Preparation of AuNP‑CQD/PDA/GO anode for MFC and its treatment of oily sewage from ships

Oily sewage discharged from ships has brought many harms to the marine environment, even endangered marine life and human life. As a new type of water treatment technology, microbial fuel cell (MFC) can efficiently treat pollutants and recover energy, which can be converted into electric energy. However, its large internal resistance restricts its development. In order to solve the problems of low power generation performance and poor biocompatibility of microbial fuel cell, a gold nanoparticle-carbon quantum dot/polydopamine/graphene oxide/bacterial cellulose (AuNP-CQD/PDA/GO/BC) electrode was prepared, and it was applied to the treatment of oily sewage from ships. Fourier transforms infrared spectroscopy, X-ray diffraction, scanning electron microscopy, gas chromatography-mass spectrometry, and contact angle measuring instrument were used to characterize the electrode. The results show that PDA bridges GO and AuNP-CQD particles through the electrostatic interaction/π-π bond/hydrogen bonding, respectively. This attracts a large number of microorganisms to attach to the surface of the porous anode material, which greatly improves the activity and quantity of microorganisms. Moreover, the maximum power density of AuNP-CQD/PDA/GO/BC electrode is 2624.91 mW/m 2 , which obviously improves the electrochemical performance of MFC. The oil content of the treated water is ≤ 15 mg/L, reaching the discharge of MARPOL 73/78 convention. Therefore, the proposed approach has paved new dimensions in not only the preparation of a new composite electrode materials but also its applications as effective degradation of ship oily sewage in MFC.

With the continuous development of global ocean development and maritime transportation, oil spill accidents at sea occur frequently, which seriously pollute the marine ecological environment and cause immeasurable economic losses (Sabikoglu 2021;Nastasi et al. 2022). Energy crisis and seawater pollution are the main challenges facing the development of the world (Wei et al. 2021;Tang et al. 2022), while traditional energyintensive sewage treatment technology can no longer meet the needs of sustainable development. How to solve the problems of energy development and utilization and seawater pollution has become a hot topic.
The microbial fuel cell (MFC) can produce clean electricity while treating organic pollutants. It is a brand-new wastewater biological treatment technology with energy recovery (Danilczuk et al. 2013;Zhang et al. 2022;Vidhyeswari et al. 2022;Raychaudhuri and Behera 2022). However, the low power output, poor biocompatibility, and slow interfacial electron transfer from microbe to anode limit the large-scale application of MFC (Elmaadawy et al. 2022;Long et al. 2019). To overcome these challenges, the anode material was modified to improve its ability to attach to microorganisms and transfer extracellular electrons (Yotprayoonsak et al. 2022;Li et al. 2021). For example, a p-PAPBA/CC electrode was prepared by electropolymerization of 3-aminophenylboric acid on a carbon cloth modified by SiO 2 (Zhao et al. 2020). And its maximum power density was 928 ± 20 mW/m 2 . PANI-SA/CB hydrogel was prepared as MFC anode to treat sodium acetate wastewater (Wang et al. 2020). The maximum power density was 515 mW/m 2 . A rGO/PANI/Pt composite as the anode catalyst was prepared by Kumar, which exhibited a maximum MFC power density of 2059 mW/m 2 with concrete life durability (Kumar et al. 2017). These MFCs are widely used in the treatment of dye wastewater and heavy metal pollution.
Moreover, there is little research on MFC using oil spill as fuel. Jeffrey first reported a single-cell microbial fuel cell constructed from diesel simulated wastewater in 2007 (Morris and Jin 2007; Morris et al. 2009;Kondee et al. 2022). It was found that the fuel cell could bioremediation oily pollutants in groundwater under anaerobic conditions. However, the hydrophobic groups on the surface of electrode are not conducive to the adhesion of electricity producing bacteria, and their biocompatibility is poor, which limits its application in marine oil spill pollution. Therefore, it is expected to improve the electrochemical performance of MFC by preparing new materials with good biocompatibility (Yang et al. 2021;Radeef and Ismail 2022).
Therefore, this paper attempts to prepare a gold nanoparticle-carbon quantum dot/polydopamine/graphene oxide/ bacterial cellulose (AuNP-CQD/PDA/GO/BC) electrode. The electrode has good biocompatibility and high electrical performance, and can be used for the treatment of marine oil spill. Moreover, constructs a dual-chamber microbial fuel cell and investigates the power generation characteristics and oil-polluted water degradation performance of the cell when it is used to treat marine oil spill pollution. Furthermore, the treatment of marine oil spill pollution and the recycling of waste are realized.

