Rapid elution and separation method for heavy metals in engineering residue soil and application of rapid elution and separation method

Abstract

The invention discloses a method for rapidly eluting and separating heavy metals in engineering residue soil and application, and belongs to the technical field of environmental geotechnical engineering and solid waste resourceful treatment.The method comprises the following steps that (1) the engineering residue soil is pretreated, then water is added to prepare slurry, and after aggregate of the slurry is screened out through two-stage hydraulic cyclone, the slurry is filtered out; a compound chelating agent is added for double-frequency ultrasonic enhanced desorption treatment, and desorbed slurry is obtained; and (2) adding magnetic nanoparticles into the desorbed slurry, carrying out magnetic sensitization conditioning, carrying out magnetic separation, carrying out plate-frame pressure filtration on the slurry, and recycling the regenerated magnetic nanoparticles, thereby completing the elution separation of the engineering residue soil heavy metals. The heavy metal removal rate in the whole process of the technology is larger than 95%, the magnetic medium recovery rate is larger than 99%, the technology has the advantages of being environmentally friendly in agent, high in anti-interference capacity, thorough in solid-liquid separation and the like, and the harmlessness and recycling level of the engineering muck is remarkably improved.

Description

Technical Field

[0001] This invention belongs to the field of environmental geotechnical engineering and solid waste resource utilization technology, and particularly relates to a rapid elution and separation method for heavy metals in engineering slag and its application. Background Technology

[0002] With the acceleration of urbanization and the increased development of underground space (such as subway tunneling and underground utility tunnel construction), a massive amount of construction waste has been generated during engineering projects. Construction waste generated from the relocation of old industrial areas, development around mining areas, and areas affected by atmospheric deposition often contains excessive levels of heavy metal pollutants such as lead (Pb), cadmium (Cd), copper (Cu), and zinc (Zn). Simply landfilling or dumping this contaminated waste not only occupies valuable land resources, but the heavy metals it contains are also highly susceptible to migration into groundwater or surrounding soil through rainwater leaching, posing ecological and environmental risks. Furthermore, this restricts the resource utilization of construction waste as recycled building materials (such as roadbed filler and raw materials for sintered bricks).

[0003] Current soil heavy metal remediation technologies mainly include solidification / stabilization, phytoremediation, bioremediation, and chemical leaching. Among these, ex-situ chemical leaching is considered an effective method for treating high-concentration heavy metal contaminated construction waste due to its short remediation cycle and ability to completely remove and reduce pollutants. However, existing leaching technologies still face the following significant technical bottlenecks and limitations when applied to large-scale construction waste treatment: First, construction waste (especially tunnel boring machine waste) typically contains a high proportion of clay particles (<0.005mm) and silt particles (0.005-0.075mm). According to soil chemisorption theory, heavy metals tend to accumulate in fine-particle components with large specific surface areas, high surface charge densities, and abundant organic matter. Fine-particle components with a particle size less than 0.075mm often carry a large amount of heavy metal pollution load in the soil. Traditional full-volume leaching processes lack fine particle size classification pretreatment, resulting in a large amount of reagent being ineffectively adsorbed or consumed on the cleaned coarse aggregate surface, leading to reagent waste. In addition, fine-particle soils easily form dense micro-aggregates, encapsulating heavy metals inside the aggregates. Traditional mechanical stirring can only clean contaminants on the particle surface, making it difficult to break up these micron-sized aggregate structures. This prevents internal contaminants from contacting the leaching agent, forming a "dead zone" with extremely high mass transfer resistance, making desorption kinetics extremely slow, often requiring several hours or even tens of hours to reach equilibrium, and the desorption rate is difficult to improve.

[0004] Secondly, existing technologies often use strong acids (such as hydrochloric acid and nitric acid) or synthetic strong chelating agents (such as EDTA and DTPA) as leaching agents. While these can effectively dissolve heavy metals, they severely damage the soil's crystal structure and physicochemical properties, leading to a significant loss of soil nutrients (N, P, K) and organic matter. The treated soil loses its reuse value, and a large amount of acidic wastewater is generated, resulting in high subsequent treatment costs. EDTA has a strong complexing ability for heavy metals, but its broad-spectrum nature is a double-edged sword. Construction waste often contains large amounts of macroelements such as calcium (Ca), magnesium (Mg), and iron (Fe) (especially calcium carbonate fragments from construction waste, where calcium content can reach tens of thousands of mg / kg). EDTA has a strong complexing ability for heavy metals... 2+ Mg 2+ It also has a strong binding capacity (LogK Ca-EDTA ≈ 10.7). During leaching, a large amount of EDTA preferentially binds to high concentrations of background cations in the soil, rather than the target trace heavy metals, leading to "reagent competition failure." To ensure the removal rate of heavy metals, the dosage of EDTA has to be increased several times over, which not only greatly increases the cost but also causes significant calcium loss and damages the soil aggregate structure. More seriously, EDTA is extremely difficult to degrade in the natural environment (its half-life can reach several months to several years). Once the residual EDTA-heavy metal complex migrates with the leachate, it will greatly increase the mobility of heavy metals in groundwater, causing serious secondary pollution risks.

[0005] Finally, the high viscosity of the mud leads to difficulties in solid-liquid separation. The leached sludge system contains a large number of colloidal fine particles (<2μm), which have extremely poor settling properties, following Stokes' Law, where the settling velocity is proportional to the square of the particle size. The natural settling of micron-sized particles takes several days. Existing technologies (such as CN101733065A) involve magnetic separation or leaching, but most of them suffer from problems such as magnetic separation being mainly used for wastewater treatment. When directly applied to high-viscosity soil sludge, magnetic nanoparticles are easily encapsulated or masked by the soil matrix, resulting in low recovery rates. There is a lack of optimized reagent formulations for high-calcium matrices, and there are defects such as competitive adsorption. Single-frequency ultrasonic treatment is prone to generating standing wave fields, resulting in uneven treatment and limited effectiveness in breaking up stubborn agglomerates.

