Electrolyte-driven H₂O₂ boost in plasma activated water: The role of sulfate in plasma electrolysis

Plasma-activated water (PAW) is recognized as a promising technology for decentralized water disinfection, yet its practical application is often constrained by the limited yields of key reactive species such as H₂O₂. To address this bottleneck, this study evaluates a plasma-induced auxiliary electrolysis strategy by comparing sulfate-based (PAW-1) and nitrate-based (PAW-2) electrolytes under identical discharge conditions. To ensure a rigorous comparison, optical emission spectroscopy (OES) was first employed to verify the consistency of plasma-phase characteristics (T_g ≈ 2300 K, T_exc ≈ 2.5 × 10⁴ K and n_e ≈ 1.6 × 10¹⁵ cm⁻³), effectively decoupling liquid-phase chemical enhancement from gas-phase physical interference. On this basis, experimental results demonstrated that the PAW-1 achieved a peak H₂O₂ concentration of 256 μmol/L, representing a twofold enhancement over the nitrate benchmark (132 μmol/L) with an energy efficiency of 490.83 μmol·kWh⁻¹. This leap in performance is attributed to the formation of peroxydisulfate (S₂O₈²⁻) and sulfate radicals (SO₄^·−), which function as long-lived oxidant reservoirs. Notably, this sulfate activation occurs within a single-cathode configuration where the liquid is grounded as the negative electrode, representing a reductive environment that stands in stark contrast to conventional electrochemical systems where sulfate oxidation strictly relies on anodic polarization. Further sterilization experiments validated the superior oxidative capacity of this system, as PAW-1 achieved a >5-log₁₀ reduction of E. coli within 20 min, whereas only ≈3-log₁₀ reduction was observed in PAW-2. These findings clarify anion-dependent redox pathways in plasma electrolysis and establish sulfate-driven chemistry as a robust, energy-efficient strategy for advanced water treatment.