Understanding the Interaction of Bacteria, Algae, and Biomolecules at Electrode Surfaces

This dissertation focused on understanding the mechanisms of biofouling control, chlorinated byproduct formation and algae photosynthetic activity at electrode surfaces as a function of electrode potential and solution conditions. Specific goals of this work were to 1) elucidate mechanisms of microorganism poration and attachment on conductive boron-doped diamond (BDD) and modified BDD surfaces, 2) develop micro-scale analytical techniques to study near surface chemistry and their effect on microorganism poration/attachment, 3) study organic and inorganic chlorinated byproduct formation during the electrochemical advanced oxidation process at Magnéli phase Ti4O7 electrodes, and 4) investigate the effect of anodic potentials on microalgae photosynthetic activity at modified BDD surfaces. These goals were accomplished through a combination of carefully designed experimental analysis and theoretical techniques. For microorganism related studies, a IrOx ultramicroelectrode (UME) was fabricated and scanning electrochemical microscopy (SECM) was used to study the real-time near surface solution pH and reactive oxygen species (ROS) detection and use this information to interpret electrode-mediated bacteria inactivation. X-ray photo electron spectroscopy (XPS) was used to characterize surface modifications on the BDD electrodes. A mathematical reactive transport model was developed to interpret the SECM data for a better understanding of the near-electrode chemistry. Hydrogen peroxide (H2O2) was the primary oxidant formed under cathodic conditions, and a combination of H2O2, Cl•, HO2• Cl2•−, and Cl2 formation likely contributed to bacteria poration at potentials as low as 0.5 V vs Ag/AgCl. Compared to current proposed mechanisms of biofilm control with electrochemical methods, this study focused on low applied potentials (e.g., −0.2 to 1 V versus Ag/AgCl) on the electrode surface and real time near surface measurements, which provided insights on the physicochemical properties of the initial stages of biofouling control. Chelation-induced biofilm control has also been studied in a solution phase for destabilization bacteria and reducing their viability. Chelators show a significant impact for antibacterial purposes by extracting divalent ions (Mg2+ and Ca2+) from the lipopolysaccharides (LPS) of the cell membrane. Based on the first study, the BDD surface was modified with an N-propyl-2-hydroxyacetamide group, which provided a surface with chelation ability for biofilm control. In this study, Pseudomonas aeruginosa (PAO1) was used as a model pathogenic organism and low potentials (e.g., −0.2 to 1 V versus Ag/AgCl) were applied on the electrodes. Results suggested that divalent ions from the outer membrane of PAO1 were chelated by N-propyl-2-hydroxyacetamide functional groups that were immobilized on a BDD optically transparent electrode (termed OH-BDD/OTE). Two- to three-fold higher percentage of porated bacteria were observed on the OH-BDD/OTE compared with BDD/OTE under applied anodic potentials between 0.1 to 0.5 V vs. Ag/AgCl. Zeta potential measurements of the PAO1 bacteria as a function of chelators and Mg2+ concentrations were performed to support the chelation hypothesis. A mathematical model was built on the nonlinear Poisson–Boltzmann equation to interpret experimental data. Natural organic matter (NOM) is also commonly found in surface and ground waters, as a result of a complex matrix of organic substances combining different hydrological, biological and geological interactions. Phenolic compounds play a central role in the structure of NOM and many industrial contaminants present in natural waters. NOM can react with chlorine and form halogenated byproducts which can contaminate drinking water and increase documented health risks, such as bladder cancer and birth defects. Electrochemical advanced oxidation processes (EAOPs) have recently been studied as possible new modular technologies for water treatment. In our study, resorcinol was used as a model organic compound, and it was oxidized in the presence of varying concentrations (1 and 5 mM) of sodium chloride (NaCl) and as a function of electrode potential. A Magnéli phase Ti4O7 electrode was used as a reactive electrochemical membrane (REM) and characterized by X-ray diffraction (XRD). Electro-oxidation was performed using a flow-through electrochemical cell with the presence of NaCl and resorcinol under a constant flow rate and different electrode potentials. High-performance liquid chromatography (HPLC), ionic chromatography (IC) and liquid chromatography–mass spectrometry (LC-MS) were used to determine the possible inorganic, organic and chlorinated byproducts. Furthermore, the chlorinated byproducts were proposed based on the chromatography results and density functional theory (DFT) simulations. For the fourth study, microalgae photosynthetic response under applied anodic potentials were investigated. Microalgae are a microorganism which can be harvested as a promising source for green energy but harmful for water systems. Electrochemical methods have been studied for algae inhibition under anodic current densities (e.g. 10 mA/cm2) for water purification purposes. Moreover, anodic oxidation process has been studied for lipid extraction for biofuel production. Applied anodic potentials might have an impact on microalgae photosynthetic ability and viability during the harvesting process. Hence, monitoring microalgae photosynthetic activity provides the insight for a comprehensive understanding of the correlation between electrode potentials and algae viability. The modified BDD used in the first study (APTES-BDD/OTE) was used as a conductive electrode for algae attachment. SECM was used for surface mapping and measuring algae photosynthetic activity using soluble redox couples as electron shuttles. Photosynthetic activity was measured as a function of the anodic potentials on the electrode. In summary, work from this dissertation advanced knowledge in the area of biological/electrode interactions with respect to water treatment applications. For biofilm control at the electrode surfaces, results indicated that applied potentials can caused H2O2 and Cl-based oxidants formation at the electrode surface which contributed to bacterial poration. Moreover, electrode modification may be a viable method to prevent biofouling of electrode surfaces that are operated at low applied potentials. The formation of Cl-based oxidants and OH• at the electrode surface can react with organic contaminants and form halogenated byproduct during EAOPs. Results indicated that chlorinated byproducts should be carefully monitored during EAOPs and multi-barrier treatment approaches are likely necessary to prevent halogenated byproducts in the treated water. In addition, photosynthetic activity measurement gives insights for monitoring algae harvesting, biomass separation and cell treatment processes for biofuel production by reactive electrochemical membrane.