Method for efficiently removing soil heavy metals based on magnetic removal technology and application

Abstract

The invention relates to the technical field of soil heavy metal removal, in particular to a method for efficiently removing soil heavy metal based on a magnetic removal technology and application. The invention provides an innovative method for combining soil heavy metal activation and magnetic removal technologies. By applying one of the activating agents EDDS, RLs and CaCl2, insoluble heavy metals in the soil can be converted into soluble heavy metals and transferred into a soil solution, the content of the heavy metals in the soil solution is remarkably increased, and then the heavy metals are efficiently removed through the adsorption effect of the magnetic material on the heavy metals. According to the method, through the pre-activation step, the problem that a traditional magnetic removal technology depends on the form of the heavy metal is solved, and the utilization efficiency of a magnetic material and the overall removal efficiency of the heavy metal are remarkably improved. The method has important practical application value and popularization prospect for rapidly repairing the heavy metal polluted farmland and guaranteeing the safety of the cultivated land and sustainable production of grains.

Description

Technical Field

[0001] This invention belongs to the field of soil heavy metal removal technology, specifically relating to a method and application for efficient removal of soil heavy metals based on magnetic removal technology. Background Technology

[0002] In recent years, with the rapid development of industry and agriculture in my country, the problem of heavy metal pollution in soil has become increasingly serious. Soil, as a key environmental element for maintaining Earth's life systems, is an important carrier of biosphere material cycling and energy flow, and a core foundation for the stability of terrestrial ecosystems. Its environmental quality directly affects the safety and quality of agricultural products and further impacts human health through the food chain. In 2014, the Ministry of Environmental Protection and the Ministry of Land and Resources jointly released the "National Soil Pollution Status Survey Bulletin," which systematically revealed the overall state of my country's soil environment for the first time. The report pointed out that the overall soil environmental quality in China is not optimistic, with a total exceedance rate of 16.1%, among which arable land pollution is particularly prominent, with about one-fifth of my country's arable land contaminated by heavy metals such as cadmium, arsenic, chromium, and lead. Given my country's large population and strong demand for food, the safe and efficient remediation of heavy metal-contaminated farmland has become an urgent environmental and social task.

[0003] Currently, remediation technologies for heavy metal pollution in soil mainly include leaching, bioremediation, passivation, and magnetic removal. Among these, magnetic removal technology utilizes magnetic materials to adsorb heavy metals from the soil solution, followed by rapid separation using an external magnetic field. This technology not only completely removes pollutants but also avoids the secondary pollution problems common in traditional remediation methods, attracting widespread attention due to its advantages such as unrestricted operation and the ability to permanently remove heavy metals. However, in practical applications, magnetic materials primarily adsorb and remove water-soluble and exchangeable cadmium, which constitutes a relatively small proportion of the total heavy metal content in soil, significantly limiting its overall remediation efficiency. Therefore, overcoming this technological bottleneck is crucial for improving the remediation effectiveness of magnetic materials.

[0004] Soil heavy metal activators are functional reagents that can regulate the speciation of heavy metals through chemical or biochemical processes. Their mechanisms of action include dissolution, coordination desorption, speciation transformation, and interfacial modification. These activators disrupt the bond between heavy metals and soil particles, promoting the transfer of heavy metals from the solid phase to the liquid phase, thus allowing them to enter the soil solution. This process creates favorable conditions for the subsequent efficient adsorption and removal by magnetic materials. Therefore, combining activators with magnetic materials to form a synergistic remediation technology holds promise for significantly improving the remediation effect on heavy metal-contaminated soils.

