Method for preparing saline-alkali soil conditioner from sludge composted materials and application of saline-alkali soil conditioner

Abstract

The invention belongs to the technical field of solid waste recycling and soil improvement, and relates to a method for preparing a saline alkali soil conditioner from sludge composted materials, which comprises the following steps: firstly, adding 0.1-0.2% of calcium acetate into lignocellulose pre-hydrolysate, precipitating to remove macromolecular lignin which is easy to cause surface precipitation, and then adding the calcium acetate into the lignocellulose pre-hydrolysate; the modified eluting agent rich in micromolecular hydrophilic lignin and organic acid is obtained; then, mixing the eluting agent with sludge composted materials according to a liquid-solid ratio of (3-5): 1 for leaching, and synchronously and efficiently removing heavy metals and soluble salts; after leaching, carrying out solid-liquid separation to obtain a purified composted material; and finally, compounding the compound with desulfurized gypsum and humic acid according to a dry basis mass ratio of (30-50): (20-40): (10-30), aging and granulating to obtain the saline alkali soil conditioner. The method treats wastes with wastes, is low in cost, can deeply remove heavy metals and salts, can obviously reduce the pH value of soil, improves organic matters, promotes plant growth, and realizes the dual goals of safe utilization of sludge and improvement of saline-alkali soil.

Description

Technical Field

[0001] This invention belongs to the field of soil stabilizers, and specifically relates to a method and application of preparing a saline-alkali soil conditioner from sludge compost. Background Technology

[0002] Aerobic fermentation technology for sludge can effectively degrade organic matter, kill pathogens, and transform unstable sludge into stable compost.

[0003] However, heavy metals such as Cd, Pb, and Cr present in sludge are the most significant environmental risks limiting its agricultural use. Direct application to soil poses long-term potential risks to ecological and agricultural product safety. Furthermore, the high soluble salt content in compost exacerbates soil salinization, damaging the basic physicochemical properties of the soil and severely limiting its application in saline-alkali soil improvement. Therefore, developing efficient and low-cost technologies for the simultaneous removal of heavy metals and soluble salts is a key bottleneck in achieving the safe large-scale utilization of sludge on land.

[0004] Currently, technologies for removing heavy metals from sludge include chemical leaching, bioleaching, and electroremediation. Among these, chemical leaching is highly efficient, but commonly used leaching agents such as inorganic acids and chelating agents like EDTA have drawbacks such as high cost, potential for secondary pollution, and damage to the nutrients in the compost. Therefore, finding a green, inexpensive, and efficient alternative leaching agent has become a research hotspot.

[0005] Pre-hydrolysis of lignocellulose raw materials is a key pretreatment process that selectively degrades and dissolves hemicellulose in the raw materials through hydrothermal treatment (such as dilute acid pre-hydrolysis or steam explosion). Its main purpose is to achieve high-value utilization of all components of biomass. The pre-hydrolysate produced in this process is a complex resource liquid containing hydrolyzed sugars, furfural, organic acids, and water-soluble lignin. Among these, organic acids such as acetic acid in the pre-hydrolysate can effectively promote the dissolution and complexation of heavy metal ions from the compost. The acidic conditions can also effectively leach easily soluble salt ions (such as chlorides and sulfates), causing them to be discharged with the leachate, thus significantly reducing the risk of secondary salinization of the soil and providing an important guarantee for the subsequent preparation of safe saline-alkali soil conditioners. On the other hand, the water-soluble lignin in the pre-hydrolysate, due to its phenolic hydroxyl and carboxyl groups, has excellent heavy metal ion chelating ability and is a potential natural bio-based heavy metal adsorbent. However, lignin has a complex structure. During pre-hydrolysis, some lignin with larger molecular weights and lower hydrophilic group content can easily combine with heavy metal ions to form insoluble precipitates, which then remain in the compost, resulting in low heavy metal ion removal rates and environmental risks of secondary release.

