Iron-nitrogen co-modified biochar catalyst as well as preparation method and application thereof

Abstract

The invention belongs to the technical field of catalyst preparation and wastewater treatment, and particularly relates to an iron-nitrogen co-modified biochar catalyst as well as a preparation method and application thereof. Comprising the following steps: (1) pyrolyzing a biomass precursor at 200-400 DEG C in an inert gas atmosphere to obtain low-temperature biochar; (2) sufficiently mixing an iron salt solution, an organic nitrogen source and the low-temperature biochar through an impregnation method to obtain a mixed solution, and drying the mixed solution to obtain an iron-nitrogen co-modified biochar precursor; and (3) pyrolyzing the iron-nitrogen co-modified charcoal precursor at 600-1000 DEG C in an inert gas atmosphere to obtain the iron-nitrogen co-modified charcoal catalyst. The iron-nitrogen co-modified biochar catalyst is used for treating organic matters in high-salt organic wastewater, can realize catalytic polymerization removal of the organic matters, and effectively solves the problems that the number of active sites of biochar is limited, the electron transfer efficiency is relatively low, and the utilization efficiency of an oxidant in a persulfate activation technology is low and the mineralization ability is poor.

Description

Technical Field

[0001] The present invention belongs to the technical fields of catalyst preparation and wastewater treatment, and particularly relates to an iron-nitrogen co-modified biochar catalyst, a preparation method thereof and an application thereof. Background Art

[0002] High-salt organic wastewater generally contains a large amount of refractory organic matters, such as phenols, polycyclic aromatic hydrocarbons, etc., and also contains various inorganic salts, posing a serious threat to the ecological environment and human health. Commonly used advanced oxidation methods for the deep treatment of organic matters in high-salt concentrated wastewater include Fenton and Fenton-like methods, ozonation, etc. The oxidative and destructive removal of such organic matters is achieved by generating strongly oxidizing species (such as O3, 1 O2, ·OH, SO4 ·- and high-valent metals, etc.) to degrade and mineralize them into small-molecule inorganic matters such as CO2 and H2O. Although the advanced oxidation methods have a fast reaction rate and a wide application range, they have limitations such as low electron efficiency, large dosage of oxidants, and incomplete oxidation. Factors such as the self-annihilation of active species and the competitive consumption of active species by salt ions in high-salt concentrated wastewater will result in the effective utilization rate of oxidants usually being less than 10%; at the same time, the widely used ozonation technology in engineering can only oxidize organic matters into small-molecule organic acids such as oxalic acid, and it is difficult to achieve the complete purification of organic matters. The organic matters in high-salt organic wastewater are both pollutants and a kind of organic resources. If a destructive advanced oxidation method that can efficiently oxidize and transform organic matters and a non-destructive enrichment method that can recycle organic matters can be coupled, it has important research and application value to achieve the enhanced recovery and removal of organic pollutants under high-salt stress.

[0003] In recent years, a brand-new organic matter removal mechanism, namely the direct oxidation transfer process, has been discovered in the catalytic oxidation process based on persulfate. The specific process is that pollutants and oxidants undergo a direct oxidation-reduction reaction of electrons on the surface of a heterogeneous catalyst, and the generated pollutant intermediates spontaneously undergo surface polymerization or coupling reactions, and the formed polymerization products are enriched on the surface of the catalyst, thereby realizing the effective removal of pollutants in sewage. The direct oxidation transfer process has the dual functions of both efficiently oxidizing and transforming organic matters and enriching and recycling organic matters in the catalytic oxidation process. It not only has high tolerance to the high-salt concentrated water environment, but also has the potential for harmless treatment and resource utilization of purifying and recycling organic pollutants in wastewater. Therefore, it has important research value to develop an efficient direct oxidation transfer process system aiming at the characteristics of organic matters in high-salt concentrated wastewater.

[0004] Biochar can construct an effective heterogeneous catalytic polymerization system and is expected to be used for the efficient catalytic polymerization purification of high-salt organic wastewater. However, the number of active sites of raw biochar is limited and the electron transfer efficiency is relatively low, and the catalytic activity and service life are not ideal. Therefore, it is necessary to appropriately modify biochar to improve its catalytic activity and durability. Summary of the Invention

[0005] In view of the defects of the prior art, the present invention provides an iron-nitrogen co-modified biochar catalyst and its preparation method and application, which aims to solve the problems of limited number of biochar active sites, low electron transfer efficiency, and low efficiency of activated persulfate in degrading organic pollutants.

[0006] According to a first aspect of the present invention, there is provided a method for preparing an iron-nitrogen co-modified biochar catalyst, comprising the following steps:

[0007] (1) Pyrolyzing a biomass precursor at 200-400° C. in an inert gas atmosphere to obtain low-temperature biochar; the low-temperature biochar is biochar obtained by pyrolysis at a low temperature of 200-400° C.

[0008] (2) fully mixing the iron salt solution, the organic nitrogen source and the low-temperature biochar by an impregnation method to obtain a mixed solution, and drying the mixed solution to obtain an iron-nitrogen co-modified biochar precursor;

[0009] (3) Pyrolyzing the iron-nitrogen co-modified biochar precursor at 600-1000° C. in an inert gas atmosphere to obtain an iron-nitrogen co-modified biochar catalyst.

