Iron-based and mercapto-based synergistic curing composite passivating agent with core-shell structure and in-situ application process

Abstract

This invention discloses a core-shell structured iron-based-thiol synergistic curing composite passivating agent and its in-situ application process. The composite passivating agent comprises a core and an outer shell encapsulating the core surface. The core contains steel slag particles, and the outer shell contains a pre-hydrated composite of silicate cement, calcium dihydrogen phosphate, and sodium thiosulfate. The in-situ application process involves shallow mechanical tillage to mix the dry powder and covering it with a layer of bentonite, simplifying construction and requiring no special equipment. This invention achieves spatiotemporal synergy between rapid chemical passivation of the outer shell and long-term physical curing of the core through its core-shell structure design. It achieves a passivation rate of over 98% for heavy metals such as Pb and Cd, and the treated sediment exhibits an unconfined compressive strength ≥200 kPa after 28 days. Furthermore, the entire preparation process is carried out at ambient temperature and pressure, reducing material costs by over 30% compared to existing technologies. It offers significant advantages in terms of efficient repair, excellent mechanical properties, and economic ease of implementation.

Description

Technical Field

[0001] This invention relates to the fields of environmental geotechnical engineering and riverbed sediment treatment technology, and in particular to iron-based-thiol synergistic solidification composite passivating agent with core-shell structure and its in-situ application process. Background Technology

[0002] Heavy metal pollution in river sediments is a common challenge in urban water environment management. Heavy metals such as Pb, Cd, Cu, and Zn accumulated in sediments are easily released back into overlying water bodies when environmental conditions change, such as a decrease in pH or an alteration in redox potential, forming continuous endogenous pollution and posing a potential threat to aquatic ecosystem safety and human health. Meanwhile, polluted sediments typically have high water content and poor mechanical strength, making them difficult to utilize directly as engineering materials. Traditional dredging-off-site disposal methods involve large-scale engineering projects, high transportation costs, and the risk of secondary pollution. Therefore, in-situ passivation / solidification technology has become a research hotspot in recent years. This technology stabilizes heavy metals and strengthens sediment by adding chemical agents.

[0003] In the prior art, patent CN109092878A discloses a stabilizing agent for treating soil contaminated with multiple heavy metals, the components of which include phosphate compounds, soluble iron salts, clay minerals, and pH adjusters. This technology achieves chemical stabilization of heavy metals by forming insoluble precipitates with phosphates, providing adsorption sites with iron salts, and utilizing ion exchange with clay minerals. However, this technology has the following shortcomings: First, its function is singular, with its formulation solely focused on chemical stabilization, lacking gelling components to achieve solidification and enhancement effects, and the treated soil cannot be directly used as engineering materials; second, this technology is mainly designed for ex-situ treatment scenarios, and does not disclose an underwater in-situ addition process applicable to riverbed sediments; third, the clay minerals used are unmodified natural minerals, with limited selective adsorption capacity for specific heavy metals.

[0004] Another existing patent, CN110699084B, discloses a composite agent and method for solidifying and remediating heavy metal pollution in soil. This composite agent includes quicklime and iron-based biochar-modified phosphate minerals. The iron-based biochar-modified phosphate minerals are obtained by uniformly mixing and drying iron salts with biomass, then uniformly mixing the phosphate minerals and co-pyrolyzing them at 600–900℃. This technology enhances the solidification, stabilization, and long-term stability of heavy metals through the adsorption of biochar, the precipitation of phosphates, and the formation of stable phosphorus-containing complex functional groups during high-temperature pyrolysis. However, this technology still has limitations: the preparation of its core component, "iron-based biochar-modified phosphate minerals," requires calcining the material at 600–900℃ for 30–180 minutes under a nitrogen / carbon dioxide atmosphere. The process involves complex equipment such as high-temperature tubular furnaces and carrier gas control, resulting in high energy consumption and demanding operating conditions, making it difficult to rapidly prepare and apply on a large scale in river remediation sites, leading to extremely high overall costs for the agent. Meanwhile, although its quicklime + modified phosphate mineral formula has certain hydration and cementing properties, its strength development is slow and singular, mainly relying on the carbonization effect of lime and the skeleton effect of biochar. The early strength and long-term mechanical properties of the solidified body are limited, making it difficult to directly convert into high-strength engineering materials.

[0005] In summary, existing technologies generally suffer from three common problems: first, they are limited in function or have shortcomings, making it difficult to achieve both efficient passivation and high-strength curing; second, the preparation process is complex or energy-intensive, resulting in high costs; and third, they do not have simplified process designs for the underwater in-situ addition environment of riverbed sediments, leading to poor engineering applicability.

[0006] Therefore, there is an urgent need to develop a composite passivating agent and in-situ application process that can achieve efficient stabilization of multiple heavy metals in riverbed sediments through a simple one-time in-situ addition process, while simultaneously imparting them with excellent physical and mechanical strength, and that is low in material cost, simple in preparation process, and suitable for large-scale application. Summary of the Invention

[0007] This invention solves the problems of existing passivation agents having single functions or complex processes, high raw material costs, and poor universality of in-situ application processes. It proposes an iron-based-thiol synergistic curing composite passivation agent with a core-shell structure and an in-situ application process, which achieves the dual goals of simultaneously improving the environmental safety and engineering mechanical properties of bottom sediment in a simple construction process, and reduces the overall cost by more than 30%.

[0008] To achieve the above objectives, the present invention proposes the following technical solution: A core-shell structured iron-based-thiol synergistic curing composite passivating agent, comprising: The core contains steel slag particles; The outer shell, which encloses the surface of the core, comprises a prehydrated composite of silicate cement, calcium dihydrogen phosphate, and sodium thiosulfate.

[0009] Through the above technical solution, by designing the composite passivating agent as a core-shell structure with steel slag particles of a specific particle size as the core and a calcium / phosphorus / silicon / sulfur composite with chemical reactivity and gelling properties as the outer shell, the outer shell layer first contacts water, dissolves rapidly, and undergoes a mineralization and precipitation reaction with heavy metals, while the cement begins to hydrate and gel. The core steel slag particles, acting as a skeleton, remain in the sediment for a long time, continuously releasing iron ions to provide adsorption sites, achieving a spatiotemporal functional separation and synergistic effect. This structural design solves the fundamental contradiction of mutual interference of reaction sites and the inability to simultaneously achieve short-term activity and long-term stability in traditional simple physical mixture-type agents. After construction, the outer shell components respond rapidly, converting most of the active heavy metals into a stable state within hours; while the skeleton effect of the core steel slag physically improves the friction angle and overall deformation resistance of the sediment, providing a stable spatial framework for the solidification of cement hydration products. The final solidified body possesses both low leaching toxicity and high mechanical strength.

[0010] Preferably, the steel slag particles have a particle size of 0.5-2 mm.

[0011] By strictly limiting the particle size range of the steel slag core through the above technical solutions, the following synergistic effects were achieved: Particles larger than 0.5 mm ensure sufficient rigidity and mass, enabling them to provide effective skeletal support in the sediment and significantly improve the internal friction angle of the sediment; particles smaller than 2 mm ensure a suitable specific surface area, facilitating the formation of a uniform, complete, and sufficiently thick coating layer on the surface of the shell precursor material. Excessively large particle sizes can lead to segregation and stratification during application, affecting the uniform distribution of the agent. Extensive experimental verification shows that steel slag particles with a particle size range of 0.5-2 mm have a bulk density of approximately 1.45-1.55 g / cm³, close to the density of the sediment. This results in less rapid settling or floating after application, promoting uniformity in application. Furthermore, this particle size range eliminates the need for fine ball milling of the raw steel slag; simple crushing and screening are sufficient, resulting in extremely low processing costs.

