Montmorillonite modified hydrothermal carbon, preparation method thereof and application of montmorillonite modified hydrothermal carbon in field of saline-alkali soil improvement

Abstract

The invention particularly relates to montmorillonite modified hydrothermal carbon, a preparation method thereof and application of the montmorillonite modified hydrothermal carbon in the field of saline-alkali soil improvement. A biomass material, surface adsorption / low-crystalline-state Fe montmorillonite and iron-containing inorganic salt are used as base materials, the montmorillonite modified hydrothermal carbon material capable of driving HO <-> based on low-crystalline-state Fe (II) is provided, waste biomass is used as the base materials, the preparation process is simple, and recycling of waste energy is achieved. The invention further provides a saline-alkali soil conditioner, which comprises the montmorillonite modified hydrothermal carbon material and desulfurized gypsum, can effectively improve the soil fertility and the biological enzyme activity when being applied to a field in a combined manner, and has important significance on coastal saline-alkali soil improvement.

Description

Technical Field

[0001] The present invention relates to the technical field of saline-alkali soil improvement, and specifically comprises a montmorillonite-modified hydrothermal carbon and a preparation method thereof, and also comprises application of the montmorillonite-modified hydrothermal carbon in the field of saline-alkali soil improvement. Background Art

[0002] The information disclosed in this background technology section is only intended to enhance understanding of the overall background of the invention and should not necessarily be regarded as an admission or any form of suggestion that the information constitutes the prior art already known to those skilled in the art.

[0003] Coastal saline-alkali lands suffer from severe salinity and alkali stress, low organic matter content, poor soil structure, and severe freshwater resource constraints, resulting in low soil fertility and, in most cases, low- to medium-yield fields (Yang Jinsong et al., 2022). Synergistically improving the quality and productivity of saline-alkali farmland is a key and challenging area of research for saline-alkali land improvement and utilization (Amini et al., 2016). Currently, adding soil amendments is an effective method for improving the physical and chemical properties of saline-alkali soils. Biochar, with its large surface area, complex pore structure, rich functional groups, and high cation exchange capacity, can effectively reduce salinity and alkalinity in saline-alkali soils, increase microbial activity, cation exchange capacity, and slow nutrient release, offering broad application prospects in improving saline-alkali and degraded soils (Wei Ying et al., 2023; Fungo et al., 2017). Biochar can be divided into biochar and hydrochar depending on its preparation method (Zhong Xinghao et al., 2025). Compared to biochar, hydrochar production does not require biomass drying, consumes less energy, and has a relatively higher carbonization yield (Khan et al., 2021). However, both biochar and hydrochar contain a high proportion of recalcitrant carbon, resulting in relatively low bioavailability (Mukherjee et al., 2014). Therefore, in addition to maintaining biochar's excellent properties of base ion retention and slow nutrient release, improving its carbon turnover capacity, thereby increasing SOC content and soil fertility, can effectively enhance the effectiveness of biochar in improving saline-alkali land.

[0004] Montmorillonite is a 2:1 expansive clay mineral with a large specific surface area, strong adsorption capacity, and high cation exchange capacity. It is currently widely used in fields such as binders and pharmaceutical carriers. Furthermore, due to isomorphous substitution and other effects, montmorillonite often contains a certain amount of Fe. Studies have shown that the reduction-oxidation process of montmorillonite can drive the formation of reactive oxygen species, hydroxyl radicals (HO·), with the oxidation of Fe(II) being a key factor (Du et al., 2021; Wang et al., 2022). HO· is highly oxidizing and can directly or indirectly oxidize and decompose macromolecular organic matter in the soil, increasing the bioavailability of organic carbon and playing a crucial role in soil carbon turnover (Du et al., 2019). Previous research by the inventors has shown that Fe-based montmorillonite can accelerate biochar carbon turnover by driving a free radical mechanism, effectively enhancing the effectiveness of biochar in saline-alkali land reclamation.

[0005] In addition, desulfurized gypsum is also one of the most widely used saline-alkali soil improvers. Desulfurized gypsum can significantly increase the Ca content of saline-alkali soil. 2+ and SO4 2- Concentration, desulfurization gypsum contains Ca 2+ Can replace Na in saline-alkali soil + , converting harmful sodium carbonate and sodium bicarbonate into harmless neutral salts such as sodium sulfate (Zhang Jishi et al., 2017). + Ca 2+ After replacement, it is conducive to the formation of soil aggregates, increase soil porosity, increase air permeability and water permeability, improve physical and chemical properties, and facilitate salt leaching and plant growth (Liu Jianhong, 2008). It should be noted that the main component of desulfurization gypsum is calcium sulfate, which can significantly increase the salt content of the soil after application, especially Ca 2+ Mg 2+ The salt content increases, while the application of organic materials can reduce the salt content (Wang Qilong et al., 2019). Summary of the Invention

[0006] Based on this research background, the present invention primarily aims to provide a hydrochar with enhanced bioavailability. By introducing iron-based montmorillonite, the material drives the oxidative decomposition of organic macromolecules in the soil by hydroxyl radicals (HO·), thereby improving soil carbon turnover. To achieve this technical objective, the present invention proposes to prepare hydrochar using montmorillonite and biomass feedstocks commonly found in the Yellow River Delta as a base material. The hydrochar will then be investigated for its effects on the physical, chemical, and biological properties of coastal saline-alkali soils, clarifying the mechanism of its highly effective action in improving saline-alkali soils.