Preparation of AuNP-CQD composite
The anhydrous citric acid was used as carbon source, and 2 g of anhydrous citric acid was weighed into an evaporation dish. Then, it was heated to 180 ℃ with an electric heating jacket and melted for 30 min. Next, the molten liquid is slowly added to 150 mL NaOH (10 g/L) solution under stirring. Adjust pH to 7.0 with HCl solution, and add 2.125 g NaNO 3 to dissolve it. Finally, an orange CQD solution is obtained.
Ten milliliters of HAuCl 4 solution (4.0 × 10 −4 mol/L, Shanghai Meiruier Chemical Technology Co., Ltd., China) and 10 ml of CQDS (0.2 g/L) solution were mixed in a beaker, magnetically stirring for 20 min, then heated in a microwave synthesizer at 200 ℃ for 30 min. And cooling to room temperature to obtain the AuNP-CQD brownishyellow compound solution.

Preparation of AuNP-CQD/PDA/GO modified biochar electrode
In this paper, the biochar electrode was prepared by laboratory firing of waste walnut shells.
Prepare graphene oxide (GO, 0.15 mg/mL; Shenzhen Hongdachang Plan Co., Ltd.) solution. Using an electrochemical workstation with biochar as a working electrode, working at a positive current of 0.4 mA/cm −2 for 360 s, the negatively charged GO molecules were electrophoresed on the surface of biochar, then run at negative current of 0.7 mA/cm −2 for 120 s. After the above working cycle for 10 times, the electrode was taken down and dried at room temperature to obtain the GO/BC electrode.
AuNP-CQD complex solution (1.5 ml), 10 mg of dopamine (Aladdin, China), and 5 ml of KMnO 4 solution (0.1 mol/L) were added to 50 mL of deionized water, and uniformly dispersing the mixed solution by ultrasonic for 3 h. The GO/BC electrode was completely immersed in the mixed solution, and placed in a refrigerator at 4 °C for 24 h. After the electrode was taken out, it was washed several times with distilled water and ethanol, and dried at room temperature to obtain AuNP-CQD/PDA/GO modified biochar (AuNP-CQD/PDA/GO/BC) composite electrode.

MFC battery assemble and operation
The H-type MFC box, a proton exchange membrane (Nafion117, DuPont), and the data acquisition card (DAQM4206I) were used in the MFC. The reference electrodes were made of graphite felt with a size of 1 × 1 cm 2 (thickness 4 mm, bulk density 0.10 g/cm 3 , carbon content 95%), biochar, and AuNP-CQD/PDA/GO/BC electrode. All cathodes were made of graphite felt. The anode and cathode were fixed at the ends of both chambers to ensure that the distance between the anode and cathode was the same in all MFCs.
In order to reduce the error caused by accidental factors, five parallel experiments were conducted, and the oil degradation rate and electrochemical performance of the stabilized battery were tested.

Material characterization and performance test
The infrared spectrometer (FTIR, FTIR-Affinity-1S of Shimadzu, Japan) and X-ray diffractometer (XRD, Bruker D8 advance) were used to analyze the structural change of electrode materials. The microstructure of the anode materials was analyzed by scanning electron microscope (SEM). Contact angle tester was used to observe the wettability and hydrophilicity of anode materials. Ultraviolet spectrophotometer (UV-1800, Shimadzu, Japan) was used to draw a standard curve based on the absorbance value at the optimum absorption wavelength. The residual oil of MFC anode was extracted, and the sterile blank group was used as the control group. The ability of degrading petroleum hydrocarbon components was analyzed by GC/MS. The output voltage and power density of MFC were measured by data acquisition card and electrochemical workstation.