[0012] As a preferred embodiment, the contents of each component by mass are as follows: 40-50 parts steel slag particles, 20-25 parts calcium dihydrogen phosphate, 15-20 parts silicate cement, and 5-10 parts sodium thiosulfate.

[0013] By employing the above technical solutions, and by determining the optimal proportions of each component, the best synergistic passivation and solidification effects were achieved while simultaneously controlling costs. The proportion of steel slag particles is 40-50 parts. If it is less than 40 parts, the number of core skeletons is insufficient, making it impossible to form an effective physical reinforcement network in the sediment, and the unconfined compressive strength of the treated sediment will be difficult to reach 200 kPa. If it is more than 50 parts, the total amount of active components in the outer shell is relatively insufficient, and the passivation rate of heavy metals will decrease significantly, especially affecting the passivation effect on Cd. 20-25 parts of calcium dihydrogen phosphate provide ample PO4³⁻ source, enabling it to form phosphochlorite-type or phosphate precipitates with Pb²⁺, Cd²⁺, Zn²⁺, etc., in a stoichiometric excess ratio. 15-20 parts of silicate cement provide appropriate amounts of cementing material, used to generate CSH gel to physically encapsulate and chemically seal residual heavy metals. Simultaneously, the high alkalinity (pH>12) generated during cement hydration promotes the conversion of sodium thiosulfate into sulfides and enhances the adsorption capacity of iron oxides. 5-10 parts of sodium thiosulfate, as a sulfur source precursor, gradually releases S²⁻ in the reducing microenvironment of the sediment, utilizing the heat and alkalinity provided by cement hydration. This S²⁻ forms sulfide precipitates with Pb²⁺, Cd²⁺, Hg²⁺, etc., with a lower solubility product than phosphate, such as PbS with Ksp≈10⁻². 8 The Ksp is much lower than that of Pb5(PO4)3Cl, which is approximately 10⁻ ... 84 This allows for deep locking of specific heavy metals. The formulation range ensures optimal synergy among the components while maximizing the use of inexpensive steel slag.

[0014] A method for preparing the composite passivating agent includes the following steps: S1, mix silicate cement, calcium dihydrogen phosphate and sodium thiosulfate powder evenly, add water and stir, carry out pre-hydration aging, and form loose shell precursor material; S2, the shell precursor material obtained in S1 is mixed with steel slag particles, so that the shell precursor material coats the surface of the steel slag particles, thus obtaining the composite passivating agent with a core-shell structure.

[0015] Through the above technical solution, by first pre-hydrating and aging the active powder to form a viscous precursor, and then mixing it with the granular core through low-speed roller coating, the core-shell structure can be constructed purely physically under normal temperature and pressure conditions. Specifically, the pre-hydration and aging process of S1 is the key control point of the entire preparation process. The addition of an appropriate amount of water causes an initial hydration reaction on the surface of silicate cement particles, generating a thin and continuous hydrated calcium silicate (CSH) gel film. This gel film has two functions: first, it provides viscosity, allowing the powdered material to adhere to the surface of the steel slag particles; second, the pre-hydrated cement has higher activity, and the hydration reaction starts faster after being added to the bottom mud, shortening the strength formation time. Calcium dihydrogen phosphate partially dissolves during this process, forming calcium phosphate salt precipitates in the alkaline micro-regions on the surface of the cement particles, intertwining with the CSH gel, enhancing the density and reactivity of the shell. Sodium thiosulfate, due to its good water solubility, is uniformly distributed in the precursor material during this process. S2 employs a non-powered or low-speed drum mixer, utilizing the material's own gravity and friction to achieve coating, thus avoiding the coating layer detachment or steel slag particle breakage that can occur with high-speed mixing. The entire preparation process requires no heating devices, organic solvents, or complex chemical synthesis equipment, resulting in low equipment investment, simple operation, and suitability for large-scale industrial production as well as temporary on-site preparation.

[0016] Preferably, in step S1, the proportion of water added to the mass of silicate cement does not exceed 10%.

[0017] Through the above technical solution, by strictly controlling the amount of water added during pre-hydration, only a discontinuous, tiny hydration film forms on the surface of cement particles, generating sufficient viscosity to encapsulate the core, without causing overall solidification and clumping. Specifically, when the water content is 5%-10% of the cement mass, the hydration degree of cement particles is controlled at approximately 5%-15%, with the hydration products mainly in a gel state. Slight adhesion occurs between particles through liquid bridging forces, and the material as a whole remains loose and powdery, facilitating subsequent encapsulation operations and storage and transportation. If the water content is less than 5%, the gel content is insufficient, and the shell precursor cannot effectively adhere to the steel slag surface, significantly reducing the encapsulation rate. If the water content exceeds 10%, the cement hydration reaction accelerates, and the material may clump and harden within the mixer or during storage, leading to product failure, and the clumped material cannot be spread during application. Experimental verification shows that when the water content is 8% of the cement mass, the encapsulation rate of the precursor material can reach over 95%, and the finished product, stored under sealed conditions for 3 months, shows no significant decrease in activity.

[0018] A method for in-situ solidification and remediation of heavy metal pollution in riverbed sediment, using the aforementioned composite passivating agent, includes the following steps: Sa, Construction: Perform underwater tillage operation, turn up the bottom mud of a preset thickness, and at the same time add the composite passivating agent dry powder into the bottom mud turbidity during the tillage process to mix it with the bottom mud; Sb, Curing: After construction, cover the surface of the mixed area at the bottom of the water with a layer of bentonite material for static water curing.

[0019] The construction method described above, by abandoning complex high-pressure jet grouting equipment, slurry preparation systems, and high-pressure pipelines, creatively adopts the simplest and most universal integrated construction method of underwater tillage and simultaneous dry powder mixing. This significantly reduces the construction difficulty of in-situ riverbed sediment remediation and the reliance on specialized construction equipment. The excavators or amphibious excavators used in this method are the most common and universal equipment in river dredging and environmental remediation projects, and equipment rental or allocation is extremely convenient. The direct addition of dry powder avoids a series of construction problems caused by pre-prepared slurry, such as water-cement ratio control, slurry sedimentation, and pipeline blockage, while also reducing water transportation and addition costs. The installation of a bentonite capping layer in Sb is a key supporting measure of this remediation method. The tillage process inevitably causes temporary disturbance to the sediment, leading to the release of some heavy metals and fine particulate matter into the overlying water. Sodium-based bentonite powder is promptly and evenly spread on the surface of the construction area. Upon contact with water, it rapidly hydrates and expands, forming a dense, low-permeability mud film covering layer. This covering layer has three functions: first, it effectively isolates the overlying water body, preventing the instantaneous release and diffusion of pollutants caused by construction disturbance; second, it slows the loss of alkaline substances required for the curing reaction in the sediment, maintaining a high pH environment in the sediment pore water; and third, it provides a settling matrix for suspended solids in the overlying water body, accelerating water clarification after construction. The entire remediation process requires only general-purpose engineering machinery and simple manual spreading operations. Operators can master the process with minimal training, making it suitable for urban rivers and small to medium-sized rivers of various widths and depths in my country.

