Modified biochar, preparation method thereof and application of modified biochar in SBR (Sequencing Batch Reactor) denitrification

Abstract

The invention discloses modified biochar, a preparation method thereof and application of the modified biochar in SBR (sequencing batch reactor) denitrification, and belongs to the technical field of environmental engineering. By modifying the biochar, C-OH,-COOH, SBET and ID / IG of the biochar are improved, so that the biochar has excellent denitrification performance. The modified biochar serves as a battery in the SBR reactor, and in the aerobic stage, the surface of the modified biochar is rich in groups such as C-O and-OH, and the modified biochar has high electron donating capacity; in the anoxic stage, the groups are oxidized by denitrifying microorganisms, and electrons are released for denitrification utilization, so that the conversion of NO3 <->-N to N2 is accelerated. Meanwhile, the modified biochar can promote enrichment of denitrifying functional bacteria; by adding the modified charcoal, the microbial community structure of the SBR reactor can be obviously changed, and the stability of the SBR reactor can be improved, so that the system can maintain relatively high denitrification efficiency in the operation process, and the demand on an external carbon source is reduced.

Description

Technical Field

[0001] The invention belongs to the technical field of environmental engineering, and particularly relates to a modified biochar, a preparation method thereof, and an application thereof in SBR denitrification. Background Art

[0002] Nitrogen pollution has become one of the most important water environmental problems, mainly due to the increase of nitrate in water bodies. - The main sources of -N pollution are: the large-scale use of nitrogen-containing fertilizers in agriculture, the excessive discharge of urban sewage and industrial wastewater, and the leakage of landfill leachate. - Once NO3 enters the water, it can easily cause eutrophication. In a eutrophic water environment, a large number of algae and phytoplankton will reproduce and grow rapidly and excessively. This process will consume a large amount of dissolved oxygen in the water, causing the dissolved oxygen to be exhausted, and then causing aquatic organisms to die due to lack of oxygen, ultimately leading to the deterioration of the water environment. - The harm of NO3 is not only reflected in the adverse effects on the aquatic ecological environment, but also causes irreversible damage to humans and animals. - -N itself is relatively weak in toxicity, and a small amount of NO3 - -N usually does not pose a serious health threat to the human body. However, once NO3 - -N accumulates continuously and is consumed by humans when it reaches a certain concentration. NO3 - -N will be transformed into nitrite (NO2 - -N), this conversion may cause methemoglobinemia. In addition, relevant studies have shown that long-term drinking of high concentrations of NO3 - -N water will not only increase the risk of thyroid tumors, but may even induce cancer. - -N can also interfere with the body's normal absorption of vitamin A, leading to a series of complications.

[0003] In removing NO3 from water - Among the many methods for removing NO3, the most common ones are physical and chemical methods such as ion exchange, adsorption, reverse osmosis and advanced oxidation, as well as biological methods such as heterotrophic denitrification (HD), autotrophic denitrification (AD), mixed nutrient denitrification and anaerobic ammonium oxidation. Compared with physical and chemical methods, biological denitrification is more effective in removing NO3 -Bio-nitrification has significant advantages in terms of reducing nitrogen and is a more economical and environmentally friendly method. This is mainly due to the low energy demand and relatively low chemical consumption during the biological denitrification process. Among various biological denitrification technologies, heterotrophic denitrification and autotrophic denitrification were studied earlier and are more widely and frequently used in practice.

[0004] As researchers explore the electrochemical properties of biochar, it has been widely used to promote nitrogen reduction in denitrification. However, research on biochar in denitrification has largely focused on the impact of its overall electrochemical properties on denitrification, lacking a comparison of the correlation between the redox activity and conductive structure of biochar, which determine its electrochemical properties, and denitrification efficiency. Summary of the Invention

[0005] In view of the above-mentioned prior art, the present invention provides a modified biochar and a preparation method thereof and application in SBR denitrification to solve the technical problem of poor denitrification effect in the existing sewage treatment process.

[0006] In order to achieve the above object, the technical solution adopted by the present invention is to provide a method for preparing modified biochar, comprising the following steps: S1: pretreating a raw material to obtain a pretreated material; the raw material is at least one of peanut shells, rice straw, sawdust, cellulose, hemicellulose, and lignin; S2: The pretreated material is heated to 300-700°C and pyrolyzed until no white smoke is produced; then air-cooled to room temperature, washed, and dried to obtain biochar; S3: modifying the biochar by soaking or co-firing; soaking modification comprises the following steps: immersing the biochar in an alkaline solution for 10 to 15 hours, discarding the supernatant, and then soaking in the alkaline solution for 10 to 15 hours, followed by washing and drying. Co-firing modification includes the following steps: mixing biochar with a modifier, grinding it, heating it to 300-700°C, holding it for 45-70 minutes, then air-cooling it to room temperature, washing it, and drying it. The modifier can be Na2S2O3, KMnO4, a Na2S2O3-KMnO4 mixture, KOH, or Na3PO4.

[0007] On the basis of the above technical solution, the present invention can also be improved as follows.

[0008] Furthermore, the raw material pretreatment method in S1 is: crushing the raw material into particles with a particle size of less than 5 mm, washing with deionized water and then drying at 60°C.

[0009] Furthermore, the heating rate of the pretreated material in S2 is 6°C / min; the cleaning method is to wash with deionized water until neutral; and the drying method is to dry at 60°C to constant weight.

[0010] Furthermore, the alkali solution is a sodium hydroxide solution with a concentration of 2M; during the soaking modification process, the biochar is soaked in the alkali solution twice for 12 hours.

[0011] Furthermore, the mass ratio of Na2S2O3 to KMnO4 in the mixture of Na2S2O3 and KMnO4 is 1:1.

[0012] Furthermore, during the co-firing modification process, the biochar and the modifier were mixed in a mass ratio of 1:1; the heating rate was 6°C / min.

