Hydrogen peroxide (H2O2) is usually considered to be an important reagent in green chemistry since water is the only by-product in H2O2 involved oxidation reactions. Early studies show that direct synthesis of H2O2 by plasma-water interactions is possible, while the factors affecting the H2O2 production in this method remain unclear. Herein, we present a study on the H2O2 synthesis by atmospheric pressure plasma-water interactions. The results indicate that the most important factors for the H2O2 production are the processes taking place at the plasma-water interface, including sputtering, electric field induced hydrated ion emission, and evaporation. The H2O2 production rate reaches ~1200 μmol/h when the liquid cathode is purified water or an aqueous solution of NaCl with an initial conductivity of 10500 μS cm−1.

Using an aqueous solution of NaCl as the cathode, we explored the atmospheric pressure Ar discharge plasma generated above the solution surface. The formation pathways of aqueous hydrogen peroxide (H 2 O 2aq ) in this system were investigated. Dimethyl sulfoxide was used as a scavenger of hydroxyl (OH) radicals to investigate the contribution of dissolved OH radicals to the H 2 O 2aq . The results indicate that the H 2 O 2aq is mainly formed by the combination of the dissolved OH radicals at the plasma-affected thin liquid layer, while the H 2 O 2 formed in the gas phase and the H 2 O 2aq formation inside water by the plasma-induced ultraviolet radiation have no contribution.

A derivative absorption spectroscopic method is used in situ to simultaneously trace and quantify the aqueous peroxide (H2O2), nitrate () and nitrite () generated during plasma–liquid interactions. The results indicate that the time evolutions of H2O2, and generated from the plasma–liquid interactions strongly depend on the solution’s pH value, which varies with the plasma treatment. The concentrations of aqueous H2O2, and increase independently from each other during the plasma treatment when the solution’s pH value is higher than 3.0. However, when the solution’s pH value is less than 3.0, most of the aqueous (~71.5%) will exist in the form of molecular nitrous acid since the pKa of nitrous acid is 3.4, the aqueous is mainly formed from the reaction between H2O2 and as well as the decomposition of molecular HNO2, which leads to a continuous increase of concentration and an appearance of the maximum concentrations of H2O2 and as the pH value of the solution reaches 3.0.

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Hydroxyl (OH) radical is the most important reactive species produced by the plasmaliquid interactions, and the OH in the liquid phase (dissolved OH radical, OHdis) takes effect in many plasma-based applications due to its high reactivity. Therefore, the quantification of the OHdis in the plasma-liquid system is of great importance, and a molecular probe method usually used for the OHdis detection might be applied. Herein we investigate the validity of using the molecular probe method to estimate the [OHdis] in the plasma-liquid system. Dimethyl sulfoxide is used as the molecular probe to estimate the [OHdis] in an air plasma-liquid system, and the partial OHdis is related to the formed formaldehyde (HCHO) which is the OHdis-induced derivative. The analysis indicates that the true concentration of the OHdis should be estimated from the sum of

We perform a series of experiments in a plasma-liquid system, aiming to explore in real time the change of a synthetic wastewater (methyl orange, MO) under an atmospheric pressure DC plasma exposure. The results indicate that the short-lived hydroxyl radicals generated from the plasma-liquid interactions are the predominant reactants in the MO molecules decomposition, while the long-lived species such as the plasma-induced hydrogen peroxide (H 2 O 2 ) are ineffective. Nevertheless, the plasma-generated H 2 O 2 can contribute to the MO decomposition by producing hydroxyl radicals from reacting with Fe 2+ (Fenton's reaction) or by producing peroxynitrite from reacting with NO 2 − . Moreover, the MO decomposition rate in the case of liquid cathode is found to be significantly faster than that in the case of liquid anode.

Treatment of methyl orange (MO) synthetic wastewater by air discharge plasma was investigated, and the solution's pH value was found to be a key parameter of influencing the treatment efficiency. The results indicate that lower pH value can induce production of high reactive species, NO+ and peroxynitrous acid, which can contribute to the MO decomposition. When the appropriate amount of Fe2+ ions is present in the MO solution, the MO decomposition can be promoted by Fenton's reaction (Fe2+ reacts with H2O2 produced from the plasma–liquid interaction). Furthermore, the lower pH value is also beneficial for the Fenton's reaction by avoiding an appearance of Fe(OH)3 precipitate that can reduce the treatment efficiency.

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