[0010] Preferably, the biomass precursor is selected from one or more of rice husks, wood chips, straw, and bamboo charcoal; the iron salt in the iron salt solution is selected from one or more of ferric chloride hexahydrate, ferrous sulfate heptahydrate, and ferric nitrate nonahydrate; and the organic nitrogen source is selected from one or two of dicyandiamide and melamine.

[0011] Preferably, the pyrolysis time of the pyrolysis in step (1) and step (3) is independently selected from 2 to 4 hours, and the heating rate is 2-10°C / min.

[0012] Preferably, the mass ratio of the low-temperature biochar, the nitrogen element in the organic nitrogen source, and the iron element in the iron salt solution is 1:(2-4):(0.025-0.126), and the concentration of the iron salt solution is 5-25 mmol / L.

[0013] According to another aspect of the present invention, there is provided an iron-nitrogen co-modified biochar catalyst prepared by the preparation method.

[0014] According to another aspect of the present invention, there is provided an iron-nitrogen co-modified biochar catalyst for treating organic matter in high-salt organic wastewater, wherein the iron-nitrogen co-modified biochar catalyst and an oxidant are added to the high-salt organic wastewater; or the iron-nitrogen co-modified biochar catalyst is mixed with quartz sand and filled into a column-type continuous flow device, and the oxidant and high-salt organic wastewater are flowed through the column-type continuous flow device from bottom to top; the salt concentration in the high-salt organic wastewater is 15 to 50 g / L.

[0015] Preferably, adding the iron-nitrogen co-modified biochar catalyst and the oxidant into the high-salt organic wastewater is specifically as follows: first, adding the iron-nitrogen co-modified biochar catalyst into the high-salt organic wastewater for pre-adsorption, and then adding the oxidant into the high-salt organic wastewater.

[0016] Preferably, the mass ratio of the iron-nitrogen co-modified biochar catalyst to the quartz sand is 1:(7.5 - 16); the mass ratio of the iron-nitrogen co-modified biochar catalyst to the volume of the high-salt organic wastewater is 0.2 - 1 g / L; the mass ratio of the oxidant to the volume of the high-salt organic wastewater is 0.1 - 0.5 g / L.

[0017] Preferably, the oxidant is persulfate; preferably, the persulfate is one or more of monopersulfate or dipersulfate; the concentration of the persulfate is 2 - 4 g / L.

[0018] Preferably, the organic matter in the high-salt organic wastewater is selected from one or more of phenolic compounds, amide compounds or carboxylic acid compounds; preferably, the phenolic compounds are selected from phenol or 4-formylphenol.

[0019] Generally speaking, compared with the prior art through the above technical solutions conceived by the present invention, the following technical advantages are mainly possessed:

[0020] 1. In the present invention, the biomass precursor is pyrolyzed at a low temperature of 200 - 400 °C to prepare low-temperature biochar for the effective anchoring of iron source and nitrogen source during the impregnation process. Then, the iron source, nitrogen source and low-temperature biochar are fully mixed by the impregnation method to obtain an iron-nitrogen co-modified low-temperature biochar precursor; then the iron-nitrogen co-modified low-temperature biochar is pyrolyzed at 600 - 1000 °C to obtain an iron-nitrogen co-modified biochar catalyst. During the pyrolysis process, the iron salt will be gradually reduced to zero-valent iron and iron oxides and anchored in the carbon network, and the nitrogen atoms will form bonds with carbon atoms during the high-temperature reconstruction of the carbon network to form nitrogen species such as pyridine nitrogen, pyrrole nitrogen and graphitic nitrogen; the effective anchoring of iron and nitrogen in the carbon plane increases the number of active sites of the original biochar material, improves the electron transfer efficiency, and synergistically enhances the catalytic activity of the iron-nitrogen co-modified biochar, thus effectively promoting the oxidation kinetics of persulfate and improving the efficiency of degrading organic pollutants.

[0021] The biochar with a higher degree of graphitization can be obtained by the two-step heating and carbonization method. Improving the graphitization is beneficial to electron conduction and oxidant activation on the one hand, and can also promote the adsorption of intermediates such as phenoxy radicals through π-π interaction; and since the iron ions are reduced by the biochar, the electron density of the iron active center is lower, and the iron with a lower electron density can more quickly activate the oxidant to generate active species, improving the efficiency of the catalytic polymerization reaction of organic matter.

[0022] 2. The iron-nitrogen co-modified biochar catalyst of the present invention can efficiently activate persulfate to catalyze the polymerization and degradation of phenol under different acid-base conditions and oxidant dosages. During the addition of the oxidant persulfate, singlet oxygen and high-valent iron (Fe IV and 1 O2) and other non-radical species are mainly generated, with good anti-anion interference ability, and have the potential to catalytically polymerize and treat organic matter in high-salt wastewater.

[0023] 3. The catalyst of the present invention is filled into a columnar continuous flow device, and the catalytic polymerization removal of high-salt organic wastewater can be achieved with a small filling amount of the iron-nitrogen co-modified biochar catalyst. The removal rate of organic pollutants is high, the system is anti-interference and has good stability, and the phenolic compounds mainly undergo an oligomerization process; pre-adsorbing by adding the iron-nitrogen co-modified biochar catalyst to the high-salt organic wastewater first, and then adding the oxidant to the high-salt organic wastewater can achieve a better adsorption effect and improve the degradation efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is a schematic flow chart for preparing the iron-nitrogen co-modified biochar catalyst provided in Example 1.

[0025] Figure 2 It is an XRD pattern of the biochar catalysts obtained in Example 1, Comparative Example 1 and Comparative Example 2.