Oil suction and oil retention test
Weighing anode material was recorded as m 1 . Put it into 150 mL oily sewage and keep it rotating for 10 min, then clamp it out with tweezers and let it stand for 30 s to remove water and oil from the surface of the material, and weighed and recorded as m 2 . Next, the tray containing the materials was tilted at 30°, and allowed to stand for 10 min, weighed, and recorded as m 3 . Finally, according to Eqs. (1) and (2), the oil retention rate (Q) and the oil absorption rate (K) are calculated.

Standard curve drawing
Petroleum ether is used to extract and dilute ship oily sewage, and oil standard solution with mass concentration of 10, 40, 50, 70, 80, and 90 mg/L is prepared. The sample was scanned at full ultraviolet wavelength, and there was an obvious absorption peak at the wavelength of 255 nm. Draw the standard curve (as shown in Fig. 1). The standard curve equation is Y = 0.01166X + 0.43585, and the linear correlation coefficient R 2 = 0.9995. It shows that the fitting degree is high, which is suitable for analyzing the calculation of petroleum pollution degradation rate.

Degradation rate test
The sample was placed in a 10,000 r/min centrifuge. After centrifugation for 10 min, the supernatant was extracted with petroleum ether. Then, the sample was tested by UV full-wavelength scanning. Combined with the crude oil standard curve formula, the biodegradation rate of oil sewage was measured.
Among then, is degradation rate (%); C 0 is blank group oil concentration (mg/L), and C 1 is sample oil concentration (mg/L). Figure 2 shows the infrared spectra of two electrode materials, biochar, and AuNP-CQD/PDA/GO/BC.  The stretching vibration corresponding to O-H at 2357 cm −1 is the hydroxyl group. It was formed by the hydrogen bond during the synthesis process. The strong absorption peak of the para-substituted benzene ring of the electron-donating group is at 1610 cm −1 . It indicates that there is π-π interaction among AuNP-CQD/PDA/GO in the synthesis process. 1300 cm −1 and 1000 cm −1 are the vibration peaks of para-substituted benzene ring, C-O and C-O-C, respectively. This indicated the existence of oxygen-containing functional groups in the composites. According to infrared spectrum analysis, the composite electrode material has more hydrophilic groups than biochar. Figure 3 shows the XRD pattern of the two electrode materials. In the curve 3a, the X-ray diffraction peak of the biochar appeared at the vicinity of 2θ = 25.6° and 51.9°. The characteristic diffraction peak of biochar in curve b did not disappear. Meanwhile, 2θ = 36.4° and 44.5° are the characteristic absorption peaks on the (111) and (200) crystal planes of AuNPs. Combined with the FT-IR spectrum, it can be concluded that the structure of biochar was not destroyed during the preparation of the composite electrode material. Moreover, the CQD-AuNP was uniformly attached to the surface of biochar.

Characterization of AuNP-CQD/PDA/GO/BC electrode
It can be seen from the SEM in Fig. 4 that compared with biochar ( Fig. 4a), it has a porous microstructure with higher surface flatness.
The AuNP-CQD/PDA/GO/BC electrode material (Fig. 4b) not only retains the original porous structure but also makes the surface rougher due to nanotube loading, increasing the number of pores and specific surface area. It is expected to improve its adsorption capacity and provide a good living environment for microorganisms.
The surface wettability of MFC anode material affects the difficulty of microbial attachment. Therefore, the contact angle of the two electrode materials was used to compare the wettability, and the results are shown in Fig. 5. The contact angles θ of biochar and AuNP-CQD/PDA/GO/ BC were 7.740° and 2.347°, respectively.
Combined with the infrared spectrum analysis, the composite electrode material contains many hydrophilic groups, such as -OH and -CO-. This makes the wettability of the electrode much better than that of the biochar electrode. As well, more hydrophilic groups can be connected with negatively charged electricity-producing bacteria through Van der Waals force and electrostatic force. This is more conducive to the attachment of microorganisms to the composite electrode material, and improves the number and activity of electricity-producing bacteria. It is beneficial to improve the power generation performance of MFCs.

