Efficient Organic Pollutant Removal by Bio/MNs Collaborating with Pseudomonas aeruginosa PAO1

: Organic pollution is one of the main sources of environmental pollution, which poses a serious threat to the ecological environment and human health. In this study, we synthesized a composite material consisting of biochar ‐ supported magnetite nanoparticles (Bio/MNs) and collaborated with Pseudomonas aeruginosa PAO1 ( P. aeruginosa PAO1) to conduct a bio ‐ chemical composite remediation approach for organic pollution. The results of the scanning electron microscope (SEM) and X ‐ ray diffractometer (XRD) show that Bio/MNs composites have been prepared successfully. Under light conditions, the highest removal rate of organic pollution by Bio/MNs synergistic P. aeruginosa PAO1 reached 81.5%. Gradient experiments revealed a direct correlation between the removal rate of organic pollution and the dosage of P. aeruginosa PAO1, as well as the input of Bio/MNs, within a specific range. Moreover, due to the positively charged nature of organic pollution, its maximum removal rate reaches 98.6% at pH = 11, exhibiting a 1.76 ‐ fold increase compared to that at pH = 3. The experimental results show that the collaboration between Bio/MNs and P. aeruginosa PAO1 expedited the electron transfer rate and increased the generation of ∙ OH and O − 2 , consequently facilitating the efficient degradation of organic pollutants. These findings inspire Bio/MNs collaborative microorganisms for providing new methods for the green and sustainable removal of organic pollutants.