[0020] Preferably, in the Sa process, underwater tillage is carried out using an excavator bucket, and the composite passivating agent dry powder is added synchronously through a hopper that is linked to the bucket's movement.

[0021] Through the above technical solution, by mechanically linking the tillage equipment and the feeding equipment, the two operations of bottom mud tillage and quantitative chemical addition can be completed simultaneously with a single device and a single action. Specifically, a hopper with an adjustable flow control valve is installed on the back of the excavator bucket or on the boom. The hopper guides the passivating agent dry powder to the bucket tillage area through a pipe. When the excavator boom performs the action of digging and turning over the bottom mud, the linkage mechanism triggers the hopper valve to open, and the dry powder is precisely sprinkled into the turned-over bottom mud flow along the trajectory of the bucket movement; when the excavator boom lifts up and transports materials, the valve automatically closes, stopping the feeding. This linked dosing method, compared to the traditional two-step method of first spreading powder and then mixing, has the following advantages: First, the contact and mixing of the agent with the bottom sediment is more thorough, and the agent particles are dispersed and encapsulated in the turbid sediment flow, avoiding premature dissolution and diffusion loss of the agent in the water; second, the spatial distribution of the agent is more uniform, and the dosage automatically follows the bucket movement to cover the area and volume, making it less likely to cause local over- or under-dosing; third, it reduces the loss of agent under water scouring and improves the utilization rate of the agent, with a measured utilization rate of over 90%, while the utilization rate of the traditional spreading method is usually only 60%-70%. The hopper valve can be mechanically linked or electrically controlled, with the mechanically linked type being simple in structure, low in cost, easy to modify and maintain on site, and more suitable for engineering site conditions.

[0022] Preferably, in the Sa, the amount of composite passivating agent dry powder added accounts for 6% to 10% of the dry weight of the sediment to be treated.

[0023] By limiting the admixture ratio of the agents described above, the waste of agents is avoided while ensuring that the passivation rate of heavy metals meets the standards and the curing strength meets the engineering requirements. This achieves the best balance between economic benefits and remediation effects. The selection of the admixture ratio needs to comprehensively consider the degree of pollution, the target remediation standards, and economic factors. For sediments with moderate pollution levels, such as Pb content of 500-1000 mg / kg and Cd content of 20-50 mg / kg, an admixture ratio of 6%-7% is usually sufficient to meet the remediation indicators. For sediments with severe pollution levels, such as Pb content >1000 mg / kg and Cd content >50 mg / kg, the admixture ratio needs to be increased to 8%-10% to ensure the passivation effect. It is worth noting that when the admixture ratio is further increased to above 10%, the increase in the passivation rate of heavy metals slows down significantly, showing diminishing marginal returns, while the cost of agents increases linearly, resulting in poor economic efficiency. Furthermore, excessive cement can lead to excessively alkaline sediment, such as pH >11, which is not conducive to subsequent ecological restoration. Therefore, an incorporation ratio of 6%-10% is the optimal range after comprehensively balancing the repair effect and the economic efficiency of the project.

[0024] Preferably, the bentonite material covering the Sb is sodium-based bentonite powder, and the dosage is 1-2 kg / m².

[0025] The above technical solution, using natural sodium-based bentonite powder as the covering material, achieves excellent sealing and isolation effects at extremely low cost. Compared to calcium-based bentonite, sodium-based bentonite has a larger interlayer spacing and a hydration expansion ratio of 10-15 times, while calcium-based bentonite typically only expands 3-5 times. This results in a denser and more continuous mud film, with a permeability coefficient as low as 1×10⁻⁻⁻⁶. 9 Below cm / s. When the dosage is 1-2 kg / m², bentonite hydration forms a continuous, dense mud film approximately 5-10 mm thick on the surface of the sediment. This mud film effectively blocks over 99% of suspended particles in the sediment from diffusing into the water. If the dosage is below 1 kg / m², the mud film coverage may be discontinuous, with gaps or weak areas, significantly reducing the sealing effect. If the dosage is above 2 kg / m², the mud film thickness is too large, hindering normal material exchange between the sediment and the overlying water, and increasing unnecessary material costs. Sodium-based bentonite is a natural mineral, non-toxic and harmless, and will not adversely affect the aquatic ecosystem. Furthermore, its market price is low, less than 1 yuan per kilogram, making it extremely cost-effective.

[0026] Preferably, the static water curing time is no less than 14 days.

[0027] By employing the above technical solutions, and by establishing a minimum curing period of 14 days, the silicate cement within the outer shell is ensured to fully complete its main hydration reaction stages, forming a solidified body with sufficient cementitious strength. The hydration process of silicate cement is divided into an induction period, an acceleration period, a deceleration period, and a stabilization period. At room temperature, the acceleration period typically lasts until about 7 days, at which point the cement has completed approximately 60%-70% of its strength development; by 14 days, the strength development can reach over 85% of the 28-day standard strength. For the in-situ remediation process described in this invention, a 14-day curing time is sufficient for the treated sediment to achieve an unconfined compressive strength of not less than 200 kPa, meeting the mechanical requirements for subsequent use as revetment filling material or ecological substrate. Simultaneously, 14 days is also a time window for the heavy metal stabilization reaction to reach full completion, the crystallization and aging processes of phosphate mineralization precipitation and sulfide precipitation are essentially complete, and the TCLP leaching concentration tends to stabilize. During the curing period, still water conditions must be maintained to avoid water flow erosion and disturbance, preventing the newly formed low-strength solidified body from being destroyed or the reagents from being lost.

[0028] Therefore, the present invention has at least the following beneficial effects: 1. Through the core-shell structure design of "steel slag core - calcium / phosphorus / silicon / sulfur composite shell", multiple heavy metals (Pb, Cd, Cu, Zn) are simultaneously and efficiently chemically passivated in one treatment, with a stabilization rate of >98%, and the physical and mechanical properties of the sediment are significantly improved. This solves the fundamental functional defects of existing technologies that can only chemically passivate and cannot physically solidify, or have complex processes and insufficient solidification strength. This allows the repaired sediment to be directly converted into revetment filling materials or ecological substrates, realizing a closed loop of waste-to-resource conversion.

[0029] 2. This technology uses widely available, inexpensive, and readily available industrial solid waste, converter steel slag, and the basic industrial chemical sodium thiosulfate to replace the expensive modified biochar and other modified minerals required by existing technologies, which involve complex chemical synthesis or high-temperature pyrolysis. Material costs are reduced by more than 30%, with raw material costs estimated to be below 500 yuan / ton. Furthermore, the entire preparation process is carried out at ambient temperature and pressure, requiring only mixing, trace water addition for aging, and roller coating. It eliminates the need for energy-intensive and complex processes such as 600-900℃ high-temperature tubular furnaces and carrier gas protection, resulting in extremely low production energy consumption and equipment investment, making it highly valuable for industrial application.

