Sludge-based nano biochar as well as preparation method and application thereof

Abstract

The invention relates to sludge-based nano biochar as well as a preparation method and application thereof. The preparation method comprises the following steps: preparing iron-containing excess sludge into a suspension, adjusting the pH value to 2-8, heating and hydrolyzing at 40-150 DEG C, collecting solids, drying and grinding to obtain sludge fine powder; calcining in an N2 atmosphere to obtain charcoal powder; the charcoal powder is subjected to acid pickling, and acidified charcoal is obtained; mixing and reacting with hydrogen peroxide in proportion, centrifuging to take supernate, and filtering to obtain a nano biochar solution; and purifying and drying the nano-biochar solution to prepare nano-biochar powder. The particle size of the nano biochar is 1t; the molar percentage content of the oxygen element is 15%-50%; the width of lattice fringes is 0.22 nm, and the total content of heavy metals is lower than 0.05%. The sludge-based nano biochar provided by the invention has good conductivity, oxidation-reduction property and low biotoxicity, and can simultaneously accelerate the extracellular electron transfer effect and intracellular electron transfer rate of microorganisms and improve the biological denitrification efficiency.

Description

TECHNICAL FIELD

[0001] The present application relates to a sludge-based nanobiocarbon and a preparation method and application thereof, and belongs to the technical field of sludge treatment and pollution control. BACKGROUND

[0002] With the rapid development of industrialization and the overuse of nitrogen fertilizer in agriculture, a large amount of nitrate is discharged into groundwater and surface water, leading to many environmental problems (such as water eutrophication and greenhouse effect) and human diseases (such as hemochromatosis, blue blood disease and carcinogenesis). At present, biological denitrification technology plays an important role in the removal of nitrate due to its simple operation, low cost and other advantages, but the low denitrification rate and incomplete denitrification restrict the development of biological denitrification technology. The main reason is the low electron transfer rate. Studies have found that biological carbon, as an electron mediator, can accelerate the extracellular electron transfer of microorganisms and promote interspecies electron transfer.

[0003] Sludge is usually rich in organic matter, heavy metals and pathogenic microorganisms, and its composition is complex and unstable. It can be regarded as a pollutant and as a potential resource. Landfill, incineration, composting and other sludge treatment methods can easily cause secondary pollution problems. Making sludge into sludge-based biochar can greatly reduce the volume of sludge, decompose organic residues in sludge and kill toxic and harmful pathogens, which is one of the effective methods to realize sludge reduction, harmless and resourceful treatment. Sludge is usually prepared into biochar by pyrolysis, hydrothermal carbonization and other methods, among which the pyrolysis method has the highest biochar yield. Compared with large-particle biochar, nanobiocarbon has better electron transfer performance.

[0004] Plácido J et al. (Plácido J, López S B, Meissner K E, et al. Multivariate analysis of biochar-derived carbonaceous nanomaterials for detection of heavy metal ions in aqueous systems [J]. Science of the total environment, 2019, 688: 751-761.) used KMnO4 chemical oxidation of biochar under high pressure to prepare nanobiocarbon, which had problems such as residual oxidizing agent, containing heavy metals, high energy consumption and low yield.

[0005] Lin et al.(Lin Q,Chen H,Cao J,et al.Facile synthesis strategy from sludge-derived extracellular polymeric substances to nitrogen-doped graphene oxide-like material and quantum dots[J].ACS omega,2021,6(38):24940-24948.) extracted extracellular polymers from sludge, pyrolyzed and carbonized, and then oxidized by hydrogen peroxide to prepare nano-biochar. The amount of hydrogen peroxide used is large, the reaction rate is slow, the yield is low, and the main component of biological cells in the sludge is not effectively utilized.

[0006] The nano-biochar prepared by the above method has a low oxygen content, and the redox property needs to be improved. SUMMARY

[0007] In view of the deficiencies of the prior art, the purpose of the present application is to provide a sludge-based nano-biochar and a preparation method and application thereof. The sludge-based nano-biochar provided by the present application has good electrical conductivity, redox property and low biological toxicity, can simultaneously accelerate the extracellular electron transfer effect and intracellular electron transfer rate of microorganisms, and improve the biological denitrification efficiency.

[0008] The first aspect of the present application provides a sludge-based nano-biochar, wherein the particle size of the nano-biochar is <20 nm, the surface contains rich oxygen-containing groups, the oxygen-containing groups are C=O quinone groups and C–OH groups, the molar percentage content of oxygen elements is 15%-50%, the total content of heavy metals is less than 0.05%, and the lattice stripe width is 0.22 nm.