[0006] Currently, commercially available sludge heavy metal leaching technologies mainly rely on inorganic acids (such as hydrochloric acid and sulfuric acid) or synthetic chelating agents (such as EDTA). While inorganic acid leaching agents are less expensive, they introduce large amounts of soluble salts while removing heavy metals, exacerbating the risk of soil salinization. Synthetic chelating agents, on the other hand, are difficult to use on a large scale due to their high cost, poor biodegradability, and potential for secondary groundwater pollution. Furthermore, although pre-hydrolyzed solutions are considered green leaching agents, some of their lignin readily forms insoluble precipitates with heavy metals, remaining in the composted material. This results in low removal rates and a risk of secondary release, limiting the feasibility of their direct application.

[0007] Therefore, there is a need to invent a method for preparing a sludge composting agent for saline-alkali soil, which can not only efficiently and cost-effectively remove heavy metals and soluble salts simultaneously, but also make full use of industrial by-products to achieve "waste treatment with waste", while ensuring the environmental safety and soil improvement value of the final product. Summary of the Invention

[0008] To address the aforementioned technical problems, this invention provides an efficient, low-cost, and environmentally friendly integrated method.

[0009] The technical solution of the present invention is as follows: A method for preparing a saline-alkali soil conditioner by leaching sludge compost with a pre-hydrolyzed solution includes the following steps: S1, Preparation of pre-hydrolyzed solution eluent: Take the pre-hydrolyzed solution and add 0.1%~0.2% (w / v, mass / volume percentage) of calcium acetate solid to it under uniform stirring. After stirring and reacting for 30~60 min, let the mixture stand to precipitate for 2~4 h, or use ordinary pressure filtration or vacuum filtration to separate the solid and liquid, and collect the supernatant or filtrate to obtain the pre-hydrolyzed solution eluent; S2, Heavy metal leaching of sludge compost: The pre-hydrolyzed liquid and sludge compost are mixed at a liquid-to-solid ratio of (3~5) L:1kg, and stirred at 50~150 rpm for 2~4h at room temperature. The leaching and complexation of heavy metals are achieved through the hydrophilic small molecule lignin and organic acids in the pre-hydrolyzed liquid. S3, Solid-liquid separation: After rinsing, solid-liquid separation is performed, the solid part is collected, and after drying, purified compost is obtained; S4, compounding of saline-alkali soil conditioner: the solid residue after leaching, namely the purified compost, desulfurized gypsum and humic acid are compounded at a dry basis mass ratio of (30~50): (20~40): (10~30) and mixed thoroughly. S5, Aging and Product Preparation: The compounded materials are aged and matured at room temperature for 5-7 days, and then dried, crushed and granulated to obtain the saline-alkali soil conditioner.

[0010] Preferably, the pre-hydrolyzed liquid raw material in S1 is lignocellulose, selected from at least one of broadleaf wood, coniferous wood, bamboo, and agricultural straw.

[0011] Preferably, the pre-hydrolyzed solution in S1 is obtained through a hydrothermal treatment process; The above-mentioned hydrothermal treatment process is selected from at least one of hydrothermal pretreatment, dilute acid prehydrolysis, and steam explosion; The above pre-hydrolyzed solution contains hemicellulose-degraded sugars, furfural, organic acids, and water-soluble lignin.

[0012] Preferably, the S4 desulfurized gypsum contains no less than 85% CaSO4·2H2O, no less than 40% organic matter content of humic acid, no less than 35% organic matter content in the purified compost, and the heavy metal content is lower than the risk screening value specified in the "Soil Environmental Quality Agricultural Land Soil Pollution Risk Control Standard" (GB15618-2018).

[0013] Preferably, after S5 is aged and matured at room temperature for 5 to 7 days, it is dried at 50 to 60°C until the moisture content is less than 15%, crushed to pass through a 100-mesh sieve, and then granulated using a disc granulator with 10% to 15% added moisture to finally obtain a granular saline-alkali soil conditioner with a particle size of 2 to 4 mm.

[0014] The above-mentioned method for preparing a saline-alkali soil conditioner from sludge compost is applied to the improvement of saline-alkali soil.