Introduction
Organic pollution constitutes a primary source of environmental contamination and a significant challenge for humanity [1,2].Furthermore, organic pollution significantly contributes to water contamination, presenting a substantial menace to both the ecological environment and human health [3].Taking methylene blue (MB) as an example, the exposure of individuals to MB can lead to symptoms such as nausea, vomiting, and respiratory distress.Prolonged exposure has the potential to induce carcinogenesis and genetic alterations [4,5].Therefore, the effective removal of organic pollutants, such as MB, stands as an urgent necessity.
Universally, adsorption and degradation emerge as the predominant techniques for the removal of organic pollution [6].Notable degradation approaches encompass photodegradation, ozone treatment, chemical oxidation, and others [7][8][9].These methods have demonstrated significant advancements in the field of MB degradation.Nevertheless, problems endure concerning the efficiency of adsorption, production costs, and dye affinity [7].In recent years, nanomaterials have shown great application prospects in the removal of dyes from wastewater due to their extensive surface area, high adsorption properties, low resistance to diffusion, and faster equilibrium rate [10,11].The researchers successfully prepared new nano silver particles and poly (styryl-nisopropylacrylamide-methacrylic acid) composite material, leading to the proficient adsorption and catalytic reduction of methylene blue [12].Similarly, the photocatalytic performance of organic pollutants can be effectively improved by utilizing nano-scale zinc oxide (nZnO) due to its exceptional sensitivity to ultraviolet light [13].Furthermore, the study indicates that iron oxides hold noteworthy promise in the decomposition of organic substances [14,15].The heterogeneous semiconductor structure between the magnetite nanoparticles (MNPs) and ore particles increases the surface reactivity, resulting in a degradation rate of MB of up to 96% [16].Notably, the semiconductor properties of MNPs also allow the photocatalyst to aggregate and oxidize the solution, increasing the durability of the catalyst and making it more efficient at degrading MB [17].In addition, a BiFeO3/Fe3O4 catalyst was mechanically synthesized, which promoted the formation of electrons and holes due to the change in the band gap under light conditions and achieved the complete degradation of MB in 40 min [3].However, single nanoparticles face the challenges of easy agglomeration, poor uniformity, and poor electrical conductivity, which will limit the degradation of MB [18].Therefore, enhancing the dispersion of nanoparticles can facilitate the separation of electron-hole pairs, thus augmenting their photocatalytic efficiency in dealing with organic pollutants [2].Based on this, it is imperative to identify an effective and cost-efficient dispersant capable of enhancing the degradation of organic pollutants by magnetite nanoparticles.
The biochar, being a highly porous material with abundant sources and a low cost, serves as an exceptional solid support substrate [19].Furthermore, as a carbonaceous material, biochar possesses the ability to effectively sequester organic pollutants [20].It is noteworthy that the exceptional electron supply capacity, electron mediating capacity, and electron acceptance capacity of biochar also contribute to its auxiliary degradation of organic pollutants [21,22].Thus, the combination of biochar and degradable materials can achieve the efficient removal of organic pollutants [23,24].For instance, studies have shown that carboxymethyl cellulose stabilized zinc oxide/biochar nanocomposites with a saturation adsorption capacity of 17.01 g/kg for MB [25].Under ultraviolet irradiation, the removal rate of MB reached 83.1%.In addition, researchers synthesized biochar supported by nano-metallic silver particles through carbothermal synthesis.Because pine biochar promotes the separation of electron-hole pairs and enhances electron transfer, the maximum net degradation of MB reached 15.88 g/kg [26].Moreover, Fe3O4/lignin nanoparticles loaded with biochar were employed for the degradation of MB, achieving a maximum removal efficiency of 621.52 mg/g [27].These composites exhibit significant potential for the degradation of organic pollutants.However, the single composite material limits the continuous production of electrons [28,29].Fortunately, it has been discovered that microorganisms with extracellular electron transport capabilities can generate and transfer electrons to external receptors through the surrounding medium.This novel finding presents a promising avenue for tackling environmental pollutants [30,31].It was found that P. aeruginosa PAO1 is a typical extracellular respiratory bacterium, which interacts with anatase and has a maximum photocurrent density of 37.72 mA/cm 2 [32].In addition, studies have shown that mineral-microbial interactions have some potential in the treatment of organic pollution.The degradation of organic pollutants may be closely related to the microbial reduction of Fe(III) minerals [33,34].Additionally, with the participation of electrogenic microorganisms, structural iron can undergo multiple redox reaction cycles to achieve sustainable reactive oxygen species (ROS) generation and organic degradation [34].
Organic pollution is an important pollutant that affects the ecological environment and human health.In this study, magnetic nanoparticles loaded with corncob biochar are synthesized by chemical methods, and the electroactive microorganism P. aeruginosa PAO1 is introduced as the exogenous electron donor for the first time to explore the degradation of organic pollutants by microbial cooperative composite materials under light conditions.Through microstructure characterization and material composition analysis of the composite, successful loading was further confirmed.Subsequently, by discussing the effect of MB removal under different conditions, a possible mechanism for the P. aeruginosa PAO1 co-degradation of organic pollutants is proposed.The results demonstrate that this method can efficiently remove MB, providing a new approach for the green and sustainable removal of organic pollutants.

Preparation of Biochar and Composites
To prepare corncob biochar, the corncob is washed and air-dried before being pulverized.Subsequently, the corncob was put into a carbonization furnace at 500 °C under oxygen-limited conditions for 2 h.The resulting unaltered corncob biochar is then cooled to the surrounding temperature using a cooler, ground, and subsequently sieved through a 200-mesh steel sieve.It is then subjected to washing with deionized water and 0.1 mol/L HCI, followed by drying at a temperature of 90 °C.The prepared biochar (marked as Bio) was stored in plastic containers for further application.
The preparation method of a corn biochar-loaded magnetite nanoparticles composite is as follows: the utilized reagents are composed of FeSO4•7H2O, FeCI3•6H2O, and KOH solutions.The salts purchased and used in the experiment were of analytical grade, and all chemicals were used without further purification.A 10% solution of FeCl3•6H2O and FeSO4•7H2O was prepared, followed by strict sonication mixing of 8 mL of the FeCl3•6H2O solution and 16 mL of the FeSO4•7H2O solution with 1 g of the corncob biochar for a duration of 10 min [35].The mixture was titrated by 100 mL of 0.15 mol/L KOH solution to prepare a corncob biochar supported by the magnetite nanoparticles (marked as Bio/MNs).Then, the Bio/MNs were filtered by a filter and dried at 100 °C for 36 h.Finally, the prepared Bio/MNs were put in the ultra-clean table (SW-CJ-1FD, Suzhou, China) for ultraviolet sterilization and stored in plastic containers for further application.