[0006] Therefore, how to provide a complete set of technologies and equipment that can organically couple fine particle size classification, efficient green selective desorption and rapid magnetic separation technology to address the characteristics of engineering waste soil such as "fine particle enrichment pollution, high matrix calcium content and difficulty in solid-liquid separation" is a technical problem that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0007] To address the aforementioned technical problems, this invention proposes a rapid washing and separation method for heavy metals in engineering waste soil and its application.

[0008] To achieve the above objectives, the present invention provides the following technical solution: A rapid elution and separation method for heavy metals in construction waste includes the following steps: (1) After pretreatment, the engineering waste soil is mixed with water to make mud slurry. After the mud slurry is screened by two-stage hydraulic cyclone to remove aggregate, a compound chelating agent is added for dual-frequency ultrasonic enhanced desorption treatment to obtain desorbed slurry. (2) Add magnetic nanoparticles to the desorbed slurry and perform magnetic sensitization conditioning. After magnetic separation, filter the mud through a plate and frame filter press and recover the regenerated magnetic nanoparticles to complete the washing and separation of heavy metals in the engineering waste soil.

[0009] Preferably, the mud in step (1) is obtained by mixing engineering waste soil and water at a solid-liquid ratio of 1g:4mL.

[0010] Beneficial effects: This ratio ensures that the slurry system has a sufficiently low apparent viscosity, which is conducive to the smooth nucleation, growth, and violent collapse of cavitation bubbles in the subsequent dual-frequency ultrasonic field, and ensures the uniform transmission of physical and mechanical shear forces throughout the field. If the solid-liquid ratio is too high (e.g., 1:2), the yield stress of the slurry increases sharply, and the attenuation coefficient of the ultrasonic waves in it increases exponentially, severely inhibiting the cavitation effect; if the solid-liquid ratio is too low (e.g., 1:10), although the rheological properties are excellent, it will lead to a significant dilution of the effective concentration of the expensive compound chelating agent, and at the same time, it will multiply the hydraulic load of the subsequent high-gradient magnetic separation system and the final wastewater treatment cost.

[0011] Preferably, in the two-stage hydrocyclone described in step (1), the separation particle size of the first-stage hydrocyclone is 2.0 mm, and coarse sand and gravel are screened out; the separation particle size of the second-stage hydrocyclone is less than 0.075 mm, and fine sand is screened out.

[0012] Beneficial effects: This invention slurries the slag, removes coarse gravel using a primary hydrocyclone (cutoff point ~2mm), and then separates fine sand using a secondary hydrocyclone (cutoff point ~0.075mm). The coarse gravel and fine sand comprise 30%-50% of the total slag mass, but are enriched with over 80% heavy metals. Furthermore, the "powder + clay" slurry component obtained after screening is used for deep treatment. This step significantly reduces the amount of earthwork required for subsequent chemical treatment, substantially lowering reagent costs and equipment load.

[0013] More preferably, before adding the compound chelating agent, the slurry after removing the aggregate is concentrated to a solids content of 15%.

[0014] Preferably, the amount of compound chelating agent added in step (1) is 0.01-0.1 mol / L.

[0015] Preferably, the compound chelating agent is obtained by mixing tetrasodium glutamate diacetate (GLDA) and citric acid (CA) in a molar ratio of 1:(1-3).

[0016] More preferably, after adding the compound chelating agent, the pH of the system is adjusted to 4.0-5.0.

[0017] Beneficial effects: GLDA in this invention is a novel green biodegradable chelating agent. Compared with common EDTA chelating agents, GLDA exhibits better performance in acidic to neutral conditions regarding Ca2+. 2+ The binding constant (LogK ~ 6.4) is much lower than that for Pb. 2+ (LogK~10.5), Cu 2+ The binding constants of heavy metals, such as (LogK~13.1), exhibit a wider selectivity window. Simultaneously, CA adjusts the system pH to a slightly acidic level (4.0-5.5), promoting the release of carbonate-bound heavy metals. Furthermore, citrate ions act as a "shuttle carrier," first forming weak complexes with heavy metals and desorbing them from the soil surface, then being captured by the more strongly binding GLDA in the liquid phase, forming stable GLDA-metal complexes. This synergistic effect effectively inhibits competition from calcium and magnesium ions, improving the extraction efficiency of heavy metals.

[0018] Preferably, in the dual-frequency ultrasonic enhanced desorption process described in step (1), the low frequency (20-28 kHz) and the high frequency (40-60 kHz) alternate, with a switching cycle of 10-30 seconds, and the ultrasonic power density is 0.3-0.6 W / cm². 2 The total reaction time is 20-60 minutes, and the stirring speed is 200 rpm.

[0019] Beneficial effects: The dual-frequency ultrasonic mechanism in this invention utilizes the high-intensity shock waves and shear forces generated by low-frequency (20-28kHz) ultrasound to physically break up soil micro-aggregates and expose encapsulated heavy metal sites; simultaneously, it utilizes the high-density cavitation bubbles and microjets generated by high-frequency (40-60kHz) ultrasound to thin the liquid film boundary layer on the particle surface, enhancing the diffusion of the agent into the micropores and the mass transfer of the complex. The alternating or orthogonal dual-frequency operating mode eliminates the standing wave dead zone, achieving uniform enhancement throughout the reactor.

