Application of silicon-based amorphous calcium carbonate in synchronous removal of cadmium in water and soil

Abstract

The application provides application of silicon-based amorphous calcium carbonate in synchronous removal of cadmium in water bodies and soil, and removal rate of Si-ACC on cadmium in water bodies is determined according to kinetic adsorption and isothermal adsorption experiments. The Si-ACC provided by the application has a new use in synchronous control of cadmium pollution in water bodies and soil, realizes "one dose for two purposes", can treat water and soil, the adsorption amount of the Si-ACC on water body Cd(II) is greater than or equal to 111 mg / g within 5 min, and the removal rate is greater than or equal to 99%; under the condition of 20 g / kg dose, the water-soluble Cd of soil is approximately equal to 0 after 7 days, the exchangeable state decreases by greater than or equal to 34%, and the carbonate combined state increases by greater than or equal to 110%; compared with the MICP process which needs 12 rounds of grouting and a total of 36-60 days, the Si-ACC process is shortened by more than 80%.

Description

Technical Field

[0001] This invention belongs to the field of heavy metal water and soil pollution control technology, specifically involving the application of silicon-based amorphous calcium carbonate in the simultaneous removal of cadmium from water and soil. Background Technology

[0002] Lead-zinc mining, the discharge of wastewater, sewage sludge for agricultural use, and the improper disposal of cadmium-containing waste have resulted in cadmium levels in soil and water bodies in some areas far exceeding the soil's environmental carrying capacity, posing a threat to plants and animals and seriously affecting human health. Current remediation methods for cadmium-contaminated soil include physical / chemical remediation and microbial remediation. However, all these methods suffer from drawbacks such as high investment costs, complex equipment, and significant environmental disturbance, limiting their application in cadmium-contaminated soil. While phytoremediation is used to treat cadmium-contaminated soil, methods for treating cadmium migrating from the soil into plants are not provided. Improper or untreated plant debris can easily cause secondary soil pollution.

[0003] Treatment methods for cadmium-containing wastewater depend on the concentration and form of cadmium ions. Currently, commonly used methods can be categorized into three main types: chemical methods (such as neutralization precipitation and reduction methods), physicochemical methods, and biological methods. Chemical methods are suitable for treating large-scale, high-concentration cadmium-containing wastewater. Physicochemical methods (such as ion exchange and membrane separation methods) can achieve resource recovery while treating heavy metal cadmium-containing wastewater. Biological methods have the advantage of being green and pollution-free, but their components are complex, and they are mostly in the research and promotion stage. Furthermore, existing cadmium-containing wastewater treatment methods often suffer from long treatment times and low removal rates. Existing technology CN113896302A describes a method for treating cadmium-containing wastewater using calcium carbonate and a method for recovering cadmium ions, where 2 g / L of amorphous calcium carbonate treats 1000 mg / L of Cd. 2+ Solution, Cd removed after 2 days 2+ The efficiency is over 99.99%. Treating a 10 mg / L Cd solution with 1 g / L amorphous calcium carbonate, achieving a Cd removal efficiency of over 99.69%, also requires 20 minutes.

[0004] One of the existing technologies (Biogenic calcium improved Cd) 2+ and Pb 2+The immobilization of soil using ureolytic bacteria *Bacillus pasteurii* (Zhang L, Wang W, Yue C, et al. *Science of the Total Environment*, 2024, 921: 171060.) employs bio-derived calcium carbonate (eggshell / oyster shell powder) combined with microbial induced calcium carbonate precipitation (MICP) technology, which can increase the increase of carbonate-bound Cd in soil by 118% (the same level as in this invention). However, it is time-consuming (12 rounds, each round consisting of 3 days of curing + 1 day of static setting), with a total cycle of 48 days. If the bacterial culture for expansion is included, the cycle is greater than 60 days.

[0005] Furthermore, existing substances that can remove cadmium either only remove cadmium from solutions (water bodies) or only remove cadmium from soil. Current technologies do not have research on the simultaneous removal of cadmium from water and soil, and existing cadmium removal and soil remediation methods are relatively slow and inefficient, and generally pose a high risk of secondary pollution.

