A composite passivation agent for reducing mercury mobility and zero-valent mercury generation rate in contaminated soil of a site and a preparation method thereof

Abstract

The application discloses a composite passivation agent for reducing mercury migration rate and zero-valent mercury generation rate in contaminated soil of a site and a preparation method thereof. The composite passivation agent comprises hematite and immobilized fulvic acid, and the mass ratio of the two is 100:(2-3). The immobilized fulvic acid is adsorbed, complexed or coated on the surface of the hematite by a water phase pre-composite mode to obtain the composite passivation agent and form a hematite-organic matter composite interface. The composite passivation agent is used for adsorbing and fixing Hg(II) in contaminated soil of the site, reducing the content of the migratable mercury in the soil, and inhibiting the reduction of Hg(II) to generate zero-valent mercury under dark conditions. According to the hematite-immobilized fulvic acid composite passivation technology, the content of the migratable mercury in the soil can be reduced and the dark reduction conversion of Hg(II) to zero-valent mercury can be inhibited from the initial risk control stage of the contaminated soil of the site.

Description

Technical Field

[0001] This invention relates to the field of heavy metal remediation technology in contaminated soil, and particularly to a composite passivating agent and its preparation method for reducing mercury migration rate and zero-valent mercury formation rate in contaminated soil. Background Technology

[0002] Mercury is a highly toxic environmental substance that seriously endangers human health and ecological safety. It is characterized by its high toxicity, easy migration, easy transformation, and ability to be transported over long distances. Long-term activities such as mercury mining, chlor-alkali industry, non-ferrous metal smelting, production of mercury-containing instruments, coal combustion emissions, and the storage of mercury-containing waste can all lead to the accumulation of mercury in the soil. Compared with ordinary farmland soil, industrial legacy sites and mining sites have higher concentrations of mercury pollution, stronger spatial heterogeneity, and more complex occurrence forms, making them more susceptible to entering the surrounding environment through rainwater leaching, groundwater migration, surface runoff, and dust dispersion.

[0003] The environmental risks of mercury in contaminated soil at sites include at least two aspects. Firstly, mercury can migrate via soil pore water, leachate, colloids, and dissolved organic matter, causing groundwater or surface water pollution. Secondly, divalent mercury (Hg(II)) can undergo dark reduction with the participation of soil organic matter, minerals, and reducing components, generating volatile mercury (zero-valent mercury), which is then released from the soil surface into the air. Zero-valent mercury has strong volatility and long-distance transport capabilities; simply reducing the leaching concentration of mercury in the soil is insufficient to comprehensively control the environmental risks of mercury-contaminated sites. It is also necessary to simultaneously inhibit the conversion of Hg(II) to zero-valent mercury.

[0004] Existing remediation technologies for mercury-contaminated soil mainly include solidification and stabilization, sulfide passivation, biochar passivation, mineral adsorption, soil washing, thermal desorption, and phytoremediation. Among these, solidification and stabilization and passivation technologies have good application prospects in the risk management of contaminated sites due to their relatively simple operation and low cost. In existing technologies, iron-containing minerals, biochar, zeolite, and organic fertilizers are often used as heavy metal passivating agents to reduce the availability or leaching of heavy metals in the soil. However, these technologies usually focus primarily on the adsorption and fixation of heavy metals or the reduction of phytoavailability, paying less attention to whether Hg(II) will be further reduced to zero-valent mercury during the passivation process.

[0005] Dissolved organic matter (DOC) is an important component in soil that influences mercury migration and transformation. DOC can complex with Hg(II) through carboxyl groups, phenolic hydroxyl groups, and sulfur-containing functional groups, thereby enhancing mercury's migration ability in pore water. Simultaneously, some DOC possesses electron-donating capabilities, promoting the reduction of Hg(II) to zero-valent mercury under dark conditions. Therefore, in mercury-contaminated soils, DOC may both enhance Hg(II) migration and promote the formation of zero-valent mercury.

[0006] Existing studies on the mineral-Hg(II)-dissolved organic matter ternary system have shown that the coexistence of certain iron minerals and dissolved organic matter can promote the dark reduction of Hg(II) and increase the formation of zero-valent mercury. These studies reveal natural interface processes, but their direct application to contaminated soil remediation may pose a risk of promoting mercury volatilization and release. Therefore, materials used for site remediation should not aim to promote Hg(II) reduction, but rather should construct low-reducing mineral-organic matter interfaces capable of immobilizing Hg(II) and inhibiting the formation of zero-valent mercury.