[0015] When pre-hydrolyzed solutions are applied directly, some large-molecule lignin with low hydrophilic group content may cause residual metal ion adsorption and precipitation. To address this challenge, this invention adds an appropriate amount of calcium acetate to the pre-hydrolyzed solution. Calcium ions preferentially precipitate this portion of lignin, and the supernatant is obtained after solid-liquid separation, yielding a pre-hydrolyzed solution leaching agent rich in small-molecule lignin with hydrophilic groups. This leaching agent uses hydrophilic small-molecule lignin as its core active ingredient, working synergistically with acetic acid to effectively promote the dissolution and complexation of heavy metals, efficiently leaching heavy metal ions from sludge compost and extracting them to the liquid phase for deep removal. This overcomes the application defects of the original pre-hydrolyzed solution and lays a solid foundation for the subsequent preparation of safe saline-alkali soil conditioners. Finally, the purified compost is compounded with other additives to prepare a high-value saline-alkali soil conditioner. This technology constructs an innovative "waste-to-waste" path, providing a resource-based solution with significant engineering application prospects for the coordinated solution of sludge disposal and saline-alkali land improvement.

[0016] This invention prepares a highly efficient bio-based leaching agent for heavy metals by treating pre-hydrolyzed liquid with calcium acetate; deeply removes heavy metals from municipal sludge compost, thus removing its agricultural use restrictions; and combines the purified compost with desulfurized gypsum and humic acid to prepare a conditioner whose heavy metal content fully meets the national agricultural land soil safety standards and can be safely applied to saline-alkali soils.

[0017] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) The core leaching agent of this invention uses pre-hydrolyzed liquid, a by-product of the papermaking or biorefining industry. Through simple pretreatment, it is transformed into a highly efficient heavy metal removal agent. At the same time, municipal sludge aerobic fermentation compost and desulfurization gypsum and other solid wastes are transformed into high-value soil conditioners, thus constructing a closed-loop technology path of "treating waste with waste". It is low in cost and has a high degree of resource utilization.

[0018] (2) By controlling the addition of calcium acetate to the pre-hydrolysate, the low content of macromolecular lignin with low hydrophilic group content that is prone to clogging and competitive adsorption is removed, reducing competitive adsorption, ensuring a stable and efficient rinsing process, and achieving deep removal of heavy metals from sludge compost.

[0019] (3) The purified sludge compost and desulfurized gypsum (providing Ca) 2+ Replacement of Na + When humic acid (which improves soil aggregate structure) and humic acid are used in the optimal ratio, they work synergistically to simultaneously improve the structure of saline-alkali soil, supplement nutrients and enhance its physical and chemical properties, thus achieving the dual goals of "pollution control" and "soil improvement".

[0020] (4) The process is simple, the equipment requirements are low, the energy consumption is low, and the operating conditions are mild, which greatly improves the feasibility and economy of the technology and makes it easy to achieve large-scale industrial production and application. Detailed Implementation

[0021] The technical solution of the present invention will be further illustrated below through specific embodiments. However, the following examples are merely simplified examples of the present invention and do not represent or limit the scope of protection of the present invention. The scope of protection of the present invention is determined by the claims.

[0022] Example 1: The sludge compost from the aerobic fermentation of municipal sludge in Zhangjiakou had heavy metal contents of 11.8 mg / kg, 0.5 mg / kg, 91.8 mg / kg, and 251.7 mg / kg, respectively, and a soluble salt content (based on EC value) of 4.99 mS / cm.

[0023] Take 1L of pre-hydrolyzed solution, add 1.0g of calcium acetate (0.1% w / v) under uniform stirring, continue the reaction for 40min, let it stand to precipitate for 3h, and collect the supernatant to obtain the modified rinsing agent. The leaching agent was mixed with 200g of the above-mentioned sludge compost (liquid-solid ratio 4:1), and the mixture was stirred and leached at 120 rpm for 3 hours at room temperature. Then, the solid and liquid were separated by pressure filtration, the filter residue was collected and dried at 80℃ to constant weight to obtain purified compost. Finally, the purified composted material is compounded with desulfurized gypsum and humic acid at a dry basis mass ratio of 50:30:20, and after being mixed evenly, a saline-alkali soil conditioner is obtained.

