| Chloronitrobenzene(CNB) is an important dye intermediates for synthesis of many products in chemical industries. CNB has many families of derivatives, of which 4-CNB is produced at the largest quantity. Owing to special physical a nd chemical properties, 4-CNB is extremely chemically stable in natural environment, leading to accumulation in water, sediment, and soil followed by possibly entering into the food chain. 4-CNB can cause acute, chronic or persistent physiological toxicity once exposing to the animal and human body in direct or indirect way. Hence, the 4-CNB has been listed as primary persistent toxic non-degradable organic pollutant by the Environmental Protection Agency of the United State of America(EPA, USA), European and China government. Using conventional methods to remove 4-CNB, such as adsorption, advanced oxidation process(AOP), and biological degradation, have been addressed to be technically feasible. However, these approaches may be problematic and unsustainable due to low treating efficiency, high cost and extensive energy input, or potential production of secondary contaminants that needs further treatments. Thus, it is of environmental importance to develop more efficient and more cost effective technologies for pretreatment of 4-CNB-containing wastewater.Recently, bioelectrochemical system(BES) has been drawing a growing interest for its remarkable potential for many applications such as recovering electric energy from wastes, producing useful resources(e. g. H2, CH4, H2O2), desalinating seawater as well as removing various kinds of pollutants in wastewater. Since these processes are driven by the anodic bioelectrocatalysis, BES will become a new promising technology to alleviating energy- and environment-relevant issues, making wastewater treatment more economical and more sustainable. The efficiency of cathodic reduction of pollutants is essentially dependent on the composition, interface and structure of the electrode in BES. The present study aims to address three problems as stated in detail below. The first part focuses on exploitation of the important role of electrode surface area and diffusion coefficient in facilitating electron transfer based on numerical modeling methods. The second part carries out the work on improving cathodic reduction of 4-CNB by fabricating graphene-Mn O2 interface with good conductivity and large surface area on conventional carbon electrode. The last part deals with the construction of continuously operated BES where the cathode was modified by graphene and Mn O2 for reduction of 4-CNB. In particular, we also investigate the applied voltage as the key factor that influences the 4-CNB reduction, the structure, composition and evolution of microbial community, attempting to discuss the mechanisms of 4-CNB reduction upon molecular microbiology.The three-dimensional structured pseudo-capacitive interface was constructed on carbon electrode by in-situ reduction of permanganate by carbon elements(reduced oxidized graphene, r GO). We would expect to enhance interfacial electron transfer by enriching the electrochemically active microbes and facilitating electron transfer via pseudo-capacitive property of Mn O2. The results show that in comparison with bare carbon electrode, the r GO is able to provide much higher surface area and active sites for the attachment of electroactive biofilm. Besides, the pseudo-capacitive property of Mn O2 is an effective way for enhancing electron transfer, and thus improving the reducing activity and stability of 4-CNB reduction. The removing efficiency of 4-CNB could reach 98.2% for r GO/Mn O2-CP, which was approximately 30% higher than that of 61% for carbon electrode. The dominant reduced products of 4-CNB is characterized as both 4-CAN and AN, suggesting the capability of r GO/Mn O2-CP for dechlorination. By optimizing the operational parameters, it was found that the removal of 4-CNB was positively related to applied voltage and decrease in initial 4-CNB concentration. The optimum 4-CNB removal was obtained at p H-neutral condition, preliminarily due to the inhibition of acid and alkaline to microbial growth.The impacts of electrode surface and mass transfer property to electrochemical reactions are investigated by using numerical modeling methods. The results demonstrated that the models based on redox electron mediator and Fick ’s law can well describe the electrochemical reactions of BES. The cell voltage and current depend on the external resistance applied, which is determined by the mass transfer and reaction process within the biofilm. At low-current density regions, the reduced mediator predominated, and the BES works at high-concentration reduced mediator condition, and vice versa. In comparison, at relatively higher current density regions, the system lacks reductive force, making it inaccessible maintain normal working condition at low reduced mediator condition. In this case, the cell voltage and current exhibits steep decline. The substrate(acetate) depends more upon time whereas the redox electron mediator relies on the inner spatial distance to the electrode surface. Increasing the electrode surface area was shown effective to alleviate the limitation originated from the lack of reduced mediator, thereby the metabolic rate could be increased owing to extend of growing space. The increase in mass transfer coefficient could increase the electrode reaction, i. e. the redox mediator with higher mass transfer coefficient is essential to improve the BES performances.Considering the practical wastewater treatment generally requires the pollutants to be removed continuously and stably, the electrochemical reduction of 4-CNB was examined in a continuous BES having the cathode fabricated with carbon felt modified by r GO and Mn O2 using the methods described in the former chapter. In addition, the 4-CNB reduction efficiency, impact factors and molecular microbiology are also investigated. The results demonstrate that the acclimated cathodic biofilms were capable of reducing 4-CNB continuously, accounting for efficiency of 99.5% at HRT of 48 h and applied voltage of 0.4 V. The 4-CNB reduction efficiency was found to increase with increase of HRT and applied voltage. The reduced products were mainly 4-CAN at low voltage(<0.4 V), while higher voltage(e. g. 0.8 V) could lead to the formation of AN resulting from the dechlorination of 4-CAN. The modified CF electrode was beneficial for the enrichment of electroactive biofilm, which was also promoted by the application of voltage. It was also observed that the micr obial diversity of biofilm formed for voltage of 0.8 V was larger than that for voltage of 0 V. Based on 16 S r RNA gene Illumina sequencing results, Enterobacter genera were found predominant at applied voltage of 0.2 V and 0.4 V, amounting to the fraction of 28.6% and 24.1%, respectively. For the applied voltage of 0.8 V, the Dysgonomonas genera predominated as 18.8% of the total microbial community. These results suggest that Enterobacter and Dysgonomonas genera are active toward electrochemically reducing the nitrobenzene substances, which plays a significant role in reduction of 4-CNB in the cathode of BES. |
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