[0007] Existing invention patents disclose soil conditioners for farmland contaminated with heavy metals, containing zeolite, biochar, iron-containing mineral particles, organic fertilizer, inorganic fertilizer, trace element supplements, and microbial agents. The iron-containing mineral particles may include ferrihydrite, magnetite, goethite, or hematite. This type of technology belongs to broad-spectrum farmland soil conditioning and nutrient reconstruction technologies. Its core is to reduce the availability of heavy metals and replenish soil nutrients. However, it does not address the material structure design for the risks of Hg-DOM migration and the dark formation of zero-valent mercury in contaminated soil, nor does it require the pre-composite formation of hematite with dissolved organic matter to form a low-release, low-electron-donating interface.

[0008] Another invention patent discloses a method for reducing mercury content in rice using binding inhibitors such as selenium salts, sulfur salts, iodine salts, ammonium salts, or EDTA. The core of this method is to utilize these substances to bond or complex with mercury, thereby inhibiting the absorption, translocation, and accumulation of mercury in rice. This type of technology primarily addresses rice cultivation and agricultural product safety issues, focusing on reducing mercury content within plants rather than synergistically controlling the risks of mercury migration and volatilization of zero-valent mercury from contaminated soil.

[0009] Therefore, there is an urgent need to develop a composite passivating agent and its preparation method suitable for contaminated soil, which can not only reduce the migration rate of mercury in the soil, but also inhibit the dark reduction of Hg(II) to zero-valent mercury, thereby reducing the risk of mercury release into groundwater and the atmosphere. Summary of the Invention

[0010] To address the shortcomings of existing technologies, the purpose of this application is to provide a composite passivating agent and its preparation method for reducing mercury migration and zero-valent mercury formation in contaminated soil. This composite passivating agent is not simply a mixture of hematite, fulvic acid, and other soil conditioners. Instead, it first uses an aqueous pre-composite process to fix fulvic acid onto the surface of the hematite, followed by solid-liquid separation and washing to remove unbound free fulvic acid, thereby forming a hematite-organic matter interface with low release and low electron-donating activity.

[0011] The objective of this invention is achieved through the following technical solution: In a first aspect, embodiments of the present invention provide a composite passivating agent for reducing the migration rate and zero-valent mercury formation rate in contaminated soil, comprising hematite and immobilized fulvic acid in a mass ratio of 100:(2-3); the immobilized fulvic acid is adsorbed, complexed, or coated onto the surface of hematite via an aqueous pre-composite method to obtain the composite passivating agent and form a hematite-organic matter composite interface; the composite passivating agent is used to adsorb and immobilize Hg(II) in contaminated soil, reduce the content of mobile mercury in the soil, and inhibit the reduction of Hg(II) to zero-valent mercury under dark conditions.

[0012] Furthermore, the hematite comprises α-Fe2O3, preferably artificially synthesized α-Fe2O3; the average particle size of the α-Fe2O3 is 50 nm-100 μm.

[0013] Furthermore, the immobilized fulvic acid is preferably Peat-derived Pahokee Peat Humic Acid (PPHA).

[0014] Furthermore, the composite passivating agent is obtained by adsorbing, complexing, or coating the immobilized fulvic acid onto the surface of hematite after aqueous pre-composite treatment, solid-liquid separation, washing, and drying; the washing step is used to remove unbound free fulvic acid, thereby reducing the release of soluble organic carbon into the soil pore water after application.

[0015] Furthermore, the composite passivating agent, when applied to mercury-contaminated soil, is characterized in that it increases the content of adsorbed mercury by 17%-29%, reduces the content of free mercury by 12%-23%, and significantly inhibits the formation rate of zero-valent mercury under dark conditions, with an inhibition degree of up to 70%.

[0016] Furthermore, the adsorbed mercury represents the mercury content adsorbed on hematite, the free mercury represents the furic acid DOM-bound mobile mercury content, and the zero-valent mercury generation rate is the amount of zero-valent mercury generated per unit mass in a light-proof, sealed system.

[0017] Secondly, the present invention also provides a method for preparing the composite passivating agent, comprising the following steps: (1) preparing an immobilized fulvic acid solution; (2) adding hematite to the prepared solution to form a hematite-fulvic acid mixed suspension; (3) adjusting the pH of the hematite-fulvic acid mixed suspension to 3.0~9.0, and stirring or shaking the suspension for 24 h under light-protected conditions; (4) separating the solid and liquid phases and washing the solid phase to remove unbound free fulvic acid.

