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Article

Bio-Coated Graphitic Carbon Nitrides for Enhanced Nitrobenzene Degradation: Roles of Extracellular Electron Transfer

by
Yuming Wang
1,
Yi Li
1,*,
Longfei Wang
1,
Wenlong Zhang
1 and
Thomas Bürgi
2
1
Key Laboratory of Integrated Regulation and Resource Development on Shallow Lake of Ministry of Education, College of Environment, Hohai University, Nanjing 210098, China
2
Department of Physical Chemistry, University of Geneva, 30 Quai Ernest-Ansermet, 1211 Geneva, Switzerland
*
Author to whom correspondence should be addressed.
Sustainability 2023, 15(23), 16372; https://doi.org/10.3390/su152316372
Submission received: 11 October 2023 / Revised: 18 November 2023 / Accepted: 23 November 2023 / Published: 28 November 2023
(This article belongs to the Special Issue Sustainability in Water Treatment)
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Abstract

:
Graphitic carbon nitrides (g-C3N4) and microorganisms could collaboratively enhance photocatalytic properties or facilitate environmental depollution through coupled photocatalytic and biological reactions, which prevented the destruction of photocatalytic stresses to ecological systems and resulted in a sustainable technology for water remediation in rivers and lakes. However, the roles of bio-substances as well as electronic interactions between inorganic and organic systems were still unclear. Herein, g-C3N4, nitrogen-deficient g-C3N4 (ND-g-C3N4), and fluorinated g-C3N4 (F-g-C3N4) were coated with representative bacteria, i.e., Escherichia coli MG 1655, and characterized using integrated spectroscopic techniques. Photocatalytic activities were then evaluated through nitrobenzene degradation performance in an aqueous solution under visible light illumination. Nano-photocatalysts were observed to be adsorbed onto bio-aggregates, and surface hydrophilicity was convinced to be determined in the toxicity of photocatalysts in dark environments. Layered structures of ND-g-C3N4 and F-g-C3N4 were revealed in XRD spectra, and surface coverage of the Luria–Bertani medium was eliminated during E. coli cultivation. Hetero-junctions between photocatalysts and bio-substances were indicated in XPS results. Red-shifts for g-C3N4 and F-g-C3N4 materials as well as a slight blue-shift for ND-g-C3N4 were demonstrated in UV-vis spectra, which might be attributed to the destruction of nitrogen defects on ND-g-C3N4. Owing to the attached bio-substances, nitrobenzene removal could reach twice that with pristine photocatalysts, and ROS quantitative analysis confirmed that hydroxyl radicals were the determined reactive species degrading nitrobenzene in the water solution. The observation of more OH species generation indicated that extracellular electron transfer of E. coli reduced electron–hole recombination and provided reduction sites during photocatalytic degradation of nitrobenzene. This work proved additional electron-transfer paths and reaction mechanisms in hybridized photocatalytic and biological processes, which indicated that bio-activities could be a great promoter of material modification and the incorporation between inorganic and organic systems successfully showed an eco-friendly and sustainable pathway to utilize photocatalysts in natural water.

