System and method for removing heavy metals from groundwater

Abstract

The application belongs to the technical field of water treatment, and particularly relates to a groundwater heavy metal removal system and method, wherein a water solution of a complexing agent is injected into groundwater at an upstream, the complexing agent comprising the following components in parts by weight: 10 parts of ethylenediamine disuccinic acid tetrasodium salt; 3-3.5 parts of sodium gluconate; 1.5-2 parts of ferrous sulfate heptahydrate; 0.8-1 part of citric acid monohydrate; and 5-6 parts of sodium bicarbonate; a reaction wall is built at a downstream, a strong reduction filler layer is arranged in the reaction wall, the strong reduction filler layer comprises sulfur-modified zero-valent iron particles and conductive particles, the strong reduction filler layer reduces high-valence heavy metals in the groundwater to generate stable precipitates, and the application can promote the heavy metals to flow to the reaction wall, avoid the heavy metals from being retained in soil, and improve the removal effect on the heavy metals.

Description

Technical Field

[0001] This invention belongs to the field of water treatment technology, and in particular to a system and method for removing heavy metals from groundwater. Background Technology

[0002] Heavy metal pollution in groundwater seriously threatens drinking water safety and the ecological environment. Heavy metals in groundwater mainly originate from industrial emissions, agricultural activities, and domestic waste pollution, specifically including lead, cadmium, mercury, hexavalent chromium, arsenic, copper, zinc, and nickel. Existing methods for removing heavy metals from groundwater include in-situ remediation and ex-situ remediation. Ex-situ remediation involves extracting groundwater and then remediating it, suitable for moderate and small-scale pollution. In-situ remediation directly remediates groundwater underground, causing less environmental disturbance and suitable for large-scale remediation. Specific methods include: injecting reducing agents (such as zero-valent iron and sulfites) to reduce highly toxic hexavalent chromium to trivalent chromium precipitate and mercury ions to elemental mercury; injecting solidifying agents such as cement, lime, and phosphates to fix heavy metals in the soil / aquifer medium through adsorption, precipitation, and ion exchange, reducing their mobility; and constructing reactive barriers along the pollution plume migration path, filled with functional materials such as zero-valent iron, activated carbon, and zeolite, where heavy metals are degraded or adsorbed as they pass through the barriers. While injecting various agents into groundwater can promote the precipitation or transfer of heavy metals into the soil, the heavy metals remain in the soil, leading to excessive levels of heavy metals in the soil. Constructing reactive barriers is a remediation method that directly removes heavy metals, fundamentally reducing the content of heavy metals in the groundwater. Examples include a utility model patent with application number CN202421158062.0 disclosing an in-situ groundwater heavy metal pollution remediation device, and an invention patent with application number CN202210421762.3 disclosing a biorenewable in-situ remediation method for groundwater in a zero-valent iron sulfide base.

[0003] Currently, the use of reactive barriers for treating heavy metals in groundwater has the following problems:

[0004] 1. Many heavy metals (such as lead, cadmium, divalent mercury, and trivalent chromium) have weak migration ability, are easily adsorbed by the aquifer medium, and are easily retained in the soil, making it impossible to effectively reach the reactive barrier.

[0005] 2. Existing reaction walls typically contain adsorption packing material, which is not effective enough for removing heavy metals. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a groundwater heavy metal removal system and method that promotes the flow of heavy metals to the reactive barrier, avoids the retention of heavy metals in the soil, and improves the removal effect of heavy metals.

[0007] To solve the above problems, the technical solution adopted in this invention is: a method for removing heavy metals from groundwater.

[0008] An aqueous solution of a complexing agent is injected into the groundwater upstream. The complexing agent comprises the following components in parts by weight:

[0009] 10 parts of tetrasodium ethylenediamine disuccinate; 3-3.5 parts of sodium gluconate; 1.5-2 parts of ferrous sulfate heptahydrate; 0.8-1 part of citric acid monohydrate; 5-6 parts of sodium bicarbonate;

[0010] A reaction wall is constructed downstream, and a strong reducing filler layer is installed inside the reaction wall. The strong reducing filler layer includes sulfur-modified zero-valent iron particles and conductive particles. The strong reducing filler layer reduces high-valent heavy metals in the groundwater to generate stable precipitates.

[0011] Further, tetrasodium ethylenediamine disuccinate, sodium gluconate, sodium bicarbonate, and water are added to the first container and stirred until completely dissolved; ferrous sulfate heptahydrate, citric acid monohydrate, and deoxygenated water are added to the second container to form a clear ferrous-citric acid composite solution; the ferrous-citric acid composite solution is slowly added to the first container and mixed evenly, and then immediately injected into the upstream groundwater.

[0012] Furthermore, after the groundwater is treated by the strong reducing filler layer, it is treated again by a mixed filler of sulfur particles and limestone particles.