[0009] The second aspect of the present application provides a preparation method of a sludge-based nano-biochar, comprising the following steps:

[0010] (1) preparing a suspension liquid from iron-containing residual sludge, adjusting the pH to 2-8, heating and hydrolyzing at 40-150 DEG C, collecting solid matter, drying and grinding to obtain sludge fine powder;

[0011] (2) calcining the sludge fine powder under N2 atmosphere to obtain biochar powder;

[0012] (3) acid washing the biochar powder to obtain acidified biochar;

[0013] (4) mixing and reacting the acidified biochar with hydrogen peroxide in a certain proportion, centrifuging to take supernatant, and filtering to obtain a nano-biochar solution;

[0014] (5) purifying and drying the nano-biochar solution to obtain nano-biochar powder.

[0015] The mass content of iron in the iron-containing residual sludge in step (1) is 1.5%-8.0%, and the total content of heavy metals is higher than 0.5%, generally 1.0%-5.0%. The heavy metals are one or more of Mn, Cr, Ni, Cu, Al, Ti, etc.

[0016] In step (1), the iron-containing residual sludge is mixed with water to form a suspension, and the mass-volume ratio of the sludge to water is 1g:10-15mL.

[0017] In step (1), an inorganic acid is used to adjust the pH of the suspension to 2-8, and the inorganic acid can be at least one of hydrochloric acid, sulfuric acid, etc.

[0018] In step (1), hydrolysis is carried out at 40-150°C, preferably 100-120°C, and the hydrolysis time is 1-8h.

[0019] In step (1), the solid can be collected by filtration, centrifugation, etc., and centrifugation is preferred.

[0020] In step (1), the drying temperature is 100-105°C, and the drying time is 4-10h.

[0021] In step (1), the grinding is to pass through a 100-500 mesh screen to obtain a sludge powder.

[0022] In step (2), the calcination temperature is 450-700°C, the heating rate is 2-8°C / min, and the calcination time is 1-4h. The calcination can be carried out in a tube furnace or the like.

[0023] In step (3), the acid pickling uses an inorganic acid solution with a mass concentration of 10%-18%, and the washing is carried out 1-3 times. The inorganic acid is at least one of hydrochloric acid, sulfuric acid, etc.

[0024] In step (4), the acidified biochar is mixed with hydrogen peroxide at a mass ratio of 1:5-10, the reaction temperature is 15-80°C, preferably 20-50°C, the rotation speed is 50-300r / min, and the reaction time is 6-60h.

[0025] After the reaction in step (4) is completed, the supernatant is obtained by centrifugation at 5000-10000g, and the nanometer biochar solution is obtained by filtration.

[0026] In step (5), the nanometer biochar solution obtained is purified by using a dialysis bag, and the molecular weight cut-off of the dialysis bag is 500-100000Da.

[0027] In step (5), the drying is preferably freeze-drying, and the temperature of the freeze-drying is -35~-50°C, and the nanometer biochar powder is obtained.

[0028] The third aspect of the present application provides application of the sludge-based nanobiocarbon in biological anaerobic denitrification removal of nitrate.

[0029] In the application, the biomass is activated sludge. The influent is composed of nitrate, electron donor, KH2PO4, K2HPO4, CaCl2, MgSO4, etc., and the pH value of the influent is 6-9. The denitrification temperature is 25-35℃.

[0030] Compared with the prior art, the present application has the following beneficial effects:

[0031] (1) The sludge-based nanobiocarbon prepared by the present application contains rich oxygen-containing groups, has low heavy metal content, and has a lattice stripe width of 0.22 nm, excellent electrical conductivity, redox property and low biological toxicity. When it is used for biological anaerobic denitrification removal of nitrate, it can act as an electron mediator, promote extracellular electron transfer and effectively improve intracellular electron transport chain activity, thereby solving the problem of low microbial denitrification efficiency and improving the anaerobic biological removal rate of nitrate.

[0032] (2) The method of the present application generates low-valence iron activated hydrogen peroxide in situ and generates a large amount of free radicals. The free radicals can crack large particle biocarbons to generate nanobiocarbons, which not only have small particle size, but also exhibit reversible oxidation-reduction peaks characteristic of quinone groups in the cyclic voltammetry curve. Compared with conventional nanobiocarbons, the nanobiocarbons have more excellent redox performance.

[0033] (3) The preparation method of the present application is green, safe and high-yield, and can realize sludge reduction, harmless and resource utilization. The prepared nanobiocarbons not only have small particle size, but also have no residual oxidizing agent, low heavy metal content and low biological toxicity, and are more green and economical. BRIEF DESCRIPTION OF DRAWINGS

[0034] Figure 1 is a transmission electron microscope image of the nanobiocarbon prepared in Example 2.

