UC Davis

Water is an essential resource for all living organisms, yet it is contaminated with a wide variety of toxins that are harmful to human beings and the marine ecosystem. The two main contaminants discussed in this dissertation will be crude oil and per- and polyfluoroalkyl substances (PFAS). Crude oil enters the marine ecosystem when there is a crude oil spill on the surface of the ocean. The application of dispersant molecules causes the oil to be broken into smaller droplets which travel deeper into the ocean leading to further contamination. While crude oil affects large bodies of water, PFAS enter our drinking water streams by industrial waste, groundwater contamination, and leachate. PFAS are fluorinated carbon chains with a terminal functional group ranging from carboxylic acids, sulfuric acids, and alcohols. Unlike crude oil these molecules do not absorb visible light, and can’t be seen in drinking water but are still very toxic. There is a need for developing robust, mechanically and chemically stable adsorbent materials to treat these contaminants in our water sources. This dissertation discusses the sol-gel synthesis of an understudied ceramic mesoporous hafnium oxide (MHO) for the adsorption of crude oil as well as PFAS. The MHO is chemically stable due to the variety of surface functional groups, thermally stable due to the fact that it is synthesized at high temperature (700°C), and has a bimodal pore network with both macropores (>50 nm), and mesopores (2-50 nm) that allow for rapid water flux and entrapment of contaminants respectively. Chapter 1 aims to provide an introduction to challenges faced by our marine environments today by discussing effects of crude oil spills on the marine ecosystem, and impacts of per-and polyfluoroalkyl substances (PFAS) on our environment. The chapter reviews current technologies used to address crude oil spills and PFAS contamination. Finally it addresses some knowledge gaps in current adsorbent materials for water remediation, as well as describing favorable materials that can begin addressing these gaps. Foundational information on sol-gel synthesis of mesoporous hafnium oxide (MHO) ceramics is discussed in detail. Chapter 2 describes the ability of monolithic mesoporous hafnium oxide ceramics as an adsorbent to sequester crude oil from crude oil-water emulsions. The MHO ceramic monolith is incorporated in a vacuum filtration setup and evaluated in comparison to other commercially available crude oil adsorbents. This chapter shows that MHO can remove 99.9% crude oil from crude oil-water emulsions at acidic, neutral, and alkaline pH. The MHO ceramic monolith could be regenerated by calcination which furthers the sustainability of these materials. Chapter 3 begins to describe PFAS remediation through biodegradation and composting of PFAS containing food service products. This work highlights results from both bench-scale and commercial-scale composting of FSP as a function of moisture content and inoculum. A moisture content of 60% was ideal for composting, whereas there was no difference in degradation with different inoculums. Unfortunately, PFAS containing FSP degraded at a fraction of the rate of non-PFAS containing FSP. Due to high moisture content during composting, PFAS from FSP dissolves in the water and can leach into groundwater sources. MHO monoliths were used to filter PFAS from water and we observed that longer chain hydrophobic PFAS had higher removal efficiencies than shorter chain PFAS. Chapter 4 discusses the adsorption of the most common PFAS molecule found in water, perfluorooctanoic acid (PFOA) on to powders of MHO using pH-modulated Bronsted acid and base sites on the surface. The kinetics and adsorption behavior of PFOA on the surface of MHO as a function of charged species on the surface are discussed. The adsorption followed a two-step model, where the first step was rapid, and the second step was much slower. The ideal pH for adsorption of PFOA by MHO is an acidic pH of 2.3 as the surface is positively charged at this pH, and the PFOA molecule exists as an anion. Finally the regeneration of spent MHO powders via calcination is discussed. Chapter 5 presents future work for PFOA treatment using electrochemical oxidation. Preliminary results using platinum mesh, nickel foil, and glassy carbon as working electrodes is presented. These electrodes showed PFOA destruction to carbon dioxide gas and unidentified fluorinated species. The degradation rates of PFOA drastically increased in more alkaline pH due to increased hydroxide radicals concentration, that are necessary for PFOA oxidation.