Electron Transfer Mechanism of Zero-Valent Iron - Bacteria Synergy in Nitrate Reduction

Abstract

As the main source of drinking water, the nitrate pollution of groundwater is a potential threat to environmental remediation and human health, as nitrate is a nonligand forming oxyanion that is soluble in water and exhibits high mobility. However, if nitrate is reduced to ammonia, recycling of nitrate pollutant may be realized. In this respect, many bacterium are capable of nitrate reduction into ammonia (NRA), but the sluggish reaction kinetics due to electron donor deficiency and the rate-limiting nitrite reduction step severely limit their practical application. On the other hand, zero valent iron (ZVI) has been widely used for water remediation due to its high reduction activity (Eh=-0.44), low cost, and environmental friendliness. However, ZVI alone cannot effectively reduce nitrate into ammonia. In addition, it suffers from severe side reactions and are prone to passivation during long-time operation. In this thesis, the author elaborated a hybrid system consisting of ZVI and Shewanella oneidensis MR-1, an environmentally-ubiquitous bacterium with unique extracellular electron transport (EET) and nitrate reduction capability, to synergistically enhance NRA process. The author demonstrated the superiority of the bio-hybrid system over the individual groups in NRA, elucidated the underlying mechanisms from the perspectives of metabolic and EET collaborations, and evaluated its feasibility for practical wastewater treatment. The main research contents and results are as follows:  

1: Nitrate was selected as the model pollutant, the structure-activity relationship between the nZVI shell structure and its nitrate reduction activity, selectivity and oxidation resistance was clarified by different spectral characterization and microscopic detection techniques. The results indicated that, the compact-structured, lesshydrophilic shell of nZVI with abundant crystalline α-Fe₂O₃  component not only enables nZVI efficient interfacial electron transfer ability to accelerate the pollutant reduction, but also serves as a protective layer to suppress the H2-evolution side reaction and the oxygen diffusion, thereby rendering the material with high decontamination activity and selectivity as well as good air stability. In contrast, the hydrophilic and loose-structured shell of nZVI, which mainly consists of amorphous FeO and FeOOH, has limited conductivity and cannot effectively protect the inner nZVI from oxidation by water nor air, thereby resulting in poor activity and stability for decontamination application. The results of this study provide an important reference for assessing the reduction activity and selectivity of nZVI for contaminant treatment and for assessing its stability during storage and transportation, and will be of great significance to guide the synthesis of nZVI with high reducing activity, high selectivity and excellent air stability.  

2: The gene editing method was combined with various analytical techniques to explore the pollutant reduction site, electron transfer direction, electron donor source and electron transfer pathway of the ZVI and S. oneidensis MR-1 bio-hybrid system fornitrate reduction, in order to analyze the internal mechanism that ZVI boosts nitrateto-ammonia bioconversion via extracellular electron donation and reduction pathway complementation. The research results showed that biological and abiotic processesperfectly complement each other’s rate-limiting steps in each single system, bacteria are mainly responsible for NO₃^--to-NO₂^- conversion, while ZVI enables efficient NO₂^--to-NH₄⁺ conversion, thereby overcoming the limitations of individual systems for theentire NRA process. ZVI provides electrons to drive biological reduction of NO₃^- to NO₂^- through both direct EET process and H₂-mediated indirect EET process, and the efficiency of both EET processes highly depend on the physical distance between ZVI and bacteria cells. The co-presence of bacteria with ZVI not only decreases the formation of vivianite on the surface of ZVI, but also promote the corrosion of ZVI for H₂-evolution, which is beneficial for both biotic and abiotic processes. The analysis of the new mechanism of the bio-hybrid system provides an important theoretical reference for its application for reclamation of nitrate wastewater.  

3: Activated carbon was used as the carrier and ZVI was filled in as the filler, S. oneidensis MR-1 with a specific concentration was pumped into the up-flow continuous flow reactor in order to complete the optimization of the bio-hybrid system, then it was applied to nitrate wastewater recycling. The results showed that the activity of bacteria could be maintained well with the adding of additional low concentration organic carbon to the actual electroplating wastewater. The decrease of “nitrate-to-ammonia” (NRA) activity was attributed to the gradual consumption of ZVI and the formation of inert iron oxide shell on the surface of ZVI that slows interfacial electron transfer, but this problem could be solved by the supplement of ZVI. When the bio-hybrid system was used for the treatment of the synthetic stainless steel pickling wastewater in an upflow anaerobic reactor, the produced ammonia concentration was 8.2~14.1 mM during a 50-hour continuous operation, achieving 95%-97.8% NO₃^- to-NH₃ selectivity. The ammonia recovery efficiency could reach 77.8% by gas extraction. The optimization of the bio-hybrid system for nitrate wastewater recycling provide a theoretical support and technical reference to the large-scale applications of this bio-hybrid system.

Date of Award	9 Nov 2022
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Kwan Sing Paul LAM (Supervisor), Wenwei Li (External Supervisor), Jianxiong ZENG (External Supervisor) & Ruquan YE (Supervisor)