[0020] Preferably, the magnetic nanoparticles in step (2) have a core-shell structure, with the core being Fe3O4 and the outer shell being a SiO2 layer coated with amino or thiol functional groups. The magnetic nanoparticles have an average particle size of 50-200 nm, a saturation magnetization greater than 50 emu / g, and an isoelectric point pH value greater than 8.0. The amount of magnetic nanoparticles added is 0.5%-2.0% of the dry soil mass.

[0021] More preferably, the magnetic nanoparticles are Fe3O4@SiO2-NH2 magnetic nanoparticles, and the amount added is 0.5-2.0% of the mass of the desorbed slurry.

[0022] More preferably, the magnetic sensitization conditioning time is 10 minutes.

[0023] Beneficial Effects: Amino-functionalized magnetic nanoparticles (Fe3O4@SiO2-NH2) possess the dual functions of adsorbent and flocculation nucleus. The amino groups (-NH2) on their surface can complex and adsorb residual heavy metal ions in the liquid phase, further reducing the concentration of the supernatant and achieving the adsorption effect. Simultaneously, by adjusting the Zeta potential, the positively charged amino-functionalized magnetic particles undergo charge neutralization and adsorption bridging with the negatively charged soil clay colloids, inducing fine soil particles to aggregate around the magnetic particles, achieving flocculation and forming micro-flocs containing magnetic cores. This process endows the originally non-magnetic soil fine particles with "magnetic responsiveness."

[0024] Preferably, the background magnetic field for magnetic separation in step (2) is 1.0-2.5T and the slurry flow rate is 0.5-3.0 cm / s.

[0025] More preferably, the magnetic separation is achieved using a high gradient magnetic separator (HGMS). The high-gradient magnetic separator is filled with a magnetically conductive medium.

[0026] Beneficial effects: This invention uses a high-gradient magnetic separator (HGMS) to magnetically separate the magnetically sensitized slurry. The magnetically conductive medium (such as stainless steel fibers) inside the HGMS generates an extremely high magnetic field gradient under a background magnetic field, producing a huge magnetic trapping force on tiny magnetic flocs, causing them to instantly overcome fluid drag and be trapped. This achieves continuous separation of fine slurry within seconds, and the separation efficiency is not limited by particle settling velocity. Preferably, the recycling and regeneration of magnetic nanoparticles in step (2) specifically includes the following steps: The magnetic nanoparticles obtained after magnetic separation are backwashed with an inorganic acid solution for 15-40 minutes. The resulting eluent is a heavy metal concentrate, and regenerated magnetic nanoparticles are obtained simultaneously.

[0027] More preferably, the inorganic acid solution is hydrochloric acid or sulfuric acid with a pH of 1.0-2.0.

[0028] Beneficial effects: The process of recycling and regenerating magnetic nanoparticles can utilize high concentrations of protons (H+). +A strong protonation-displacement reaction occurs. The extreme acidic environment rapidly disrupts the coordination bonds between GLDA and heavy metals, and maximizes the positive charge of the amino functional groups on the surface of the magnetic particles. This causes heavy metal ions to rapidly desorb from the particle surface due to electrostatic repulsion of like charges, dissolving into a very small amount of eluent to form a concentrated heavy metal solution. After desorption, the magnetic nanoparticles retain their core magnetic properties and surface functional group activity, allowing them to be repeatedly pumped into the upstream magnetic sensitization conditioning cell for reuse. This closed-loop regeneration design minimizes the consumption of expensive nanomaterials, greatly improving the engineering and economic feasibility of this technology.

[0029] Application of a rapid elution and separation method for heavy metals in engineering waste soil in the resource utilization of engineering waste soil. Compared with the prior art, the present invention has the following advantages and technical effects: This invention discloses a rapid elution and separation method for heavy metals in engineering waste soil and its application in the resource utilization of engineering waste soil. Addressing the enrichment pattern of heavy metals in fine particles, this invention first introduces a multi-stage hydraulic cyclone classification process to achieve source reduction and precise treatment. Simultaneously, to address the difficulties in desorption of fine particle aggregates and interference from high-calcium background, this invention innovatively employs a "dual-frequency ultrasound + GLDA / citric acid compound system." These two components synergistically achieve uniform enhancement throughout the reactor and effectively suppress competition between calcium and magnesium ions, thereby improving the extraction efficiency of heavy metals. Furthermore, to address the difficulty in settling fine particles, this invention introduces the concept of "magnetization," endowing originally non-magnetic soil fine particles with "magnetic responsiveness." Combined with HGMS separation, it achieves continuous separation of fine slurry within seconds, and the separation efficiency is not limited by particle settling velocity. Finally, the process provided by this invention is simple, the raw materials are readily available, and it can be widely applied. Attached Figure Description

[0030] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings: Figure 1 This is a process flow diagram of the rapid washing and separation method for heavy metals in engineering waste soil in Embodiment 1 of the present invention. Detailed Implementation

[0031] 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.

[0032] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0033] Unless otherwise specified, all raw materials used in the embodiments of this invention were purchased through commercial channels; The construction waste soil was taken from a typical high-calcium heavy metal contaminated site (former electroplating plant) in southern China. The soil had a pH of 7.9, an organic matter content of 3.5%, and a calcium carbonate content as high as 4.2%. The main heavy metal pollutants were lead (Pb) at 1850 mg / kg, cadmium (Cd) at 12.4 mg / kg, and copper (Cu) at 960 mg / kg. Particle size analysis showed that particles <0.075 mm accounted for 42.5% of the total mass, but were enriched with 89% of Pb and 91% of Cd.

