The sulfate-based solutions such as Na2SO4 have been widely used as ‘inert’ supporting electrolytes for electro-degradation of contaminants at BDD anodes, where the electrooxidation pathways have been widely explained by the role of OH radicals and other reactive oxygen species. At the same time, the electrochemical production of persulfate from sulfate ions has also been reported and typically explained via the formation of sulfate radicals (SO4-) from SO42- either directly or through the reaction with OH . Similar to OH, SO4- are known to be strong oxidants in chemical oxidation processes, reporting high degradation rates when employed for the removal of organic contaminants. In this work, the role of sulfate ions in the electrooxidation of organic contaminants at BDD anodes was re-examined. The electrochemical removal of several persistent organic pollutants was performed in sulfate anolytes and compared to other electrolytic solutions such as nitrate, while investigating the effect of different operating conditions. Results showed that electrooxidation kinetics of seven persistent organic contaminants at BDD anode were quantified to be significantly enhanced by 10 – 15 folds in the presence of sulfate ions compared to sulfate-free electrolytes at identical conductivities. Even the lowest investigated sulfate concentration of 1.56 mM (i.e., ~150 mg L-1) operated with the lowest current density (100 A m-2), yielded higher degradation rate for diatrizoate than nitrate electrolyte with 60 mM concentration and 200 A m-2 current density. The potential role of persulfate and hydrogen peroxide in sulfate-based electrochemical oxidation systems was elucidated by conducting chemical oxidation reactions of persulfate and hydrogen peroxide with the seven persistent organic contaminants in the absence of electric current, and in the absence of an activator such as metal ions, heat or high pH. Results revealed that both have slow oxidation kinetics when reacting with these persistent organic contaminants in the absence of any known activator, and were ruled out.Since the inert systems employing nitrate anolytes are expected to oxidize these persistent organic contaminants primarily via their electrogenerated OH radicals, this superiority of sulfate anolyte, by one order of magnitude, advocates the possible formation and participation of electrochemically generated sulfate-based oxidizing species in the electrooxidation of these persistent organic contaminants.

Impact of sulfate ions on electrochemical oxidation of recalcitrant organic compounds using boron-doped diamond anodes

253rd National Meeting (2017)

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