[0007] Therefore, in a first aspect of the present invention, a method for preparing montmorillonite-modified hydrothermal carbon is provided, comprising the following steps: The biomass material, surface adsorbed / low crystalline Fe montmorillonite and iron-containing inorganic salt are mixed evenly, water is added to make the solid-liquid ratio of the mixed system be 1:8~12, and the mixed system is reacted at a temperature of 220±5℃ and a pressure of 2.5-3.0MPa for 1~3h to obtain the product.

[0008] Possible biomass materials include agricultural and forestry waste (including corn straw, wheat straw, rice husks, wood chips, bark, dead wood, fallen leaves, etc.), fruit shells and food processing residues (coconut shells, nut shells, fruit pits, etc.), dewatered sludge, and animal manure. Organic materials with high cellulose content or a certain water content are preferred. In the above-mentioned research on the preparation of montmorillonite-modified hydrochar, the raw materials for hydrochar preparation were selected based on: 1) the resource utilization of biomass materials; and 2) compared to biochar, hydrochar preparation is milder, more cost-effective, has higher yields, and is easy to operate, showing promising application prospects.

[0009] In an embodiment of the present invention with better effects, the mass ratio of the biomass material, surface adsorption / low crystalline Fe montmorillonite and iron-containing inorganic salt is 45~95:8~10:0.8~1.2.

[0010] In one embodiment, the biomass material is dewatered sludge with a moisture content of 75-85%. The mass ratio of the dewatered sludge, surface-adsorbed / low-crystalline Fe montmorillonite, and iron-containing inorganic salt is 45-55:8-10:0.8-1.2. When water is added, the "solids" in the solid-to-liquid ratio is calculated based on the dry weight of the dewatered sludge. Due to the complex composition of dewatered sludge, which contains a large amount of metal ions in addition to organic matter, to prevent irreversible precipitation of metal ions during the hydrothermal reaction, the above raw materials are mixed and acid is added to adjust the pH to 4.0-5.0 before being transferred to the hydrothermal reactor for subsequent reactions.

[0011] In another embodiment, the biomass material is straw, and the mass ratio of the straw, surface adsorbed / low crystalline Fe montmorillonite and iron-containing inorganic salt is 85-95:8-10:0.8-1.2.

[0012] The above-mentioned iron-containing inorganic salt is selected from one or more of ferric chloride (FeCl3), ferric sulfate (Fe2(SO4)3), ferric nitrate (Fe(NO3)3), ferric phosphate (FePO4), ferric bromide (FeBr3), ferric fluoride (FeF3), and ferric iodide (FeI3).

[0013] The basis for using surface-adsorbed / low-crystalline Fe montmorillonite and iron-containing inorganic salts as raw materials is: 1) HO· can be produced during the reduction and reoxidation process of the above-mentioned montmorillonite. It has extremely strong oxidizing properties and can directly or indirectly oxidize and decompose macromolecular organic matter in the soil, thereby improving the bioavailability of organic carbon and playing a very important role in soil carbon turnover; 2) The above research results show that surface-adsorbed / low-crystalline Fe(II) is the key factor driving the formation of HO·. Combined with the previous research results of the inventors, by adding iron-containing inorganic salts to increase the surface-adsorbed / low-crystalline Fe(II) in montmorillonite, it is expected to increase HO· production. In addition, the hydrothermal process can increase the interlayer size of montmorillonite to form a product similar to iron-pillared montmorillonite.

[0014] In one embodiment verified by the present invention, the preparation method of the surface adsorption / low crystalline Fe montmorillonite is as follows: (1) Reduction: Mix hematite slag, sodium dithionite and sodium citrate-sodium bicarbonate buffer under anaerobic conditions, heat in a water bath at 65-75°C for 3-5 hours, and separate, retain and wash the solids in the heated product; (2) Oxidation: The solid washed in step (1) is exposed to air and placed in darkness for reaction for 45 to 50 hours, during which H2O2 solution is added in stages; the H2O2 solution is added in the following manner: the initial concentration is 4 to 6 mmol / L, and the solid is added at a dosage ratio of 1 g to 2 to 6 mL, and then 1 to 3 mmol / L of H2O2 solution is added every hour at a dosage ratio of 1 g to 1 to 3 mL; the product after the oxidation is mixed to obtain a suspension, and 1 to 3 times the volume of 0.4 to 0.6 M hydrochloric acid solution is added and stirred for reaction for 1 to 3 hours to obtain surface adsorbed / low crystalline Fe montmorillonite.

[0015] Furthermore, in step (1), the dosage ratio of the hematite slag, sodium dithionite and sodium citrate-sodium bicarbonate buffer solution is 0.03-0.06 g: 0.08-0.12 g: 20-30 mL.

[0016] Furthermore, in step (1): in the buffer solution, the concentration of sodium citrate is 0.003-0.004 M, the concentration of sodium bicarbonate is 0.3-0.4 M, and an inert gas is introduced in advance for a period of time to remove dissolved oxygen in the solution, and the introduction time is 20-40 minutes.

[0017] Furthermore, in step (1): the heated product can be obtained by centrifugal separation and washed with sodium chloride solution and deionized water.

[0018] In a second aspect, a montmorillonite-modified hydrothermal carbon material prepared by the method described in the first aspect is provided.