[0026] Figure 3 It is the scanning electron microscope, transmission electron microscope and element distribution maps of the biochar catalysts obtained in Comparative Example 1, 2 and Example 1; where (a) is Comparative Example 1; (b) is Comparative Example 2; (c) is Example 1.

[0027] Figure 4 It is a high-resolution transmission electron microscope image of the biochar catalysts obtained in Comparative Example 2 and Example 1; where (a) is Comparative Example 2; (b) is Example 1.

[0028] Figure 5 It is the nitrogen adsorption-desorption curve and pore size distribution map of the biochar catalysts obtained in Comparative Example 1, 2 and Example 1; where (a) is the nitrogen adsorption-desorption curve of Comparative Example 1; (b) is the pore size distribution map of Comparative Example 1; (c) is the nitrogen adsorption-desorption curve of Comparative Example 2; (d) is the pore size distribution map of Comparative Example 2; (e) is the adsorption-desorption curve of Example 1; (f) is the pore size distribution map of Example 1.

[0029] Figure 6 It is the infrared spectrum and Raman spectrum of the biochar catalysts obtained in Comparative Example 1, 2 and Example 1; where (a) is the infrared spectrum; (b) is the Raman spectrum.

[0030] Figure 7 X-ray photoelectron spectroscopy diagrams of the biochar catalysts obtained in Comparative Example 2 and Example 1; among them, (a) is the N1s spectrum of Comparative Example 2; (b) is the Fe 2p spectrum of Comparative Example 2; (c) is the N1s spectrum of Example 1; (d) is the Fe2p spectrum of Example 1.

[0031] Figure 8 Phenol (PhOH) degradation activity diagrams of the biochar catalysts obtained in Example 1, Comparative Example 1, and Comparative Example 2; among them, (a) is the linear diagram of phenol degradation; (b) is the bar chart of phenol degradation rate and apparent reaction rate constant.

[0032] Figure 9 Effect of oxidant dosage on the catalytic polymerization efficiency of the biochar catalysts obtained in Example 1 and Comparative Example 2; among them, (a) is the oxidant dosage of 0.1 g / L; (b) is the oxidant dosage of 0.2 g / L; (c) is the oxidant dosage of 0.3 g / L; (d) is the oxidant dosage of 0.4 g / L.

[0033] Figure 10 Effect of pH value on the catalytic polymerization efficiency of the biochar catalysts obtained in Example 1 and Comparative Example 2; among them, (a) is pH = 3.0; (b) is pH = 6.0; (c) is pH = 9.0.

[0034] Figure 11 Identification diagrams of key active species during the reaction of Comparative Example 1, Comparative Example 2, Example 1 with persulfate; among them, (a) is the quenching experiment result of Comparative Example 1; (b) is the quenching experiment result of Comparative Example 2; (c) is the quenching experiment result of Example 1; (d) PMSO conversion experiment; (e) is Fe IV O 2+ 、 1 O2 and the proportion of free radical pathways.

[0035] Figure 12 Electron paramagnetic resonance diagrams of different reaction systems obtained in Example 2; among them, (a) is the spectrum of Comparative Example 1 with DMPO as the capturer; (b) is the spectrum of Comparative Example 2 with DMPO as the capturer; (c) is the spectrum of Example 1 with DMPO as the capturer; (d) is the spectrum of Comparative Example 1 with TEMP as the capturer; (e) is the spectrum of Comparative Example 2 with TEMP as the capturer; (f) is the spectrum of Example 1 with TEMP as the capturer.

[0036] Figure 13Experimental results of the nitroblue tetrazolium (NBT) probe molecules in different reaction systems obtained in Example 2; among them, (a) is the full ultraviolet scan spectrum of Comparative Example 1; (b) is the full ultraviolet scan spectrum of Comparative Example 2; (c) is the full ultraviolet scan spectrum of Example 1; (d) is the production of superoxide radicals (O2 ·- ) in the systems of Comparative Example 1, Comparative Example 2 and Example 1 obtained by quantifying the NBT probe molecules.

[0037] Figure 14 Spectrogram of the direct electron transfer process (ETP) mechanism for phenol degradation in different catalyst reaction systems in Example 2; among them, (a) is the experimental device diagram of the anodic oxidation process (GOP); (b) is the phenol degradation situation of the biochar obtained in Comparative Example 1; (c) is the phenol degradation situation of the biochar obtained in Comparative Example 2; (d) is the phenol degradation situation of the biochar obtained in Example 1.

[0038] Figure 15 Effects of different inorganic anions and background organic matter (HA) on the performance of the biochar obtained in Example 1 for catalytic degradation of phenol; among them, (a) is Cl - (b) HCO3 - (c) CO3 2- (d) NO3 - (e) SO4 2- (f) Background organic matter (HA)

[0039] Figure 16 COD removal effect of the iron-nitrogen co-modified biochar for catalytic polymerization treatment of high-salt organic wastewater in Example 3 (the inset is a schematic diagram of the column continuous flow device).

[0040] Figure 17 Three-dimensional fluorescence spectrogram of high-salt organic wastewater before and after treatment by the column continuous flow device in Example 3; among them, (a) is the three-dimensional fluorescence spectrogram of the column continuous flow influent; (b) is the three-dimensional fluorescence spectrogram of the column continuous flow effluent; (c) is the three-dimensional fluorescence spectrogram of the continuous flow effluent after treatment with activated carbon; (d) is the three-dimensional fluorescence spectrogram of the continuous flow effluent after treatment with polyferric sulfate.