Characterization of the Composites
The prepared Bio/MNs were passed through a 400-mesh steel sieve for characterization analysis.The Bio/MNs were identified with an X-ray diffractometer (XRD, Ultima IV, Tokyo, Japan) under Cu kα radiation operating at 40 kV and 40 mA, the morphological features were measured by a scanning electron microscope (SEM, JSM-6510, Tokyo, Japan) that operated at 10 kV, and the working distance was 10 mm.

Cultivation of Microorganisms
P. aeruginosa PAO1 was used in the experiment.A single colony of P. aeruginosa PAO1 was picked up and aerobically cultured in 100 mL of Luria-Bertani no-sodium chloride medium (LBNS: 10 g/L tryptone, 5 g/L yeast extract).Subsequently, it was continuousshock-cultured in an oscillating incubator (ZQPL-200, Tianjin, China) at 35 °C and 150 rpm for 15 h to obtain a P. aeruginosa PAO1-containing suspension.The suspension was then subjected to low-speed intelligent centrifugation (LSC-50H, Wuxi, China) at 4000× g rpm for 10 min.The resulting concentrated solution was washed twice with a phosphatebuffered medium (containing 5 g/L tryptone, 0.25 g/L yeast extract, 19.2 g/L Na2HPO4•12H2O, and 7.8 g/L NaH2PO4•2H2O) at pH 7.0.Finally, the cells were diluted in the phosphate buffer medium to obtain a bacterial concentrate on an ultra-clean table.
In order to complete batch experiments, bacteria were cultured in the phosphatebuffered medium.Tryptone and yeast extract were used as the carbon source for P. aeruginosa PAO1.Notably, these mediums were sterilized using the autoclave (SN510C, Shanghai, China) at 121 °C for 20 min, and all these works were carried out on the ultraclean table.The pH value was measured using a pH meter (PH610, Beijing, China).

Measurement of the Adsorption and Removal of MB
A 50 mg/L MB reserve solution was prepared using a methylene blue reagent for the MB removal experiment, which was conducted in a 100 mL conical bottle, and the conical bottle mouth was sealed with tissue culture sealing film during the experiment.In order to avoid accidental phenomena and reduce experimental errors, the degradation experiment was repeated three times for each group.An external light-emitting diode (LED) with a cooling system served as the light source, with an operating wavelength ranging from 400 to 700 nm and a light intensity of 60 mW/cm 2 (FGH-1, Photoelectric Instrument Factory, Beijing Normal University, Beijing, China).During the experiment, the LED was placed on the side of the conical bottle, about 5 cm away, for irradiation.Before the experiment began, various experimental materials were first put into a conical bottle with MB, and the magnetic stirrer (MS5, Beijing, China) was started to achieve a certain adsorption saturation state.Then, the LED was started to analyze the effects of light exposure, the P. aeruginosa PAO1 injection volume (1-4 mL), the biochar dosage (1-4 g/L), and the pH values of the solutions (2, 4, 6, 8) on MB removal.The concentration of MB was determined using an ultraviolet-visible spectrophotometer (SP-756P, Shanghai, China) at a wavelength of 664 nm.First, standard solutions of MB with concentrations ranging from 2 to 40 μg/mL were prepared, and the standard curve for the MB concentration was established using UV-VIS spectrophotometry.Finally, the absorbance of the solution was measured at different time intervals to determine the concentration of MB.Finally, the data statistics and the drawing of the graph were completed by the drawing software (Origin 2021).