[0013] Furthermore, after the heavy metals are reduced by the strong reducing filler layer, the precipitate generated is adsorbed by the mixed filler of hydroxyapatite particles and limonite sand.

[0014] Furthermore, the sulfur-modified zero-valent iron particles comprise a zero-valent iron core, an FeSx gradient layer, and a porous carbon shell arranged sequentially from the inside out, wherein x is 0.8-1.1.

[0015] Furthermore, the preparation process of sulfur-modified zero-valent iron particles is as follows:

[0016] S1. Dissolve ferric nitrate nonahydrate, sodium thiosulfate and glucose in a mixed solvent of deionized water and ethanol, and then hydrothermally react at 120-180°C for 6-12 hours to obtain the precursor composite sol.

[0017] S2. Spray dry the precursor composite sol to obtain precursor microspheres;

[0018] S3. Place the precursor microspheres in a fluidized bed reactor and preheat them to 300-400°C under inert gas protection. First, introduce a mixed gas containing sulfur vapor and reducing gas to form a sulfur-rich sulfidation layer on the surface of the precursor microspheres. Then, introduce carbon source gas to form a carbon shell on the outer wall of the sulfur-rich sulfidation layer.

[0019] S4. In a high-purity hydrogen atmosphere, the precursor microspheres coated with a sulfur-rich sulfide layer and a carbon shell are heated to 300-500℃ at a rate of 5°C / min, then heated to 500-700℃ at a rate of 2°C / min, and held at 500-700℃ for 1-2 hours.

[0020] S5. Cool to room temperature in a hydrogen or inert gas atmosphere.

[0021] The groundwater heavy metal removal system used in the above-mentioned groundwater heavy metal removal method includes a reaction wall installed downstream of the groundwater and multiple complexing agent injection mechanisms installed upstream of the groundwater. The reaction wall is provided with a strong reducing filler layer.

[0022] Furthermore, the reactive wall includes a permeable wall, a first water-proof wall, and a second water-proof wall, which are arranged sequentially along the flow direction of groundwater. A water storage chamber is provided between the permeable wall and the first water-proof wall, and a treatment chamber is provided between the first water-proof wall and the second water-proof wall. One end of the water storage chamber is connected to one end of the treatment chamber through a connecting hole located at the bottom of the first water-proof wall, and the other end of the treatment chamber is connected to the groundwater layer downstream of the reactive wall. The strong reducing filler layer is located in the treatment chamber.

[0023] Furthermore, the treatment chamber is connected to the groundwater layer downstream of the reaction wall via a pumping mechanism. The connection hole leads to the treatment chamber of the pumping mechanism, where multiple strong reducing packing layers, at least one auxiliary reducing packing layer, and at least one sedimentation and adsorption layer are sequentially arranged. A third baffle is arranged between each pair of adjacent packing layers, and the third baffle divides the treatment chamber into a wave-shaped water flow channel. The upper surfaces of the strong reducing packing layer, the auxiliary reducing packing layer, and the sedimentation and adsorption layer are all provided with an anaerobic-aerobic alternating treatment packing layer.

[0024] Furthermore, the auxiliary reduction packing layer includes sulfur particles and limestone particles, the precipitation adsorption layer includes hydroxyapatite particles and limonite sand, and the anaerobic-aerobic alternating treatment packing layer includes porous ceramic or plastic biological packing.

[0025] The beneficial effects of this invention are: by injecting a complexing agent into the groundwater upstream, the complexing agent can complex the heavy metals that are retained and adsorbed in the soil as the groundwater flows to the reaction wall, thereby promoting the migration of heavy metals to the reaction wall, so as to remove heavy metals more effectively and reduce the heavy metal content in the soil.

[0026] Complexing agents include tetrasodium ethylenediaminedisuccinate, sodium gluconate, ferrous sulfate heptahydrate, citric acid monohydrate, and sodium bicarbonate. Among them, tetrasodium ethylenediaminedisuccinate is a highly efficient and environmentally friendly chelating agent with extremely strong complexing ability. The complexes formed have good water solubility and high stability, playing the main complexing function and effectively complexing heavy metals such as trivalent chromium, divalent lead, divalent cadmium, divalent copper, divalent nickel, divalent mercury, and divalent zinc. Sodium gluconate is a weak hydroxycarboxylic acid complexing agent with moderate complexing ability for alkaline earth metals such as calcium, magnesium, iron (II / III), and manganese, as well as transition metals (which tetrasodium ethylenediaminedisuccinate struggles to complex), playing an auxiliary role in complexation without interfering with the complexation reaction of tetrasodium ethylenediaminedisuccinate.