[0041] Figure 18 Ultraviolet-visible absorption spectrogram of high-salt organic wastewater before and after treatment by the column continuous flow device in Example 3; among them, (a) is the ultraviolet-visible absorption spectrogram of the column continuous flow influent and effluent; (b) is the ultraviolet-visible absorption spectrogram of the continuous flow effluent before and after treatment with activated carbon at different concentrations; (c) is the ultraviolet-visible absorption spectrogram of the continuous flow effluent before and after treatment with polyferric sulfate at different concentrations.

[0042] Figure 19This is a color comparison chart of the continuous effluent water before and after treatment with different concentrations of activated carbon and polyferric sulfate in Example 3; (a) is the activated carbon treatment, and (b) is the polyferric sulfate treatment).

[0043] Figure 20 It is the water quality analysis of high-salt organic wastewater and the catalytic polymerization mechanism diagram of Example 3; wherein (a) is the secondary mass spectrum of phenol; (b) is the secondary mass spectrum of phenol dimer (phenols); (c) is the secondary mass spectrum of phenol dimer (quinones); (d) is the time-of-flight mass spectrum; and (e) is the catalytic polymerization mechanism diagram. DETAILED DESCRIPTION

[0044] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0045] The present invention provides a method for preparing an iron-nitrogen co-modified biochar catalyst, comprising:

[0046] (1) pyrolyzing a biomass precursor at 200-400°C in an inert gas atmosphere to obtain low-temperature biochar;

[0047] (2) fully mixing the iron salt solution, the organic nitrogen source and the low-temperature biochar by an impregnation method to obtain a mixed solution, and drying the mixed solution to obtain an iron-nitrogen co-modified biochar precursor;

[0048] (3) Pyrolyzing the iron-nitrogen co-modified biochar precursor at 600-1000° C. in an inert gas atmosphere to obtain an iron-nitrogen co-modified biochar catalyst.

[0049] Furthermore, the mass ratio of the low-temperature biochar, the nitrogen element in the organic nitrogen source, and the iron element in the iron salt solution is 1:(2-4):(0.025-0.126), and the concentration of the iron salt solution is 5-25 mmol / L. By adjusting the iron loading, the catalytic activity of the obtained iron-nitrogen co-modified biochar catalyst can be adjusted.

[0050] Furthermore, the pyrolysis time of the pyrolysis in step (1) and step (3) is 2 to 4 hours, and the heating rate is 2 to 10°C / min.

[0051] Furthermore, step (1) is specifically as follows: further crushing the biomass precursor and passing it through a 60-100 mesh sieve; washing it with ultrapure water several times to remove surface impurities, and drying it; and then pyrolyzing it at 200-400° C. in a nitrogen atmosphere to obtain low-temperature biochar.

[0052] Further, in step (2), the mixing is sufficient mixing by magnetic stirring.

[0053] Further, the iron-nitrogen co-modified biochar catalyst obtained in step (3) is washed with ultrapure water and dried.

[0054] Reaction mechanism: First, the biomass precursor is pyrolyzed at a low temperature of 200 - 400 °C to produce low-temperature biochar for the effective anchoring of iron source and nitrogen source during the impregnation process. Then, the iron source, nitrogen source, and low-temperature biochar are fully mixed by the impregnation method to obtain an iron-nitrogen co-modified low-temperature biochar precursor; the iron-nitrogen co-modified low-temperature biochar is pyrolyzed at 600 - 1000 °C to obtain an iron-nitrogen co-modified biochar catalyst. During the pyrolysis process, the iron salt will gradually be reduced to zero-valent iron and iron oxides and anchored in the carbon network. Nitrogen atoms will form bonds with carbon atoms during the high-temperature reconstruction of the carbon network to form nitrogen species such as pyridine nitrogen, pyrrole nitrogen, and graphitic nitrogen; the effective anchoring of iron and nitrogen in the carbon plane increases the number of active sites of the original biochar material, improves the electron transfer efficiency, synergistically enhances the catalytic activity of the iron-nitrogen co-modified biochar, and further affects the catalytic mechanism.

[0055] The preparation method adopted in the present invention significantly improves the graphitization degree of biochar; iron species such as zero-valent iron and iron oxides are introduced into the carbon network, and the introduction of nitrogen-containing ligands is beneficial to the regulation of the coordination environment of Fe sites. At the same time, nitrogen sites such as pyridine N, pyrrole N, and graphitic N are also introduced into the carbon network, significantly increasing the catalytic active sites on the biochar surface. By coupling the interaction between metal iron species, non-metal nitrogen species, and the carbon network, the charge on the carbon plane is redistributed, accelerating the electron transfer rate, thereby improving the efficiency of activating persulfate to degrade organic pollutants.

[0056] The present invention also provides a method for treating high-salt organic wastewater using the iron-nitrogen co-modified biochar catalyst described above, adding the iron-nitrogen co-modified biochar catalyst and an oxidant into the high-salt organic wastewater; or mixing the iron-nitrogen co-modified biochar catalyst with quartz sand and filling it into a column continuous flow device, and flowing the oxidant and high-salt organic wastewater through the column continuous flow device from bottom to top; the salt concentration in the high-salt organic wastewater is (15 - 50) g / L.