Characterization of Composites
SEM analyses were conducted to investigate the microstructure of the original biochar and Bio/MNs.The morphology and microstructure of both the original corn biochar and the Bio/MNs are depicted in Figure 1.The surface of the raw corn biochar has a smooth texture and contains a small amount of debris while exhibiting an intricate network of channels and pore structures (Figure 1a,b).In contrast, the microstructure of Bio/MNs exhibits irregularity, with scattered magnetite nanoparticles forming clusters and filling in the pores and channels of the biochar (Figure 1c,d).Therefore, drawing from the SEM findings, it can be inferred that the integration of magnetite nanoparticles and biochar was successful.The SEM elemental mapping of the Bio/MNs and biochar is presented in Figure 2a,b, revealing clear observations of C (carbon) and O (oxygen) in both samples, with a uniform distribution indicated by the brightness of the map.Notably, Fe is exclusively found uniformly on the surface of Bio/MNs, reaching a high content of 45%.Furthermore, EDS elemental mapping analysis demonstrated that biochar contains 62.38% C and 37.62% O (Figure 2d).Compared with biochar, the surface carbon content of Bio/MNs is reduced to 41.96% due to the coverage of Fe3O4, while the oxygen and iron contents are increased to 46.14% and 11.9% (Figure 2c), respectively.The experimental results demonstrate the effective loading of Fe3O4 on biochar and the successful preparation of the Bio/MNs complex.The X-ray photoelectron spectroscopy (XPS) analysis of biochar and Bio/MNs (Figure 4) provides new evidence supporting the above conclusion.Figure 4a displays the full spectrum scan of biochar and Bio/MNs, revealing characteristic binding energy peaks of O1s and C1s in both samples at binding energies of 529 eV and 280 eV, respectively.However, only Bio/MNs exhibit characteristic binding energy peaks of Fe2p, with a binding energy around 708 eV (Figure 4a).Notably, the strength of the C1s diffraction peak decreases significantly, while that of the O1s diffraction peak slightly increases after combining biochar with Fe3O4.In addition, the XPS spectrum of Fe2p in Bio/MNs is presented in Figure 4b, illustrating the presence of two distinct forms of iron: Fe (III) and Fe (II).The relative abundances of Fe (III) and Fe (II) were determined to be 67.14% and 32.86%, respectively.Furthermore, the high-resolution Fe2p spectrum of Bio/MNs exhibits prominent peaks corresponding to Fe2p1/2 and Fe2p3/2, with binding energies measured at 725.5 eV and 712.3 eV, respectively.This further confirmed that Fe3O4 was successfully loaded with biochar and changed the elemental composition and content of the original biochar.

