Enhanced bioreduction of nitrobenzene by reduced graphene oxide materials: effects of surface modification and coexisting soluble electron shuttles

Geobacter sulfurreducens promoted the biosynthesis of reduced graphene oxide and coupled it for nitrobenzene reduction

In order to explore an efficient and green method to deal with nitrobenzene (NB) pollutant, reduced graphene oxide (rGO) as an electron shuttle was applied to enhance the extracellular electron transfer (EET) process of Geobacter sulfurreducens, which was a typical electrochemically active bacteria (EAB). In this study, rGO biosynthesis was achieved via the reduction of graphene oxide (GO) by G. sulfurreducens PCA within 3 days. Also, the rGO-PCA combining system completely reduced 50-200 µmol/L of NB to aniline as end product within one day. SEM characterization revealed that PCA cells were partly wrapped by rGO, and therefore the distance of electron transfer between strain PCA and rGO material was reduced. Beside, the ID/IG of GO, rGO, and rGO-PCA combining system were 0.990, 1.293 and 1.31, respectively. Moreover, highest currents were observed in rGO-PCA-NB as 12.950 µA/-12.560 µA at -408 mV/156 mV, attributing to the faster electron transfer efficiency in EET process. Therefore, the NB reduction was mainly due to: (I) direct EET process from G. sulfurreducens PCA to NB; (II) rGO served as electron shuttle and accelerated electron transfer to NB, which was the main degradation pathway. Overall, the biosynthesis of rGO via GO reduction by Geobacter promoted the NB removal process, which provided a facile strategy to alleviate the problematic nitroaromatic pollution in the environment.

Nitrobenzene reduction promoted by the integration of carbon nanotubes and Geobacter sulfurreducens

Electron shuttles (ES) can mediate long-distance electron transfer between extracellular respiratory bacteria (ERB) and the surroundings. However, the effects of graphite structure in ES on the extracellular electron transfer (EET) process remain ambiguous. This work investigated the function of graphite structure in the process of nitrobenzene (NB) degradation by Geobacter sulfurreducens PCA, in which highly aromatic carbon nanotubes (CNTs) was studied as a typical ES. The results showed that the addition of 1.5 g L^-1 of CNTs improved the NB biodegradation up to 81.2%, plus 18.8% NB loss due to the adsorption property of CNTs, achieving complete removal of 200 μM NB within 9 h. The amendment of CNTs greatly increased the EET rate, indicating that graphite structure exhibited excellent electron shuttle performance. Furthermore, Raman spectrum proved that CNTs obtained better graphite structure after 90 h of cultivation with strain PCA, resulting in higher electrochemical performance. Also, CNTs was perceived as the "Contaminant Reservoir", which alleviated the toxic effect of NB and shortened the distance of EET process. Overall, this work focused on the effects of material graphite structure on the EET process, which enriched the understanding of the interaction between CNTs and ERB, and these results might promote their application in the in-situ bioremediation of nitroaromatic-polluted environment.

Electrochemical Degradation of Nitrobenzene Wastewater: From Laboratory Experiments to Pilot-Scale Industrial Application

In this study, the electrochemical degradation of nitrobenzene (NB) was conducted on the Ti/SnO2-Sb/Ce-PbO2 anode with excellent functional performance. The effect of applied current density, electrode distance, pH value and initial concentration on the reaction kinetics of NB was systematically studied. The total organic carbon (TOC) removal rate reached 91.5% after 60 min of electrolysis under optimal conditions. Eight aromatic intermediate products of NB were identified by using a gas chromatography coupled with a mass spectrometer, and two aliphatic carboxylic acids were qualitatively analyzed using a high-performance liquid chromatograph. The electrochemical mineralization mechanism of NB was proposed based on the detected intermediates and the identified key active oxygen specie. It was supposed that the hydroxyl radical produced on an anode attacked NB to form hydroxylated NB derivatives, followed by the benzene ring opening reactions with the formation of aliphatic carboxylic acids, which mineralized to CO2 and H2O. In addition, NB was reduced to less stable aniline on the cathode surface, which resulted in actualized mineralization. The successful pilot-scale industrial application in combination with wastewater containing NB with the influent concentration of 80–120 mg L−1 indicated that electrochemical oxidation has great potential to abate NB in practical wastewater treatment.