[0027] When ferrous sulfate heptahydrate and citric acid monohydrate dissolve in water, the citrate ion forms a relatively stable ferrous-citric acid mixture with the ferrous ion. This slows down the oxidation of ferrous ions by oxygen in the water, allowing them to migrate long distances to the reaction wall. The complexation reaction between citrate and ferrous ions is reversible. A small amount of free ferrous ions exist in the ferrous-citric acid system. When these ions encounter hexavalent chromium, they can reduce it to low-toxicity, easily precipitated trivalent chromium, reducing the acute toxicity and oxidation potential of the pollutant plume and lessening the treatment burden on the strong reducing packing layer. Trivalent chromium readily complexes with tetrasodium ethylenediaminedisuccinate, ensuring effective migration of chromium to the reaction wall. Some heavy metals, when left in groundwater for extended periods, are adsorbed and encapsulated by the surface of aquifer sand particles and iron-manganese oxide particles, making release difficult. In this invention, citric acid itself is an excellent natural complexing agent with a strong affinity for ferric iron. During migration, it etches natural metal oxides onto the surface of solid particles in the aquifer (such as sand and iron-manganese oxides), thereby releasing "aged" heavy metal pollutants (such as arsenic and lead encapsulated in iron oxides) that were firmly adsorbed or co-precipitated by the natural oxides. These pollutants are then captured by the main complexing agent, tetrasodium ethylenediamine disuccinate. Therefore, the addition of ferrous sulfate heptahydrate and citric acid monohydrate can promote the migration of heavy metals that have remained in the soil for a long time to the reactive barrier, where they are removed.

[0028] When groundwater reaches the strong reducing filler layer, zero-valent iron (ZVFe) directly reduces heavy metals in the water, oxidizing itself to ferrous ions. Simultaneously, ZVFe provides an extremely low redox potential, disrupting the complexation equilibrium of ferrous ions and rapidly releasing them. This dual replenishment of ferrous ions sustains the reducing capacity of the strong reducing filler layer. Ferrous ions, acting as electron carriers, diffuse into the pore water far from the ZVFe surface, transferring electrons to heavy metal ions. These heavy metal ions are reduced and oxidized to ferric ions. When ferric ions diffuse to the ZVFe surface, they gain electrons again and are reduced back to ferrous ions, continuously cycling this process. This efficiently transports electrons from the ZVFe surface into the pores, expanding the effective reaction space and overcoming the limitation of traditional ZVFe only being able to perform surface contact reactions. Furthermore, traditional ZVFe surfaces easily form a ferric hydroxide passivation film, leading to reduced activity. Citrate ions, however, have a strong complexing ability for ferric ions and can etch away the existing passivation film, restoring the activity of the ZVFe. Attached Figure Description

[0029] Figure 1 This is a cross-sectional schematic diagram of the groundwater heavy metal removal system of the present invention;

[0030] Figure 2 This is a top view of the reactive barrier;

[0031] Figure 3 yes Figure 2 Schematic diagram of the cross section of AA;

[0032] Figure 4 yes Figure 2 Cross-sectional view of BB;

[0033] Reference numerals: 1—permeable wall; 2—first water-blocking wall; 3—second water-blocking wall; 4—water storage chamber; 5—connection hole; 6—pumping mechanism; 7—auxiliary reduction packing layer; 8—strong reduction packing layer; 9—sedimentation adsorption layer; 10—third water-blocking plate; 11—complexing agent injection mechanism; 12—anaerobic-aerobic alternating treatment packing layer. Detailed Implementation

[0034] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0035] The groundwater heavy metal removal system of the present invention, such as Figure 1 As shown, it includes a reaction wall located downstream of the groundwater and multiple complexing agent injection mechanisms 11 located upstream of the groundwater. A strong reducing filler layer 8 is provided inside the reaction wall.

[0036] The complexing agent injection mechanism 11 is used to inject an aqueous solution of complexing agent into the groundwater upstream. Specifically, the complexing agent injection mechanism 11 can be an injection pipe driven to the depth where the groundwater is located. The injection pipe has multiple evenly distributed injection holes. The injection pipe is connected to a drive pump, which uses the drive pump to pressurize the aqueous solution of complexing agent into the groundwater. Multiple complexing agent injection mechanisms 11 can be set to inject the aqueous solution of complexing agent into the groundwater in a dispersed and uniform manner, ensuring the diffusion range of the complexing agent.

[0037] A reactive barrier is constructed at a suitable location downstream of the groundwater. The reactive barrier captures heavy metals in the groundwater, reducing their content. A strong reducing filler layer 8 is installed within the reactive barrier. This strong reducing filler layer 8 includes sulfur-modified zero-valent iron particles and conductive particles. The conductive particles can be made of stable conductive materials such as graphite particles. The strong reducing filler layer 8 reduces the high-valent heavy metals in the groundwater, generating stable precipitates.

[0038] The role of the complexing agent is that, as the groundwater flows to the reaction wall, it can complex the heavy metals that are retained and adsorbed in the soil, thereby promoting the migration of heavy metals to the reaction wall, so as to remove heavy metals more effectively and reduce the heavy metal content in the soil.

