Method for preparing sludge-based phosphorus-rich biochar material through Fenton modification

Abstract

The invention belongs to the technical field of harmless treatment and resource utilization of municipal sludge, and particularly relates to a method for preparing a sludge-based phosphorus-rich biochar material through Fenton modification. Aiming at the problems of low phosphorus fixation rate, high heavy metal leaching risk and the like in sludge treatment in the prior art, the method comprises the following steps: performing Fenton modification on sludge according to a TOC (Total Organic Carbon) value, and performing pyrolysis and calcination on the modified sludge at 500 DEG C and 700-900 DEG C in sequence to obtain the phosphorus-rich biochar. The heavy metal leaching rate of the phosphorus-rich biochar prepared by the method is greatly reduced, meanwhile, the phosphorus fixation rate is increased, and the method has remarkable progress.

Description

Technical Field

[0001] The present invention belongs to the technical field of harmless treatment and resource utilization of municipal sludge, and specifically relates to a method for preparing sludge-based phosphorus-rich biochar material through Fenton modification. Background Art

[0002] The treatment and resource utilization of municipal sludge is a core challenge in the field of environmental engineering. It contains large amounts of organic matter, phosphorus, and heavy metal pollutants (such as cadmium, lead, and arsenic). While phosphorus, a non-renewable resource, has recycling value, the presence of heavy metals severely restricts the resource utilization of sludge. Achieving "reduction, harmlessness, and resource utilization" in the treatment of sludge has become a core issue that urgently needs to be addressed in the field of environmental engineering.

[0003] While traditional heat treatment processes can partially fix phosphorus and remove heavy metals, the presence of complex organic matter in sludge (such as humus and lipids) severely hinders the effective release and stabilization of phosphorus, resulting in low phosphorus fixation rates (typically 78%-85%), insufficient bioavailability (30%-40%), and a high risk of heavy metal leaching (e.g., Cd: 0.3 mg / L, Pb: 0.5 mg / L). Furthermore, the treatment of Fenton iron sludge in existing technologies is mostly limited to the recovery of iron resources (e.g., the preparation of ferrous sulfate), failing to achieve the synergistic resource utilization of organic and inorganic matter in sludge.

[0004] Fenton oxidation technology efficiently degrades organic pollutants through hydroxyl radicals (·OH), but its application in sludge pretreatment still has limitations. For example, in the iron-containing sludge (Fenton iron sludge) produced after the Fenton reaction, the iron element is mostly aggregated in the form of hydroxyl oxides. Direct use in the pyrolysis-calcination process will lead to uneven iron distribution, affecting the catalytic performance of biochar. Existing patents (such as CN118416889A) propose a method of combining acid and alkali treatment with high-temperature pyrolysis. Although it improves the dispersibility of iron elements, it still does not solve the inhibitory effect of organic matter on phosphorus release, and does not fully utilize the pretreatment potential of the Fenton reaction.

[0005] Therefore, there is an urgent need to develop a sludge treatment process that can simultaneously optimize the degradation efficiency of organic matter, release more fixed phosphorus, and promote the uniform loading of iron elements, ultimately achieving efficient phosphorus fixation, improved bioavailability and heavy metal risk control. Summary of the Invention

[0006] In view of this, the present invention provides a preparation method that can effectively improve the bioavailability of sludge-based phosphorus-rich biochar, which can remove the contamination of heavy metal ions such as As, Cd, Cu, Ni, Pb and Zn in biochar and the problem of large-scale loss of phosphorus.

[0007] The method for preparing sludge-based phosphorus-rich biochar material by Fenton modification according to the present invention comprises the following steps: (1) Heating and drying municipal sludge, grinding, crushing, and screening to obtain sludge particles; (2) Add a certain proportion of deionized water to the sludge particles to prepare sludge slurry; (3) After the sludge slurry was dispersed evenly by ultrasonic treatment and the pH was adjusted, ferrous sulfate heptahydrate (FeSO4·7H2O) and hydrogen peroxide (H2O2) were Fenton-modified to obtain modified sludge; Fe 2+ The molar ratio with H2O2 is 1:1; (4) Place the modified sludge into a centrifuge for centrifugal dehydration, followed by washing and drying; (5) The dried modified sludge is pyrolyzed using a microwave pyrolysis device, and sludge semi-coke is obtained after cooling; (6) The sludge semi-coke is calcined in a nitrogen atmosphere and cooled to obtain phosphorus-rich biochar.