[0024] The removal rates of heavy metals Pb, Cd, Cr, and Ni in the sludge compost were measured according to the "Determination of Metal Elements in Solid Waste by Inductively Coupled Plasma Mass Spectrometry" (HJ 766-2015) and calculated using the following formula: Removal rate (%) = (Initial concentration – Post-treatment concentration) / Initial concentration × 100%, which were 69.3%, 72.2%, 56.7%, and 68.4%, respectively. At the same time, the soluble salt content (the soluble salt concentration was measured using a conductivity meter and expressed as the conductivity EC value) decreased to 1.50 mS / cm (a decrease of 69.9%). The heavy metal content of the compounded soil conditioner was lower than the Class A risk screening value specified in the "Standard for Pollutant Control of Agricultural Sludge" (GB 4284-2018). When the saline-alkali soil conditioner was applied to saline-alkali soil at a dosage of 35 g / kg relative to dry soil (initial pH=8.7), the soil pH dropped to 7.69 after 40 days, the soil organic matter increased by 67.1%, the height growth rate of maize seedlings planted in the improved soil was 30.6%, and the soluble sugar and soluble protein were 9.74 and 12.54 mg / g, respectively, which were 39.54% and 30.08% higher than the blank control group.

[0025] Example 2: Take 1L of pre-hydrolyzed solution, add 1.0g of calcium acetate (0.1% w / v) under uniform stirring, continue the reaction for 40min, let it stand to precipitate for 3h, and collect the supernatant to obtain the modified rinsing agent. The rinsing agent was mixed with 200g of sludge compost from Example 1 (liquid-solid ratio 4:1), and the mixture was stirred and rinsed at 120rpm for 3 hours at room temperature. Then, the solid and liquid were separated by pressure filtration, the filter residue was collected and dried at 80°C to constant weight to obtain purified compost. Finally, the purified composted material is compounded with desulfurized gypsum and humic acid at a dry basis mass ratio of 50:40:10, and after being mixed evenly, a saline-alkali soil conditioner is obtained.

[0026] The removal rates of heavy metals Pb, Cd, Cr, and Ni (determined according to the "Determination of Metal Elements in Solid Waste by Inductively Coupled Plasma Mass Spectrometry" (HJ 766-2015) and calculated using the following formula: Removal rate (%) = (Initial concentration – Post-treatment concentration) / Initial concentration × 100%) were 70.0%, 71.5%, 55.3%, and 67.9%, respectively. Meanwhile, the soluble salt content (determined using a conductivity meter and expressed as EC conductivity) decreased to 1.58 mS / cm (a reduction of 68.3%). The heavy metal content of the compounded soil conditioner was lower than the Class A risk screening value specified in the "Standard for Pollutant Control of Agricultural Sludge" (GB 4284-2018). When the saline-alkali soil conditioner was applied to saline-alkali soil at a dosage of 35 g / kg relative to dry soil (initial pH=8.7), the soil pH dropped to 7.74 after 40 days, the soil organic matter increased by 60.8%, the height of maize seedlings planted in the improved soil increased by 16.6%, and the soluble sugar and soluble protein were 8.21 and 11.84 mg / g, respectively, which were 17.62% and 22.82% higher than the blank control group.

[0027] Example 3: The purified compost material obtained in Example 2 was used; the purified compost material was compounded with desulfurized gypsum and humic acid at a dry basis mass ratio of 50:10:40. After being mixed evenly, the saline-alkali soil conditioner was obtained. The heavy metal content of the compounded soil conditioner was lower than the Class A risk screening value specified in the "Standard for Pollutant Control of Agricultural Sludge" (GB 4284-2018).

[0028] The saline-alkali soil conditioner was applied to saline-alkali soil (initial pH=8.7) at a dosage of 35 g / kg relative to dry soil. After 40 days, the soil pH dropped to 7.70, the soil organic matter increased by 74.2%, and the height of maize seedlings planted in the improved soil increased by 23.5%. The soluble sugar and soluble protein were 8.37 and 12.18 mg / g, respectively, which were 19.91% and 26.35% higher than those in the blank control group.