[0039] Specifically, the complexing agent comprises the following components in parts by weight:

[0040] 10 parts of tetrasodium ethylenediamine disuccinate; 3-3.5 parts of sodium gluconate; 1.5-2 parts of ferrous sulfate heptahydrate; 0.8-1 parts of citric acid monohydrate; 5-6 parts of sodium bicarbonate.

[0041] Among them, tetrasodium ethylenediamine disuccinate is a highly efficient and environmentally friendly chelating agent with extremely strong complexing ability. The complex formed has good water solubility and high stability, playing the main complexing function and effectively complexing heavy metals such as trivalent chromium, divalent lead, divalent cadmium, divalent copper, divalent nickel, divalent mercury, and divalent zinc.

[0042] Tetrasodium ethylenediaminedisuccinate has poor complexing ability with alkaline earth metals and transition metals, while sodium gluconate, a weak hydroxycarboxylic acid complexing agent, has moderate complexing ability with alkaline earth metals and transition metals such as calcium, magnesium, iron (II / III), and manganese, playing an auxiliary role in complexation and improving the overall complexing effect. Since the complexing ability of sodium gluconate is lower than that of tetrasodium ethylenediaminedisuccinate, it does not interfere with the complexation reaction of tetrasodium ethylenediaminedisuccinate.

[0043] When ferrous sulfate heptahydrate and citric acid monohydrate dissolve in water, the citrate ion and ferrous ion form a relatively stable ferrous-citric acid, which can slow down the rate of oxidation of ferrous ions by oxygen in water, allowing ferrous ions to migrate long distances to the reaction wall.

[0044] Tetrasodium ethylenediaminedisuccinate has difficulty complexing hexavalent chromium. The complexation reaction between citrate and ferrous ions is reversible. A small amount of free ferrous ions exist in the ferrous-citric acid system. When ferrous ions encounter hexavalent chromium, they can reduce hexavalent chromium to trivalent chromium, which is less toxic and easier to precipitate. This reduces the acute toxicity and oxidation potential of the pollutant plume and reduces the treatment burden on the strong reducing packing layer. Trivalent chromium readily complexes with tetrasodium ethylenediaminedisuccinate, ensuring that hexavalent chromium can also effectively migrate to the reaction wall.

[0045] Some heavy metals, when retained in groundwater and soil for extended periods, are adsorbed onto the surfaces of aquifer sand particles and iron-manganese oxide particles, and then encapsulated by natural oxides, making them difficult to release. In this invention, citric acid itself is an excellent natural complexing agent with a strong affinity for ferric and aluminum. During migration, it etches the natural metal oxides on the surface of aquifer solid particles (such as sand and iron-manganese oxides), thereby releasing "aged" heavy metal pollutants (such as arsenic and lead encapsulated by iron oxides) that were firmly adsorbed or co-precipitated by the natural oxides. These pollutants are then captured by the main complexing agent, tetrasodium ethylenediamine disuccinate. Therefore, the addition of ferrous sulfate heptahydrate and citric acid monohydrate can promote the migration of heavy metals that have remained in the soil for a long time to the reactive barrier, where they are removed.

[0046] Sodium bicarbonate adjusts the pH of groundwater to a neutral range (6.5-8.0), ensuring efficient complexation reactions and providing a stable chemical environment for subsequent treatment.

[0047] In the strong reducing filler layer 8, the particle size of zero-valent iron particles is smaller than that of conductive particles, which increases the porosity of the filler layer. Furthermore, the conductive particles overlap with each other to form a conductive path that runs through the strong reducing filler layer 8, significantly reducing the resistance to electron transfer.

[0048] When groundwater reaches the strong reducing filler layer 8, zero-valent iron (ZVFe) directly reduces the heavy metals that have moved to its surface, and is itself oxidized to ferrous ions. Simultaneously, ZVFe provides an extremely low redox potential, disrupting the complexation equilibrium of ferrous ions and rapidly releasing ferrous ions. This dual replenishment of ferrous ions sustains the reducing capacity of the strong reducing filler layer. Ferrous ions, acting as electron carriers, diffuse into the pore water far from the ZVFe surface, transferring electrons to the heavy metal ions. The heavy metal ions are reduced and oxidized to ferric ions. When the ferric ions diffuse to the ZVFe surface, they gain electrons again and are reduced back to ferrous ions. This cycle repeats continuously, efficiently transporting electrons from the ZVFe surface into the pores, expanding the effective reaction space, and overcoming the limitation of traditional ZVFe only being able to undergo surface contact reactions. In addition, the traditional zero-valent iron surface is prone to forming a passivation film of ferric hydroxide, which leads to reduced activity. Citrate has a strong complexing ability for ferric ions, and citrate can etch the already formed passivation film (ferric hydroxide film) to restore the activity of zero-valent iron.

