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Date of Award
Doctor of Philosophy (PhD)
Global water demands continue to rise, especially in lieu of population growth and consequential economic and energy needs. Furthermore, global climate change has placed additional stresses on the future availability of freshwater. As a result, it is now becoming crucial to replenish water sources via drinking water and wastewater treatment. Due to advancements in nanotechnology, engineered and manufactured nanoparticles are increasingly entering water and wastewater treatment plants. In addition, naturally occurring nanoparticles can also form or exist in drinking water and wastewater treatment facilities. Therefore, to design better water treatment systems, it is important to understand how these nanoparticles will affect water chemistry as well as the efficacy of drinking water and wastewater treatment processes. This work consists of environmental perspectives of natural and engineered nanoparticle interactions in two complex aqueous environments: wastewater treatment and drinking water treatment systems.
First, impacts of natural and engineered nanoparticles on wastewater treatment process efficacy were investigated in the following three systems: (1) heterogeneous iron (hydr)oxide formation on organic coated substrates, (2) homogeneous iron (hydr)oxide formation, and (3) mixed homo/heterogeneous nucleation of iron (hydr)oxide on CeO2 engineered nanoparticles. In System (1), heterogeneous precipitation of naturally occurring iron (hydr)oxide nanoparticles in model wastewater systems was investigated. Quartz, polyaspartate and alginate coated substrates were used to model abundant mineral substrates found downstream of wastewater treatment, anionic polyelectrolytes used in coagulation processes, and extracellular materials in biofilm during biochemical treatment, respectively. For the first time, iron (hydr)oxide nanoparticle formation on polymeric substrates was monitored in situ. Results indicated that substrate surface hydrophilicity was more dominant than electrostatic interactions in predicting nucleation. In System (2), homogeneous iron (hydr)oxide nucleation and phase transformation was then investigated as a function of synthesis conditions. Iron (hydr)oxides are highly reactive and effective sorbents for wastewater contaminants and formation conditions can determine their sorption efficacy. Therefore, in this study, the Fe(III) hydrolysis kinetics and cooling rates were altered to investigate the simultaneous formation of mixtures of hematite and 6-line ferrihydrite iron (hydr)oxide nanoparticles. Complementary in situ and ex situ analytical techniques revealed that understanding in situ physicochemical properties can control ex situ nanoparticle characteristics. Also, separate, distinct hematite and 6-line ferrihydrite phases were generated simultaneously and 6-line ferrihydrite removed more As(V), a model wastewater contaminant, compared to hematite. Moreover, iron (hydr)oxides can also form in the presence of engineered nanomaterials in wastewater which can affect contaminant transport downstream as well as wastewater stream chemistry. Therefore, in System (3), iron (hydr)oxide formation on engineered cerium oxide (CeO2) nanoparticles by redox reactions with Fe2+ (a reagent used in advanced oxidation processes in wastewater treatment) and Cr(VI)(aq) (a pre-existing wastewater contaminant) was investigated. The coexistence of Fe2+ and Cr(VI)(aq) were found to greatly promote the colloidal stability and to inhibit the dissolution of CeO2 nanoparticles while promoting the formation of an iron (hydr)oxide surface coating layer via redox reactions at the CeO2 nanoparticle surface. This is more prominent in the presence of Cr(VI)(aq) compared to systems without Cr(VI)(aq) ions. Engineered nanoparticles could act as heterogeneous nucleation sites and adsorption sites when released into the environment, incorporating toxic elements and molecules into a “hybrid” engineered/natural nanoparticle composite. As such, tt is essential to understand surface redox chemistry which nanoparticles could experience during wastewater treatment processes.
Second, effects of naturally forming colloids and membrane-surface-modifier nanomaterials on reverse osmosis (RO) drinking water treatment processes were studied. Fouling by calcium carbonate (CaCO3) and calcium sulfate (CaSO4) and other brackish water constituents can clog RO membrane pores and reduce the amount of purified water produced, and as a result, engineered nanomaterials have been used to reduce fouling on membrane surfaces. In this work, two scientific challenges related to colloid interactions during RO were addressed: (4) mineral scaling on polyethylene glycol (PEG)-modified RO membranes, and (5) mineral scaling, organic fouling and biofouling on multifunctional membrane surfaces. In System (4), hydrophilic, polyamide RO membrane surface modification using grafted PEG was studied as a remedy to reduce fouling from mineral scalants (i.e., CaCO3 and CaSO4) and humic acid which exists in high concentrations in brackish water. In batch systems without humic acid, the PEG-grafted membranes were successful in reducing mineral scale formation at the membrane surface; however, in the presence of humic acid, a specific interaction between SO4, PEG, and humic acid resulted in promoted CaSO4 scaling at the membrane surface. Findings of this work indicate that multiple RO feed water constituents should be considered when determining the efficacy of membrane surface modifications. In System (5), to simultaneously combat colloidal fouling from CaCO3 and CaSO4, organic fouling (e.g., humic acid) and biofouling (e.g., Escherichia coli), a multifunctional membrane was fabricated. Graphene oxide (GO) nanosheets, gold nanostars (AuNS), and PEG were combined on polyamide RO membrane surfaces and demonstrated to significantly reduce fouling from the three major fouling classes. Bacterial inactivation at the RO membrane surface was achieved by irradiating the membrane with an 808 nm laser activating the photothermal properties of the Au nanostars. Our newly developed novel, multifunctional membrane surface was therefore able to significantly reduce mineral scaling, organic fouling, and biofouling during RO without additional chemical or thermal treatments.
The findings from this systematic, mechanistic study investigating natural and engineered nanoparticle interactions in complex, dynamic systems can help improve the understanding of the fate, transport, and transformations of nanoparticles in water and wastewater treatment processes in response to increasing quantities and applications of nanoparticles in aquatic systems. Furthermore, through our unique engineered design, we have provided promising solutions for drastically improving water treatment processes in complex feed solutions.
Daniel Giammar, Benjamin Gilbery, John Fortner, Srikanth Singamaneni, Yinjie Tang
Available for download on Thursday, August 15, 2115