Biochar-humic acid-calcium alginic acid composite saline-alkali soil modifier and preparation and application method thereof

Abstract

The invention relates to the technical field of soil remediation, in particular to a biochar-humic acid-calcium alginic acid composite saline-alkali soil modifier and a preparation and application method thereof. The modifier adopts a multi-layer structure design: biochar is pre-adsorbed by calcium chloride, compounded with sodium humate, and then crosslinked with sodium alginate to form a gel to form a composite inner core; secondly, sulfobetaine methacrylate and vinyl phosphonic acid are introduced into a sodium alginate molecular chain through graft polymerization, and modified sodium alginate is obtained to serve as an outer layer coating material; and finally, dipping the composite inner core into the outer layer solution, and carrying out graded recalcification by calcium chloride and short-time treatment by sodium sulfate to obtain a final product. Through the synergistic effect of calcium ion slow release of the inner core and zwitterionic characteristics of the outer layer, efficient replacement and long-term stable regulation and control of soil sodium ions are achieved, the pH value, the conductivity and the alkalization degree of soil are remarkably reduced, the water holding capacity is effectively improved, and the soil conditioner is suitable for sustainable improvement of moderate and severe saline-alkali soil.

Description

Technical Field

[0001] This invention relates to the field of soil remediation technology, and in particular to a biochar-humic acid-alginate composite saline-alkali land conditioner and its preparation and application methods. Background Technology

[0002] Traditional methods for treating saline-alkali soils rely on calcium-containing materials such as gypsum and phosphogypsum to reduce soil alkalinity through calcium-sodium ion exchange. However, these materials have high solubility, and calcium ions are rapidly leached away after irrigation or rainfall, achieving only short-term desalination of the surface layer. This makes it difficult to continuously supply effective calcium sources to deeper soil layers, leading to repeated accumulation of sodium ions and a rebound in alkalinity. Especially in moderately to severely saline-alkali areas, where soil colloids are highly dispersed and have poor permeability, a single calcium source is easily fixed or inactivated during migration, making it impossible to establish a stable ion exchange gradient.

[0003] While organic soil conditioners such as humic acid and organic fertilizers can improve soil structure, their high salt content and rapid mineralization rate mean they easily decompose in saline-alkali environments, producing secondary salts and exacerbating the increase in electrical conductivity. Furthermore, organic matter has a weak affinity for sodium ions and lacks active sites for directional sodium ion exchange, thus contributing little to reducing the percentage of exchangeable sodium. Current technologies attempt to combine organic and inorganic materials, but these often remain at the level of physical mixing, resulting in weak interfacial bonds between components and asynchronous calcium ion release and organic complexation processes, failing to create a synergistic microenvironment.

[0004] Sodium alginate and other natural polymers can serve as calcium ion carriers, but their gel networks exhibit poor stability under saline-alkali conditions. High pH values ​​and high sodium ion concentrations can lead to gel shrinkage and premature release of calcium ions. Some studies have employed chemical cross-linking or coating processes to delay calcium ion dissolution, but the outer membrane material often has poor compatibility with the soil medium, is easily encapsulated by colloids or biodegraded, and loses its controlled-release function. Furthermore, existing soil amendments lack specific adsorption mechanisms for sodium ions in the soil, resulting in low sodium ion removal efficiency. They also fail to consider the simultaneous improvement of soil pore structure and water transport, making it difficult for crop roots to grow stably in the amended layer.

[0005] In recent years, biochar has been introduced into saline-alkali land improvement systems due to its porous structure and surface functional groups. However, raw biochar has limited calcium ion loading capacity and is easily competitively adsorbed by sodium ions in high-salt environments, reducing calcium ion availability. Relying solely on the physical adsorption of biochar without chemical bridging with components such as humic acid and alginate makes it impossible to establish a long-term calcium-sodium ion exchange cycle. Existing composite soil conditioners mostly focus on short-term desalination indicators, failing to achieve synergistic regulation from multiple dimensions such as ion exchange kinetics, aggregate formation mechanisms, and interfacial stability, resulting in large fluctuations in actual field application effects and insufficient sustainability. Summary of the Invention

[0006] In view of this, the purpose of this invention is to propose a biochar-humic acid-alginate composite saline-alkali land conditioner and its preparation and application method, so as to solve the problem that existing saline-alkali land conditioners have short-term improvement effects and slow recovery of soil physicochemical properties due to excessively rapid release of calcium ions, insufficient replacement of sodium ions and unstable aggregate structure, making it difficult to achieve sustainable ecological restoration.

