Authors: Joseph Groele
Source: Deep Blue (University of Michigan) (article, 2023-01-01)
Field: Chemistry — Spectroscopy

Plasma-based Decomposition of Per- and Polyfluoroalkyl Substances in Water: Plasma Source Exploration, Batch Kinetics, Effects of Water Conductivity, and an Architecture for Continuous Flow Processing Demand for freshwater is rapidly rising due to expanding agriculture and industrialization, further exacerbated by population growth, changing climate, accidental spills, and deteriorating infrastructure. Amidst this, providing access to safe drinking water around the globe remains a grand challenge of the 21st century. A significant part of the solution is to recycle and re-use water; however, wastewaters contain contaminants of emerging concern which are not adequately being addressed by conventional water treatment technologies. Per- and polyfluoroalkyl substances (PFAS) have received attention lately after being detected in &gt;98% of the US population’s blood. Meanwhile, most municipal water treatment plants are not equipped to remove these compounds. Human exposure to PFAS has been linked to a multitude of health problems, and these chemicals are found in a wide range of industrial and consumer products. The high strength of the carbon-fluorine bonds found in PFAS are both the source of their industrial appeal and the reason for their persistence in the environment. While activated carbon filtration has shown efficacy for removing long-chain PFAS from water, the compounds are not decomposed, but simply stored in different media that must be dealt with, thus leading to high overall costs and concerns about reintroducing the contamination back into the environment. Non-thermal plasmas are a promising emerging technology for the destruction of PFAS in water owing to their ability to drive multiple decomposition mechanisms including advanced oxidation and reduction. This thesis explores two plasma-water reactor configurations for the decomposition of PFAS in contaminated groundwaters collected from legacy sites: an underwater plasma jet and a barrier discharge with water dielectrics. Based on the removal percent of perfluorooctanesulfonic acid (PFOS), air was found to be a suitable working gas, and both AC and ns-pulsed power supplies could generate the plasmas for these reactors. Plasma-water contact area was a critical variable governing the decomposition performance. Furthermore, it was found that the mass removal efficiency was higher when treating higher PFAS concentrations, suggesting that a concentration step followed by plasma treatment can improve the cost-effectiveness of the treatment process. Batch processing of the groundwater revealed that long-chain PFAS followed empirical first-order decay, and notably, the two most prevalent PFAS in the groundwater were removed to below Michigan EPA limits within 90 minutes of treatment. Throughout the treatment, the water conductivity increased approximately fourfold; however, a separate experiment indicated that the plasma discharge properties did not change significantly with increasing water conductivity. Specifically, plasma density, gas temperature, and discharge power remained approximately constant. Water conductivities below 200 µS/cm were associated with higher peak discharge voltages, higher electron densities and gas temperatures, and more intense UV emission, which is thought to be associated with a spark-type discharge that forms due to longer charge relaxation times and streamer propagation lengths, thus enabling a transient conductive channel between electrodes. Finally, an architecture for converting the batch-mode plasma treatment into a scalable continuous flow configuration is proposed, along with a model for estimating the system size and performance based on the batch reactor kinetics. This architecture features membrane concentration of PFAS followed by a staged plasma micro-reactor treatment, each with internal water recirculation. The cost for removing PFOS to below Michigan EPA limits from an initial concentration of 200 ppt is estimated at 1.30 USD per thousand liters at a continuous flow rate of 170 L/min. Decomposition ; Kinetics ; Plasma ; Conductivity ; Continuous flow ; Flow (mathematics) ; Chemistry ; Chemical engineering ; Environmental science ; Process engineering ; Physical chemistry ; Organic chemistry ; Physics ; Mechanics ; Engineering Mass Spectrometry Techniques and Applications