[0049] The reduced heavy metals eventually form stable precipitates. For example, chromium is converted into chromium hydroxide and chromium sulfide precipitates, lead into lead sulfide precipitates, cadmium into cadmium sulfide and cadmium hydroxide precipitates, arsenic into arsenic sulfide and iron oxide precipitates, mercury into elemental mercury (liquid) and mercury sulfide precipitates, etc. These precipitates are adsorbed by the strong reducing filler layer 8.

[0050] The preparation process of the aqueous solution of the complexing agent is as follows:

[0051] Add tetrasodium ethylenediaminedisuccinate, sodium gluconate, sodium bicarbonate, and water to the first container and stir until completely dissolved. The amount of water can vary, as long as it is sufficient to completely dissolve the three materials. Add ferrous sulfate heptahydrate, citric acid monohydrate, and deoxygenated water to the second container to form a clear ferrous-citric acid composite solution. The amount of deoxygenated water should be minimized. Slowly add the ferrous-citric acid composite solution to the first container and mix thoroughly. Immediately after mixing, inject the mixture into the upstream groundwater. If it cannot be used immediately, it needs to be sealed and stored away from light.

[0052] In this invention, the reactive barrier includes a permeable wall 1, a first water-blocking wall 2, and a second water-blocking wall 3. The permeable wall 1, the first water-blocking wall 2, and the second water-blocking wall 3 are arranged sequentially along the direction of groundwater flow. The permeable wall 1 is a wall that groundwater can penetrate; existing technology can be used, and it has a certain filtering function, preventing soil particles from entering the interior of the reactive barrier with the groundwater. The first water-blocking wall 2 and the second water-blocking wall 3 can be made of concrete or metal plates, preventing groundwater from penetrating them.

[0053] A water storage chamber 4 is provided between the permeable wall 1 and the first water-proof wall 2, and a treatment chamber is provided between the first water-proof wall 2 and the second water-proof wall 3. One end of the water storage chamber 4 is connected to one end of the treatment chamber through a connecting hole 5 located at the bottom of the first water-proof wall 2, and the other end of the treatment chamber is connected to the groundwater layer downstream of the reaction wall. The strong reducing filler layer 8 is located in the treatment chamber. A water-proof plate is also provided at the bottom of the treatment chamber to prevent groundwater from entering or leaving the treatment chamber through the bottom wall.

[0054] Groundwater typically flows horizontally, meaning it flows slowly along a direction roughly parallel to the ground surface. Upon reaching the reactive barrier, it seeps into the interior of the barrier from its entire surface. Reactive barriers are usually long and thin, making it difficult to guarantee the retention time of surface water. In this invention, a long, narrow water storage chamber 4 and a treatment chamber are separated inside the reactive barrier. Groundwater seeps evenly into the water storage chamber 4 through the permeable wall 1, then enters one end of the treatment chamber through the connecting hole 5. It then flows along the treatment chamber; the long flow path ensures sufficient retention time for the removal of heavy metals, and finally, it is discharged from the other end of the treatment chamber into the downstream groundwater layer.

[0055] Groundwater flows relatively slowly, therefore, removing heavy metals from groundwater using reactive barriers is extremely time-consuming, potentially lasting decades. To facilitate faster groundwater flow to the reactive barrier, in this invention, the treatment chamber is connected to the groundwater layer downstream of the reactive barrier via a pumping mechanism 6. A connection hole 5 leads to the treatment chamber of the pumping mechanism 6. Multiple strong reducing packing layers 8, at least one auxiliary reducing packing layer 7, and at least one sedimentation and adsorption layer 9 are sequentially arranged within this chamber. A third baffle plate 10 is positioned between each pair of adjacent packing layers, dividing the treatment chamber into wavy water flow channels. Anaerobic-aerobic alternating treatment packing layers 12 are provided on the upper surfaces of the strong reducing packing layer 8, the auxiliary reducing packing layer 7, and the sedimentation and adsorption layer 9.

[0056] The strong reducing packing layer 8 plays a major role in reducing heavy metals. After treatment by the strong reducing packing layer 8, a small amount of residual heavy metals remain in the groundwater, along with residual complexing agents such as tetrasodium ethylenediamine disuccinate and citric acid. The auxiliary reducing packing layer 7 and the precipitation adsorption layer 9 can further treat the residues. Specifically:

[0057] The auxiliary reducing packing layer 7 comprises sulfur particles and limestone particles, and is inoculated with sulfur-oxidizing bacteria and sulfate-reducing bacteria. After the groundwater is treated by the strong reducing packing layer 8, a mixed packing of sulfur and limestone particles is used for further treatment. The groundwater has low oxygen content; sulfur, acting as an electron acceptor, promotes the growth of sulfur-oxidizing bacteria, which oxidize sulfur to sulfate ions under anaerobic conditions. These sulfate ions are then converted to S²⁻ by sulfate-reducing bacteria, forming stable sulfide precipitates with residual trace heavy metals. Limestone provides a buffer and calcium source.

