Application of glow discharge plasma for wastewater treatment

Charge transfer processes at the interface of plasmas and liquid electrolytes have been studied for over 100 years. Both plasmas and liquid electrolytes contain charged species, and interactions between the two phases represent a unique combination of physics, chemistry, and materials science that is fundamentally different than that between solid electrodes and liquid electrolytes. Unfortunately, scientific progress over the last century has been slow because of several key challenges including the inability to stabilize nonthermal plasmas at atmospheric pressure, and the lack of fundamental understanding of the mechanisms for charge transfer. Within the last decade, significant strides have been made to overcome these challenges. Here, the authors review this fascinating area of study, highlighting the history, development of experimental systems, mechanistic aspects, and emerging applications.

Charge transfer processes at the interface between plasmas and liquids

Cathodic corrosion is a process that etches metal electrodes under cathodic polarization. This process is presumed to occur through anionic metallic reaction intermediates, but the exact nature of these intermediates and the onset potential of their formation is unknown. Here we determine the onset potential of cathodic corrosion on platinum electrodes. Electrodes are characterized electrochemically before and after cathodic polarization in 10 M sodium hydroxide, revealing that changes in the electrode surface start at an electrode potential of -1.3 V versus the normal hydrogen electrode. The value of this onset potential rules out previous hypotheses regarding the nature of cathodic corrosion. Scanning electron microscopy shows the formation of well-defined etch pits with a specific orientation, which match the voltammetric data and indicate a remarkable anisotropy in the cathodic etching process, favouring the creation of (100) sites. Such anisotropy is hypothesized to be due to surface charge-induced adsorption of electrolyte cations.

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Anisotropic etching of platinum electrodes at the onset of cathodic corrosion.

The critical aspects of the corrosion of metal electrodes in cathodic reductions are covered. We discuss the involved mechanisms including alloying with alkali metals, cathodic etching in aqueous and aprotic media, and formation of metal hydrides and organometallics. Successful approaches that have been implemented to suppress cathodic corrosion are reviewed. We present several examples from electroorganic synthesis where the clever use of alloys instead of soft neat heavy metals and the application of protective cationic additives have allowed to successfully exploit these materials as cathodes. Because of the high overpotential for the hydrogen evolution reaction, such cathodes can contribute toward more sustainable green synthetic processes. The reported strategies expand the applications of organic electrosynthesis because a more negative regime is accessible within protic media and common metal poisons, e.g., sulfur-containing substrates, are compatible with these cathodes. The strongly diminished hydrogen evolution side reaction paves the way for more efficient reductive electroorganic conversions.

Cathodic Corrosion of Metal Electrodes-How to Prevent It in Electroorganic Synthesis.

Contact glow discharge electrolysis (CGDE)/plasma electrolysis (PE) which is associated with the formation of a light emitting plasma around an electrode in a high conductivity electrolyte solution at moderate voltages up to ~1 kV, has in recent years attracted considerable interest as a tool for generating a large quantity of heat and a high yield of solvent-split radicals. These potentialities of CGDE/PE have, in fact, been exploited by a large number of investigators for applications ranging over areas as varied as synthetic chemistry, waste water treatment, degradation of polymers, electrosurgical tools, surface engineering, nanoparticle fabrication, machining and micro-machining, hydrogen production with very encouraging results. The article reviews comprehensively these results.

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Contact Glow Discharge Electrolysis: A Novel Tool for Manifold Applications

The question of how normal electrolysis (NE) at high voltages breaks down and transforms into contact glow discharge electrolysis (CGDE) was addressed with the anode as the preferred center for the phenomenon. Because close analogies in physical characteristics exist in the NE-to-CGDE transition and the natural convection-to-stable film boiling transition observed during pool boiling of liquids, the applicability of Helmholtz-Taylor's boiling heat transfer theory to the breakdown of NE and the transition to CGDE was examined. The approach was justified by the proportionality relationships observed between the critical flux of heat dissipated by the Joule effect near the anode and the anolyte surface tension or the radius of the anode wire. The results led to a detailed mechanism of the transition of NE to CGDE at the anode consisting of the following sequences: local solvent vaporization, setting up of stable counterflows between the vapor bubbles and the liquid anolyte at the anode, collapse of this flow pattern and formation of an unstable vapor film with an overlying heavier liquid, stabilization of the vapor film, and incipience of glow discharges across the film.

Mechanism of the Breakdown of Normal Electrolysis and the Transition to Contact Glow Discharge Electrolysis

A study of scavenging effects of iso-propanol, n-propanol, n-butanol and acetone on H• radicals, precursors for nonfaradaic products of anodic contact glow discharge electrolysis (CGDE) in aqueous media, has been carried out. A kinetic analysis of the competing reactions of H• with different species in the system leads to a primary yield of 9.8 mol (mol electron)−1 each of H• and OH• radicals in the liquid-phase reaction zone of anodic CGDE. The method is considerably more convenient to apply than those reported earlier based on the use of OH• scavengers.

Scavenging Effects of Aliphatic Alcohols and Acetone on H• Radicals in Anodic Contact Glow Discharge Electrolysis: Determination of the Primary Yield of H• Radicals

Non faradaic yields of anodic contact glow discharge electrolysis (CGDE) originate through H· and OH· radical generated during the process. Scavenging effects of Fe(CN)
 6
 4−
 on OH· radicals, in alkaline media have been investigated. A kinetic analysis of the competing reactions of O− with different species in the system leads to an yield of 9.8 mol mol electron−1 of OH· and H· radicals each in the liquid phase reaction zone of anodic CGDE in good agreement with the yield reported from a study involving H· radical scavengers.

Effects of Alkaline Ferrocyanide on Non-faradaic Yields of Anodic Contact Glow Discharge Electrolysis: Determination of the Primary Yield of OH · Radicals

Urvashi Gangal

Papers

Mechanism of the Breakdown of Normal Electrolysis and the Transition to Contact Glow Discharge Electrolysis

Scavenging Effects of Aliphatic Alcohols and Acetone on H• Radicals in Anodic Contact Glow Discharge Electrolysis: Determination of the Primary Yield of H• Radicals

Effects of Alkaline Ferrocyanide on Non-faradaic Yields of Anodic Contact Glow Discharge Electrolysis: Determination of the Primary Yield of OH · Radicals