Electrochemical oxidation of pharmaceuticals in fresh urine

Abstract

Electrochemical oxidation of fresh human urine is a promising method to prevent pharmaceuticals from being discharged into the environment. Urine comprises only a small (~1%) volumetric fraction of municipal wastewater, but represents a dominant source of pharmaceuticals, many of which may pass through conventional wastewater treatment and pose risks to aquatic ecosystems. Point-source treatment of source-separated urine presents a unique opportunity to degrade pharmaceuticals before dilution with wastewater, and electrochemical advanced oxidation processes are one increasingly investigated option. However, they often lead to the formation of oxidation byproducts including chlorate, perchlorate, haloacetic acids, trihalomethane, etc. The work presented in this dissertation is broadly focused on developing safe and effective electrochemical methods for the treatment in fresh human urine to mitigate the pharmaceutical contamination in the environment. First, I demonstrate that the electrogenerated hydroxyl radicals (•OH) enables the selective oxidation of pharmaceutical during the electrolysis of fresh human urine, while •OH is quenched by HCO3− in hydrolyzed urine leading to a slower pharmaceutical degradation rate during the electrolysis of hydrolyzed urine. Additionally, I show that urea at the concentration present in fresh urine suppresses the oxidation pathways of Cl− to ClO3− and ClO4−. Taken all together, it is advantageous to employ the electrochemical advanced oxidation processes in fresh human urine rather than the hydrolyzed urine. Next, I focus on understanding the underlying chemistry and fundamental kinetics of pharmaceutical oxidation during electrolysis of fresh human urine. I use the selective radical quenchers to study the significance of individual oxidants for the oxidation of pharmaceutical in electrolyte only containing chloride ions. I show that electro-generated dissolved Cl2 adjacent to the anode surface could contribute the pharmaceutical degradation which has been overlooked by the scientific community. Furthermore, I demonstrate that organic urine constituents (urea, creatinine, and uric acid) quench the dissolved oxidants leading to a mass-transfer limited reaction, while the inorganic urine constituents could quench chemisorbed oxidants resulting in a mixed kinetically and mass-transfer controlled reaction. Moreover, increasing the mass transfer rates and anode potential is essential for achieving faster pharmaceutical oxidation. Finally, I design and develop an alternating scheme of oxidation and reduction using interdigitated working electrodes (IWEs) for the oxidation of pharmaceuticals while minimizing the formation of chlorinated byproducts. The alternating scheme is achieved by using (1) a convention method where a bipotentiostat applies a constant anodic potential to one side of the IWE and a cathodic potential to the other side of the IWE while a stir bar directly on top of the planar IWEs moves fluid across the IWE (with separate counter and reference electrodes in the cell) and (2) an alternating current method where at any given time one side of the IWE is the working electrode and the other side is the counter electrode but over time the WE and CE are alternated by the potentiostat at a rate of up to 100 Hz. I demonstrate that the generation of chlorate, chloroacetic acids and chloroform is significantly suppressed for both methods above. However, the rates of pharmaceutical degradation are substantially enhanced for the alternating scheme achieved by convection method, while their rates decrease largely using alternating current method. Collectively, the work included in this dissertation demonstrates considerable progress toward understanding the kinetics of electrochemical oxidation of pharmaceuticals in human urine and developing the safe and effective processes that could be implemented at the source of generation, with focus on the rapid pharmaceutical degradation and minimum generation of toxic byproducts.