Sustainable Oxidation: Generating Chemical-Free Ways to Remove Contaminants of Emerging Concern

Current methods of contaminant removal in water and wastewater treatment are expensive and typically employ processes that produce toxic disinfection byproducts. Nanobubbles (NBs), or spherical packages of gas under 1000 nm in size, have the potential to create a paradigm shift in advanced oxidation processes (AOPs) by offering a chemical-free alternative to chlorine and ozone oxidants. However, NBs remain relatively enigmatic, and in particular, recent literature fails to consistently describe NB coalescence and collapse. This study aimed to collect preliminary data for the evaluation of several dynamic stimuli (UV-A, microbubble (MB) aeration, and NaCl) on NB size and on methylene blue (mBlu) degradation.

Bulk NBs were generated using a pilot-size NB generator (25XTB). DI H₂O was circulated at 25 gal/min in a 55-gallon drum containing 30 gallons of DI H₂O until air NBs reached saturation. Dynamic stimuli were then introduced to sub-samples of NB DI H₂O. Relative frequency of particle diameter was measured for the UV-A, MB aeration, and NaCl sub-samples using dynamic light scattering (DLS). Additionally, degradation of mBlu, a sample organic contaminant, was measured for the UV-A and MB aeration sub-samples using UV-Vis spectroscopy. Aeration was investigated using 0.5, 2.0, and 5.0-micron bubbles.

NB DI H₂O samples exposed to UV-A showed approximately an 80% decrease in mBlu concentration, compared to control DI H₂O samples which demonstrated only a 50% decrease in mBlu concentration with UV-A exposure. The DLS results for air NBs in DI H₂O treated with UV-A showed a narrowing size distribution as well as a decreasing average size over time (0, 60, and 90 min). Similarly, NB DI H₂O samples exposed to aeration of 0.5, 2.0, and 5.0-microns showed up to a 10% decrease in mBlu concentration. NB DI H₂O with 500 mM NaCl showed an increasing average size over time (0, 30, and 60 min).

These results support the hypothesis that NB size can be controlled with dynamic stimuli, as well as the prediction that NBs have the potential to degrade organic contaminants. More generally, this work provides a basis to further explore NBs as a sustainable method of contaminant removal in water and wastewater treatment applications.