[0007] To achieve the above objectives, the present invention provides a method for preparing a biochar-humic acid-alginate composite saline-alkali land conditioner, comprising the following steps:

[0008] (1) Disperse biochar in deionized water, then add hydrogen peroxide solution to activate it, wash and dry to obtain activated biochar;

[0009] (2) The activated biochar was dispersed in a calcium chloride solution for pre-adsorption, filtered, and then placed in an acidic aqueous solution containing sodium humate to form a composite core A; core A was added to an aqueous solution of sodium alginate, and calcium chloride solution was added dropwise to form a gel to obtain core B; core B was treated with sodium chloride solution for a short time and then dried to obtain a biochar-humic acid-alginate composite core.

[0010] (3) Preparation of modified sodium alginate: sulfobetaine methacrylate and vinylphosphonic acid were introduced into an aqueous solution of sodium alginate and grafted onto the solution under acidic conditions in the presence of an initiator to obtain modified sodium alginate.

[0011] (4) The composite core is immersed in the modified sodium alginate outer layer solution, and then recalcified in calcium chloride solution of different concentrations, short-term treatment in sodium sulfate solution, and recalcified again. After washing, drying and sieving, a biochar-humic acid-alginate composite saline-alkali land conditioner is obtained.

[0012] Preferably, the biochar in step (1) is corn stalk pyrolysis biochar, which is obtained by heating corn stalks at 500°C for 60 minutes under nitrogen and then pulverizing them through a 40-mesh sieve.

[0013] Preferably, the hydrogen peroxide solution in step (1) has a mass fraction of 30%.

[0014] Preferably, the activation treatment temperature in step (1) is 60°C and the time is 1.5-2.5h.

[0015] Preferably, the ratio of biochar, deionized water and hydrogen peroxide solution in step (1) is 5g:50mL:4-6g.

[0016] Preferably, the concentration of the calcium chloride solution used for pre-adsorption in step (2) is 0.04-0.06 mol / L, and the treatment time is 45-75 min.

[0017] Preferably, the acidic aqueous solution containing sodium humate in step (2) is prepared by mixing sodium humate and deionized water at a ratio of 9-11g:500mL, and then adjusting the pH to 4.8-5.4 with 1mol / L nitric acid.

[0018] Preferably, the compounding time in the acidic aqueous solution containing sodium humate in step (2) is 60-120 min.

[0019] Preferably, in step (2), the molar ratio of mannuronic acid to guluronic acid in sodium alginate used for gelation is 1:2.

[0020] Preferably, the concentration of the calcium chloride solution used for gelation in step (2) is 0.04-0.06 mol / L.

[0021] Preferably, the gelation time in step (2) is 8-12 min.

[0022] Preferably, in step (2), after the kernel B is gelled, it is treated with a sodium chloride solution with a concentration of 0.04-0.06 mol / L for a short time of 2-4 min.

[0023] Preferably, in step (2), the ratio of activated biochar, calcium chloride solution for pre-adsorption, acidic aqueous solution containing sodium humate, sodium alginate aqueous solution, calcium chloride solution for gelation, and sodium chloride solution for short-term treatment after gelation is 5g:40mL:50mL:80mL:40mL:30mL.

[0024] Preferably, in step (3), the mass ratio of sodium alginate, sulfobetaine methacrylate and vinylphosphonic acid is 90-110:4-8:2-4.

[0025] Preferably, the initiator in step (3) is cerium ammonium nitrate.

[0026] Preferably, in step (4), the graded recalcification is carried out with calcium chloride solutions of 0.18-0.22 mol / L, 0.09-0.11 mol / L and 0.28-0.32 mol / L in sequence, and the two recalcifications are treated with sodium sulfate solution of 0.008-0.012 mol / L for 2 min between each recalcification.

[0027] Preferably, in step (4), the modified sodium alginate outer layer solution is prepared by mixing modified sodium alginate and deionized water at a ratio of 18-22g:900-1100mL.

[0028] Preferably, in step (4), the total amount of calcium chloride solution used in the composite core, modified sodium alginate outer layer solution, and graded recalcification, and the ratio of sodium sulfate solution to calcium chloride solution used in the recalcification are 4.5-5.5g:9-11mL:60mL:20mL:20mL.

[0029] Preferably, the immersion time in the modified sodium alginate outer layer solution in step (4) is 8-12 min.

[0030] Furthermore, the present invention also provides a biochar-humic acid-alginate composite saline-alkali land conditioner, which is obtained by the above preparation method.