[0018] Furthermore, the DOC concentration of the fulvic acid solution is 10-14 mg C / L, and the concentration of hematite added is 1.5-2.5 g / L.

[0019] Thirdly, the present invention also provides an application of the composite passivating agent in reducing the mercury migration rate and zero-valent mercury formation rate in contaminated soil. The application specifically involves adding the composite passivating agent to the mercury-contaminated soil and ensuring it is in full contact with the contaminated soil through stirring, tilling, in-situ mixing, injection, covering, or burying. The soil moisture content is adjusted or maintained at 40%-100% of the maximum water holding capacity of the soil, preferably 60%-90%, so that the composite passivating agent reacts with Hg(II) in the soil, thereby reducing the mercury migration rate and zero-valent mercury formation rate.

[0020] Furthermore, the mercury-contaminated site soil includes soil from mercury mining areas, soil from chlor-alkali industrial contaminated sites, soil from non-ferrous metal smelting sites, soil from mercury-containing waste storage sites, soil from decommissioned industrial sites, tailings, river and lake sediments, or groundwater-soil interface contaminated media.

[0021] The beneficial effects of this invention are as follows: This invention utilizes a hematite-immobilized fulvic acid composite passivation technology to reduce the content of mobile mercury in soil from the initial stage of risk management for contaminated soil sites, and inhibits the dark reduction conversion of Hg(II) to zero-valent mercury. Compared to traditional passivating agents that only focus on reducing the effective state of heavy metals, this invention can simultaneously reduce the risk of mercury migration into water bodies and volatilization into the atmosphere. It has advantages such as convenient implementation, wide availability of materials, good environmental compatibility, and applicability to in-situ risk management of contaminated soil sites. Attached Figure Description

[0022] To more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0023] Figure 1 This is a schematic diagram of the preparation process of the hematite-fulvic acid composite passivating agent in this invention; Figure 2 This invention describes the kinetics of zero-valent mercury formation in the hematite-fulvic acid composite system. Figure 3 This refers to the proportion of adsorbed Hg, dissolved Hg, and reduced elemental zero-valent mercury in the hematite-fulvic acid composite system of this invention. Detailed Implementation

[0024] Exemplary embodiments will now be described in detail. The embodiments described in the following exemplary embodiments do not represent all embodiments consistent with this application.

[0025] The terminology used in this application is for the purpose of describing particular embodiments only and is not intended to be limiting of the application. The singular forms “a,” “the,” and “the” used in this application and the appended claims are also intended to include the plural forms unless the context clearly indicates otherwise.

[0026] This invention provides a composite passivating agent for reducing the migration rate and zero-valent mercury formation rate in contaminated soil, comprising hematite and immobilized fulvic acid in a mass ratio of 100:(2-3). The immobilized fulvic acid is adsorbed, complexed, or coated onto the surface of hematite via an aqueous pre-composite method to obtain the composite passivating agent and form a hematite-organic matter composite interface. The composite passivating agent is used to adsorb and immobilize Hg(II) in contaminated soil, reduce the content of pore water mercury, leached mercury, and migratable mercury in the soil, and inhibit the reduction of Hg(II) to zero-valent mercury under dark conditions.

[0027] Specifically, the hematite is α-Fe2O3, preferably artificially synthesized α-Fe2O3; the average particle size of the α-Fe2O3 is 50 nm-100 μm.

[0028] Specifically, the immobilized fulvic acid is preferably Peat-derived Pahokee Peat Humic Acid (PPHA).

[0029] Specifically, the composite passivating agent is obtained by pre-compounding the immobilized fulvic acid in an aqueous phase, separating the solid and liquid phases, washing and drying, and then adsorbing, complexing or coating it onto the surface of hematite; the washing step is used to remove unbound free fulvic acid, thereby reducing the release of soluble organic carbon into the soil pore water after application.

[0030] Specifically, after the composite passivating agent is applied to mercury-contaminated soil, it increases the content of adsorbed mercury by 17%-29%, reduces the content of free mercury by 12%-23%, and significantly inhibits the formation rate of zero-valent mercury under dark conditions, with an inhibition degree of up to 70%.

[0031] Specifically, the adsorbed mercury represents the mercury content adsorbed on hematite, the free mercury represents the furic acid DOM-bound migratory mercury content, and the zero-valent mercury generation rate is the amount of zero-valent mercury generated per unit mass in a light-proof, sealed system.