[0034] Reagent: GLDA (40% aqueous solution, industrial grade); Citric acid (analytical grade); The Fe3O4@SiO2-NH2 magnetic nanoparticles were prepared in-house, with an average particle size of 80 nm, a saturation magnetization of 65 emu / g, and an isoelectric point pH greater than 8.0. The preparation method includes the following steps: (1) Preparation of Fe3O4 nanoparticles by coprecipitation method: FeCl3·6H2O and FeCl2·4H2O were weighed and dissolved in deionized water at a molar ratio of 2:1. Under the protection of extremely vigorous mechanical stirring and the introduction of high-purity nitrogen to remove dissolved oxygen, the solution was heated to 80°C in a water bath. Ammonia water with a mass fraction of 25% was added dropwise at a uniform rate until the pH of the solution reached above 10, and a dense black precipitate was formed instantly. After the aging reaction continued for 1 hour, the precipitate was collected by magnetic separation using an external neodymium iron boron strong magnet and repeatedly washed with deionized water until neutral to obtain pure superparamagnetic Fe3O4 nanoparticles; (2) SiO2 coating using the Stöber method: The superparamagnetic Fe3O4 nanoparticles prepared above were ultrasonically dispersed in a mixed dispersion system containing a high proportion of anhydrous ethanol (200 ml), deionized water (50 ml), and a very small amount of catalyst ammonia water (5 ml), with a volume ratio of 40:10:1. Tetraethyl orthosilicate (TEOS) was added dropwise very slowly under continuous strong mechanical stirring. The system reacted at room temperature for 20 hours, during which TEOS hydrolyzed and condensed in situ on the Fe3O4 surface, forming a uniform and dense SiO2 protective shell. After magnetic separation and washing, Fe3O4@SiO2 core-shell intermediates were obtained; (3) Surface amino functionalization: 1 g of dried Fe3O4@SiO2 powder was redispersed in 100 ml of anhydrous ethanol solvent, and 3.3 ml of silane coupling agent 3-aminopropyltriethoxysilane (APTES) was added. The reaction system was heated to 60 °C and refluxed, and stirred vigorously for 24 hours. During this process, the alkoxy groups of APTES molecules condensed with the silanol groups on the surface of the silica shell, covalently grafting free amino groups with positive charge potential onto the particle surface. The final product was washed alternately with deionized water and ethanol, and then freeze-dried under vacuum to obtain targeted magnetic nanoparticles with an average particle size in the range of 50-200 nm, a saturation magnetization of more than 65 emu / g, and an isoelectric point pH greater than 8.0.

[0035] In this embodiment of the invention, the Zeta potential was measured using a Malvern potentiometer.

[0036] Unless otherwise specified, room temperature or normal temperature in the embodiments of the present invention refers to 25±3℃.

[0037] Example 1 A rapid elution and separation method for heavy metals in construction waste, such as Figure 1 As shown, it includes the following steps: (1) Slag Classification and Reduction: 1000 kg of construction slag was crushed and screened to remove construction waste, and then sent to a slurry mixing tank to prepare a slurry with a solid-liquid ratio of 1:4 (g:mL). This slurry was pumped into a first-stage hydrocyclone (separating particles of 2.0 mm). Coarse sand and gravel (approximately 160 kg) were discharged from the underflow of the first-stage hydrocyclone and then sequentially conveyed to an aggregate spiral washer. Under the combined washing action of strong mechanical friction and high-pressure water spray, the thin layer of contaminated clay adhering to the surface was thoroughly removed. The overflow from the first-stage hydrocyclone entered a second-stage hydrocyclone (separating particles of 0.075 mm). Fine sand (approximately 400 kg) was discharged from the underflow of the second-stage hydrocyclone, with a Pb content of 48 mg / kg, meeting the standard for recycling. Finally, the fine-particle slurry (dry weight approximately 440 kg) overflowing from the second-stage hydrocyclone was obtained and collected, with a Pb content enriched to approximately 4100 mg / kg, and was then sent to the next unit for further treatment. This step achieved a 56% reduction in the amount of waste at the source, significantly reducing the load on subsequent reactors.

[0038] (2) Dual-frequency ultrasonic enhanced desorption: The fine-particle slurry obtained in step (1) was transported to the enhanced reaction vessel. After adjusting the solid content to 15%, GLDA (30 mmol / L) and citric acid (30 mmol / L) (molar ratio 1:1) were added as compound chelating agents. After adjusting the pH of the system to stabilize at 4.5, the array ultrasonic transducer was turned on. The low frequency of 28 kHz (power 300W) and the high frequency of 40 kHz (power 300W) were set to run alternately, with a switching cycle of 20 seconds. The total reaction time was 40 minutes, the ultrasonic power density was 0.375 W / cm², and the mechanical stirring speed was 200 rpm. After the reaction, the desorbed slurry was obtained. Among them, the sampling analysis showed that the heavy metal desorption rate was extremely fast in the first 10 minutes of the reaction, reaching 78% of the total removal amount, and the apparent kinetic constant was 0.12 min. -1 .

[0039] (3) Magnetic Sensitization Conditioning: 1.0% by mass of Fe3O4@SiO2-NH2 magnetic nanoparticles were added to the desorbed slurry, and stirring was continued for 10 minutes to obtain the conditioned slurry. The conditioned slurry was then pumped into a high gradient magnetic separator (HGMS), with the background magnetic field set to 1.5 T and the flow rate to 1.0 cm / s. The slurry flowing out after magnetic separation quickly clarified, and the slurry was filtered through a plate and frame filter press. The slurry cake had a low water content, with Pb content of 180 mg / kg (removal rate >95%) and Cd content of 0.8 mg / kg (removal rate >93%), fully meeting the standards for safe use of industrial land.