[0031] Furthermore, the present invention also provides a method for applying a biochar-humic acid-alginate compound saline-alkali soil conditioner, wherein the biochar-humic acid-alginate compound saline-alkali soil conditioner is mixed evenly with the topsoil at a depth of 10cm at a dosage of 50kg per mu, and applied before sowing.

[0032] The beneficial effects of this invention are:

[0033] This invention utilizes a composite structure of biochar, humic acid, and sodium alginate to achieve the graded and slow release of calcium ions and the efficient replacement of sodium ions in saline-alkali soils. The porous framework of biochar provides a stable storage space for calcium ions, while the synergistic bridging effect of humic acid and sodium alginate allows for the gradual release of calcium ions into the soil, avoiding the short-term failure problem caused by the rapid dissolution of calcium ions in traditional soil conditioners. This structure can continuously maintain the calcium ion concentration gradient around soil colloids, promote the deep desorption and leaching of sodium ions, and significantly reduce alkalinity.

[0034] The calcium alginate gel network in the soil conditioner's core forms an interpenetrating structure with the biochar-humic acid complex, enhancing the water stability and pore connectivity of soil aggregates. This network expands when wet, capturing sodium ions and promoting their downward migration with water; it shrinks during the dry phase, maintaining soil aeration and inhibiting salt accumulation on the surface. This dynamic response mechanism effectively improves the physical structure of saline-alkali soils, increasing field water holding capacity and root penetration.

[0035] The sulfobetaine and vinylphosphonic acid groups grafted onto the outer layer endow the modifier with zwitterionic properties, enabling it to maintain interface cleanliness in high-salt environments and preventing organic-inorganic colloids from covering the active sites. The strong coordination of the phosphonic acid groups with calcium ions further stabilizes the cross-linked network, allowing the modifier to maintain its morphological integrity and functional activity under long-term irrigation or rainfall conditions, avoiding performance degradation caused by physical breakage or chemical degradation.

[0036] This soil conditioner achieves a time-series regulation from rapid desalination to long-term stability through a synergistic design of multi-layered calcium ion storage and release and sodium ion adsorption. Initially, it initiates sodium ion replacement by controllably releasing calcium ions from the core; in the middle stage, it maintains ion exchange momentum through intelligent response of the outer layer; and in the later stage, it relies on the persistent support of the organic-inorganic complex for soil ecological restoration. The entire process aligns with the natural laws of soil salinization reversal, thus achieving sustainable improvement of saline-alkali land without excessive reliance on external irrigation or chemical agents. Detailed Implementation

[0037] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments.

[0038] Example 1:

[0039] (1) 500g of corn stalk pyrolysis biochar (corn stalks were kept at 500℃ for 60min under nitrogen and crushed through a 40-mesh sieve) was put into 5000mL of deionized water, 400mL of 30wt% hydrogen peroxide solution was added, stirred at 60℃ for 1.5h, filtered, washed 3 times with deionized water, and dried at 80℃ to constant weight to obtain activated biochar.

[0040] (2) Disperse 500g of activated biochar in 4000mL of 0.04mol / L calcium chloride dihydrate solution, stir at 25℃ for 45min, filter to obtain activated biochar adsorbed with calcium ions, then dissolve 90g of sodium humate (from Sigma-Aldrich, catalog number H102874) in 5000mL of deionized water, adjust the pH to 4.8 with 1mol / L nitric acid, add the activated biochar adsorbed with calcium ions, stir at 25℃ for 60min, filter, and wash three times with deionized water. Core A was obtained; 180g of sodium alginate (M / G = 1:2) was dissolved in 8000mL of deionized water and stirred at 25℃ for 3h. Core A was added, and then 4000mL of calcium chloride dihydrate solution with a concentration of 0.04mol / L was added dropwise. The mixture was stirred at 25℃ for 8min and filtered to obtain core B. Core B was then added to 3000mL of sodium chloride solution with a concentration of 0.04mol / L and stirred at 25℃ for 2min. The mixture was filtered and dried at 65℃ for 24h to obtain biochar-humic acid-alginate composite core.

