Physicochemical Properties and Time Stability of Plasma Activated Water by a Liquid-Cathode Glow-Type Discharge in Air: The Effect of Air Confinement

Nonthermal discharges in atmospheric pressure air in contact with water produce large amounts of reactive species in the gas phase that can enter into the water by diffusion, thus inducing the formation of secondary reactive species in the liquid phase, including those long-lived species such as NO
$_{2}^{-}$
, NO
$_{3}^{-}$
, and H
$_{2}$
O
$_{2}$
. Depending on the controllable parameters of the discharge, the plasma activated water (PAW) may acquire different physicochemical properties, resulting in various applications. Physicochemical measurements of PAW obtained by means of a water-cathode glow-type discharge in atmospheric pressure air operating in open and closed reactor conditions are reported. The discharge was operated in a millisecond pulsed-dc regime at an rms current value of 100 mA and a power of 100 W. A large volume of 1 L of distilled water was treated for 30 min. In both cases, low pH values of
$\sim$
2.5 and very high levels of NO
$_{3}^{-}$
(up to 250 mg/L) in PAW were obtained; however, in the closed system, no H
$_{2}$
O
$_{2}$
was found and high concentrations of nitrite (120 mg/L) were measured, while in the open system, large levels of H
$_{2}$
O
$_{2}$
were observed (45 mg/L) and no NO
$_{2}^{-}$
was found. Likewise, the electrical conductivity value for the closed reactor (
$\approx $
2000
$\mu $
S/cm) was significantly higher than for the open reactor (
$\approx $
1000
$\mu $
S/cm). The reasons for these different behaviors in terms of PAW chemistry are discussed. Also, the time stability of PAW was measured.