[0013] Furthermore, the raw material for preparing biochar is rice straw, and the pyrolysis temperature of rice straw when preparing biochar is 300°C; the modification method of the obtained biochar is to immerse the biochar in 2M sodium hydroxide solution for 12 hours, then discard the supernatant, add 2M sodium hydroxide solution again and soak for 12 hours, then wash and dry.

[0014] Furthermore, the raw material for preparing biochar is rice straw, and the pyrolysis temperature of rice straw when preparing biochar is 700°C; the modification method of the obtained biochar is to mix biochar and Na3PO4 in a mass ratio of 1:1, and then heat it to 700°C at a heating rate of 6°C / min, keep it warm for 60 minutes, and then air-cool it to room temperature, and then wash and dry it.

[0015] The invention also discloses a modified biochar, which is prepared by the above-mentioned preparation method.

[0016] The present invention also discloses the application of the modified biochar in wastewater denitrification based on SBR.

[0017] Furthermore, the sewage composition was: 150-600 mg / L COD, 600 mg / L NaHCO3, 50 mg / L KNO3, 30 mg / LMgCl2·6H2O, 6.4 mg / L CaCl2, 5.0 mg / L FeSO4·7H2O, 0.12 mg / L MnCl2·4H2O, 0.03 mg / LCuSO4·5H2O, 0.12 mg / L ZnSO4·7H2O, 1500 mg / L EDTA and 0.03 mg / L CoCl·6H2O.

[0018] Furthermore, the dosage of modified biochar in the SBR reactor was 1 g / L; during the SBR denitrification process, O / A was 0.5~2.6, C / N was 3~12, and the operation time was 21~31 days.

[0019] The beneficial effects of the present invention are: 1. The present invention improves the C-OH, -COOH, SBET and I D / I G , so that the modified biochar has excellent denitrification performance.

[0020] 2. The present invention achieves efficient removal of NO3 in sewage by strengthening the SBR reaction through modified biochar - -N, the denitrification mechanism is mainly reflected in the regulation of electron supply and reception, the improvement of microbial system activity and the optimization of microbial communities. Firstly, the modified biochar can act as a "battery" in the SBR reactor, and the redox functional groups on its surface can dynamically regulate the flow of electrons in different operating stages. In the aerobic stage, the surface of the modified biochar is rich in reducing groups such as CO and -OH, which have strong electron-donating ability; in the anoxic stage, these groups are oxidized by denitrifying microorganisms, releasing electrons for denitrification, thereby accelerating NO3 - Under aerobic conditions, the proportion of reducing groups (such as -OH and -COOH) in biochar is high, while under anoxic conditions, these groups decrease, accompanied by an increase in the proportion of oxidizing groups (such as C=O). This indicates that biochar with a high content of redox active parts is more conducive to the charging and discharging of electrons during the OA alternation process than biochar with a high content of conductive structures.

[0021] Secondly, modified biochar can improve the activity of microbial systems, including NAR, NIR and ETSA. The enhancement of NAR and NIR helps NO3 - -N and NO2 - -N conversion, thereby reducing the intermediate product NO2 - -N accumulation, improving the overall denitrification efficiency.

[0022] Thirdly, the high specific surface area and rich pore structure of modified biochar provide an ideal environment for microorganisms to attach and grow, promoting the enrichment of denitrification functional bacteria; the addition of modified biochar can significantly change the microbial community structure of the SBR reactor and improve its stability, enabling the system to maintain a high denitrification efficiency during operation. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is a diagram of the denitrification batch experiment simulation device; 1. Water sampling tube; 2. Gas sampling tube; 3. Nitrogen purge tube; Figure 2 NO3 in autotrophic denitrification of different biochars - -N removal rate; Figure 3 (a) EDC, EAC, and EEC of different biochars; Figure 3 (b) EDC, EAC, and EEC of the control modified biochar and modified biochar; Figure 4 (a) and Figure 4 (b) CV curves and C of different biochars p ; Figure 4 (c) and Figure 4 (d) CV curves and C of control modified biochar and modified biochar, respectively. p ;; Figure 5 (a) and Figure 5 (b) Nyquist plots and R of different biochars ct ; Figure 5 (c) and Figure 5 (d) Nyquist plots and R of control modified biochar and modified biochar, respectively. ct ; Figure 6 NO3 in autotrophic denitrification of different biochars - -N removal rate and its C p (a) R ct (b), linear regression plots of EAC (c), EDC (d), and EEC (e); Figure 7 SEM images of S-300, P-300, R-300, R-500, R-700, R-Na, and R-NaP; Figure 8 Raman spectra (a), XRD structures (b), S BET histogram (c) and average pore size histogram (d); Figure 9 FTIR spectra (a), XPS total spectra (b), C element peak area ratio histogram (c), O element peak area ratio histogram (d), and N element peak area ratio histogram (e) of S-300, P-300, R-300, R-500, R-700, R-Na, and R-NaP; Figure 10 This is a diagram of the SBR experimental device; 4, microorganisms; 5, aeration head; 6, biochar; 7, aeration pump; 8, flow meter; Figure 11 NO3 in different SBR reactors at different operating stages - -N removal rate (a), NO2 - -N concentration (b), NH4 + -N concentration (c) and COD removal rate (d); Figure 12 NO3 in different SBR reactors at different operating stages --N average removal rate (a), NO2 - -N average concentration (b), NH4 + -N average concentration (c) and COD average removal rate (d). DETAILED DESCRIPTION

[0024] The specific implementation methods of the present invention are described in detail below with reference to the embodiments.

[0025] Example 1: Preparation and characterization of biochar 1. Preparation of Biochar The raw material selection and preparation temperature for biochar production are shown in Table 1. Biochar was prepared using organic solid waste (peanut shells, rice straw, and poplar sawdust) and pure biochemical components (PBC) (cellulose, xylan, and lignin). Peanut shells, rice straw, and sawdust were selected based on their primary biomass components: cellulose, hemicellulose, and lignin, respectively.

