The investigation of microbial denitrification processes for the removal of nitrate from water using bio-electrochemical methods and carbon nano-materials

摘要

With ever increasing regulation of the quality of drinking water and wastewater treatment, there is a need to develop methods to remove nitrogenous compounds from water. These processes are mediated by a variety of micro-organisms that can oxidise ammonia to nitrate, and then reduced to gaseous nitrogen by another set of organisms. This two stage process involves the relatively slow oxidation of ammonia to nitrate followed a relatively fast reduction of nitrate to nitrogen. Nitrate reduction normally requires anaerobic environments and the addition of organic matter to provide reducing power (electrons) for nitrate reduction. In practical situations the nitrate reduction can be problematic in those precise quantities of organic matter to ensure that the process occurs while not leaving residual organic matter. The aim of this study was to investigate microbial denitrification using electrochemical sources to replace organic matter as a redactant. The work also involved developing a system that could be optimised for nitrate removal in applied situations such as water processing in fish farming or drinking water, where high nitrate levels represent a potential health problem. Consequently, the study examined a range of developments for the removal of nitrate from water based on the development of electrochemical biotransformation systems for nitrate removal. This also offers considerable scope for the potential application of these systems in broader bio-nanotechnology based processes (particularly in bioremediation). The first stage of the study was to investigate the complex interactions between medium parameters and their effects on the bacterial growth rates. The results proved that acetate is a good carbon source for bacterial growth, and therefore it was used as an organic substrate for the biological process. High nitrate removal rate of almost 87% was successfully achieved by using a microbial fuel cell (MFC) enriched with soil inocula with the cathodes cells fed with nitrate and the anode fed with acetate. The maximum power density obtained was 1.26 mW/m2 at a current density of 10.23 mA/m2. The effects of acetate, nitrate and external resistance on current generation and denitrification activity were investigated, and the results demonstrated that nitrate removal was greatly dependent on the magnitude of current production within the MFC. Increase of acetate (anode) and nitrate (cathode) concentrations improved the process, while increasing external resistance reduced the activity. Furthermore, for a clear understanding of the nitrate reduction process, the analysis of the associated bacteria was performed through biochemical tests and examination of morphological characteristics. A diversity of nitrate reducing bacteria was observed; however a few were able to deliver complete denitrification. Pure cultures in MFC were examined and the voltage output achieved was about 36% of that obtained by mixed cultures. The nitrate removal gained was 56.2%, and this is almost 31% lower than that obtained by the mixed bacterial experiment. In an attempt to improve the MFC, modifications to the electrochemical properties of the electrode were investigated through the use of a cyclic voltammetry using carbon nanomaterials to coat the graphite felts electrodes. Among all the nanomaterials used in this study, graphitised carbon nanofibres (GCNFs) was selected for further investigation as it offered the best electrochemical performance and was thought to provide the largest active surface area. The performance of the MFC system coupled with the GCNFs modified electrodes was evaluated and significant improvements were observed. The highest voltage output achieved was about 41 mV with over 95% nitrate removal. The work is discussed in the context of improved MFC performance, potential analytic applications and further innovations using a bio-nanotechnology approach to analyse cell-electrode interactions.