[0040] (4) Media regeneration: After HGMS adsorption saturation, the magnetism is turned off, and 0.1 mol / L HCl solution is introduced to backwash the magnetic nanoparticles after magnetic separation for 20 minutes. The resulting eluent is the heavy metal concentrate, and at the same time, regenerated magnetic nanoparticles are obtained. The magnetic properties of the regenerated magnetic nanoparticles are restored to 98% of the initial value and can be put back into use.

[0041] Example 2 A rapid elution and separation method for heavy metals in construction waste includes the following steps: (1) Slag Classification and Reduction: 1000 kg of construction slag was crushed and screened to remove construction waste, and then sent to a slurry mixing tank to prepare a slurry with a solid-liquid ratio of 1:4. This slurry was pumped into a first-stage hydrocyclone (separating particles of 2.0 mm). Coarse sand and gravel (approximately 150 kg) were discharged from the underflow of the first-stage hydrocyclone and then sequentially conveyed to an aggregate spiral washer. Under the combined washing action of strong mechanical friction and high-pressure water spray, the thin layer of contaminated clay adhering to the surface was thoroughly removed. The overflow from the first-stage hydrocyclone entered a second-stage hydrocyclone (separating particles of 0.075 mm). Fine sand (approximately 410 kg) was discharged from the underflow of the second-stage hydrocyclone, with a Pb content of 49 mg / kg, meeting the standard for recycling. Finally, the fine-particle slurry (dry weight approximately 440 kg) overflowing from the second-stage hydrocyclone was obtained and collected, with a Pb content enriched to approximately 4150 mg / kg, and was then sent to the next unit for further treatment.

[0042] (2) Dual-frequency ultrasonic enhanced desorption: The fine-particle slurry obtained in step (1) was transported to the enhanced reaction vessel. After adjusting the solid content to 15%, GLDA (5 mmol / L) and citric acid (5 mmol / L) (molar ratio 1:1) were added as a compound chelating agent. After adjusting the pH of the system to stabilize at 4.5, the array-type ultrasonic transducer was turned on. The low frequency of 20 kHz (power 225 W) and the high frequency of 40 kHz (power 225 W) were set to alternate, with a switching cycle of 10 seconds. The total reaction time was 20 minutes, the ultrasonic power density was 0.375 W / cm², and the mechanical stirring speed was 200 rpm. After the reaction was completed, the desorbed slurry was obtained.

[0043] (3) Magnetic Sensitization Conditioning: 0.5% Fe3O4@SiO2-NH2 magnetic nanoparticles were added to the desorbed slurry, and the mixture was stirred for 10 minutes to obtain the conditioned slurry. The conditioned slurry was then pumped into an HGMS with a background magnetic field of 1.0 T and a flow rate of 0.5 cm / s. The slurry flowing out after magnetic separation quickly became clear. The slurry was then filtered through a plate and frame filter press. The slurry cake had a low water content, with a Pb content of 380 mg / kg (removal rate of approximately 90.8%) and a Cd content of 1.2 mg / kg (removal rate of approximately 90.3%), fully meeting the standards for safe use of industrial land.

[0044] (4) Media regeneration: After HGMS adsorption saturation, the magnetism is turned off, and 0.1 mol / L HCl solution is introduced to backwash the magnetic nanoparticles after magnetic separation for 15 minutes. The resulting eluent is the heavy metal concentrate, and at the same time, regenerated magnetic nanoparticles are obtained. The magnetic properties of the regenerated magnetic nanoparticles are restored to 96% of the initial value and can be put back into use.

[0045] Example 3 A rapid elution and separation method for heavy metals in construction waste includes the following steps: (1) Slag Classification and Reduction: 1000 kg of construction slag was crushed and screened to remove construction waste, and then sent to a slurry mixing tank to prepare a slurry with a solid-liquid ratio of 1:4. This slurry was pumped into a first-stage hydrocyclone (separating particles of 2.0 mm). Coarse sand and gravel (approximately 155 kg) were discharged from the underflow of the first-stage hydrocyclone and then sequentially conveyed to an aggregate spiral washer. Under the combined washing action of strong mechanical friction and high-pressure water spray, the thin layer of contaminated clay adhering to the surface was thoroughly removed. The overflow from the first-stage hydrocyclone entered a second-stage hydrocyclone (separating particles of 0.075 mm). Fine sand (approximately 405 kg) was discharged from the underflow of the second-stage hydrocyclone, with a Pb content of 45 mg / kg, meeting the standard for recycling. Finally, the fine-particle slurry (dry weight approximately 440 kg) overflowing from the second-stage hydrocyclone was obtained and collected, with a Pb content enriched to approximately 4120 mg / kg, and was then sent to the next unit for further treatment.

[0046] (2) Dual-frequency ultrasonic enhanced desorption: The fine-particle slurry obtained in step (1) was transported to the enhanced reaction vessel. After adjusting the solid content to 15%, GLDA (25 mmol / L) and citric acid (75 mmol / L) (molar ratio 1:3) were added as a compound chelating agent. After adjusting the pH of the system to stabilize at 4.5, the array-type ultrasonic transducer was turned on, and the low frequency of 28 kHz (power 300 W) and the high frequency of 60 kHz were set to run alternately, corresponding to the ultrasonic power density being pushed up to the upper limit of 0.6 W / cm². The switching cycle was 30 seconds, the total reaction time was 60 minutes, the ultrasonic power density was 0.375 W / cm², and the mechanical stirring speed was 200 rpm. After the reaction was completed, the desorbed slurry was obtained.

