This NSF award by the Chemical and Biological Separations Program supports work by Professor Alexander P. Mathews of Kansas State University to investigate methods and mechanisms to enhance gas-liquid mass transfer and reaction rates using nanoparticles and nanoshells as shuttles. Nanoparticles and mesoporous nanoshells possess unique properties such as high surface area and mobility, and these can be used to advantage in changing bubble properties in gas-liquid mass transfer. Nanoparticle sizes are in the range of hydrodynamic boundary layers, and can be used to transport mass across the boundary layer. Moreover, adsorbent nanoparticles can sorb solutes from the liquid phase and rapidly shuttle the solutes to the gas phase. This process is expected to provide several fold increase in mass transfer rates due to (1) a parallel liquid-solid-gas transfer mechanism in addition to liquid-gas mass transfer, (2) increased bubble residence time, and (3) cavitating bubbles and under ultrasonic fields. This research will examine the mechanisms by which inert and adsorbent nanoparticles will affect mass transfer rates in the transfer of dissolved organic contaminants from water to the air phase in the presence and absence of ultrasonic fields. Gas-liquid mass transfer processes are important in natural and engineered systems. The application of knowledge gained from this work will provide more efficient means of conducting mass transfer operations in drinking water purification, multiphase reactions in the process industries, and in the removal of volatile organic compounds from contaminated groundwaters and wastewaters.
The goal of this project was to investigate the effects of nanoparticles in enhancing the remediation efficiency of waters contaminated with volatile organic compounds (VOCs) that are harmful to human health and the environment. Magnetite nanoparticles containing different ratios of Fe2+ and Fe3+ and with differing magnetic properties and surface areas were synthesized in the laboratory. Silica coated magnetite nanoparticles were also synthesized in the laboratory. Gas-liquid mass transfer studies were conducted for the removal of contaminants such as methyl tert-butyl ether (MTBE) and trichloroethylene (TCE) by air stripping in the presence of small amounts of nanoparticles. The effect of ultrasound cavitation effects on gas-liquid mass transfer rates was also investigated. Synthesis procedures were developed to produce magnetites with tunable surface areas and magnetic properties. The surface areas of the laboratory synthesized magnetites (153 to 316 m2/g) were two to four times that of commercially available magnetite. The high surface area magnetites provided high removal efficiencies for toxic metals such as mercury from water samples. Gas-liquid mass transfer studies indicate that the addition of magnetite nanoparticles tend to increase mass transfer rates during air stripping of TCE and MTBE from water. However, aggregation of the magnetite nanoparticles need to be controlled by coating the nanoparticles with surfactants to produce stable suspensions. The remediation efficiency is decreased by the formation of nanoparticle aggregates. Ultrasound irradiation was found to enhance gas-liquid mass transfer efficiency. However, the addition of magnetite nanoparticles to this system did not enhance mass transfer rates due to the absorption of ultrasound energy by magnetite particle aggregates. Ultrasound irradiation in the presence of silica coated magnetite nanoparticles was found to provide about 32% enhancement in mass transfer efficiency when compared to a system without nanoparticles. The removal of the magnetite nanoparticles from the process effluent for recycle was studied using high gradient magnetic filtration (HGMF). A filtration efficiency of 98% or higher could be achieved in the laboratory with a 0.5 Tesla magnet. The filtered magnetite was recovered for reuse by demagnetizing and backwashing with a solution at pH 10 or higher. This project has broad applications to gas-liquid mass transfer in three phase systems. Potential applications include the stripping of VOCs from contaminated waters, oxygen absorption in biological treatment and fermentation systems, and the removal of toxic metals using high surface area magnetites. Postdoctoral scholars, graduate students, undergraduate students, and one high school student were trained in this field as part of this project.