1. Introduction

Recalcitrant pollutants have been catching increasing attention for escaping from conventional wastewater treatment and increasing potential ecological risks. Recently, photocatalysis was vastly researched to figure out the most suitable and eco-friendly photocatalysts to be efficiently applied in wastewater treatment. The unique physical and chemical properties of graphitic carbon nitrides constantly inspired researchers to actively explore their potential applications in various fields such as solar-energy harvesting, electro-chemical cells, and environmental decontamination [1]. Bacterial inactivation of g-C3N4 has been vastly reported in the last decade and its efficient application in water disinfection and volatile organic compound (VOC) removal has been demonstrated [2,3,4]. Inactivation effects of photocatalytic activities on bio-activities and bio-systems hindered practical and long-term applications in polluted water remediation and led to great risks to sustainability in rivers and lakes. The coexistence of g-C3N4 and attached bacterial cells or biofilms attracted great interest due to enhanced photocatalytic properties by bio-substances, as well as coupled photocatalysis and biological reactions [5]. For example, reduced graphene oxide (rGO) aggregates could be used for bacterium isolation and the caged microorganisms would be permanently inactivated even by near-infrared irradiation [6]. Synergistic effects of GO and abiotic cellulose facilitated the inactivation of microbes due to alternated surface hydrophilicity [7]. Researchers have also attempted to incorporate dual catalysts of TiO2/g-C3N4 with photosynthetic bacteria, and the increased degradation of dyes demonstrated positive effects of bio-substances on photocatalytic activities [8].
Generally, more electricity and higher voltages could be obtained in microbial fuel cells (MFCs) when biofilms were formed in 3D graphene scaffolds bound to bio-anodes [9]. Photo-activated electrons generated by nanomaterials, e.g., CdS nanotubes, could even take the position of bio-electrons in dinitrogen bio-reduction into ammonia, which were conventionally generated through microbial metabolisms along with huge consumption of dissolved organics [10,11]. Non-photosynthetic bacteria could also be self-photosensitized through the hybridization with CdS quantum materials and efficiently converted CO2 into acetic acids [12]. Although great efforts have been devoted to investigating the synergistic effects of photocatalyst–microorganism hybrids in past years, great challenges remain in the way of optimizing such hybrids and turn them into practical applications [13]. One of the most important issues was the missing knowledge of physical connections and electronic interactions between photocatalytic materials and microbial substances, and how they could be influenced by surface properties of catalysts such as g-C3N4 materials [14].
Heteroatom doping, a facile method for surface modification of photocatalysts to obtain superior photoactivities, has been vastly explored and the dopants could be either metallic (Au, Ag, and Pt, etc.) or non-metallic (N, S, and F, etc.) depending on expected functional improvements [15,16,17,18,19]. Considering the possible toxicity of released metal ions, non-metallic atoms seemed to be more suitable for the surface modification of g-C3N4 to tune their surface properties and photocatalytic activities when hybridized with biological substances [20,21]. Previous studies indicated that fluorination of g-C3N4 could strongly increase surface-adsorbed hydroxyl groups, and obvious blue-shift in UV-vis absorbance spectra and greater electron-transfer resistances were also uncovered due to the strong electronegativity of F atoms [22,23]. Meanwhile, nitrogen defects were observed to change the surface energies of g-C3N4 particles and provided a lower conductive band, which significantly strengthened the photocatalytic performance [24]. Consequently, surface properties of g-C3N4 materials could be tuned through either fluorination or nitrogen defect introduction by means of alkali pretreatments, and the roles of surface properties in photocatalyst and bio-substance hybrids could be analyzed by comparing their photocatalytic activities when incorporated with bio-substances [22,24,25,26].
Secreted proteinaceous pilin filaments could extend to tens of micrometers long and serve as nanowires that transport electrons over long distances through a network, which was also termed bio-conductivity by the means of extracellular electron transfer (EET) [27]. Polymerized aromatic rings along extracellular cytochromes and filaments were basic units in EET and a series of oxidation and reduction reactions initiated by electron concentration gradients or electric fields were the determined mechanism of EET [28,29]. Extended electron-transfer paths through active π electrons in g-C3N4 were already seen in MFCs and π–π interactions were expected in the connection points between g-C3N4 and bacteria [30]. Reduced recombination of photo-generated electrons and holes was reasonably expected because of more conductive bio-substances coated on the surface of photocatalysts, which would significantly increase the generation of reactive oxygen species (ROS) [31]. Bio-electrochemical bacteria such as Escherichia and Enterobacter might serve as electron acceptors due to possible electron consumption during their metabolisms to reduce nitrobenzene or other chemicals [28,32]. However, solid evidence was urgently needed to uncover the roles of adsorbed bio-substances in photocatalyst–microbe hybrids, as well as their synergistic effects [14]. As a typical non-biodegradable organic pollutant, nitrobenzene has been attracting numerous concerns due to its discharge from the petroleum industry and could pose potential ecological risks even at low levels. Researchers have devoted great attention to highly efficient and eco-friendly methods to eliminate nitrobenzene from wastewater, and photocatalysis undoubtedly needed to be studied for nitrobenzene transformation with photoactivities. Previous literature demonstrated that free-electron transfer through graphene sheets was determined in photocatalytic nitrobenzene degradation [33,34,35,36,37].
In this work, g-C3N4, g-C3N4 with nitrogen defects (ND-g-C3N4), and fluorinated g-C3N4 (F-g-C3N4) materials were fabricated for in situ bio-coating with the pre-cultured Escherichia coli MG 1655 strain. Various characterization methods were employed to figure out the alteration of the properties of g-C3N4-based materials before and after bio-coating. The effects of EET through conductive pilins and other membrane compartments were investigated using electrochemical impedance spectroscopy (EIS), and mechanisms of the inner circuits during photocatalytic processes were proposed based on mathematical simulations for charge transfer resistances (Rct). Finally, the photocatalytic performance was evaluated through photocatalytic degradation of nitrobenzene, as well as ROS generation measurements using electron-spin-resonance spectroscopy (ESR). Our findings could be an important advancement to uncover interactions between photocatalysis and biological processes, which might lead to multiple functions acquired through microbes and specific materials, and provide an eco-friendly and sustainable pathway for applications of innovative materials in natural water, as well as long-term recoveries of polluted rivers and lakes.