Results of MB Degradation by Different Repair Materials
Figure 5 illustrates the impact of various repair materials on MB degradation under light (L) and dark (D) conditions, revealing a significant disparity between single and compound repair agents in terms of their effectiveness.The dosage of P. aeruginosa PAO1 and Bio/MNs in this process is 3 mL and 0.2 g, respectively, while maintaining a pH value of 7. The degradation rate of MB by a single remediation material reached 36.4% at the 8th hour.It is noteworthy that, under identical conditions, the composite (Bio/MNs) with P. aeruginosa PAO1 exhibited an impressive degradation rate of up to 81.5% (Figure 5a).The results demonstrated that the combination of Bio/MNs and P. aeruginosa PAO1 exhibited superior efficacy in the degradation of MB, surpassing the performance of individual remediation materials.The predominant factor lies in the porous structure of biochar, which effectively encapsulates microorganisms, providing them with an optimal living environment while preventing direct contact with organic pollution within a short timeframe [36].Consequently, this mechanism sustains the activity of microorganisms.Additionally, it is evident that light plays a crucial role in facilitating the degradation of MB.Under dark conditions, the degradation rate of MB by Bio/MNs and P. aeruginosa PAO1 was 45.2%.However, upon exposure to light, the degradation rate increased significantly to 81.5%, representing a 1.8-fold increase compared to that observed under dark conditions.Compared to the results of the previous study, the degradation rate of MB was increased by 73.5% [37].This is due to the semiconducting nature of magnetite, which becomes excited under light conditions and generates photoelectrons and holes [38].As an electroactive microorganism, P. aeruginosa PAO1 interacts with magnetite to enhance its extracellular electron transfer process [39,40], consequently accelerating the pace of MB degradation.In order to further study the degradation process of MB in various repair materials, the kinetic curves of MB degradation were fitted (Figure 5b).The degradation process of MB of various restorative materials follows the pseudo-first-order kinetic equation.Among these, the removal rate constant of Bio/MNs synergistic P. aeruginosa PAO1 is 0.239 ± 0.025 d −1 , which is the largest in the degradation of MB under illumination (Table 1).The fitting index value indicates a satisfactory level of fit (R 2 = 0.945).Therefore, the fitting results showed that light promoted P. aeruginosa PAO1 and Bio/MNs in effectively removing MB.In addition, the results of multiple experiments showed that Bio/MNs and P. aeruginosa PAO1 had good cycling stability under light conditions.

Effect of the Microbial Dose on the Degradation of MB
To further investigate the degradation process of Bio/MNs by P. aeruginosa PAO1, various doses of P. aeruginosa PAO1 were examined in conjunction with Bio/MNs (0.2 g) to degrade MB under light.As shown in Figure 6a, when the Bio/MNs and pH were 0.2 g and 7, respectively, the degradation rate of MB increased with the increase in the P. aeruginosa PAO1 dose.The degradation rate of MB significantly increased when the dose of P. aeruginosa PAO1 was escalated from 1 mL to 4 mL.Notably, at a dosage of 4 mL, the degradation rate of MB reached its peak (77.1%), exhibiting a 2.3-fold enhancement compared to the P. aeruginosa PAO1-free group (32.7%).This degradation rate is 2.1 times that of previous studies [41].The application of P. aeruginosa PAO1 combined with Bio/MNs resulted in a significant enhancement of MB removal by 46.3 mg/g [42].It is worth noting that an increase in the number of microorganisms can directly release more electrons [40].Furthermore, the increase in the number of bacteria further promotes the redox reactions interaction between microorganisms and magnetite, releasing more Fe (II) and thereby reducing MB [43].In order to reveal the degradation kinetics of MB by the P. aeruginosa PAO1 dose, the fitted MB degradation time curve is shown in Figure 6b.The results show that the degradation process of MB follows the first-order kinetic equation.The ln (C/C0) value of MB showed a good linear relationship with the reaction time, and the fitting index value indicated a good fit (R 2 > 0.85) (Table 2).When adding 0 mL, 1 mL, 2 mL, 3 mL, and 4 mL P. aeruginosa PAO1 to the system, the removal rate constants of MB were 0.071 ± 0.011, 0.129 ± 0.026, 0.180 ± 0.031, 0.233 ± 0.035, and 0.281 ± 0.048 d −1 , respectively (Table 2).The fitting results further revealed that the degradation rate of MB exhibited a positive correlation with the P. aeruginosa PAO1 concentration.