[0026] Table 1 Raw material selection and preparation temperature for biochar preparation

[0027] The biochar preparation process using organic solid waste as raw material is as follows: the raw material is first crushed into small particles with a particle size of less than 5 mm using a pulverizer. The material is then washed with deionized water and dried at 60°C. The dried material is placed in a muffle furnace and heated at a rate of 6°C / min from room temperature to the target temperature (300, 500, or 700°C). Pyrolysis is performed at the target temperature in an oxygen-limited atmosphere until no white smoke is emitted, followed by natural cooling to room temperature. Finally, the fired biochar, which has been sieved through a 100-mesh sieve, is washed with deionized water to a pH of 7.0 ± 0.2 and then dried at 60°C for later use. The preparation method using cellulose, xylan, and lignin as raw materials is the same as above, with a target temperature of 300°C. A total of 12 biochar types were obtained, labeled PX, RX, SX, CeX, Hemi-X, and Lig-X, where P, R, S, Ce, Hemi, and Lig represent peanut shells, rice straw, poplar sawdust, cellulose, hemicellulose, and lignin, respectively, and X represents the preparation temperature.

[0028] To optimize the electrochemical performance of biochar, the biochar with larger EEC (R-300) or smaller R ct (R-700) biochar was modified to obtain a biochar with a larger EEC and / or smaller R ctBiochar. Modification reagents and methods are shown in Table 2. Sodium hydroxide (NaOH), sodium thiosulfate (Na2S2O3), potassium permanganate (KMnO4), and a mixture of Na2S2O3 and KMnO4 (1:1 mass ratio) were used as chemical modifiers to modify the control biochar (R-300) to increase the redox active fraction of the biochar. Two modification methods were used: one was immersion modification, in which the biochar was mixed with a 2M NaOH solution at a solid-to-liquid ratio of 1:50 (g:mL). After standing for 12 hours, the supernatant was discarded, and the same volume of 2M NaOH solution was added and allowed to stand for 12 hours. Finally, the mixture was filtered and washed with 1M HCl until the pH of the washing solution reached 7.0±0.2. Another method involves co-firing modification. The preparation process involves mixing biochar with Na₂S₂O₃ or KMnO₄, or a mixture of Na₂S₂O₃ and KMnO₄, in a 1:1 mass ratio in a mortar. The mixture is ground for 15 minutes to achieve a uniform mix. The mixture is then heated in a muffle furnace at a rate of 6°C / min to a target temperature of 300°C, followed by co-pyrolysis for 1 hour. The mixture is then naturally cooled to room temperature. Finally, the biochar is washed with deionized water to a pH of 7.0 ± 0.2 and dried at 60°C for later use. The resulting four biochars are designated R-Na, R-NaS, R-KMn, and R-NaS / KMn. Biochar prepared at 700°C (R-700) was selected as a control. Sodium phosphate (Na₃PO₄) and potassium hydroxide (KOH) were used as chemical modifiers to increase the conductive structure of the biochar. The modification method was the same as the co-firing modification described above: R-700 was modified by grinding and co-pyrolysis at 700°C for 1 hour. The two modified biochars obtained were designated R-NaP and RK, respectively. The cleaning procedures were the same as above. All biochars were sieved through a 100-mesh sieve before cleaning.

[0029] Table 2 Biochar modification directions, reagents and methods

[0030] 2. Characterization of Biochar 1. Biochar enhances the efficiency of autotrophic nitrogen removal (1) Enrichment and inoculation of denitrification sludge Experimental sludge was collected from the anaerobic denitrification and aerobic nitrification zones of the 10th Urban Wastewater Treatment Plant in China. The culture was maintained at a constant temperature (25°C) in a sealed 4.5L reaction flask for 60 days to enrich autotrophic and heterotrophic denitrifying bacteria. The composition and dosage of the autotrophic sludge enrichment medium were 20mM Na2S2O3·5H2O, 30mM NaHCO3, 20mM KNO3, 18.7mM NH4Cl, 3.25mM MgCl2·6H2O, 0.08mM FeSO4·7H2O, and 0.05mM CaCl2. The electron donor in the heterotrophic sludge enrichment medium was changed from 20mM Na2S2O3·5H2O in the autotrophic medium to 20mM C6H 12 O6, the rest are the same as autotrophic. Replace 2L culture supernatant every 2 days and monitor the effluent NO3 - -N concentration. When NO3 - When the -N removal rate is greater than 80% and tends to be stable, it is considered that the autotrophic and heterotrophic denitrifying bacteria are successfully enriched.

[0031] Before inoculating autotrophic and heterotrophic denitrification sludge, it is necessary to wash it with physiological saline and remove residual NO3 by sedimentation, filtration and flushing. - -N and SO4 2- , until the conductivity of the wash solution remains stable. Subsequently, the treated seed sludge was inoculated into 1.8 L of simulated wastewater used in autotrophic or heterotrophic batch experiments as inoculum sludge for the autotrophic or heterotrophic batch experiments. The autotrophic inoculum had a suspended solids (MLSS) concentration of 6610 mg / L and a volatile suspended solids (MLVSS) concentration of 4297 mg / L, while the heterotrophic inoculum had a MLSS concentration of 5964 mg / L and a MLVS concentration of 3996 mg / L.

