Kinetics and Mechanism of Cr(VI) Reduction in a Water Cathode Induced by Atmospheric Pressure DC Discharge in Air

The process of reduction of Cr6+ ions (solution of potassium dichromate, K2Cr2O7) in a water cathode was studied during a DC discharge in air. The concentration range of Cr6+ was (5.7-19) x10(-5) mol/l and discharge current range was 20-80 mA. Cr6+ ions were shown to be reversibly reduced under a discharge action. The equilibrium degree of reduction increased with increasing initial concentration of the solution at fixed discharge current. At fixed initial concentration the reduction degree increased with increasing discharge current. The reduction degrees so obtained were 0.34-0.84. A kinetic scheme of the processes taking place in a solution was proposed. The calculated data obtained as a result of application of this scheme described well the experimental results on Cr6+ kinetics. The main processes of Cr6+ reduction and Cr3+ oxidation were revealed. HO (2) (center dot) radicals and hydrogen peroxide were shown to be responsible for Cr6+ reduction whereas (OH)-O-center dot radicals and O-2 molecules provide the reverse process of Cr3+ oxidation to Cr6+. The mechanism of action of phenol additives improving the process efficiency is discussed. The efficiency of phenol action as a radical scavenger was shown to be determined with its mass-transfer to the reaction area rather than chemical reaction rate.