Interfacial Electron Transfer Mechanism In Biological And Chemical Transformation Of Pollutants

Interfacial electron transfer plays an essential role in a broad range of biological reactions, chemical systems and environmental remediation, including energy flow and conversion. The interfacial reaction could be promoted by giving a deep insight into the electron transfer mechanisms and improving the electron transfer efficiency. This study aims to elucidate the interfacial electron transfer mechanism in biological and chemical transformations of pollutants. A high energy conversion efficiency could be achieved by controlling electron flow routes between microbial terminal oxidoreductase and electron acceptors. The effective pollutant transformation could be accomplished by designing contaminant-targeted photocatalytic nanomaterials to reveal the interaction mechanisms and structural evolution of pollutant molecules on catalyst surface. Main contents and results of this dissertation are as follows:1. Biological transformation of pollutants could be improved by screening and using high efficient electrochemically active bacteria (EAB). Hexagonal tungsten oxide (h-WO3) nanocluster was developed as a high-throughput probe for identification of EAB, the "star" microorganisms in the fields of environment, energy, microbiology, and biogeochemistry. Here, the mechanism for screening EAB by the WO3nanocluster is explored using molecular modeling and electrochemical experiments. The electron transfer efficiency is found to be governed by the special molecular configuration of cytochrome and the solvent exposed heme with respect to the nanocluster surfaces. The axial bishistidine residues bounded to the porphyrin plane enable interfacial electron transfer to occur with a low thermodynamic barrier. The apparent electron transfer rate is dependent primarily on the transfer distance. This work provides deep insights into the interfacial electron transfer at microbe-nanocluster, and is useful for molecular design of highly-efficient electron-capturing nanomaterials.2. Microbial extracellular electron transfer (EET) could be tuned through modifying anode surface. After a molecular-level investigation, it is revealed that EET from the heme group of c-type cytochrome (c-Cyt), an outer membrane protein, to the graphite nanosheet electrode, a widely used electron acceptor, is significantly influenced by the molecular conformations of the surrounding amino acid residues of c-Cyt and the solvent exposed heme with respect to the EA surface. The lysine residue presented on the surface of c-Cyt provides a weak hydrogen bond to shorten the electron transfer distance and hence accelerate EET. The thermodynamic and kinetic analyses demonstrate that the hydrophilic oxygen-containing groups on graphite surface contribute greatly to the EET through lowering the thermodynamic barrier and allowing rapid reaction equilibrium. Effective control of the c-Cyts/graphite interaction is beneficial to accelerating and engineering EET for designing highly-efficient biological energy-conversion systems.3. Phenazines, as a type of electron shuttle, are involved in various biological processes to facilitate microbial energy metabolism and electron transfer. They are constituted of a large group of nitrogen-containing heterocyclic compounds, and can be produced either by a diverse range of bacteria or by artificial synthesis. They vary significantly in their properties, depending mainly on the nature and positions of substitutent groups. Thus, it is of great interest to find out the most favorable substituent type and molecular structure of phenazines for electron transfer routes. Here, the impacts of the substituent group on the reduction potentials of phenazine-type redox mediators in aqueous solution were investigated by using quantum chemical calculations, and the calculation results were further validated with experimental data. The results show that the reaction free energy was substantially affected by the location of substituent groups on the phenazine molecule and the protonated water clusters. For the main proton addition process, the phenazines substituted with electron-donating groups and those with electron-withdrawing groups interacted with different protonated water clusters, attributed to the proximity effect of water molecules on proton transfer. Thus, high energy conversion efficiency could be achieved by controlling electron flow route with appropriate substituted phenazines to reduce the biological energy acquisition. This study provides useful information for designing efficient redox mediators to promote electron transfer between microbes and terminal acceptors, which are essential to bioenergy recovery from wastes and environmental bioremediation.4. Interfacial electron transfer involves in photocatalytic nitrification and denitrification. Nitrate could be formed from abundant atmospheric nitrogen and oxygen on nano-sized titanium dioxide surfaces under ultraviolet or sunlight irradiation. This is an undiscovered nitrate formation process that occurs universally. We suggest that nitric oxide is an intermediate product in this process, and elucidate its formation mechanisms using first-principles density functional theory (DFT) calculations. The results indicate that the conduction band mechanism with a low energy barrier (Ea) can be the dominant pathway for the NO formation, and the valence band mechanism can also possibly occur with h+at a low reaction rate. However, nitrate causes severe ecological and health risks, and nitrate contamination of drinking water sources has become one of the most important water quality concerns all over the world. Photocatalytic reduction of nitrate to molecular nitrogen presents a promising approach to remove nitrate from drinking water sources. We have found an efficient, selective and sustainable bioelectro-photocatalytic nitrate-reducing system by utilizing TiO2nanoparticles as the photocatalyst and bio-electrons from microbial metabolism as the hole scavenger. Compared with the conventional denitrification mechanism, such a bioelectro-photocatalytic reaction pathway has a lower Ea, suggesting that the complete photocatalytic reduction of nitrate to N2without accumulation of toxic intermediates is energetically feasible. The mechanisms of the highly-selective nitrate reduction were elucidated by DFT calculations. The scavenging of holes by the bio-electrons avoids the occupation of extra adsorption competition anions onto the active surface, resulting in the selective and efficient photocatalytic denitrification.5. Crystal surface of TiO2photocatalyst can be tuned to promote the degradation of pollutants in the practical applications. TiO2-based materials have been widely investigated for the photocatalytic degradation of various persistent pollutants. Especially, modification of TiO2surface with nanosized metallic clusters is found to accelerate the degradation process, but the underlying mechanism and the effective crystal surface for pollutant degradation remain unclear. Photodegradation of nitrobenzene (NB), a model pollutant, on the surface of Pt cluster-loaded anatase TiO2(Pt/TiO2) catalyst is investigated by combining DFT quantum chemistry calculations with experimental studies. The configurations of absorbed organic molecules, the active sites of the composite catalysts as well as the interactions in the NB photodegradation in aqueous phase are elucidated. The mechanism of the oxidative and reductive reactions on the TiO2(001) and (101) surface is proposed, which is found to be appropriate to explain the improved light adsorption and electron-hole separation of anatase TiO2and accelerated NB photodegradation by loading Pt clusters. Furthermore, the thermodynamic analysis and liquid chromatography mass spectrometry results reveal that an oxidative degradation of NB on (001) surface is favored for the Pt/TiO2. Thus, the efficiency of NB degradation in aqueous solution might be enhanced by exposing large ratio of (001) surface in the synthesis of Pt/TiO2. This work provides implications for designing contaminant-targeted photocatalytic nanomaterials.