[0006] Therefore, it is of great significance to study a technology for the simultaneous, efficient and rapid removal of cadmium from water and soil. Summary of the Invention

[0007] The present invention aims to provide the use of silicon-based amorphous calcium carbonate in the simultaneous and rapid removal of cadmium from water and soil, overcoming the shortcomings of the prior art.

[0008] Specifically, this invention provides an application of silicon-based amorphous calcium carbonate in the simultaneous removal of cadmium from water and soil.

[0009] Furthermore, the removal rate of cadmium in water by Si-ACC (silicon-based amorphous calcium carbonate) was determined based on kinetic adsorption and isothermal adsorption experiments.

[0010] Furthermore, the kinetic adsorption experimental steps are as follows: Step 1: Add 0.5~3 g of Si-ACC to 800~1200 mL of CdCl2·5H2O solution; Step 2: Periodically collect subsamples from the mother liquor; Step 3: Subsample centrifugation (4000 RPM, 1 min), the supernatant is filtered through a 0.45 μm filter membrane; Step 4: After the filtered supernatant is acidified with 2%~3% HNO3, the concentrations of Cd, Ca, Si and P in the supernatant are determined by inductively coupled plasma optical emission spectrometry (ICP-OES).

[0011] Furthermore, in the first step, the initial pH of the CdCl2·5H2O solution was 4-6, and the initial cadmium concentration was 80-120 mg / L; in the second step, subsamples were collected at 5, 10, 20, 30, 40, and 50 min after the start of the experiment.

[0012] Furthermore, the isothermal adsorption experiment steps are as follows: Step 1: Add 0.5~3 g of Si-ACC to 1000 mL of CdCl2·5H2O solutions of different concentrations; Step 2: Collect subsamples from mother liquors with different CdCl2·5H2O concentrations after 60 minutes of the experiment; Step 3: Subsample centrifugation (4000 RPM, 1 min), supernatant filtered through a 0.45 μm filter membrane; Step 4: After the filtered supernatant is acidified with 2%~3 vol% HNO3, the concentrations of Cd, Ca, Si and P in the supernatant are determined by inductively coupled plasma optical emission spectrometry (ICP-OES).

[0013] Furthermore, in step 2, the concentrations of different CdCl2·5H2O were set to 0, 200, 400, 600, and 800 mg / L, respectively.

[0014] Furthermore, the application steps for removing cadmium from soil using silicon-based amorphous calcium carbonate are as follows: Step S1: Prepare cadmium-contaminated soil; Step S2: Add Si-ACC (silicon-based amorphous calcium carbonate) to the cadmium-contaminated soil.

[0015] Furthermore, the specific steps of step S1 are as follows: S11: Cadmium chloride (CdCl2) was selected as the pollution source and added to the soil at a ratio of 50 mg / kg to prepare cadmium-contaminated soil. S12: The water holding capacity of cadmium-contaminated soil remains at 80%; S13: Place in a constant temperature incubator (25℃) for aging for one week, stirring once a day to obtain cadmium-contaminated soil.

[0016] Furthermore, the specific steps of step S2 are as follows: S21: Add Si-ACC at a ratio of 20 g / kg to cadmium-contaminated soil and continue to maintain the soil water holding capacity at 80%; S22: After stirring evenly, continue to incubate in a constant temperature incubator at 25℃ for one week, stirring once a day, so that Si-ACC is evenly mixed in cadmium-contaminated soil. According to the Tessier fractional extraction method, different forms of cadmium are extracted from the treated soil sample stepwise, and the concentration of different forms of Cd in the extract is determined by inductively coupled plasma mass spectrometry (ICP-MS).

[0017] Furthermore, the soil samples were dried in an oven at 50°C after cultivation, and then ground through a 100-mesh sieve. The ground soil samples were then stored in a dry and cool environment for later use.

