We examined the decomposition of toluene by atmospheric pressure microwave discharge using carbon felt (CF) as an intriguing medium for sustaining plasma, and studied the decomposition behavior. The discharge was generated easily at 200W microwave power using CF25 which was thermally pretreated at 2500ºC and had a relatively low electric resistance of 0.16Ωcm. The discharge was generated at the gap between each two pieces of CF, however, not generated at the inside of each CF. The temperature of the gap and the intensity of emission from discharging gaps were increased with seconds until they reached an equilibrium state in about 10 seconds. The XPS analysis revealed that the graphite structure of each carbon fiber making CF was maintained through the discharge, that is, the surface of carbon fiber was kept chemically unchanged without binding nitrogen and oxygen atoms. Toluene disappeared completely, and H2 was generated stoichiometrically by discharge in nitrogen or even in air atmosphere. Furthermore, in the case of the discharge in air with toluene, O2 disappeared, and CO and CO2 were generated. H2O, NOx and O3 were not detected. The reaction kinetic consideration revealed that the decomposition of Toluene followed the first-order reaction, the apparent decomposition rate constant of toluene was increased with the number of gaps between CFs in a reaction vessel, and the decomposition rate of toluene was independent of the concentration of O2.
Experiments were conducted to investigate selective sulfuration of copper, zinc and nickel contained in a simulated plating wastewater using H2S as a sulfurating agent. The sulfuration of single solutions of copper, zinc and nickel, and mixed solutions of these metals with H2S was conducted in a bubble column reactor. As a result, it was found that heavy metals can be selectively precipitated from the mixed-metal plating wastewater at a pH value of 1.5 for copper, 4.5 for zinc and 6.7±0.2 for nickel. The purities of precipitates based on total amount of metal contained were 95.5%, 87.4% and 94.7% in the case of copper, zinc and nickel, respectively. When the results were compared with those obtained for the selective precipitation with Na2S, it was found that higher purities of metal sulfides were achieved with H2S. It was considered that local increases of pH resulting from an addition of alkaline sulfurating agent of Na2S were avoided in the case of H2S. Consequently, an undesirable co-precipitation of zinc and nickel sulfides at a pH value of 1.5 corresponding to copper selective precipitation was reduced. Similarly, the precipitation of nickel sulfide was partially avoided during the precipitation of zinc sulfide at a pH value of 4.5.
The dissolution rate and dissolution morphology of an electroplated nickel film in acidic electrolyzed water (AEW) of a sodium chloride aqueous solution and in a hydrochloric acid aqueous solution of the same pH were studied. As a result, it was found that the dissolution rate of the electroplated nickel layer in the AEW was about two times higher than that in the hydrochloric acid aqueous solution of the same pH. On the surface of the electroplated nickel film immersed in the AEW, the oxide layer was removed, and microscopic pits due to local dissolution were observed. The pit has a dish-like shape with a depth of less than 0.5μm, and its surface roughness decreased when compared to the one before soaking. On the other hand, as for the one immersed in the hydrochloric acid aqueous solution, no such phenomenon was observed, and its surface roughness increased. Moreover, when the AEW was heated to 40ºC and sprayed, the dissolution of the electroplated nickel surface was promoted, the surface had a satin-like finish, and the possibility of etching was suggested.