[0058] The precipitate adsorption layer 9 comprises hydroxyapatite particles and limonite sand. After the strong reducing filler layer 8 reduces heavy metals, some tiny precipitates flow with the groundwater and are adsorbed by the mixed filler of hydroxyapatite particles and limonite sand to form precipitates. Hydroxyapatite undergoes a calcium displacement reaction with the precipitates, where heavy metal ions replace calcium in the hydroxyapatite lattice, generating more stable heavy metal apatite with a lower solubility product than sulfides, essentially achieving permanent fixation and completely preventing secondary leaching. Limonite sand is a porous natural iron oxide with high porosity and a large specific surface area, capable of trapping nanoscale fine precipitates. Simultaneously, the hydroxyl sites on its surface adsorb the precipitate particles, which are then encapsulated by iron oxide colloids, locking the precipitates within the pores and preventing loss. Furthermore, the ferric ions released by the limonite sand can form ferrites (such as FeCr2O4 and FeAsO4) with the reduced arsenic and chromium, co-precipitating to solve the fixation problem of difficult-to-sulfurize and easily lost heavy metals, compensating for the shortcomings of the strong reducing filler layer 8 in treating arsenic.

[0059] The anaerobic-aerobic alternating treatment packing layer 12 includes porous ceramic or plastic biological packing. Under anaerobic conditions, denitrification occurs, and under aerobic conditions, an aerobic biofilm is cultivated. This biodegrades and simultaneously degrades complexing agent residues such as tetrasodium ethylenediamine disuccinate and citric acid, eliminating the risk of secondary pollution.

[0060] When the water level in the second treatment chamber reaches the set level, the pumping mechanism 6 pumps the groundwater in the treatment chamber downstream, causing the water levels in the treatment chamber and the storage chamber 4 to gradually decrease. When the water level reaches the bottom of the anaerobic-aerobic alternating treatment packing layer 12, pumping stops, and the groundwater naturally seeps into the storage chamber 4, causing the water levels in the storage chamber 4 and the treatment chamber to gradually rise. This process is repeated, allowing the anaerobic-aerobic alternating treatment packing layer 12 to alternately be submerged and exposed to air, creating a continuously alternating anoxic-aerobic environment that naturally allows for deep treatment of residual complexing agents.

[0061] Meanwhile, the natural flow direction of groundwater is from the upstream high water level area to the low water level area of ​​the reaction wall. When the water level in the water storage chamber 4 decreases, the water level difference between the water storage chamber 4 and the upstream groundwater increases, and the driving force of hydraulic infiltration is enhanced, which can promote the groundwater to enter the water storage chamber 4 more quickly and improve the efficiency of groundwater treatment.

[0062] By setting multiple third water-blocking plates 10, the groundwater can flow up and down, ensuring that the groundwater can pass through the strong reducing filler layer 8.

[0063] This invention can use conventional sulfur-modified zero-valent iron particles, but conventional sulfur-modified zero-valent iron particles have problems such as easy agglomeration, easy corrosion and passivation, and uneven sulfur distribution. The sulfur-modified zero-valent iron particles of this invention include a zero-valent iron core, an FeSx gradient layer and a porous carbon shell layer arranged sequentially from the inside to the outside, wherein x is 0.8-1.1.

[0064] The preparation process of sulfur-modified zero-valent iron particles is as follows:

[0065] S1. Ferric nitrate nonahydrate, sodium thiosulfate, and glucose are dissolved in a mixed solvent of deionized water and ethanol, and then hydrothermally reacted at 120-180°C for 6-12 hours to obtain a precursor composite sol. The ratio of ferric nitrate nonahydrate, sodium thiosulfate, and glucose satisfies the following molar ratio: iron, sulfur, and carbon is 1:(0.1-0.3):(0.5-2). During this process, sodium thiosulfate decomposes to produce S²⁻, which reacts with some Fe²⁺ to form FeS nanocrystals. Glucose undergoes preliminary carbonization, forming a uniform Fe(OH)₃, FeS, and carbon precursor composite sol.

[0066] S2. Spray drying of the precursor composite sol, with an inlet temperature of 180-220°C, causes the droplets to dry instantly after spraying, resulting in precursor microspheres with a particle size of 50-200 micrometers.

[0067] S3. Place the precursor microspheres in a fluidized bed reactor and preheat to 300-400°C under inert gas (such as nitrogen) protection. First, introduce a mixture of sulfur vapor and reducing gas (such as hydrogen). The sulfur vapor preferentially reacts with the iron on the surface of the precursor microspheres, forming a thin and relatively uniform sulfur-rich sulfide layer (mainly FeS2) on the surface of the precursor microspheres. Maintain the temperature at 300-400°C, and then introduce a carbon source gas. The carbon source gas is a dilute mixture such as propylene / ethylene. The carbon source undergoes chemical vapor deposition on the surface of the precursor microspheres, thereby forming a porous amorphous carbon shell on the outer wall of the sulfur-rich sulfide layer. This carbon shell encapsulates and fixes the sulfur and iron inside.

