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Description
Nitrate is the most prevalent water pollutant limiting the use of groundwater as a potable water source. The overarching goal of this dissertation was to leverage advances in nanotechnology to improve nitrate photocatalysis and transition treatment to the full-scale. The research objectives were to (1) examine commercial and synthesized photocatalysts, (2) determine the effect of water quality parameters (e.g., pH), (3) conduct responsible engineering by ensuring detection methods were in place for novel materials, and (4) develop a conceptual framework for designing nitrate-specific photocatalysts. The key issues for implementing photocatalysis for nitrate drinking water treatment were efficient nitrate removal at neutral pH and by-product selectivity toward nitrogen gases, rather than by-products that pose a human health concern (e.g., nitrite). Photocatalytic nitrate reduction was found to follow a series of proton-coupled electron transfers. The nitrate reduction rate was limited by the electron-hole recombination rate, and the addition of an electron donor (e.g., formate) was necessary to reduce the recombination rate and achieve efficient nitrate removal. Nano-sized photocatalysts with high surface areas mitigated the negative effects of competing aqueous anions. The key water quality parameter impacting by-product selectivity was pH. For pH < 4, the by-product selectivity was mostly N-gas with some NH4+, but this shifted to NO2- above pH = 4, which suggests the need for proton localization to move beyond NO2-. Co-catalysts that form a Schottky barrier, allowing for localization of electrons, were best for nitrate reduction. Silver was optimal in heterogeneous systems because of its ability to improve nitrate reduction activity and N-gas by-product selectivity, and graphene was optimal in two-electrode systems because of its ability to shuttle electrons to the working electrode. "Environmentally responsible use of nanomaterials" is to ensure that detection methods are in place for the nanomaterials tested. While methods exist for the metals and metal oxides examined, there are currently none for carbon nanotubes (CNTs) and graphene. Acknowledging that risk assessment encompasses dose-response and exposure, new analytical methods were developed for extracting and detecting CNTs and graphene in complex organic environmental (e.g., urban air) and biological matrices (e.g. rat lungs).
ContributorsDoudrick, Kyle (Author) / Westerhoff, Paul (Thesis advisor) / Halden, Rolf (Committee member) / Hristovski, Kiril (Committee member) / Arizona State University (Publisher)
Created2013
Description
Electrospinning is a means of fabricating micron-scale diameter fiber networks with enmeshed nanomaterials. Polymeric nanocomposites for water treatment require the manipulation of fiber morphology to expose nanomaterial surface area while anchoring the nanomaterials and maintaining fiber integrity; that is the overarching goal of this dissertation. The first investigation studied the effect of metal oxide nanomaterial loadings on electrospinning process parameters such as critical voltage, viscosity, fiber diameter, and nanomaterial distribution. Increases in nanomaterial loading below 5% (w/v) were not found to affect critical voltage or fiber diameter. Nanomaterial dispersion was conserved throughout the process. Arsenic adsorption tests determined that the fibers were non-porous. Next, the morphologies of fibers made with carbonaceous materials and the effect of final fiber assembly on adsorption kinetics of a model organic contaminant (phenanthrene, PNT) was investigated. Superfine powdered activated carbon (SPAC), C60 fullerenes, multi-walled carbon nanotubes, and graphene platelets were added to PS and electrospun. SPAC maintained its internal pore structure and created porous fibers which had 30% greater PNT sorption than PS alone and a sevenfold increase in surface area. Carbon-based nanomaterial-PS fibers were thicker but less capacious than neat polystyrene electrospun fibers. The surface areas of the carbonaceous nanomaterial-polystyrene composites decreased compared to neat PS, and PNT adsorption experiments yielded decreased capacity for two out of three carbonaceous nanomaterials. Finally, the morphology and arsenic adsorption capacity of a porous TiO2-PS porous fiber was investigated. Porous fiber was made using polyvinylpyrrolidone (PVP) as a porogen. PVP, PS, and TiO2 were co-spun and the PVP was subsequently eliminated, leaving behind a porous fiber morphology which increased the surface area of the fiber sevenfold and exposed the nanoscale TiO2 enmeshed inside the PS. TiO2-PS fibers had comparable arsenic adsorption performance to non-embedded TiO2 despite containing less TiO2 mass. The use of a sacrificial polymer as a porogen facilitates the creation of a fiber morphology which provides access points between the target pollutant in an aqueous matrix and the sorptive nanomaterials enmeshed inside the fiber while anchoring the nanomaterials, thus preventing release.
ContributorsHoogesteijn von Reitzenstein, Natalia Virginia (Author) / Westerhoff, Paul (Thesis advisor) / Hristovski, Kiril (Committee member) / Perreault, Francois (Committee member) / Herckes, Pierre (Committee member) / Arizona State University (Publisher)
Created2018
Description
Ion exchange sorbents embedded with metal oxide nanoparticles can have high affinity and high capacity to simultaneously remove multiple oxygenated anion contaminants from drinking water. This research pursued answering the question, “Can synthesis methods of nano-composite sorbents be improved to increase sustainability and feasibility to remove hexavalent chromium and arsenic simultaneously from groundwater compared to existing sorbents?” Preliminary nano-composite sorbents outperformed existing sorbents in equilibrium tests, but struggled in packed bed applications and at low influent concentrations. The synthesis process was then tailored for weak base anion exchange (WBAX) while comparing titanium dioxide against iron hydroxide nanoparticles (Ti-WBAX and Fe-WBAX, respectively). Increasing metal precursor concentration increased the metal content of the created sorbents, but pollutant removal performance and usable surface area declined due to pore blockage and nanoparticle agglomeration. An acid-post rinse was required for Fe-WBAX to restore chromium removal capacity. Anticipatory life cycle assessment identified critical design constraints to improve environmental and human health performance like minimizing oven heating time, improving pollutant removal capacity, and efficiently reusing metal precursor solution. The life cycle environmental impact of Ti-WBAX was lower than Fe-WBAX as well as a mixed bed of WBAX and granular ferric hydroxide for all studied categories. A separate life cycle assessment found the total number of cancer and non-cancer cases prevented by drinking safer water outweighed those created by manufacture and use of water treatment materials and energy. However, treatment relocated who bore the health risk, concentrated it in a sub-population, and changed the primary manifestation from cancer to non-cancer disease. This tradeoff was partially mitigated by avoiding use of pH control chemicals. When properly synthesized, Fe-WBAX and Ti-WBAX sorbents maintained chromium removal capacity while significantly increasing arsenic removal capacity compared to the parent resin. The hybrid sorbent performance was demonstrated in packed beds using a challenging water matrix and low pollutant influent conditions. Breakthrough curves hint that the hexavalent chromium is removed by anion exchange and the arsenic is removed by metal oxide sorption. Overall, the hybrid nano-sorbent synthesis methods increased sustainability, improved sorbent characteristics, and increased simultaneous removal of chromium and arsenic for drinking water.
ContributorsGifford, James McKay (Author) / Westerhoff, Paul (Thesis advisor) / Hristovski, Kiril (Thesis advisor) / Chester, Mikhail (Committee member) / Arizona State University (Publisher)
Created2016