[0047] (3) Magnetic Sensitization Conditioning: 2% Fe3O4@SiO2-NH2 magnetic nanoparticles were added to the desorbed slurry, and stirring was continued for 10 minutes to obtain the conditioned slurry. The conditioned slurry was then pumped into an HGMS with a background magnetic field of 2.5 T and a flow rate of 3.0 cm / s. The slurry flowing out after magnetic separation quickly clarified. The slurry was then filtered through a plate and frame filter press. The slurry cake had a low water content, with a Pb content of 74 mg / kg (removal rate >95%) and a Cd content of 0.3 mg / kg (removal rate >93%), fully meeting the standards for safe use of industrial land.

[0048] (4) Media regeneration: After HGMS adsorption saturation, the magnetism is turned off, and 0.1 mol / L HCl solution is introduced to backwash the magnetic nanoparticles after magnetic separation for 40 minutes. The resulting eluent is the heavy metal concentrate, and at the same time, regenerated magnetic nanoparticles are obtained. The magnetic properties of the regenerated magnetic nanoparticles are restored to 99.5% of the initial value and can be put back into use.

[0049] Comparative Examples 1-4 were designed to verify the superiority of the compound chelating agent in this invention. Under the same ultrasonic conditions (dual frequency, 40 min), the treatment effects of different combinations of agents were compared.

[0050] Comparative Example 1 The only difference from Example 1 is that the compound chelating agent used in step (2) is GLDA, and the addition amount is 60 mmol / L. Specifically, the steps include: Step (1) is the same as in Example 1; (2) Dual-frequency ultrasonic enhanced desorption: The fine-particle slurry obtained in step (1) was transported to the enhanced reaction vessel. After adjusting the solid content to 15%, GLDA (60 mmol / L) was added as a compound chelating agent. After the pH of the system was adjusted to stabilize at 4.5, the array-type ultrasonic transducer was turned on. The low frequency of 28 kHz (power 300W) and the high frequency of 40 kHz (power 300W) were set to alternate, with a switching cycle of 20 seconds. The total reaction time was 40 minutes, and the ultrasonic power density was 0.375 W / cm³. 2 The mechanical stirring speed was 200 rpm. After the reaction was completed, the desorbed slurry was obtained.

[0051] Steps (3)-(4) are the same as in Example 1.

[0052] Comparative Example 2 The only difference from Example 1 is that the compound chelating agent used in step (2) is citric acid, and the addition amount is 60 mmol / L. Specifically, the steps include: Step (1) is the same as in Example 1; (2) Dual-frequency ultrasonic enhanced desorption: The fine-particle slurry obtained in step (1) was transported to the enhanced reaction vessel. After adjusting the solid content to 15%, citric acid (60 mmol / L) was added as a compound chelating agent. After the pH of the system was stabilized at 4.5, the array-type ultrasonic transducer was turned on. The low frequency of 28 kHz (power 300W) and the high frequency of 40 kHz (power 300W) were set to alternate, with a switching cycle of 20 seconds. The total reaction time was 40 minutes, and the ultrasonic power density was 0.375 W / cm³. 2 The mechanical stirring speed was 200 rpm. After the reaction was completed, the desorbed slurry was obtained.

[0053] Steps (3)-(4) are the same as in Example 1.

[0054] Comparative Example 3 The only difference from Example 1 is that the compound chelating agent used in step (2) is EDTA, and the addition amount is 60 mmol / L. Specifically, the following steps are included: Step (1) is the same as in Example 1; (2) Dual-frequency ultrasonic enhanced desorption: The fine-particle slurry obtained in step (1) was transported to the enhanced reaction vessel. After adjusting the solid content to 15%, EDTA (60 mmol / L) was added as a compound chelating agent. After the pH of the system was stabilized at 4.5, the array-type ultrasonic transducer was turned on. The low frequency of 28 kHz (power 300W) and the high frequency of 40 kHz (power 300W) were set to alternate, with a switching cycle of 20 seconds. The total reaction time was 40 minutes, and the ultrasonic power density was 0.375 W / cm³. 2 The mechanical stirring speed was 200 rpm. After the reaction was completed, the desorbed slurry was obtained.

[0055] Steps (3)-(4) are the same as in Example 1.

[0056] Comparative Example 4 The only difference from Example 1 is that the compound chelating agent used in step (2) is a compound composition of GLDA and CaCl2. Specifically, it includes the following steps: Step (1) is the same as in Example 1; (2) Dual-frequency ultrasonic enhanced desorption: The fine-particle slurry obtained in step (1) was transported to the enhanced reaction vessel. After adjusting the solid content to 15%, GLDA (60 mmol / L) and CaCl2 (30 mmol / L) were added as compound chelating agents. After adjusting the pH of the system to stabilize at 4.5±0.1, the array-type ultrasonic transducer was turned on. The low frequency of 28 kHz (power 300W) and the high frequency of 40 kHz (power 300W) were set to alternate, with a switching cycle of 20 seconds. The total reaction time was 40 minutes, and the ultrasonic power density was 0.375 W / cm³. 2 The mechanical stirring speed was 200 rpm. After the reaction was completed, the desorbed slurry was obtained.

[0057] Steps (3)-(4) are the same as in Example 1.

[0058] Technical effects: The initial pH and final pH of the slurry before and after the addition of the compound chelating agent in Example 1 and Comparative Examples 1-4 were tested. The heavy metal and calcium contents in the regenerated magnetic nanoparticles obtained in Example 1 and Comparative Examples 1-4 were determined by microwave digestion-ICP-MS, and the heavy metal removal rate and calcium dissolution amount were calculated. The results are shown in Table 1.

