Pulsed Discharge Plasma over the Surface of an Aqueous Solution to Induce Lignin Decomposition

In this work, non-equilibrium atmospheric-pressure plasma was introduced over an aqueous solution as a medium to depolymerise lignin employing an applied voltage of 10 kV and a frequency of 10 kHz. As a starting material, lignin was extracted from Japanese cedar wood flour utilising a deep eutectic solvent (DES) at 50–120 °C. The Fourier-transform infrared (FT–IR) spectra indicated that lignin was extracted from the Japanese cedar wood flour in this temperature range. Additionally, the ultraviolet–visible (UV–Vis) spectra of the liquid fraction of the products at 280 nm revealed the high lignin content of the extracts. Gel permeation chromatography (GPC) revealed that the average molecular weight of the lignin was 400–700 Da. Matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry (MALDI-TOF MS) illustrated that the prolonged discharge plasma treatment concurrently promoted the further depolymerisation reaction and repolymerisation of the lignin-derived compounds.