[0008] Preferably, in step (1), the municipal sludge is heated and dried in an oven at 105°C, ground and crushed, and then passed through a 100-200 mesh sieve; Preferably, the pH in step (3) is 3-4, and the reaction time of Fenton modification is 2h. Specifically, according to the TOC index of the sludge, corresponding concentrations of ferrous sulfate heptahydrate (FeSO4·7H2O) and hydrogen peroxide (H2O2) are added for Fenton modification. When TOC is ≤ 500mg / L, the Fe 2+ The addition amount is 30mM, the addition amount of H2O2 is 30mM; when TOC ≥ 500mg / L, add Fe 2+ 50mM, H2O250mM; Preferably, the centrifuge speed in step (4) is 9000 r / min, the centrifugation time is 10 min, the drying temperature is 80° C., and the drying time is 24 h, so that the modified sludge is dried to a constant weight.

[0009] Preferably, high-purity nitrogen is introduced to replace the air atmosphere in the microwave pyrolysis device before pyrolysis in step (5), and the nitrogen flow rate during pyrolysis is greater than 100 mL / min; the pyrolysis temperature is 500°C, the heating rate is 10°C / min, and the pyrolysis time is 60 min.

[0010] Preferably, in step (6), a tubular furnace is used for calcination, and nitrogen is introduced before calcination to replace the air in the furnace. The calcination temperature is 700°C-900°C, the heating rate is 10°C / min, the calcination time is 30min-90min, and the nitrogen flow rate is greater than 100mL / min.

[0011] Preferably, the calcination temperature in step (6) is 800°C-900°C.

[0012] After pyrolysis at 500°C and calcination at 700°C-900°C, the organic matter in the sludge targeted by Fenton modification is basically completely decomposed or carbonized. At this time, Fenton modification is not only inefficient, but also destroys the stability of the biochar itself and increases the risk of leaching of heavy metals and phosphorus. Therefore, the present invention first performs Fenton modification and then pyrolysis and calcination, without involving Fenton modification of the calcined biochar.

[0013] The Fenton-modified phosphorus-rich biochar prepared by the present invention can be used in phosphate rock or phosphate fertilizer. One of the core contributions of Fenton modification is heavy metal removal. Hydroxyl radicals (·OH) can break the bonds between heavy metals and organic matter, for example, dissociating cadmium-humic acid complexes into free Cd 2+ The free heavy metals are then adsorbed or co-precipitated by the Fe(OH)3 colloid generated by hydrolysis, which greatly reduces their mobility. During the high-temperature calcination stage, the remaining heavy metals and Fe 0 The reaction generates stable oxides (such as CdO and PbFeO4), which are further solidified in the biochar matrix. Experiments have shown that after Fenton pretreatment, the leaching concentration of cadmium can be reduced from 0.3 mg / L to below 0.1 mg / L. The improvement in heavy metal removal efficiency is due to the synergistic effect of multiple stages. The Fenton modification stage initially fixes heavy metals through oxidative decomposition and iron-based co-precipitation; the pyrolysis stage further reduces them to elemental metals or low-toxic oxides; and the calcination stage completely reduces the leaching risk through high-temperature solidification. Taking cadmium as an example, the leaching concentration after Fenton treatment is reduced by more than 40%, and the removal efficiency of heavy metals such as lead and arsenic is also significantly improved.

[0014] The introduction of Fenton modification technology may lead to an increase in total costs. Studies have shown that when Fe 2+The effects were significant and similar when the molar ratio to H₂O₂ ranged from 1:1 to 4:1. Given the inherent iron content in sludge, a 1:1 ratio was used in this study to minimize costs. Optimizing the pyrolysis temperature is also crucial. A pyrolysis temperature of 500°C balances organic matter pyrolysis efficiency with energy costs. Temperatures too low result in incomplete pyrolysis, while temperatures too high may damage the carbon skeleton structure. The heating rate was controlled at 10°C / min to avoid local overheating or pore collapse caused by rapid heating. The choice of calcination temperature directly influences the physicochemical properties of biochar. Within the 700°C-900°C range, the specific surface area increases significantly with increasing temperature. However, above 900°C, the carbon skeleton may collapse due to excessive ablation, resulting in a decrease in specific surface area. Therefore, this temperature range is preferred as the optimal calcination temperature, balancing pore development and structural stability. The specific surface area and pore structure of biochar directly influence its application potential. The phosphorus-rich biochar produced by this method has a specific surface area of ​​30-120 m² / g, with mesopores accounting for over 70%, making it suitable for applications in adsorption and catalysis. SEM-EDS characterization revealed that iron is evenly distributed on the carbon skeleton surface, forming a high density of active sites, providing a foundation for subsequent environmental remediation applications.