[0019] The present invention applies the montmorillonite-modified hydrothermal carbon material prepared in the first aspect to the field. The results show that the application of hydrothermal carbon can significantly increase the SOC content, especially the soluble and easily oxidizable organic carbon components. Furthermore, the present invention intends to apply desulfurized gypsum on the basis of modified hydrothermal carbon to improve saline-alkali land, attempting to rely on the advantages of the two modifiers to achieve the purpose of synergistically improving the quality and production capacity of saline-alkali land. The results show that the simultaneous application of hydrothermal carbon and desulfurized gypsum can significantly increase the activity of enzymes related to carbon conversion and anti-peroxidation in the soil, improve the biological properties of the soil, and provide a good foundation for microbial-driven biogeochemical processes.

[0020] Based on the above research results, the third aspect of the present invention provides a composition, which includes an active dose of the montmorillonite-modified hydrothermal carbon material according to the second aspect and also includes desulfurization gypsum.

[0021] In some embodiments of the above composition, the dosage ratio of the montmorillonite-modified hydrothermal carbon to the desulfurized gypsum is 1-3:1-3, and specific examples are 3:1, 2:1, 1:1, 1:2 or 1:3.

[0022] In some other embodiments, the above-mentioned composition also includes single or compound fertilizers, and the single fertilizers include nitrogen fertilizers, phosphorus fertilizers, and potassium fertilizers, and specific examples include urea, ammonium sulfate, ammonium bicarbonate, superphosphate, calcium magnesium phosphate fertilizer, phosphate rock, potassium chloride, and potassium sulfate; the compound fertilizers include binary compound fertilizers, ternary compound fertilizers, and special compound fertilizers, and specific examples include potassium dihydrogen phosphate, potassium nitrate, nitrogen-phosphorus-potassium compound fertilizers, organic-inorganic compound fertilizers, special fertilizers for fruits and vegetables, or special fertilizers for rice.

[0023] The fourth aspect of the present invention provides the use of the montmorillonite-modified hydrothermal carbon material of the second aspect or the composition of the third aspect in the field of saline-alkali soil improvement.

[0024] The application in the field of saline-alkali soil improvement described in the fourth aspect above may be for any one or a combination of the following purposes: 1) Reduce salinity and alkali stress in saline-alkali soil; 2) Increase the organic matter content in saline-alkali soil; 3) Improve the fertility of saline-alkali soil; 4) Increase aggregates in saline-alkali soils; 5) Soil porosity increases, and air permeability and water permeability increase.

[0025] The saline-alkali soil is not limited to the cause and region, and possible causes include seawater backflow, salt accumulation caused by drought / semi-arid climate, or unreasonable artificial irrigation, etc., and the above-mentioned modified hydrothermal carbon material or composition can be applied.

[0026] In a fifth aspect, the present invention provides a saline-alkali soil conditioner, which is composed of the montmorillonite-modified hydrothermal carbon material and desulfurization gypsum described in the second aspect, and the two are not mixed before use.

[0027] In a sixth aspect, the present invention provides a method for improving saline-alkali soil, comprising applying the montmorillonite-modified hydrothermal carbon material and desulfurized gypsum described in the second aspect to the soil to be treated, with the application dosage being 0.8-1.2% of the mass of the soil to be treated.

[0028] Compared with the prior art, the present invention has the following beneficial effects: 1. This invention first uses iron-containing slag as raw material and, through redox modification, provides a high-yield HO· surface-adsorbed / low-crystalline Fe montmorillonite. Compared to previous studies, this study modified the oxidation method, introducing hydrogen peroxide to accelerate HO· production. Results showed significant differences in the performance of different iron-containing slag types treated with the aforementioned oxidation method, with hematite exhibiting significantly improved HO· production using the improved oxidation method.

[0029] 2. This invention utilizes surface-adsorbed / low-crystalline Fe montmorillonite and iron-containing inorganic salts to modify hydrochar. This increases the surface-adsorbed / low-crystalline Fe content. This accelerates carbon turnover in the hydrochar by driving a free radical mechanism (producing HO·), increasing soil SOC content and its active components, and improving the physical, chemical, and biological properties of saline-alkali soils. Furthermore, this invention incorporates waste biomass resources as a substrate in its hydrochar modification research, achieving high-value utilization of waste energy.

[0030] 3. The present invention is designed to reduce salt stress in saline-alkali soils, improve soil fertility, and transform them into productive farmland by accelerating biochar carbon turnover and replacing harmful ions in the soil through a free radical mechanism. Based on this idea, the present invention provides a synergistic treatment method for montmorillonite-modified hydrothermal carbon materials and desulfurized gypsum. It has been verified that the combined application of these two components further increases the content of water-stable aggregates compared to the single application of montmorillonite-modified hydrothermal carbon materials, significantly improves the activity of enzymes related to carbon conversion and anti-peroxidation in the soil, and achieves a "1+1>2" effect. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The accompanying drawings, which constitute a part of the present invention, are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

