| Wastewater resource utilization is critical for resolving the supply-demand gap in water resources,minimizing water pollution,and maintaining water ecological security.Wastewater may appear to be a dirty substance in general,but it is still an underutilized resource,especially the potential of organic matter to be converted into bioenergy.Microbial fuel cell(MFC)is a promising technology for directly using wastewater to produce bioelectricity energy.MFC system uses electroactive bacteria(EAB)to harvest electrons by metabolizing organic matter in wastewater,and then transfer electrons from their interior to anodes by various extracellular electron transfer(EET)mechanisms.MFC technology has good application prospects because it can convert organic matter in sewage into bioenergy-electricity.Constructed wetland(CW)is a kind of wastewater treatment technology that mimics the natural wetland system.It has been developed for decades and is used for a wide range of varieties of wastewater treatments worldwide.CW technology is constantly updated and intensified as the discharging protocols for specific water quality parameters become stricter.CW system has natural redox conditions,i.e.,the anaerobic environment within the deeper wetland and the aerobic environment on the upper wetland section towards the surface.This could satisfy the anode and cathode criteria of MFC.Thus,MFC was introduced into CW,forming a novel technology,referred to as the CW-MFC system.CW-MFC shows the potential as a sustainable approach to alleviate the growing energy gap while also treating wastewater effectively and affordably.Therefore,CW-MFC is a novel technology for sustainable utilization of wastewater resources.The removal and impact of refractory emerging contaminants,such as pesticides,antibiotics,and perfluoroalkyl substances and polyfluoroalkyl substances(PFASs)in the wastewater resource utilization process should also be explored urgently.PFASs are artificial refractory organic pollutants that are widely present in the aqueous environment.Due to the unquiet strength of the highly polarized carbon-fluorine bond(C-F)and its hydrophobic/lipophobic features as well as biological persistence properties,the remediation and treatment of PFASs is a major challenge.Preliminary studies indicate that a few kinds of technical approaches could remove or transfer PFASs,but their effectiveness is not as high as expected or limited while most of the techniques are only tested at laboratory scale.CW-MFC integrates physical,chemical,and enhanced biological processes,plus the wetland plants’ functions,with strong eco-friendly features for the comprehensive removal of PFASs.However,there are still many problems that need to be studied and solved in CW-MFC technology in terms of system design,performance optimization,energy recovery efficiency,and the removal of emerging contaminants.Based on the above discussion,this thesis carried out the following five aspects:(1)A bibliometric analysis using VOSviewer based on Web of Science data was conducted to provide an overview by tracing the development footprint of this technology.The countries,institutions,authors,key terms,and keywords were tracked and a corresponding map was generated.From 2012 to September 2020,442 authors from 129 organizations in 26 countries published 135 papers in 42 journals,with a total citation of 3,139 times.The key terms analysis revealed four clusters: bioelectricity generation performance,mechanism study,removal of refractory pollutants,and enhanced removal of conventional contaminants.Other research themes include: exploration of the biochemical properties of electrochemically active bacteria;removal of emerging contaminants;effective bioelectricity harvesting and use;and biosensor development and scaling-up for real-world application.The bibliometric results provide valuable references and information on potential research directions for future studies.(2)The cathode performance is the main limiting factor for bioelectricity generation.CWMFC exhibited a performance discrepancy between using granular activated carbon(GAC)and columnar activated carbon(CAC)as air-cathode materials.No doubt,this is linked with electrochemical performance and decontaminant characteristics in the CW-MFC system.To provide insight into this performance discrepancy,three CW-MFCs were designed with different carbon-materials to construct varied shapes of air-cathodes.The results showed that the ringshaped cathode filled with GAC yielded the highest voltage of 458 m V with a maximum power density of 13.71 m W·m-2 and > 90% COD removal in the CW-MFC system.The electrochemical characteristics and the electron transport system activity(ETSA)are the driving forces to deliver the GAC with better electron transportation and oxygen reduction reaction(ORR).This will help elucidate the underlying mechanisms of different activated carbons for aircathode applications and thus promote their large application.