[0015] The phosphorus fixation rate and bioavailability of phosphorus-rich biochar are key indicators of process success. Due to the coupled mechanism between phosphorus and heavy metals, while reducing heavy metal content, the phosphorus fixation rate should be maintained as much as possible, ideally increasing it. The present invention utilizes Fenton pretreatment to release encapsulated phosphorus, combined with stabilization through pyrolysis and calcination, to increase the phosphorus fixation rate to over 77%. The phosphorus form is converted from inert hydroxyapatite to soluble pyrophosphate, and the bioavailable fraction reaches over 50%, significantly exceeding the 30%-40% of conventional processes. Compared to the traditional two-step process (i.e., pyrolysis and calcination alone without Fenton pretreatment), the present invention offers advantages in multiple dimensions. The phosphorus fixation rate is significantly increased, the bioavailable fraction increases from 37.4% to 53.2%, the heavy metal leaching concentration is reduced by 60%-90%, and the specific surface area is expanded by 60%-313%. Detailed data from the examples further support the practicality of the present invention. For example, when treating municipal sludge with a TOC of 350 mg / L, the phosphorus fixation rate reached 99.43%, and the zinc and copper leaching concentrations were less than 0.2 mg / L and 0.15 mg / L, respectively. When treating municipal sludge with a TOC of 510 mg / L, the arsenic leaching concentration was less than 0.05 mg / L, demonstrating the effectiveness of the Fenton pretreatment and pyrolysis-calcination synergistic process, as well as its wide applicability to complex sludge. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 This is the elemental mapping diagram of the phosphorus-rich biochar prepared in Example 3 obtained by scanning transmission electron microscopy combined with energy spectrum analysis.

[0017] Figure 2 This is the elemental mapping diagram of the biochar prepared in Comparative Example 3 using scanning transmission electron microscopy combined with energy spectrum analysis. DETAILED DESCRIPTION

[0018] The present invention is described in detail below with reference to the embodiments. The embodiments described in the following exemplary embodiments do not represent all embodiments consistent with the present invention. Obviously, the embodiments described are only some embodiments of the present invention, rather than all embodiments. In the absence of conflict, the embodiments in this application and the features in the embodiments can be combined with each other. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0019] Those skilled in the art will readily appreciate other embodiments of the present invention after considering the invention disclosed herein in the specification and examples. This application is intended to cover any variations, uses, or adaptations of the present invention that follow the general principles of the invention and include common knowledge or customary techniques in the art not disclosed herein. The description and examples are to be considered as exemplary only.

[0020] To better illustrate the technical means and product effects of the present invention, the preferred embodiments of the present invention are described below. The microwave pyrolysis furnace used in the embodiment is model CY-PY1100C-M, and the tubular furnace used for calcination is model 181-GSL-1200X, a two-way mass flow meter open tubular electric furnace.

[0021] Example 4 and Comparative Example 4 used municipal sludge 2 as the sludge raw material, while the other examples and comparative examples used municipal sludge 1 as the sludge raw material. The industrial analysis of the municipal sludge used in the examples was performed according to GB / T 17664-1999, mainly including fixed carbon (FC%), volatile matter (V%), ash (A%), and moisture (M%), where FC = 100% - VAM. Elemental analysis was performed using the EA 3000 element analyzer, mainly including C (%), H (%), N (%), S (%), and O (%), where O = 100% - CHSNA. The test results are shown in Table 4: Table 4 Main components and contents of municipal sludge

[0022] Example 1

[0023] (1) First, the excess sludge from the municipal sewage treatment plant is heated and dried in an oven at 105°C, then crushed and ground, and passed through a 200-mesh sieve to obtain sludge particles; (2) Take 40g of sludge particles, add 400ml of deionized water, and stir evenly under ultrasonic conditions to prepare sludge slurry; (3) After adjusting the pH of the sludge slurry to 3, its TOC was measured to be 350 mg / L. The dosage of FeSO4·7H2O in the slurry was 30 mM, and the dosage of H2O2 was 30 mM. The reaction was carried out for 2 hours to perform Fenton modification and obtain modified sludge. (4) Place the modified sludge in a centrifuge at 9000 r / min for 10 min to dehydrate, wash it after dehydration, dry it at 80°C for 24 h, and then pass it through a 100-mesh sieve; (5) The dried modified sludge was placed in a microwave pyrolysis furnace, and the pyrolysis conditions were set. High-purity nitrogen was introduced to replace the air atmosphere in the furnace. After the replacement was completed, the microwave oven was started to pyrolyze the sludge. The pyrolysis temperature was 500 °C, the heating rate was 10 °C / min, and the pyrolysis time was 60 min. Nitrogen was continuously introduced during the pyrolysis process, and the gas flow rate was greater than 100 mL / min. After the pyrolysis was completed, the sludge was cooled to obtain sludge semi-coke. (6) The sludge semi-coke was placed in a tubular furnace for calcination. Before calcination, nitrogen was introduced to replace the air in the furnace. The calcination was carried out under a nitrogen atmosphere with a gas flow rate greater than 100 mL / min, a calcination temperature of 700 °C, a heating rate of 10 °C / min, and a calcination time of 60 min. After cooling, phosphorus-rich biochar was obtained.

