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Water Treatment Using Graphite Adsorbents With Electrochemical Regeneration
[Thesis]. Manchester, UK: The University of Manchester; 2012.
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Abstract
Increased public awareness, stricter legislation standards, and environmental and health effects associated with water pollution are driving the development of improved wastewater treatment techniques. In order to meet these challenges, a novel and cost effective process has been developed at the University of Manchester to treat water contaminated with dissolved organics by exploiting a combination of adsorption and electrochemical regeneration. Adsorption of organics takes place on the surface of a non-porous and highly electrically conductive graphite adsorbent, followed by anodic electrochemical regeneration leading to oxidation of the adsorbed organic contaminants. The mechanism of degradation of adsorbed organics during electrochemical regeneration is particularly important from the point of view of the breakdown products. Ideally, complete oxidation of the adsorbed organics to CO2 and H2O should occur, but it is also possible that intermediate by-products may be formed. These breakdown products could be released into the water, be released as gases during the regeneration process or may remain adsorbed on the surface of the adsorbent. Information about the breakdown products is an important requirement for the commercial application of the process. This PhD project focused on an investigation of the formation of intermediate oxidation products released into the water (liquid phase) and with the regeneration gases. Phenol was chosen as a model pollutant and a graphite intercalation compound (GIC) adsorbent, Nyex®1000 (Arvia® Technology Ltd) was used. The main oxidation products formed during both batch and continuous adsorption with electrochemical regeneration were 1,4-benzoquinone, maleic acid, oxalic acid, 4-chlorophenol and 2,4-dichlorphenol. These compounds were detected in small concentrations compared to the overall concentration of the phenol removed. Two mechanisms of organic oxidation during electrochemical regeneration of the GIC adsorbents were identified. The first was the complete oxidation of the adsorbed species on the surface of the adsorbent and the second involved the indirect electrochemical oxidation of organics in solution. Breakdown products were found to be formed due the indirect oxidation of organics in solution. The formation of (chlorinated and non-chlorinated) breakdown products was found to be dependant on current density, pH, initial concentration, chloride content and the electrolyte used in the cathode compartment. The concentrations of chlorinated breakdown products can be minimized by using low current density, low initial concentrations, a chloride-free environment and/or treating the water over a number of adsorptions and regeneration cycles. On the other hand, non-chlorinated breakdown products can be minimized by applying higher current density and treating the solution over several cycles of adsorption and regeneration. Therefore, selection of optimum conditions is important to reduce the formation of undesirable breakdown products. The formation of free chlorine during batch electrochemical regeneration was also investigated under a range of operating conditions including the initial concentration of chloride ions, current density and pH. The outcomes of this study have important implications in optimising the conditions for the formation of chlorinated breakdown products and in exploring the role of electrochlorination for water disinfection.Analysis of the regeneration gases has revealed that the main components of the gases generated during the electrochemical regeneration of GIC adsorbents were CO2 and H2O. A preliminary mass balance has suggested that about 60% of the adsorbed phenol was oxidised completely to CO2. However, further work is needed to determine the fate of the remaining phenol.The surface characterization of the GIC adsorbent during adsorption and electrochemical regeneration was carried out using surface techniques including Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, Energy dispersive X-ray spectroscopy (EDS) and Boehm titration. FTIR and Raman spectroscopy were found to be unsuitable for determining the concentration changes at the surface of the adsorbent during adsorption and regeneration. However, Boehm titration has shown that the GIC adsorbent has phenolic, carboxylic and lactonic groups. The concentrations of phenolic groups were found to be higher after phenol adsorption and to decrease during electrochemical regeneration. The results of EDS analysis gave results which were consistent with these observations.Another important aspect of this PhD project was to explore the potential application of adsorption and electrochemical regeneration using GIC adsorbents to water disinfection. A model microorganism E. coli was selected for adsorption and electrochemical regeneration studies under a range of experimental conditions. This study has provided evidence that the process of adsorption and electrochemical regeneration using GIC adsorbents can be used for disinfection of water. Disinfection of water was found to be a combination of two processes: the adsorption of microorganisms followed by their deactivation on the surface; and electrochemical disinfection in solution due to indirect oxidation. The possible disinfection mechanisms involved in these processes include electrochlorination, pH changes and deactivation by direct oxidation of microorganisms. Scanning electron microscopy was found to be a useful tool for investigating changes in surface morphology of microorganisms during adsorption and electrochemical regeneration. The disinfection of a variety of bacteria, fungi and yeasts was tested and evaluated. However, disinfection of protozoa including C. parvum was not demonstrated successfully. It was also demonstrated that the process of adsorption with electrochemical regeneration using GIC adsorbents can be used to simultaneously remove organics and to disinfect microorganisms.