Abstract
Atmospheric pressure non-equilibrium plasma (APNP) and its derivative, plasma-activated liquid (PAL), hold great promise for pollutant degradation and microbial inactivation. However, whether the biochemical efficacy of PAL can be fully replicated by a PAL mimic (PAL-M) based solely on long-lived reactive species remains debated. In this study, a systematic comparison framework for direct plasma treatment, PAL, and PAL-M was established in a disodium phosphate-citric acid buffer system (pH 3.0 and 7.0) with strictly controlled conductivity (∼12 mS/cm). Three representative dyes-indigo carmine (IC), methylene blue (MB), and methyl orange (MO) - and three microorganisms with gradient resistance- Escherichia coli, Staphylococcus aureus, and Bacillus subtilis spores-were selected as probes to comprehensively evaluate the equivalence of the three treatment methods. A 37-pin array needle-to-liquid plasma device driven by a 10 kV DC power supply was used, exhibiting self-pulsing discharge characteristics (peak current ∼30 mA, frequency ∼0.7 MHz). After 5 min of treatment, the accumulated concentrations of H2O2, NO2-, and NO3- in pH 3.0 and 7.0 liquids were 1.21/1.75 mM, 0.19/0.69 mM, and 4.51/2.91 mM, respectively. Dye decolorization experiments showed that direct plasma treatment achieved near-complete loss of the characteristic absorbance of IC (>95%) and approximately 76%-80% decolorization of MB and MO within 5 min. In contrast, PAL and PAL-M failed to effectively decolorize any of the three dyes, even after 25 min (residual rates >90%). Microbial inactivation experiments further revealed that at pH 3.0, direct plasma treatment achieved approximately 5.0 log and 3.5 log inactivation of E. coli and S. aureus, respectively; PAL contributed about 2.5 log and 2.2 log inactivation; while the inactivation effect of PAL-M was not significantly different from the pH 3.0 liquid control. B. subtilis spores exhibited the highest resistance, achieving only about 1 log inactivation at pH 3.0. Using IC as a model, concentration gradient experiments with specific scavengers for OH (D-mannitol), 1O2 (DABCO), and O2- (p-BQ) revealed that scavenging OH alone had no significant inhibition; DABCO produced the strongest dose-dependent inhibition at pH 7.0, suggesting the involvement of a DABCO-sensitive, 1O2-associated pathway, and scavenging O2- only showed a discernible inhibitory effect under neutral conditions. Control experiments adding the same scavengers to PAL showed no measurable scavenger-sensitive IC decolorization activity, consistent with the rapid decay of short-lived ROS after PAL preparation. The combined action of all three scavengers significantly delayed the kinetics of IC decolorization, further supporting the involvement of short-lived ROS-related pathways in the aqueous-phase oxidation process. By integrating chemical and biological probes into a strictly controlled pH buffer system, these results suggest that short-lived ROS-related pathways contribute importantly to the higher efficacy of direct plasma treatment under the present conditions. In contrast, PAL, which primarily retains long-lived species, can only account for a limited fraction of the total efficacy. Neither PAL nor the PAL-M strategy based on H2O2/NO2-/NO3- can adequately replicate the effects of direct plasma treatment, and the observed differences among the three treatment methods are highly dependent on the target and pH. The advantage of direct plasma treatment may be associated with the combined contributions of continuously generated short-lived reactive species, accumulated long-lived species, and the local plasma-liquid environment.