Effect of the Bio/MNs Dosage on the Degradation of MB
The impact of the Bio/MNs dosage on MB degradation was further investigated while maintaining a fixed amount of P. aeruginosa PAO1 (3 mL), pH = 7, and light exposure.The degradation of MB increases with the increase in Bio/MNs, as depicted in Figure 7a.In the absence of Bio/MNs, the removal rate of MB at the fifth hour is merely 32.2%.By contrast, an escalation in Bio/MNs usage from 0.1 g to 0.4 g leads to a corresponding enhancement in the removal rate of MB from 67.9% to 98.8%, respectively.Compared to the results of the previous study, the removal rate of MB increased by 83.4% [44].A possible reason for this result is that the attachment of Fe/Mn minerals with redox reaction activity to biochar will change the properties of biochar [45] and participate in the electron transfer process of the contaminant fixed biochar [46].Moreover, biochar acts as an electron mediator in the reduction in Fe (III) minerals by reducing bacteria [47].As the Bio/MNs dosage is heightened, certain soluble groups within the biochar matrix will disperse into the solution and engage in the redox reaction process within the solution [43].This also revealed that, with the increase in the Bio/MNs dose, the electron mediators also increased, and more Fe (III) was reduced, thereby increasing the removal rate of MB.In order to reveal the degradation kinetics of MB by the addition of Bio/Mns, the fitted MB degradation time curve is shown in Figure 7b.The results show that the degradation process of MB conforms to the first-order kinetic equation.The ln (C/C0) values of MB showed a good linear relationship with the reaction time, and the fitting index values were well fitted (R 2 > 0.95) (Table 3).In addition, when 0.4 g of Bio/MNs is added, the degradation rate constant of MB is 1.059 ± 0.138 d −1 .Consequently, the fitting results also demonstrated a positive correlation between the dosage of Bio/MNs and the removal rate of MB.

Effect of pH on the Degradation of MB
The impact of pH on the removal rate of MB by P. aeruginosa PAO1 with Bio/MNs under light conditions was investigated across a range of initial values (3)(4)(5)(6)(7)(8)(9)(10)(11).As depicted in Figure 8a, when the Bio/MNs and P. aeruginosa PAO1 were 0.2 g and 3 mL, respectively, an increase in the pH value corresponded to an increase in the MB removal rate.When the initial pH value is 3, the removal rate of MB is 56.0% at the fifth hour.In contrast, when the initial pH was increased to 11, the removal rate increased to 98.6%, which is 1.76 times greater than that observed at pH = 3.It is worth noting that the removal rate of MB is 93.4% higher than that of previous studies, and the removal effect is very significant [23].These results indicate that the initial pH of the solution has a significant effect on the removal of MB by P. aeruginosa PAO1 in collaboration with Bio/MNs.This is because there are functional groups and surface charges in Bio/MNs, so the initial pH value has a very important influence on the degradation of MB in the composite [5].Furthermore, the capacity of H + binding sites on biochar is limited under acidic conditions.Notably, the researchers found that the biochar and Fe3O4 complex had a zero potential pH of 1.74.As the pH value increases from 4 to 11, the surface of the complex becomes negatively charged [48].However, MB is a positively charged cationic dye [49].Therefore, under the condition of a low pH value, there is electrostatic repulsion between MB and Bio/MNs, resulting in a lower removal rate of MB.As the pH value increases, there is an electrostatic attraction between MB and Bio/MNs, leading to the enhanced degradation of MB.To delve deeper into the influence of the initial pH value on MB removal, the kinetics curve of MB degradation was subjected to fitting.The degradation process of MB under various pH conditions follows the first-order kinetic equation (Figure 8b).In particular, when the pH value is 11, the removal rate constant of Bio/MNs synergistic P. aeruginosa PAO1 is 1.048 ± 0.083 d −1 (Table 4).Moreover, the fitting index revealed a high degree of fit (R 2 = 0.98).Therefore, the fitting results show that the removal rate of MB increases with the increase in the initial pH value.