[0057] Further, adding the iron-nitrogen co-modified biochar catalyst and an oxidant into the high-salt organic wastewater specifically means: first adding the iron-nitrogen co-modified biochar catalyst into the high-salt organic wastewater for pre-adsorption, and then adding the oxidant into the high-salt organic wastewater.

[0058] Further, the oxidant is one or two of monopersulfate or dipersulfate, preferably monopersulfate.

[0059] Further, the dosage of the oxidant in the sewage is 0.2 - 1 g / L; the dosage of the iron-nitrogen co-modified biochar catalyst in the sewage is 0.1 - 0.5 g / L.

[0060] Further, the organic matter in the high-salt organic wastewater is selected from one or more of phenolic compounds, amide compounds or carboxylic acid compounds; preferably, the phenolic compounds are selected from phenol or 4-formylphenol.

[0061] Further, after the reaction is completed, when relevant tests are required, 1 mL of the solution is filtered through a 0.22 μm polyethersulfone microporous membrane, and 50 μL of methanol is added and mixed evenly to terminate the possible degradation reaction in the sample.

[0062] Further, the mass ratio of the iron-nitrogen co-modified biochar catalyst to the quartz sand is 1:(7.5 - 16).

[0063] Further, the concentration of the persulfate is 2.0 - 4.0 g / L, the pH of the persulfate solution is 5.0 - 7.0; the influent flow rate of the persulfate is 2.0 - 3.0 mL / min.

[0064] Further, the residual organic compounds and chromaticity in the effluent of the column continuous flow device can be further removed by activated carbon and / or polyferric sulfate.

[0065] Example 1

[0066] A preparation method of an iron-nitrogen co-modified biochar catalyst is as Figure 1 shown, and its steps are as follows:

[0067] (1) Pyrolyze rice husk at 200 °C in an inert gas atmosphere to obtain low-temperature biochar (BC400);

[0068] (2) Disperse 1.0 g of BC400 and 4.0 g of dicyandiamide in 90.0 mL of 0.025 M ferric chloride hexahydrate solution, and magnetically stir for 8 h to fully impregnate and mix. Then place the above mixed solution in an oven and dry overnight at 80 °C to obtain iron-nitrogen co-modified low-temperature biochar; place the iron-nitrogen co-modified low-temperature biochar in a tubular furnace, pyrolyze at 800 °C in a nitrogen atmosphere for 2 h, with a heating rate of 5 °C / min, wash with ultrapure water after cooling to room temperature, and dry overnight at 80 °C to obtain BC-N / Fe25.

[0069] Comparative Example 1

[0070] A preparation method of a biochar catalyst, the specific steps are as follows:

[0071] (1) Pyrolyze rice husk at 200 °C in an inert gas atmosphere to obtain low-temperature biochar (BC400);

[0072] (2) The low-temperature biochar (BC400) was placed in a tube furnace and pyrolyzed at 800 °C in a nitrogen atmosphere for 2 h with a heating rate of 5 °C / min. After cooling to room temperature, it was washed with ultrapure water and dried overnight at 80 °C to obtain high-temperature biochar (BC800).

[0073] Comparative Example 2

[0074] Preparation of low-iron-loading iron-nitrogen co-modified biochar (BC-N / Fe5): 1.0 g of BC400 and 4.0 g of dicyandiamide were dispersed in 90.0 mL of 0.005 M ferric chloride hexahydrate solution and magnetically stirred for 8 h for sufficient impregnation and mixing. Then the above mixed solution was dried in an oven at 80 °C to obtain iron-nitrogen co-modified low-temperature biochar; the iron-nitrogen co-modified low-temperature biochar was placed in a tube furnace and pyrolyzed at 800 °C in a nitrogen atmosphere for 2 h with a heating rate of 5 °C / min. After cooling to room temperature, it was washed with ultrapure water and dried overnight at 80 °C to obtain BC-N / Fe5.

[0075] Figure 2 , Figure 3 , Figure 4 XRD, SEM, STEM, HRTEM and EDS spectra of the catalysts prepared in Example 1 and Comparative Examples 1 and 2 are shown respectively. As Figure 2 can be seen, compared with BC800, characteristic diffraction peak signals of iron species such as Fe 0 and Fe3O4 appeared in the XRD pattern after iron-nitrogen co-modification. The obvious lattice fringes of Fe 0 and Fe3O4 in the HRTEM image further proved the successful loading of iron species on the carbon plane. Figure 3 Further characterized the morphology of the catalysts. It can be seen that BC800 has an amorphous structure. Iron species particles appeared on the carbon surface after iron-nitrogen co-modification, and with the increase of the impregnated iron concentration, the iron species particles increased in number and size. At the same time, the corresponding EDS spectra also indicated the successful introduction of nitrogen species into the carbon network. Based on the above characterizations, we confirmed the successful preparation of iron-nitrogen co-modified biochar, and iron-nitrogen co-modification increased the number of surface active sites of biochar.

[0076] Figure 5 , Figure 6 , Figure 7 Further showed the specific surface area, pore size distribution, functional group structure, defect degree / graphitization degree and bonding situation of the catalysts prepared in Example 1 and Comparative Examples 1 and 2 respectively. As Figure 5 shown in (d), (e), (f) therein, the three biochar catalysts are all mesoporous structures; the specific surface area of the catalysts was obtained by measuring the nitrogen adsorption-desorption curve. As Figure 5It can be seen from (a), (b), and (c) that compared with BC800, the specific surface area of the material after iron-nitrogen co-modification decreases with the increase in the impregnated iron concentration, which may be due to the loading of the corresponding iron species on the surface of the biochar. The three biochar catalysts have similar functional group structures, but after iron-nitrogen co-modification, the degree of defect of the biochar decreases significantly, while the degree of graphitization increases significantly. This is due to the carbon plane reconstruction induced by iron species at high temperature. The increase in the degree of graphitization may endow the biochar catalyst with more delocalized electrons, thus having better electron transfer efficiency. In addition, XPS further proves the formation of active sites such as Fe 0 , pyridine N, pyrrole N, and graphitic N in the iron-nitrogen co-modified biochar, and there is also coordination bonding between Fe and N.