[0030] 3. The material preparation process is extremely simplified, requiring only conventional powder mixing and conveying equipment. It can be pre-produced on a large scale in a factory or quickly prepared on-site using a simple production line, without being limited by site or equipment conditions. The construction method eliminates the need for specialized equipment such as high-pressure jet grouting, employing the most commonly used amphibious excavators in river dredging projects. Only a simple linked hopper needs to be added to the bucket to carry out integrated underwater tillage and simultaneous dry powder mixing. The entire construction process has a low technical threshold; operators can master it with simple training. It is applicable to urban rivers, small and medium-sized rivers, and nearshore areas of lakes of various widths and depths in my country, possessing extremely high construction versatility and promotion potential. The beneficial effects of this invention are: Detailed Implementation

[0031] 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 application examples. Example

[0032] The raw materials used in each embodiment and comparative example, as well as their manufacturers, brands / specifications, are detailed in Table 1.

[0033] Table 1. Raw material sources and specifications steel slag Maanshan Iron & Steel Co., Ltd. Converter steel slag, hot quenching method After crushing and magnetic separation to recover large pieces of iron, the material is then sieved through a circular vibrating screen to obtain particles with a diameter of 0.5-2 mm, an apparent density of 3.15 g / cm³, and a free calcium oxide content of 2.3%. Silicate cement Anhui Conch Cement Co., Ltd. P·O 42.5 ordinary Portland cement Specific surface area 350 m² / kg, initial setting time 145 min, final setting time 210 min, 3-day compressive strength 26.8 MPa, 28-day compressive strength 48.2 MPa calcium dihydrogen phosphate Sichuan Chuanheng Holding Group Co., Ltd. <![CDATA[Industrial grade, the main content of Ca(H2PO4)2·H2O ≥ 98%]]> <![CDATA[White crystalline powder, passing through a 200-mesh standard sieve, water-soluble P2O5 content ≥ 55%, pH value (1% aqueous solution) 3.0 - 3.5]]> Sodium thiosulfate Shanghai Zhongtai Chemical Co., Ltd. <![CDATA[Industrial grade, main content Na2S2O3·5H2O ≥ 99%]]> Colorless and transparent crystalline granules, pulverized in a universal grinder and passed through a 200-mesh standard sieve before use to ensure fine powder. Sodium bentonite Liaoning Chaoyang Dongda Mining and Metallurgical Research Institute Natural sodium bentonite, NF grade Montmorillonite content ≥85%, expansion ratio ≥15ml / g, gel value ≥45ml / 15g, passing rate through a 200-mesh standard sieve ≥98%. Ferrous sulfate Tianjin Dingshengxin Chemical Co., Ltd. <![CDATA[Industrial grade, FeSO4·7H2O ≥ 98%]]> Light green crystals, used for comparative preparation Example 3 quicklime Jiangxi Xinyu Huasheng Calcium Industry Co., Ltd. Industrial grade, CaO ≥ 92% Blocky, crushed and passed through a 200-mesh sieve before use, used in comparative preparation examples 3 and 4. Ordinary sodium bentonite Shandong Huawai Bentonite Co., Ltd. Drilling mud grade Montmorillonite content ≥75%, 200 mesh, used for comparative preparation Example 3 Straw biomass Shandong Jingrui Machinery Co., Ltd. corn stalks Wash, dry, pulverize, and pass through a 2mm sieve for later use in comparative preparation example 4. Ferric chloride Shanghai Aladdin Biochemical Technology Co., Ltd. <![CDATA[Industrial grade, FeCl3·6H2O≥99%]]> Example 4 for comparison Superphosphate Yunnan Yuntianhua Co., Ltd. <![CDATA[Industrial grade, effective P2O5 ≥ 16%]]> For comparison, preparation example 4 Nitrogen Air Liquide (China) Investment Co., Ltd. Industrial grade, purity ≥99.9% Carrier gas for comparative preparation Example 4 (tube furnace) Preparation examples of composite passivating agents: Preparation Example 1: As the optimal formulation of a core-shell structured passivating agent, the composite passivating agent with a core-shell structure described in this invention is prepared according to the following detailed steps: (1) Powder premixing: Using an electronic platform scale with an accuracy of ±0.1 kg, accurately weigh 250 kg of calcium dihydrogen phosphate, 150 kg of P·O 42.5 ordinary Portland cement, and 100 kg of sodium thiosulfate (crushed through a 200-mesh sieve). Add the three powders sequentially to a double-cone V-type mixer with an effective volume of 1500 L. The double-cone V-type mixer is a V-1500 model manufactured by Wuxi Minghai Mixing Equipment Factory, with a mixing speed set to 30 r / min and a mixing time of 15 minutes. After mixing, stop the machine, open the observation port, and visually inspect the sample. The material should be a uniform white-light yellow powder, without obvious color spots or particle agglomeration, indicating uniform powder. Weigh the total weight of the mixed powder and compare it with the initial feed amount; the material loss rate should be less than 0.2%.

[0034] (2) Pre-hydration and aging: While the V-type mixer continues to operate at 30 r / min, the metering pump and atomizing nozzles are activated through the mixer's built-in liquid addition system to uniformly spray metered deionized water into the tumbling powder. The amount of water added is strictly calculated according to 8% of the silicate cement mass, i.e., 12 kg, accurate to ±0.1 kg. The water spraying rate is controlled at 3 kg / min to ensure that the water mist is evenly dispersed in the powder and to avoid local over-wetting. After water spraying is completed, the mixer speed is reduced to 20 r / min, and mixing and aging continue for 20 minutes. During the aging process, the initial hydration reaction occurs on the surface of the cement particles, calcium dihydrogen phosphate partially dissolves and forms calcium phosphate salt precipitate in the alkaline micro-region, and sodium thiosulfate is evenly distributed in the material system. After aging is completed, the discharge valve is opened to discharge the material. The material should be in a loose, dust-free state, able to be clumped when lightly pinched with fingers, and easily dispersed when lightly pressed, with a moisture content of approximately 2.4% of the total material. This material is temporarily stored in a moisture-proof hopper with a plastic liner and is designated as "shell precursor material".

[0035] (3) Core-shell structure assembly: Weigh 400 kg of steel slag particles with a particle size of 0.5-2 mm after screening, and add them together with approximately 512 kg of all the shell precursor materials obtained in step (2) into a non-powered drum mixer with an effective volume of 2000 L. The non-powered drum mixer is a GTH-2000 model manufactured by Zhengzhou Teda Mining Machinery Co., Ltd. The drum body of this mixer has several lifting plates evenly distributed along the axial direction inside, which are used to lift and throw the materials. The drum rotation speed is set to 15 r / min, and continuous rolling and mixing is performed for 5 minutes. During the low-speed rolling process, the steel slag particles are lifted and thrown down by the lifting plates, and repeatedly come into contact, collide, and rub against the shell precursor materials. The sticky shell precursor powder gradually and evenly coats the surface of the steel slag particles, forming a continuous grayish-white shell with a thickness of approximately 0.1-0.3 mm. After the mixing is completed, open the discharge port to discharge the material, and remove any possible small amount of clumps or large uncoated particles through a 3 mm sieve. The sieved product is a core-shell structured granular composite passivating agent with a "steel slag core and a calcium / phosphorus / silicon / sulfur composite outer shell". The finished product is packaged in moisture-proof woven bags lined with polyethylene film, each bag weighing 25 kg net. The bags are sealed with a sealing machine and stored in a cool, dry, and well-ventilated warehouse for later use. The product obtained in this preparation example is labeled as Sample A1.

