Inactivation of Salmonella in Water by Bubble-Spark Cold Plasma: Physicochemical Properties and Kinetic Modeling using a Modified Chick–Watson Model

Abstract Cold plasma is an emerging non-thermal technology for decontaminating food and water. This study examined how discharge frequency (500–2500 Hz), treatment time (0–300 s), and treatment method (direct vs. indirect) affect Salmonella inactivation and the formation of key reactive species. Direct treatment of a three-strain Salmonella cocktail in a micropulse bubble-spark reactor produced continuous reductions up to 6.3 log CFU/mL. Inactivation correlated with a decreased pH (approximately 3.5) and increased conductivity (170 µS/cm) and rising concentrations of ozone (0.63 mg/L), hydrogen peroxide (4.8 mg/L), nitrite (7.4 mg/L), and nitrate (94 mg/L). Higher frequency (2500 Hz) accelerated inactivation, achieving a 5-log reduction in 1 min with 17.1 kWh/m 3 energy consumption, significantly faster and more efficient than the 5 min required at 500 Hz (22.6 kWh/m 3 ). Indirect treatment (mixing inoculum into pre-activated water) achieved only a 1.3-log reduction, with ozone rapidly depleted, likely due to reactive species, such as hydroxyl radicals, atomic oxygen, and peroxynitrite, not replenished. A revised Chick–Watson model, utilising dissolved ozone as a measurable proxy for reactive species accurately described Salmonella inactivation during direct treatment (R 2 > 0.94, RMSE < 0.83 log CFU/mL), demonstrating that increased discharge frequencies enhance the microbial inactivation rate, presumably due to short-lived species attaining elevated dynamic balance concentrations at higher frequencies.