[0029] Example 4: Take 1L of pre-hydrolyzed solution, add 2.0g of calcium acetate (0.2% w / v) under uniform stirring, continue the reaction for 40min, let it stand to precipitate for 3h, and collect the supernatant to obtain the modified elution agent. The rinsing agent was mixed with 200g of sludge compost from Example 1 (liquid-solid ratio 4:1), and the mixture was stirred and rinsed at 120rpm for 3 hours at room temperature. Then, the solid and liquid were separated by pressure filtration, the filter residue was collected and dried at 80°C to constant weight to obtain purified compost. Finally, the purified composted material is compounded with desulfurized gypsum and humic acid at a dry basis mass ratio of 50:30:20, and after being mixed evenly, a saline-alkali soil conditioner is obtained.

[0030] The removal rates of heavy metals Pb, Cd, Cr, and Ni (determined according to the "Determination of Metal Elements in Solid Waste by Inductively Coupled Plasma Mass Spectrometry" (HJ 766-2015) and calculated using the following formula: Removal rate (%) = (Initial concentration – Post-treatment concentration) / Initial concentration × 100%) were 63.7%, 58.9%, 45.8%, and 64.8%, respectively. Meanwhile, the soluble salt content (distinguished by a conductivity meter and expressed as EC conductivity) decreased to 1.86 mS / cm (a reduction of 60.7%). The heavy metal content of the compounded soil conditioner was lower than the Class A risk screening value specified in the "Standard for Pollutant Control of Agricultural Sludge" (GB 4284-2018). When the saline-alkali soil conditioner was applied to saline-alkali soil at a dosage of 35 g / kg relative to dry soil (initial pH=8.7), the soil pH dropped to 7.71 after 40 days, the soil organic matter increased by 65.7%, and the height of corn seedlings planted in the improved soil increased by 28.9%.

[0031] Example 5: Take 1L of pre-hydrolyzed solution, add 2.0g of calcium acetate (0.2% w / v) under uniform stirring, continue the reaction for 40min, let it stand to precipitate for 3h, and collect the supernatant to obtain the modified elution agent. The leaching agent was mixed with 200g of sludge compost from Example 1 (liquid-solid ratio 4:1), and the mixture was stirred and leached at 120 rpm for 3 hours at room temperature. The mixture was then separated into solid and liquid by pressure filtration. The filter residue was collected and dried at 80°C to constant weight to obtain purified compost. Finally, the purified composted material is compounded with desulfurized gypsum and humic acid at a dry basis mass ratio of 50:40:10, and after being mixed evenly, a saline-alkali soil conditioner is obtained.

[0032] The removal rates of heavy metals Pb, Cd, Cr, and Ni (determined according to the "Determination of Metal Elements in Solid Waste by Inductively Coupled Plasma Mass Spectrometry" (HJ 766-2015) and calculated using the following formula: Removal rate (%) = (Initial concentration – Post-treatment concentration) / Initial concentration × 100%) were 63.4%, 59.1%, 45.0%, and 63.2%, respectively. Meanwhile, the soluble salt content (distinguished by a conductivity meter and expressed as EC value) decreased to 1.95 mS / cm (a reduction of 60.9%). The heavy metal content of the compounded soil conditioner was lower than the Class A risk screening value specified in the "Standard for Pollutant Control of Agricultural Sludge" (GB 4284-2018). When the saline-alkali soil conditioner was applied to saline-alkali soil at a dosage of 35 g / kg relative to dry soil (initial pH=8.7), the soil pH dropped to 7.70 after 40 days, the soil organic matter increased by 57.7%, and the height of corn seedlings planted in the improved soil increased by 15.4%.

[0033] Example 6: The purified compost material obtained in Example 5 was used; the purified compost material was compounded with desulfurized gypsum and humic acid at a dry basis mass ratio of 50:10:40. After being mixed evenly, the saline-alkali soil conditioner was obtained. The heavy metal content of the compounded soil conditioner was lower than the Class A risk screening value specified in the "Standard for Pollutant Control of Agricultural Sludge" (GB 4284-2018).