[0036] Preparation Example 2: The operation steps and equipment of this preparation example are exactly the same as those of Preparation Example 1. The only difference is that the proportions of each component have been adjusted to verify the endpoints of the proportion range described in this invention and the trend of effect changes. Specifically, the following are the ingredients: 200 kg of calcium dihydrogen phosphate, 200 kg of P·O 42.5 ordinary Portland cement, 50 kg of 200-mesh sodium thiosulfate, 16 kg of prehydration water, and 500 kg of steel slag particles with a particle size of 0.5-2 mm. The product was obtained by following steps (1) to (3) of Preparation Example 1. It was observed that the density of visible steel slag particles in the product increased significantly due to the increase of the steel slag proportion to 50 parts; the outer powder color turned slightly white due to the reduction of sodium thiosulfate to 5 parts. The product obtained in this preparation example is labeled as sample A2.

[0037] Preparation Example 3: Core-shell structure passivating agent with adjusted pre-hydration water addition The operation steps and equipment of this preparation example are exactly the same as those of Preparation Example 1, except that the amount of water added during prehydration in step (2) was adjusted to verify the endpoints of the water addition range described in this invention and its impact on the finished product state. Specifically, the amount of water added was strictly calculated as 5% of the mass of silicate cement, i.e., 7.5 kg, accurate to ±0.1 kg. Other component ratios and operating parameters were the same as in Preparation Example 1. Due to the reduced amount of water added, the aged shell precursor material was drier and looser than that in Preparation Example 1, and the stickiness was slightly weaker when pinched by fingers, but it could still form an effective coating on the steel slag surface. The product obtained in this preparation example was marked as Sample A3.

[0038] Comparative Preparation Example 1: Simple Physical Hybrid Passivating Agents Without Core-Shell Structure This comparative preparation example aims to verify the performance improvement effect of the "pre-hydration aging-roller coating" core-shell construction process of the present invention compared with simple physical mixing. The same components and amounts as in Preparation Example 1 were used: 250 kg of calcium dihydrogen phosphate, 150 kg of P·O 42.5 silicate cement, 100 kg of sodium thiosulfate, and 400 kg of steel slag. Instead of pre-hydration aging and roller coating, all the above materials were added at once to a horizontal twin-shaft forced mixer with an effective volume of 2000 L. The horizontal twin-shaft forced mixer was manufactured by Zhengzhou Songwei Machinery Manufacturing Co., Ltd., model WH-2000, with a stirring speed set to 60 r / min, and mixed for 30 minutes until visually uniform. Since no water was involved, the materials were all dry powder and dry granules, and there was no sticky coating process. The material was unloaded, and a very small amount of agglomeration, possibly caused by moisture absorption, was removed using a 5 mm mesh sieve. The mixture was then sealed in a moisture-proof woven bag lined with polyethylene film for later use. The product obtained in this comparative preparation example is designated as Sample B1.

[0039] Comparative preparation example 2: This comparative preparation example is used to verify the unique contribution of sodium thiosulfate component in the present invention to the passivation effect, especially to the passivation effect of specific heavy metals such as Cd. The preparation process is exactly the same as in Preparation Example 1, except that sodium thiosulfate is removed from the formula and replaced with an equal mass of calcium dihydrogen phosphate to maintain the total mass of the shell components. The specific formula is: 350 kg of calcium dihydrogen phosphate, 150 kg of P·O 42.5 silicate cement, and 400 kg of steel slag. Following steps (1) to (3) of Preparation Example 1, the amount of water added during prehydration remains 8% of the cement mass, i.e., 12 kg. The product obtained in this comparative preparation example is labeled as Sample B2.

[0040] Comparative preparation example 3: This comparative preparation example is used to directly compare the effects of the present invention with the technical solution of patent CN109092878A, which represents the general prior art. A representative set of stabilizing agents was prepared according to the preferred formulation range disclosed in the patent specification. The specific formulation is as follows: Phosphate compounds: Calcium dihydrogen phosphate was selected, weighed in the amount of 3.5 parts by weight (70 kg in this preparation example); Soluble iron salt: Use ferrous sulfate, weigh 2.0 parts by weight, i.e. 40 kg; Unmodified clay minerals: ordinary sodium-based bentonite was selected, weighed 1.0 part by weight, i.e. 20 kg; pH adjuster: Use quicklime, weigh 3.5 parts by weight, i.e. 70 kg; All four materials were added to a V-type mixer and mixed at 30 r / min for 30 minutes until homogeneous. The mixture was then discharged, bagged, and sealed for later use. The product was a light brownish-gray powder with a distinct lime odor. The product obtained in this comparative preparation example is designated as sample B3.

[0041] Comparative preparation example 4: This comparative preparation example is used to directly compare the effects of the present invention with the prior art patent CN110699084B, which represents high-temperature chemical modification to achieve curing and repair. The preparation was carried out strictly according to the technical solution of Example 2, which exhibits the best strength performance, as described in the patent specification.

[0042] First, prepare iron-based biochar-modified superphosphate: (1) Preparation of iron-based biomass: 50 kg of corn stalk biomass (washed, dried, crushed, and sieved through a 2 mm sieve) was weighed and placed in a 500 L mixing tank. 25 kg of ferric chloride (FeCl3·6H2O) and sufficient deionized water were added at a mass ratio of iron salt to biomass of 0.5:1. The mixture was stirred for 2 hours to allow the iron salt to fully impregnate and adsorb onto the biomass, forming a uniform slurry. The slurry was placed in a stainless steel tray and then placed in an electric heating drying oven at 105 °C until constant weight was achieved, yielding iron-based biomass blocks. After cooling, the blocks were pulverized using a hammer mill and sieved through a 2 mm sieve to obtain iron-based biomass powder.

[0043] (2) Co-pyrolysis: The above-mentioned iron-based biomass powder and superphosphate were mechanically mixed in a horizontal twin-shaft mixer for 2 hours until homogeneous. This mixture was then placed in an alumina crucible and placed in a vacuum atmosphere tube furnace. The vacuum atmosphere tube furnace was a GSL-1600X model manufactured by Hefei Kejing Materials Technology Co., Ltd. First, a vacuum was drawn, and then a mixed carrier gas of nitrogen (N2) and carbon dioxide (CO2) (volume ratio 1:1) was introduced, with the gas flow rate controlled at 200 ml / min. Under the protection of the carrier gas, the temperature was increased to 800℃ at a heating rate of 10℃ / min, and then calcined at 800℃ for 60 minutes. After calcination, the furnace was cooled to room temperature under continuous carrier gas protection. The product was removed, ground using a ball mill, and passed through a 100-mesh sieve to obtain iron-based biochar modified superphosphate.

[0044] Then, prepare the compound medicine: According to the formula in Example 2 of this patent, quicklime accounts for 30%, and iron-based biochar modified superphosphate accounts for 70%. 70 kg of the self-made iron-based biochar modified superphosphate and 30 kg of quicklime were weighed and mixed in a V-type mixer for 30 minutes until homogeneous. The mixture was then bagged and sealed for later use. The product is a dark black powder with a lime odor. The product obtained in this comparative preparation example is labeled as Sample B4.

[0045] Collection and characterization of polluted sediment samples: In order to objectively and scientifically evaluate the remediation effect of the passivating agents obtained from the preparation examples and comparative examples on heavy metal polluted sediments, this invention selected a typical polluted river sediment as the test sample and characterized its basic physicochemical properties and heavy metal pollution characteristics in detail.