[0024] Example 2 The difference between this embodiment and embodiment 1 is that the calcination temperature in step (6) is set to 800° C., and the other preparation methods are the same as those in embodiment 1.

[0025] (1) First, the excess sludge from the municipal sewage treatment plant is heated and dried in an oven at 105°C, then crushed and ground, and passed through a 200-mesh sieve to obtain sludge particles; (2) Take 40g of sludge particles, add 400ml of deionized water, and stir evenly under ultrasonic conditions to prepare sludge slurry; (3) After adjusting the pH of the sludge slurry to 3, its TOC was measured to be 350 mg / L. The dosage of FeSO4·7H2O in the slurry was 30 mM, and the dosage of H2O2 was 30 mM. The reaction was carried out for 2 hours to perform Fenton modification and obtain modified sludge. (4) Place the modified sludge in a centrifuge at 9000 r / min for 10 min to dehydrate, wash it after dehydration, dry it at 80°C for 24 h, and then pass it through a 100-mesh sieve; (5) The dried modified sludge was placed in a microwave pyrolysis furnace, and the pyrolysis conditions were set. High-purity nitrogen was introduced to replace the air atmosphere in the furnace. After the replacement was completed, the microwave oven was started to pyrolyze the sludge. The pyrolysis temperature was 500 °C, the heating rate was 10 °C / min, and the pyrolysis time was 60 min. Nitrogen was continuously introduced during the pyrolysis process, and the gas flow rate was greater than 100 mL / min. After the pyrolysis was completed, the sludge was cooled to obtain sludge semi-coke. (6) The sludge semi-coke was placed in a tubular furnace for calcination. Before calcination, nitrogen was introduced to replace the air in the furnace. The calcination was carried out in a nitrogen atmosphere with a gas flow rate greater than 100 mL / min, a calcination temperature of 800 °C, a heating rate of 10 °C / min, and a calcination time of 60 min. After cooling, phosphorus-rich biochar was obtained.

[0026] Example 3 The difference between this embodiment and embodiment 1 is that the calcination temperature in step (6) is set to 900° C., and the other preparation methods are the same as those in embodiment 1.

[0027] (1) First, the excess sludge from the municipal sewage treatment plant is heated and dried in an oven at 105°C, then crushed and ground, and passed through a 200-mesh sieve to obtain sludge particles; (2) Take 40g of sludge particles, add 400ml of deionized water, and stir evenly under ultrasonic conditions to prepare sludge slurry; (3) After adjusting the pH of the sludge slurry to 3, its TOC was measured to be 350 mg / L. The FeSO4·7H2O dosage in the slurry was 30 mM, and the H2O2 dosage was 30 mM. The reaction was carried out for 2 h to perform Fenton modification and obtain modified sludge. (4) Place the modified sludge in a centrifuge at 9000 r / min for 10 min to dehydrate, wash it after dehydration, dry it at 80°C for 24 h, and then pass it through a 100-mesh sieve; (5) The dried modified sludge was placed in a microwave pyrolysis furnace, and the pyrolysis conditions were set. High-purity nitrogen was introduced to replace the air atmosphere in the furnace. After the replacement was completed, the microwave oven was started to pyrolyze the sludge. The pyrolysis temperature was 500 °C, the heating rate was 10 °C / min, and the pyrolysis time was 60 min. Nitrogen was continuously introduced during the pyrolysis process, and the gas flow rate was greater than 100 mL / min. After the pyrolysis was completed, the sludge was cooled to obtain sludge semi-coke. (6) The sludge semi-coke was placed in a tubular furnace for calcination. Before calcination, nitrogen was introduced to replace the air in the furnace. The calcination was carried out in a nitrogen atmosphere with a gas flow rate greater than 100 mL / min, a calcination temperature of 900 °C, a heating rate of 10 °C / min, and a calcination time of 60 min. After cooling, phosphorus-rich biochar was obtained.