[0032] Figure 1 The flowchart of the preparation method and application of the montmorillonite-modified hydrothermal charcoal and saline-alkali soil conditioner of the present invention is as follows; Figure 2The formation of HO· during the reduction and reoxidation of montmorillonite and its relationship with different forms of Fe; Figure 2 (a) is a bar graph of the hydroxyl radical content of 22 iron-containing montmorillonites. The "am" in the bar graph represents a statistically significant difference. The two are arranged in order according to the expression of hydroxyl radicals. If the letters are the same, it means there is no statistical difference between the two. If there are multiple letters, it means there is a difference but it is not significant. Figure 2 Middle (b) shows the correlation between surface adsorbed / low crystalline Fe(II) and free hydroxyl expression; Figure 2 Middle (c) shows the correlation between high crystalline Fe and free hydroxyl expression; Figure 2 Middle (d) is the correlation between silicate-bound Fe and free hydroxyl expression; Figure 3 This is a diagram showing the improvement effect of montmorillonite-modified hydrothermal carbon combined with desulfurized gypsum on saline-alkali soil in a field experiment; Figure 3 Middle (a) is the soil organic carbon content in the improved saline-alkali soil; Figure 3 Middle (b) is the graph of soluble organic carbon content in saline-alkali soil after improvement; Figure 3 Middle (c) is the hot water extractable organic carbon content in the improved saline-alkali soil; Figure 3 Middle (d) is the graph of the easily oxidizable organic carbon content in the improved saline-alkali soil; Figure 3 Middle (e) is the graph of β-glucosidase activity in saline-alkali soil after improvement; Figure 3 Middle (f) is the graph of catalase activity in saline-alkali soil after improvement; Figure 3 Middle (g) is the graph of polyphenol oxidase activity in saline-alkali soil after improvement; Figure 3 Middle (h) is the graph of sucrase activity in saline-alkali soil after improvement; Figure 3 In the figure, CK represents the control; G represents desulfurized gypsum; H represents montmorillonite-modified hydrothermal carbon; GH represents desulfurized gypsum + montmorillonite-modified hydrothermal carbon; ab in the bar graph represents statistically significant differences. If the letters are the same, it means that there is no statistical difference between the two. If there are multiple letters, it means that there is a difference but it is not significant. DETAILED DESCRIPTION

[0033] It should be noted that the following detailed descriptions are illustrative and intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which the present invention belongs.

[0034] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present invention. As used herein, unless the context clearly indicates otherwise, the singular form is intended to include the plural form. In addition, it should be understood that when the terms "comprise" and / or "include" are used in this specification, they indicate the presence of features, steps, operations, devices, components and / or combinations thereof.

[0035] In the context of this specification, the word "comprising" is understood to mean "including especially". It should not be interpreted as "consisting only of".

[0036] Terminology explanation section: SOC:soil organic carbon, soil organic carbon; HO·:hydroxyl radical, hydroxyl free radical.

[0037] As described in the background, synergistically improving the quality and productivity of saline-alkali farmland is a key and challenging area of research in saline-alkali land improvement and utilization. To address these technical challenges, the present invention provides a saline-alkali soil conditioner that improves soil fertility, reduces the proportion of harmful salts in saline-alkali soil, and simultaneously improves soil fertility. To achieve this objective, the present invention first performs a reduction-oxidation modification on iron-containing slag raw materials to drive the formation of hydroxyl radicals. Montmorillonite containing high-yield HO· is then selected and carbonized with biomass materials and iron salts. Field experiments are then conducted to determine the application method of the carbonized material.

[0038] 1. Reduction and reoxidation modification of montmorillonite The present invention relates to reducing and reoxidizing commercially available iron-containing slag to obtain a montmorillonite material with a high HO yield. The relevant research is as follows: (1) Reduction: 0.05 g of slag, 0.10 g of sodium dithionite, 5 mL of sodium citrate (0.018 M), and 20 mL of sodium bicarbonate (0.48 M) buffer were mixed in a sealed glass bottle in an anaerobic cabinet. The sealed bottle was then removed from the anaerobic cabinet and heated in a water bath at 70 °C for 4 h. The heated product was centrifuged at 4000 rpm for 10 min. The supernatant was poured out of the anaerobic cabinet and 5 mL of 1 M NaCl solution and 5 mL of deionized water were added in sequence, and each was washed three times.

[0039] All liquid reagents used in the reduction experiment needed to be deoxygenated by nitrogen flow for 30 minutes before use.

[0040] (2) Oxidation: Add 100 mM terephthalic acid (TPA, detection reagent) solution and a small amount of water to the slag product after reduction in step (1) to a final TPA concentration of 1 mM. Then, each slag product is divided into two parts and exposed to air together. The reaction is carried out on a shaker (200 rpm) in the dark for 48 h. During the reaction, H2O2 is added to one of the slag products in stages, with an initial concentration of 5 mmol / L and a solid-liquid ratio of 1 g:4 mL. 2 mmol / L H2O2 solution is supplemented every hour at a dosage ratio of 1 g:2 mL. The suspension is then sampled, filtered using a 0.45 μm membrane, and tested for HO·.

[0041] The present invention screened 11 common iron-containing slags in the art. After reduction, the 11 iron-containing slags were divided into two parts, one part of which was subjected to hydrogen peroxide oxidation treatment, thereby obtaining 22 types of oxidized iron-containing montmorillonites. The types of the iron-containing slags and the HO· test results of the 22 iron-containing montmorillonites are shown in Table 1 below: Table 1. HO· test results for 11 iron-containing slags and 22 iron-containing montmorillonites Figure 2 There are 22 columns from left to right in (a) corresponding to the numbers 1-22 in Table 1. The above research results show that all 11 slags have considerable HO· formation during oxidation, with the amount of HO· ranging from 39.12 to 1993.56 μmol / kg, but different montmorillonites show different orders of magnitude ( Figure 2 a) According to Table 1, after the introduction of hydrogen peroxide, the HO· expression trends in the iron-containing montmorillonite products varied. The introduction of hydrogen peroxide into the iron-containing montmorillonite, ferrihydrite, lepidocrocite, hematite, and iron hydroxide increased HO· production, while the HO· production of chlorite, nano-zero-valent iron, pyrite, magnetite, goethite, and iron oxalate decreased. Among them, hematite had the highest HO· content after reduction-oxidation modification.