[0046] (1) Sample Collection: The sediment samples were collected from a section of a polluted river near the site of the former electroplating workshop of a long-established auto parts manufacturing plant in Nanjing, Jiangsu Province. This river had historically received heavy metal-containing electroplating wastewater and rinsing wastewater discharged from the plant, resulting in severe heavy metal accumulation in the sediment. A gravity columnar sediment sampler (model ZYC-2, developed by the Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences) was used. Ten sampling points were set up along the centerline of the river, spaced 20 m apart. Sediment columnar samples were collected from each sampling point at a depth of 0-50 cm. All samples were collected in a clean plastic container and thoroughly mixed manually with a stainless steel shovel for at least 30 minutes to eliminate local differences and obtain a representative mixed sample. The total volume of the mixed sample was approximately 500 kg. The samples were sealed in plastic containers on-site and transported to the laboratory by refrigerated truck for further processing and analysis.

[0047] (2) Sample Pretreatment: The collected mixed sediment sample was evenly spread on multiple stainless steel trays in a laboratory fume hood, with a thickness of about 2-3 cm. It was allowed to air dry at room temperature until the moisture content was about 20%, ideally when it was no longer sticky and could be crumbled. It was turned several times a day to accelerate moisture evaporation. After air drying, visible foreign matter such as gravel, bricks, plastic fragments, and plant debris was removed. The removed sediment was then gently broken up with a wooden mallet and passed through a standard sieve with a 5 mm aperture to remove large sand and gravel particles and any remaining impurities. The material passing through the sieve was the homogeneous, finely ground sediment sample for testing, stored in a sealed plastic box for later use. A portion of the sample that passed through the 5 mm sieve was further ground using an agate mortar until it passed through a 100-mesh nylon sieve for total heavy metal analysis and physicochemical property testing.

[0048] (3) Determination of basic physicochemical properties: pH value: The pH value was determined using the potentiometric method according to the "Determination of Soil pH" (NY / T 1377-2007). 10.0 g of air-dried sediment sample (passed through a 100-mesh sieve) was weighed and placed in a 50 ml beaker. 25 ml of deionized water (water-to-soil ratio 2.5:1) was added. The mixture was vigorously stirred with a magnetic stirrer for 2 minutes and allowed to stand for 30 minutes. The pH value of the supernatant was then measured using a precision pH meter (Mettler-Toledo, FE28-Standard type) calibrated with standard buffer solutions. The measurement was repeated three times, and the average value was taken. The result was 6.78.

[0049] Moisture content: Refer to the drying method in the "Standard for Geotechnical Testing Methods" (GB / T 50123-2019). Weigh approximately 50g of air-dried sediment sample (accurate to 0.01g) that has passed through a 5mm sieve, place it in a pre-weighed aluminum box, and put it in an electrically heated drying oven at 105-110℃ until constant weight (approximately 8 hours). After removal, cool to room temperature in a desiccator and weigh. Calculate the moisture content of the sample. The result is 19.8% (after air drying). Based on this, the original sediment moisture content can be inferred to be approximately 85%.

[0050] Organic matter content: Following the potassium dichromate oxidation-external heating method in the "Soil Organic Matter Determination Method" (NY / T 1121.6-2006), the result was 4.52%.

[0051] Particle size distribution: The particle size distribution was determined using a laser particle size analyzer (Mastersizer 3000, Malvern Instruments Ltd., UK). A small amount of air-dried sediment sample that had passed through a 2mm sieve was taken, deionized water was added, and the sample was ultrasonically dispersed for 5 minutes before being analyzed. The results showed that sand (>50μm) accounted for 18%, silt (2-50μm) accounted for 56%, and clay (<2μm) accounted for 26%. The soil type is silty clay loam.

[0052] (4) Determination of total heavy metal content and leaching toxicity: Total heavy metal content: Refer to the "Microwave Digestion Method for Total Metal Element Content in Soil and Sediment" (HJ 832-2017). Weigh approximately 0.2 g (accurate to 0.0001 g) of air-dried sediment sample that has passed through a 100-mesh sieve, place it in a polytetrafluoroethylene microwave digestion vessel, add 6 ml of analytical grade nitric acid, 2 ml of analytical grade hydrochloric acid, and 2 ml of analytical grade hydrofluoric acid, seal the vessel, and place it in a microwave digester (CEM Corporation, MARS 6 model, USA) according to the set program for digestion. After digestion, remove the acid until nearly dry, transfer the solution with 1% dilute nitric acid solution, and dilute to a 50 ml volumetric flask. Use inductively coupled plasma mass spectrometry (ICP-MS, Thermo Fisher Scientific, iCAP RQ model, USA) to determine the concentrations of Pb, Cd, Cu, Zn, etc. in the solution.

[0053] TCLP leaching toxicity: The leaching was performed according to the USEPA Method 1311 (Toxicity Characteristic Leaching Procedure). A suitable leachate was selected based on the pH of the sediment sample. Since the sediment pH was <5.0, leachate 1 (glacial acetic acid buffer solution, pH = 4.93 ± 0.05) was selected. 100.0 g of air-dried sediment sample (passed through a 5 mm sieve) was weighed and added to 2000 ml of leachate at a liquid-to-solid ratio of 20:1. The sample was placed on a rotary shaker (Beijing Zhongke Haobo Technology Co., Ltd., HB-TCLP-12 type) and shaken at 30 ± 2 r / min for 18 ± 2 hours. After shaking, the sample was filtered under reduced pressure through a 0.6-0.8 μm glass fiber membrane. The filtrate was collected, and the concentrations of various heavy metals in the filtrate were determined by ICP-MS.

[0054] Table 2 summarizes the results of heavy metal total content and TCLP leaching toxicity determination of the tested sediment samples.

[0055] Table 2 Heavy metal content and leaching characteristics of the tested sediment Total heavy metal content (mg / kg) 1256.3 87.5 612.8 1845.2 TCLP leaching concentration (mg / L) 18.52 3.21 15.84 48.36 Limits (mg / L) for Leaching Toxicity Identification of Hazardous Waste (GB 5085.3-2007) 5 1 100 100 Surface Water Environmental Quality Standard (GB 3838-2002) Class IV Limit (mg / L) 0.05 0.005 1.0 2.0 As shown in Table 2, the tested sediment is a sediment contaminated with multiple heavy metals, with Pb and Cd pollution being particularly severe. Their total amounts far exceed the background values ​​for general soil, and their TCLP leaching concentrations exceed the limits set by the "Hazardous Waste Identification Standard" by 3.7 times and 3.2 times, respectively, posing a high environmental risk. Therefore, this sediment is very suitable as a test model for the remediation target of this invention.

[0056] An example of effectiveness verification using a simplified in-situ curing repair method: The in-situ solidification and remediation method for heavy metal pollution in riverbed sediments described in this invention was validated in a laboratory-scale simulated river flume. The flume was constructed by bonding together 10mm thick colorless transparent plexiglass sheets, with internal dimensions of 2.0m length, 0.5m width, and 1.0m height. The flume was designed as a closed-loop circulation system, with contaminated sediment at the bottom and simulated river water at the top.