[0005] CN113546952B discloses a magnetic biochar soil remediation agent for heavy metal pollution in soil and its application. This invention prepares magnetic biochar for adsorbing and removing heavy metals from soil, and the magnetic biochar can effectively remove heavy metals from soil. However, the removal capacity of the magnetic biochar for heavy metals in soil is relatively low. This is mainly because heavy metals in soil exist in different forms, and magnetic materials can only remove easily adsorbed forms such as water-soluble and exchangeable forms (Wang et al., 2024; Gong et al., 2021), thus resulting in limited removal efficiency of magnetic materials for heavy metals in soil. Summary of the Invention

[0006] The main objective of this invention is to address the limitation of magnetic removal technology in removing heavy metals from soil due to the restrictive forms they exist in. To achieve this goal, this invention proposes improvements and innovations to existing technologies, combining soil heavy metal activation with magnetic removal technology. By applying an activator, sparingly soluble heavy metals in the soil can be converted into soluble forms and transferred into the soil solution, significantly increasing the heavy metal content in the soil solution. This allows for efficient removal through the adsorption of heavy metals by magnetic materials. This method can significantly improve the overall removal efficiency of magnetic removal technology for heavy metals in soil, which is of great significance for promoting the safe utilization of farmland and ensuring food security in my country.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] A method for efficiently removing heavy metals from soil based on magnetic materials includes the following steps: Add contaminated soil to a centrifuge tube, add an activator, and then add CEOS magnetic material. Place the centrifuge tube in a shaker and shake for 1-12 hours. After the reaction is complete, pour it into a beaker and use a strong magnet to recover the CEOS from the soil. Repeat the above process 1-3 times. Take out the reacted soil, let it air dry naturally, grind and sieve it, and then test the total cadmium content of the soil.

[0009] Furthermore, the contaminated soil is cadmium-contaminated soil with a cadmium content of at least 0.3 mg / kg.

[0010] Furthermore, the amount of the CEOS magnetic material used is 1~8 wt%.

[0011] Furthermore, this invention selects at least one of the following three representative activators that have minimal impact on soil or are biodegradable: ethylenediamine disuccinic acid (EDDS), rhamnolipids (RLs), and calcium chloride (CaCl2). The dosage range of these activators is 0.5–1.0 mM EDDS, 0.5–5.0% RLs solution, or 0.0002–0.02 M CaCl2.

[0012] Preferably, the activator is 1 mM EDDS, 1.0% RLs solution, or 0.002 M CaCl2. More preferably, the activator is 1 mM EDDS.

[0013] Compared with the prior art, the advantages and beneficial effects of the present invention are as follows: Traditional magnetic removal technology, while capable of adsorbing and removing heavy metals through magnetic materials, is largely limited by the form in which heavy metals exist in the soil, often resulting in limited removal efficiency. This invention proposes an innovative method combining soil heavy metal activation with magnetic removal technology. By applying one of the following activators—EDDS, RLs solution, or CaCl2—poorly soluble heavy metals in the soil can be converted into soluble forms and transferred into the soil solution, significantly increasing the heavy metal content in the soil solution. This allows for efficient removal through the adsorption of heavy metals by CEOS magnetic materials. This method, through a pre-activation step, solves the problem of dependence on the form of heavy metals in traditional magnetic removal technology, significantly improving the utilization efficiency of magnetic materials and the overall removal efficiency of heavy metals. This invention is the first to propose a combination of "activator + magnetic material," which has significant practical application value and promising prospects for the rapid remediation of farmland contaminated with heavy metals and for ensuring arable land safety and sustainable food production.

[0014] This invention optimizes the dosage of the activator to ensure the overall removal efficiency of heavy metals. If the concentration of the activator is too high, the removal of heavy metals in the soil is mainly due to the activator, while the effect of the magnetic material is very weak. If the concentration of the activator is too low, the activation effect on heavy metals in the soil is weak, and the effect of the magnetic material on removing heavy metals from the soil is also weak. Attached Figure Description

[0015] Figure 1 The effects of different activator concentrations on the removal of total cadmium from soil by single activator solutions and synergistic remediation.

[0016] Figure 2 To investigate the effects of different treatment times on the removal of total cadmium from soil by synergistic technologies.

[0017] Figure 3 The effect of different amounts of CEOS added on the removal of total cadmium from soil by synergistic technology.

[0018] Figure 4 The effect of the number of treatments using synergistic technologies on the removal of total cadmium from soil.

