There is an increasing demand for the provision of cleaner safer water. In the last 5 years, the global water supply industry has spent > £57 billion on purification treatments. With an increasing population and energy costs, investment is predicted to increase over the next 10 years. Moreover, the industry is attempting to move towards more efficient and sustainable processes for the treatment of a wide range of contaminants. This project focuses on two novel sustainable technologies for remediation of common waste water contaminants: photoelectrocatalysis (pathogens and 2,4-DCP) and biosorption (heavy metals - HMs). The application of semiconductor photocatalysis in waste water treatment has been intensively investigated over the past decade. These studies mainly involve nano dimensional titanium dioxide as a photocatalyst using ultra-violet light as an energy source. However, practical applications are still limited by its poor visible light activity. In this study a photoelectrocatalytic batch cell (PECB) and photoelectrocatalytic fuel cell with a flow through configuration (PECFC) containing a visible light active tungsten trioxide (W03) photocatalyst have been optimised and assessed for contaminant remediation. The potential for the PECB to disinfect a surrogate human pathogen, the lux-marked E. coli HE 101 pUCD607, is investigated in Chapter 3. Disinfection experiments indicated that a > 99 % decrease in CFU/rnl occurred within 15 min. Although, this experiment showed that bacterial disinfection can be achieved by light alone (photolysis), the results indicated that disinfection rates were enhanced considerably by using the immobilised thin film W03 photoelectrocatalyst. This alternative catalyst was further assessed in a flow through PECFC system. The combination of the visible light enhanced W03 and the proton exchange membrane fuel cell (PEMFC) technology to remediation of 2,4-DCP in waste waters is investigated in Chapter 4. Degradation of 2,4-DCP was monitored over a period of 24 hrs. A total decrease of 74 % in 2,4-DCP concentration was observed, from which ea. 54 % were accountable to photoelectrocatalytic degradation processes and 20 % due to losses by adsorption or volatilisation. This decreased further to > 98 % removal over 6 days. A combination of chemical (HPLC) and bacterial biosensor (lux-marked Escherichia coli HB101 pUCD607) toxicity responses confirmed degradation of the parent compound with a concomitant increase in toxicity due to formation of intermediates, respectively. The reduction in 2,4-DCP concentration was observed to follow first order kinetics assuming a perfect flow model for the PECFC. However, more work is required to improve sustain ability of this technique as reduced efficiency of the PECFC occurred with prolonged use of the MEA (potentially due to occlusion of the catalytic sites), leading to loss of membrane conductivity. A major constraint with PECFC is the presence of eo-contaminants such as HMs that limit the efficiency of the MEA. Therefore, Chapter 5 assesses the efficacy and mechanisms for a sustainable biosorbent (distillery spent grain - DSG) to remove HMs from contaminated waters. A batch system was employed to determine the sorption of five different HMs from aqueous solution to DSG. Adsorption occurred up to a saturation point of 11.8, 14.1, 11.2, 38.1 and 14.6 mg of Co, Cu, Ni, Pb and Zn / g DSG, respectively. Adsorption for all HMs conformed to the Freundlich isotherm model, indicating heterogeneity of the DSG surface. The sorption of HM followed the pseudo- second-order kinetic model, indicating that the rate-controlling step in the process was chemical interaction between the HM ions and the functional groups on the DSG surface. An increased sorption efficiency of the DSG occurred with increased storage time as decomposition of the organic matrix resulted in increased number of active sorption sites. However, deterioration in the aesthetic quality of the DSG meant that a balance was required between optimum performance and ease of handling in the application of this material; an optimum storage period of 3 months has been proposed. The batch equilibrium sorption experiments estimated sorption under optimal conditions where there was no limiting rate of interaction between HM and DSG active sites. A leaching set up more reminiscent of a 'real life' in-stream remediation scenario is assessed in Chapter 6. Successful sorption of all five HMs was observed but this was significantly reduced compared to batch equlibia. Moreover, an assessment of the effect of competing ions (NaCl) on HM sorption efficiency of the DSG indicated that increasing the ionic strength of the HM solution generally resulted in a decrease in HM sorption capacity of DSG at lower initial HM concentrations but the opposite effect was observed at the highest initial HM concentration. Sequential extractions, carried out on the BM-laden DSG after leaching experiments indicated that all five HMs studied were strongly bound within the organic matrix of the DSG as < 10 % of the sorbed HMs were loosely bound on labile or exchangeable sites. A preliminary investigation of DSG as a potential sorbent for 2,4-DCP is described in Chapter 7. For two concentrations (16.3 and 40.75 mg/l) , 66.0-68.9- % and 39.6-44.3 % of the 2,4-DCP was removed in batch and leaching experimental set-ups, respectively. The W03 photoelectrocatalytic fuel cells (batch PECB and continuous flow PECFC) and waste-derived biosorbent investigated during the course of this study are both promising emerging technologies for sustainable waste water treatment technologies. Moreover, there is potential for both technologies to act as complementary systems in a treatment train with the DSG deployed upstream of the PECFC (Chapter 8). This DSG- PECFC arrangement could potentially improve the efficiency of the PECFC to degrade organic contaminants, as the DSG will sorb both HM and organic pollutants, thereby reducing the contaminant concentration load stream entering the PECFC. This proposed set-up could in principle be adapted for application in-line of existing waste water treatment systems.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:553824 |
| Date | January 2011 |
| Creators | Scott-Emuakpor, Efetobor |
| Publisher | University of Aberdeen |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://digitool.abdn.ac.uk:80/webclient/DeliveryManager?pid=182250 |
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