[0041] (3) Dissolve 90g of sodium alginate (M / G = 1:2) in 3000mL of deionized water, add 4g of sulfobetaine methacrylate and 2g of vinylphosphonic acid at a constant temperature of 40℃, adjust the pH to 4.0 with 1mol / L nitric acid, stir continuously and add 1g of cerium ammonium nitrate, react under nitrogen protection for 50min, cool down to 25℃, dialyze with deionized water for 40h, concentrate under reduced pressure, and vacuum dry at 60℃ for 10h to obtain modified sodium alginate;

[0042] (4) Add 18g of modified sodium alginate to 900mL of deionized water and stir for 8min to obtain an outer layer solution. Add 450g of biochar-humic acid-alginate composite core to the outer layer solution and soak at 25℃ for 8min. Filter the solution and add it to 2000mL of calcium chloride dihydrate solution with a concentration of 0.18mol / L. Stir for 8min and add it to 2000mL of calcium chloride dihydrate solution with a concentration of 0.09mol / L. Stir for 4min and filter the solution. Add it to 2000mL of sodium sulfate solution with a concentration of 0.008mol / L. Stir for 2min and add it to 2000mL of calcium chloride dihydrate solution with a concentration of 0.28mol / L. Stir for 4min and filter the solution. Wash the solution 3 times with deionized water, dry it at 65℃ for 24h, and pass it through a 40-mesh sieve to obtain a biochar-humic acid-alginate composite saline-alkali soil conditioner.

[0043] Example 2:

[0044] (1) 500g of corn stalk pyrolysis biochar (corn stalks were kept at 500℃ for 60min under nitrogen and crushed through a 40-mesh sieve) was put into 5000mL of deionized water, 500mL of 30wt% hydrogen peroxide solution was added, stirred at 60℃ for 2h, filtered, washed 3 times with deionized water, and dried at 80℃ to constant weight to obtain activated biochar.

[0045] (2) Disperse 500g of activated biochar in 4000mL of 0.05mol / L calcium chloride dihydrate solution, stir at 25℃ for 60min, filter to obtain activated biochar adsorbed with calcium ions, then dissolve 100g of sodium humate (from Sigma-Aldrich, catalog number H102874) in 5000mL of deionized water, adjust the pH to 5.1 with 1mol / L nitric acid, add the activated biochar adsorbed with calcium ions, stir at 25℃ for 90min, filter, and wash three times with deionized water. Core A was obtained; 200g of sodium alginate (M / G = 1:2) was dissolved in 8000mL of deionized water and stirred at 25℃ for 4h. Core A was added, and then 4000mL of calcium chloride dihydrate solution with a concentration of 0.05mol / L was added dropwise. The mixture was stirred at 25℃ for 10min and filtered to obtain core B. Core B was then added to 3000mL of sodium chloride solution with a concentration of 0.05mol / L and stirred at 25℃ for 3min. The mixture was filtered and dried at 65℃ for 24h to obtain a biochar-humic acid-alginate composite core.

[0046] (3) Dissolve 100g of sodium alginate (M / G = 1:2) in 3000mL of deionized water, add 6g of sulfobetaine methacrylate and 3g of vinylphosphonic acid at a constant temperature of 40℃, adjust the pH to 4.2 with 1mol / L nitric acid, stir continuously and add 2g of cerium ammonium nitrate, react under nitrogen protection for 60min, cool down to 25℃, dialyze with deionized water for 48h, concentrate under reduced pressure, and vacuum dry at 60℃ for 12h to obtain modified sodium alginate;

[0047] (4) Add 20g of modified sodium alginate to 1000mL of deionized water and stir for 10min to obtain an outer layer solution. Add 500g of biochar-humic acid-alginate composite core to the outer layer solution, soak at 25℃ for 10min, filter, and then add to 2000mL of calcium chloride dihydrate solution with a concentration of 0.2mol / L. Stir for 10min, then add to 2000mL of calcium chloride dihydrate solution with a concentration of 0.1mol / L. Stir for 5min, filter, then add to 2000mL of sodium sulfate solution with a concentration of 0.01mol / L. Stir for 2min, then add to 2000mL of calcium chloride dihydrate solution with a concentration of 0.3mol / L. Stir for 5min, filter, wash 3 times with deionized water, dry at 65℃ for 24h, and pass through a 40-mesh sieve to obtain biochar-humic acid-alginate composite saline-alkali soil conditioner.

[0048] Example 3:

[0049] (1) 500g of corn stalk pyrolysis biochar (corn stalks were kept at 500℃ for 60min under nitrogen and crushed through a 40-mesh sieve) was put into 5000mL of deionized water, 600mL of 30wt% hydrogen peroxide solution was added, stirred at 60℃ for 2.5h, filtered, washed 3 times with deionized water, and dried at 80℃ to constant weight to obtain activated biochar.