[0019] Figure 5 The effects of mixing different activators (all at their original concentrations) with magnetic materials on the synergistic remediation of soil. Detailed Implementation

[0020] The technical solution of the present invention will be further described in detail below with reference to specific embodiments. It should be understood that the following embodiments are merely illustrative and explanatory of the present invention, and should not be construed as limiting the scope of protection of the present invention. All technologies implemented based on the above content of the present invention are covered within the scope of protection intended by the present invention. Unless otherwise stated, the raw materials and reagents used in the following embodiments are commercially available products, or can be prepared by known methods.

[0021] The actual cadmium-contaminated soil used in this invention was collected from a location in Shaoguan City, Guangdong Province. Topsoil (0-20 cm) was collected using a grid method and mixed using a quartering method. After the collected soil was naturally air-dried, plant and animal remains, gravel, and other debris were removed, and the soil was ground and sieved. The total cadmium content of this soil was 2.3 mg / kg, and the pH value was 6.2. According to the "Soil Environmental Quality - Agricultural Land Soil Pollution Risk Control Standard (Trial)" (GB15615-2018), the cadmium control standard for paddy fields is 0.4 mg / kg, exceeding the standard by 5.7 times.

[0022] Example 1 Synergistic effect of different concentrations of activators with magnetic materials Experimental Procedure: 5.0 g of contaminated soil was weighed into a 50 mL centrifuge tube. 25 mL of activators at different concentrations were added: 10, 5, 2, 1, 0.5, and 0.1 mM EDDS; 0.2, 0.002, 0.0002, and 0.00005 M CaCl2; and 10%, 5%, 2%, 1%, 0.5%, and 0.1% RLs. Then, 3% CEOS was added, and the centrifuge tube was placed in a shaker and shaken for 6 h. After the reaction was complete, the mixture was poured into a beaker, and CEOS was recovered from the soil using a strong magnet. Five recovery operations were performed on each sample. The reacted soil was removed, allowed to air dry naturally, ground, sieved, and the total cadmium content of the soil was tested. For the removal of cadmium from the soil using a single activator, no CEOS was added; only the activator was added to the soil, and the reaction was carried out under the above conditions. The CEOS treatment group does not add activator, but only adds CEOS material. The amount added and the treatment time are the same as the activator + COES treatment group.

[0023] Experimental Results: The effects of activator concentration on the removal of total cadmium from soil by two remediation modalities (single activator and synergistic remediation) were investigated. The results are as follows: Figure 1 As shown. Figure 1Figure (a) shows that the removal rate of total cadmium in soil increased with increasing EDDS concentration. Under low concentrations (0.1, 0.5, and 1 mM) of EDDS, the synergistic treatment significantly improved the removal of total cadmium in soil compared to EDDS alone. After remediation with EDDS alone, the total cadmium content in soil was 2.02, 1.75, and 1.48 mg / kg, respectively. After synergistic remediation, the total cadmium content decreased by 1.61, 1.36, and 0.99 mg / kg, respectively. Under high concentrations (2, 5, and 10 mM) of EDDS, there was no significant difference in the remediation effect of the two treatments on soil cadmium. Based on the above experimental results, 1 mM EDDS solution combined with CEOS showed the best synergistic effect on soil cadmium removal, therefore, 1 mM EDDS solution was selected for subsequent synergistic remediation experiments. Furthermore, lower EDDS concentrations had a weaker activation effect on soil heavy metals, thus reducing the synergistic remediation effect of EDDS and CEOS. At high concentrations of EDDS, there was no significant difference in the effects between the EDDS treatment and the EDDS-CEOS synergistic treatment group. This is because high concentrations of EDDS inhibit the effect of CEOS, so the reduction of heavy metals in the soil is mainly attributed to the effect of EDDS, while the effect of CEOS is relatively weakened.