[0032] The present invention also provides a method for preparing the composite passivating agent, comprising the following steps: (1) preparing an immobilized fulvic acid solution; (2) adding hematite to the prepared solution to form a hematite-fulvic acid mixed suspension; (3) adjusting the pH of the hematite-fulvic acid mixed suspension to 3.0~9.0, and stirring or shaking the suspension for 24 h under light-protected conditions; (4) separating the solid and liquid phases and washing the solid phase to remove unbound free fulvic acid.

[0033] Preferably, in this embodiment, the pH value is selected as 3.0, 5.0, 7.0 and 9.0.

[0034] Specifically, the DOC concentration of the fulvic acid solution is 10-14 mg C / L, and the concentration of hematite added is 1.5-2.5 g / L.

[0035] This invention also provides an application of the aforementioned composite passivating agent in reducing mercury migration and zero-valent mercury formation in contaminated soil, wherein the application specifically includes: (1) Detect the total mercury content, mobile mercury content, pore water mercury concentration, pH, organic matter content and dark generation potential of zero-valent mercury in the contaminated soil of the site; (2) Determine the dosage of the composite passivating agent based on the total mercury content, mobile mercury content, pore water mercury concentration and soil organic matter content measured in step (1); the dosage of the composite passivating agent is 0.5%-10% of the dry weight of the contaminated soil, preferably 1%-5%; wherein, when the total mercury content of the soil is high or the proportion of mobile mercury exceeds 20% of the total mercury content, the dosage of the composite passivating agent is increased (the application amount is preferably 3%-10%); when the degree of pollution is low, a lower dosage can be used (the application amount is preferably 0.5%-3%). (3) Add the composite passivating agent to the mercury-contaminated soil and make it fully contact the contaminated soil by stirring, tilling, in-situ mixing, injection, covering or burying. (4) Adjust or maintain the soil moisture content to 40%-100% of the maximum water holding capacity of the soil, preferably 60%-90%, so that the composite passivating agent can fully react with Hg(II), fulvic acid and pore water in the soil, thereby reducing the mercury migration rate and the zero-valent mercury formation rate; (5) After the reaction is completed, the concentrations of mobile mercury, pore water mercury, zero-valent mercury formation rate and / or zero-valent mercury volatilization flux are measured to evaluate the passivation effect.

[0036] The results show that the hematite-immobilized fulvic acid composite passivator provided by the present invention can promote the adsorption and immobilization of Hg(II) on the mineral surface and reduce the content of mobile Hg(II) in the system. At the same time, since the composite passivator weakens the fulvic acid-mediated Hg(II) reduction process, it significantly inhibits the conversion of Hg(II) to Hg(0), reduces the zero-valent mercury generation rate and volatilization risk, and achieves synergistic control of mercury migration and release.

[0037] It should be noted that, in this application, the amount of zero-valent mercury generated refers to the content of zero-valent mercury generated by the reduction of Hg(II) in a unit volume of the reaction system under light-proof or dark conditions, which can be evaluated by purging and collecting zero-valent mercury in a closed reactor. The mercury mobility can be evaluated by the content of adsorbed mercury and dissolved mercury.

[0038] It should be noted that the research experiments conducted by this invention have demonstrated that when hematite and fulvic acid act together on a system containing Hg(II), the following effects are mainly produced: 1) The active sites such as Fe-OH and Fe-O on the surface of hematite can undergo surface complexation and adsorption with Hg(II), causing Hg(II) to transfer from the aqueous phase or pore water to the solid phase surface, thereby reducing the proportion of free mercury and mobile mercury.

[0039] 2) The carboxyl, phenolic hydroxyl, carbonyl, aromatic structures, and sulfur-containing functional groups in immobilized fulvic acid can complex with Hg(II). This immobilized organic matter, together with the hematite surface sites, can enhance the immobilization effect of Hg(II) on the composite interface.

[0040] 3) By pre-compounding, washing and drying in aqueous phase, the release of free fulvic acid into soil pore water during application can be reduced, thus avoiding the free DOM complexing mercury and increasing its mobility.

[0041] 4) Hematite can fix some of the electron-donating active components in fulvic acid, reduce the electron donor capacity of the system that can participate in the reduction of Hg(II), thereby inhibiting the reduction of Hg(II) to volatile zero-valent mercury under dark conditions.