[0034] The saline-alkali soil conditioner was applied to saline-alkali soil (initial pH=8.7) at a dosage of 35 g / kg relative to dry soil. After 40 days, the soil pH dropped to 7.75, the soil organic matter increased by 73.9%, and the height of maize seedlings planted in the improved soil increased by 27.9%.

[0035] Comparative Example 1: The aerobic sludge compost was rinsed with clean water (preparation method and reaction conditions are the same as in Example 1). The results showed that the removal rates of Pb, Cd, Cr, and Ni (method as in Example 1) were 11.3%, 13.2%, 10.6%, and 12.9%, respectively, and the soluble salt content (method as in Example 1) decreased to 3.35 mS / cm.

[0036] A soil conditioner was prepared by mixing the leached compost with desulfurized gypsum and humic acid at a dry basis mass ratio of 50:30:20. When this saline-alkali soil conditioner was applied to saline-alkali soil (initial pH=8.7) at a dosage of 35 g / kg relative to the dry basis soil, the soil pH dropped to 8.12, the organic matter increased by 50.5%, and the growth rate of corn seedling height increased by 10.2%, confirming that the effect of water leaching on the removal of heavy metals is limited.

[0037] Comparative Example 2: The original pre-hydrolyzed solution was used for rinsing (preparation method and reaction conditions were the same as in Example 1). The results showed that the removal rates of Pb, Cd, Cr, and Ni (method as in Example 1) were 36.4%, 39.1%, 33.8%, and 32.9%, respectively, and the soluble salt content (method as in Example 1) decreased to 2.95 mS / cm.

[0038] Soil conditioner was prepared by compounding the leached compost with desulfurized gypsum and humic acid at a dry basis mass ratio of 50:30:20. When this saline-alkali soil conditioner was applied to saline-alkali soil (initial pH=8.7) at a dosage of 35 g / kg relative to the dry basis soil, the soil pH dropped to 7.98, the organic matter increased by 65.8%, and the growth rate of corn seedling height increased by 17.8%, indicating that the leaching effect of the unmodified pre-hydrolyzed solution was not good.

[0039] Comparative Example 3: The rinsing was performed using a pre-hydrolyzed rinsing agent modified with 0.05% calcium acetate (other conditions were the same as in Example 1).

[0040] The results showed that the removal rates of Pb, Cd, Cr, and Ni (using the same method as in Example 1) were 54.2%, 57.6%, 50.3%, and 54.2%, respectively, and the soluble salt content (using the same method as in Example 1) decreased to 1.72 mS / cm.

[0041] A soil conditioner was prepared by compounding the leached compost with desulfurized gypsum and humic acid at a dry basis mass ratio of 50:30:20. When this saline-alkali soil conditioner was applied to saline-alkali soil (initial pH=8.7) at a dosage of 35 g / kg relative to the dry basis soil, the soil pH dropped to 7.79, the organic matter increased by 64.7%, and the growth rate of corn seedling height increased by 26.4%, proving that low-dose calcium acetate modification can significantly improve the leaching effect.

[0042] Comparative Example 4: The rinsing was performed using a pre-hydrolyzed solution rinsing agent modified with 0.3% calcium acetate (other conditions were the same as in Example 1).

[0043] The results showed that the removal rates of Pb, Cd, Cr, and Ni (using the same method as in Example 1) were 59.1%, 53.9%, 49.2%, and 54.8%, respectively, and the soluble salt content (using the same method as in Example 1) was 2.18 mS / cm.

[0044] A soil conditioner was prepared by mixing the leached compost with desulfurized gypsum and humic acid at a dry basis mass ratio of 50:30:20. When this saline-alkali soil conditioner was applied to saline-alkali soil (initial pH=8.7) at a dosage of 35 g / kg relative to the dry basis soil, the soil pH dropped to 7.76, the organic matter increased by 65.9%, and the growth rate of corn seedling height increased by 27.5%, confirming that excessive calcium acetate would reduce the treatment effect due to competitive inhibition.