Stimulation of pyrolytic carbon materials as electron shuttles on the anaerobic transformation of recalcitrant organic pollutants: A review

Pyrolytic carbon materials (PCMs) with various surface functionalities are widely used as environmentally friendly and cost-efficient adsorbents for the removal of organic and inorganic pollutants. Recent studies have illustrated that PCMs as electron shuttles (ESs) could also show excellent performances in promoting the anaerobic transformation of recalcitrant organic pollutants (ROPs). Numerous studies have demonstrated the excellent electron-shuttle capability (ESC) of PCMs to stimulate the anaerobic reductive transformation of ROPs. However, there is a lack of consistent understanding of the mechanism of ESC formation in PCMs and the stimulation mechanism for ROPs anaerobic transformation. To gain a more comprehensive understanding of the latest developments in the study of PCMs as ESs for ROPs anaerobic transformation, this review summarizes the formation mechanism, influencing factors, and stimulation mechanisms of ESC. ESC benefits from redox functional groups (quinone and phenol groups), persistent free radicals (PFRs), redox-active metal ions, conductive graphene phase, and porous nature of their surface. The factors influencing ESC include the highest treatment temperature (HTT), feedstocks, modification methods, and environmental conditions, of which, the HTT is the key factor. PCMs promote the reductive transformation of ROPs under anaerobic conditions via abiotic and biotic pathways. Eventually, the prospects for the ROPs anaerobic transformation enhanced by PCMs are proposed.

Acceleration of the bio-reduction of methyl orange by a magnetic and extracellular polymeric substance nanocomposite

Extracellular electron transfer (EET) plays an important role in bio-reduction of environmental pollutants. Extracellular polymeric substances (EPS), a kind of biogenic macromolecule, contain functional groups responsible for acceleration of EET. In this study, azo dye-methyl orange (MO) was chosen as a model pollutant, and a Fe₃O₄ and EPS nanocomposite (Fe₃O₄@EPS) was prepared to evaluate its promotion on the bio-reduction of MO. The flower-like core-shell configuration of Fe₃O₄@EPS with a 12 nm of light layer of EPS was confirmed by TEM. The redox ability of EPS was well reserved on Fe₃O₄@EPS by FTIR and electrochemical test. The application of Fe₃O₄@EPS on sustained acceleration of MO decolorization were confirmed by batch experiments and anaerobic sequenced batch reactors. Due to biocompatibility of the biogenic shell, the as-prepared Fe₃O₄@EPS exhibited low toxic to microorganisms by the Live/dead cell test. Moreover, negligible leaching of EPS under high concentration of various anions and less than 10% of EPS was released under extreme acidic and basic pH condition. The results of study provided a new preparation method of biological intimate and environmentally friendly redox mediators and suggested a feasible way for its use on bio-reduction of pollutants.

Anaerobic reduction of high-polarity nitroaromatic compounds by electrochemically active bacteria: Roles of Mtr respiratory pathway, molecular polarity, mediator and membrane permeability

Electrochemically active bacteria (EAB) are effective for the bioreduction of nitroaromatic compounds (NACs), but the exact reduction mechanisms are unclear yet. Therefore, 3-nitrobenzenesulfonate (NBS) was used to explore the biodegradation mechanism of NACs by EAB. Results show that NBS could be anaerobically degraded by Shewanella oneidensis MR-1. The generation of aminoaromatic compounds was accompanied with the NBS reduction, indicating that NBS was biodegraded via reductive approach by S. oneidensis MR-1. The impacts of NBS concentration and cell density on the NBS reduction were evaluated. The removal of NBS depends mainly on the transmembrane electron transfer of S. oneidensis MR-1. Impairment of Mtr respiratory pathway was found to mitigate the reduction of NBS, suggesting that the anaerobic biodegradation of NBS occurred extracellularly. Knocking out cymA severely impaired the extracellular reduction ability of S. oneidensis MR-1. However, the phenotype of ΔcymA mutant could be compensated by the exogenous electron mediators, implying the trans-outer membrane diffusion of mediators into the periplasmic space. This work provides a new insight into the anaerobic reduction of aromatic contaminants by EAB.