Cyclic voltammetry test
Cyclic voltammetry (CV) is one of the important methods for detecting the electron transfer efficiency and electrochemical activity of an electrode. As shown in Fig. 6, all three CV curves showed obvious redox peaks, and the curve oxidation current increased with the increase of potential. The maximum peak current output is in the order of AuNP-CQD/PDA/GO/BC anode > biochar anode > GF anode. This indicates that the composite anode has good electrochemical reversibility.
The reason was that after modification by AuNPs-CQDs, there was no inhibition of induced current, and the obtained electric double-layer current was higher, thus confirming the successful deposition. As a high conductivity and high specific surface area material, CQDs can significantly increase the electrode surface area (Han et al. 2021;Wang et al. 2022;Youh et al. 2021). The addition of graphene oxide can reduce the charge transfer resistance and accelerate electron transfer in the electrode material. At the same time, PDA has good dispersibility, which can avoid damage to microorganisms and improve the biocompatibility of the composite anode.

Electrode antipolarization test
As can be seen from Fig. 7, the output voltage of the MFC gradually decreases with the increase of current density, and the AuNP-CQD/PDA/GO/BC anode is the smallest. The output voltage of the GF anode drops sharply in the activated polarization region of 0-2100 mA/m 2 , indicating that the anode has been polarized. However, the output voltage attenuation of the AuNP-CQD/PDA/GO/BC anode was small, and the curve shows a linear relationship, which indicates that there is no polarization in the composite electrode. It shows that under the same conditions, the total internal resistance of the three anodes is GF > biochar > composite anode, and the anti-polarization ability of the composite anode is stronger. This is due to the porous surface and high specific surface area of the composite-modified electrode, which can adsorb more microorganisms. Furthermore, the modification of PDA can increase the hydrophilicity of the electrode surface, introduce positive charge into the electrode surface, promote electrostatic interaction with the negative charge of microorganisms, and further increase the adhesion and adhesion of microorganisms. Moreover, these can increase the supply rate and electron transfer rate of redox substances and increase the current density and conductivity, thus increasing the power generation of MFC.

Output voltage test
As can be seen from Fig. 8, the output voltage of MFC rises rapidly at the beginning of the work. Then, the output voltage is stable, and finally slowly decreases. This is due to the series voltage division of the external resistor. The MFC output voltage of the biochar anode is higher than that of the GF anode. The output voltage of the AuNP-CQD/PDA/GO/BC anode is the largest, with a voltage value of 757.14 mV. Moreover, the stability time of MFC output voltage is c > b > a.
The voltage output of MFC is mainly affected by the number of microbial loads and internal resistance. Among them, the output voltage of the AuNP-CQD/PDA/GO/BC anode is greatly increased, which indicates that the composite material could effectively utilize its good adsorption performance and high biocompatibility to increase the number of microorganisms on the anode. Furthermore, AuNPs-CQDs are uniformly loaded on the electrode, so that electrons can be transferred quickly, and the resistance can be reduced, thus having stronger electron output capability.

Power density test
The power density is one of the standards to measure MFC production efficiency. As can be seen in Fig. 9, the maximum power densities for the GF anode, the biochar anode, and the AuNP-CQD/PDA/GO/BC anode are 448.05 mW/ m 2 , 1423.76 mW/m 2 , and 2624.91 mW/m 2 , respectively.
Compared with the biochar anode, the maximum power density of the composite anode is increased by 84.36%. This is because biochar is loaded by AuNPs-CQDs, which increases the number of electrogenic microorganisms attached to its surface. At the same time, there were new hydrogen bond associations on the surface of the composite anode, which built a better conductive network. Thus, it makes up for the shortcoming of low electron transfer efficiency of biochar.
Therefore, the electricity production mechanism of this microorganism may be dominated by direct electron transfer. This is because AuNPs-CQDs and GO can be uniformly adsorbed on the surface of waste walnut biochar after composite modification, which significantly enhances the conductivity of the electrode and increases the effective area of the electrode. At the same time, the introduction of PDA with good dispersibility and biocompatibility increases the hydrophilicity of the electrode surface, introduces positive charge to the surface, and promotes the electrostatic interaction with the negative charge of microorganisms. These comprehensive effects may promote the electro-generating bacteria to adhere to the surface of the composite anode more directly, increase the electron transfer rate, and improve the MFC.