[0032] (2) Experimental steps Biochar (2.5 g), inoculum sludge (100 mL) and synthetic wastewater (400 mL) were added to Figure 1 In the experimental device shown, 1 is the water sampling tube, 2 is the gas sampling tube, and 3 is the nitrogen purge tube. A blank group and a control group without biochar but with inoculated sludge were also set up. All batch experiments were set up with replicates. The composition and dosage of the simulated wastewater used in the autotrophic batch experiment were 2.65mM Na2S2O3·5H2O, 3.57mM NaHCO3, 3.57mM KNO3, 1.30mM NH4Cl, 3.25mM MgCl2·6H2O, 0.08mM FeSO4·7H2O, 0.05mM CaCl2, and the pH was 7.0±0.2; in the heterotrophic batch experiment, the electron donor was changed from 2.65mM Na2S2O3·5H2O to 1.28mM C6H 12Except for O6, other compositions and dosages were consistent with those of autotrophic culture. Before cultivation at 25°C and 120 rpm, each flask was purged with N2 for 15 minutes to remove dissolved oxygen (DO). 25 mL of water was sampled from the water sampling tube at 12, 24, 36, 48, 60, 72, and 84 hours after cultivation. The collected water samples were filtered through a 0.45 μm filter and NO3 was determined. - -N, NO2 - -N、NH4 + -N concentration and ORP.

[0033] NO3 in autotrophic denitrification of 18 prepared biochars - The comparison results of -N removal rate are as follows Figure 2 As shown. Figure 2 It can be seen that at 300℃, the biochar with R as raw material performs best, NO3 - -N removal rate was 44.3%, higher than that of P and S. As the temperature increased, the NO3 - -N removal rate generally decreased, especially when R and P were used as raw materials. Secondly, the NO3 - -N removal rate was significantly higher than that of biochar (R, S and P) with organic solid waste as raw material, and NO3 - -N removal efficiency increased by 27.4~30.6%. Among them, Hemi performed best with a removal efficiency of 71.7%, followed by Ce and Lig. Furthermore, the low-temperature modified biochars (R-Na, R-NaS / KMn, R-NaS and R-KMn) had a higher removal efficiency for NO3 than the control modified biochar R-300. - -N removal effect increased by 21~30.3%. Among them, R-Na has a great effect on NO3 - -N removal effect is the best, with a removal rate of 74.6%, and its autotrophic nitrogen removal effect is higher than that of PBC biochar. Compared with the control modified biochar R-700, the high temperature modified biochar (RK and R-NaP) has a better effect on NO3 - -N removal effect was improved by 33.2~36.6%. Although the improvement effect of high-temperature modified biochar was higher than that of low-temperature modified biochar, R-NaP (70.3%) with the best denitrification effect was still lower than R-Na.

[0034] 2. Characterization of electrochemical properties of biochar Chemical titration was used to quantify the redox properties of biochar. In this experiment, EEC consisted of EDC and EAC of biochar. EDC was related to the phenolic functional groups of biochar, while EAC was related to the quinone functional groups of biochar. Figure 3 As shown. Figure 3As can be seen from (a), at 300℃, R-300 has stronger electron donating ability and better electron exchange capacity than S-300 and P-300. PBC biochar has larger EDC and EEC than R-300, S-300 and P-300, which is related to its denitrification efficiency and C p Among them, Hemi-300 has the largest EDC and EEC values, which are 0.562mmol e - / g and 1.252mmol e - / g. The EAC of R, S and P increases with the increase of preparation temperature, while the EDC and EEC decrease with the increase of preparation temperature. This is because the oxygen-containing functional groups gradually decrease with the increase of temperature, especially the phenolic functional groups related to the redox of biochar, thus forming quinone structures and electroactive aromatic structures. Figure 3 As can be seen from (b), R-Na in low-temperature modified biochar has the largest EDC and EEC values ​​among all biochars, and its EDC is 0.076~0.124mmol e higher than that of other biochars. - / g, EEC is 0.028~0.115mmol e higher than other biochars - / g. The co-firing modification of Na3PO4 and biochar significantly increased the EEC of biochar (R-NaP), but it was still lower than that of R-Na. This further shows that Na3PO4 as a modifier is effective in reducing the EEC of biochar R ct At the same time, it will also increase the EEC of biochar, thereby increasing its ability to reduce electrons, which is one of the main reasons for improving the denitrification capacity of biochar.

[0035] The specific capacitance of biochar was measured by CV. Figure 4 As shown. Figure 4 As shown in a and b, at 300 °C, the CV curve of R-300 shows a wider closed area and a higher redox peak than that of P-300 and S-300, indicating a higher C p (9.37F / g), indicating that it has stronger charge storage capacity and good redox reaction reversibility than P-300 and S-300. With the increase of preparation temperature, the closed area of ​​CV curves of R, S and P gradually narrows, the redox peak gradually decreases, and the C p The decrease is consistent with the change trend of denitrification with preparation temperature. PBC biochar also shows good electrochemical performance. The change trend of its CV closed area, C p The change trend is consistent with its denitrification trend, that is, Hemi-300>Ce-30>Lig-300, and all are higher than R-300, S-300, and P-300. Figure 4c shows that modification can increase the closed area and redox peak of biochar CV, and R-Na has the largest closed area and redox peak current. Figure 4 d shows that the C of low temperature modified biochar is higher than that of R-300. p Increased by 0.62~1.98F / g, C of R-Na p Up to 11.35F / g; C of biochar p The larger the value, the stronger the electron transfer capacity of the sludge-biochar system is and the higher the denitrification efficiency is. p C of biochar modified at lower temperature p The conductivity of biochar is lower than that of R-Na, especially R-NaP, which is 3.56 F / g lower than that of R-Na. However, their denitrification effects in denitrification are only 4.3% different, indicating that the role of conductivity of biochar in the electron transfer rate in denitrification cannot be ignored.