[0042] The slag products after the reduction and oxidation of the above hematite were mixed evenly to obtain a suspension, and 5 mL of the suspension was measured and subjected to the following operations: 1) Add 10 mL of 0.5 M HCl and extract for 2 h to obtain montmorillonite with surface adsorbed / low crystalline Fe; 2) Add 10 mL of 1.8 M H2SO4 and extract for 24 h to obtain highly crystalline iron-rich montmorillonite; 3) Add 10 mL of 1.3 M HF and extract for 24 h to obtain montmorillonite containing Fe silicate.

[0043] For the three montmorillonites with different iron forms, 1 mL of the extract was taken from each of them and filtered through a 0.45 μm filter membrane. The Fe(Ⅱ) concentration in the solution was determined by measuring the absorbance at 510 nm using an ultraviolet-visible spectrophotometer using the o-phenanthroline method. The results are shown in Table 1. Figure 2 (bd) Correlation analysis found that HO· formation was significantly positively correlated with surface adsorption / low crystalline Fe(II) ( Figure 2 b, R 2 =0.607). The oxidation of Fe(II) in montmorillonite is a key factor driving the formation of HO·. Combined with the inventors' previous research (Wang LL, Du H. Y, Xu HC, Li H., Li LN, 2022. Insights into phenanthrene attenuation by hydroxyl radicals from reduced iron-bearing mineral oxygenation. Journal of Hazardous Materials, 439, 129658.), surface-adsorbed / low-crystalline Fe(II) plays an important role in driving the formation of HO·. Therefore, surface-adsorbed / low-crystalline Fe montmorillonite was selected for subsequent research.

[0044] 2. Montmorillonite modified hydrothermal carbon The above-mentioned montmorillonite with surface adsorption / low crystalline Fe was selected and further combined with biomass materials and FeCl3 as the base material to prepare montmorillonite-modified hydrothermal carbon.

[0045] During the research phase, high-water-content sewage sludge was selected as the biomass material for hydrothermal carbon production. The preparation method was as follows: municipal sludge was dehydrated to a moisture content of approximately 80%. The dehydrated sludge, montmorillonite with surface-adsorbed / low-crystalline Fe, and FeCl₃ were mixed uniformly in a mass ratio of 50:10:1. Dilute hydrochloric acid was added to adjust the pH of the mixture to 4.0-5.0. The mixture was loaded into a high-pressure hydrothermal reactor, set at 220±5°C, maintained at a pressure of 2.5-3.0 MPa, and reacted for 2 hours. After the reaction, the solid fraction was centrifuged, dried at 105°C for 12 hours, and then ball-milled to a 200-mesh size to obtain montmorillonite-modified hydrothermal carbon.

[0046] 3. Screening saline-alkali soil conditioners through field experiments Taking the coastal saline-alkali land in Dongying Agricultural High-tech Zone as the research object, the improvement effects of montmorillonite-modified hydrothermal carbon, desulfurized gypsum and their combination were studied through field experiments.

[0047] 1. Improvement effect of desulfurization gypsum The experiment included 4 treatments: 0 t / hm 2 (S0), 2 t / hm 2 (S2), 4 t / hm 2 (S4) and 6 t / hm 2 (S6) Desulfurized gypsum. Each treatment had four replicate plots, each measuring 6 m × 8 m, with 1 m between plots. Each treatment was treated with 160 kg N / hm2. 2 、90 kg P2O5 / hm 2 and 90 kg K2O / hm 2 60% of the nitrogen fertilizer and all of the phosphorus and potassium fertilizers were used as basal fertilizer, with the remaining 40% of the nitrogen fertilizer used as topdressing. Urea, superphosphate, and potassium sulfate were used as commercially available nitrogen, phosphorus, and potassium fertilizers, respectively. Desulfurized gypsum was purchased from Hebei Yousheng Company and contains approximately 85% CaSO₄·2H₂O. Other components include approximately 4% CaCO₃, approximately 9% water, and approximately 2% other impurities. The cropping system employed a corn-ryegrass rotation, with the corn variety Zhengdan 958 and the ryegrass variety Dongmu 70. Fertilizer and desulfurized gypsum were applied only during the corn season; no fertilizer was applied during the ryegrass season. Soil samples were collected from 0–20 cm depth after corn harvest and tested for indicators such as total soluble salts.

[0048] The results of the desulfurization gypsum treatment experiment on saline-alkali land showed that there was no significant difference in soil SOC and soluble total salt content after adding desulfurization gypsum (Table 2, p>0.05). Different desulfurization gypsum application rates had different effects on the content of water-soluble ions in the soil. Compared with the S0 treatment, the Ca content in the S2, S4 and S6 treatments was significantly higher than that in the S0 treatment. 2+ The contents of Na + The contents of soil Mg decreased by 16.49%, 3.83% and 11.60% respectively. 2+ and K + The above results show that the addition of desulfurized gypsum can effectively improve the ion balance in the soil solution of saline-alkali land, but cannot effectively increase SOC and reduce the total amount of soluble salts.