[0028] like Figure 1 and Figure 2 As shown, Figure 1 The sludge-based biochar after Fenton modification was shown. Figure 2 The figure shows the unmodified sludge-based biochar. Observing the figure, we can see that after Fenton modification, the number of micropores and mesopores in the biochar increases significantly, its specific surface area also increases, and the phosphorus density also increases. Furthermore, the distribution of iron (Fe) becomes more uniform. These changes demonstrate the successful modification process and its positive impact on phosphorus fixation.

[0029] Example 4 The difference between this embodiment and embodiment 3 is that the sludge composition is different, the TOC in step (3) is 510 mg / L, the FeSO4·7H2O dosage in the slurry is 50 mM, and the H2O2 dosage is 50 mM. The other preparation methods are the same as those in embodiment 3.

[0030] (1) First, the excess sludge from the municipal sewage treatment plant is heated and dried in an oven at 105°C, then crushed and ground, and passed through a 200-mesh sieve to obtain sludge particles; (2) Take 40g of sludge particles, add 400ml of deionized water, and stir evenly under ultrasonic conditions to prepare sludge slurry; (3) After adjusting the pH of the sludge slurry to 3, its TOC was measured to be 510 mg / L. The FeSO4·7H2O dosage in the slurry was 50 mM, and the H2O2 dosage was 50 mM. The reaction was carried out for 2 h to perform Fenton modification and obtain modified sludge. (4) Place the modified sludge in a centrifuge at 9000 r / min for 10 min to dehydrate, wash it after dehydration, dry it at 80°C for 24 h, and then pass it through a 100-mesh sieve; (5) The dried modified sludge was placed in a microwave pyrolysis furnace, and the pyrolysis conditions were set. High-purity nitrogen was introduced to replace the air atmosphere in the furnace. After the replacement was completed, the microwave oven was started to pyrolyze the sludge. The pyrolysis temperature was 500 °C, the heating rate was 10 °C / min, and the pyrolysis time was 60 min. Nitrogen was continuously introduced during the pyrolysis process, and the gas flow rate was greater than 100 mL / min. After the pyrolysis was completed, the sludge was cooled to obtain sludge semi-coke. (6) The sludge semi-coke was placed in a tubular furnace for calcination. Before calcination, nitrogen was introduced to replace the air in the furnace. The calcination was carried out in a nitrogen atmosphere with a gas flow rate greater than 100 mL / min, a calcination temperature of 900 °C, a heating rate of 10 °C / min, and a calcination time of 60 min. After cooling, phosphorus-rich biochar was obtained.

[0031] Comparative Example 1 This comparative example differs from Example 1 in that there is no Fenton modification treatment.

[0032] (1) First, the excess sludge from the municipal sewage treatment plant is heated and dried in an oven at 105°C, then crushed and ground, and passed through a 200-mesh sieve to obtain sludge particles; (2) The sludge particles were placed in a microwave pyrolysis furnace, and the pyrolysis conditions were set. High-purity nitrogen was introduced to replace the air atmosphere in the furnace. After the replacement was completed, the microwave oven was started to pyrolyze the sludge. The pyrolysis temperature was 500°C, the heating rate was 10°C / min, and the pyrolysis time was 60 min. Nitrogen was continuously introduced during the pyrolysis process, and the gas flow rate was greater than 100 mL / min. After the pyrolysis was completed, the sludge was cooled to obtain sludge semi-coke. (3) The sludge semi-coke was placed in a tubular furnace for calcination. Before calcination, nitrogen was introduced to replace the air in the furnace. The calcination was carried out under a nitrogen atmosphere with a gas flow rate greater than 100 mL / min, a calcination temperature of 700 °C, a heating rate of 10 °C / min, and a calcination time of 60 min. Biochar was obtained after cooling.

[0033] Comparative Example 2 This comparative example differs from Example 2 in that there is no Fenton modification treatment.

[0034] (1) First, the excess sludge from the municipal sewage treatment plant is heated and dried in an oven at 105°C, then crushed and ground, and passed through a 200-mesh sieve to obtain sludge particles; (2) The sludge particles were placed in a microwave pyrolysis furnace, and the pyrolysis conditions were set. High-purity nitrogen was introduced to replace the air atmosphere in the furnace. After the replacement was completed, the microwave oven was started to pyrolyze the sludge. The pyrolysis temperature was 500°C, the heating rate was 10°C / min, and the pyrolysis time was 60 min. Nitrogen was continuously introduced during the pyrolysis process, and the gas flow rate was greater than 100 mL / min. After the pyrolysis was completed, the sludge was cooled to obtain sludge semi-coke. (3) The sludge semi-coke was placed in a tubular furnace for calcination. Before calcination, nitrogen was introduced to replace the air in the furnace. The calcination was carried out in a nitrogen atmosphere with a gas flow rate greater than 100 mL / min, a calcination temperature of 800 °C, a heating rate of 10 °C / min, and a calcination time of 60 min. Biochar was obtained after cooling.

