Adsorption and degradation of environmental contaminants exemplified by arsenic, vinyl fluoride and nitrate
Date
2016
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Publisher
University of Delaware
Abstract
The prevalence of legacy and emerging contaminants has increasingly stressed our limited water resources, and caused impaired water quality in many parts of the world. To meet our growing demand for clean water in this century, it is of the utmost importance to develop more effective means to remove/degrade pollutants in water. In this research, three novel materials/processes were investigated for the adsorption or degradation of three important contaminants: arsenic, fluorinated alkenes, and nitrate. Arsenic is a common groundwater contaminant that poses a serious health threat to populations in the U.S. and other countries. Fluorinated organics are widespread in consumer and industrial products, and many of them are persistent due to the high stability of the carbon-fluorine bond. Nitrate is the most ubiquitous pollutant in U.S. groundwater. Nitrate is not only toxic at high concentrations, but is also a leading cause of water quality impairment. We have developed a new nano-magnetite-based sorbent to remove arsenic, investigated an effective catalyst for the reduction of fluorinated alkenes, and determined the capacity of a biochar to promote microbial nitrate reduction.
Magnetite nanoparticle composite (MNPC) was synthesized for the removal of arsenic from water. We have shown that magnetite nanoparticles (MNP) possess high capacities and superior kinetics for adsorption of arsenic. In addition, to enable treatment applications, a new method was developed to embed MNPs into a silica network (MNPC). MNPC exhibited high adsorption capacities for arsenite and arsenate, 159.7 and 165.1 mg g-1, respectively, comparable to the adsorption capacity of MNPs under anaerobic conditions. MNPC could retain over 99.99% of the MNPs in its structure. Moreover, the embedment prevented exposure of MNPC to oxygen and thereby extended its service life. Our results suggest that MNPC may represent a viable technology for arsenic removal from groundwater and drinking water.
Rhodium on alumina was used as a catalyst to activate hydrogen gas for the reduction of vinyl fluoride (VF) as a model compound for fluorinated alkenes. VF is the monomer of fluoropolymer, a high production volume compound, and a probable (group 2A) carcinogen. We studied the kinetics of VF reduction in the presence of water. The rate-limiting step for the reduction was determined to be the mass transfer of VF from bulk water to the catalyst surface. Based on the product distribution, the reaction paths were found to consist of reductive defluorination, followed by hydrogenation, and hydrogenation only, producing ethane and fluoroethane, respectively, as final product. When water was absent, the kinetics was too fast to be measured producing mainly fluoroethane as the final product. The experiment with humidified hydrogen gas showed that even layers of adsorbed water molecules on the surface of the catalyst would dramatically shift the reaction rate and product distribution. By revealing the crucial role of water in controlling both the reaction kinetics and pathway, this study could be an important step toward the development of effectively catalytic treatment for fluorocarbons.
We demonstrated for the first time that biochar could serve as an electron donor to support microbial nitrate reduction. This new discovery could be a basis of novel engineered treatment/remediation systems to degrade nitrate, the most prevalent pollutant in the U.S. groundwater. Geobacter metallireducens (GS-15) was used to investigate the role of redox active functional groups in biochar to nitrate reduction by exoelectrogenic bacteria. We showed that both biologically and chemically reduced biochar could support nitrate reduction. Results of this study suggest that biochar could be a bioaccessible electron storage medium in bioretention cells and other engineered systems, and this finding may also be applied to other black carbon.
Each approach in this dissertation represents a breakthrough in contaminant treatment. Results of each investigation either form a basis for new and improved treatment methods or have implications, for the fate of contaminants in natural systems. Both are discussed in Chapter 5.