[0045] Table 1. Effects of different calcium acetate addition amounts on the leaching effect of sludge compost.

[0046] Note: The experimental conditions were uniformly set as follows: initial compost material Pb=11.8 mg / kg, Cd=0.5 mg / kg, Cr=91.8 mg / kg, Ni=251.7 mg / kg, EC=4.99 mS / cm; the rinsing process was uniformly controlled at a liquid-to-solid ratio of 4:1, a reaction time of 3 hours, and a rotation speed of 120 rpm at room temperature. After rinsing, the sludge compost material was dried to measure the heavy metal removal rate and EC value.

[0047] Comparative analysis of the examples and a series of comparative examples (see Table 1) shows that when the calcium acetate addition is 0.1%, the system achieves the best treatment effect, with removal rates of 69.3%, 72.2%, 56.7%, and 68.4% for Pb, Cd, Cr, and Ni, respectively. Simultaneously, the EC value decreases to 1.50 mS / cm, indicating effective control of salt content. An appropriate amount of calcium acetate (0.1%) can effectively precipitate acid-insoluble lignin with large molecular weight and low hydrophilic group content in the pre-hydrolyzed solution, eliminating the risk of forming insoluble precipitates with heavy metals. At the same time, it retains and activates acid-insoluble lignin with small molecular weight and abundant functional groups, fully utilizing its phenolic hydroxyl and carboxyl groups for heavy metal complexation. When excessive calcium acetate is added, the treatment effect decreases due to the competitive inhibition effect of calcium ions. This finding provides key process parameter basis for the industrial application of this invention, ensuring the reliability and economy of the technology.

[0048] Table 2. Effects of the mixing ratio of sludge compost with desulfurization gypsum and humic acid on soil improvement. Group Clinker: Desulfurized gypsum: Humic acid Soil pH Organic matter growth rate (%) Plant height growth rate (%) Soluble sugar content of the plant (mg / g) Soluble protein content of the plant (mg / g) Example 1 50：30：20 7.69 67.1 30.6 9.74 12.54 Example 2 50：40：10 7.74 60.8 16.6 8.21 11.84 Example 3 50：10：40 7.70 74.2 23.5 8.37 12.18 Note: The experimental conditions were uniformly set as follows: initial compost material Pb=11.8 mg / kg, Cd=0.5 mg / kg, Cr=91.8 mg / kg, Ni=251.7 mg / kg, EC=4.99 mS / cm; leaching was carried out using a pre-hydrolyzed solution treated with 0.1% calcium acetate, and the leaching process was uniformly controlled at a liquid-to-solid ratio of 4:1, a reaction time of 3 hours, and a rotation speed of 120 rpm at room temperature; the purified compost material obtained after leaching was compounded with desulfurized gypsum and humic acid at a certain dry basis mass ratio to prepare a soil conditioner, which was applied to saline-alkali soil (initial pH=8.7) at a dosage of 35 g / kg relative to the dry basis soil.

[0049] Table 2 shows that when the pre-hydrolyzed solution treated with 0.1% calcium acetate was used to wash the sludge compost, and the sludge compost obtained after washing and drying was compounded with desulfurized gypsum and humic acid to prepare a soil conditioner, the soil conditioner showed the best comprehensive improvement effect when the mass ratio was 50:30:20: the soil pH dropped significantly from the initial 8.7 to 7.69, which is close to the suitable range for plant growth; the organic matter content increased by 67.1%, and the soil fertility was significantly improved; the plant height growth rate reached 30.6%, which was significantly better than other ratio groups.

[0050] Under this ratio, the decomposed material provides basic fertility as organic matter and nutrient base, desulfurized gypsum effectively replaces sodium ions in the soil by providing calcium ions, and humic acid significantly enhances the formation of soil aggregates. The synergistic effect of the three establishes an optimal balance between salt regulation, structural improvement, and fertility enhancement, avoiding the risk of soil compaction that may be caused by excessive desulfurized gypsum, and preventing the increased costs and loose structure problems caused by excessive humic acid.

