Surface Modification of Electrode Surfaces for Water Treatment and Sensing Applications

This dissertation summarizes different modifications of porous and non-porous electrode surfaces for electrochemical water and wastewater treatment and sensing applications. Specifically, the research is focused on the preparation of porous substoichiometric TiO2 electrodes (TinO2n-1, n = 4 - 10), modification of TinO2n-1 and non-porous boron-doped diamond (BDD) electrodes, electrode characterization using analytical and electrochemical techniques, and their use for trace contaminant detection, oxidation and reduction of organic and inorganic contaminants from water matrices. The major objectives of the research are: 1) to study the electrochemical detection of antibiotics in wastewater matrices with high sensitivity and selectivity using modified electrodes, 2) to investigate the effects of different surface fluorination methods on perchlorate (carcinogenic) formation inhibition and organic contaminant oxidation during electrochemical water treatment, 3) to investigate the use of bimetallic catalysts for electrocatalytic nitrate reduction in water matrices, and 4) to determine the effect of doped metal oxide (bismuth doped tin oxide) catalysts for the enhancement of hydroxyl radical production to enhance the electrochemical oxidation of organic contaminants. This dissertation reports on the successful fulfilment of the research objectives as follows: The overuse and presence of ciprofloxacin (CFX), a second generation fluoroquinolone, an antibiotic for treating diseases associated with gram-positive and gram-negative bacteria and can cause the emergence of potential drug resistant bacteria. Electrochemical techniques are the most extensively used methods compared to other analytical techniques as they are cost effective, energetically viable, robust, highly sensitive, quick and easy to use. They have been mainly used for CFX detection in urine sample, milk, serum and tablets. However, CFX detection in natural or wastewater matrices has not been investigated. In this work an electrochemical sensor was fabricated by modifying the boron-doped diamond (BDD) electrode surface with a porous nafion multi-walled carbon nanotube composite film. X-ray photo electron spectroscopy (XPS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were utilized to characterize the sensor. The sensors were able to detect CFX using differential pulse voltammetry (DPV) in the presence of other antibiotics (i.e., amoxicillin), other non-target water components and several commonly present organic compounds. The sensor was capable of CFX detection in the presence of WWE matrix with excellent sensitivity and selectivity. Different concentration of different organic compounds can foul the sensor but the fouling was easily removed using short cathodic current confirming its effectiveness for contaminant detection in diverse water matrices. Electrochemical advanced oxidation process (EAOPs) has been widely used for organic compound oxidation in various water matrices. However, the use of EAOPs are limited by perchlorate formation, which is a carcinogenic byproduct formed via extended electrolysis of chloride ions present in solution. To address this problem, fluorination was performed on BDD electrodes using radio frequency (RF) plasma in the presence of H2 and CF4, perfluorooctanoic acid (PFOA) electrochemical oxidation, and silanization using aliphatic and aromatic silane. XPS was performed to characterize the effectiveness of the fluorination methods. Chronoamperometry experiments were used to monitor perchlorate production and organic compound oxidation as a function of electrode coating. The most effective method to prevent perchlorate production was modification of the BDD electrode with an aliphatic silane (1H, 1H, 2H, 2H perfluorodecyltrichlorosilane) self-assembled monolayer (SAM). The SAM was able to completely inhibited perchlorate formation with only minor effects on organic compound oxidation. The surface coverage, thickness the blocking layer and steric effect hindered the electron transfer rate and the result was supported by XPS and density function theory (DFT) simulations determined film length. Nitrate (NO3 -) is one of the most common pollutants in the natural environment. Therefore the US Environmental Protection Agency (EPA) has set maximum contaminant levels (MCLs) for NO3 - and NO2 - of 700 and 70 M, respectively. Destructive methods for NO3 - removal have been extensively researched over the years. Catalytic and electrocatalytic nitrate reduction using bimetallic catalysts have shown promise, but these methods are limited by storage and delivery of an external electron donor (e.g., H2) and formation of high concentrations of undesired NO2 - and NH3, respectively. In this study two different bimetallic catalysts (e.g. Pd- Cu and Pd-In) were used to modify reactive electrochemical membranes (REMs) using the incipient wetness method and the modified REMs were characterized by X-ray diffraction (XRD), SEM/EDS, and ICP-OES. Electrocatalytic nitrate reduction was performed with respect to electrode placement, flow rate, electrode potential, NO3 - concentration, and under various solution conditions using chronoamperometry in flowthrough mode. This study showed that NO3 - was reduced from a 1 mM feed concentration to below the EPA’s regulatory MCL (700 M) in a single pass through the REM (residence time ~ 2 s), with low product selectivity (<2 %) of NO2 -/NH3, high current efficiency (105 ± 5.4%), and low energy consumption (1.1 to 1.3 kWh mol-1). Nitrate reduction was not affected by dissolved oxygen and carbonate species and only slightly decreased in a surface water sample due to Ca2+ and Mg2+ scaling. Research has shown that REMs composed of TinO2n-1 are very effective for EAOPs. However, at high membrane fluxes the removal of contaminants can become kinetically limited. This work showed that the deposition of bismuth doped tin oxide (BDTO) catalysts to REMs could enhance the hydroxyl radical and thus increase electrochemical oxidation rates. The catalysts were deposited by pulsed electrodeposition and pulsed laser deposition techniques and characterized using XPS, XRD, SEM/EDS, TEM, linear scan voltammetry (LSV), and ICP-OES. Terephthalic acid (TA) was used as a hydroxyl radical probe, whereas atrazine (ATZ) and clothianidin (CDN) were chosen as potential harmful herbicide and pesticide, respectively. TA, ATZ, and CDN oxidation rate constants increased upon BDTO deposition. At the highest applied potential (3.5 V/SHE), TA, ATZ and CDN concentrations were below the detection limit in the permeate for REM/BDTO electrodes. Chemical oxygen demand (COD) analysis of TA and total N analysis of ATZ and CDN showed complete mineralization of the compounds at the highest applied potential. DFT simulations provided potential dependent activation energies for ATZ and CDN, which were supported by experimental data. The dissertation showed modification of porous and non-porous electrodes for excellent sensitivity and selectivity for antibiotics detection in water matrices, minimization of perchlorate formation, nitrate reduction and subsequent removal from water matrices and excellent organics oxidation and mineralization. The results of the dissertation have opened several new research directions. The modification of BDD electrodes can be performed using selectively edge functionalized (-COOH) vertically aligned carbon nanotubes and porous nafion composite film which may increase the selectivity and sensitivity. Due to the toxicity of fluorocarbons, electrodes can be modified using metal oxide materials (Al2O3, ZrO2, TiO2) for perchlorate formation inhibition with significant organics oxidation by creating a blocking layer. A series of bimetallic catalyst deposited REMs can be used for highly concentrated nitrate reduction and removal in reverse osmosis and ion exchange brine solution. Bimetallic catalyst deposited REM and bismuth doped tin oxide catalyst deposited REM can be used as cathode and anode, respectively for nitrate reduction and organics oxidation simultaneously in a single setup.