[0077] Example 2

[0078] The performance of three biochar catalysts prepared in Example 1, Comparative Example 1, and Comparative Example 2 for activating peroxymonosulfate (PMS) to degrade phenol (PhOH) was investigated comparatively, and the effects of the oxidant dosage and pH value during the catalytic reaction on the polymerization efficiency of the catalyst BC-N / Fe25 prepared in Example 1 and the catalyst BC-N / Fe5 in Comparative Example 2 were examined. The specific steps are as follows:

[0079] Weigh 0.02 g of the three biochar catalysts prepared in Example 1, Comparative Example 1, and Comparative Example 2 respectively, disperse them in 50 mL of an aqueous solution containing 20 mg / L PhOH, mix well with magnetic stirring. After pre-adsorbing for 30 min (no oxidant was added during pre-adsorption, and oxidant PMS was added after the end of pre-adsorption), add 0.2 g / L of PMS to initiate the catalytic reaction. Take samples at regular intervals, filter them with a 0.22 μm disposable filter head, and detect the concentration change of residual PhOH by high performance liquid chromatography. At the same time, set up 2 additional parallel groups, take 20 mL of samples at the reaction time of 0 min and 30 min respectively, filter them with a 0.22 μm disposable filter head, inject them into a TOC tube pre-added with 0.1 g of anhydrous sodium sulfite, and detect the TOC change before and after the catalytic reaction by a total organic carbon analyzer to calculate the polymerization efficiency of the biochar catalyst. Figure 8 , Figure 9 , Figure 10 where, [PhOH]0 and [PhOH] represent the initial concentration of PhOH and the concentration at the sampling time respectively, and PE represents the corresponding polymerization efficiency (the ratio of TOC removal rate to phenol removal rate).

[0080] As Figure 8As shown, the catalytic performance of BC800 is very poor, but after iron-nitrogen co-modification, the catalytic activity of the biochar has been significantly improved. Among them, BC-N / Fe25 shows the best phenol degradation performance and can achieve complete removal of phenol within 30 min. The degradation efficiency is 13.38 times and 4.3 times that of BC800 and BC-N / Fe5, respectively. During the catalytic reaction process, the dosage of the oxidant and the pH value have an impact on the catalytic polymerization efficiency of the iron-nitrogen co-modified biochar as Figure 9 , Figure 10 shown. Even at a relatively low oxidant dosage (0.1 g / L), BC-N / Fe25 still has a high catalytic polymerization efficiency (64.69%), which is significantly higher than that of BC-N / Fe5 (48.35%); at the same time, after increasing the oxidant dosage, although the catalytic polymerization efficiency of both iron-nitrogen co-modified biochar catalysts increases, the catalytic polymerization efficiency of BC-N / Fe25 is still relatively high, reaching about 70%. Since the difference in the catalytic polymerization efficiency of the two iron-nitrogen co-modified biochar catalysts is relatively large when the oxidant dosage is 0.1 g / L, the influence of the pH value of the reaction system of the two iron-nitrogen co-modified biochars on the catalytic polymerization efficiency of the materials was explored at this oxidant dosage; within a wide pH range (pH = 3.0 - 9.0), BC-N / Fe25 has a high catalytic polymerization efficiency, reaching about 70%, while the catalytic polymerization efficiency of BC-N / Fe5 gradually decreases with the increase of pH. The above results indicate that the prepared BC-N / Fe25 has the ability to efficiently activate PMS for catalytic polymerization and / or degrade phenol, and can maintain a high catalytic polymerization efficiency under different oxidant dosages and pH values, and is expected to achieve efficient catalytic polymerization removal of organic pollutants in sewage with a low oxidant consumption.

[0081] The differences in the active species generated by the BC800, BC-N / Fe5, and BC-N / Fe25 catalysts prepared in Example 1, Comparative Example 1, and Comparative Example 2 during the degradation of organic matter in the activated persulfate system were investigated separately. Quenching experiments, electron paramagnetic resonance (EPR), polyisopropylidene acetone (PMSO) conversion experiments, and nitroblue tetrazolium chloride (NBT) probe molecule experiments were used for characterization respectively, and the results are as Figure 11 , Figure 12 , Figure 13 shown. Figure 11 Among them, Control, MeOH (methanol), DMSO (dimethyl sulfoxide), TBA (tert-butanol), and FFA (furfuryl alcohol) represent the biochar catalytic systems without quenching agent and with 100 mM methanol, 100 mM dimethyl sulfoxide, 100 mM tert-butanol, and 5 mM furfuryl alcohol quenching agents added respectively. Among them, MeOH can quench sulfate radicals SO4 ·- and hydroxyl radicals ·OH, and DMSO can quench high-valent metals (Fe IV) TBA can quench ·OH, and FFA can quench singlet oxygen 1 O2. The quenching results show that there is almost no SO4 in the system ·- acting with ·OH. It is mainly Fe IV and 1 non-radical species such as O2. The formation of Fe IV was further demonstrated through the PMSO conversion experiment.