Facilitated bioreduction of nitrobenzene by lignite acting as low-cost and efficient electron shuttle

The searching for efficient and economical redox mediators to promote the treatment of wastewater containing recalcitrant organic compounds is greatly needed. In this study, the redox mediator activities of four different lignite samples to facilitate the bioreduction of nitrobenzene by Shewanella oneidensis MR-1 were tested for the first time. The initial nitrobenzene reduction rate was increased by 40.4%-90.3% in the presence of 50 mg/L of different lignite samples. Lignite collected from Xinjiang (XJL) having more oxygenated groups performed better in enhancing nitrobenzene bioreduction. The stimulating effects increased with the increase of lignite dosage (0-200 mg/L) and the decrease of lignite particle size (150-0.1 μm). However, the pristine XJL samples with assorted sizes of particles exhibited better stimulating effects than size-fractionated ones, implying that different-sized XJL particles might have synergetic effects on the bioreduction process. When humic acid or iron was removed from XJL, its promoting effects were decreased, demonstrating the crucial roles of both components in lignite-enhanced nitrobenzene bioreduction. Nitric acid treatment could form more oxygenated moieties on lignite surface, which played vital roles in promoting nitrobenzene bioreduction. The initial nitrobenzene bioreduction rate in the presence of HNO₃-treated XJL was 80.8% higher than that obtained with pristine XJL. This study proposed an effective and readily available redox mediator that could be applied to promote the bioreduction of recalcitrant electrophilic pollutants.

The catalytic degradation of nitrobenzene by the Cu-Co-Fe-LDH through activated oxygen under ambient conditions

Efficient and low-cost catalysts for catalytic wet air oxidation (CWAO) under ambient conditions are of great significance for the degradation of hydrophobic organic contaminants. In this study, four LDH catalysts were prepared and their catalytic performance was studied by the degradation of nitrobenzene. The CuCoFe-LDH shows the best catalytic activity with an NB removal efficiency of 41.2%. The CuCoFe-LDH exhibited a typical layer structure, with a specific surface area of 167.32 m² g^-1, and Cu²⁺, Co²⁺ and Fe³⁺ were evenly dispersed on the crystal. The NB removal efficiency was increased by 12.5% through adding formic acid. After five recycling processes, the NB removal efficiency was 18.9% because 3.8 mg g^-1 of Co was leached out of the LDH. In the CWAO process, H₂O₂, ˙OH, ˙O₂^- and ¹O₂ were successfully formed through activated oxygen by the CuCoFe-LDH catalyst under ambient conditions. This work further broadens the application scope of layered double hydroxides (LDHs) in the degradation of organic pollutants by CWAO under ambient conditions.

Degradation of nitrobenzene by synchronistic oxidation and reduction in an internal circulation microelectrolysis reactor

The degradation of nitrobenzene by synchronistic oxidation and reduction was investigated using an internal circulation microelectrolysis (ICE) reactor with an active volume of 0.018 m³. Compared with a conventional fixed bed reactor with and without aeration, the ICE reactor exhibited a markedly higher nitrobenzene degradation efficiency. The effects of various operational parameters such as reaction time, aeration rate, initial nitrobenzene concentration, initial pH, and a volume ratio of iron and carbon (Fe/C) were also investigated. The optimal operating conditions (reaction time = 60 min, aeration rate = 5 × 10^-4 m³/s, initial concentration of nitrobenzene = 300 mg/L, pH = 3.0, Fe/C = 1:1) gave removal efficiencies of nitrobenzene and chemical oxygen demand of 98.2% and 58%, respectively. The biodegradability index of the treated nitrobenzene solution was 0.45, which is 22 times that of the original solution. The reaction intermediates were identified through high-performance liquid chromatography, ultraviolet-visible spectroscopy, Fourier transform infrared spectroscopy, gas chromatography-mass spectrometry, and ion chromatography. The primary intermediates were determined to be aniline, phenol, and carboxylic acids, indicating that nitrobenzene was synchronously oxidized and reduced in the ICE reactor. Based on the identified intermediates, a possible pathway for nitrobenzene degradation in the ICE reactor is proposed.