[0049] Table 2. Physical and chemical properties of soils treated with different desulfurization gypsum additions 2. Montmorillonite modified hydrothermal carbon and desulfurized gypsum To investigate the effects of montmorillonite-modified hydrochar and desulfurized gypsum on saline-alkali soil amelioration, a combined application experiment was conducted. Four treatments were designed: a control (CK), desulfurized gypsum (G), hydrochar (H), and both desulfurized gypsum and hydrochar (GH). Prior to the soil microplot experiment, the soil was adjusted to 60% of its field capacity and pre-incubated at 25°C for two weeks. Each treatment was supplemented with a compound fertilizer (N-P₂O₅-K₂O ratio of 15-15-15) at a ratio of 1% by weight of the soil. Both desulfurized gypsum and montmorillonite-modified hydrochar were added at a ratio of 1% by weight of the soil. All additives were thoroughly mixed with the soil. Each treatment was replicated three times, with water losses regularly replenished. After one year of incubation, soil samples were collected and analyzed for physical, chemical, and biological properties, including soil organic carbon (SOC) and soluble salts. The application methods and results are shown in Table 3.

[0050] Table 3. Effects of hydrochar combined with desulfurization gypsum on soil physical and salinity.

[0051] Four treatments: CK, control; G, desulfurized gypsum; H, montmorillonite-modified hydrothermal carbon; GH, desulfurized gypsum + montmorillonite-modified hydrothermal carbon.

[0052] The results showed that the addition of montmorillonite-modified hydrochar significantly increased the SOC content, especially the soluble and easily oxidizable organic carbon components. The SOC of the H and GH treatments reached 12.83±0.90 g / kg and 12.18±0.70 g / kg, respectively, which were 35.21% and 28.26% higher than those of the CK treatment ( Figure 3 While the application of desulfurized gypsum alone cannot significantly increase the SOC content, it can significantly increase the easily oxidizable organic carbon component, increasing it by about 12.65% ( Figure 3 d). The application of amendments can significantly increase the activities of enzymes related to soil neutralization and carbon transformation and anti-peroxidation ( Figure 3 fh). β-glucosidase activity increased by 23.47%-40.52% in all three treatments compared to the CK treatment; polyphenol oxidase activity increased more than 2-fold in the montmorillonite-modified hydrochar treatments (H and GH); sucrase activity in the G treatment was 34.23% higher than in the CK treatment; and activity in the G and GH treatments was 24.76% and 8.83% higher than in the CK treatment, respectively. These results indicate that the application of amendments, especially the combination of montmorillonite-modified hydrochar and desulfurized gypsum, significantly improved soil biological properties, providing a favorable foundation for microbially driven biogeochemical processes.

[0053] In addition, montmorillonite-modified hydrochar is a black solid product with carbon as the main component and rich in oxygen functional groups. When applied to soil, it can effectively increase soil porosity and promote the formation of water-stable aggregates, significantly improving soil physical properties (Table 3). For example, the application of montmorillonite-modified hydrochar has a tendency to reduce soil bulk density, especially in the GH treatment, from 1.52 ± 0.06 g / cm 3 Reduced to 1.40 ± 0.06 g / cm 3 , which decreased by 8.32%; the GH treatment showed a trend of increasing the number of large aggregates (>0.25 mm) in water-stable aggregates, with the proportion of large aggregates reaching 19.82 ± 0.82%, an increase of about 8.27% compared with CK. Moreover, montmorillonite-modified hydrochar has a large specific surface area and rich functional groups, showing a strong adsorption capacity for metal elements. Its application significantly reduced the total salt content in the soil solution, of which the H and GH treatments reduced it by 26.90% and 21.15% compared with the CK treatment, respectively. Desulfurized gypsum combined with montmorillonite-modified hydrochar (GH) significantly increased the Ca content in the soil solution. 2+ Mg 2+ and K + The content of base ions is closer to equilibrium.

[0054] In summary, the combined advantages of desulfurized gypsum and montmorillonite-modified hydrothermal carbon in improving saline-alkali land can effectively improve the physical, chemical, and biological properties of the soil, laying the foundation for improving the quality and efficiency of saline-alkali land. Therefore, the montmorillonite-modified hydrothermal carbon prepared by this invention and a certain amount of desulfurized gypsum are recommended for improving coastal saline-alkali land.

[0055] In order to enable those skilled in the art to more clearly understand the technical solution of the present invention, the technical solution of the present invention will be described in detail below with reference to specific embodiments.

[0056] Example 1 In this embodiment, a method for preparing surface-adsorbed / low-crystalline Fe montmorillonite is provided, comprising the following steps: (1) Reduction: 0.05 g of hematite slag, 0.10 g of sodium dithionite, and 25 mL of pre-deoxygenated buffer solution were mixed in a sealed glass bottle in an anaerobic cabinet. The buffer solution contained 0.0036 M sodium citrate and 0.38 M sodium bicarbonate. The mixture was heated in a water bath at 70°C for 4 h. The heated product was centrifuged at 4000 rpm for 10 min. The supernatant was removed in an anaerobic cabinet and washed three times with 1 M NaCl solution and deionized water, respectively.

[0057] (2) Oxidation: The product after washing in step (1) was exposed to air and reacted on a shaker (200 rpm) in the dark for 48 h. During the reaction, H2O2 was added at an initial concentration of 5 mmol / L, with a solid to H2O2 solution dosage ratio of 1 g:4 mL. Subsequently, 2 mmol / L H2O2 solution was added every hour at a dosage ratio of 1 g:2 mL. After the reaction, 2 volumes of 0.5 M HCl were added to the product and extracted for 2 h to obtain montmorillonite with surface adsorbed / low crystalline Fe.