[0018] Furthermore, the atomic molar ratio of Ca:Si in Si-ACC is ≥0.5.

[0019] Compared with the prior art, the present invention has the following outstanding features and advantages: 1. This invention provides a novel application of silicon-based amorphous calcium carbonate (Si-ACC) in the simultaneous control of cadmium pollution in water and soil, achieving "two uses in one agent"; 2. This invention breaks through the kinetic bottleneck: the adsorption capacity of Cd(II) in water is ≥111mg / g within 5min, the removal rate is ≥99%, and the calcium and silicon contained in silicon-based amorphous calcium carbonate are common elements in nature, with extremely low risk of secondary pollution; 3. This invention overcomes the capacity bottleneck: at a dosage of 20g / kg, the water-soluble Cd in the soil decreases below the detection limit within 7 days, the exchangeable Cd decreases by ≥34%, and the carbonate-bound Cd increases by ≥110%; 4. This invention uses 1 g / L silica-based amorphous calcium carbonate to treat 100 mg / L Cd. 2+ Solution to remove Cd 2+ With an efficiency of over 99.99%, it only takes 5 minutes; 5. This invention achieves an increase of ≥110% in carbonate-bound state within 7 days. Figure 5 As shown in F3, the time required for the Microbial Induced Calcium Carbonate (MICP) process is more than 80% shorter than that of the Microbial Induced Calcium Carbonate Precipitation (MICP) process, which requires 12 rounds of grouting and a total of 36–60 days. Attached Figure Description

[0020] Figure 1 The adsorption kinetics curves (A) of Si-ACC-0 and Si-ACC-1 for cadmium in Example 1 and Comparative Example 1 and the changes in solution pH (B) are shown. Figure 2 The release of calcium, silicon, and phosphorus during cadmium fixation using Si-ACC-0 (A) and Si-ACC-1 (B) in Example 1 and Comparative Example 1 is shown. Figure 3The adsorption isotherms (A) and pH changes (B) of cadmium by Si-ACC-0 and Si-ACC-1 in Example 2 and Comparative Example 2 are shown. Figure 4 The release of calcium, silicon and phosphorus when different concentrations of cadmium were fixed in Si-ACC-0 (A) and Si-ACC-1 (B) in Example 2 and Comparative Example 2; Figure 5 The changes in cadmium speciation and pH value in cadmium-contaminated soils of Example 3 and Comparative Example 3 after treatment with Si-ACC-0 and Si-ACC-1 are shown. Detailed Implementation

[0021] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. The technical solutions of the present invention will be further described below with reference to implementation examples.

[0022] Si-ACC-0 represents calcite; Si-ACC-1 represents silicon-based amorphous calcium carbonate with Ca:Si = 1:1 (atomic molar ratio).

[0023] Example 1 This embodiment provides an application of silicon-based amorphous calcium carbonate in the removal of cadmium from water. The specific steps are as follows: Step 1: Add 1.0 g of Si-ACC-1 to 1000 mL of CdCl2·5H2O solution (initial pH 5.0, initial cadmium concentration 100 mg / L); Step 2: Subsamples were collected at 5, 10, 20, 30, 40, and 50 minutes after the start of the experiment. Step 3: Subsample centrifugation (4000 RPM, 1 min), the supernatant is filtered through a 0.45 μm filter membrane; Step 4: After the filtered supernatant is acidified with 3% HNO3, the concentrations of Cd, Ca, Si and P in the supernatant are determined by inductively coupled plasma optical emission spectrometry (ICP-OES).

[0024] Comparative Example 1 This comparative example provides an application of calcite (Si-ACC-0) in the removal of cadmium from water. The specific steps are as described in Example 1. The difference between this comparative example and Example 1 is that Si-ACC-0 is used to replace Si-ACC-1 in Example 1.

[0025] like Figure 1As shown, based on the adsorption kinetic curve ( Figure 1 A) and the adsorption kinetic parameters (Table 1) show that Si-ACC-1 can fix cadmium (Qe, exp) up to 111.12 mg / g, while Si-ACC-0 can fix cadmium only 9.49 mg / g.