[0024] from Figure 1 As shown in (b), the removal efficiency of the synergistic technology for soil cadmium increases with increasing CaCl2 solution concentration, but decreases under 0.02 M CaCl2 solution conditions. Under both 0.0002 and 0.002 M CaCl2 solution conditions, the synergistic technology significantly outperformed the single CaCl2 solution treatment for soil cadmium removal, indicating that at these two concentrations, the synergistic technology showed better synergistic removal of total soil cadmium. Among these, the synergistic effect of 0.002 M CaCl2 solution with CEOS was the best, reducing the total soil cadmium content to 1.10 mg / kg. Based on these experimental results, the synergistic effect of 0.002 M CaCl2 solution with CEOS was the best for removing heavy metals from soil; therefore, 0.002 M CaCl2 solution was selected for subsequent experiments on synergistic soil remediation. Similarly, at low concentrations of CaCl2 solution (0.00005 M), its activation efficiency for soil heavy metals was weak, resulting in relatively limited removal of heavy metals. At a high concentration of CaCl2 solution (0.2 M), there was no significant difference in the removal of heavy metals from soil between the CaCl2 treatment and the CaCl2-CEOS synergistic treatment group. This is because the high concentration of CaCl2 inhibited the effect of CEOS, so the reduction of heavy metals in the soil was mainly attributed to the effect of CaCl2, while the effect of CEOS was relatively weakened.

[0025] from Figure 1As shown in (c), the synergistic technology significantly outperformed single RLs treatment in removing cadmium from soil, and the removal efficiency of single RLs solution was relatively similar across different concentrations. After single RLs treatment, the total cadmium content in the soil ranged from 1.43 to 1.97 mg / kg. For synergistic remediation, the removal efficiency of different RLs concentrations also showed relatively small differences, with total cadmium content ranging from 1.08 to 1.69 mg / kg. Among these, the synergistic technology with 1.0% RLs showed the best removal efficiency, reducing the total cadmium content in the soil to 1.08 mg / kg. Based on these experimental results, the synergistic effect of 1% RLs solution and CEOS was the best for removing heavy metals from soil. Therefore, 1.0% RLs solution was selected for subsequent synergistic soil remediation experiments. Similarly, at lower concentrations (0.1%), the removal efficiency of the RLs-CEOS synergistic treatment group decreased, while at higher concentrations (10%), the difference in heavy metal removal efficiency between the single activation treatment and the synergistic treatment group was relatively small. This also indicates that the concentration of the activator has a significant impact on the synergistic remediation of heavy metals in soil. If the concentration of the activator is too high, the removal of heavy metals in soil is mainly due to the activator, while the effect of the magnetic material is very weak. If the concentration of the activator is too low, the activation effect on heavy metals in soil is weak, and the effect on improving the removal of heavy metals in soil by the magnetic material is also weak.

[0026] Example 2 Optimization of collaborative repair time Experimental procedure: Weigh 5.0 g of soil into a 50 mL centrifuge tube, add 25 mL of 1 mM EDDS, 0.002 M CaCl2, and 1% RLs, respectively, and then add 3% CEOS material. Place the centrifuge tube in a shaker and shake for 0.5, 1, 2, 3, 5, and 10 h. After the reaction is complete, pour the mixture into a beaker and follow the procedure in Example 1. Finally, test the total cadmium content of the soil.

[0027] Experimental results: The effect of treatment time for synergistic remediation on the removal efficiency of total cadmium in soil is as follows: Figure 2 As shown, with increasing treatment time, the synergistic treatment of CaCl2 and RLs solutions with CEOS gradually increased the removal rate of cadmium in the soil, reaching adsorption equilibrium at 3 h and 2 h, respectively. The synergistic treatment of EDDS solution with CEOS reached adsorption equilibrium within 1 h. After reaching adsorption equilibrium, the total cadmium levels in the soil treated with EDDS, CaCl2, and RLs solutions with CEOS were 0.81, 1.02, and 0.93 mg / kg, respectively. Based on these experiments, all synergistic technologies reached adsorption equilibrium within 3 h; therefore, 3 h was chosen as the condition for subsequent experiments.

[0028] Example 3 Optimization of magnetic material addition amount Experimental Procedure: Weigh 5.0 g of SW soil into a 50 mL centrifuge tube, and add 25 mL of 1 mM EDDS, 0.002 M CaCl2, and 1% RLs solution, respectively. Then add 1%, 3%, 5%, 8%, and 10% CEOS, respectively. Place the centrifuge tube in a shaker and shake for 3 h. After the reaction is complete, recover the CEOS and continue the experiment according to the above procedure.