[0058] Example 2 In this embodiment, another method for preparing surface-adsorbed / low-crystalline Fe montmorillonite is provided, comprising the following steps: (1) Reduction: 0.03 g of hematite slag, 0.08 g of sodium dithionite, and 20 mL of pre-deoxygenated buffer solution were mixed in a sealed glass bottle in an anaerobic cabinet. The buffer solution contained 0.004 M sodium citrate and 0.4 M sodium bicarbonate. The mixture was heated in a water bath at 65°C for 5 h. The heated product was centrifuged at 4000 rpm for 10 min. The supernatant was removed in an anaerobic cabinet and washed three times with 1 M NaCl solution and deionized water, respectively.

[0059] (2) Oxidation: The product after washing in step (1) was exposed to air and reacted on a shaker (200 rpm) in the dark for 50 h. During the reaction, H2O2 solution was added in stages with an initial concentration of 4 mmol / L at a solid to H2O2 solution ratio of 1 g:6 mL. 3 mmol / L H2O2 solution was then added every hour at a ratio of 1 g:1 mL. After the reaction, 1 volume of 0.6 M HCl was added to the product and extracted for 1 h to obtain montmorillonite with surface adsorbed / low crystalline Fe.

[0060] Example 3 In this embodiment, another method for preparing surface-adsorbed / low-crystalline Fe montmorillonite is provided, comprising the following steps: (1) Reduction: 0.06 g of hematite slag, 0.12 g of sodium dithionite, and 25 mL of pre-deoxygenated buffer solution were mixed in a sealed glass bottle in an anaerobic cabinet. The buffer solution contained 0.003 M sodium citrate and 0.3 M sodium bicarbonate. The mixture was heated in a water bath at 75°C for 3 h. The heated product was centrifuged at 4000 rpm for 10 min. The supernatant was removed in an anaerobic cabinet and washed three times with 1 M NaCl solution and deionized water, respectively.

[0061] (2) Oxidation: The product after washing in step (1) was exposed to air and reacted on a shaker (200 rpm) in the dark for 45 h. During the reaction, H2O2 solution was added in stages. The initial concentration was 6 mmol / L. The solid was added at a dosage ratio of 1 g:2 mL. 1 mmol / L H2O2 solution was then added every hour at a dosage ratio of 1 g:3 mL. 3 volumes of 0.4 M HCl were added to the product after the reaction and extracted for 3 h to obtain montmorillonite with surface adsorbed / low crystalline Fe.

[0062] The montmorillonite with surface adsorption / low crystalline Fe described in the above Examples 1-3 has basically the same technical effects.

[0063] Example 4 In this embodiment, a montmorillonite-modified hydrothermal carbon and a preparation method thereof are provided, comprising the following steps: (1) Municipal sewage was filtered through a plate and frame filter to reduce the water content to 80%, and then crushed into particles with a particle size of ≤5 mm to obtain dewatered sludge for use. The dewatered sludge, the montmorillonite with surface adsorbed / low crystalline Fe in Example 1, and FeCl3 were mixed uniformly in a mass ratio of 50:10:1, and dilute hydrochloric acid was added to adjust the pH of the mixture to 4.0-5.0.

[0064] (2) The mixture from step (1) was placed in a high-pressure hydrothermal reactor, set at 220 ± 5°C, maintained at a pressure of 2.5-3.0 MPa, and reacted for 2 h. After the reaction, the solid portion was centrifuged and dried at 105°C for 12 h, then ball-milled to 200 mesh to obtain montmorillonite-modified hydrothermal carbon.

[0065] Example 5 In this embodiment, another montmorillonite-modified hydrothermal carbon and its preparation method are provided, which differs from Example 4 in that: In step (1), the dewatered sludge, the montmorillonite with surface adsorption / low crystalline Fe in Example 2 and Fe2(SO4)3 are mixed uniformly in a mass ratio of 45:8:0.8, and dilute sulfuric acid is added to adjust the pH of the above mixture to 4.0-5.0.

[0066] Example 6 In this embodiment, another montmorillonite-modified hydrothermal carbon and its preparation method are provided, which differs from Example 4 in that: In step (1), the dewatered sludge, the montmorillonite with surface adsorption / low crystalline Fe in Example 3 and Fe(NO3)3 are mixed uniformly in a mass ratio of 55:10:1.2, and dilute nitric acid is added to adjust the pH of the above mixture to 4.0-5.0.

[0067] Example 7 In this embodiment, another montmorillonite-modified hydrothermal charcoal and its preparation method are provided. The difference from Example 4 is that the biomass material is straw. The preparation method is as follows: Straw was chopped into pieces ≤1 cm in size for later use. A high-pressure hydrothermal reactor was used to react the stalks with water in a weight ratio of 90 parts straw, 9 parts montmorillonite, and 1 part Fe₂(SO₄)₃. The reaction parameters were: a raw material to water mass-volume ratio of 1:10, 200°C, 8 MPa, and a reaction time of 1 h. After the reaction, the stalks were washed with deionized water and dried and ground to obtain montmorillonite-modified hydrothermal carbon.

[0068] The montmorillonite-modified hydrothermal carbon described in Examples 4-7 above has basically the same technical effects.

[0069] Example 8 In this embodiment, a saline-alkali soil conditioner is provided, which includes the montmorillonite-modified hydrothermal carbon prepared in any one of Examples 4-7 and desulfurized gypsum.

[0070] The application method for the aforementioned amendments is as follows: Adjust the soil's water holding capacity to 60%. After a one- to two-week pre-incubation period, add compound fertilizer, montmorillonite-modified hydrocharcoal, and desulfurized gypsum, and mix thoroughly. Water the soil regularly for six months to a year. Each of the three ingredients should be added at a rate of 1% by weight of the soil. The compound fertilizer should contain 15% each of nitrogen (N), phosphorus (P2O5), and potassium (K2O), for a total nutrient content of 45%.

