This study evaluates underwater non-thermal plasma (UNTP) as a reagent-free process for the complete mineralization of oxalic acid, a major chelating agent in nuclear decontamination effluents. Quantitative assessment was based on total organic carbon (TOC) removal and stable carbon isotope tracing with uniformly labeled ¹³C-oxalic acid. TOC and ion chromatography (IC) analyses demonstrated complete mineralization within 60 min at ≤ 300 ppm (k = 0.120, 0.100, 0.042 min⁻¹; t₉₀ = 19.2, 23.0, 55.0 min), whereas at 450 ppm partial mineralization remained (TOC 25.4 mg C/L after 60 min). At higher concentrations (1000–2000 ppm), TOC removal was restricted to 25–57% with rate constants decreasing to 0.008 and 0.003 min⁻¹ (t₉₀ = 288, 767 min); at 3000 ppm, reaction nearly stagnated (k = 0.0009 min⁻¹; t₉₀ ≈ 2558 min). Energy yield peaked at low/intermediate concentrations (0.9–1.3 g-C kWh⁻¹; 1.1 g-C kWh⁻¹ at 450 ppm) but declined to 0.9, 0.3, and 0.2 g-C kWh⁻¹ at 1000, 2000, and 3000 ppm. Mechanistic profiling showed that both glyoxylic and formic acids remained below the method detection limits (LOD) throughout the treatment period, supporting that a predominantly direct mineralization pathway to CO₂ was operative. Critically, ¹³C tracer experiments (300 ppm, 60 min) yielded δ¹³C = + 5702‰ (~ 7.0 atom % ¹³C), confirming the presence of substrate-derived carbon in the evolved CO₂. No solids or carbonate byproducts were detected, consistent with a nearly closed carbon balance. Bulk temperatures remained ≤ 40 °C under all conditions, confirming non-thermal operation. These findings establish TOC-based kinetics and isotopic evidence of oxalic acid mineralization, define a practical operating window (≤ 2000 ppm), and support UNTP as a sustainable route for treating chelating agents in decontamination effluents.

Mineralization of oxalic acid by underwater non-thermal plasma: TOC removal and 13C isotope tracer
    Abstract
    1 Introduction
    2 Materials and methods
        2.1 Reagents and solution preparation
        2.2 Underwater plasma reactor
        2.3 Operating conditions and sampling
        2.4 Analytical methods
        2.5 ¹³CO₂ isotope tracer experiment
        2.6 Kinetic analysis and energy efficiency calculation
    3 Results and discussion
        3.1 Concentration-dependent oxalic acid removal and mineralization behavior
        3.2 Accumulation of intermediates and degradation pathway
        3.3 Verification of final products through ¹³C stable isotope analysis
        3.4 Kinetic analysis and energy efficiency
        3.5 Maintenance of non-thermal plasma conditions
    4 Conclusions
    References

• Daeseok Hong(Radwaste Management Center, Korea Atomic Energy Research Institute, Daejeon 34057, Korea)
• Ki-baek Shin(Radwaste Management Center, Korea Atomic Energy Research Institute, Daejeon 34057, Korea, Department of Nuclear and Quantum Engineering, Korea Advanced Institute of Science and Technology, Daejeon 34141, Korea)
• Gyuseong Cho(Department of Nuclear and Quantum Engineering, Korea Advanced Institute of Science and Technology, Daejeon 34141, Korea) Corresponding author
• Geon Woo Yang(Institute of Plasma Technology, Korea Institute of Fusion Energy, Gunsan 54004, Korea)
• Heejae Lee(Institute of Plasma Technology, Korea Institute of Fusion Energy, Gunsan 54004, Korea, Department of Applied Plasma and Quantum Beam Engineering, Jeonbuk National University, Jeonju 54896, Korea)
• Yong Cheol Hong(Institute of Plasma Technology, Korea Institute of Fusion Energy, Gunsan 54004, Korea, KFE-School, University of Science and Technology, Daejeon 34113, Korea)
• Kangil Kim(Institute of Plasma Technology, Korea Institute of Fusion Energy, Gunsan 54004, Korea, KFE-School, University of Science and Technology, Daejeon 34113, Korea)