[0036] The R of biochar was determined by EIS. ct , the results are as follows Figure 5 As shown, the diameter of the semicircle in the high-frequency region of the EIS curve represents the charge transfer resistance R of the biochar. ct , which is related to its conductivity. Figure 5 As shown in a and b, at 300 °C, the semicircle diameter of the high-frequency region of the EIS curve of R-300 is smaller than that of P-300 and S-300, but smaller than that of R of PBC biochar. ct The change of semicircle diameter in the high frequency region of the EIS curve of PBC biochar and R ct The change of the value is consistent with the change trend of the corresponding organic solid waste as raw material biochar, that is, Hemi-300<Ce-300<Lig-300. Temperature and modification method are the factors affecting biochar R ct The main reason. Figure 5 a and b show that the semicircle diameters and R ct The value is negatively correlated with the preparation temperature. For example, the semicircle diameter of the high-frequency region of the EIS curve of R is R-300>R-500>R-700. Figure 5 c It can be seen that compared with the control modified biochar, different modification methods can reduce the semicircle diameter of the high frequency region of the biochar EIS curve. Combined with the EIS curve fitting value ( Figure 5 d) It can be seen that the redox active part of the modified biochar R ct The sizes are R-KMn<R-NaS / KMn<R-NaS<R-Na, which are all lower than the control modified biochar R-300. R-NaP shows the smallest Rct among all biochars, R ctThe smaller the value, the better the conductivity and the more conducive it is to electron transfer, indicating that R-NaP has significant advantages in promoting electron transfer, which can increase the activity of the electron transport system in the autotrophic denitrification process and thus promote the denitrification effect.

[0037] In addition, the correlation between the autotrophic denitrification efficiency of biochar and the physical and chemical properties of biochar itself was studied. Figure 6 As shown. Figure 6 As can be seen from the data, the autotrophic denitrification performance of biochar is positively correlated with Cp, EDC, and EEC, with Pearson correlation coefficients r of 0.69, 0.82, and 0.93, respectively. This means that higher Cp, EDC, and EEC values ​​are associated with better autotrophic denitrification performance. In contrast, the Rct of biochar exhibits a clear negative correlation with autotrophic denitrification performance, with an r of -0.14. This suggests that as Rct increases, the autotrophic denitrification performance of biochar may decrease, and that the charge transfer resistance limits the autotrophic denitrification performance of biochar to a certain extent. When examining the effect of biochar's EAC on its autotrophic denitrification performance, data analysis revealed no significant correlation between the two, with an r of only 0.16. This suggests that EAC is not a decisive factor affecting the autotrophic denitrification performance of biochar. Combined with the previous analysis of the denitrification performance, electrochemical properties and electron exchange capacity of biochar, seven relatively representative biochars were selected as the objects of subsequent characterization and analysis from the perspectives of raw materials, preparation temperature and modification effect, namely S-300, P-300, R-300, R-500, R-700, R-Na and R-NaP.

[0038] 3. Characterization of the physical and chemical properties of biochar The microstructure of biochar was analyzed by SEM. Figure 7 As shown. Figure 7As can be seen in the figure, at 300°C, S-300 exhibits a blocky structure with a smooth surface covered with pores, while P-300 and R-300 exhibit a layered honeycomb structure with a smooth surface. R-300 exhibits a richer honeycomb structure than P-300, which may be the reason for the smaller EIS of S-300 and P-300. R-300, R-500, and R-700 demonstrate the effect of temperature on the structure of straw biochar. Low temperatures produce a more complete structure. As the preparation temperature increases, the surface morphology of the biochar becomes rougher due to the pyrolysis of the organic components in the biomass, resulting in more cracks and pores. High temperatures increase the porosity of the biochar and promote a more uniform distribution of pores. Among the modified biochars, R-Na exhibits more small pores on its surface after alkaline corrosion, but its layered structure remains unchanged. Compared with the control biochar (R-700), the structure of R-NaP collapsed, the surface morphology became rougher, the pore structure became richer, and it showed multi-layered and porous characteristics, which indicated that the Na3PO4 modification treatment increased the surface defects and active sites of biochar. These changes helped to improve the electron transport capacity of biochar and reduce the R ct , enhancing its electron transfer rate and electron storage performance in autotrophic denitrification.

[0039] Figure 8 (a) shows the Raman spectrum of biochar. -1 (D band) and 1590cm -1 The two characteristic peaks at (G band) represent disordered defect carbon and ordered graphitic carbon respectively; the D band is closely related to the defects and disordered structures in carbon materials, which mainly originate from edge defects, impurities or structural irregularities in carbon materials; the G band mainly originates from the sp 2 Hybrid structure. The intensity ratio of D to G (ID / IG) is an important indicator to measure the degree of defects in carbon materials. At the same preparation temperature (300°C), the ID / IG value of biochar varies with different raw materials, R-300>S-300>P-300, and the ID / IG value of R-type biochar increases with the increase of preparation temperature. Among them, high temperature (700°C) has the greatest effect on the degree of defects in biochar. The ID / IG value of R-700 is 0.18 higher than that of R-300. SEM results also show that the surface of biochar is rougher and the pores are richer at high temperatures. R-NaP has the largest ID / IG value (0.90) and the largest degree of surface defects. The increase in surface defects can expose more active sites, which helps to reduce R ct , improving its electron transport capacity, thereby promoting the reduction process of autotrophic denitrification.

[0040] The crystal structure of biochar was analyzed by XRD. Figure 8 (b) is shown. Figure 8As can be seen in (b), the seven types of biochar have peaks of amorphous carbon (002) and crystalline carbon (100) between 20-30° and 40-50°, respectively. At 300°C, P-300, S-300, and R-300 have no obvious crystallization peaks, indicating that their carbonization degree is low. With the increase of preparation temperature, R-type biochar shows obvious crystallization peaks in the XRD spectrum, especially at 700°C, the crystallization peak of the R-700 sample is significantly enhanced, indicating that its graphitization degree is high. This shows that with the increase of carbonization temperature, the structure of biochar gradually changes from amorphous to crystalline carbon material, and the degree of graphitization increases. After NaOH modification, the (002) peak of R-Na shifted to the left, the peak intensity remained basically unchanged, the (100) peak weakened, and the amorphous structure increased; R-NaP showed more complex structural characteristics, and the (002) peak at 23° was raised and pointed, indicating that the P element may have been introduced during the modification process, changing its crystal structure or surface defects. The (100) peak intensity at 40~50° did not change significantly. This result shows that the surface defects of R-NaP increased, which can expose more active sites, which helps to improve its electron transport ability, which is consistent with the results of Raman spectroscopy analysis.