[0050] (2) Disperse 500g of activated biochar in 4000mL of 0.06mol / L calcium chloride dihydrate solution, stir at 25℃ for 75min, filter to obtain activated biochar adsorbed with calcium ions, then dissolve 110g of sodium humate (from Sigma-Aldrich, catalog number H102874) in 5000mL of deionized water, adjust the pH to 5.4 with 1mol / L nitric acid, add the activated biochar adsorbed with calcium ions, stir at 25℃ for 120min, filter, and wash with deionized water for 3 minutes. Next, core A was obtained; 220g of sodium alginate (M / G = 1:2) was dissolved in 8000mL of deionized water and stirred at 25℃ for 5h. Core A was added, and then 4000mL of calcium chloride dihydrate solution with a concentration of 0.06mol / L was added dropwise. The mixture was stirred at 25℃ for 12min and filtered to obtain core B. Core B was then added to 3000mL of sodium chloride solution with a concentration of 0.06mol / L and stirred at 25℃ for 4min. The mixture was filtered and dried at 65℃ for 24h to obtain biochar-humic acid-alginate composite core.

[0051] (3) Dissolve 110g of sodium alginate (M / G = 1:2) in 3000mL of deionized water, add 8g of sulfobetaine methacrylate and 4g of vinylphosphonic acid at a constant temperature of 40℃, adjust the pH to 4.4 with 1mol / L nitric acid, stir continuously and add 3g of cerium ammonium nitrate, react under nitrogen protection for 70min, cool down to 25℃, dialyze with deionized water for 56h and concentrate under reduced pressure, dry under vacuum at 60℃ for 14h to obtain modified sodium alginate;

[0052] (4) Add 22g of modified sodium alginate to 1100mL of deionized water and stir for 12min to obtain an outer layer solution. Add 550g of biochar-humic acid-alginate composite core to the outer layer solution, soak at 25℃ for 12min, filter, and then add to 2000mL of calcium chloride dihydrate solution with a concentration of 0.22mol / L. Stir for 12min, then add to 2000mL of calcium chloride dihydrate solution with a concentration of 0.11mol / L. Stir for 6min, filter, then add to 2000mL of sodium sulfate solution with a concentration of 0.012mol / L. Stir for 2min, then add to 2000mL of calcium chloride dihydrate solution with a concentration of 0.32mol / L. Stir for 6min, filter, wash 3 times with deionized water, dry at 65℃ for 24h, and pass through a 40-mesh sieve to obtain biochar-humic acid-alginate composite saline-alkali soil conditioner.

[0053] Application method: Apply 50 kg of biochar-humic acid-alginate compound saline-alkali soil conditioner per mu, mix it evenly with the topsoil to a depth of 10 cm, and apply before sowing.

[0054] Comparative Example 1:

[0055] The difference between Comparative Example 1 and Example 2 is that the step of pre-adsorbing calcium ions by stirring activated biochar in a 0.05 mol / L calcium chloride dihydrate solution at 25°C for 60 min is omitted, while the other conditions are the same as in Example 2.

[0056] Comparative Example 2:

[0057] The difference between Comparative Example 2 and Example 2 is that after the core B gels, it is not subjected to a short-term treatment of stirring in a 0.05 mol / L sodium chloride solution at 25°C for 3 minutes. The other conditions are the same as in Example 2.

[0058] Comparative Example 3:

[0059] The difference between Comparative Example 3 and Example 2 is that vinylphosphonic acid was not added, while the other conditions were the same as in Example 2.

[0060] Comparative Example 4:

[0061] The difference between Comparative Example 4 and Example 2 is that sulfobetaine methacrylate was not added, while the other conditions were the same as in Example 2.

[0062] Comparative Example 5:

[0063] The difference between Comparative Example 5 and Example 2 is that cerium ammonium nitrate was not added, and initiation grafting was not performed, while the other conditions were the same as in Example 2.

[0064] Comparative Example 6:

[0065] The difference between Comparative Example 6 and Example 2 is that the fractional recalcification process does not use the concentration order of 0.2, 0.1, 0.3 mol / L, but instead uses calcium chloride dihydrate solution of 0.2, 0.2, 0.2 mol / L in sequence. The time is the same as in Example 2, and the other conditions are the same as in Example 2.

[0066] Comparative Example 7:

[0067] The difference between Comparative Example 7 and Example 2 is that the mannuronic acid / guluronic acid ratio of sodium alginate is 2:1 instead of 1:2, while the other conditions are the same as in Example 2.