[0035] Figure 2 is a cyclic voltammetry curve diagram of the nanobiocarbon prepared in Example 2. DETAILED DESCRIPTION

[0036] The technical solutions of the present application and their effects will be described in detail below in combination with specific examples. The examples are implemented on the premise of the technical solutions of the present application, and detailed implementation modes and specific operation processes are given, but the protection scope of the present application is not limited to the following examples.

[0037] In the following examples, the experimental methods are conventional methods in the art unless otherwise specified. The experimental materials used in the following examples are purchased from conventional biochemical reagent stores unless otherwise specified.

[0038] In the embodiment of the present application, the particle size of the prepared nano-biochar is detected and analyzed by a transmission electron microscope. The nitrate nitrogen concentration is determined by GB7480-87 "Water quality-determination of nitrate-nitrogen-phenyldisulfonic acid spectrophotometric method".

[0039] Example 1

[0040] Take 10 g of residual sludge (iron content is 1.5%, total heavy metal content is 1.0%) and add 100 mL of water to make a suspension, adjust the pH to 2 and hydrolyze at 100 ℃ for 2 h; after hydrolysis, remove the supernatant by centrifugation, and dry the precipitate in an 80 ℃ oven to constant weight; grind the obtained dry solid and pass it through a 200 mesh screen to obtain sludge fine powder; place the sludge fine powder in a tube furnace and calcine it at 700 ℃ under N2 atmosphere for 2 h, with a heating rate of 5 ℃ / min; wash the obtained biochar powder with 10% hydrochloric acid and ultrapure water for 2 times respectively to obtain acidified biochar; under the condition of 50 ℃ and 100 r / min, hydrogen peroxide and acid-washed biochar are reacted for 24 h according to the mass ratio of 7:1, and after 7000 g centrifugation, the nano-biochar solution is obtained by passing through a 100 nm filter membrane. The obtained nano-biochar solution is purified using a 50000 Da dialysis bag, and then freeze-dried to obtain nano-biochar powder.

[0041] Example 2

[0042] Take 8 g of residual sludge (iron content is 3%, total heavy metal content is 2.0%) and add 100 mL of water to make a suspension, adjust the pH to 4 and hydrolyze at 100 ℃ for 2 h; after hydrolysis, remove the supernatant by centrifugation, and dry the precipitate in an 80 ℃ oven to constant weight; grind the obtained dry solid and pass it through a 200 mesh screen to obtain sludge fine powder; place the sludge fine powder in a tube furnace and calcine it at 600 ℃ under N2 atmosphere for 2 h, with a heating rate of 5 ℃ / min; wash the obtained biochar powder with 10% hydrochloric acid and ultrapure water for 2 times respectively to obtain acidified biochar; under the condition of 30 ℃ and 100 r / min, hydrogen peroxide and acid-washed biochar are reacted for 24 h according to the mass ratio of 7:1, and after 5000 g centrifugation, the biochar solution is obtained by passing through a 100 nm filter membrane. The obtained nano-biochar solution is purified using a 1000 Da dialysis bag, and then freeze-dried to obtain nano-biochar powder.

[0043] From the attached Figure 1 It can be seen that the particle size of the prepared nano-biochar is <20 nm, and the lattice fringe width is 0.22 nm. From the attached Figure 2 It can be seen that in the cyclic voltammetry curve, the reversible oxidation-reduction peak (-0.3-0.1 V) of the prepared nano-biochar is the unique reversible oxidation-reduction peak of C=O quinone group.

[0044] Example 3

[0045] The same as example 2, except that calcination is carried out at 450℃ under N2 atmosphere. The biochar powder is finally prepared.

[0046] Comparative example 1

[0047] The same as example 2, except that the process of step (1) is not taken, and the iron-containing residual sludge is directly dried and ground into sludge fine powder. The biochar powder is finally prepared.

[0048] Comparative example 2

[0049] The same as example 2, except that the acid washing process of step (3) is not taken, and the biochar powder calcined in step (2) is directly mixed with hydrogen peroxide. The biochar powder is finally prepared.

[0050] Comparative example 3

[0051] The same as example 2, except that potassium permanganate is used instead of hydrogen peroxide. The biochar powder is finally prepared.

[0052] Comparative example 4

[0053] The same as example 2, except that the residual sludge does not contain iron. The biochar powder is finally prepared.

