This item is under embargo and not available online per the author's request. For access information, please visit http://libanswers.wustl.edu/faq/5640.
Date of Award
Doctor of Philosophy (PhD)
To alleviate global water scarcity and improve public health, engineered water treatment and management systems have been developed for purifying contaminated water and desalinating brackish or ocean water. These engineered systems provide substantial amounts of potable water and lessen environmental concerns about the release of contaminated water. Wastewater treatment plants (WWTPs), water desalination plants (WDPs), and managed aquifer recharge systems (MARs) are three representative sustainable water management (SWM) systems. But the operation of all three poses two fundamental questions: (1) What is the fate of nanoscale solids (e.g., engineered nanomaterials, naturally occurring nanoparticles) in SWM systems and how will their physicochemical properties be changed when they encounter other water constituents, including cations and anions, reactive radical species, and organic matter? (2) How can our current knowledge enable more stable, scalable, and sustainable nanomaterial-based technologies for next-generation water treatment? To seek answers to these two questions, this dissertation focuses on the interface of chemistry and environmental engineering in 3 Systems: advanced oxidation processes (AOPs), managed aquifer recharge (MAR), and membrane distillation (MD), to (i) pursue in-depth and systematic investigations on solid-liquid interfacial interactions between nanoparticles and different water constituents (e.g., organic matter) in both water treatment and subsurface systems, and (ii) to utilize the knowledge obtained from fundamental mechanistic studies to develop nature-inspired nanomaterial-based membranes for sustainable water treatment.First, System 1 focused on investigating the surface chemistry of engineered nanomaterials (ENMs) in advanced oxidation processes (AOPs). The widespread industrial applications of ENMs, such as titanium oxide, cerium oxide, and graphene-based carbon materials, have increased the likelihood of their release into aquatic systems, including engineered water treatment systems, where they can undergo surface chemistry changes induced by water components. Using cerium oxide nanoparticles (CeO2 NPs) as representative ENMs, I examined on the effects of both reactive oxygen species (ROS) generated during UV/H2O2 treatment and dissolved organic matter (DOM) on the NPsՠcolloidal stability and surface chemistry. During UV/H2O2 treatment, superoxide radicals (O2_) dominated in neutralizing the surface charge of CeO2 NPs, leading to decreased electrostatic repulsive forces between nanoparticles and a higher extent of sedimentation. DOM was found to complex with the CeO2 NPsՠsurface and to act as a protective layer, making direct reactions between ROS and CeO2 and their impacts on colloidal stability insignificant in a short reaction period. These new findings have important implications for understanding the colloidal stability, sedimentation, and surface chemical properties of CeO2 NPs in aqueous systems where DOM and ROS are present.Second, System 2 aimed at investigating sustainable water management by managed aquifer recharge (MAR). To alleviate groundwater over-drafting, MAR has widely applied the engineered injection of secondary water sources into aquifers. However, groundwater chemistry changes induced by recharged water can significantly affect arsenic mobility in subsurface reservoir systems. Elevated arsenic mobility can result from increased oxidative dissolution of arsenic-bearing sulfide minerals, including arsenopyrite (FeAsS). In System 2, the effects of different water components, such as abundant oxyanions (i.e., phosphate, silicate, and bicarbonate) and DOM (natural and effluent organic matter), on the arsenic mobility from FeAsS were studied. Suwannee River DOM (SRDOM) was found to decrease arsenic mobility in the short term (< 6 hours) by inhibiting arsenopyrite oxidative dissolution, but it increased arsenic mobility over a longer experimental time (7 days) by inhibiting secondary iron(III) (hydr)oxide precipitation and decreasing arsenic adsorption onto iron(III) (hydr)oxide. In situ grazing incidence small-angle X-ray scattering (GISAXS) measurements suggested that SRDOM decreased iron(III) (hydr)oxide nucleus sizes and growth rates. A combined analysis of SRDOM and other proteinaceous or labile DOM (alginate, polyaspartate, and glutamate) revealed that DOM with higher molecular weights caused more increased arsenic mobility. In addition to DOM, phosphate showed a time-dependent reversed effect on arsenic mobility. In the short term (6 hours), phosphate promoted the dissolution of FeAsS through monodentate mononuclear surface complexation, while over a longer experimental time (7 days), the enhanced formation of secondary minerals, such as iron(III) (hydr)oxide (maghemite, _-Fe2O3) and iron(III) phosphate (phosphosiderite, FePO4塲H2O), helped to decrease arsenic mobility through re-adsorption. Over the entire 7-day reaction, silicate increased arsenic mobility, and bicarbonate decreased arsenic mobility in our batch experiments. The phosphate system showed the highest amount and largest sizes of secondary precipitates among the three oxyanions (phosphate, silicate, and bicarbonate). These new observations advance our understanding of the impacts of DOM and oxyanions in injected water on arsenic mobility and on secondary precipitate formation during the geochemical transformation of arsenic-containing sulfide minerals in MAR.In many natural and engineered aquatic