| Extracellular respiratory bacteria (ERB), which are capable of exocellular transfer of electrons along respiratory chain to reduce extracellular electron acceptors, have an environmental ubiquity and important applications in the fields of biogeochenmistry, environment, microbiology and bioenergy. Inorganic functional nanomaterials, especially the electrochrome nanomaterials and photocatalysts, have attracted a wide interest because of their special microstructure and responses to the external stimuli (e.g., light, electricity). Main contents and results are as follows:1. By using an electrochromic material, WO3 nanoclusters, in a designed 96-well plate, a novel method to rapidly detect ERB and quantitatively evaluate their extracellular electricity transfer ability was developed based on the direct electron transfer process from cytochrome of ERB to WO3 nanoclusters. The extracellular electricity transfer of ERB has been quantitatively assessed according to the intensity of coloration within 5 minutes and further validated using the conventional microbial fuel cell tests. To evaluate this photometric method, Shewanella oneiensis MR-1, a typical ERB, and its ten mutants were tested. Also, the well-known Geobacter sulfurreducens and Pelobacter carbinolicus were used to validate this method. The mechanisms for electron transfer from ERBs to WO3 nanoprobe were elucidated. Results show that the color developments of various mutants were well consistent with their electrochemical activities. ERB can also be isolated using this method because of its high sensitivity and effectiveness. Three strains of ERB were isolated from mixed samples with this method and their electrochemical activities were evidenced by MFC tests. Among them, Kluyvera cryocrescens with the highest current density in this study has not been recognized to have the ability of electricity generation in literature.2. Microbial fuel cell is a bioelectrochemical system that utilizes electrons transferred outside ERB to produce electricity from organic substrates. In an MFC the open circuit voltage could reach only 0.80 V, and such a low-voltage electricity could be used to supply an extra electricity to a photoelectrocatalytic (PEC) system and to achieve a higher photocatalytic efficiency. A bioelectrochemical system for a utilization of MFC electricity and an effective degradation of organic pollutants was constructed with p-nitrophenol as a target pollutant. This system is composed of a coupled TiO2-mediated photoelectrocatalytic oxidation reactor and an air-cathode MFC. The MFC is used to provide an external anodic bias to the PEC reactor for the degradation of p-nitrophenol, a typical refractory, hazardous and priority toxic compound. Kinetic analysis shows that the MFC-assisted photoelectrocatalytic (MPEC) system exhibits more rapid p-nitrophenol degradation at a rate two times of the sum rate by the individual photocatalytic and electrochemical methods. The results demonstrate that the power generated from an MFC could be utilized to supply the extra bias potential to reduce the recombination between the photogenerated electrons and holes, and thus to enhance the photocatalytic efficiency. The electrons transferred from the ERB and energy captured by ERB from organic wastes were utilized to enhance the degradation of biorefractory compounds in the PEC reactor.3. Nitrogen photofixation process for nitrate formation from N2 and O2 in air with photocatalysts under ambient conditions has never been reported. Both indoor and outdoor experiments demonstrate that nitrate could be readily formed from abundant N2 and O2 in air on nano-sized titanium dioxide (TiO2) surface under UV or sunshine irradiation. Such a ubiquitous and important nitrogen photofixation process could occur universally. The nitrogen photofixation rate constant initially increased with the increasing relative humidity, but decreased at a higher relative humidity because of the reduced illumination intensity on the photocatalyst surface in the presence of mist. Furthermore, the formation of nitric oxide (NO) was also observed as an intermediate gas product in such a process and the reaction process was elucidated. A higher nitrogen photofixation rate was achieved by using ERB to transfer electrons to the nitrogen photofixation system. In this process N2 and O2 in air could be converted into nitrate on the widely used TiO2 surface under UV or sunshine irradiation. Such a catalytic reaction has a double-edged consequence:it could be used as an alternative method to the Ostwald process for nitrate production at a much cheaper cost; it also has a potential to alter the global nitrogen cycle and even becomes an increasing pollution source to environment and human beings.4. Photocatalytic reduction of nitrate by TiO2 and silver-doped TiO2 prepared by photodepositon in the present of several hole scavengers and negative electricity supplied by MFC was investigated. Denitrification process was observed when negative electricity was supplied by MFC to the photoelectrocatalytic system, confirming that electrons transfer from ERB to the TiO2 in this system could act as a scavenger of the photogenerated holes. The independent or competitive relationship between the compounds and electrons transfered from ERB acting as hole scavenger was demonstrated. Such an integrated process has a great potential to be used for photocatalytic reduction of nitrate. |
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