Mechanistic Analysis
The mechanism by which Bio/MNs collaborate with P. aeruginosa PAO1 to degrade MB organic pollution is illustrated in Figure 9. Initially, the porous structure of biochar rapidly adsorbs MB, thereby impeding its further diffusion [5].Additionally, the abundant pores within biochar provide favorable sites for microbial attachment, thereby affording a protective niche for microorganisms [36].Notably, P. aeruginosa PAO1 acts as the main electron donor throughout the system.Magnetite and biochar act as electron mediators in the system.In particular, part of the extracellular electrons produced by P. aeruginosa PAO1 will enter the magnetite valence band and then transfer to the conduction band and transfer electrons under light excitation.The primary factor lies in the photoexcited electron transfer pathway within the Bio/MNs-P.aeruginosa PAO1-MB system [50].Upon light excitation, Bio/MNs generate photogenerated electrons and holes.P. aeruginosa PAO1 utilizes bioelectrons generated through metabolism to effectively scavenge light-induced holes, thereby facilitating the separation of photogenerated electron-hole pairs and enhancing the current output [51], thus speeding up the extracellular electron transfer process of P. aeruginosa PAO1.In addition, biochar can increase the release of O2 − and •OH [52].Therefore, Bio/MNs collaborate with P. aeruginosa PAO1 to significantly remove MB organic pollutants under light conditions.

Conclusions
In summary, Bio/MNs nanocomposites were successfully prepared by the chemical method and used to degrade MB in collaboration with P. aeruginosa PAO1.Comprehensive experimental analysis showed that Bio/MNs synergistic P. aeruginosa PAO1 could remove 81.5% of MB at 8 h under light conditions.Gradient experiments unveiled a proportional relationship between MB removal and the dosage of P. aeruginosa PAO1, as well as the incorporation of Bio/MNs within a specified range.Furthermore, MB functions as a cationic dye, and in acidic settings, due to the constraints on H+ binding sites on biochar, the pinnacle MB removal rate reaches 98.6% at pH = 11.This is because biochar not only alleviates the agglomeration problem of magnetite nanoparticles but also mediates the electron transfer between P. aeruginosa PAO1 and magnetite, thus promoting the production of photohole-electron pairs.In addition, the semiconductor properties of magnetite itself and the electric generation properties of P. aeruginosa PAO1 interact to further enhance the electron generation rate, thus speeding up the generation of •OH and O -

Figure 2 .
Figure 2. SEM elemental mapping image (a,b) and EDS elemental mapping images (c,d) of biochar and Bio/MNs.

Figure 3
Figure3displays the XRD patterns of raw corn biochar and magnetic biochar derived from corn biochar with magnetite nanoparticles (Bio/MNs).The original biochar shows two diffraction peaks at 21.9° and 36.2°, and the corresponding material composition is organic matter.Notably, the overall peak intensity of Bio/MNs is relatively low, consisting of small diffraction peaks.And Bio/MNs show characteristic peaks at 2θ angles of 21.3°, 36.8°,41.7°, and 53.6°, corresponding to the (111), (311), (400), and (422) crystal faces of magnetite nanoparticles.In addition, the Bio/MNs also contain the diffraction peaks of organic matter and an Fe2O3 characteristic.Fe2O3 may be caused by the oxidation of part of Fe3O4 during the preparation of Bio/MNs.Evidently, the successful preparation of Bio/MNs composites is demonstrated, with magnetite nanoparticles displaying a favorable crystallization effect within the biochar matrix.

Figure 4 .
Figure 4.The full spectrum of XPS for biochar and Bio/MNs (a); the narrow scan XPS spectrum of Fe2p (b).

Figure 5 .
Figure 5. MB degradation of different repair materials (a) and kinetic curves (b).

Figure 6 .
Figure 6.MB degradation curve (a) and kinetic fitting curve (b) of different microbial contents.

Figure 7 .
Figure 7. MB degradation (a) and kinetic fitting curve (b) of Bio/MNs at different doses.

Figure 8 .
Figure 8. MB degradation curve (a) and kinetic fitting curve (b) of different pH values.

Table 1 .
Kinetic parameter fitting of various experimental materials.

Table 2 .
Kinetic parameter fitting of the bacterial gradient reaction.

Table 3 .
Kinetic parameter fitting of Bio/MNs gradient reaction.

Table 4 .
Kinetic parameter fitting of the pH gradient reaction.