2. Materials and Methods

Dicyandiamide and Luria–Bertani (LB) broth were purchased from Sigma Aldrich. Escherichia coli MG 1655 was purchased from the American Type Culture Collection (Manassas, VA, USA). Other chemicals such as urea and ammonium fluoride (NH4F) were used directly without any pretreatment. Milli-Q water was used for the preparation of all solutions, while sterile water was used for material–microbe hybridization.
Bulk g-C3N4 (yellow) was prepared by directly annealing dicyandiamide (transparent crystal) at 550 °C for 4 h and then milled into powder after cooling down to room temperature. G-C3N4 (1 g) and ammonium fluoride (2.5 g) were uniformly ground and transferred into a Teflon-lined stainless-steel autoclave with 100 mL Milli-Q water. The autoclave was heated at 180 °C for 8 h and then cooled down to room temperature. Bulk F-g-C3N4 (brown) was obtained by centrifugation at 10,000 rpm for 10 min and washed with ethanol and Milli-Q water repeatedly until the pH of the supernatant was close to neutral. Fifty mg g-C3N4 and F-C3N4 were then immersed in 200 mL milli-Q water, respectively, and sonicated for 1 h. Few-layered g-C3N4 and F-g-C3N4 stocking solutions were obtained by eliminating residual sedimentation. Bulk ND-g-C3N4 materials were synthesized through alkali pretreatment before urea calcination [24]. Typically, 15 g of urea was diluted in sodium hydroxide solution (0.01 g in 30 mL H2O) and transferred into a 50 mL alumina crucible. The resulting solution was left to evaporate overnight at 80 °C and the solid mixtures were then calcined at 550 °C in a muffle furnace for 4 h and ground into powders. Fifty mg of as-obtained powder was further suspended in 200 mL milli-Q water and sonicated for 1 h to acquire a few-layered ND-g-C3N4 stocking solution.
E. coli MG-1655 was cultured in 15 mL of LB broth medium at 37 °C overnight on a shaker (150 rpm). Bacterial cells were harvested by centrifugation (14,000 rpm, 5 min) and diluted in 10 mL milli-Q water, and mixed with nanomaterials stocking solutions in different ratios (volumetric ratio = 5:5, 4:6, 3:7, 2:8, and 1:9), followed by shaking for another 6 h. Bio-hybrid GM nanomaterials were finally washed 3 times using milli-Q water and diluted in 10 mL milli-Q water to obtain a bio-hybrid stocking solution. Reference experiments without any E. coli cells or g-C3N4-based materials were synthesized using similar procedures [12].
The photocatalytic activity of the synthesized photocatalyst was evaluated by using nitrobenzene as the target recalcitrant pollutant to measure the photocatalytic activities of bio-coated g-C3N4, ND-g-C3N4, and F-g-C3N4 materials. Briefly, 2 mL of bio-hybrid material was mixed with 50 mL of nitrobenzene solution (20 mg/L) in a cylindrical quartz tube and placed into a commercial photocatalytic reactor (Beijing China Education Au-light Co., Ltd., Beijing, China) equipped with a 100 W Xe lamp (wavelength ranging from 600 to 650 nm). The solutions were kept in the dark for 30 min under stirring to reach saturated adsorption of nitrobenzene on the catalyst surface. Afterward, the lamp was turned on to initiate the photocatalytic degradation of nitrobenzene. Samples were withdrawn at appropriate time intervals and filtered through a 0.22 μm membrane before high-performance liquid chromatography (HPLC) measurements. An Agilent 1260 system equipped with a diode array detector and a 4.6 × 250 mm Eclipse Plus C18 column at 30 °C (Agilent Technologies, Santa Clara, CA, USA) were used and the mobile phase (methanol:water (1:1)) was injected at a flow rate of 1.0 mL min−1, with the detection wavelength at 254 nm.
Surface morphology images were acquired via a Hitachi SU3500 scanning electron microscope. Transmission electron microscopy images were obtained using a JEOL JEM-1400 microscope at an accelerating voltage of 100 kV. UV-vis absorption spectra were recorded on a UV-3600 spectrometer (Shimadzu, Kyoto, Japan) equipped with an integrating sphere from 200 to 800 nm. BaSO4 was used as a reference. XPS (Thermo ESCALAB 250) was performed using mono-chromated Al Ka radiation (1486.8 eV). XRD patterns were obtained on a Bruker D8 Advance X-ray diffractometer with Cu-Ka radiation (l = 0.15418 nm). FTIR measurements were recorded using an FTIR spectrometer (Perkin-Elmer) with KBr as the supporting material.
Electrochemical impedance spectroscopy (EIS) was performed using a CHI660D workstation (Shanghai Chenhua, Shanghai, China) with a three-electrode configuration using the prepared samples as the working electrodes, a Pt plate as the counter electrode, Ag/AgCl as the reference electrode, and a 300 W Xe lamp equipped with a 420 nm cut-off filter as the light source. Na2SO4 aqueous solution (0.5 M) was used as the electrolyte. The working electrodes were prepared as follows: 10 mg of the as-prepared F-g-C3N4 nanosheets were dispersed in absolute ethanol, and the suspension was directly deposited onto an indium–tin oxide (ITO) glass plate and then dried at 80 °C in a vacuum oven. Electrochemical impedance measurements were performed within a frequency range of 0.1 kHz to 100 kHz. The cell voltage measurements across the resistor were recorded once every five minutes by using an eDAQ e-corder data acquisition system (Bronjo Medi, Singapore) equipped with Chart software (PowerChrom 280).

