Spectroscopic investigation of nanosecond pulsed discharges in underwater plasma for flowing water treatment applications

Abstract Ensuring access to clean water, particularly in industrial settings, remains a significant challenge in the 21st century. Conventional wastewater treatment methods often suffer from inefficiencies, high operational costs, and limited scalability, necessitating the development of advanced, cost-effective, and sustainable alternatives. This study investigates the potential of underwater plasma discharge for flowing wastewater treatment under high fluid flow rates, focusing on the impact of discharge parameters—particularly applied voltage and frequency—on plasma dynamics. Optical emission spectroscopy was used to characterize key plasma parameters, including electron temperature ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>T</mml:mi> <mml:mrow> <mml:mtext>e</mml:mtext> </mml:mrow> </mml:msub> </mml:mrow> <mml:mo stretchy="false">)</mml:mo> <mml:mo>,</mml:mo> </mml:mrow> </mml:math> gas temperature <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:mo>(</mml:mo> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>T</mml:mi> <mml:mrow> <mml:mtext>g</mml:mtext> </mml:mrow> </mml:msub> </mml:mrow> </mml:mrow> <mml:mo>)</mml:mo> </mml:mrow> </mml:mrow> </mml:math> , and electron number density <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mrow> <mml:mo>(</mml:mo> <mml:mrow> <mml:mrow> <mml:msub> <mml:mi>n</mml:mi> <mml:mrow> <mml:mtext>e</mml:mtext> </mml:mrow> </mml:msub> </mml:mrow> </mml:mrow> <mml:mo>)</mml:mo> </mml:mrow> </mml:mrow> </mml:math> . Degradation experiments were conducted in both oxygen-enriched and ambient air conditions at a fixed flow rate of 0.3 l min −1 , achieving a degradation efficiency of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mn>63.57</mml:mn> <mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">%</mml:mi> </mml:mrow> </mml:mrow> </mml:mrow> </mml:math> for methylene blue (MB) and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mn>65.24</mml:mn> <mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">%</mml:mi> </mml:mrow> </mml:mrow> </mml:mrow> </mml:math> for rhodamine B (Rh-B) in the presence of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mn>400</mml:mn> <mml:mrow> <mml:mtext> SCCM </mml:mtext> </mml:mrow> </mml:mrow> </mml:math> oxygen flow rate. In contrast, degradation efficiencies in air were <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mn>52.47</mml:mn> <mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">%</mml:mi> </mml:mrow> </mml:mrow> </mml:mrow> </mml:math> for MB and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mn>55.50</mml:mn> <mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">%</mml:mi> </mml:mrow> </mml:mrow> </mml:mrow> </mml:math> for Rh-B. Compared to conventional dye degradation techniques, this method offers rapid and efficient pollutant breakdown in continuously flowing samples, demonstrating its potential as a scalable and energy-efficient plasma-based water treatment technology.