Modular Advanced Oxidation Processes Enabled by Cathodic Hydrogen Peroxide Production
- Author(s): Barazesh, James Michael
- Advisor(s): Sedlak, David L
- et al.
Point-of-entry and point-of-use drinking water treatment have emerged as an alternative means of providing potable water in locations where centralized water treatment is unable to provide safe water or where access to treated water is impractical. Although such systems could be useful in many applications, adoption of the technology requires cost-effective, reliable removal of pathogens, trace organic contaminants, and pollutant metals. Electrochemical water treatment technologies are advantageous for small-scale, distributed water treatment because they can be installed quickly without large capital investment and reactants can be generated on demand, solely though the input of electrical energy. However, their adoption has been hampered by a variety of issues including limited understanding of removal mechanisms, variable treatment efficiency, uncertainties about durability, and relatively high costs of operations associated with power consumption. For example, anodic treatment of water sources containing chloride and bromide has been shown to attenuate parent compounds but produce potentially toxic products under certain conditions, which could preclude consumption of treated water. Much of the prior work regarding electrochemical treatment has been purely empirical or has been conducted in simplified electrolytes that are not representative of conditions encountered in treatment systems. Therefore, this research attempts to bridge this gap by providing both a theoretical understanding and practical application of electrochemical treatment of natural waters. At the same time, we identify novel electrochemical remediation strategies that challenge the status quo and address the major limitations barring the uptake of anodic oxidation for the treatment of organic pollutants from waste streams.
To identify conditions which anodic oxidation would be appropriate for treatment of organic contaminants in various drinking water sources, electrolysis of a suite of trace organic contaminants including pharmaceuticals, pesticides, and personal care products was evaluated under representative conditions (Chapter 2). Key reaction pathways, including transformation on anode surfaces and reactions with homogenous species including hydroxyl radicals, carbonate radicals, chlorine, and bromine were assessed for different classes of contaminants on both titanium-iridium oxide (Ti-IrO2) and boron-doped diamond electrodes. Assessment of transformation rates indicate that many of the contaminants were transformed by halide species and that halogenated byproducts such as trihalomethanes and haloacetic acids were often produced in excess of drinking water quality standards prior to complete removal of the contaminant.
To provide a means of employing electrochemical treatment without the production of toxic disinfection byproducts, an alternative approach was developed by integrating cathodically-driven electrolysis with ultraviolet (UV) photolysis to produce a low-cost system capable of transforming trace organic contaminants (Chapter 3). Using a carbon-based gas-permeable cathode, hydrogen peroxide (H2O2) was produced from ambient air with high efficiency. H2O2-containing water was then exposed to UV irradiation followed by passage through an anode chamber. The performance of the system was evaluated using a suite of trace organic contaminants that spanned a range of reactivity with UV light and hydroxyl radical. Results indicate that organic contaminants could be removed in flow-through reactors without the formation of toxic byproducts at a lower cost and energy demand than observed in conventional anodic oxidation systems.
Finally, the cathode-driven electrolysis system was applied for the removal of toxic trace elements in drinking water sources. Initial experiments conducted in contaminated groundwater collected from Colusa, CA indicated that dissolved arsenic, lead, and copper were converted into colloid-associated forms that could be removed by filtration when water passed through the treatment system (Chapter 4). Results indicate that the pollutant elements were associated with iron-containing colloids which were formed when dissolved Fe(II) in the groundwater was exposed to O2 and H2O2. Experiments conducted with water containing solutes typically present in groundwater were used to gain insight into the mechanism through which toxic elements were removed. These experiments demonstrated that UV light caused changes in natural organic matter that altered its affinity for iron, and that loss of ligating ability was accelerated in the presence of Ca2+ and Mg2+.
Collectively, results of the experiments included in this dissertation indicate that the modular advanced oxidation system integrating electrolysis with UV photolysis holds promise as a point-of-use or point-of-entry treatment system. Under conditions expected in water treatment systems, the device is capable of simultaneously removing trace organic contaminants and pollutant metals. Additional research is needed to further develop and deploy the system to assess its performance under field conditions.