3. Results

3.1. Surface Morphology and Photo-Chemical Analysis

Morphologic pictures of as-synthesized bio-coated g-C3N4, ND-g-C3N4, and F-g-C3N4 materials are shown in Figure 1. E. coli bio-substances served as supports for g-C3N4 and ND-g-C3N4 materials, and more bio-substances were produced during E. coli strain cultivation in the presence of g-C3N4. SEM pictures of pristine g-C3N4, ND-g-C3N4, and F-g-C3N4 in Figure S1 demonstrated that the size of ND-g-C3N4 particles was smaller than for g-C3N4 and F-g-C3N4. Meanwhile, F-g-C3N4 nanomaterials were covered by bio-substances and cellular structures were abundantly detected in bio-coated F-g-C3N4, probably implying that F-g-C3N4 was non-toxic to E. coli cells due to the hydrophilic surface of F-g-C3N4 particles. E. coli MG-1655 cultivation was strongly inhibited by ND-g-C3N4, while cellular aggregation and broken cells were observed in the presence of g-C3N4 materials. Extracellular polymeric substances (EPS) were reported to be a self-protection mechanism of bacteria, resulting in cell aggregation and the smooth surface of bio-aggregates in SEM pictures [7].
From the above results, we can infer that EPS were produced by E. coli MG-1655 in the presence of all three inorganic photocatalysts, which means that direct contact with inorganic photocatalysts disturbed regular metabolisms of E. coli MG-1655 cells. Additionally, g-C3N4 demonstrated adverse effects such as inactivation and inhibition on E. coli MG-1655, while fluorination could remit the toxicity of nanomaterials.
A broad peak in the XRD around 22.5° was observed for E. coli (pretreated through freeze drying, Figure S2). The same peak remained in bio-coated g-C3N4, ND-g-C3N4, and F-g-C3N4 photocatalysts, which could be regarded as background and pre-eliminated. The spectra of pristine and bio-coated g-C3N4 were similar to JCPDS No.50-1512 which have unique signals around 27.87°, 32.53°, 46.53°, and 57.16°, representing (110), (200), (111), and (220) lattice planes, respectively. The (200) peak shifted from 32.53° to 32.59° for nitrogen-deficient g-C3N4, indicating a decrease in the interplanar stacking distance, while it shifted from 32.53° to 32.48° for F-g-C3N4, originating from the enlarged interplanar stacking distance due to F atoms. After bio-coating, the lattices of g-C3N4-based materials were left unchanged, while a few particles were covered with bio-substances, leading to lower intensities of all peaks.

3.2. Molecular Characterization

To further analyze the chemical bonds and elemental compositions, the three materials coated with E. coli were analyzed using XPS. The survey spectra of bio-coated g-C3N4, ND-g-C3N4, and F-g-C3N4 materials are shown in Figure 2b and the intensity versus binding energy plots for C1s and N1s are shown in Figure 3. Signals of C, N, O, Na, and Cl elements were recorded for bio-coated photocatalysts, while the signal of F was hardly seen for bio-coated F-g-C3N4 due to the much smaller amount of F dopants compared to other elements. The atomic rates of carbon and nitrogen in g-C3N4, ND-g-C3N4, and F-g-C3N4 can be seen in Table S1, and compared with pristine g-C3N4, the nitrogen rate decreased by 4% in ND-g-C3N4, which confirmed the formation of nitrogen defects in ND-g-C3N4. Sources of Na and Cl could be both bio-substances and the adsorbed LB medium, which contained 5 g/L NaCl to support E. coli cultivation. C1s high-resolution spectra in Figure 3 indicated that signals for C consisted of three peaks with binding energies of 284.6, 286.1, and 287.9 eV, which were denoted to graphitic C=C bonds, the sp2 C atoms bonded to N inside the aromatic structure (C-N), and the sp2 C atoms in the aromatic rings (C-NH2), respectively. Contrary to previous publications, more C=C bonds were observed in bio-coated g-C3N4, ND-g-C3N4, and F-g-C3N4 than in C-NH2. This phenomenon could be ascribed to the adsorption of E. coli cells and EPS onto the surfaces of catalysts, which contained numerous C=C structures. The N1s high-resolution spectra of bio-coated g-C3N4 in Figure 3 demonstrated that N signals could be recognized as a single peak situated at 399.50 eV, corresponding to tricoordinate (N3C) nitrogen atoms. It could be assumed that N-C was the chemical bond formed between g-C3N4 materials and E. coli bio-substances.
Chemical structures of bulk bio-coated photocatalysts were confirmed using FTIR. As shown in Figure 2c, the broad peaks between 3600 and 3100 cm−1 represented N-H stretches due to partial hydrogenation of some nitrogen atoms and -NH- in bio-substances, respectively. Furthermore, O-H stretches of attached bio-substances and water overlapped with the N-H signals. The peak at 3280 cm−1 represented C-H stretching vibration, and the difference between the three samples originated from more bio-substances on g-C3N4 than F-g-C3N4, followed by ND-g-C3N4. The peak around 3040 cm−1 represented unsaturated C-H bonds, and multi-peaks ranging from 3000 to 2800 cm−1 came from saturated C-H bonds. The signal of C-H bonds confirmed the existence of bio-coatings and the intensities were consistent with SEM results, demonstrating that E. coli cultivation was strongly inhibited by ND-g-C3N4 photocatalysts. Peaks at 1610 cm−1 were assigned to C-N, while peaks around 1530 cm−1 were the unique signals of aromatic compounds. Furthermore, signals between 1250 cm−1 and 1100 cm−1 might come from C-O or C-OH from attached bio-substances.
UV-vis absorbance spectra of pristine and bio-coated g-C3N4, ND-g-C3N4, and F-g-C3N4 are depicted in Figure 4. As seen in all curves, pure and modified g-C3N4, as well as their bio-hybrids, exhibited similar absorbance in the UV–visible range except a slight peak between 250 and 290 nm, probably implying the presence of adsorbed LB medium and attached E. coli cells in bio-coated g-C3N4 materials. Additional peaks between 300 and 350 nm observed for both uncoated and coated ND-g-C3N4 could be the evidence responsible for the formation of an extra defect band below the conductive band of g-C3N4 materials due to the introduction of nitrogen defects through alkali pretreatment of urea [38]. Enhanced absorbance of UV and visible irradiation for g-C3N4 and F-g-C3N4 convinced the positive role of bio-substances, while decreased visible absorbance for ND-g-C3N4 after bio-coating resulted from the interaction between bio-substances and nitrogen defects. Slight red-shifts were shown in the presence of bacterium cells for g-C3N4 and F-g-C3N4; however, the spectra of ND-g-C3N4 were blue-shifted. Red-shifts predominantly originated from electronic interaction between surface photoactivation and microbial metabolisms, and E. coli substances attached provided a lower band for g-C3N4 and F-g-C3N4 catalysts. However, the blue-shift of bio-coated ND-g-C3N4 originating from partially destroyed surface nitrogen defects was probably due to heterojunctions formed on activated N atoms. Tauc plots were applied to analyze band-gaps of bio-coated photocatalysts. Calculated bandgaps of pristine g-C3N4, ND-g-C3N4, and F-g-C3N4 materials were 2.68, 2.27, and 2.76 eV, respectively, while these values altered to 2.46, 2.51, and 2.48 eV after bio-coating. Multiple electron transfer paths due to E. coli substances made these bio-hybrids more efficient in harvesting light energy, which was crucial in photocatalytic applications.