[0026] Table 1 ; The cadmium fixation capacity of Si-ACC-1 is 11.8 times that of Si-ACC-0. Si-ACC-1 exhibits higher cadmium removal efficiency, reaching equilibrium in just 5 minutes with a fixation capacity of 111.01 mg / g, close to 99.9% of its equilibrium fixation capacity. In contrast, Si-ACC-0 fixes only 2.94 mg / g of cadmium in the same timeframe, representing 30.98% of its equilibrium fixation capacity. The fixation capacity of Si-ACC-0 increases slowly and continuously over 60 minutes, from 2.94 mg / g at 5 minutes to 9.49 mg / g at 60 minutes.

[0027] Figure 1 In Table 1, Qt represents the adsorption amount at that time (X-axis), and Qe represents the maximum adsorption amount derived from Qt.

[0028] The change in solution pH is mainly driven by the dissolution of CaCO3. Both Si-ACC-0 and Si-ACC-1 contain a large amount of CaCO3. When these solutions come into contact with a CdCl2 solution with an initial pH of acidic (pH 5), the CaCO3 dissolves and releases HCO3-. - and CO3 2- These ions can neutralize hydrogen ions in the solution, causing the solution pH to rise. Among carbonate minerals, calcite has the lowest solubility, while amorphous calcium carbonate (ACC) has the highest solubility; this characteristic is one of the main reasons for the pH changes in the solutions of different experimental groups. As the OH- ions in the solution increase... - As the concentration increases, the pH of the solution continues to rise. This is because, at the same temperature (25℃), the solubility product of CdCO3 (5.2 × 10⁻⁶) is significantly higher. -12 () is less than CaCO3 (2.8 × 10) -9 Therefore, theoretically, CdCO3 precipitate will also form in CaCO3 solution, which will help to further remove Cd from the solution. 2+ Meanwhile, the solubility of Cd(OH)₂(S) reaches its minimum at pH 9.84–13.31, and the solubility product of Cd(OH)₂ (2.5 × 10⁻⁶) is [missing information]. -14 The solubility product of Ca(OH)₂ is 5.5 × 10⁻⁶. -6 The small value means that under high pH conditions, Cd...2+ Compared to Ca 2+ More likely to preferentially react with OH in the solution - They combine to form Cd(OH)2 precipitate. Therefore, Cd(OH)2 should also be present during the Cd removal process of Si-ACC-1.

[0029] like Figure 2 As shown, since the initial cadmium solution is weakly acidic, Si-ACC-0 (A) and Si-ACC-1 (B) materials partially dissolve upon contact with the cadmium solution, releasing Ca, Si, and P. Si-ACC-0, due to its higher crystallinity and stability, dissolves more slowly, releasing less Ca. 2+ At least, at 60 min, Ca 2+ The highest concentration was found in [specific concentration not specified], at 6.79 mg / L. Si-ACC-1 released the most Ca (73.57–86.83 mg / L) and Si (27.92–60.49 mg / L). Among these, Si release showed a pattern of initial increase followed by stabilization. However, Ca... 2+ The release trend showed an initial increase followed by a decrease, which was closely related to the pH change trend. Meanwhile, the cadmium removal rate did not change significantly. It is speculated that this might be due to Ca... 2+ With CO3 - The recombination formed CaCO3 precipitate, causing Ca in the solution to... 2+ With CO3 - Concentration decreases, H + The concentration increases relatively, which leads to a decrease in pH value.

[0030] Example 2 This embodiment provides an application of silicon-based amorphous calcium carbonate in the removal of cadmium from water. The specific steps are as follows: Step 1: Add 1.0 g of Si-ACC-1 to 1000 mL of CdCl2·5H2O solution (CdCl2·5H2O concentration set at 0, 200, 400, 600, and 800 mg / L respectively); Step 2: Collect subsamples from mother liquors with different CdCl2·5H2O concentrations after 60 minutes of the experiment; Step 3: Subsample centrifugation (4000 RPM, 1 min), supernatant filtered through a 0.45 μm filter membrane; Step 4: After the filtered supernatant is acidified with 3V% HNO3, the concentrations of Cd, Ca, Si and P in the supernatant are determined by inductively coupled plasma optical emission spectrometry (ICP-OES).