[0068] S4. In a high-purity hydrogen atmosphere, the precursor microspheres coated with a sulfur-rich sulfide layer and a carbon shell are heated to 300-500℃ at a rate of 5°C / min. During this stage, the outer amorphous carbon shell is stabilized and strengthened, and the internal FeS2 begins to convert to FeS, releasing some active sulfur. The high-purity hydrogen atmosphere can specifically be a mixture of hydrogen and argon, with a hydrogen molar content of 5-10%.

[0069] The temperature is then increased to 500-700℃ at a rate of 2°C / min and held at this temperature for 1-2 hours. Under a strong reducing atmosphere, the internal iron oxides (ferric oxide and magnetite) are completely reduced to zero-valent iron, forming the core. The released active sulfur reacts with some of the newly formed zero-valent iron and FeS to form a FeSx gradient layer with varying composition, transitioning from sulfur-rich near the carbon shell to sulfur-poor near the zero-valent iron core. The carbon shell acts as a confinement layer, preventing the zero-valent iron particles from excessively sintering and growing at high temperatures, and limiting the loss of active sulfur.

[0070] This step can be performed in a tube furnace.

[0071] S5. Cool to room temperature in a hydrogen or inert gas atmosphere, or cool with the furnace.

[0072] The sulfur-modified zero-valent iron particles prepared by this process have a porous carbon shell that protects the active components, preventing the zero-valent iron core from directly contacting groundwater and preventing ineffective corrosion of the zero-valent iron core, thus reducing the formation of a passivation film. The porous carbon shell also provides additional adsorption sites, improving particle flowability and resulting in excellent particle dispersibility, preventing agglomeration and caking.

[0073] The porous structure of the carbon shell and the loose structure of the gradient layer enable the specific surface area of ​​sulfur-modified zero-valent iron to remain at 80-150 m² / g, which far exceeds that of traditional sulfur-modified zero-valent iron.

[0074] Traditional sulfur-modified zero-valent iron has a single sulfur phase, either FeS or FeS2, resulting in an uncontrollable S²⁻ release rate. Furthermore, the sulfur is simply loaded on the surface, making it easily washed away by water or oxidized to sulfate, thus failing to provide a long-term S²⁻ supply. This process first constructs a sulfur-rich layer and a carbon shell on the surface using a fluidized bed. Then, under the confinement effect of the carbon shell's "nanoreactor," internal reduction and sulfidation occur, achieving precise control over the gradient distribution and active forms of sulfur from the outside to the inside, avoiding random distribution and loss of sulfur. During the reaction, the sulfur-rich outer layer (x≈1.1) preferentially reacts with the reduced heavy metals to rapidly form stable sulfide precipitates, while the sulfur-poor inner layer (x≈0.8) tightly binds to the zero-valent iron core, slowly releasing active sulfur to continuously replenish the sulfur source, preventing sulfur loss and achieving long-term sulfidation.

[0075] Traditional sulfur-modified zero-valent iron releases electrons through direct transfer via the particle surface, resulting in high resistance, easy electron accumulation, slow reduction reaction kinetics, and low throughput per unit time. In contrast, the sulfur-modified zero-valent iron of this invention uses a zero-valent iron core as a strong electron donor, an FeSx gradient layer as an electron transfer medium, and a porous carbon shell as an electron transport channel, forming an efficient electron pathway. This significantly reduces electron transfer resistance and improves the reduction rate.

[0076] The sulfur-modified zero-valent iron obtained by this process has a particle size of 100-500μm, and the conductive particles are graphite with a particle size of 200-500μm. The sulfur-modified zero-valent iron and graphite are mixed evenly at a weight ratio of 3:1 to obtain the strong reducing filler of the strong reducing filler layer 8.

[0077] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for removing heavy metals from groundwater, characterized in that: a water solution of a complexing agent is injected into the groundwater upstream, the complexing agent comprising the following components in parts by weight: tetrasodium ethylenediamine disuccinate 10 parts; sodium gluconate 3-3.5 parts; ferrous sulfate heptahydrate 1.5-2 parts; citric acid monohydrate 0.8-1 part; sodium bicarbonate 5-6 parts; a reaction wall is built downstream, and a strong reduction filler layer (8) is arranged in the reaction wall, the strong reduction filler layer (8) comprising sulfur-modified zero-valent iron particles and conductive particles, and the strong reduction filler layer (8) reduces high-valence heavy metals in the groundwater to generate stable precipitates. The tetrasodium ethylenediamine disuccinate, sodium gluconate, sodium bicarbonate, and water are added to a first container and stirred until completely dissolved; the ferrous sulfate heptahydrate, citric acid monohydrate, and deoxygenated water are added to a second container to form a clear ferrous-lithium acid complex solution; the ferrous-lithium acid complex solution is slowly added to the first container and mixed evenly, and immediately after mixing, the solution is injected into the groundwater upstream. After the groundwater is treated by the strong reduction filler layer (8), the groundwater is treated again using a mixed filler of sulfur particles and limestone particles. After the heavy metals are reduced by the strong reduction filler layer (8), the generated precipitates are adsorbed using a mixed filler of hydroxyapatite particles and brown iron ore sand. The sulfur-modified zero-valent iron particles comprise, from the inside out, a zero-valent iron core, a FeSx gradient layer, and a porous carbon shell layer, where x is 0.8-1.