[0035] Comparative Example 3 This comparative example differs from Example 3 in that there is no Fenton modification treatment.

[0036] (1) First, the excess sludge from the municipal sewage treatment plant is heated and dried in an oven at 105°C, then crushed and ground, and passed through a 200-mesh sieve to obtain sludge particles; (2) The sludge particles were placed in a microwave pyrolysis furnace, and the pyrolysis conditions were set. High-purity nitrogen was introduced to replace the air atmosphere in the furnace. After the replacement was completed, the microwave oven was started to pyrolyze the sludge. The pyrolysis temperature was 500°C, the heating rate was 10°C / min, and the pyrolysis time was 60 min. Nitrogen was continuously introduced during the pyrolysis process, and the gas flow rate was greater than 100 mL / min. After the pyrolysis was completed, the sludge was cooled to obtain sludge semi-coke. (3) The sludge semi-coke was placed in a tubular furnace for calcination. Before calcination, nitrogen was introduced to replace the air in the furnace. The calcination was carried out in a nitrogen atmosphere with a gas flow rate greater than 100 mL / min, a calcination temperature of 900 °C, a heating rate of 10 °C / min, and a calcination time of 60 min. Biochar was obtained after cooling.

[0037] Comparative Example 4 This comparative example differs from Example 4 in that there is no Fenton modification treatment.

[0038] (1) First, the excess sludge from the municipal sewage treatment plant is heated and dried in an oven at 105°C, then crushed and ground, and passed through a 200-mesh sieve to obtain sludge particles; (2) The sludge particles were placed in a microwave pyrolysis furnace, and the pyrolysis conditions were set. High-purity nitrogen was introduced to replace the air atmosphere in the furnace. After the replacement was completed, the microwave oven was started to pyrolyze the sludge. The pyrolysis temperature was 500 °C, the heating rate was 10 °C / min, and the pyrolysis time was 60 min. Nitrogen was continuously introduced during the pyrolysis process, and the gas flow rate was greater than 100 mL / min. After the pyrolysis was completed, the sludge was cooled to obtain sludge semi-coke. (3) The sludge semi-coke was placed in a tubular furnace for calcination. Before calcination, nitrogen was introduced to replace the air in the furnace. The calcination was carried out in a nitrogen atmosphere with a gas flow rate greater than 100 mL / min, a calcination temperature of 900 °C, a heating rate of 10 °C / min, and a calcination time of 60 min. Biochar was obtained after cooling.

[0039] Comparative Example 5 The difference between this comparative example and Example 3 is that the sludge particles prepared in step (1) are first subjected to microwave pyrolysis, then subjected to Fenton modification, and finally subjected to calcination.

[0040] (1) First, the excess sludge from the municipal sewage treatment plant is heated and dried in an oven at 105°C, then crushed and ground, and passed through a 200-mesh sieve to obtain sludge particles; (2) The sludge particles were placed in a microwave pyrolysis furnace, and the pyrolysis conditions were set. High-purity nitrogen was introduced to replace the air atmosphere in the furnace. After the replacement was completed, the microwave oven was started to pyrolyze the sludge. The pyrolysis temperature was 500°C, the heating rate was 10°C / min, and the pyrolysis time was 60 min. Nitrogen was continuously introduced during the pyrolysis process, and the gas flow rate was greater than 100 mL / min. After the pyrolysis was completed, the sludge was cooled to obtain sludge semi-coke. (3) Take 40g of sludge semi-coke, add 400ml of deionized water, and stir evenly under ultrasonic environment to prepare sludge semi-coke slurry; (4) After adjusting the pH of the sludge semi-coke slurry to 3, its TOC was measured to be 210 mg / L. The dosage of FeSO4·7H2O in the slurry was 30 mM, and the dosage of H2O2 was 30 mM. The reaction was carried out for 2 h to perform Fenton modification and obtain modified sludge semi-coke. (5) Place the modified sludge semi-coke into a centrifuge at 9000 r / min for 10 min for dehydration, wash it after dehydration, dry it at 80°C for 24 h, and then pass it through a 100-mesh sieve; (6) The dried modified sludge semi-coke was placed in a tubular furnace for calcination. Nitrogen was introduced before calcination to replace the air in the furnace. The calcination was carried out under a nitrogen atmosphere with a gas flow rate greater than 100 mL / min, a calcination temperature of 900 °C, a heating rate of 10 °C / min, and a calcination time of 60 min. Biochar was obtained after cooling.