[0031] Comparative Example 2 This comparative example provides an application of calcite (Si-ACC-0) in the removal of cadmium from water. The specific steps are as described in Example 2. The difference between this comparative example and Example 2 is that Si-ACC-0 is used to replace Si-ACC-1 in Example 2.

[0032] Detailed procedures for determining the concentrations of Cd, Ca, Si, and P in the supernatant using inductively coupled plasma optical emission spectrometry (ICP-OES) in Examples 1-2 and Comparative Examples 1-2: The Cd concentration (supernatant) during the Si-ACC Cd fixation process was determined using inductively coupled plasma optical emission spectrometry (Varian Vista PRO, Australia). Test conditions: high-frequency generator power 0.9 kW, atomization flow rate 1.3 L / min, plasma cooling gas flow rate 15.0 L / min, auxiliary gas flow rate also 15.0 L / min, integration time 5 s.

[0033] like Figure 3 As shown, the removal capacity of cadmium by Si-ACC-1 exhibits a continuous increasing trend with the increase of equilibrium concentration, rising from 12.07 mg / g (Cd equilibrium concentration of 0) to 463.33 mg / g (Cd equilibrium concentration of 525.65 mg / L). When the Cd equilibrium concentration increases from a low 0.02 mg / L to 28.00 mg / L, the removal capacity of cadmium by Si-ACC-0 also rapidly increases from 5.88 mg / g to 28.26 mg / g. However, when the Cd equilibrium concentration is higher, the removal capacity of cadmium by Si-ACC-0 decreases, dropping to 7.88 mg / g at a Cd equilibrium concentration of 646.88 mg / L. This indicates that when the initial Cd concentration is too low (≤ 100 mg / L), Si-ACC-1 can almost completely remove cadmium from the solution, bringing the cadmium equilibrium concentration close to 0 mg / L. As for Si-ACC-0, when the initial Cd concentration is < 50 mg / L, its removal of cadmium has not reached saturation, demonstrating its effective removal capability for low concentrations of cadmium.

[0034] In a cadmium solution with an initial pH of 5.0, cadmium mainly exists as Cd. 2+ It exists in the form of cadmium. Based on the relationship between cadmium equilibrium concentration and equilibrium pH (… Figure 3 (B) In all experimental groups, the equilibrium pH initially decreased rapidly and then decreased slowly with increasing cadmium equilibrium concentration. Specifically, when using Si-ACC-1 to remove cadmium, if the initial Cd concentration was ≤ 300 mg / L, the equilibrium pH was high (pH > 8.7), at which point the Cd equilibrium concentration was extremely low, and the removal rate could reach 100%. It is speculated that in this case, Si-ACC-1 mainly removes cadmium by dissolving CaCO3 to increase the pH of the solution, thereby promoting Cd removal.2+ With OH - Cadmium removal is achieved by combining the reaction of Si-ACC-1 with the formation of Cd(OH)₂ precipitate. When the initial Cd concentration is in the range of 650–1000 mg / L, the equilibrium pH of the solution after the reaction with cadmium is close to neutral. When Si-ACC-0 is used to treat cadmium solutions with lower concentrations (initial concentration ≤ 20 mg / L), the pH of the solution after the reaction is alkaline (9.5–10.0); while when the initial Cd concentration exceeds 20 mg / kg, the equilibrium pH is neutral or acidic (6.3–7.0). These results once again confirm that adjusting the pH is one of the effective means to improve cadmium removal.

