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

Abstract

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 NO 3 − and H 2 O 2 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 H 2 O 2 , 20-180mg l −1 for NO 3 − and 0.5-14mg l −1 for NO 2 −. 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 NO 2 − , whose recombination is found to be determined by the H 2 O 2 radical. In order to control the H 2 O 2 concentration and thus the NO 2 − 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.

Plasma‐activated water to improve the stress tolerance of barley

“…The efficiency in the active species production of the two systems is very similar (40 vs. 49 μmol/kJ), although with different compositions. Nevertheless, as shown in our previous work, the composition of PAW prepared with the N 2 jet can be tuned by varying the treatment distance. Both systems are feasible for scale‐up development, however, with respect to operating costs, one would prefer using a less‐expensive gas (nitrogen vs. argon) and a higher repetition rate excitation (kHz vs. Hz).…”

“…PAW is characterized by its composition and pH. During the plasma treatment, several plasma species—such as NO and NO 2 molecules, N 2 + and O 2 + ions, N and O atoms, and electrons—interact with the water surface or dissolve in the water, and contribute to the creation of different short‐ and long‐lived species . For example, the electrons by dissociating the water molecules create OH molecules, which recombine into H 2 O 2 .…”

“…The short‐lived species are difficult to access and they contribute mostly to the liquid chemistry, therefore, the PAW can be characterized by the long‐lived species, which have been identified to be the H 2 O 2 , NO 2 − , and NO 3 − . The concentration of NO 2 − , NO 3 − , and H 2 O 2 , and the pH of samples are measured with QUANTOFIX® test strips and evaluated with a QUANTOFIX® Relax unit (by Macherey‐Nagel, GmbH) . The QUANTOFIX Relax reader allows fast, quantitative analysis of strips with high accuracy.…”

“…The evolution of the radical concentrations in PAW#1 (closed symbols) and PAW#2 (open symbols) during storage in a closed container kept in the dark at room temperature. The measuring error has been determined to be less than 10% (typically 6–8%) . PAW, plasma‐activated water…”

“…During the plasma treatment, several plasma species-such as NO and NO 2 molecules, N 2 + and O 2 + ions, N and O atoms, and electrons-interact with the water surface or dissolve in the water, and contribute to the creation of different short-and long-lived species. [51,52] [52] The QUANTOFIX Relax reader allows fast, quantitative analysis of strips with high accuracy. This method makes possible the analysis of the samples F I G U R E 2 Scheme of the hybrid plasma reactor.…”