[0041] Performance Analysis 1. Analysis of heavy metal content in phosphorus-rich biochar An inductively coupled plasma emission spectrometer (ICP) was used to detect the content of heavy metal ions in the biochar samples prepared in each embodiment and comparative example. The specific method was as follows: 0.1 g of sample was weighed and placed in a polytetrafluoroethylene digestion tank of a microwave digester. 8 mL of 65% nitric acid, 1 mL of 30% hydrogen peroxide, and 2 mL of 30% hydrofluoric acid were added to the sample, and then the sample was digested using microwave heating. The digestion method was as follows: first, the temperature was raised to 150° C. and kept warm for 10 minutes, then the temperature was continued to be raised to 180° C. and kept warm for 5 minutes, and then the temperature was continued to be raised to 210° C. and kept warm for 30 minutes to complete the digestion process. The microwave power during the entire digestion process was 2000 W.

[0042] After the digested sample was cooled, it was filtered using a 0.45 μm filter and fixed to volume. The heavy metal concentration in the sample was detected by ICP. The results are shown in Table 1.

[0043] Table 1 Heavy metal content in the biochar materials prepared in various examples and comparative examples (mg / kg) As Cd Cr Cu Ni Pb Zn total Comparative Example 1 50.8 2 290.3 314.9 96.5 76.0 1124.7 1955.2 Example 1 46.2 0 253.7 299.4 81.8 74 893.0 1648.1 Comparative Example 2 46.1 1.9 270.0 317.2 98.6 43.2 876.3 1653.3 Example 2 43.2 0.0 267.5 313.2 87.6 39.4 538.0 1288.9 Comparative Example 3 60.7 1.8 270.0 331.2 97.3 3.1 485.6 1248.9 Example 3 41.3 0 216.0 315.9 81.7 0 144.3 799.3 Comparative Example 4 63.1 0 251.5 345.5 91 0 375.5 1126.6 Example 4 41.1 0 199.9 320.3 73.8 0 112.3 747.4 Comparative Example 5 48.3 0.0 256.4 278.4 97.9 0 444.2 1125.1 Comparison of Table 1 with the corresponding comparative examples shows that the phosphorus-rich biochar prepared in each example can effectively reduce the total amount of heavy metals. Compared with Comparative Example 3, the total amount of heavy metals in the phosphorus-rich biochar prepared in Example 3 was reduced by 36%. Compared with Comparative Example 4, the total amount of heavy metals in the phosphorus-rich biochar prepared from high-TOC sludge in Example 4 was reduced by 34%. Comparative Example 5 shows that the biochar prepared by pyrolysis followed by Fenton modification has a less significant effect on reducing heavy metals.

[0044] 2. Analysis of phosphorus fixation rate in phosphorus-rich biochar A standardized phosphorus speciation method (SMT) developed under the framework of the European Committee for Standardization and Testing was used to detect the phosphorus fixation rate in the biochar samples prepared in each embodiment and comparative example. The specific method is as follows: 20 ml of 3.5 M hydrochloric acid was added to 0.2 g of sample, and the sample was extracted with constant temperature shaking for 16 hours. The supernatant was extracted using a 0.45 μm filter and the phosphorus content in the sample was analyzed by molybdenum blue test. The results are shown in Table 2.

[0045] Phosphorus yield R in the sample yield The calculation formula is as follows: R yield =M2 / M1×100% (1) Where M1 and M2 are the masses of the sample before and after calcination, respectively.

[0046] The formula for calculating the phosphorus fixation rate ΨP in the sample is as follows: ψP=C2 / C1×R yield ×100% (2) Where C1 and C2 are the phosphorus concentrations in the sample before and after calcination, respectively.

[0047] Table 2 Phosphorus detection results in various examples and comparative examples Calcination temperature (T) / ℃ Yield (%) <![CDATA[C1(mg / g)]]> <![CDATA[C2(mg / g)]]> Fixed rate (%) Comparative Example 1 700 95.03 53.65 42.13 74.62 Example 1 700 92.47 53.65 44.85 77.30 Comparative Example 2 800 91.21 53.65 57.22 97.28 Example 2 800 88.92 53.65 59.99 99.43 Comparative Example 3 900 88.66 53.65 49.08 81.11 Example 3 900 86.01 53.65 50.78 81.41 According to Table 2, compared with the corresponding comparative examples, the phosphorus concentration and fixation rate in the phosphorus-rich biochars prepared by calcining at different temperatures in Examples 1 to 3 were improved.