[0042] 5) Unlike some iron mineral-DOM systems that can promote the formation of zero-valent mercury, Hg in the hematite-immobilized fulvic acid system mainly exists in the adsorbed state, with low zero-valent mercury formation, making it more suitable for the synergistic control of mercury migration and zero-valent mercury volatilization in mercury-contaminated sites.

[0043] Example 1 Preparation of hematite-immobilized fulvic acid composite passivating agent like Figure 1 As shown, a pre-composite preparation experiment of hematite and fulvic acid was carried out using the composite passivating agent of the present invention. The specific process is as follows: Step 1: Preparation of hematite material. Take commercial hematite powder α-Fe2O3 and wash it three times with deionized water to remove surface impurities. Dry the washed hematite at 60 ℃, grind it, and pass it through a 100-mesh sieve for later use.

[0044] Step 2: Preparation of fulvic acid solutions with different pH values. Extract soluble organic matter from fulvic acid (PPHA) to prepare fulvic acid solutions with a DOC concentration of 12 mg C / L. Adjust the pH of the solutions to 3.0, 5.0, 7.0, and 9.0 using 0.1 mol / L HCl or NaOH.

[0045] Step 3, Pre-binding reaction. Add hematite to the fulvic acid solution to achieve a hematite concentration of 2 g / L. React under light-protected conditions at 25 °C with shaking for 24 h to allow fulvic acid to be fully adsorbed or bound to the hematite surface.

[0046] Step 4, Free DOM Removal. After the reaction is complete, the solid phase is collected by centrifugation and gently washed 2-3 times with deionized water to remove unbound free fulvic acid. This step is used to reduce the risk of releasing free DOM into pore water after the composite passivating agent is applied to the soil.

[0047] Example 2 Inhibitory effect of hematite-fulvic acid complex on the formation of zero-valent mercury under different pH conditions The inventors conducted an experiment to inhibit the formation of zero-valent mercury using the composite passivating agent of this invention. The specific procedure is as follows: Three systems were set up: fulvic acid-Hg, hematite-Hg, and hematite-fulvic acid-Hg. The concentrations of fulvic acid, initial Hg(II), mineral dosage, and reaction volume were kept consistent in each system. Furthermore, to investigate the effect of pH on the performance of the composite passivating agent of this invention, hematite-fulvic acid-Hg systems were set up under pH conditions of 3, 5, 7, and 9. The hematite-fulvic acid composite passivating agent was added to the simulated system containing Hg(II), and the pH of the system was adjusted to 3, 5, 7, and 9 using HCl or NaOH. The reaction was carried out at 25°C in the dark for 24 h. Samples were collected at different time points during the reaction to determine the amount of zero-valent mercury generated, adsorbed Hg, and free Hg.

[0048] like Figure 2 As shown, the experimental results indicate that the amount of zero-valent mercury formed in the fulvic acid-Hg system is relatively high, suggesting that free fulvic acid can participate in the dark reduction of Hg(II). The addition of hematite significantly altered the formation behavior of zero-valent mercury in the system. Specifically, the amount of zero-valent mercury in the hematite-Hg system decreased significantly with reaction time, from an initial 0.58 μg / L to approximately 0.04 μg / L. The amount of zero-valent mercury formed in the hematite-fulvic acid-Hg system reached its lowest level at the end of the reaction, significantly lower than that in the fulvic acid-Hg system. Furthermore, the hematite-fulvic acid composite passivator inhibited the formation of zero-valent mercury within the pH range of 3-9. The inhibition effect was particularly significant under weakly acidic to neutral conditions.

[0049] The results show that the hematite-immobilized fulvic acid composite system can significantly inhibit the reduction of Hg(II) to zero-valent mercury under dark conditions, thus reducing the risk of mercury volatilization and release. This composite passivating agent is suitable for contaminated soils under different pH conditions, and is particularly suitable for the remediation and risk management of weakly acidic to neutral mercury-contaminated soils.

[0050] Example 3 Effect of hematite-fulvic acid complex on Hg speciation Based on Example 2, the speciation of Hg in different systems was further determined. Hg in the reaction system was classified into adsorbed Hg, free Hg, and zero-valent mercury. Adsorbed Hg refers to Hg fixed by minerals or composite passivating agents, free Hg refers to migratable Hg remaining in the aqueous phase, and zero-valent mercury refers to volatile mercury generated during reduction.