[0057] (1) Simulated river channel construction: The air-dried sediment samples collected and pretreated by passing through a 5mm sieve were used to calculate the required water volume based on an actual moisture content of 85%. The sediment was then mixed with deionized water to form a uniform slurry with a moisture content of 85%. The slurry was carefully pumped into a water tank to a thickness of 0.5m, and the surface was roughly smoothed with a scraper. The mixture was allowed to stand for 48 hours to allow the sediment to settle and compact under its own weight, and some of the overlying water was drained. Then, simulated river water was slowly injected from one end of the water tank at a very low flow rate. Clean upstream water from the river channel was used after being filtered through a 0.45μm filter membrane until the water depth reached 0.3m. Care was taken to avoid impacting the sediment surface during the injection process. After the water tank was constructed, it was allowed to stand for 24 hours to allow the entire system to reach a stable state and for the sediment-water interface to become clear.

[0058] (2) Repair Construction, Tillage and Mixing: This application example simulates the underwater tillage and synchronous material feeding method using an excavator bucket. Due to the size limitations of the water tank, a manual method is used for equivalent simulation. A simple simulated tillage and material feeding device is designed: a small, openable hopper is welded to the head of a 1.5m long stainless steel toothed rake. The hopper outlet is guided to the tillage direction of the toothed rake through a thin tube and is equipped with a manual valve. The repair agents are samples A1 obtained from Preparation Example 1, A2 obtained from Preparation Example 2, and A3 obtained from Preparation Example 3, respectively. The agent admixture ratio (dry weight of passivating agent / dry weight of bottom mud) is set to 8% for all samples. Based on the bottom mud laying volume (2m × 0.5m × 0.5m = 0.5m³) and density, the estimated dry bottom mud mass is approximately 350kg, therefore, 28kg of passivating agent needs to be added to each treatment channel. During construction, the handheld simulator inserts the rake into the bottom mud to a depth of approximately 20 cm, manually opens the hopper valve, and then drags the rake at a uniform speed (approximately 1.5 m / min) along the length of the trough. As the rake moves, the passivating agent powder is continuously and quantitatively released through valves and guide pipes into the turbid mud flow stirred up by the rake, mixing with it. Each trough is tilled and mixed three times to ensure thorough mixing of the agent and the bottom mud. During construction, a brief period of turbidity can be observed in the water overlying the tilled area, but it gradually clarifies after about 30 minutes. This operation simulates the core actions of excavator bucket tilling and simultaneous material feeding in actual engineering projects.

[0059] (3) Surface Sealing Covering: Immediately after the tillage and mixing are completed, a surface layer of bentonite is applied. Commercially available sodium-based bentonite powder is manually and evenly sprinkled above the water surface in the work area of ​​the water tank. The powder penetrates the water layer and settles evenly onto the bottom mud surface. The application rate is 1.5 kg / m². After application, the water becomes clearer, and after about one hour, the water returns to its clear and transparent state.

[0060] (4) Curing and Sampling: The water tank is covered with a transparent plexiglass cover with ventilation holes to reduce moisture evaporation. Curing is carried out under still water conditions at room temperature (20-28℃). After curing for the specified ages of 7, 14, and 28 days, a 50mm inner diameter plexiglass sampling tube is carefully inserted into the sediment to extract undisturbed columnar core samples. The top layer of bentonite covering the extracted core sample is removed, leaving the uniformly repaired sediment section in the middle for subsequent testing of various indicators.

[0061] Repair effect comparison test and result analysis: The passivating agents obtained from each preparation example and comparative example were used to treat samples B3 and B4 according to the construction and maintenance methods described in the application examples above. Samples B3 and B4 were also mixed in by tilling at a dry weight ratio of 8%. Three parallel experiments were set up for each agent.

[0062] To verify the effectiveness of heavy metal chemical passivation, a TCLP leaching toxicity test was conducted. After 28 days of curing, core samples of the restored sediment from each group were taken and subjected to TCLP leaching toxicity testing according to USEPA Method 1311, following the same procedures as described above. The concentrations of Pb, Cd, Cu, and Zn in the filtrate were determined, and the passivation rate (stabilization rate) of each heavy metal was calculated. The formula for calculating the passivation rate is as follows: Passivation rate (%) = [(C0-Ct) / C0] × 100%; Wherein, C0 is the TCLP leaching concentration of the sediment before remediation (see Table 2), and Ct is the average of the three groups of TCLP leaching concentrations of the sediment after 28 days of remediation and curing.

[0063] Table 3 Comparison of TCLP leaching toxicity and passivation rate of each treatment group after 28 days Sample A1 (Preparation Example 1, optimal for this invention) Leaching concentration (mg / L) 0.28 0.03 0.45 1.12 passivation rate (%) 98.49 99.07 97.16 97.68 Sample A2 (Preparation Example 2, formulation endpoint) Leaching concentration (mg / L) 0.35 0.04 0.52 1.25 passivation rate (%) 98.11 98.75 96.72 97.41 Sample A3 (Preparation Example 3, low water addition) Leaching concentration (mg / L) 0.31 0.04 0.49 1.18 passivation rate (%) 98.33 98.75 96.91 97.56 Sample B1 (Comparison 1, without core-shell structure) Leaching concentration (mg / L) 0.65 0.12 0.92 2.15 passivation rate (%) 96.49 96.26 94.19 95.55 Sample B2 (Comparison 2, without sodium thiosulfate) Leaching concentration (mg / L) 0.96 0.31 1.05 3.42 passivation rate (%) 94.82 90.34 93.37 92.93 Sample B3 (Comparison 3, prior art CN109092878A) Leaching concentration (mg / L) 1.46 0.34 1.27 4.38 passivation rate (%) 92.12 89.41 91.98 90.94 Sample B4 (Comparison 4, prior art CN110699084B) Leaching concentration (mg / L) 0.86 0.18 0.91 3.01 passivation rate (%) 95.35 94.39 94.25 93.77 As can be seen from the data in Table 3 above: The absolute advantage of the passivation effect of this invention: The optimal solution of this invention, sample A1, achieves a passivation rate of over 97% for all heavy metals, especially an astonishing 99.07% for Cd, which is the most difficult to stabilize. This is comprehensively superior to the existing technical solutions B3 and B4, where B4 represents a high-temperature modification technology, achieving a passivation rate of nearly 5 percentage points higher for Cd and 3 percentage points higher for Pb.

[0064] The key roles of core-shell structure and sulfur source: The significant decrease in passivation rate of samples B1 (without core-shell) and B2 (without sulfur source) further demonstrates that the core-shell structure and sodium thiosulfate, the two core innovations of this invention, are indispensable for performance improvement.

[0065] To verify the physical and mechanical properties, unconfined compressive strength (UCS) tests were conducted. After 28 days of curing, core samples of the repaired sediment were collected from each group. The sediment core samples were cylinders with a height-to-diameter ratio of 2:1, and the unconfined compressive strength was tested according to the "Standard for Geotechnical Testing Methods" (GB / T 50123-2019). The unconfined compression mode of a strain-controlled triaxial apparatus (Nanjing Soil Instrument Factory, TSZ-1 type) was used, and the loading rate was controlled at 1% / min. The average value of three parallel samples from each group was taken.

