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.