[0041] The nitrogen adsorption and desorption experiments on biochar were carried out, and the results were as follows: Figure 8 (c) and 8(d). Figure 8 As can be seen in (c), the S BET Mainly affected by temperature and Na3PO4 modification. BET The impact is small, R-300R, S-300 and P-300 S BET All between 2.6 and 4 meters 2 / g range. BET It increases with the increase of preparation temperature, which is due to the loss of volatile organic compounds and the increase of micropores formed by the transformation of amorphous carbon into graphite crystal phase. BET Among them, the effect of medium temperature (500℃) on the S BET The biggest impact, compared to R-300, R-500's S BET Increased by 46.2m 2 / g. R-Na S BET (2.92m 2 / g) compared with the control biochar (S BET -R-300=2.93m 2 / g) remained basically unchanged, and the S BET Compared with the control biochar (R-700), the 2 / g. From Figure 8(d) As can be seen, the average pore size of the seven biochars all remained between 5 and 17 nm, indicating that the biochar structure was mostly at the mesoporous level (2 nm < D < 50 nm). At 300°C, R-300 had the largest average pore size, at 17.18 nm. The average pore size of the R-type biochar decreased with increasing preparation temperature, decreasing from 17.18 nm to 5.38 nm when the preparation temperature increased from 300°C to 700°C. NaOH modification reduced the average pore size of the biochar from 17.18 nm to 10.04 nm. The average pore size of the biochar modified with Na₃PO₄ remained essentially unchanged, increasing only 0.44 nm compared to the control biochar (R-700), but the average pore size remained relatively low, indicating a highly developed porous structure, consistent with its Raman and XRD results.

[0042] FTIR of biochar Figure 9 (a) shows 3456cm -1 The characteristic peak at 1640 cm is attributed to the bending vibration of -OH / NH. -1 The characteristic peak at 1056cm is attributed to the stretching vibration of C=C / C=O. -1 The characteristic peak at 1640 cm is attributed to the stretching of -COOH. -1 There is an obvious absorption peak near 3456cm -1 The peak strength at 1056cm is R-300>S-300>P-300; among them, R-300 is at 1056cm -1 There is also an obvious absorption peak at 1640 cm. These results indicate that R-300 retains more oxygen-containing functional groups than S-300 and P-300. With the increase of carbonization temperature, the -OH / NH and C=C / C=O absorption peaks in the FTIR spectrum gradually weaken, especially at 1640 cm -1 region, indicating that the oxygen-containing functional groups on the biochar surface decreased significantly during the high-temperature carbonization process. For the modified biochar, R-Na and R-NaP were significantly higher at 3456 cm-1 than those of the control biochar. -1 and 1640cm -1 The absorption peaks near the R-Na were enhanced, especially for R-Na, which indicated that the content of oxygen-containing functional groups on the surface of biochar increased during the modification process, which was related to its higher EDC.

[0043] The elemental valence and content of biochar were analyzed by XPS. The results are as follows: Figure 9 As shown in (b)~(e). Figure 9(b) is the overall spectrum of 7 kinds of biochar. The 7 kinds of biochar have similar peaks at 284.8, 532.5 and 400eV, corresponding to C 1S, O1S and N 1S, respectively. At 300℃, R-300 has stronger absorption peaks at 284.8eV and 532.5eV than S-300 and P-300, and an obvious N 1S peak appears at 400eV. With the increase of preparation temperature, the C 1S and O 1S peaks of biochar gradually weaken, and the N 1S peak disappears. The modification has the most obvious effect on enhancing the O 1S peak of biochar, especially R-Na. The proportion of C atoms is about 1.3%. Figure 9 As shown in (c), the reducing groups CC / CH, CO and -COOH are dominant, accounting for 57.58~83.31% of the total atoms, among which low-temperature biochar has the highest proportion. The proportion of reducing groups decreases with the increase of preparation temperature. When the temperature rises from 300℃ to 700℃, the content of reducing groups decreases from 80.66% to 60.13%. The modification has little effect on the content of reducing groups. R-Na and R-NaP decrease by 1.36% and 2.54% respectively compared with the control biochar, mainly due to the reduction of CC / CH. The changes in the proportion of O atoms are shown in Figure 2. Figure 9 As shown in (d), at low temperature, the C-OH and -COOH contents of R-300 are higher than those of S-300 and P-300, accounting for 50.31% and 31.7% respectively. C-OH and -COOH can donate electrons in biochar and improve its reducibility. This may be the reason why R-300 has larger Cp, EDC and EEC than S-300 and P-300. With the increase of preparation temperature, the content of C=O gradually increases from 6.63% to 37.69%; the content of CO, C-OH and -COOH gradually decreases. This result is consistent with the change trend of its EAC and EDC. After modification, the contents of CO, C-OH and -COOH of R-NaP and R-Na increase. Among them, R-Na has the largest atomic proportion of C-OH and -COOH in all biochars, which is consistent with the FTIR results. The atomic proportion of N element is as follows: Figure 9 As shown in (e), the proportion of Pyridine N atoms first increases and then decreases with the increase of preparation temperature, and the proportion of Pyrrolic N and Graphitic N atoms increases with the increase of preparation temperature. After modification, except for the decrease in the proportion of Pyridine N in R-NaP, the proportions of other nitrogen-containing peaks increase.