[0071] Example 9 In this embodiment, a saline-alkali soil conditioner is provided, which is different from Example 8 in that when the conditioner is applied, the added amount of the montmorillonite-modified hydrothermal charcoal and the desulfurized gypsum is 0.8% of the soil weight.

[0072] Example 10 In this embodiment, a saline-alkali soil conditioner is provided, which is different from Example 8 in that when the conditioner is applied, the added amount of the montmorillonite-modified hydrothermal charcoal and the desulfurization gypsum is 1.2% of the soil weight.

[0073] The saline-alkali soil conditioners described in Examples 8-10 above can achieve similar soil improvement effects.

[0074] The foregoing description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Those skilled in the art will readily appreciate that various modifications and variations of the present invention are possible. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention are intended to be within the scope of protection of the present invention.

Claims

1. A method for preparing montmorillonite-modified hydrothermal carbon, characterized in that: The steps include: The biomass material, surface adsorbed / low crystalline Fe montmorillonite and iron-containing inorganic salt are mixed evenly, water is added to make the solid-liquid ratio of the mixed system 1:8-12, and the mixed system is reacted at a temperature of 220±5°C and a pressure of 2.5-3.0MPa for 1-3 hours to prepare the product; The mass ratio of the biomass material, surface adsorption / low crystalline Fe montmorillonite and iron-containing inorganic salt is 45-95:8-10:0.8-1.

2.

2. The method for preparing montmorillonite-modified hydrothermal carbon according to claim 1, wherein: The biomass material is dewatered sludge having a water content of 75-85%. The mass ratio of the dewatered sludge, surface adsorbed / low-crystalline Fe montmorillonite, and iron-containing inorganic salt is 45-55:8-10:0.8-1.

2. When water is added, the "solid" in the solid-liquid ratio is calculated based on the dry weight of the dewatered sludge. After mixing the above raw materials, acid is added to adjust the pH to 4.0-5.0, and then transferred to a hydrothermal reactor for subsequent reaction.

3. The method for preparing montmorillonite-modified hydrothermal carbon according to claim 1, wherein: The biomass material is straw, and the mass ratio of the straw, surface adsorption / low crystalline Fe montmorillonite and iron-containing inorganic salt is 85-95:8-10:0.8-1.

2.

4. The method for preparing montmorillonite-modified hydrothermal carbon according to claim 1, wherein: The iron-containing inorganic salt is selected from one or more of ferric chloride, ferric sulfate, ferric nitrate, ferric phosphate, ferric bromide, ferric fluoride, and ferric iodide.

5. The method for preparing montmorillonite-modified hydrothermal carbon according to claim 1, wherein: The preparation method of the surface adsorption / low crystalline Fe montmorillonite is as follows: (1) Reduction: Mix hematite slag, sodium dithionite and sodium citrate-sodium bicarbonate buffer under anaerobic conditions, heat in a water bath at 65-75°C for 3-5 hours, and separate, retain and wash the solids in the heated product; (2) Oxidation: The solid washed in step (1) is exposed to air and placed in darkness for reaction for 45 to 50 hours, with H2O2 solution added in stages during the reaction; the H2O2 solution is added in the following manner: the initial concentration is 4 to 6 mmol / L, and the solid is added at a dosage ratio of 1 g: 2 to 6 mL, and then 1 to 3 mmol / L of H2O2 solution is added every hour at a dosage ratio of 1 g: 1 to 3 mL; the product after the oxidation is mixed to obtain a suspension, and 1 to 3 times the volume of 0.4 to 0.6 M hydrochloric acid solution is added and stirred for reaction for 1 to 3 hours to obtain surface adsorbed / low crystalline Fe montmorillonite; In step (1), the dosage ratio of the hematite slag, sodium dithionite and sodium citrate-sodium bicarbonate buffer solution is 0.03-0.06 g: 0.08-0.12 g: 20-30 mL; in the buffer solution, the concentration of sodium citrate is 0.003-0.004 M, and the concentration of sodium bicarbonate is 0.3-0.4 M. Inert gas is introduced in advance for a period of time to remove dissolved oxygen in the solution, and the introduction time is 20-40 min.

6. Montmorillonite-modified hydrothermal carbon material prepared by the method according to any one of claims 1 to 5.

7. A composition, characterized in that The composition includes an active dose of the montmorillonite-modified hydrothermal carbon material according to claim 6 and also includes desulfurized gypsum; the dosage ratio of the montmorillonite-modified hydrothermal carbon to the desulfurized gypsum is 1-3:1-3.

8. Use of the montmorillonite-modified hydrothermal carbon material according to claim 6 or the composition according to claim 7 in the field of saline-alkali soil improvement; the improvement purpose is selected from any one or a combination of the following: 1) Reduce salinity and alkali stress in saline-alkali soil; 2) Increase the organic matter content in saline-alkali soil; 3) Improve the fertility of saline-alkali soil; 4) Increase aggregates in saline-alkali soils; 5) Soil porosity increases, and air permeability and water permeability increase.

9. A saline-alkali soil conditioner, characterized in that: The improver is composed of the montmorillonite modified hydrothermal carbon material according to claim 6 and desulfurization gypsum, and the two are not mixed before use.

10. A method for improving saline-alkali soil, characterized in that: The method comprises applying the montmorillonite-modified hydrothermal carbon material and desulfurized gypsum according to claim 6 to the soil to be treated, with the application dosage being 0.8-1.2% of the mass of the soil to be treated.

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