[0068] Performance testing:

[0069] Cultivation Experiment: To evaluate the soil improvement effects of the examples and comparative examples in the soil system, the top 0-10cm layer of air-dried soil from the same plot of severely saline-alkali farmland was selected and sieved through a 2mm sieve for later use. The dosage was calculated based on 50kg per acre, mixed with 10cm of topsoil, and applied before sowing, and adjusted to indoor conditions, with a soil bulk density of 1.3t / m³. 3 The calculated equivalent application rate was 0.58 g / kg dry soil. Sealed plastic culture pots (approximately 26 cm inner diameter and 22 cm height) were used, each containing 10 kg of dry soil. 5.78 g of the sample from the examples or comparative samples was weighed and evenly spread, mixed thoroughly for 5 minutes, and then allowed to stand for 48 hours to pre-equilibrate to 70% of field capacity. The soil was then incubated in a constant temperature and humidity chamber for 30 days (25 ± 1℃, 60% ± 5% relative humidity, protected from light). Each treatment had three replicates. During the cultivation period, the soil was weighed daily, and deionized water was added by spraying using the mass difference method to control the moisture content fluctuation within ±1%. The pot mouth was covered with a perforated polyethylene film to inhibit evaporation and ensure gas exchange. Sampling time points were set at 0d, 7d, and 30d. Each time, about 100g of representative soil sample was taken from each pot using the S-shaped five-point mixing method for the determination of pH, conductivity, total water-soluble salts, cation exchange capacity, exchangeable sodium and field water holding capacity. The remaining samples were sealed to prevent moisture and stored at 4℃ for later use.

[0070] Soil pH: The pH was measured at 25℃ using the potentiometric method according to HJ 962-2018, with water as the extraction solvent and a water-to-soil ratio of 2.5:1. The initial soil pH was 9.21. The pH was measured at 7 days and 30 days, and the results are shown in Table 1.

[0071] Soil electrical conductivity (EC): According to the HJ 802-2016 electrode method, air-dried soil and water were mixed at a ratio of 1:5 (m / V), shaken at 20℃ for 30 min, and then allowed to stand / centrifuged to separate the supernatant. The reading was taken at 25℃ and calibrated with potassium chloride standard solution. The initial EC was 220 mS / m. The EC was measured for 30 days, and the results are shown in Table 1.

[0072] Total water-soluble salts: determined according to NY / T 1121.16-2006, the initial total water-soluble salts were 5.8 g / kg, and the total water-soluble salts were determined over 30 days. The results are shown in Table 1.

[0073] Cation exchange capacity (CEC), exchangeable sodium, and exchangeable sodium percentage (ESP): CEC was determined according to HJ 889-2017 hexaamminecobalt trichloride leaching-spectrophotometric method; exchangeable sodium was determined according to NY / T 1615-2008 (ammonium chloride-ethanol exchange, flame photometric method); ESP = (exchangeable sodium / CEC) × 100%. The initial CEC was 12.1 cmol(+)·kg. -1 The exchangeable sodium content is 2.8 cmol(+)·kg. -1 The ESP was 23.3%. The CEC, exchangeable sodium and ESP were measured after 30 days, and the results are shown in Table 1.

[0074] Field water holding capacity: Measured according to NY / T 1121.21-2008, the initial field water holding capacity (mass fraction) was 18.3%, and the field water holding capacity was measured for 30 days. The results are shown in Table 1.

[0075] Emergence and aboveground fresh weight of potted crops: A randomized block pot experiment was conducted according to NY / T 2271-2016 "Requirements for Testing and Evaluation of the Effect of Soil Conditioners". The same batch of conventional wheat seeds were sown (20 seeds per pot, covered with 1.5cm of soil). The greenhouse temperature was 25℃, relative humidity was 60%-70%, and light intensity was 12h / d. The substrate and application were the same as in the previous culture experiment. The emergence rate was counted on day 14, and the aboveground fresh weight was harvested on day 30. The results are shown in Table 1.

