Dye Degradation by Plasma-Generated RONS

A 1 g/L stock solution of MB, MO and CR is prepared by dissolving the required amount of analytical grade dye in Millipore water. The experimental solutions (200 mg/L) are obtained by diluting the stock solution in accurate proportions. Degradation of the dyes is monitored via a UV-Vis spectrophotometer S-3100, with a wavelength resolution, accuracy, and reproducibility of 0.95 nm, ±0.5 nm and ±0.02 nm, respectively. The absorption is measured at 660 nm, 460 nm, and 500 nm for MB, MO and CR, respectively, and the degradation percentage and energy efficiency (g/kWh) are calculated using a previously reported method22 (link). The H₂O₂ concentration is measured using titanyl ions34 (link)35 (link) in the presence of sodium azide to control the H₂O₂ degradation by nitrites. The NO concentration is detected using 4-amino-5-methylamino-2′,7′-difluorofluorescein (DAF-FM)36 , and the OH concentration is measured using terephthalic acid (20 mM) by means of the procedure described earlier in ref. 37 (link). The NO₂⁻ concentration is measured using Griess reagent supplied by Aldrich Chemical Co. (USA), whereas the NO₃⁻ concentration is obtained using the Acorn Series ION 6 meter (pH/mV/°C Meter), nitrate electrode, from Oakton Instruments, USA. After exposure of both plasmas (generated from the ID- and D-APPJs) to deionized (DI) water and the three different solutions, the pH and temperature of the water and of the solutions are measured using a pH meter (Eutech Instruments, Singapore) and Infrared (IR) camera (Fluke Ti100 Series Thermal Imaging Cameras, UK). All measurements are performed in triplicate. The density of the dyes in the solution is measured using an Anton-Paar DSA 5000 with an accuracy in temperature ±0.01 K, whereas the uncertainty in the density is ±0.00005 g cm⁻³. Prior to the measurements, the instrument is calibrated with DI water and dry air as standards at 293.15 K38 (link)39 (link)40 (link).
The filtrates are measured by a high performance liquid chromatography method using HPLC-UV (Agilent 1200, USA) containing a ZORBAX SB-C18 column (2.1 mm 150 mm, 5 mm). 30/70 (v/v) of acetonitrile/10% acetic acid is used as the mobile phase for MB, MO and CR in HPLC-UV. The analysis is performed at 25 °C with 0.8 mL/min flow rate of an injected 100 mL volume of sample. The final degradation products are analyzed using LC[QTOF]MSMS. The LCMSMS consists of a TripleTOF 5600 (quadrupole-Time of flight) tandem mass spectrometer (ABSciex). The TOF mass range is 40,000 m/z with a maximum resolution power of MS 25,000@m/z 195, MS/MS 35,000@m/z 965, and a mass accuracy of <0.05 ppm.
To understand the effect of various RONS, generated by the plasma, on the decolorization/degradation of the dyes, we also treat the dye solutions with ozone and nitric oxide. Ozone (O₃) is generated using an ozone machine (Model: LAB-II, Company: OZONE TECH). Applying this technique we generate 5580 ppm of ozone, the concentration of which is measured using the detection tubes obtained from Gastec (Product No. 18M and 18L, Gastec, Japan). These tubes contain a reagent which changes its color after coming into contact with the ozone. Note that these tubes have an accuracy of about ±10% due to the presence of other interfering species. Nitric oxide (NO) is generated using a microwave plasma system. This system consists of a magnetron, waveguide component (WR-340 for 2.45 GHz) and a microwave plasma torch apparatus, as described in our earlier work41 , which can generate 5712 ppm of NO.