[0082] In the EPR spectrum ( Figure 12 ), carbon-centered radicals and 1 the signal of O2 were detected in the BC800 / PMS system. In the BC-N / Fe5 / PMS system and the BC-N / Fe25 / PMS system, the characteristic peak signal of DMPOX (possibly originating from high-valent metal Fe IV or 1 O2) and 1 the signal of O2 were detected. However, no radical signals (SO4 ·- , ·OH, and O2 ·- ) were detected in all three systems.

[0083] As Figure 13 shown, the NBT probe molecule experiment proved that only trace amounts of O2 ·- were generated in the three biochar systems, and O2 ·- generally only served as an intermediate species for the catalyst to activate PMS to convert into ·OH and 1 O2. Therefore, its contribution can be ignored. The above experimental characterization results show that 1 O2 is the main active species in the BC800 catalytic system, while Fe IV and 1 O2 are the main active species in the two iron-nitrogen co-modified biochar systems.

[0084] Figure 14 It is a map of the direct electron transfer process (ETP) mechanism for the degradation of phenol in different catalyst reaction systems in Example 2. Since the biochar catalytic system can achieve the degradation of organic pollutants through the direct electron transfer process (ETP), the ETP mechanism in the three biochar-activated persulfate systems was explored through the galvanostatic oxidation process (GOP) experiment. The specific GOP experiment was as follows: The biochar material was coated on the surface of the carbon paper to make an electrode. Using a 50 mM Na2SO4 solution as the electrolyte, the phenol solution and the persulfate solution were placed in two electrolytic cells respectively, separated by an ion exchange membrane in the middle, and the two electrodes were connected by a wire to start the reaction. During the reaction, samples were taken regularly to detect the change in phenol concentration. As Figure 14 shown, in the GOP experiment, almost no degradation of phenol occurred, so the existence of the ETP mechanism can be excluded.

[0085] Figure 15 The effects of different inorganic anions and background organic matter (HA) on the performance of the biochar-catalyzed degradation of phenol obtained in Example 1 were investigated. According to the catalytic degradation experiment of PhOH in Example 2, the catalyst BC-N / Fe25 prepared in Example 1 was tested for its resistance to anion interference. The results are as Figure 15 shown. Among them, Control refers to the case where no anion is added to the catalytic system of BC-N / Fe25, and Cl - , HCO3 - , CO3 2- , NO3 - , SO4 2- refer to the common anions in water that affect the catalytic performance, and HA refers to background organic matter such as humic acid that may exist in water. During the experiment, sodium chloride, sodium bicarbonate, sodium carbonate, sodium nitrate, sodium sulfate, and humic acid with different concentration gradients were added to the biochar-activated persulfate system to simulate the interference of actual water bodies on the catalyst performance. Cl - has a promoting effect on the degradation of phenol. Low concentrations of HCO3 - , CO3 2- also have a promoting effect on the degradation of phenol in the early stage of the reaction, while high concentrations of NO3 - , SO4 2- only have a slight inhibitory effect on the degradation of phenol; however, HA has an obvious inhibitory effect on the degradation of phenol, probably due to competing with phenol for active species. The above results indicate that BC-N / Fe25 has good resistance to anion interference.

[0086] Example 3

[0087] A method and application for catalytic polymerization treatment of actual high-salt organic wastewater by activating persulfate with an iron-nitrogen co-modified biochar catalyst BC-N / Fe25. The specific steps are as follows: The iron-nitrogen co-modified biochar catalyst is mixed with quartz sand and then filled into a columnar continuous flow device. The oxidant and high-salt organic wastewater are mixed by a peristaltic pump and then flow through the columnar continuous flow device from bottom to top. The high-salt organic wastewater has an extremely high salt content and a COD as high as 386.8 mg / L, mainly containing phenolic compounds such as phenol and 4-formylphenol, as well as other organic pollutants such as amides and carboxylic acids. The filling amount of the iron-nitrogen co-modified biochar catalyst in the columnar continuous flow device is 20.0 g; the filling amount of quartz sand in the columnar continuous flow device is 1500 g. The inlet PMS concentration is 4.0 g / L, and the inlet pH is 7.0. The water flow rate is 2.67 mL / min, and the hydraulic retention time is 2.65 h.

[0088] As Figure 16As shown, the column continuous flow device can stably remove COD in high-salt organic wastewater for a long time, and the COD removal rate can reach 67.5% - 77.3%. As Figure 17 , Figure 18 shown, the three-dimensional fluorescence spectrum and ultraviolet-visible absorption spectrum further show that the column continuous flow device can effectively remove organic pollutants in high-salt organic wastewater, and after the continuous effluent is treated with activated carbon or polyferric sulfate, the residual organic matter and chromaticity in the wastewater can be further removed, as Figure 19 described. It shows strong practical application potential.

[0089] The water quality composition of high-salt organic wastewater was analyzed by mass spectrometry and time-of-flight mass spectrometry, and the catalytic polymerization mechanism of iron-nitrogen co-modified biochar was explored. As Figure 20 shown, high-salt organic wastewater mainly contains phenolic compounds such as phenol and 4-formylphenol, as well as other organic pollutants such as amides and carboxylic acids. During the catalytic process, phenol is first converted into a phenoxy radical cation, and then couples with another molecule of phenoxy radical cation or phenol through C-C bond or C-O bond to form a dimer product, and may further couple to form a larger polymerization product.

