Plasma treatment in aqueous solutions for environmental applications
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Date
2026-07-07
Authors
Βασιλειάδου, Φοίβη
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Abstract
Water pollution caused by industrial effluents containing synthetic organic dyes constitutes a significant environmental challenge. Despite their widespread application, conventional wastewater treatment methods often exhibit limited effectiveness towards persistent pollutants. Among the emerging advanced oxidation processes, non-thermal plasma (NTP) technology has attracted consid-erable attention due to its ability to generate highly reactive species under ambient conditions, with-out the addition of chemicals. These characteristics make NTP a promising and environmentally friendly alternative for wastewater treatment applications.
In the present work, a dielectric barrier discharge (DBD) plasma jet reactor with the high-voltage electrode immersed in the liquid phase was developed for the degradation of bromothymol blue (BTB) aqueous solutions (100 mL). Argon and atmospheric air were employed as working gases, while plasma generation was achieved using a positive DC pulsed power supply. Experiments were per-formed using initial BTB concentrations of 3.7 and 10 mg L⁻¹ at pH 13.3, gas flow rates of 2 L min-1, electrode gaps of 0.5 and 2 cm and various applied voltages and operating frequencies, in order to evaluate their effect on degradation performance and energy efficiency. Optical emission spectros-copy (OES), UV-Vis spectroscopy, chemical analyses and electrical characterization were employed to study the plasma-liquid system.
In addition, an equivalent electrical circuit was developed to describe the electrical behaviour of the reactor. Although several equivalent circuit models have been proposed for DBD systems operating with a gas-filled electrode gap, studies involving plasma reactors where the discharge is gener-ated directly in contact with liquids remain limited. The model provided an excellent fit to the exper-imental impedance data (R² = 0.9967), while the good agreement between calculated and experimental power values further validated the equivalent circuit model.
OES analysis revealed the presence of excited argon species together with Hα and atomic oxygen emissions in argon plasma, while excited nitrogen species were detected in atmospheric air plasma. Chemical analyses confirmed the formation of H₂O₂, NO₂⁻ and NO₃⁻ during plasma treatment, sup-porting the proposed degradation mechanisms. The initial BTB concentration, electrode gap, applied voltage and operating frequency had a limited influence on the degradation efficiency, with the most stable plasma observed at 2 kHz. The degradation experiments demonstrated the effective removal of BTB using both argon and atmospheric air plasmas, with maximum conversions of 77.19% and 98.12%, respectively. The degradation process followed first-order kinetics, with correlation coeffi-cients ranging from 0.86 to 0.997. Atmospheric air plasma exhibited higher degradation rates than argon plasma. Although both plasmas achieved comparable energy yields, atmospheric air plasma showed the highest value (0.39 g kWh⁻¹) compared to argon plasma (0.34 g kWh⁻¹). On the other hand, argon plasma operated with a lower energy consumption (30.36 kJ L⁻¹) than atmospheric air plasma (42.69 kJ L⁻¹).
The obtained results demonstrate the effectiveness of DBD plasma technology for the degradation of dye-containing wastewaters and contribute to a better understanding of plasma-liquid interactions and the electrical behaviour of plasma-liquid reactors.
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Keywords
Non-thermal plasma, Dielectric barrier discharge (DBD), Wastewater treatment, Advanced oxidation processes (AOPs), Equivalent electrical circuit, Optical emission spectroscopy (OES), Plasma-liquid interactions