Ultrasound-assisted electrochemical treatment of wastewaters containing organic pollutants by using novel Ti/Ta2O5-SnO2 electrodes

Advanced oxidation processes (AOPs) are modern methods using reactive hydroxyl radicals for the mineralization of organic pollutants into simple inorganic compounds, such as CO₂ and H₂O. Among AOPs, electrochemical oxidation (EO) is a method suitable for colored and turbid wastewaters. The degradation of pollutants occurs on electrocatalytic electrodes. The majority of electrodes contain in their structure either expensive materials (diamond and Pt-group metals) or are toxic for the environment compounds (Sb or Pb). One of the main disadvantages of electrochemical method is the polarization and contamination of electrodes due to the deposition of reaction products on their surface, which results in diminishing of the process efficiency. Ultrasound combined with the electrochemical degradation process eliminates electrode contamination because of the continuous mechanical cleaning effect produced by the formation and collapse of acoustic cavitation bubbles near to the electrode surface. Moreover, high frequency ultrasound generates hydroxyl radicals at water sonolysis. Ultrasound-assisted EO is a nonselective method for oxidation of different organic compounds with high degradation efficiencies.The aim of this research was to develop novel sustainable and cost-effective electrodes working as electrocatalysts and test their activity in electrocatalytic oxidation of organic compounds such as dyes and organic acids. Moreover, the goal of the research was to enhance the efficiency of electrocatalytic degradation processes by assisting it with ultrasound to eliminate the main drawbacks of a single EO such as electrode polarization and passivation. Novel Ti/Ta₂O₅-SnO₂ electrodes were developed and found to be electrocatalytically active toward water (with 5% Ta content, 10 oxide film layers) and organic compound oxidation (with 7.5% Ta content, 8 oxide film layers), and therefore these electrodes can be applicable in both environmental and energy fields. The synergetic effect of combined electrolysis and sonication was shown while conducting sonoelectrochemical (EO/US) degradation of methylene blue (MB) and formic acid (FA). Complete degradation of MB and FA was achieved after 45 and 120min of EO/US process, respectively, in neutral media. Mineralization efficiency of FA over 95% was obtained after 2h of degradation using high frequency ultrasound (381, 863, and 1176kHz) combined with 9.1mA/cm² current density. EO/US degradation of MB provided over 75% mineralization in 8h. High degradation kinetic rates and mineralization efficiencies of model pollutants obtained in EO/US experiments provide the preconditions for further extrapolation of this treatment method to pilot scale studies with industrial wastewaters.