Tuning the composition of plasma-activated water by a surface-wave microwave discharge and a kHz plasma jet

An atmospheric pressure surface-wave microwave discharge and a kHz plasma jet are used to activate purified water. It is shown, that by varying the treatment distance and the initial Ar/N-2/O-2 mixture composition of the surface-wave microwave discharge the concentration ratio of NO3- and H2O2 radicals created in the plasma activated water (PAW) can be varied over three orders of magnitude, which can be preserved during months of storage at room temperature. At the same time, with the 5 min treatment of the 32 ml water the absolute radical concentrations are varied in the range of 0.5-85 mg l(-1) for H2O2, 20-180 mg l(-1) for NO3- and 0.5-14 mg l(-1) for NO2-. In the case of the N-2 kHz plasma jet this concentration ratio can be tuned within one order of magnitude by varying the treatment distance. By treating different volumes very similar concentration ratios are obtained, which evolve differently during storage, as the ageing dynamics is determined by the absolute concentration of radicals. In general, the radical most affected by ageing is NO2-, whose recombination is found to be determined by the H2O2 radical. In order to control the H2O2 concentration and thus the NO2- radicals recombination, the application of a Fenton type reaction is suggested, which is implied by inserting a copper surface into PAW during or after plasma treatment.