[0048] From the above analysis, it can be seen that the sludge Fenton modified in the present invention is subjected to a two-step heat treatment of pyrolysis-calcination, and the Fe 2+ and H2O2 dosage, Fenton-modified phosphorus-rich biochar was prepared, achieving excellent results at a lower cost. It can not only reduce the risk of heavy metal pollution during the resource utilization of sludge-based phosphorus-rich biochar, but also obtain a higher phosphorus yield and fixation rate, indicating that the present invention can achieve the preset goal of harmless and resource-based treatment of municipal sludge.

[0049] 3. Analysis of specific surface area of ​​phosphorus-rich biochar The specific surface area of ​​sludge-based phosphorus-rich biochar was measured using a fully automatic specific surface and porosity analyzer (BET), and the results are shown in Table 3.

[0050] Table 3 Specific surface area (m2) of biochar materials prepared in various examples and comparative examples 2 / g) Comparative Example 1 Example 1 Comparative Example 2 Example 2 Comparative Example 3 Example 3 Comparative Example 4 Example 4 Specific surface area 14.39 29.43 44.54 71.34 22.93 94.76 26.99 84.25 According to Table 3, Fenton modification can significantly increase the specific surface area of ​​sludge-based phosphorus-rich biochar, providing more binding sites for phosphorus fixation.

Claims

1. A method for preparing sludge-based phosphorus-rich biochar material by Fenton modification, characterized in that: Includes the following: (1) Heating and drying municipal sludge, grinding, crushing, and screening to obtain sludge particles; (2) Add a certain proportion of deionized water to the sludge particles to prepare sludge slurry; (3) After the sludge slurry is dispersed evenly by ultrasonic treatment and the pH is adjusted, ferrous sulfate and hydrogen peroxide are added for Fenton modification to obtain modified sludge; Fe 2+ The molar ratio with H2O2 is 1:1; (4) Place the modified sludge into a centrifuge for centrifugal dehydration, followed by washing and drying; (5) The dried modified sludge is pyrolyzed using a microwave pyrolysis device, and sludge semi-coke is obtained after cooling; (6) The sludge semi-coke is calcined in a nitrogen atmosphere and cooled to obtain phosphorus-rich biochar.

2. The method for preparing sludge-based phosphorus-rich biochar material by Fenton modification according to claim 1, characterized in that: In step (1), the municipal sludge is heated and dried at 105°C, ground and crushed, and then passed through a 100-200 mesh sieve.

3. The method for preparing sludge-based phosphorus-rich biochar material by Fenton modification according to claim 1, characterized in that: In step (3), the pH is 3-4, and the reaction time of Fenton modification is 2 h.

4. The method for preparing sludge-based phosphorus-rich biochar material by Fenton modification according to claim 1, characterized in that: According to the TOC index of sludge, ferrous sulfate and hydrogen peroxide were added for Fenton modification. When TOC was ≤500mg / L, Fe 2+ The addition amount is 30mM, the addition amount of H2O2 is 30mM; when TOC≥500mg / L, Fe 2+ The added amount was 50 mM, and the added amount of H2O2 was 50 mM.

5. The method for preparing sludge-based phosphorus-rich biochar material by Fenton modification according to claim 1, characterized in that: The centrifuge speed in step (4) is 9000 r / min, the centrifugation time is 10 min, and the drying temperature is 80° C., so that the modified sludge is dried to a constant weight.

6. The method for preparing sludge-based phosphorus-rich biochar material by Fenton modification according to claim 1, characterized in that: In step (5), nitrogen is introduced to replace the air atmosphere in the microwave pyrolysis device before pyrolysis, and the nitrogen flow rate during pyrolysis is greater than 100 mL / min; the pyrolysis temperature is 500°C, the heating rate is 10°C / min, and the pyrolysis time is 60 min.

7. The method for preparing sludge-based phosphorus-rich biochar material by Fenton modification according to claim 1, characterized in that: In step (6), calcination is carried out in a nitrogen atmosphere at a calcination temperature of 700°C-900°C, a heating rate of 10°C / min, a calcination time of 30min-90min, and a nitrogen flow rate greater than 100mL / min during calcination.

8. The method for preparing sludge-based phosphorus-rich biochar material by Fenton modification according to claim 7, characterized in that: The calcination temperature in step (6) is 800°C-900°C.

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