[0029] Experimental Results: The effect of CEOS addition on total cadmium removal from soil in co-remediation is as follows: Figure 3 As shown in the figure, the experimental results indicate that the removal rate of cadmium in soil by the three synergistic technologies gradually increases with the increase of CEOS addition. Within the addition range of 1%, 3%, and 5%, the 5% CEOS addition significantly improved the removal efficiency of cadmium in soil compared to 1% and 3%. However, within the addition range of 5%, 8%, and 10%, the difference in Cd removal efficiency between the 5% and 8% CEOS treatments was small. Considering cost-effectiveness, the 5% CEOS addition is more economical while maintaining high removal efficiency; therefore, 5% was chosen as the condition for subsequent experiments.

[0030] Example 4 Optimization of collaborative repair times Experimental Procedure: Weigh 5.0 g of SW soil into a 50 mL centrifuge tube. Add 25 mL each of 1 mM EDDS solution, 0.002 M CaCl2 solution, and 1% RLs solution, followed by 5% CEOS. Place the centrifuge tube in a shaker and shake for 3 h. After the reaction is complete, recover the CEOS and return the soil solution to the centrifuge tube. Repeat the above process twice. Finally, collect the reacted soil and perform the experiment according to the above procedure.

[0031] Experimental Results: The effect of the number of co-remediation treatments on the removal of total cadmium from the soil was investigated. The results are as follows: Figure 4 As shown, the removal rate of cadmium in the soil increased significantly with the number of treatments. The synergistic treatment with 0.002 M CaCl2, 1 mM EDDS, and 1% RLs solution reduced the total cadmium content in the soil by 58.4%, 67.9%, and 63.2%, respectively. Three consecutive remediation treatments significantly improved the removal of cadmium from the soil compared to single or two treatments. Based on these experimental results, three remediation treatments were determined as the conditions for subsequent experiments.

[0032] Example 5 The impact of activator mixtures and synergistic effects of magnetic materials on soil remediation Experimental Procedure: Eight treatment groups were set up: CK, 1 mM EDDS + CEOS, 0.002 M CaCl2 + CEOS, 1% RLs + CEOS, a mixture of 1 mM EDDS and 0.002 M CaCl2 + CEOS, a mixture of 1 mM EDDS and 1% RLs + CEOS, a mixture of 0.002 M CaCl2 and 1% RLs + CEOS, and a mixture of three activators + CEOS. 5.0 g of SW soil was weighed into 50 mL centrifuge tubes. The activators for each treatment group were added, followed by 5% CEOS. The centrifuge tubes were placed in a shaker at 25℃ and 150 r / min for 3 h. After the reaction was complete, the CEOS was recovered, and the above treatment process was repeated twice.

[0033] Experimental Results: Building upon the successful synergistic remediation effects of single activators and magnetic materials in previous studies, this experiment explored the synergistic remediation of soil with mixed activators (maintaining their original concentrations) and magnetic materials to further improve the removal efficiency of cadmium from soil, aiming to achieve even better remediation results. The results showed that... Figure 5 As shown, synergistic treatment with different activators and CEOS significantly reduced the total cadmium content in the soil. Among the different synergistic treatments, the synergy of 1 mM EDDS, 1 mM EDDS + 1% RLs with CEOS showed better cadmium removal efficiency, while the synergy of 1 mM EDDS + 0.002 M CaCl2, 1 mM EDDS + 1% RLs + 0.002 M CaCl2 with CEOS showed slightly lower cadmium removal efficiency than the synergistic treatment with 1 mM EDDS alone. The synergistic effect of 1% RLs + 0.002 M CaCl2 with CEOS also showed slightly lower cadmium removal efficiency than the synergistic treatment with 1% RLs alone. Furthermore, the synergistic treatment of the binary mixture of 1 mM EDDS + 1% RLs did not significantly improve cadmium removal efficiency compared to the synergistic treatment with 1 mM EDDS alone. Based on these experimental results, the synergistic removal efficiency of cadmium from soil by multiple activators and magnetic materials did not significantly improve the efficiency; therefore, the synergistic remediation of soil by using a single activator and magnetic materials is more suitable.

