Reduction and Removal of Aqueous Cr(VI) by Glow Discharge Plasma at the Gas–Solution Interface

Abstract

Aqueous chromium(VI) reduction and removal induced by glow discharge taking place at the gas-solution interface in an argon atmosphere was studied. The effect of initial pH and hydroxyl radical scavenger (ethanol) on the reduction efficiency was examined. High reduction efficiency was obtained when initial pH ≤ 2.0 or ≥ 8.0. In particular, addition of ethanol into the solution substantially increased the reduction efficiency and facilitated chromium removal from the solution in the form of sediment after discharge. The optimum pH values for Cr(VI) removal were within 6.0-7.0. Fourier transform-infrared (FTIR) spectroscopy and X-ray diffraction (XRD) analysis confirmed that the main constituent of the sediment is chromium hydroxide.

Assessment of Damage of Glutathione by Glow Discharge Plasma at the Gas–Solution Interface through Raman Spectroscopy

“…GSH can scavenge all the water primary radicals, and therefore reactions (1)-(5) may exist during plasma treatment. As shown in Figure 4 that about 30-40% damaged GSH were converted to GSSG detected by Raman spectroscopy, it is reasonable to estimate that >30-40% damaged GSH were converted to GS (reaction (1) and (3)) assuming that GSSG were mainly formed via reaction (6). GSSG is also susceptible to chemical reactive species, and therefore it was also being damaged during plasma treatment.…”

“…It means that when the formed GSSG accumulated a certain concentration, the appearance of destruction of GSSG by plasma treatment became dominant. In addition, produced GS can also decay via other reaction pathways besides reaction (6). For example, thiyl radicals can decay via intramolecular rearrangement reaction by which the hydrogen of CH 2 group at the a-position transfers to sulfur atom leading to the formation of carbon-centered radicals.…”

“…Since non-thermal discharge plasma formed in contact with liquid water can be employed to irradiate on various types of materials such as biological substances, it has been widely applied for many practical usages, including chemical analysis, [1] material processing, [2][3][4] pollution control, [5][6][7][8][9][10][11] and inactivation of cells and microbes. [8,[12][13][14][15][16] As a form of advanced oxidation-reduction processes (AORP), discharge plasma can induce the generation of large number of oxidative and reductive free radicals, such as hydroxyl radical, hydrogen radical, aqueous electron, and so on.…”

“…[17][18][19] Most applications of this technique are dependent on the reactions of the dissolved materials/pollutants with the generated chemical reactive species. For example, as for the environmental application, the generated reactive species can be used directly to degrade or remove pollutants from solution, [5][6][7][8] or to modify the physicochemical properties of absorbents. [9][10][11] As no costly equipment is required plasma sterilization is easily adaptable to complex devices and conventional processes, such as surgical instruments.…”

“…The proportion of the GSH converted to GSSG to the reduced GSH detected by this method is about 40-70%, much higher than the data determined by Raman spectroscopy. The reason for this difference is that high concentration of hydrogen peroxide was formed in solution by discharge treatment [6] and more GSSG was formed due to the action of produced hydrogen peroxide on GSH during storage of the treated sample. Because the fluorescence method is much more time-consuming and the determination cannot be accomplished immediately after discharge treatment, therefore, here it shows that in this case the Raman spectroscopy method is indeed a more effective way which can provide a more accurate evaluation.…”