Plasma-activated water (PAW) is gaining significant attention these days due to its potential use as a disinfectant, pesticide, food preservative, cancer cell treatment, fertilizer, etc. These applications of PAW depend on its reactivity (oxidizing-potential) and electrical conductivity (EC). In the present work, we have studied the effect of process parameters, viz., air flow rate, water stirrer speed, and the temperature of water during plasma–water interaction on the reactivity and EC of PAW using a three-way full factorial design of experiments. We have also attempted to optimize these process parameters. At optimum values of these parameters, we studied how the physicochemical properties of PAW vary by changing the volume of water and discharge power. Furthermore, we studied the physicochemical properties of the plasma-activated solution (PAS) and how the EC and pH of virgin solutions affect these properties. The obtained results of the present investigation showed that the flow rate of air, plasma treatment time with water, volume of water, and plasma discharge power play a significant role in controlling the reactivity and EC of PAW. Moreover, the pH and EC of virgin solution do not have a significant (p < 0.05) impact on the reactivity of PAS. This investigation also shows the study of aging time on reactive oxygen–nitrogen species and its effect on the physicochemical properties of PAW. Additionally, observed changes in physicochemical properties, NO3− ions, and H2O2 concentration in PAW were less than 10%. However, NO2− ions and dissolved O3 concentration in PAW decreased significantly over time.

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This work shows a comparative study of a change in properties of plasma-activated water (PAW) when prepared by using two different dielectric barrier discharge (DBD) configurations named a pencil plasma jet (PPJ) and a plasma device (PD). The air plasma produced from the DBD-PPJ and DBD-PD is characterized by voltage-current characteristics, and plasma species/radicals are identified using optical emission spectroscopy. Moreover, the present work emphasizes the trapping of reactive species (O3, NOx, etc.) carried by post-discharge residual gases during PAW production. The trapping of these gases' reactive species is carried out in water, which provides a useful by-product named plasma processed water (PPW). The results revealed a higher concentration of reactive oxygen species (dissolved O3 and H2O2) and a lower concentration of reactive nitrogen species (NO3− and NO2− ions) in PAW prepared by the DBD-PPJ configuration compared to the DBD-PD configuration. The trapping of reactive species (O3 and NOx) present in post-discharge residual gases is confirmed by determining the change in physicochemical properties and reactive oxygen–nitrogen species (RONS) concentration in virgin water used as a trapping medium. The high concentration of RONS in PPW showed a high concentration of reactive species in post-discharge residual gases and vice versa. Therefore, the reduction in reactive species downstream of post-discharge residual gases is shown by a substantial decrease in the concentration of RONS and physicochemical properties of PPW. Thus, PAW and PPW (by-product) prepared in this work could be used for multiple applications such as microbial inactivation, food preservation, and agriculture.

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The present work showed the role of plasma-forming gases (air, nitrogen (N2), argon (Ar), helium (He), and their mixture with oxygen (O2)) on the properties of plasma-activated water (PAW). Electrical diagnosis and optical emission spectroscopy were performed to characterize plasma and identify plasma radicals/species. The PAW is characterized by studying its physicochemical properties and dissolved reactive oxygen-nitrogen species (RONS) concentration in it. The results showed introducing O2 in N2, Ar and He plasma suppresses the emission lines intensity of NOϒ band in N2 plasma, OH band in Ar and He plasma, and N2 second positive system in He plasma. Also, adding O2 to Ar and He plasma changes the plasma discharge characteristic from glow discharge to filamentary micro-discharge. The PAW prepared by air and its mixture with O2 showed improved physicochemical properties and RONS concentration in it compared to other plasma forming gases and their mixture with O2. In addition, increasing plasma-water exposure time significantly affects the physicochemical properties and RONS concentration in PAW. Therefore, plasma forming gas and plasma-water exposure time gives better control over the properties of PAW. Hence, these parameters play a significant role in deciding the applications of PAW.

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In the present work, we study the physicochemical changes that arise in water named plasma processed water (PPW) when it is exposed to the downstream low-pressure discharge of ammonia (NH3) gas. Optical emission spectroscopy and voltage-current characteristics of NH3 plasma are studied to identify species formed in NH3 plasma along with plasma characterization. A three-way full factorial design of experiment is performed to study the effect of process parameters named applied voltage, post-discharge gas-water interaction time, and NH3¬ gas pressure on physicochemical properties of PPW. The obtained results are analyzed using analysis of variance, standardized effect estimation, regression analysis, and response surfaces. The optimum values of these properties and PPW process parameters are estimated using MATLAB fmincon solver with experimental constraints. The emission spectrum of NH3 plasma showed strong intensity N2+ lines along with weak intensity N2, NH, and N+ lines. The obtained results showed the post-discharge gas-water interaction time and applied voltage had a significant impact on physicochemical properties and ammonium ions concentration in PPW. The obtained optimum value of voltage and time is 550 V and 15 min with given experimental constraints.