[0044] Example 2: SBR experiment 1. Start-up of the reactor The activated sludge of the SBR reactor was taken from the aerobic zone of the Tenth Urban Sewage Treatment Plant in China, with an MLSS of 6630 mg / L and an MLVSS of 4442 mg / L. The sludge concentration MLVSS was diluted to 3000 mg / L, added to the SBR reactor, aerated for 1 day, and then started to be acclimated. During the acclimation period, the operating cycle of the SBR reactor was 3 cycles per day, each cycle lasting 8 hours, and the running time of each cycle was: aeration for 4 hours, standing for 2.5 hours, standing for 1 hour, and water inflow and outflow for 0.5 hours. The siphon method was used for water inflow and outflow, and 900 mL was discharged each time. During the acclimation period, the COD and NO3 of the effluent were monitored. - -N removal rate to observe the denitrification effect. After two weeks of operation, the reactor effluent COD and NO3 - The -N removal rate is higher than 60% and tends to be stable, indicating that the reactor has entered the stable operation stage.

[0045] 2. Experimental setup and operating conditions Four groups of reactors were set up, including microbial control group (Mro), unmodified biochar plus microbial treatment group (R-300+Mro), NaOH modified biochar treatment group plus microbial treatment group (R-Na+Mro), and Na3PO4 modified biochar plus microbial treatment group (R-NaP+Mro). The simulation diagram of the reaction device is shown in the figure. Figure 10 As shown in the figure, 4 represents the microorganism, 5 represents the aeration head, 6 represents the biochar, 7 represents the aeration pump, and 8 represents the flowmeter. After reactor startup, 1 g / L of the corresponding biochar was added to the R-300+Mro, R-Na+Mro, and R-NaP+Mro reactors, respectively. The experiment was divided into five phases and ran for a total of 125 days. The specific operating parameters for each phase are shown in Table 3. Phases I–III focused on the effects of modified biochar on SBR denitrification under different aerobic / anoxic (O / A) ratios; Phases II, IV, and V focused on the effects of modified biochar on SBR denitrification under different carbon-nitrogen ratios (C / N). The inlet and outlet water were discharged according to the operation cycle, with 900 mL of water each time. The composition and concentration of the inlet water were 150-600 mg / L COD (CH3COONa·3H2O), 600 mg / L NaHCO3, 50 mg / L KNO3, 30 mg / L MgCl2·6H2O, 6.4 mg / L CaCl2, 5.0 mg / L FeSO4·7H2O, 0.12 mg / L MnCl2·4H2O, 0.03 mg / L CuSO4·5H2O, 0.12 mg / L ZnSO4·7H2O, 1500 mg / L EDTA, and 0.03 mg / L CoCl·6H2O. Water samples were collected every two days after the anaerobic treatment was completed, with 50 mL of water sampled each time. The water samples were filtered through a 0.45µm hydrophobic membrane and then NO3 - -N, NO2- -N、NH4 + -N, COD and ORP analysis.

[0046] Table 3 SBR operating parameters

[0047] Nitrate removal efficiency of biochar-enhanced SBR NO3 in different SBR reactors at different operating stages - -N removal rate (a), NO2 - -N concentration (b), NH4 + -N concentration (c) and COD removal rate (d) as Figure 11 As shown in the figure; NO3 in different SBR reactors at different operating stages - -N average removal rate (a), NO2 - -N average concentration (b), NH4 + -N average concentration (c) and COD average removal rate (d) as shown Figure 12 As shown. Figure 11 (a) and 12 (a) show that under different O / A ratios and C / N ratios, the addition of biochar can promote the NO3 - -N removal, of which R-Na has the best promotion effect. - -N removal, it was found that O / A=0.8 was more conducive to NO3 - -N removal. Under the condition of C / N of 6, the influence of different O / A ratios on SBR is O / A=0.8>O / A=2.6>O / A=0.5, indicating that appropriately extending the anoxic time helps microorganisms to more fully degrade NO3 - -N; Figure 12 The dark blue arrows in (a) point to the NO3 - -N average removal rate and NO3 removal in the stable period when O / A=0.5 R-Na+Mro (64.70%) and R-NaP+Mro (62.02%) - -N average removal rate, comparing the removal rates of the three, it was found that when O / A=0.5, the SBR reactor with modified biochar had a higher removal rate of NO3 - -N removal can reach or even exceed that of NO3 when O / A=2.6 - -N removal effect, and the aeration time of the SBR reactor with O / A=0.5 was shortened by 4.3 times compared with that of the SBR reactor with O / A=2.6, which shows that the modified biochar can effectively promote the removal of NO3 --N, saving electricity costs. Comparing Phase II, Phase IV and Phase V, we can see that ( Figure 11 (a)), SBR reactor for NO3 - The removal effect of -N increases with the increase of C / N ratio, and the effect of biochar on SBR decreases with the increase of C / N. When C / N=6, biochar has the greatest promoting effect on SBR, which is 4.29~21.38% higher than Mro, especially when modified. When C / N rises to 12, the SBR reactor with biochar added only increases by 4.21~8.10% compared with Mro. Figure 12 The magenta arrow in (a) indicates that when C / N=6, the NO3 - The -N removal rates were 88.65% and 87.19%, respectively, which were only 2.33~3.79% lower than the Mro (90.98%) when C / N=12, indicating that the addition of modified biochar can reduce the dosage of organic carbon source in the SBR reactor by half. Combine Figure 11 (b) and 12 (b), under different O / A, the NO2 - The average concentration of -N first increased and then decreased with the decrease of O / A. The NO2 - The average concentration of -N decreases first and then increases with the decrease of O / A. The reason for this may be that when O / A=0.8, the NO3 - -N removal is most effective, when electrons are used for NO3 - -N to NO2 - -N conversion step, resulting in NO2 of Mro and R-300+Mro - -N accumulation, while in R-Na+Mro and R-NaP+Mro reactors, the larger C p and EEC mitigated NO2 - -N accumulation. Under different C / N, NO2 - -N average concentration increases with the increase of C / N, among which, NO2 - The average concentration of -N is Mro>R-300+Mro>R-NaP+Mro>R-Na+Mro, which further shows that modified biochar is more conducive to the denitrification nitrogen conversion process.