[0034] Through systematic optimization of experimental conditions, the optimal parameters for synergistic remediation of cadmium-contaminated soil using a single activator and magnetic materials were determined: 0.002 M CaCl2, 1 mM EDDS and 1.0% RLs solution were synergistically used with CEOS, remediation time was 3 h, material addition was 5%, and treatment was performed three times.

[0035] Example 6 Comparison of activators, magnetic materials, and magnetic materials + activators in the synergistic removal of cadmium from soil Experimental procedure: (1) The cadmium-contaminated soil was remediated using the best activator and magnetic material synergistic conditions (0.002 MCaCl2, 1 mM EDDS and 1% RLs solution were synergistic with CEOS, remediation time was 3 h, material addition was 5%, and treatment was performed three times). The total cadmium content in the soil was tested after adsorption.

[0036] (2) Cadmium-contaminated soil was treated with 0.002 M CaCl2, 1 mM EDDS and 1% RLs solution respectively. The amount of activator added and the treatment time were the same as those of the co-remediation treatment.

[0037] (3) CEOS magnetic material was used to remediate cadmium-contaminated soil. The amount of CEOS added, the remediation time, and the number of remediations were consistent with those of the co-remediation treatment.

[0038] Experimental Results: As shown in Table 1, the synergistic remediation of soil by magnetic materials and activators resulted in a total cadmium removal rate of 58.6%–68.4%, significantly higher than that of magnetic materials alone and activators alone. Furthermore, the synergistic remediation of activators and magnetic material CEOS significantly improved the removal of total cadmium by CEOS by 27.5%–37.3%, demonstrating good remediation efficacy. The 1 mM EDDS+CEOS treatment group showed the best effect in removing total cadmium from the soil, with an enhancement effect of up to 37.3% on the magnetic material, indicating good application potential.

[0039] Table 1. Comparison of synergistic removal of cadmium from soil by activators, magnetic materials, and magnetic materials + activators.

Claims

1. A method for efficiently removing heavy metals from soil based on magnetic materials, characterized in that, Specifically, the following steps are included: Add contaminated soil to a centrifuge tube, add an activator, and then add CEOS magnetic material. Place the centrifuge tube in a shaker and shake. After the reaction is complete, pour it into a beaker and use a strong magnet to recover the CEOS from the soil. Repeat the above process multiple times. Take out the reacted soil, let it air dry naturally, grind and sieve it, and then test the total cadmium content of the soil.

2. The method for efficiently removing heavy metals from soil based on magnetic materials according to claim 1, characterized in that, The contaminated soil is cadmium-contaminated soil with a cadmium content of at least 0.3 mg / kg.

3. The method for efficiently removing heavy metals from soil based on magnetic materials according to claim 1, characterized in that, The activator includes at least one of ethylenediamine disuccinic acid (EDDS), rhamnolipid, and calcium chloride (CaCl2).

4. The method for efficiently removing heavy metals from soil based on magnetic materials according to claim 1, characterized in that, The activator is 0.002 M CaCl2.

5. The method for efficiently removing heavy metals from soil based on magnetic materials according to claim 1, characterized in that, The activator is 1 mM EDDS.

6. The method for efficiently removing heavy metals from soil based on magnetic materials according to claim 1, characterized in that, The activator is a 1.0% RLs solution.

7. The method for efficiently removing heavy metals from soil based on magnetic materials according to claim 1, characterized in that, The amount of the CEOS magnetic material used is 1~8 wt%.

8. The method for efficiently removing heavy metals from soil based on magnetic materials according to claim 1, characterized in that, The centrifugal oscillation time is 3 hours.

9. The method for efficiently removing heavy metals from soil based on magnetic materials according to claim 1, characterized in that, The collaborative repair process is repeated 1 to 3 times.

10. A method for efficiently removing heavy metals from soil based on magnetic materials according to any one of claims 1-9, characterized in that, This method reduced the total cadmium content in the soil by 58.4% to 67.9%.

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