[0051] like Figure 3 As shown, the experimental results indicate that in the hematite-fulvic acid-Hg system, Hg mainly exists in the adsorbed form, with adsorbed Hg accounting for the highest proportion of total Hg, while free Hg and zero-valent mercury account for a relatively low proportion. In contrast, the proportion of zero-valent mercury is higher in the fulvic acid-Hg system; in other iron mineral-fulvic acid-Hg systems, the distribution behavior of Hg differs from that in the hematite system, with the proportion of zero-valent mercury increasing in some systems.

[0052] The results show that the hematite-fulvic acid composite passivator can mainly fix Hg in the adsorbed state, thereby reducing both the migration risk of free Hg and the volatilization risk of zero-valent mercury.

[0053] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments, and various changes and modifications can be made without departing from the spirit and scope of the invention; all such changes and modifications fall within the scope of protection claimed by the present invention.

Claims

1. A composite passivating agent for reducing mercury migration and zero-valent mercury formation rates in contaminated soil, characterized in that, The mixture comprises hematite and immobilized fulvic acid in a mass ratio of 100:(2-3). The immobilized fulvic acid is adsorbed, complexed, or coated onto the surface of hematite through an aqueous pre-composite method to obtain a composite passivating agent and form a hematite-organic matter composite interface. The composite passivating agent is used to adsorb and fix Hg(II) in the contaminated soil of the site, reduce the content of mobile mercury in the soil, and inhibit the reduction of Hg(II) to zero-valent mercury under dark conditions.

2. The composite passivating agent according to claim 1, characterized in that, The hematite is α-Fe2O3, preferably artificially synthesized α-Fe2O3; the average particle size of the α-Fe2O3 is 50 nm-100 μm.

3. The composite passivating agent according to claim 1, characterized in that, The immobilized fulvic acid is preferably fulvic acid (PPHA) derived from Pahokee Peat Humic Acid.

4. The composite passivating agent according to claim 1, characterized in that, The composite passivating agent is obtained by pre-compounding the immobilized fulvic acid in an aqueous phase, separating the solid and liquid phases, washing and drying, and then adsorbing, complexing or coating it onto the surface of hematite; the washing step is used to remove unbound free fulvic acid, thereby reducing the release of soluble organic carbon into the soil pore water after application.

5. The composite passivating agent according to claim 1, wherein after the composite passivating agent is applied to mercury-contaminated soil, it is characterized in that, It increases the content of adsorbed mercury by 17%-29%, reduces the content of free mercury by 12%-23%, and significantly inhibits the formation rate of zero-valent mercury under dark conditions, with an inhibition degree of up to 70%.

6. The composite passivating agent according to claim 5, characterized in that, The adsorbed mercury represents the mercury content adsorbed on hematite, the free mercury represents the furic acid DOM-bound migratory mercury content, and the zero-valent mercury generation rate is the amount of zero-valent mercury generated per unit mass in a light-proof, sealed system.

7. A method for preparing the composite passivating agent according to any one of claims 1-6, characterized in that, Includes the following steps: (1) Prepare an immobilized fulvic acid solution; (2) Add hematite to the prepared solution to form a hematite-fulvic acid mixed suspension; (3) Adjust the pH of the hematite-fulvic acid mixed suspension to 3.0~9.0 and stir or shake the mixture for 24 h under light-protected conditions; (4) Separate the solid and liquid phases and wash the solid phase to remove unbound free fulvic acid.

8. The preparation method according to claim 7, characterized in that, The DOC concentration of the fulvic acid solution is 10-14 mg C / L, and the concentration of hematite added is 1.5-2.5 g / L.

9. The application of the composite passivating agent according to any one of claims 1-6 in reducing the mercury migration rate and zero-valent mercury formation rate in contaminated soil, characterized in that, The specific application involves adding the composite passivating agent to the soil of a mercury-contaminated site, and then ensuring that the composite passivating agent is in full contact with the contaminated soil through stirring, tilling, in-situ mixing, injection, covering, or burying. The soil moisture content is adjusted or maintained at 40%-100% of the maximum water holding capacity of the soil, preferably 60%-90%, so that the composite passivating agent reacts with Hg(II) in the soil, thereby reducing the mercury migration rate and the zero-valent mercury formation rate.

10. The method according to claim 9, characterized in that, The mercury-contaminated soil includes soil from mercury mining areas, soil from chlor-alkali industrial sites, soil from non-ferrous metal smelting sites, soil from mercury-containing waste storage sites, soil from decommissioned industrial sites, tailings, river and lake sediments, or groundwater-soil interface contaminants.

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