[0066] Table 4 Comparison of unconfined compressive strength of each treatment group at 28 days Sample A1 (Preparation Example 1) 235 4.8 Hard, with good integrity and smooth cut surface Sample A2 (Preparation Example 2) 258 4.2 It is hard, has good overall integrity, and a strong sense of texture in its skeleton. Sample A3 (Preparation Example 3) 225 5.1 Relatively hard, with good overall integrity Sample B1 (Comparative Preparation Example 1) 201 6.5 Relatively hard, slightly loose Sample B2 (Comparative Preparation Example 2) 228 5.5 harder Sample B3 (Comparative preparation example 3, prior art CN109092878A) not applicable — The sample was too soft to obtain a complete core sample for testing. Sample B4 (Comparative preparation example 4, prior art CN110699084B) 185 7.8 It is relatively hard, but brittle, and the ends of the core sample are easily broken. As can be seen from the data in Table 4 above: The decisive advantage of the curing function: Existing technical solution B3 cannot achieve any strength at all, while solution B4, although it has a certain curing effect, has a 28-day strength of only 185 kPa, which is lower than all the solutions in this invention, and does not even reach the recommended threshold of 200 kPa for resource utilization. This is because in solution B4, the curing effect mainly relies on the physical framework of biochar and the carbonization and cementation of lime, and the cementation reaction is slow and the strength of the product is much lower than that of cement hydration products.

[0067] This invention achieves high strength and high toughness: Sample A1 of this invention not only has higher strength than B4, but its failure strain of 4.8% is also much smaller than that of B4 (7.8%), indicating that the solidified body of this invention is more dense and harder. The steel slag core and CSH gel form a "concrete"-like structure, which is the key to obtaining a high-strength, high-integrity solidified body.

[0068] Economic cost analysis: Taking the production of 1 ton of finished passivating agent as an example, the main material costs of each scheme are estimated. The unit price of each raw material is estimated based on the price of bulk purchase at the ton level and transportation to the project location, with market reference prices in 2025.

[0069] Table 5 Comparison of material costs for each processing group (Unit: RMB / ton of finished product) Dosage (kg) / Cost (RMB) Dosage (kg) / Cost (RMB) Dosage (kg) / Cost (RMB) Dosage (kg) / Cost (RMB) steel slag 80 400 / (32) 500 / (40) — — Silicate cement 450 150 / (67.5) 200 / (90) — — calcium dihydrogen phosphate 2800 250 / (700) 200 / (560) 350 / (980) — Sodium thiosulfate 2200 100 / (220) 50 / (110) — — Sodium-based bentonite (common) 600 — — 200 / (120) — Ferrous sulfate 500 — — 200 / (100) — quicklime 400 — — 350 / (140) 300 / (120) Superphosphate 1500 — — — 700 / (1050) Straw biomass 800 — — — 500 / (400) Ferric chloride 3500 — — — 250 / (875) Nitrogen, energy consumption, etc. (estimated) — — — — Approximately (800) total — Approximately 1020 yuan Approximately 800 yuan Approximately 1340 yuan Approximately 3245 yuan Note: The above is a direct material cost estimate. The preparation of sample B4 involves high-temperature calcination in a tubular furnace, which consumes a large amount of energy (electricity) and protective gas (N2 / CO2), and has high equipment depreciation and maintenance costs.

[0070] As can be clearly seen from Table 5 above, the material cost of sample preparation example 1 of this invention is approximately RMB 1020 / ton, while that of preparation example 2 can be reduced to approximately RMB 800 / ton through formula optimization. Compared with the comparative preparation example 3, which represents the general prior art and costs approximately RMB 1340 / ton, the cost is reduced by approximately 23%-40%. Compared with the comparative preparation example 4, which represents the high-temperature pyrolysis modification route, considering materials, energy consumption, and equipment depreciation, the estimated cost exceeds RMB 3000 / ton, representing a cost reduction of up to 68%-75%. This invention completely eliminates the high-temperature, high-energy-consumption chemical modification process, achieving superior repair effects while realizing unparalleled cost control.

[0071] In conclusion, the core-shell structured composite passivating agent provided by this invention completely eliminates the need for high temperature, high pressure, organic solvents, and complex chemical synthesis steps in its preparation process. It can be prepared simply by mixing, pre-hydration aging, and roller coating at room temperature and pressure. The process has a very low technical threshold, requires very little equipment investment, and is highly feasible.

[0072] In the in-situ remediation of heavy metal contaminated sediment, this composite passivating agent has demonstrated a chemical passivation effect and physical and mechanical properties that far exceed those of two typical existing technologies (CN109092878A and CN110699084B). It truly achieves the dual effect of "high-efficiency detoxification" and "high-strength solidification". The treated sediment can be directly transformed into resource-based materials such as revetment or ecological substrate.

[0073] Economic analysis shows that the overall cost of the product of this invention is reduced by more than 23% compared with general chemical stabilization technology and by more than 68% compared with high-temperature pyrolysis modification and curing technology, which has a very significant cost advantage and is highly in line with the actual needs of my country's large-scale, low-cost and high-efficiency pollution control projects.

[0074] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A core-shell structured iron-based-thiol synergistic curing composite passivating agent, characterized in that, Include: The core contains steel slag particles; The outer shell, which encloses the surface of the core, comprises a prehydrated composite of silicate cement, calcium dihydrogen phosphate, and sodium thiosulfate.

2. The composite passivating agent according to claim 1, characterized in that, The particle size of the steel slag particles is 0.5-2 mm.

3. The composite passivating agent according to claim 1, characterized in that, The components, by mass, are: 40-50 parts steel slag particles, 20-25 parts calcium dihydrogen phosphate, 15-20 parts silicate cement, and 5-10 parts sodium thiosulfate.

4. A method for preparing the composite passivating agent as described in any one of claims 1-3, characterized in that, Includes the following steps: S1, mix silicate cement, calcium dihydrogen phosphate and sodium thiosulfate powder evenly, add water and stir, carry out pre-hydration aging, and form loose shell precursor material; S2, the shell precursor material obtained in S1 is mixed with steel slag particles, so that the shell precursor material is coated on the surface of the steel slag particles, thus obtaining the composite passivating agent with the core-shell structure.

5. The preparation method according to claim 4, characterized in that, In S1, the proportion of water added to the mass of silicate cement does not exceed 10%.

6. A method for in-situ solidification and remediation of heavy metal pollution in riverbed sediment, characterized in that, The composite passivating agent according to any one of claims 1-3 is used, comprising the following steps: Sa, Construction: Perform underwater tillage operation, turn up the bottom mud of a preset thickness, and at the same time add the composite passivating agent dry powder into the bottom mud turbidity during the tillage process to mix it with the bottom mud; Sb, Curing: After construction, cover the surface of the mixed area at the bottom of the water with a layer of bentonite material for static water curing.

7. The in-situ curing repair method according to claim 6, characterized in that, In the Sa process, underwater tillage is carried out using an excavator bucket, and the composite passivating agent dry powder is added synchronously through a hopper that is linked to the bucket's movement.

8. The in-situ curing repair method according to claim 6, characterized in that, In the Sa, the amount of composite passivating agent dry powder added accounts for 6% to 10% of the dry weight of the sediment to be treated.

9. The in-situ curing repair method according to claim 6, characterized in that, In the Sb, the bentonite material covering it is sodium-based bentonite powder, and the dosage is 1-2 kg / m².

10. The in-situ curing repair method according to claim 6, characterized in that, The static water curing time shall be no less than 14 days.