3.3. Electrochemical Impedance Spectroscopy (EIS) Analysis

To better understand the electron transfer properties of the synthesized materials, EIS analysis of pristine and E. coli-coated materials was investigated. EIS spectra presented as Nyquist plots were widely applied to investigate the interfacial charge transfer processes occurring in photocatalytic reactions. As shown in Figure 5, the radius of semicircle arcs was reduced with the incorporation of E. coli cells, which demonstrated that the electronic conductivity of g-C3N4-based photocatalysts was facilitated through bio-coating engineering. The strongest effect of E. coli was seen in ND-g-C3N4, which was consistent with UV-vis spectra results showing a large blue-shift that originated from nitrogen defects destruction by microbial aggregates. The least change was witnessed in F-g-C3N4 due to highly electronegative F atoms serving as a sink of photo-activated electrons. EIS spectra were simulated based on the R(CR) model using ZSimWin software (ZSimpWin 3.30), and the charge transfer resistances (Rct) for pristine g-C3N4, ND-g-C3N4, and F-g-C3N4 with adsorbed LB medium were calculated to be 220.8, 425, and 239 kΩ, respectively. However, these values decreased to 66.8, 55, and 194 kΩ for materials with E. coli coatings, respectively. Simulation results confirmed that E. coli coating could strongly increase the electronic conductivity of g-C3N4-based materials. In comparison, ND-g-C3N4 was the most sensitive material towards bio-modification in electron-transfer efficiency while F-g-C3N4 was the least. Consequently, electron transfer within g-C3N4-based photocatalysts was accelerated by attached bio-substances through EET, and surface properties had a pronounced effect on electronic interaction between photocatalytic materials and bio-substances.

