Submerged Arc Breakdown of Methylene Blue in Aqueous Solutions

Low voltage, low energy submerged pulsed arcs between a pair of carbon or iron electrodes with a pulse repetition rate of 100 Hz, energies of 2.6–192 mJ and durations of 20, 50 and 100 μs were used to remove methylene blue (MB) contamination from 30 ml aqueous solutions. The MB concentration decreased exponentially with rates of 0.0006–0.0143 s−1 during processing with the carbon electrode pair. With the iron electrodes, the MB concentration initially decreased faster (0.030 s−1) than with the carbon electrodes, but later saturated. However when microparticles produced with the iron electrodes were periodically filtered, the high removal rate was maintained. Under these conditions, the volume of water which can be treated per unit energy expenditure was much higher with the submerged arc than with other plasma processes. A kinetic model based on MB degradation by OH· radicals formed by the discharge was formulated. The higher initial MB removal rate with iron electrodes is explained by additional OH· production from Fenton’s reaction between Fe++ and H2O2 produced by the discharge. This rate is maintained if the eroded iron particles are filtered, but if eroded iron particles accumulate, degradation slows down and stops, possibly because the iron particles catalytically decompose H2O2 and hence stops Fenton’s reaction, and either directly or via increased Fe++ dissolved from the particles, scavenge the OH· radicals.