Nanosecond pulsed plasma discharge for remediation of simulated wastewater containing thiazine and azo dyes as model pollutants

Plasmas generated over and under the surface of water have immense scope for wastewater remediation due to in situ generation of redox-rich species. Remediation of simulated wastewater using underwater plasma discharge is reported in this paper. During the plasma discharge, an increase in energy coupling from source to discharge channel was noted with an increase in the applied pulsed DC voltage from $5$ to $9kV$⁠. Higher energy coupling resulted in elevation of gas temperature and electron temperature. Concentrations of hydroxyl radicals and hydrogen peroxide, estimated spectrochemically in effluent water, also increased with the applied voltage. On the other hand, ozone concentration showed a decreasing trend with increasing applied voltage and the peak ozone density was obtained at the lower end of the applied voltage, i.e., at $5kV$⁠. With higher energy dissipation, a pronounced discharge and, hence, greater emission intensities of ultraviolet radiation, atomic oxygen, and atomic hydrogen were observed in discharge spectra. The oxidative-reductive stress produced by underwater discharge resulted in a degradation efficiency of $75.5%$ for methylene blue solution (thiazine dye) corresponding to energy yields of $3.55g/kWh$⁠. Similarly, the degradation efficiency of $81.4%$ for methyl orange solution (azo dye) with an energy yield of $4.73g/kWh$ was achieved with this discharge. From the considerations of collision theory, activation energies of $67.17kJ/mol$ for methylene blue and $75.32kJ/mol$ for methyl orange degradation were estimated for the plasma discharge. Fourier-transform infrared spectroscopy was utilized to further reveal the possible effects of plasma therapy on different bonds of the dyes. Additionally, gas chromatography mass spectrometry analyses were used to suggest the possible degradation pathways of the two dyes. Instant dye degradation in flowing samples was compared with contemporary dye degradation results listed in literature to show the effectiveness of the used technique.