[0054] Test example

[0055] In a 60 mL anaerobic serum bottle, 50 mL of influent water is added. The composition of the influent water is: 0.425 g / L NaNO3, 2.24 g / L C3H5O3Na, 0.02 g / L CaCl2, 7.22 g / L K2HPO4, 1.16 g / L KH2PO4, 0.10 g / L MgSO4, 100 mg / L of sludge-based nano-biochar. After N2 is passed, the serum bottle is sealed with a rubber plug and an aluminum cap, and then autoclaved. After cooling, 2.5 g VSS / L of activated sludge is added. The test is divided into three groups: group 1 is the control group without adding nano-biochar, group 2 is the control group without adding activated sludge, and group 3 is the control group with adding large particle biochar (100 nm). After incubation in a 30℃ incubator for 6 h, the test results are shown in Table 1.

[0056] Table 1 Test results of nano-biochar prepared by examples and comparative examples for removing nitrate

[0057]

[0058] As can be seen from the test results in Table 1, the nano-biochar prepared by the method of the present application has smaller particle size, lower heavy metal content, and better nitrate removal effect.

Claims

1. A sludge-based nanobiocarbon, characterized by: The nano-biochar has a particle size of less than 20 nm, and a surface containing rich oxygen-containing groups, wherein the oxygen-containing groups are C=O quinone groups and C-OH groups, and the molar percentage of oxygen elements is 15%-50%.

2. The nanobiocarbon according to claim 1, characterized in that: The nano-biochar has a total heavy metal content of less than 0.05% and a lattice fringe width of 0.22 nm.

3. A method of producing a sludge-based nanobiocarbon according to claim 1 or 2, characterized by The method comprises the following steps: (1) preparing a suspension by mixing the iron-containing residual sludge with water, adjusting the pH to 2-8, and heating and hydrolyzing at 40-150°C to collect solid matter, dry and grind the solid matter to obtain a sludge fine powder; (2) calcining the sludge fine powder in a N2 atmosphere to obtain a biochar powder; (3) acid washing the biochar powder to obtain an acidified biochar; (4) mixing the acidified biochar with hydrogen peroxide according to a mass ratio, reacting, centrifuging to obtain supernatant, and filtering to obtain a nano-biochar solution; (5) purifying and drying the nano-biochar solution to obtain a nano-biochar powder.

4. The method of claim 3, wherein: In step (1), the iron-containing residual sludge has a mass content of iron of 1.5%-8.0% and a total heavy metal content of not less than 0.5%.

5. The method of claim 4, wherein: In step (1), the total heavy metal content is 1.0%-5.0%, and the heavy metals are one or more of Mn, Cr, Ni, Cu, Al and Ti.

6. The method of claim 3 or 4, wherein: In step (1), the iron-containing residual sludge is mixed with water to prepare a suspension, and the mass-volume ratio of the sludge to water is 1g:10-15mL.

7. The method of claim 3, wherein: In step (1), an inorganic acid is used to adjust the pH of the suspension to 2-8, and the inorganic acid is at least one of hydrochloric acid and sulfuric acid.

8. The method of claim 3, wherein: In step (1), the heating and hydrolyzing is performed at 40-150°C, preferably 100-120°C, and the hydrolyzing time is 1-8h.

9. The method of claim 3, wherein: In step (1), the drying temperature is 100-105°C, and the drying time is 4-10h; the sludge fine powder is ground to pass through a 100-500 mesh screen.

10. The method of claim 3, wherein: In step (2), the calcining temperature is 450-700°C, the temperature rising rate is 2-8°C / min, and the calcining time is 1-4h.

11. The method of claim 3, wherein: In step (3), the acid washing uses an inorganic acid solution with a mass concentration of 10%-18%, and the washing is performed 1-3 times; the inorganic acid is at least one of hydrochloric acid and sulfuric acid.

12. The method of claim 3, wherein: In step (4), the acidified biochar is mixed with hydrogen peroxide according to a mass ratio of 1:5-10, the reaction temperature is 15-80°C, preferably 20-50°C, the rotation speed is 50-300r / min, and the reaction time is 6-60h.

13. The method of claim 3, wherein: In step (4), after the reaction is completed, the supernatant is obtained by centrifuging at 5000-10000g, and the nano-biochar solution is obtained by filtering.

14. The method of claim 3, wherein: In step (5), the obtained nano-biochar solution is purified by using a dialysis bag, and the molecular weight cut-off of the dialysis bag is 500-100000 Da.

15. The method of claim 3, wherein: In step (5), the drying is preferably freeze-drying, and the freeze-drying temperature is-35~-50°C.

16. A use of the sludge-based nano-biochar of claim 1 or 2 or the sludge-based nano-biochar prepared by the method of any one of claims 3-15 in biological anaerobic denitrification to remove nitrate.

17. Use according to claim 16, characterized in that: The organism is activated sludge; the composition of the influent is nitrate, an electron donor, KH2PO4, K2HPO4, CaCl2 and MgSO4, and the pH value of the influent is 6-9; and the denitrification temperature is 25-35°C.

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