Hydrodynamic Modifications to the Plasma-Liquid Interface for Reactivity Control in Environmental Remediation Applications

Abstract

The interaction of atmospheric pressure non-thermal plasma with liquid can drive liquid phase chemical reactions far from equilibrium, presenting opportunities to address environmental remediation problems such as water pollution and plastic recycling. Non-thermal plasma, where the electrons are vastly more energetic than the heavy ion and neutral species, can provide chemical reactivity and selectivity without bulk fluid heating. When produced on or near liquid water, these species are capable of driving oxidation and reduction reactions which facilitates the mineralization of organic contaminants in the water. However, while small scale efficacy of such plasma treatments have been demonstrated, scaling plasma-liquid discharges to real-world treatment volumes has proven challenging. Plasma driven reactions are inherently limited by species diffusion through the liquid interface, leaving the most efficient reactors to have low throughput. This work explores opportunities to enhance the plasma-liquid interface to increase transport of reactive species into the bulk liquid, aiming to bring this technology to the treatment of larger scale environmental processes.

This thesis focuses on further understanding how transport at the plasma-liquid interface controls the reactivity of the solution, particularly when the interface and bulk fluid is physically perturbed. Turbulence, where velocity fluctuations and surface perturbations occur over a range of length scales, is inherently stochastic. The complexities of turbulent flow in the presence of the direct application of plasma to a liquid is not well understood, with many coupled processes occurring both hydrodynamically and within the gas phase discharge - both of which are able to alter the transport of reactive species to the bulk fluid.

In this work, a series of experiments were carried out to modify the plasma-liquid interface using turbulent flow and the acoustic introduction of Faraday waves. These experiments aimed to elucidate the processes of hydrodynamic mixing and surface area enhancements. Local and bulk plasma modifications were also explored in conjunction with reactive species uptake into the bulk liquid. Multiple model dye compounds dissolved in solution, possessing different surface activities, indicated that while all were impacted, turbulence effects were most pronounced in the decoloration of the non-surfactant compounds. It was found that in addition to there being bulk mixing and increased plasma-liquid contact area, the propagation of surface ionization waves induced in the discharge were locally modified along water flows with complex surface features. Scalability of the plasma water reactor was considered, including power requirements and once-through treatment of contaminants.

For the remediation of plastic waste, non-thermal plasma can functionalize polymer surfaces or completely decompose the polymers into lighter hydrocarbons over time. Both of these conditions were explored, including the treatment of polymers in liquid. These plasmas may operate without consumables and only require electricity, making them a potentially environment-friendly solution to growing contamination issues. Ultimately, the goal of this work is to explore ways to control the plasma-liquid interface as a means to increase the delivered dose over a larger surface area while simultaneously improving mixing into the bulk. These results provide insight into how to engineer the plasma interface for incorporation into real-world treatment trains, such as the treatment of highly concentrated waste streams. These efforts target pressing environmental issues including the removal of recalcitrant contaminants of emerging concern as well as the depolymerization or decomposition of plastic waste.