[0076] Table 1 Performance Test Results

[0077]

[0078]

[0079] Data Analysis:

[0080] As can be seen from the data in Examples 1-3 in Table 1, the composite modifier prepared in this invention, under the hierarchical structure of activated biochar-humic acid bridging-alginate gelling-outer double grafting, exhibits a continuously enhanced salinity-alkali improvement effect with the increase of outer layer functional density and hierarchical complex calcium strength. The series of examples shows a synergistic trend of continuously decreasing pH, synchronously decreasing conductivity and water-soluble salts, steadily increasing cation exchange capacity, and a linkage decrease in exchangeable sodium and alkalinity. Furthermore, soil water storage and crop emergence and growth indicators corroborate this, indicating that the multi-scale calcium ion storage and release system of active calcium pool, organic buffer, and carbon skeleton can efficiently replace exchangeable sodium in the early stages, and maintain interface cleanliness and cross-linking stability in the later stages through the zwitterionic and phosphonic acid coordination sites in the outer layer, thus meeting the sequential requirements of replacement followed by stabilization. The possible reasons are as follows: the outer double grafting enhances the repeated capture and relocking of calcium ions, inhibiting deactivation caused by organic fouling; the strong distribution of the core with insufficient recalcification and the outer strong recalcification makes the early release friendly and the later structure stable; the biochar pores and calcium alginate gel network together improve the formation of aggregates and the water retention and aeration state, forming a comprehensive optimization of physical, chemical and biological processes.

[0081] As can be seen from the data in Table 1 for Example 2 and Comparative Example 1, the exchange efficiency of exchangeable sodium was significantly slowed down and the decrease in alkalinity was insufficient after the lack of pre-adsorption of calcium ions by activated biochar. The main reason is that pre-adsorption provides bridging sites between highly active calcium ions and humic acid at the interface. The absence of this step reduces the size of the active calcium pool, resulting in insufficient initial ion exchange driving force. Subsequent outer layer calcium overcalcification cannot compensate for the shortcomings of the core, leading to a lag in salt migration and aggregate reconstruction.

[0082] As can be seen from the data in Table 1 for Example 2 and Comparative Example 2, the short-term sodium chloride treatment after removing core B resulted in insufficient exchangeable calcium ions to trigger rapid replacement in the early stages. Although this improved gradually in the later stages, the timing of the replacement deteriorated. The main reason is that the short-term sodium chloride treatment can establish a reversible calcium ion gradient, promoting replacement followed by stabilization. The absence of this step would cause the cross-linked network to densify prematurely, affecting ion migration and pore connectivity.

[0083] As can be seen from the data in Example 2 and Comparative Example 3 in Table 1, the lack of strongly coordinating phosphonic acid sites limits both the decrease in exchangeable sodium and the improvement in basicity. The main reason is that vinylphosphonic acid provides multidentate coordination, which can stably bind calcium ions and enhance secondary calcium recalcification under high ionic strength. After its absence, the outer layer relies solely on carboxylate coordination, resulting in a decrease in calcium ion reuse rate and interfacial stability.

[0084] As can be seen from the data in Table 1 for Example 2 and Comparative Example 4, the conductivity and later stability of Comparative Example 4 were both affected. The main reason is that the strong hydration layer and electrically neutral shell of sulfobetaine can effectively inhibit organic fouling and colloidal coverage, maintain the unobstructed crosslinking channels and reduce surface energy. After its absence, the outer layer is more easily contaminated by humic substances and high-valence ions, resulting in a decrease in calcium ion cycling efficiency.

[0085] As can be seen from the data in Table 1 for Example 2 and Comparative Example 5, the overall performance of Comparative Example 5 is significantly degraded. The main reason is that when grafting does not occur, the outer layer is only a physical coating, with low density of functional groups and poor binding stability, making it difficult to form a durable site distribution and cross-linking network. Therefore, it is more prone to loosening under irrigation disturbances, increasing the risk of salt return.

[0086] As can be seen from the data in Example 2 and Comparative Example 6 in Table 1, when using equal concentrations of recalcium, the early calcium retention is too rapid while the effective replacement is insufficient, and the later stability is also not superior. The main reason is the lack of a weak-to-strong progression of 0.2, 0.1, and 0.3 mol / L, which hinders the sequential construction of replacement followed by stabilization, reducing the aggregation rate and durability.

[0087] As can be seen from the data in Table 1 for Example 2 and Comparative Example 7, short-term desalination is relatively rapid, but alkalinity and crop growth are not advantageous. The main reason is that the high proportion of mannuronic acid makes the gel softer and the pores more open, which is conducive to early salt migration, but reduces strength and calcium retention capacity, resulting in limited exchangeable sodium removal and later stability. It can be seen that there is a significant synergy between alginate configuration and recalcification program, and only by optimizing both can an unexpected balance be achieved.

[0088] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the invention is limited to these examples; within the framework of the invention, the technical features of the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other variations of the different aspects of the invention as described above, which are not provided in detail for the sake of brevity.