systems, including MAR, acid mine drainage, and hydraulic fracturing systems, poorly crystalline iron(III) (hydr)oxide nanoparticles with sizes on the order of 1б0 nm form ubiquitously. In particular, newly formed iron(III) (hydr)oxide nanoparticles can precipitate heterogeneously on substrates, altering the substrateճ surface reactivity and serving as powerful sorbents for heavy metals (Cu, Zn, Pb, or Cd), anionic contaminants (As, Cr), and organic pollutants. Yet the thermodynamic and kinetic parameters, i.e., the effective interfacial (_') and apparent activation (Ea) energies of iron(III) (hydr)oxide nucleation on earth-abundant mineral surfaces, have not been determined, which hinders accurate prediction and control of iron(III) (hydr)oxide formation and its interactions with other water constituents. Using a flow-through, time-resolved, and in situ grazing incidence small-angle X-ray scattering (GISAXS) method, the work experimentally obtained the interfacial and activation energies of iron(III) (hydr)oxide heterogeneous nucleation on quartz. GISAXS measurements successfully enabled the detection of the nucleation rates of iron(III) (hydr)oxides under different supersaturations (_, by varying pH between 3.3_3.6) and temperatures (12 弃_35 弃). Quantifying these rates led to the quantification of _' and Ea, respectively, which were not previously available. The thermodynamic and kinetic parameters obtained benefit predictions using reactive transport models and controlling iron(III) (hydr)oxideճ formation, as well as understanding its effects on pollutantճ fate and transport in natural and engineered water systems.Third, System 3 was developed to apply mechanistic knowledge gained from studies of solidзater interfaces to the development of nature-inspired nanomaterial-based membranes for sustainable desalination. In remote or underdeveloped areas, it is challenging to produce clean water because centralized water treatment techniques require high energy input and management cost. To support resilient community development, water treatment techniques for these areas should be sustainable in terms of material design and energy consumption. To address these needs, a new water treatment system based on membrane distillation (MD) has been developed. In this novel MD system, called photothermal membrane distillation (PMD), the membrane is embedded with light-absorbing photothermal materials that harvest solar energy and generate localized heat at the water-membrane interface to drive the MD process. To develop several PMD membranes with high solar conversion efficiency, polydopamine (PDA), which possesses the advantages of easy synthesis, good biocompatibility, and excellent light-to-heat conversion, was used as the photothermal material. First, a simple, stable, and scalable PDA-coated polyvinylidene fluoride (PVDF) membrane was synthesized for PMD. In a direct contact membrane distillation (DCMD) system under 0.75 kW/m2 solar irradiation, the membrane showed a high solar energy conversion efficiency (45%) and a high water flux (0.49 kg/m2塨) This performance was facilitated by the PDA coating, whose broad light absorption and outstanding photothermal conversion properties enabled a higher transmembrane temperature difference and increased the driving force for vapor transport. In addition, the excellent hydrophobicity achieved by fluoro-silanization gave the membrane great wetting resistance and high salt rejection. More importantly, the robustness of the membrane, stemming from the excellent underwater adhesion of the PDA, made it an outstanding candidate for real-world applications. Further, to increase the solar energy conversion efficiency, bacterial nanocellulose (BNC) was utilized to replace commercial PVDF membranes to decrease heat conductive loss from the photothermal layer to the cold distillate. A new photothermal membrane was thermally-engineered to incorporate a bilayered structure composed of two environmentally sustainable materials, PDA particles and BNC. The size-optimized PDA particles on the top layer maximized sunlight absorption and sunlight-to-heat conversion, and the bottom BNC aerogel insulating layer achieved high vapor permeability and low conductive heat loss. This thermally engineered design enabled a permeate flux of 1.0 kg/m2塨 under 1 sun irradiation, and a record high solar energy-to-collected water efficiency of 68%, without ancillary heat or heat recovery systems. Moreover, the membrane showed effective bactericidal activity and was easily cleaned, increasing its lifespan. This study provides a new paradigm for using photothermal material incorporated in an aerogel to sustainably purify water. Using renewable solar energy, the PMD system can also provide decentralized desalination for remote or underdeveloped areas and can support resilient community development.In summary, the work described in this dissertation offers an in-depth and mechanistic understanding of the fate of nanoscale solids (e.g., engineered nanomaterials and naturally occurring nanoparticles) in SWM systems in the presence of different water constituents (e.g., anions, reactive radical species, and organic matter). It also provides insights for designing more stable, scalable, and sustainable nanomaterial-based membranes for water treatment and desalination. Ultimately, this research will better define the chemistry of nanoscale solids and organic matter in water management systems, benefiting the design of next-generation water treatment systems that are environmentally safer and more sustainable.
Zhen He, Srikanth Singamaneni, Yinjie Tang, Patricia Weisensee,
Available for download on Saturday, October 17, 2020