[0059] Table 1. Effects of different compound chelating agent formulations on heavy metal removal rate and calcium leaching in high-calcium slag. As shown in Table 1, the heavy metal removal rate of Example 1 was significantly higher than that of Comparative Examples 1 and 2. This is because GLDA is difficult to dissolve the carbonate / oxide cement containing heavy metals under alkaline conditions; although citric acid alone is strong enough to dissolve the cement, its chelating ability is weak, and the desorbed heavy metals are easily re-adsorbed by the soil. Example 1 combines the "acid dissolution / shuttle" function of CA with the "strong chelation / anti-reverse adsorption" function of GLDA, achieving a 1+1>2 effect. Comparing Example 1 and Comparative Example 3, although the heavy metal removal rates of the two are similar, the Ca dissolution amount of Comparative Example 3 is 2.6 times that of Example 1 (4800 vs 1850 mg / L). This confirms that EDTA lacks selectivity for Ca, and a large amount of the agent is consumed by Ca. In contrast, GLDA has a much larger stability constant for Pb / Cd than for Ca in the pH range of 4-5, exhibiting an excellent selectivity window, thereby protecting the soil structure and improving the effective utilization rate of the agent.

[0060] Comparative Examples 5-7 verified the necessity of dual-frequency ultrasound in this invention by conducting comparative experiments under different sound field conditions.

[0061] Comparative Example 5 The only difference from Example 1 is that step (2) does not include dual-frequency ultrasonic treatment, but only mechanical stirring. Specifically, it includes the following steps: Step (1) is the same as in Example 1; (2) The fine-particle slurry obtained in step (1) was transported to an enhanced reaction vessel. After adjusting the solids content to 15%, GLDA (30 mmol / L) and citric acid (30 mmol / L) (molar ratio 1:1) were added as a compound chelating agent. After adjusting the pH of the system to stabilize at 4.5, the reaction was mechanically stirred at 200 rpm for 60 minutes. After the reaction was completed, the desorbed slurry was obtained.

[0062] Steps (3)-(4) are the same as in Example 1.

[0063] Comparative Example 6 The only difference from Example 1 is that step (2) does not include dual-frequency ultrasound processing, but instead performs single-frequency ultrasound processing. Specifically, it includes the following steps: Step (1) is the same as in Example 1; (2) The fine-particle slurry obtained in step (1) was transported to an enhanced reaction vessel. After adjusting the solids content to 15%, GLDA (30 mmol / L) and citric acid (30 mmol / L) (molar ratio 1:1) were added as a compound chelating agent. After the pH of the system was stabilized at 4.5, the array-type ultrasonic transducer was turned on, the low frequency was set to 28 kHz (power 300W), the total reaction time was 60 minutes, and the mechanical stirring speed was 200 rpm. After the reaction was completed, the desorbed slurry was obtained.

[0064] Steps (3)-(4) are the same as in Example 1.

[0065] Comparative Example 7 The only difference from Example 1 is that step (2) does not include dual-frequency ultrasound processing, but instead performs single-frequency ultrasound processing. Specifically, it includes the following steps: Step (1) is the same as in Example 1; (2) The fine-particle slurry obtained in step (1) was transported to an enhanced reaction vessel. After adjusting the solid content to 15%, GLDA (30 mmol / L) and citric acid (30 mmol / L) (molar ratio 1:1) were added as a compound chelating agent. After the pH of the system was stabilized at 4.5, the array-type ultrasonic transducer was turned on, the high frequency was set to 40 kHz (power 300W), the total reaction time was 60 minutes, and the mechanical stirring speed was 200 rpm. After the reaction was completed, the desorbed slurry was obtained.

[0066] Steps (3)-(4) are the same as in Example 1.

[0067] Technical effects: The content of heavy metal Pb in the regenerated magnetic nanoparticles obtained in Example 1 and Comparative Examples 1-4 was determined by microwave digestion-ICP-MS, and the Pb removal rate was calculated. The kinetic constant k was also calculated.

[0068] The effects of different ultrasound modes on reaction kinetics are shown in Table 2.

[0069] Table 2 As shown in Table 2, the reaction rate constant k of dual-frequency ultrasound (Example 1) is almost nine times that of mechanical stirring (Comparative Example 5), and the reaction time is shortened by more than 33%. This is mainly attributed to the strong shock waves generated by low-frequency ultrasound effectively breaking down the solid micro-agglomerates formed by clay particles, exposing deep-seated contamination sites; while the high-density microbubbles generated by high-frequency ultrasound enter the micropores, reducing the liquid film thickness through microjets and accelerating the diffusion of complexes. Both are indispensable and work synergistically to achieve deep and rapid extraction of fine-particle heavy metals.

[0070] Comparative Examples 8-10 were used to verify the advantages of magnetically sensitized separation by comparing the solid-liquid separation effects of fine mud.

[0071] Comparative Example 8 The only difference from Example 1 is that steps (3) and (4) are omitted; instead, the desorbed slurry is allowed to settle naturally to remove the sludge. Specifically, the following steps are included: Steps (1)-(2) are the same as in Example 1; (3) After the desorbed slurry has settled naturally for 24 hours, the supernatant and the lower layer of sludge are separated.

[0072] Comparative Example 9 The only difference from Example 1 is that steps (3) and (4) are omitted; instead, the sludge is removed only by chemical flocculation of the desorbed slurry. Specifically, the following steps are included: Steps (1)-(2) are the same as in Example 1; (3) Add 0.1% flocculant (PAM) to the desorbed slurry, let it stand for 45 minutes, and then separate the supernatant from the lower sludge.

[0073] Comparative Example 10 The only difference from Example 1 is that steps (3) and (4) are not included; instead, the desorbed slurry is simply filtered through a plate and frame filter press to remove the sludge. Specifically, the following steps are included: Steps (1)-(2) are the same as in Example 1; (3) The desorbed slurry is filtered through a plate and frame filter press to separate the water and sludge in the slurry.