3.4. Photocatalytic Performance

Nitrobenzene degradation performance by pristine and bio-coated g-C3N4, ND-g-C3N4, and F-g-C3N4 was plotted in Figure 6. Zeta potentials of bio-coated g-C3N4-based materials were measured in advance to determine the optimum conditions for photocatalytic degradation experiments and the results were plotted in Figure S5. Nitrobenzene removal was clearly strengthened for all bio-coated photocatalysts compared to pristine materials. More than 60% nitrobenzene was decomposed within 6 h by E coli-coated ND-g-C3N4, while the value was less than 30% for pristine g-C3N4. After calculation, degradation rates were observed to follow the order g-C3N4 < F-g-C3N4 < ND-g-C3N4 < E. coli-coated g-C3N4 < E. coli-coated F-g-C3N4 < E. coli-coated ND-g-C3N4. Previous studies have confirmed that F-atom doping could increase the amount of surface-bound hydroxyl groups, then transformed into hydroxyl species upon irradiation by visible light [22]. Moreover, N-defects lowered the conduction band of ND-g-C3N4 materials, and bio-substances also led to lower conductive bands compared to original materials and served as electron traps. According to previous research, nitrobenzene photocatalytic removal followed first-order kinetics [39], and the reaction rate constant k was calculated and listed in Table 1. The results demonstrated that k-values for bio-hybrids were twice as high as pristine photocatalysts, and the highest value of 0.0023 min−1 made bio-coated ND-g-C3N4 (5:5) the most efficient bio-hybrid for nitrobenzene degradation.
Effects of E. coli loadings were also investigated, and nitrobenzene degradation rates increased with the growing addition of E. coli storing solution during bio-hybrid synthesis (Table 1). Heterogenous catalysis comprises both adsorption and reaction stages, which made the adsorption rate an important parameter in the photocatalytic degradation of nitrobenzene by bio-coated g-C3N4 materials [39]. Adsorption ratios of nitrobenzene on pristine and bio-coated g-C3N4, ND-g-C3N4, and F-g-C3N4 are listed in Table 2. Nitrobenzene adsorption was enhanced with bio-substance loading and bio-coated g-C3N4 ranked the highest, which was attributed to the compromise of surface hydrophilicity and adsorption-site competition. Stable tests were conducted through six cycles of photocatalytic nitrobenzene degradation experiments under visible light. Nitrobenzene degradation decreased to around 60% of the first cycle performance for all three bio-coated g-C3N4-based photocatalysts (see Figure S7). SEM pictures of three photocatalysts after the six-cycle experiments indicated few differences compared to the original states, which demonstrated great stabilities of bio-coated g-C3N4-based photocatalysts (see Figure S8).

3.4.1. Role of Dissolved Oxygen

Dissolved oxygen played a vital role in photocatalytic procedures due to its participation in the generation of ROS, which were determined as important intermediates for nitrobenzene decomposition [40,41,42,43]. Figure 7a shows an increase of 12% in nitrobenzene degradation by E. coli-coated g-C3N4 under visible-light irradiation in an anaerobic environment, which might be attributed to direct nitrobenzene reduction by photoactivated electrons [44]. Bio-substances not only served as absorbents for nitrobenzene but also as a conductive material intimately contacted with the photocatalysts, providing reaction sites for photo-generated electrons and adsorbed nitrobenzene molecules. In addition, anaerobic conditions also favored the viability of E. coli cells in the presence of abundant LB medium. The enhancement of bioactivity was proven by 15% more nitrobenzene residual detected after a 6 h reaction when bio-coated g-C3N4 materials were pretreated by UV irradiation for 30 min. Aniline production was also detected after a 6 h photocatalytic reaction under visible-light irradiation, and its yield (aniline concentration/original nitrobenzene concentration) is depicted in Figure 7b. More aniline was produced by E. coli-coated g-C3N4 in anaerobic conditions, and the aniline production amount was ordered as ND-g-C3N4 < g-C3N4 < F-g-C3N4, which agreed with attached amounts of E. coli substances on photocatalysts. Consequently, adhered E. coli could not only reduce electron–hole recombination due to extracellular electron transfer paths through pilins and cytochromes, which were more conductive than g-C3N4-based photocatalysts but also promote nitrobenzene reduction into aniline by providing reduction sites.

3.4.2. Reactive Oxygen Species Generation

Free radicals such as hydroxyl radicals (HO•) are extremely unstable, thus spin traps (nitroxides, e.g., DMPO) are used to stabilize them temporarily in order to obtain ESR spectra by forming DMPO-OH adducts. Figure 8 shows typical signals of DMPO-OH spin adducts (1:2:2:1 quartets) in both pristine and E. coli-coated g-C3N4, ND-g-C3N4, and F-g-C3N4. Quantitative analysis of ESR peak intensities revealed that more hydroxyl radicals were generated by g-C3N4 materials incorporated with E. coli, and similar results were also observed for ND-g-C3N4 and F-g-C3N4 in Figure S3. The reason might be as follows: the electron–hole recombination could be reduced due to highly conductive bio-substances, which could also quickly convey photo-activated electrons away from holes. In addition, both bio-coated ND-g-C3N4 and F-g-C3N4 showed an enhanced generation of HO• species compared to g-C3N4, which was attributed to the lower conduction band of ND-g-C3N4 extending photo-energy harvesting and more adsorbed OH species on F-g-C3N4 surfaces prior to photo-activation. Interestingly, many more hydroxyl radicals were generated for pristine F-g-C3N4 than ND-g-C3N4, while differences disappeared after bio-coating treatments (Figure S4). The reason could be ascribed to the competition for photo-generated electrons between bio-substances and dissolved oxygen. Since more nitrobenzene was degraded by pure ND-g-C3N4 than F-g-C3N4, both superoxide and HO• radicals could react with nitrobenzene. However, similar degradation results were observed after bio-modification for ND-g-C3N4, which indicated that hydroxyl radicals were the main reactive oxygen species in nitrobenzene degradation by E. coli-coated g-C3N4 materials. ROS scavenger experiments also demonstrated that hydroxyl radicals were the leading motivation for nitrobenzene degradation (Figure S6).