[0090] In summary, in the embodiment of the present invention, the biomass precursor is first pyrolyzed at low temperature to obtain low-temperature biochar, and then the low-temperature biochar is fully impregnated and mixed with iron salt and organic nitrogen source by the impregnation method to obtain an iron-nitrogen co-modified biochar precursor, and then the precursor is pyrolyzed at high temperature to obtain iron-nitrogen co-modified biochar (BC-N / Fe25). During the pyrolysis process, iron species will be gradually reduced to zero-valent iron and other iron species are successfully anchored in the carbon network. Nitrogen species will participate in the reconstruction of the carbon skeleton at high temperature to form nitrogen sites such as pyridine N, pyrrole N, and graphite N, and will coordinate with iron species. The effective construction of iron and nitrogen sites increases the number of active sites on the biochar surface, improves the graphitization degree of the biochar, rearranges the surface charge of the biochar, and effectively improves its electron transfer efficiency. Therefore, BC-N / Fe25 can efficiently activate persulfate for catalytic polymerization and / or degradation of phenol, and shows strong anti-anion interference ability. Furthermore, in the column continuous flow experiment of activating persulfate for catalytic polymerization to treat high-salt organic wastewater, the oligomerization removal of phenolic substances in high-salt organic wastewater can be achieved, and other organic pollutants can be efficiently degraded.

[0091] Those skilled in the art can easily understand that the above are only the preferred embodiments of the present invention, and are not used to limit the present invention. Any modifications, equivalent replacements, and improvements made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for preparing an iron-nitrogen co-modified biochar catalyst, characterized in that: The following steps are involved: (1) pyrolyzing a biomass precursor at 200-400°C in an inert gas atmosphere to obtain low-temperature biochar; (2) fully mixing the iron salt solution, the organic nitrogen source and the low-temperature biochar by an impregnation method to obtain a mixed solution, and drying the mixed solution to obtain an iron-nitrogen co-modified biochar precursor; (3) Pyrolyzing the iron-nitrogen co-modified biochar precursor at 600-1000° C. in an inert gas atmosphere to obtain an iron-nitrogen co-modified biochar catalyst.

2. The method for preparing an iron-nitrogen co-modified biochar catalyst according to claim 1, characterized in that: The biomass precursor is selected from one or more of rice husks, wood chips, straw, and bamboo charcoal; the iron salt in the iron salt solution is selected from one or more of ferric chloride hexahydrate, ferrous sulfate heptahydrate, and ferric nitrate nonahydrate; and the organic nitrogen source is selected from one or two of dicyandiamide and melamine.

3. The method for preparing an iron-nitrogen co-modified biochar catalyst according to claim 1, characterized in that: The pyrolysis time of the pyrolysis in step (1) and step (3) is independently selected from 2 to 4 hours, and the heating rate is 2-10°C / min.

4. The method for preparing an iron-nitrogen co-modified biochar catalyst according to claim 1, characterized in that: The mass ratio of the low-temperature biochar, the nitrogen element in the organic nitrogen source and the iron element in the iron salt solution is 1:(2-4):(0.025-0.126), and the concentration of the iron salt solution is 5-25 mmol / L.

5. The iron-nitrogen co-modified biochar catalyst prepared according to the preparation method according to any one of claims 1 to 4.

6. The iron-nitrogen co-modified biochar catalyst according to claim 5 is used for treating organic matter in high-salt organic wastewater, characterized in that: adding the iron-nitrogen co-modified biochar catalyst and an oxidant into high-salt organic wastewater; Alternatively, the iron-nitrogen co-modified biochar catalyst is mixed with quartz sand and filled into a column-type continuous flow device, and the oxidant and high-salt organic wastewater are passed through the column-type continuous flow device from bottom to top; the salt concentration in the high-salt organic wastewater is 15 to 50 g / L.

7. The iron-nitrogen co-modified biochar catalyst according to claim 6 is used for treating high-salt organic wastewater, characterized in that: The specific steps of adding the iron-nitrogen co-modified biochar catalyst and the oxidant into the high-salt organic wastewater are as follows: firstly, the iron-nitrogen co-modified biochar catalyst is added into the high-salt organic wastewater for pre-adsorption, and then the oxidant is added into the high-salt organic wastewater.

8. The iron-nitrogen co-modified biochar catalyst according to claim 6 is used for treating high-salinity organic wastewater, characterized in that: The mass ratio of the iron-nitrogen co-modified biochar catalyst to the mass ratio of the quartz sand is 1:(7.5-16); the volume ratio of the iron-nitrogen co-modified biochar catalyst to the high-salt organic wastewater is 0.2-1g / L; the volume ratio of the mass of the oxidant to the high-salt organic wastewater is 0.1-0.5g / L.

9. The iron-nitrogen co-modified biochar catalyst according to claim 6 is used for treating high-salt organic wastewater, characterized in that: The oxidant is persulfate; preferably, the persulfate is one or more of peroxymonosulfate or peroxydisulfate; the concentration of the persulfate is 2-4 g / L.

10. The iron-nitrogen co-modified biochar catalyst according to claim 6 is used for treating high-salinity organic wastewater, characterized in that: The organic matter in the high-salt organic wastewater is selected from one or more of phenol compounds, amide compounds or carboxylic acid compounds; preferably, the phenol compound is selected from phenol or 4-formylphenol.

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