Which chemicals are best degraded by electrical plasmas?

By Dean Williams on December 14, 2016

This article belongs to the special issue on Emerging Leaders, which features invited work from the best early-career researchers working within the scope of JPhysD. This project is part of the Journal of Physics series‘ 50th anniversary celebrations in 2017. Professor Thagard was selected by the Editorial Board of JPhysD as an Emerging Leader.

Plasma-based water treatment (PWT) utilizes electrical discharge plasma and is capable of degrading most chemicals, whether via oxidation, reduction, thermal decomposition or other means. In fact, we have shown in this study that perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA), which are notoriously difficult to degrade by conventional processes, are rapidly degraded by plasma.

However, the efficiency with which PWT degrades different chemicals can vary over several orders of magnitude. The viability of PWT is thus dependent on what contaminants are meant to be removed, which makes the choice of application for this technology as important as its design and development. To assist in the process of choosing which applications to target and thus which contaminants to study, we developed a general mechanistic model that provided insights into how a compound’s relative treatability can be predicted based on just a simple survey of its molecular structure.

The model reflects a proposed triple-mechanism scheme, wherein three sets of reactions, sub-surface, surface and above-surface, all contribute to the degradation of contaminants (illustrated in the figure below). While the extent to which each set of reactions contributes varies between compounds, the combined contribution from surface and above-surface reactions is dominant, particularly for the contaminants that are most efficiently degraded. This indicates that the interfacial concentration of a compound is the most important factor that determines degradation rate, which implies that surfactant-like contaminants are most vulnerable to degradation by plasma and thus represent the most promising target for PWT.

Illustration of the triple-mechanism scheme, featuring PFOA (highly surfactant-like), fluoxetine (moderately surfactant-like) and caffeine (not surfactant-like). Hydroxyl radicals (HO∙) were assumed to be the primary reactive species involved in surface and sub-surface reactions; argon ions (Ar+) and free electrons (e-) were assumed to be the reactive species involved in above-surface reactions. Image taken from Selma Mededovic Thagard et al 2017 J. Phys. D: Appl. Phys. 50 014003, © IOP Publishing, All Rights Reserved.

While we quantified each compound’s surface activity using large sets of surface tension data, the degree to which a contaminant might behave as a surfactant can be estimated by evaluating the degree to which its molecular structure resembles a surfactant. Important surfactant-like characteristics include large, well-defined, strongly hydrophobic groups in addition to, but not comingled with, well-defined hydrophilic groups. If a contaminant has these characteristics, it will likely be degraded efficiently by PWT, and it may represent a promising application for the technology. However, if the contaminant is, for instance, small and hydrophilic, it will likely be slowly degraded by PWT, and is probably not worth the effort of investigating further. Conducting this simple evaluation prior to committing to a full investigation can help improve the value of research in this field and the likeliness of PWT achieving viability.

The full article is now available on IOPscience, along with the rest of our Emerging Leaders collection.

About the authors

Selma Mededovic Thagard is an associate professor in the Department of Chemical and Biomolecular Engineering at Clarkson University. Her research interests include electrical discharge plasma processes with a focus on theoretical and experimental investigations of fundamental plasma chemistry in single and multiphase plasma environments.

Gunnar R. Stratton is a PhD candidate in Chemical and Biomolecular Engineering at Clarkson University. His research focuses on the design and scale up of plasma-based water treatment processes and the characterization of phenomena involved in the degradation of chemical contaminants by plasma.

Fei Dai obtained her PhD degree in Environmental Science and Engineering from Clarkson University in August 2016. Her PhD study focused on pulsed electrical discharges for the degradation of organic compounds in water, particularly the effects of key process conditions on the degradation rate of different compounds.

Christopher L. Bellona is an Assistant Professor in the Civil and Environmental Engineering Department at the Colorado School of Mines. His research focuses on the use of membrane and advanced treatment technologies for the treatment of unconventional water resources including wastewater, seawater and oil and gas produced water.

Thomas M. Holsen is currently the Jean S. Newell Distinguished Professor in Engineering and a professor in Civil and Environmental Engineering at Clarkson University and director of the Clarkson Center for Air Resources Engineering and Sciences. His primary research interests include the transport, transformations and fate of hydrophobic organic chemicals, mercury, metals, and ions in a wide array of environmental systems.

Douglas G. Bohl is an Associate Professor in the Department Mechanical and Aeronautical Engineering at Clarkson University. He specializes in the development and application of optical diagnostics for the investigation of fluid flow systems. Dr. Bohl has applied these techniques to the study of mixing and transport in fluid systems, unsteady aerodynamics, and two phase flows.

Eunsu Paek is an assistant professor in the Chemical and Biomolecular Engineering Department at Clarkson University. Her research interests include molecular simulation of interfacial chemistry and ion transport, and chemical functionalization in nanostructured materials for energy and environmental applications.

Dr. Eric R. V. Dickenson is currently a Project Manager for the Water Quality Research and Development group at the Southern Nevada Water Authority. His research primarily focuses on the occurrence, formation and treatment of contaminants of emerging concern in wastewater, water reuse and drinking water treatment systems.

This work is licensed under a Creative Commons Attribution 3.0 Unported License. Image taken from Selma Mededovic Thagard et al 2017 J. Phys. D: Appl. Phys. 50 014003, © IOP Publishing, All Rights Reserved.