[0035] The removal mechanisms of cadmium by calcite mainly include ion exchange, surface coordination adsorption, and surface precipitation. For example... Figure 4 As shown, by comparing the release of Ca, Si, and P under different cadmium equilibrium concentrations, it can be found that with the increase of cadmium equilibrium concentration, the release of Ca in the Si-ACC-1 supernatant increases. 2+ The concentrations of all three elements showed an increasing trend, which is exactly the opposite of the trend of equilibrium pH. 2+ The change curve can also be divided into two stages: when the equilibrium pH is alkaline and when the equilibrium pH is neutral or acidic. When the cadmium equilibrium concentration is low, the equilibrium pH of the solution is high, and the Ca in the solution... 2+ The levels are low and relatively stable, indicating that the OH- concentration in the solution is low and relatively stable. - More, Cd 2+ More prone to reacting with OH - Cadmium is removed by combining with the precipitate formed from Cd(OH)₂. As the equilibrium concentration of cadmium increases, the Ca in the supernatant... 2+ The concentration rises rapidly, while the solution pH remains neutral. At this point, the precipitation effect of Cd(OH)2 is weak, and cadmium removal likely relies primarily on the displacement of Ca from CaCO3. 2+ To remove Cd from the solution by forming CdCO3 precipitate. 2+ Unlike Ca, as the equilibrium concentration of Cd increases, Si released into the solution tends to revert to a solid form. This means that when treating high-concentration cadmium solutions, Si may participate in the formation of Si-containing solid precipitates, which could affect the cadmium removal efficiency.

[0036] Example 3 This embodiment provides an application of silicon-based amorphous calcium carbonate in the removal of cadmium from soil. The specific steps are as follows: S11: Cadmium chloride was selected as the pollution source and added to the soil at a ratio of 50 mg / kg to prepare cadmium-contaminated soil. S12: The water holding capacity of cadmium-contaminated soil remains at 80%; S13: Place in a constant temperature incubator (25℃) for aging for one week, stirring once a day to obtain cadmium-contaminated soil; S14: Add Si-ACC-1 at a ratio of 20 g / kg to cadmium-contaminated soil and continue to maintain the soil moisture content at 80%; S15: After stirring evenly, continue to incubate in a constant temperature incubator at 25℃ for one week, stirring once a day. According to the Tessier fractional extraction method, different forms of cadmium were extracted from the treated soil samples stepwise, and the concentration of different forms of Cd in the extract was determined by inductively coupled plasma mass spectrometry (ICP-MS).

[0037] Comparative Example 3 This comparative example provides an application of calcite (Si-ACC-0) in the removal of cadmium from soil. The specific steps are as described in Example 3. The difference between this comparative example and Example 3 is that Si-ACC-0 is used to replace Si-ACC-1 in Example 3.

[0038] Following the Tessier fractional extraction method (China Geological Survey, 2005; Tessier et al., 2002), different forms of cadmium were extracted from treated soil samples in a stepwise manner, mainly including water-soluble (F1), exchangeable (F2), carbonate-bound (F3), iron-manganese oxide-bound (F4), strongly organic-bound (F5), and residual (F6) forms. After extraction, the concentration of different forms of Cd in the extract was determined by inductively coupled plasma mass spectrometry (ICP-MS). The test conditions for ICP-MS (Thermo Scientific iCAP RQ, USA) were as follows: plasma cooling gas flow rate of 14.0 L / min, auxiliary gas flow rate of 0.8 L / min, nebulizer flow rate of 1.0 L / min, nebulizer gas pressure of 3.18 bar, sampling time of 60 s, and residence time of 0.02 s. To ensure that the sample meets the instrument's optimal testing accuracy requirements, the supernatant is filtered through a filter membrane and then diluted with dilute nitric acid solution (2-3 vol%).