4. Discussion

Nitrobenzene hydrogenation to aniline was previously reported to occur on the surface of metal nanoparticles such as Ag, Au, and Pt, etc., where hydrogen atoms were dissociated [45,46,47]. Azobenzene was mostly oxidized by photo-generated ROS on the surface of g-C3N4. Furthermore, enhanced microbial reduction of nitrobenzene into aniline in previous publications indicated that electrons were conveyed from nitrobenzene to bacteria through reduced graphene oxides [1,48]. Therefore, a possible mechanism for photocatalytic degradation of nitrobenzene by E. coli-coated g-C3N4 could be inferred. As shown in Figure 9, photo-activated electrons were efficiently transferred to bio-substances where nitrobenzene was concentrated, and adsorbed nitrobenzene molecules could be directly reduced into aniline by photo-activated electrons. Free electrons could also be consumed by the E. coli metabolisms as an electron donor in microbial reduction processes, making attached bio-substance electron traps in these photocatalysts. Separated holes would react with OH−, generating highly reactive hydroxyl radicals. Dissolved oxygen might compete with bio-substances for photo-activated electrons. Synergetic mechanisms of coupled photocatalysis and biological reactions for nitrobenzene removal could be solid ground for the application of photocatalyst–bacteria hybrids in other fields such as nitrate reduction and sludge digestion.

5. Conclusions

Eco-friendly and sustainable methodologies to apply innovative materials in wastewater treatment and polluted water remediation have been catching increasing attention in research concerning sustainability in rivers and lakes. In this work, synthesized g-C3N4, ND-g-C3N4, and F-g-C3N4 materials were incorporated with Escherichia coli MG 1655 substances and systematically characterized using various spectroscopic techniques to demonstrate a sustainable pathway to apply chemical stresses such as photocatalytic activities in water environments. Morphologic results indicated inhibition effects of ND-g-C3N4 on E. coli cultivation, and EPS generation due to direct contacts between E. coli cells and inorganic photocatalysts. The crystallinity of nano-photocatalysts was reduced by the adsorbed LB medium and recovered in the presence of E. coli cells. Owing to surface hydrophilicity, less LB medium and more E. coli cells were coated on F-g-C3N4 materials. Bio-coatings were confirmed by XPS techniques and C-N bonds might be the bridge between g-C3N4 photocatalysts, E. coli cells, and extracellular substances. A red-shift of UV–vis absorbance was witnessed for both bio-coated g-C3N4 and F-g-C3N4 materials, while a slight blue-shift was observed for ND-g-C3N4, which might be attributed to the destruction of nitrogen defects. Extracellular electron transfer of E. coli substances reduced electron–hole recombination and provided reduction sites during the photocatalytic degradation of nitrobenzene. ROS quantitative analysis confirmed that hydroxyl radicals were the reactive species when E. coli-coated g-C3N4 was applied to degrade nitrobenzene in water solutions. Electron transfer between inorganic photocatalytic and biological processes was indicated to be determined in hybridized systems with chemical stress and bio-activities. Future work should focus on the incorporation of graphic carbon nitride with an as-known biological process such as nitrate reduction, where bio-generated reductive electrons could be replaced by other electron sources to clearly figure out additional electron paths and interaction mechanisms, which could encourage both practical photocatalytic applications in wastewater treatment and potential applications in medical fields.

Supplementary Materials

The following are available online at https://www.mdpi.com/article/10.3390/su152316372/s1.

