyst can be an effective method for treatment of effluent obtained from rare-earth processing.