Adsorption properties of anode materials
When treating oily sewage with MFC, if the anode material has excellent oil absorption, the oil can be adsorbed on the anode surface in a short time, which is also beneficial to the startup of microorganisms. The oil absorption properties of different anode materials are shown in Table 1.
As can be seen from Table 1, the composite anode has a higher oil retention rate and oil absorption rate, and it is easier to absorb oil stains. This may be because the composite anode has abundant pores, which makes it easier to

Degraded oily sewage
The degradation rate and COD removal rate of MFC containing different anode materials for oily sewage are shown in Figs. 10 and 11, respectively. Among them, AuNP-CQD/PDA/GO/BC anode has a degradation rate of 88.38% and COD removal rate of 79.89%, respectively, which are higher than those of the MFC with the other two anodes. Combined with the oil absorption test, this is because the AuNP-CQD and GO modified electrodes have good electrocatalytic activity, which increases the specific surface area of the electrode and accelerate the electron transfer rate. At the same time, with the addition of PDA with high biocompatibility, more electro-generating bacteria are attached to the anode, which will accelerate the degradation of organic matter, so the COD removal rate is improved. The more organic substances used in sewage, the better the biodegradability of microorganisms. In addition, the oil content in the degraded solution is 10.8 mg/:, which is lower than the 15 mg/L required by MARPOL 73/78.

Degradation analysis of components of petroleum hydrocarbons
GC/MS was used to analyze the degradation ability of components in oily sewage by MFC with AuNP-CQD/ PDA/GO/BC electrode. The results are as shown in Figs. 12 and 13.
After the calculation, normal alkanes (82.17%), naphthenes (17.51%), and polycyclic aromatic hydrocarbons (0.32%) were detected in oily sewage sample from ship. After 10 days of degradation, more than 88% of normal alkanes were decomposed, which indicated that MFC had a remarkable degradation effect on normal alkanes. It can be used as an effective tool to treat oily sewage from ship.

Conclusions
In order to solve the problem of poor biocompatibility of microbial fuel cells, AuNP-CQD/PDA/GO/BC electrode materials were prepared from waste walnut shell biomass carbon, and a dual-chamber microbial fuel cell was constructed. The electricity generation characteristics and degradation performance of oil-contaminated water were studied. The specific conclusions are as follows: 1) After the composite modification of AuNPs-CQDs, PDA, and GO, the highest stable output voltage of the composite electrode was 757.14 mV. The maximum power density was 2624.91 mW/m 2 , which increased by 84.36% compared with the microbial fuel cell constructed by waste  walnut biochar. The degradation rate of the microbial fuel cell was 88.38%, the COD removal rate was 79.89%, and the oil content of the treated water was 10.8 mg/L, which met the emission requirements of MARPOL 73/78. 2) FT-IR, XRD, SEM, and other characterization analyses showed that AuNPs-CQDs and GO in the composite electrode were uniformly loaded on the surface of biochar, which significantly enhanced the conductivity of the electrode and increased the effective area of the electrode. The introduction of PDA with good dispersibility and biocompatibility increased the hydrophilicity of the electrode surface, introduced positive charges to the surface, and promoted the electrostatic interaction with the negative charges of microorganisms. These combined effects can promote the electricity-producing bacteria to attach more directly to the composite anode surface, increase the electron transfer rate, and improve the electricity production efficiency of MFC. 3) This study provides a new promising option for marine environmental protection, because if this technology is applied on a large scale, a large amount of electricity can be generated while treating oil pollution, realizing the treatment of marine oil spill pollution and the recycling of waste walnut shells.

Declarations
Disclaimer The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.
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Conflict of interest
The authors declare no competing interests.
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