Claims

1. A method for preparing a biochar-humic acid-calcium alginate composite salt-alkali soil improver, characterized in that, The method comprises the following steps: (1) dispersing biochar in deionized water, adding hydrogen peroxide solution to activate, washing, drying, and obtaining activated biochar; (2) pre-adsorbing the activated biochar in a calcium chloride solution, filtering, and then compounding in an acid aqueous solution containing sodium humate to obtain core A; adding core A into a sodium alginate aqueous solution, and adding dropwise a calcium chloride solution to form a gel to obtain core B; treating core B with a short-time sodium chloride solution and drying to obtain a biochar-humic acid-calcium alginate composite core; (3) preparing modified sodium alginate: introducing sulfobetaine methacrylate and vinyl phosphonic acid in a sodium alginate aqueous solution, and grafting and polymerizing under the action of an initiator in an acidic condition to obtain modified sodium alginate; (4) immersing the composite core in the modified sodium alginate outer solution, and sequentially compounding with calcium in different concentrations of calcium chloride solutions, treating with a short-time sodium sulfate solution, and then compounding with calcium again, washing, drying, and sieving to obtain a biochar-humic acid-calcium alginate composite saline-alkali soil modifier. In step (2), the concentration of the calcium chloride solution used for pre-adsorption is 0.04-0.06 mol / L, and the treatment time is 45-75 min; in step (2), the concentration of the calcium chloride solution used for gel formation is 0.04-0.06 mol / L, and the gel formation time is 8-12 min; after the gel formation of core B in step (2), the short-time treatment with a 0.04-0.06 mol / L sodium chloride solution is performed for 2-4 min; In step (3), the mass ratio of sodium alginate, sulfobetaine methacrylate, and vinyl phosphonic acid is 90-110:4-8:2-4; in step (4), the graded calcium compounding uses 0.18-0.22 mol / L, 0.09-0.11 mol / L, and 0.28-0.32 mol / L calcium chloride solutions, respectively, and the 0.008-0.012 mol / L sodium sulfate solution is used for treatment for 2 min between the two calcium compoundings.

2. The preparation method of the biochar-humic acid-calcium alginate composite saline soil improver according to claim 1, characterized in that, In step (1), the biochar is corn straw pyrolysis biochar, which is obtained by pyrolyzing corn straw at 500℃ for 60 min under nitrogen, crushing, and passing through a 40-mesh sieve.

3. The preparation method of the biochar-humic acid-calcium alginate composite saline soil improver according to claim 1, characterized in that, In step (1), the treatment temperature for activation is 60℃, and the time is 1.5-2.5 h.

4. The preparation method of the biochar-humic acid-calcium alginate composite salt-alkali soil improver according to claim 1, characterized in that, In step (1), the amount ratio of biochar, deionized water, and hydrogen peroxide solution is 5 g:50 mL:4-6 g, and the mass fraction of the hydrogen peroxide solution is 30%.

5. The method of claim 1, wherein the biochar-humic acid-calcium alginate composite salt-affected soil amendment is prepared by the steps of: In step (2), the acid aqueous solution containing sodium humate is prepared by mixing 9-11 g of sodium humate with 500 mL of deionized water, and then adjusting the pH to 4.8-5.4 with 1 mol / L nitric acid, and the compounding time in the acid aqueous solution containing sodium humate is 60-120 min.

6. The method of claim 1, wherein the biochar-humic acid-calcium alginate composite salt-affected soil amendment is prepared by the steps of: In step (2), the molar ratio of mannuronic acid to guluronic acid of the sodium alginate used for gel formation is 1:

2.

7. The preparation method of the biochar-humic acid-alginate composite saline-alkali land conditioner according to claim 1, characterized in that, In step (3), the initiator is cerium ammonium nitrate.

8. The preparation method of the biochar-humic acid-alginate composite saline-alkali land conditioner according to claim 1, characterized in that, The modified sodium alginate outer layer solution in the step (4) is prepared by using modified sodium alginate and deionized water in a ratio of 18-22 g: 900-1100 mL; and the dipping time in the modified sodium alginate outer layer solution in the step (4) is 8-12 min.

9. A biochar-humic acid-calcium alginate composite salt-alkali soil improver, characterized in that, The preparation method of the biochar-humic acid-calcium alginate composite saline-alkali soil modifier according to any one of claims 1-8.

10. A method of applying a biochar-humic acid-calcium alginate composite salt-affected soil amendment, characterized by, Comprise: The biochar-humic acid-calcium alginate composite saline-alkali soil modifier according to claim 9 is mixed and stirred with the surface soil of 10 cm in depth at a dosage of 50 kg per mu, and is applied before sowing.

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