[0074] Technical effects: The turbidity of the supernatant, sludge moisture content, zeta before and after the addition of magnetic nanoparticles, and the content of heavy metals and calcium in the sludge were measured in Examples 1 and Comparative Examples 8-10. The content of heavy metals and calcium was determined by microwave digestion-ICP-MS. The results are shown in Table 3.

[0075] Table 3 Comparison of treatment effects of different solid-liquid separation methods As shown in Table 3, compared to Comparative Examples 8-10, this invention achieves the separation of water and sludge within 10 seconds, with extremely low risk of heavy metal loss with the sludge. Simultaneously, the turbidity of the resulting supernatant is <20, improving sludge recycling. Furthermore, in Example 1, the addition of aminated magnetic nanoparticles increased the Zeta potential of the soil colloid from -35 mV to -15 mV, weakening the electrostatic repulsion. This is because, during the magnetosensitization process, the aminated magnetic nanoparticles (positively charged, Zeta potential ~+30 mV @ pH 4.5) undergo strong electrostatic adsorption and charge neutralization with the negatively charged sludge clay colloids (Zeta potential ~-30 mV), acting as "nuclei" to induce the aggregation of clay particles into dense magnetic flocs. When flowing through HGMS, these micro-flocs experience extremely high magnetic gradient forces (Fm>10). 5 N / m 3 The trace amounts of free Pb remaining in the liquid phase were instantly captured by the steel wool. 2+The sludge is further adsorbed by the -NH2 groups on the surface of the magnetic particles, further reducing the liquid concentration. This not only achieves second-level separation of water and sludge, but also significantly reduces the difficulty of subsequent sludge dewatering due to the compact structure of the magnetic flocs, which entrain less water.

[0076] A system comparison between the examples and comparative examples shows that the compound chelating agent improves selective desorption efficiency, dual-frequency ultrasound enhances mass transfer and aggregate breakage, magnetosensitization imparts magnetic responsiveness to particles, and high-gradient magnetic separation achieves second-level solid-liquid separation. These four elements constitute a synergistic system of "desorption-enhancement-magnetization-separation," and the absence of any one of these steps leads to a significant decrease in overall efficiency, demonstrating a clear synergistic effect.

[0077] The above are merely preferred embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for rapid elution and separation of heavy metals in engineering waste soil, characterized in that, Includes the following steps: (1) After pretreatment, the engineering waste soil is mixed with water to make mud slurry. After the mud slurry is screened by two-stage hydraulic cyclone to remove aggregate, a compound chelating agent is added for dual-frequency ultrasonic enhanced desorption treatment to obtain desorbed slurry. (2) Add magnetic nanoparticles to the desorbed slurry and perform magnetic sensitization conditioning. After magnetic separation, filter the mud through a plate and frame filter press and recover the regenerated magnetic nanoparticles to complete the washing and separation of heavy metals in the engineering waste soil.

2. The method for rapid elution and separation of heavy metals in engineering waste soil according to claim 1, characterized in that, The mud mentioned in step (1) is obtained by mixing engineering waste soil and water at a solid-liquid ratio of 1g:4mL.

3. The method for rapid elution and separation of heavy metals in engineering waste soil according to claim 1, characterized in that, In step (1), the separation particle size of the first stage hydrocyclone is 2.0 mm, and coarse sand and gravel are removed; the separation particle size of the second stage hydrocyclone is less than 0.075 mm, and fine sand is removed.

4. The method for rapid elution and separation of heavy metals in engineering waste soil according to claim 1, characterized in that, The amount of compound chelating agent added in step (1) is 0.01-0.1 mol / L.

5. The method for rapid elution and separation of heavy metals in engineering waste soil according to claim 4, characterized in that, The compound chelating agent is obtained by mixing tetrasodium glutamate diacetate and citric acid in a molar ratio of 1:(1-3).

6. The method for rapid elution and separation of heavy metals in engineering waste soil according to claim 1, characterized in that, In step (1), during the dual-frequency ultrasonic enhanced desorption process, the low frequency (20-28 kHz) and the high frequency (40-60 kHz) alternate, with a switching cycle of 10-30 seconds, and the ultrasonic power density is 0.3-0.6 W / cm². 2 The total reaction time is 20-60 minutes, and the stirring speed is 200 rpm.

7. The method for rapid elution and separation of heavy metals in engineering waste soil according to claim 1, characterized in that, The magnetic nanoparticles described in step (2) have a core-shell structure, with the core being Fe3O4 and the outer shell being a SiO2 layer coated with amino or thiol functional groups. The magnetic nanoparticles have an average particle size of 50-200 nm, a saturation magnetization greater than 50 emu / g, and an isoelectric point pH value greater than 8.

0. The amount of magnetic nanoparticles added is 0.5%-2.0% of the dry soil mass.

8. The method for rapid elution and separation of heavy metals in engineering waste soil according to claim 1, characterized in that, The background magnetic field for magnetic separation in step (2) is 1.0-2.5T, and the slurry flow rate is 0.5-3.0 cm / s.

9. The method for rapid elution and separation of heavy metals in engineering waste soil according to claim 1, characterized in that, The recycling and regeneration of magnetic nanoparticles in step (2) specifically includes the following steps: The magnetic nanoparticles obtained after magnetic separation are backwashed with an inorganic acid solution for 15-40 minutes. The resulting eluent is a heavy metal concentrate, and regenerated magnetic nanoparticles are obtained simultaneously.

10. The application of the rapid elution and separation method for heavy metals in engineering waste as described in any one of claims 1-9 in the resource utilization of engineering waste.

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