[0039] Comparative Example 4 The soil used in this comparative example was uncontaminated farmland soil from Hefei City, Anhui Province, with a pH of 7.76. After contamination with cadmium chloride, the cadmium content in the soil was 9 mg / kg. 150 g of contaminated soil was taken and thoroughly mixed with a 10% suspension of *Bacillus pasteurellii* (total bacterial count 2.5 × 10⁸ cells / mL), and then 2% (by mass) of eggshell or oyster shell powder (purchased from a farmers' market in Hefei City) was added. After 35 days of treatment under flooded conditions (through 12 rounds of grouting), different forms of cadmium were extracted from the soil samples using the Tessier fractional extraction method. The cadmium content was analyzed using inductively coupled plasma atomic emission spectrometry (ICP-AES). After 35 days, *Bacillus pasteurellii* + eggshell powder and *Bacillus pasteurellii* + oyster shell powder increased the carbonate-bound cadmium content in the soil by 114.72% and 118.81%, respectively.

[0040] As can be seen from Example 3 and Comparative Example 4, the present invention achieves an increase of ≥110% in carbonate-bound state within 7 days, which is more than 80% shorter than the MIP process, which requires 12 rounds of grouting and a total of 36 days.

[0041] Figure 5 This shows the content and distribution of different forms of cadmium in soil after CdCl2 pollution. For example... Figure 5 As shown in Figure A, the soil pH of the control group (CK) without soil remediation treatment was 8.5. The concentrations of various forms of cadmium, from highest to lowest, were as follows: exchangeable cadmium (F2), 17.78 mg / kg (40.8%) > carbonate-bound cadmium (F3), 9.29 mg / kg (21.4%) > organic matter-bound cadmium (F5), 7.32 mg / L (16.8%) > iron-manganese oxide-bound cadmium (F4), 7.18 mg / kg (16.5%) > residual cadmium (F6), 1.92 mg / L (4.4%) > water-soluble cadmium (F1), 0.04 mg / kg (less than 0.1%). The order of the proportions of exchangeable, carbonate-bound, iron-manganese oxide-bound, and residual cadmium was similar to that observed in other studies of artificially contaminated cadmium (CdCl2) soils. However, the content of organically bound cadmium was significantly higher in this study, which may be due to the higher organic matter content of the soil samples collected in this study. This result reveals that in this study, exchangeable cadmium was the most prevalent form of cadmium in the soil contaminated with CdCl2. This is because CdCl2, as a highly soluble water-soluble cadmium source, can significantly increase the content of exchangeable cadmium in the soil upon its addition.

[0042] For water-soluble cadmium (F1), the content in the soil after Si-ACC-1 treatment was close to zero. The content of exchangeable cadmium (F2) decreased significantly, to 11.60 mg / kg, a decrease of 34.7%. The content of carbonate-bound cadmium (F3) increased significantly, rising to 19.53 mg / kg, an increase of 110.2%. The content of iron-manganese oxide-bound cadmium (F4) also increased slightly, reaching 9.32 mg / kg, an increase of 29.8%. The content of organically bound cadmium (F5) decreased to 4.27 mg / kg, a decrease of 41.7%. The content of residual cadmium (F6) was 0.79 mg / kg, a decrease of 58.9%. In contrast, compared with the control group, the content of carbonate-bound cadmium in the soil after Si-ACC-0 treatment increased to 10.53 mg / kg, an increase of 13.3%. However, the increase in exchangeable cadmium content was more significant, rising to 20.64 mg / kg, an increase of 16.1%. Meanwhile, the contents of iron-manganese oxide-bound cadmium, organically bound cadmium, and residual cadmium decreased to 5.93 and 5.66 mg / kg, respectively, representing decreases of 17.5%, 22.6%, and 23.3%. These results indicate that Si-ACC-1 can effectively convert exchangeable, organically bound, and residual cadmium in the soil into more stable carbonate-bound and iron-manganese oxide-bound cadmium. Si-ACC-0, on the other hand, promoted the conversion of iron-manganese oxide-bound, organically bound, and residual cadmium into exchangeable and carbonate-bound cadmium. Furthermore, the changes in the contents of exchangeable and carbonate-bound cadmium were most significant among all soil experimental groups.