Author Contributions

Conceptualization, Y.W. and Y.L.; methodology, Y.W. and L.W.; software, W.Z.; validation, L.W. and T.B.; formal analysis, T.B.; investigation, Y.W.; resources, Y.L.; data curation, L.W.; writing—original draft preparation, Y.W.; writing—review and editing, T.B.; visualization, Y.L.; supervision, Y.L.; project administration, W.Z.; funding acquisition, Y.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Special Fund for Carbon Peak and Carbon Neutrality Science and Technology Innovation of Jiangsu Province in 2022, grant number BE2022601; the National Key R&D Program of China, grant number 2019YFC0408301; the Foundation for Innovative Research Groups of the National Natural Science Foundation of China, grant number 51421006; the Six Talent Peaks Project in Jiangsu Province, grant number 2016-JNHB-007; the 333 High Level Talents Training Project of Jiangsu Province; and the Fundamental Research Funds for the Central Universities, grant number 2018B47814.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author. The data are not publicly available due to privacy or ethical restriction.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Scanning electron microscopy (SEM) pictures of bio-coated g-C3N4 (a), ND-g-C3N4 (b), and F-g-C3N4 (c).
Figure 1. Scanning electron microscopy (SEM) pictures of bio-coated g-C3N4 (a), ND-g-C3N4 (b), and F-g-C3N4 (c).
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Figure 2. (a) X-ray diffraction (XRD), (b) X-ray photoelectron spectroscopy (XPS), and (c) Fourier transform infrared (FTIR) spectra of pristine and bio-coated g-C3N4 (black), ND-g-C3N4 (red), and F-g-C3N4 (blue).
Figure 2. (a) X-ray diffraction (XRD), (b) X-ray photoelectron spectroscopy (XPS), and (c) Fourier transform infrared (FTIR) spectra of pristine and bio-coated g-C3N4 (black), ND-g-C3N4 (red), and F-g-C3N4 (blue).
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Figure 3. C1s and N1s high-resolution (XPS) spectra of bio-coated g-C3N4, ND-g-C3N4, and F-g-C3N4.
Figure 3. C1s and N1s high-resolution (XPS) spectra of bio-coated g-C3N4, ND-g-C3N4, and F-g-C3N4.
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Figure 4. Ultraviolet–visible spectroscopy (left) and tauc plots (right) of pristine (black lines) and bio-coated (red lines) g-C3N4, ND-g-C3N4, and F-g-C3N4.
Figure 4. Ultraviolet–visible spectroscopy (left) and tauc plots (right) of pristine (black lines) and bio-coated (red lines) g-C3N4, ND-g-C3N4, and F-g-C3N4.
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Figure 5. Electrochemical impedance spectra (EIS) within specific setups of pristine and bio-coated g-C3N4, ND-g-C3N4, and F-g-C3N4.
Figure 5. Electrochemical impedance spectra (EIS) within specific setups of pristine and bio-coated g-C3N4, ND-g-C3N4, and F-g-C3N4.
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Figure 6. Photocatalytic reduction of nitrobenzene by bio-coated g-C3N4, ND-g-C3N4, and F-g-C3N4.
Figure 6. Photocatalytic reduction of nitrobenzene by bio-coated g-C3N4, ND-g-C3N4, and F-g-C3N4.
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Figure 7. Roles of biotic activities and dissolved oxygen on photocatalytic degradation of nitrobenzene. (a) Nitrogen removal and (b) aniline production after 6 h experiment.
Figure 7. Roles of biotic activities and dissolved oxygen on photocatalytic degradation of nitrobenzene. (a) Nitrogen removal and (b) aniline production after 6 h experiment.
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Figure 8. ROS measurements of pristine (black line) and E. coli-coated (red line) g-C3N4, ND-g-C3N4, and F-g-C3N4.
Figure 8. ROS measurements of pristine (black line) and E. coli-coated (red line) g-C3N4, ND-g-C3N4, and F-g-C3N4.
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Figure 9. Schematic mechanisms of photocatalytic degradation by bio-coated g-C3N4-based photocatalysts.
Figure 9. Schematic mechanisms of photocatalytic degradation by bio-coated g-C3N4-based photocatalysts.
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Table 1. Reaction constant (k) values of nitrobenzene photocatalytic degradation by bio-coated g-C3N4, ND-g-C3N4, and F-g-C3N4.
Table 1. Reaction constant (k) values of nitrobenzene photocatalytic degradation by bio-coated g-C3N4, ND-g-C3N4, and F-g-C3N4.
Reaction Constant (k) Value
5:5 *4:6 *3:7 *2:8 *1:9 *Pristine
g-C3N40.0020.0018330.0017140.00150.0011110.0009
ND-g-C3N40.00230.002050.00158570.001350.00145560.0012
F-g-C3N40.00220.00200.00140.120.0010.001
* The ratios of stocking solution of E. coli and photocatalysts for synthesized bio-coated photocatalysts.
Table 2. Adsorption rates of nitrobenzene by bio-coated g-C3N4, ND-g-C3N4, and F-g-C3N4.
Table 2. Adsorption rates of nitrobenzene by bio-coated g-C3N4, ND-g-C3N4, and F-g-C3N4.
Adsorption Rate Value
5:5 *4:6 *3:7 *2:8 *1:9 *Pristine
g-C3N40.5280.3630.3390.3070.3120.250
ND-g-C3N40.4770.4330.3840.3380.2790.235
F-g-C3N40.5170.4510.3690.3290.2950.243
* The ratios of stocking solution of E. coli and photocatalysts for synthesized bio-coated photocatalysts.
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Wang, Y.; Li, Y.; Wang, L.; Zhang, W.; Bürgi, T. Bio-Coated Graphitic Carbon Nitrides for Enhanced Nitrobenzene Degradation: Roles of Extracellular Electron Transfer. Sustainability 2023, 15, 16372. https://doi.org/10.3390/su152316372

AMA Style

Wang Y, Li Y, Wang L, Zhang W, Bürgi T. Bio-Coated Graphitic Carbon Nitrides for Enhanced Nitrobenzene Degradation: Roles of Extracellular Electron Transfer. Sustainability. 2023; 15(23):16372. https://doi.org/10.3390/su152316372

Chicago/Turabian Style

Wang, Yuming, Yi Li, Longfei Wang, Wenlong Zhang, and Thomas Bürgi. 2023. "Bio-Coated Graphitic Carbon Nitrides for Enhanced Nitrobenzene Degradation: Roles of Extracellular Electron Transfer" Sustainability 15, no. 23: 16372. https://doi.org/10.3390/su152316372

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