(337d) Pursuing a Viable Plasma-Based Water Treatment Process: Identifying Transport Limitations and Investigating the Effect of Reactor Design On the Degradation of Bisphenol A

To improve the feasibility of plasma-based water treatment technology, a study was conducted to identify and characterize the design parameters that influence treatment effectiveness and the physical phenomena that determine treatment efficiency.  A compound’s octanol-water partition coefficient (i.e., hydrophobicity) has been determined as the major factor that influences the compounds’ relative ability to diffuse into the plasma channel. Experiments conducted with nine different solutes have shown that hydrophilic compounds tend to stay in the bulk liquid whereas hydrophobic compounds such as Bisphenol A (BPA) readily diffuse into the plasma channel where they react with OH and other radicals. Thus the electrical discharge as an advanced oxidation technology appears to be the most effective for the degradation of compounds with limited solubility in water.

The effects of several reactor design parameters were investigated, including electrode geometry, electrode spacing, reactor geometry, and the ground electrode plate area, on the degradation of BPA. The hybrid-series reactor in which the high voltage (plasma) electrode is in water and the grounded electrode is above the water’s surface was about four times as effective as the reference configuration with both electrodes in water. The reactor geometry, particularly the distance between the high voltage electrode and the reactor wall, was observed to have a major impact on the characteristics of the electrical discharge, which in turn influenced the BPA removal rate. Tripling the reactor radius yielded a threefold increase in the peak current and thus the power deposited into plasma. Similarly, the area of the grounded electrode plate was found to affect the discharge characteristics with a twofold increase in the plate area corresponding to an increase of about 40% in the peak current and input power. The distance between the electrodes was identified as the key parameter. It was found that the optimum distance depends on the input voltage and solution conductivity, but is generally near the minimum distance at which no sparking between the electrodes occurs. At the same input power of 50 W, the optimization increased the extent of removal of a 5 mg/L BPA solution from 32% to 94% in less than one hour.