1.

2. The method of removing heavy metals from groundwater of claim 1, wherein: The preparation process of the sulfur-modified zero-valent iron particles is as follows:

3. The method of claim 1, wherein: S1. Dissolving ferric nitrate nonahydrate, sodium thiosulfate, and glucose in a mixed solvent of deionized water and ethanol, and then hydrothermally reacting at 120-180°C for 6-12h to obtain a precursor composite sol; 4. The method of claim 1, wherein: S2. Spray drying the precursor composite sol to obtain precursor microspheres; 5. The method of claim 1, wherein: S3. Placing the precursor microspheres in a fluidized bed reactor, preheating to 300-400°C under inert gas protection, first passing in a mixed gas containing sulfur vapor and reducing gas to form a sulfur-rich sulfidation layer on the surface of the precursor microspheres; and then passing in a carbon source gas to form a carbon shell on the outer wall of the sulfur-rich sulfidation layer; 6. The method of claim 5, wherein: S4. Under an atmosphere of high-purity hydrogen, heating the precursor microspheres coated with the sulfur-rich sulfidation layer and the carbon shell at a rate of 5°C / min to 300-500°C, then heating at a rate of 2°C / min to 500-700°C, and maintaining the temperature at 500-700°C for 1-2h; S5. Cooling to room temperature under a hydrogen or inert gas atmosphere. The method comprises a reaction wall arranged downstream of the groundwater, a plurality of complexing agent injection mechanisms (11) arranged upstream of the groundwater, and a water solution of a complexing agent injected by the complexing agent injection mechanisms (11) into the groundwater upstream; The complexing agent comprises the following components in parts by weight: tetrasodium ethylenediamine disuccinate 10 parts; sodium gluconate 3-3.5 parts; ferrous sulfate heptahydrate 1.5-2 parts; citric acid monohydrate 0.8-1 part; sodium bicarbonate 5-6 parts; a reaction wall is built downstream, and a strong reduction filler layer (8) is arranged in the reaction wall, the strong reduction filler layer (8) comprising sulfur-modified zero-valent iron particles and conductive particles, and the strong reduction filler layer (8) reduces high-valence heavy metals in the groundwater to generate stable precipitates.

7. A groundwater heavy metal removal system for use in the method of removing heavy metals from groundwater according to claim 1, characterized by: ​ ​ ​ ​ The strong reduction filler layer (8) comprises sulfur-modified zero-valent iron particles and conductive particles, and is used for reducing high-valence heavy metals in groundwater to generate stable precipitates.

8. The groundwater heavy metal removal system of claim 7, wherein: The reaction wall comprises a permeable wall (1), a first waterproof wall (2) and a second waterproof wall (3), which are sequentially arranged along the flow direction of the groundwater; a water storage cavity (4) is arranged between the permeable wall (1) and the first waterproof wall (2), and a treatment cavity is arranged between the first waterproof wall (2) and the second waterproof wall (3); one end of the water storage cavity (4) is communicated with one end of the treatment cavity through a connecting hole (5) arranged at the bottom of the first waterproof wall (2), the other end of the treatment cavity is communicated with the groundwater layer downstream of the reaction wall, and the strong reduction filler layer (8) is located in the treatment cavity.

9. The groundwater heavy metal removal system of claim 8, wherein: The treatment cavity is communicated with the groundwater layer downstream of the reaction wall through a water pumping mechanism (6), the connecting hole (5) is arranged in the treatment cavity of the water pumping mechanism (6), a plurality of strong reduction filler layers (8), at least one auxiliary reduction filler layer (7) and at least one precipitation adsorption layer (9) are sequentially arranged in the treatment cavity, a third waterproof plate (10) is arranged between every two adjacent filler layers, the third waterproof plate (10) divides the treatment cavity into wave-shaped water flow channels, and the upper surfaces of the strong reduction filler layer (8), the auxiliary reduction filler layer (7) and the precipitation adsorption layer (9) are all provided with an anaerobic-aerobic alternating treatment filler layer (12).

10. The groundwater heavy metal removal system of claim 9, wherein: The auxiliary reduction filler layer (7) comprises sulfur particles and limestone particles, the precipitation adsorption layer (9) comprises hydroxyapatite particles and limonite sand, and the anaerobic-aerobic alternating treatment filler layer (12) comprises porous ceramic or plastic biological filler.

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