[0048] Depend on Figure 11 (c) and 12(c), NH4 +-N concentrations in the O / A=0.8, C / N=6 and O / A=0.8, C / N=12 stages were lower than the instrument detection limit, indicating that the nitrification-denitrification process in the SBR reactor was relatively complete under these conditions. However, NH4 + The accumulation of -N indicates that ammonia nitrogen cannot be completely converted under low carbon and low oxygen exposure environments, affecting the denitrification process.

[0049] The overall change pattern of COD and NO3 in different stages - -N removal trend is consistent ( Figure 11 (d) and 12(d)), COD removal first increases and then decreases with the decrease of O / A ratio, and increases with the increase of C / N ratio. - -N removal is different. When O / A=0.8 and C / N=3, the COD removal of each group of reactors can reach 87.07~88.08%, while at this time NO3 - -N removal was only 53.03~74.41%. This is because under low carbon source, even if COD removal is improved, the electrons that can be provided in the reactor are still limited, which limits the removal of NO3 - -N removal. Under different O / A ratios and C / N ratios, the modified biochar promoted COD removal most significantly, indicating that the addition of modified biochar enhanced the utilization rate of the reactor for carbon sources, which was similar to the NO3 - The removal of -N is consistent with that of

[0050] In summary, biochar, especially modified biochar (R-Na and R-NaP), can effectively increase NO3 - -N removal rate, reducing NO2 - -N and NH4 + -N accumulation, improving the COD removal effect; and the addition of biochar can not only provide additional carbon source to alleviate the problem of insufficient carbon source, but also shorten the aeration time and reduce the addition of organic carbon source, thereby saving operating costs.

[0051] Although the specific embodiments of the present invention have been described in detail in conjunction with the embodiments, this should not be construed as limiting the scope of protection of this patent. Within the scope described by the claims, various modifications and variations that can be made by those skilled in the art without creative work still fall within the scope of protection of this patent.

Claims

1. A method for preparing modified biochar, characterized in that: The following steps are involved: S1: pretreating a raw material to obtain a pretreated material; the raw material is at least one of peanut shells, rice straw, sawdust, cellulose, hemicellulose, and lignin; S2: The pretreated material is heated to 300-700°C and pyrolyzed until no white smoke is produced; then air-cooled to room temperature, washed, and dried to obtain biochar; S3: modifying the biochar by soaking or co-firing; soaking modification comprises the following steps: immersing the biochar in an alkaline solution for 10 to 15 hours, discarding the supernatant, and then soaking in the alkaline solution for 10 to 15 hours, followed by washing and drying. The co-firing modification includes the following steps: mixing the biochar with the modifier, grinding it, heating it to 300-700°C, keeping it warm for 45-70 minutes, then air-cooling it to room temperature, and then washing and drying it to obtain the biochar; the modifier is Na2S2O3, KMnO4, a Na2S2O3-KMnO4 mixture, KOH or Na3PO4.

2. The preparation method according to claim 1, characterized in that The raw material pretreatment method in S1 is as follows: the raw material is crushed into particles with a particle size of less than 5 mm, washed with deionized water and then dried at 60°C.

3. The preparation method according to claim 1, wherein: The heating rate of the pretreated material in S2 is 6°C / min; the cleaning method is to wash with deionized water until neutral; and the drying method is to dry at 60°C to constant weight.

4. The preparation method according to claim 1, wherein: The alkali solution is a sodium hydroxide solution with a concentration of 2M; during the soaking modification process, the biochar is soaked in the alkali solution twice for 12 hours.

5. The preparation method according to claim 1, wherein: During the co-firing modification process, biochar and modifier were mixed in a mass ratio of 1:1; the heating rate was 6°C / min.

6. The preparation method according to claim 1, wherein: The raw material for preparing the biochar is rice straw, and the pyrolysis temperature of rice straw when preparing biochar is 300°C; the obtained biochar is modified by immersing the biochar in a 2M sodium hydroxide solution for 12 hours, then discarding the supernatant, adding a 2M sodium hydroxide solution and soaking for 12 hours, and then washing and drying.

7. The preparation method according to claim 1, wherein: The raw material for preparing the biochar is rice straw, and the pyrolysis temperature of rice straw when preparing biochar is 700°C; the modification method of the obtained biochar is to mix the biochar and Na3PO4 in a mass ratio of 1:1, then heat it to 700°C at a heating rate of 6°C / min, keep it warm for 60 minutes, then air-cool it to room temperature, and then wash and dry it.

8. A modified biochar, characterized in that: The method is prepared according to any one of claims 1 to 7.

9. The use of the modified biochar according to claim 8, characterized in that: The modified biochar is used for wastewater denitrification based on SBR.

10. The use according to claim 9, characterized in that The sewage composition is: 150-600 mg / L COD, 600 mg / L NaHCO3, 50 mg / L KNO3, 30 mg / L MgCl2·6H2O, 6.4 mg / L CaCl2, 5.0 mg / L FeSO4·7H2O, 0.12 mg / L MnCl2·4H2O, 0.03 mg / L CuSO4·5H2O, 0.12 mg / L ZnSO4·7H2O, 1500 mg / LEDTA and 0.03 mg / L CoCl·6H2O; the dosage of the modified biochar in the SBR reactor is 1 g / L; during the SBR denitrification process, the O / A ratio is 0.5-2.6, the C / N ratio is 3-12, and the operation time is 21-31 days.

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