[0043] Among all cadmium forms, water-soluble cadmium and exchangeable cadmium have the highest mobility and bioavailability. They have weak binding ability with other solid adsorption media and are easily converted into ionic cadmium, which is then absorbed and utilized by plants. These are the main forms of cadmium pollution in soil, hence they are also called bioavailable cadmium. Experimental results show that Si-ACC-1 can effectively reduce the content of exchangeable cadmium in Cd-contaminated soil. pH, as one of the most important parameters in soil physicochemical properties, is a key factor affecting the adsorption and desorption of heavy metals in soil; its changes can alter the form of heavy metals in the soil. Carbonate-bound cadmium is very sensitive to pH changes. After Si-ACC-1 treatment, the soil pH rose to 9.2, higher than the pH of the CK group (8.5). However, after treatment with crystalline calcite (Si-ACC-0), the soil pH (8.6) did not change significantly, and the content of carbonate-bound cadmium only increased by 13.3%. In summary, Si-ACC-1 releases Ca through dissolution. 2+ and CO3 2- On the one hand, it alters the distribution of cadmium speciation in the soil by adjusting soil pH; on the other hand, it interacts with Cd... 2+The reaction produces CdCO3, converting some exchangeable cadmium in the soil into carbonate-bound cadmium. These effects reduce the bioavailability of Cd in the soil. In contrast, crystalline calcite (Si-ACC-0) has a smaller effect on the changes in cadmium speciation in the soil, but it slightly increases the content of exchangeable cadmium. Previous studies have shown that Cd... 2+ and Ca 2+ Its charge and ionic radius are similar to those of Cd. 2+ In comparison, Ca 2+ It has a higher soil affinity. Therefore, after Si-ACC-0 enters the soil, it dissolves and releases Ca. 2+ Will with Cd 2+ Competition for adsorption sites on soil colloid surfaces leads to the exchange of Cd in the soil solid phase into the soil solution, resulting in an increase in the content of exchangeable Cd in the soil under constant soil pH.

[0044] In summary, Si-ACC-1 is highly effective in reducing the content of exchangeable cadmium in soil and promoting the conversion of cadmium to carbonate-bound form. This is mainly because Si-ACC-1 can dissolve and release Ca into the soil. 2+ and CO3 2- By adjusting soil pH and generating CdCO3, the bioavailability of Cd is reduced by altering its speciation. While Si-ACC-0 treatment of Cd-contaminated soil increases the content of carbonate-bound cadmium, it also increases the content of exchangeable cadmium, which may pose a challenge to the immobilization and stability of cadmium in the soil.

[0045] The embodiments described above are merely examples of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention.

Claims

1. The application of silicon-based amorphous calcium carbonate in the removal of cadmium from soil, characterized in that, The silicon-based amorphous calcium carbonate is a silicon-based amorphous calcium carbonate with a Ca:Si atomic molar ratio of 1:1; the application steps of silicon-based amorphous calcium carbonate for removing cadmium from soil are as follows: Step S1: Prepare cadmium-contaminated soil; Step S2: Add Si-ACC to cadmium-contaminated soil; The specific steps of step S1 are as follows: S11: Cadmium chloride was selected as the pollution source and added to the soil at a ratio of 50 mg / kg to prepare cadmium-contaminated soil. S12: The water holding capacity of cadmium-contaminated soil remains at 80%; S13: Place in a constant temperature incubator for aging for one week, stirring once a day, to obtain cadmium-contaminated soil.

2. The application according to claim 1, characterized in that, The specific steps of step S2 are as follows: S21: Add Si-ACC at a ratio of 20 g / kg to cadmium-contaminated soil and continue to maintain the soil water holding capacity at 80%; S22: After stirring evenly, continue to incubate in a constant temperature incubator at 25℃ for one week. According to the Tessier fractional extraction method, different forms of cadmium were extracted from the treated soil samples stepwise, and the concentration of different forms of Cd in the extract was determined by inductively coupled plasma mass spectrometry.

3. The application according to claim 1, characterized in that, This also includes drying the soil in a 50°C oven after cultivation, grinding the dried soil through a 100-mesh sieve, and storing the ground soil samples in a dry and cool environment for later use.

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