This study has been carried out to achieve high energy efficiency for a water purification method using a plasma generation with cavitation bubbles. In the method proposed herein, the pressure loss in the water flow path of the treatment reactor is reduced by removing the nozzle for the generation of cavitation. Cavitation bubbles can be generated between the electrodes installed in the flow path by adjusting the water flow rate. With and without nozzle, a similar degree of Escherichia coli sterilization was achieved. In the proposed system, the power consumption of the pump was reduced by removing the nozzle, and as a result, the total power consumption of the apparatus could be reduced.
Our research group has recently revealed that a novel sensing catalyst, amorphous RuO2-Ta2O5 mixed oxide, can detect and quantify hydrogen phosphate (HPO4 2-) by electrochemical oxidation, of which the current density is linear to the concentration in the wide range from 10-6 mol/L to 10-2 mol/L [1], while no other catalyst has been found for electrochemical sensing of HPO4 2-. This new catalyst and sensing method can be applied to measure the concentrations of hydrogen phosphate or phosphorus in environmental water, blood, foods, medicines, and agricultural products. For practical uses of the amorphous oxide catalyst for the detecting electrode, it is important to know the effects of the temperature of the test solution on the sensitivity to HPO4 2-, especially for continuous monitoring of phosphorus in environmental water. In this paper, we report the effects of temperature on the onset potential, the oxidation current density, and the sensitivity of the oxidation of HPO4 2- examined by cyclic voltammetry and chronoamperometry using rotating disk electrode (RDE). The amorphous RuO2-Ta2O5 catalyst was prepared on a titanium disk for RDE by calcination of the precursor solution containing Ru (III) and Ta (V), in which the Ru mole ratio was 50 mol% and thermal decomposition was carried out at 260 oC. Electrochemical measurements were performed using a conventional three-electrode cell and 50 mmol/L KCl solutions with or without various concentrations of Na2HPO4 at a temperature from 3 oC to 50 oC. The cyclic voltammograms showed the oxidation wave or plateau of HPO4 2-, and the onset potential shifted negatively and the maximum current density increased with increasing temperature of the test solutions. From the results, the potential to measure the oxidation current by chronoamperometry was determined at each temperature, so that the applied potential was more positive than that at which the maximum oxidation current in the cyclic voltammograms was observed and was less than the onset potential of oxygen evolution. The relationship between the oxidation current density and the concentrations of HPO4 2- obtained at 3 oC, 10 oC, 20 oC, 30 oC, 40 oC, or 50 oC revealed that the linear relationship was observed at all the temperatures and the slope increased with increasing temperature, suggesting that the sensitivity is enhanced. However, the linear region was seen in the concentration range from 0.4 mmol/L to 10 mmol/L, which was independent of temperature. These results are reasonable, because chronoamperometry measured the diffusion-limited current of HPO4 2- oxidation with RDE and the diffusion co-efficient becomes large at higher temperature. It was also found that the sensitivity was proportional to the temperature, which implies that it is easy to calibrate the sensitivity in practical uses. More detailed results will be shown in this paper. This work was supported by “Kyoto Area Super Cluster Program” of Japan Science and Technology Agency (JST). Reference [1] T. Tsukuma and M. Morimitsu, The 66th Annual Meeting of International Society of Electrochemistry, Abs# s01-014, Taipei, Taiwan (2015).
Zinc electrowinning uses an acidic zinc sulfate solution prepared from zinc ore, which contains some other metal ions such as Mn2+. The solution after zinc electrowinning goes back to the solution preparation process, in which the zinc concentration is recovered to the appropriate level, while that of Mn2+ is increased, if no Mn2+ is consumed by anodic deposition in electrowinning cell, which is possible with amorphous RuO2-Ta2O5/Ti anodes [1]. Therefore, the situation that Mn2+ concentration becomes high enough to recover Mn2+ as manganese oxide is expected, and it would resemble the commercial production process of EMD (Electrolytic Manganese Dioxide) that is famous as the positive electrode material of some batteries. For such a novel manganese recovering system in zinc electrowinning, the anode for deposition of MnO2 from zinc electrowinning solution is needed, with which MnO2 is deposited on and harvested from the anode that should be durable for the deposition and harvesting cycle. In this study, we prepared the titanium electrode covered with manganese oxide as the catalytic layer for MnO2 deposition from zinc electrowinning solution, and the obtained electrode was characterized by XRD, SEM, and EDX. The polarization measurement and continuous electrolysis of the anode in Mn2+ containing solutions were also performed to examine the obtained product on the anode and the current efficiency. The manganese oxide coated titanium electrode was prepared by thermal decomposition of the precursor solution containing Mn2+ painted on a titanium substrate which had been degreased in acetone and etched in 10% oxalic acid. The obtained electrode was analyzed by XRD, SEM, and EDX. The anodic polarization of the anode in H2SO4 solution with and without Mn2+ was examined by cyclic voltammetry with a conventional three-electrode cell equipped with a platinum plate counter electrode and an Ag/AgCl reference electrode in saturated KCl solution. Constant current electrolysis was also performed to obtain MnO2 on the anode, and the weight of the anode before and after the electrolysis was measured to know the amount of the product which was characterized by XRD. The electrolytic solution was 2 mol/L H2SO4 + MnSO4 and used at a temperature from 40 oC to 75 oC. The catalytic layer prepared by thermal decomposition was found to be Mn2O3 by XRD measurements. Constant current electrolysis with the anode was carried out under different conditions, in which the typical examples are 40 o C or 75 o C as the electrolyte temperature with electrolysis at 5 mA/cm2 for 30 min. For these conditions, the product on the anode was obtained with the increase of the anode’s weight by electrolysis, and the XRD results showed the diffraction pattern corresponding to MnO2, although the diffraction peak intensity was weak for the product obtained at 40 oC compared to that at 75 oC. The current efficiency for MnO2 deposition was calculated with the assumption that no oxygen evolution occurs, and the results were 17.5% at 40 oC and 70.6% at 75 oC. It was also found that the electrolyte was transparent before constant current electrolysis, which was unchanged by the electrolysis at 75 oC, while that became dark red at 40 oC, implying that Mn3 + is generated at 40 oC and the oxidation of Mn2+ to MnO2 is not completed. More detailed results on the effects of the electrolysis conditions on the obtained product and the current efficiency will be shown in this paper. This work was financially supported by “Kyoto Super Cluster Program” of Japan Science and Technology Agency (JST). Reference [1] T. Zhang, Ph.D. Thesis, Doshisha University (2015).
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