(3)As an essential component in the CW-MFC system,the macrophytes play multiple roles in electricity generation and decontaminant performance.However,the interrelation between macrophyte roots and cathode has not been fully investigated,despite the fact that plant cultivation strategy is a critical issue in practice.This study was designed for the first time to explore the interaction between macrophytes and cathode in CW-MFC by planting Cyperus altrnlifolius in different positions for treating synthetic wastewater under continuous feed mode.The results showed that plants exhibited higher bioelectricity generation and dramatically improved pollution removal.More significantly,the relative locations between the plant roots and the cathode could lead to different cathode working patterns,while the optimal cathode pattern,"plant root-assisted bio-and air-cathode," was formed when the plant roots are directly placed on the air-cathode layer in CW-MFC.Thus,achieving a quick and stable voltage output(550-600 m V)and excellent performance in wastewater treatment(removal rates > 90% for COD,> 85% for TN and TP).The insight into the plant root and cathode relationship lies in whether the "multi-function cathode" can be established.This study contributes to increasing the knowledge regarding the presence and behavior of plant roots and cathodes throughout a CWMFC system.(4)The waste mineral,pyrite(Fe S2),was directly used as electrodes in the CW-MFC systems to investigate the performance of each system with the removal of contaminants,and to explore the bioelectrochemical characteristics,as well as to reveal their advantages and disadvantages.The findings revealed that incorporating pyrite into electrodes(both anode and cathode)improves TP removal(6–10% greater)while having no detrimental influence on organic degradation in the CW-MFC system.However,the sole-use of pyrite could limit the N removal(TN removal decreased by 36.95%)and bioelectricity generation(29.5% lower)owing to the deterioration of cathode oxidation conditions and the restriction of nitrification in the system.The underlying mechanism was discovered with ammonium oxidation being the rate-limiting stage of N removal due to the anoxic conditions and microbial toxicity exerted by pyrite.Moreover,the poor cathodic performance of a single natural pyrite was caused by a complex of factors,including pyritesnatching electron acceptor;iron oxide and iron hydroxide precipitation and Fe-P compound to form blockage together;and toxicity to plants and microorganisms.Furthermore,iron ions and sulfate ions released by pyrite may have an influence on effluent quality and pose environmental risks.(5)Based on a review of the PFASs research,this section discusses the feasibility of using CW-MFC systems to treat wastewater containing PFASs,and then thoroughly investigates the effects of PFASs on the microbial population structure and plant characteristics in the system.Both closed-circuit and open-circuit operation of the CW-MFC system demonstrated over 96%removal performance of PFASs in wastewater.However,with the efficient removal of PFASs,several adverse effects on system performance have occurred,including decreased N removal(TN removal reduction by approximately 7.22%)and low bioelectricity generation(voltage output decreased by 7.32%)during two months of operation.The analysis of plant ecophysiological features found that PFASs harmed the plant’s photosynthetic system while activating its antioxidant defense system.In addition,PFASs not only changed the microbial community structure but also indirectly inhibited microbial enzyme activities,which conspired to worsen system performance.Meanwhile,toxic/inhibitory effects on plants and microorganisms limit the system’s performance for bioelectricity generation and conventional pollutant removal.This can provide theoretical support for PFASs remediation technologies in the aquatic environment.CW-MFC system,as a novel wastewater treatment technology,integrates physical,chemical,enhanced biological processes,and wetland plant activities,resulting in the efficient removal of conventional and emerging contaminants in wastewater.Turning waste into resources in the CWMFC system have a wide range of applications.This work has increased our understanding of the microbial electrochemical characteristics,the removal of nutrients,the interaction mechanism between plant roots and cathodes,and the effects of emerging contaminants on the CW-MFC system.Furthermore,the application of CW-MFC technology in engineering practice is guided by three aspects: engineering simplicity,economic feasibility,and environmental sustainability. |
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