Claims

1. A method for preparing a saline-alkali soil conditioner from sludge compost, characterized in that, The steps are as follows: S1, Preparation of modified pre-hydrolyzed solution rinsing agent: Take the pre-hydrolyzed solution, add calcium acetate solid to it while stirring continuously at a uniform speed, and let it react for 30-60 minutes. Then, separate the solid and liquid of the reaction solution, collect the supernatant, and obtain the modified pre-hydrolyzed solution eluent. The mass ratio of calcium acetate solid to pre-hydrolyzed solution volume is 0.1-0.3:100 (g / mL). The pre-hydrolyzed solution is a hydrolyzed solution obtained from the hydrolysis of plants containing lignocellulose. S2, rinsing heavy metals from sludge compost: The modified pre-hydrolyzed leaching agent obtained from S1 was mixed with sludge compost containing heavy metals at a liquid-to-solid ratio of (3~5) L:1kg, and stirred at 50~150 rpm for 2~4 hours at room temperature. S3, Solid-Liquid Separation: The heavy metal leaching liquid of the sludge compost obtained from S2 is subjected to solid-liquid separation, the solid is collected and dried to obtain purified compost. S4, compound saline-alkali soil conditioner: The desulfurized gypsum, humic acid and the purified composted material obtained from S3 were mixed evenly in a dry basis mass ratio of (20~40):(10~30):(30~50); S5, aging and product preparation: The material after being mixed evenly in S4 is aged and matured at room temperature for 5-7 days, then dried, crushed, and granulated to obtain a saline-alkali soil conditioner.

2. The method for preparing a saline-alkali soil conditioner from sludge compost as described in claim 1, characterized in that, The raw material for the pre-hydrolyzed liquid in S1 is lignocellulose, which is derived from at least one of broadleaf wood, coniferous wood, bamboo, and agricultural straw.

3. The method for preparing a saline-alkali soil conditioner from sludge compost as described in claim 1, characterized in that, The pre-hydrolyzed solution in S1 is obtained through a hydrothermal treatment process; The hydrothermal treatment process is selected from any one of hydrothermal pretreatment, dilute acid prehydrolysis, and steam explosion. The pre-hydrolyzed solution contains hemicellulose-degraded sugars, furfural, organic acids, and water-soluble lignin.

4. The method for preparing a saline-alkali soil conditioner from sludge compost as described in claim 1, characterized in that, The desulfurized gypsum in S4 contains no less than 85% CaSO4·2H2O, the organic matter content of the humic acid is no less than 40%, the organic matter content of the purified compost is no less than 35%, and the heavy metal content is lower than the risk screening value specified in the "Soil Environmental Quality Agricultural Land Soil Pollution Risk Control Standard" (GB15618-2018).

5. The method for preparing a saline-alkali soil conditioner from sludge compost as described in claim 1, characterized in that, In S5, after aging and maturing at room temperature for 5-7 days, it is dried at 50-60℃ until the moisture content is less than 15%, then crushed and passed through a 100-mesh sieve. Water accounting for 10%-15% of the weight of the crushed material is added, and granulation is used to obtain granular saline-alkali soil conditioner with a particle size of 2-4mm.

6. The method for preparing a saline-alkali soil conditioner from sludge compost as described in claim 1, characterized in that, The desulfurized gypsum and humic acid in the compound saline-alkali soil conditioner S4 are mixed with the purified composted material obtained from S3 at a dry basis mass ratio of 30:20:

50.

7. The method for preparing a saline-alkali soil conditioner from sludge compost as described in claim 1, characterized in that, The ratio of the mass of calcium acetate solid in S1 to the volume of the pre-hydrolyzed solution is 0.1:100 in g / mL.

8. The method for preparing a saline-alkali soil conditioner from sludge compost as described in claim 1, characterized in that, In S2, the modified pre-hydrolyzed leaching agent obtained in S1 is mixed with sludge compost containing heavy metals, with a liquid-to-solid ratio of 4 L:1 kg.

9. The application of the saline-alkali soil conditioner prepared by the method described in claims 1 to 5 in improving saline-alkali soil.

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