Molecular composition changes of soil dissolved organic matter (SDOM) during non-thermal plasma treatment

Non-thermal plasma (NTP) is a promising technology for soil remediation. However, its effects on soil dissolved organic matter (DOM) remain poorly understood. This study systematically investigated the structural evolution of natural soil DOM during dielectric barrier discharge (DBD) treatment using a suit of complementary techniques. These included UV-Vis, Fourier transform infrared (FTIR) spectroscopy combined with two-dimensional correlation spectroscopy (2D-COS), three-dimensional excitation-emission matrix (3D-EEM) fluorescence spectroscopy, and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) characterizations. Key findings reveal that the DOM concentration increased 8-fold, from 20.44 to 182.4 mg L^-1, over 120 min of treatment, while its aromaticity and molecular weight exhibited U-shaped trends. FTIR spectroscopy indicated the rapid decomposition of lignin and carbohydrates in the initial 20 min, followed by the formation of nitrogen-rich amides up to 90 min. This transformation pathway was confirmed at the molecular level by FT-ICR-MS, which showed an initial phase of oxidative fragmentation reducing molecular weight, succeeded by a period of nitrogen incorporation and condensation that increased molecular weight. Prolonged treatment led to re-aromatization, risking the formation of refractory compounds. Furthermore, the electron-donating of DOM initially decreased from 0.51 to 0.39 before recovering to 0.47, a shift correlated with changes in hydrophobicity. The treatment also preferentially mobilized soil cations, notably increasing Ca²⁺ concentration from 23.83 to 268.83 mg L^-1, with metal dissolution strongly correlated with DOM oxidation (r > 0.77). Optimal treatment duration of 2-20 min generated biodegradable DOM which enhanced wheat germination rates by 25 % and microbial activity. In contrast, prolonged discharge promoted the formation recalcitrant compounds. These results provide molecular-level insights into DOM changes during NTP treatment and establish guidelines for balancing remediation efficiency with soil health preservation.