Coupling method for removing ultra-low concentration mercury in wastewater of metal mine

Abstract

This invention belongs to the field of wastewater treatment in metal mines and proposes a coupled method for removing ultra-low concentration mercury from metal mine wastewater. It employs a combined process of "coupled chemical precipitation + flocculation precipitation + sulfur-loaded activated carbon adsorption." First, stannous chloride is added to generate a hydroxide gel, which is then chemically precipitated in conjunction with amorphous FeS nanoparticles. Next, hydrophobically modified cationic polyacrylamide is used for flocculation precipitation. Finally, modified sulfur-loaded activated carbon is used for deep adsorption. This treatment process has a specific removal effect on mercury ions, and the final effluent mercury meets the Class II surface water standard, with stable effluent quality. It also solves the problem of sulfur loss from ordinary sulfur-loaded activated carbon.

Description

Technical Field

[0001] This invention belongs to the field of wastewater treatment in metal mines, and particularly relates to a coupled method for removing ultra-low concentration mercury from metal mine wastewater, thereby achieving the harmless treatment of ultra-low concentration mercury wastewater. Background Technology

[0002] Mercury is a highly toxic, persistent, bioaccumulative, and long-distance migrating heavy metal commonly found in industrial and mining wastewater. Such wastewater typically features large volumes, relatively low but fluctuating mercury concentrations, and complex mercury forms (potentially including dissolved, colloidal, organic mercury, and mercury attached to suspended particles). Currently, the main technologies for treating mercury-containing wastewater include physical, chemical, and biological methods. Among these, chemical and physical methods are the most widely applied.

[0003] Chemical methods include chemical precipitation and electrochemical methods. Chemical precipitation typically involves adding chemical agents to the water, such as sulfides (sodium sulfide, sodium hydrosulfide, TMT, etc.) or metal salts (iron salts, aluminum salts, etc.) coagulants, to cause mercury ions to form insoluble precipitates (such as mercuric sulfide) or remove them through co-precipitation or adsorption. Conventional chemical precipitation can usually reduce mercury concentration to mg / L or hundreds of μg / L, but residual dissolved mercury, colloidal mercury, or incompletely precipitated complexed mercury still exists. The removal effect is particularly poor for organic mercury, making it difficult to meet ultra-low concentration emission requirements. Furthermore, chemical precipitation produces a large amount of sludge, posing subsequent hazardous waste disposal problems. For example, existing technology CN102583685B discloses a method for removing trace amounts of mercury from aqueous solutions. This method utilizes adsorption and co-precipitation principles, adjusts the pH of the mercury-containing aqueous solution, and adds a complex or compound formed by one or more of a sulfur-containing substance and a zinc-containing, copper-containing, or iron-containing reagent to remove trace amounts of mercury from the aqueous solution. The mercury content in the effluent is below 10 μg / L. Electrochemical methods involve dissociating mercury compounds into mercury ions at the anode under the action of direct current, and then reducing them into metallic mercury at the cathode, thereby removing mercury from wastewater. Classic methods include electrocoagulation, electrodialysis, and electrodeposition. However, these methods have high power consumption, high investment costs, and are prone to generating toxic mercury vapors that cause secondary pollution. Moreover, they are not very effective at removing low concentrations of mercury.

[0004] Physical methods commonly used include membrane separation, ion exchange, and adsorption. Membrane separation technologies, such as reverse osmosis and nanofiltration, utilize the selective permeability of semi-permeable membranes to retain mercury ions and mercury-containing compounds. However, they have high investment and operating costs. Suspended solids, colloids, organic matter, and scaling ions in mine water can easily lead to membrane fouling, shortening membrane life. Furthermore, the high-concentration mercury concentrate produced after membrane filtration is difficult and costly to treat. Ion exchange methods use ion exchange resins with specific selectivity for mercury ions for removal. Specialized resins are expensive, and the regeneration process requires strong acids, strong alkalis, or special reagents, making regeneration complex and generating mercury-containing regeneration wastewater pollution. In addition, mass transfer efficiency decreases at ultra-low mercury concentrations, requiring longer contact times or multi-stage treatment to achieve ultra-low emission standards, resulting in poor economic efficiency. Adsorption methods utilize the physicochemical effects on the surface of adsorbents such as activated carbon, modified mineral materials, and novel nanomaterials to capture mercury ions. However, ordinary adsorbents have limited adsorption capacity for ultra-low concentrations of mercury, are easily affected by coexisting ions (such as calcium, magnesium, and organic matter), and are difficult to regenerate. Moreover, many adsorbents have low adsorption performance for organic mercury. For mercury-containing mine water with ultra-low concentration emissions, it may be difficult to meet emission standards or the cost may be too high if physical methods are used alone.

[0005] It is evident that both chemical and physical methods have limitations in single-technology treatment, especially for low-concentration mercury-containing wastewater, making it difficult to consistently meet the requirements for ultra-low concentration emissions. Summary of the Invention

[0006] To overcome the technical problems of poor treatment effect, poor effluent stability, and difficulty in consistently meeting emission standards for mercury-containing metal mine wastewater in existing technologies, this invention proposes a coupled method for removing ultra-low concentration mercury from metal mine wastewater. The combined process of "coupled chemical precipitation + flocculation precipitation + sulfur-loaded activated carbon adsorption" has a specific removal effect on mercury ions, especially for mine water containing low concentrations of mercury in metal mines. The effluent mercury can even meet the Class II surface water standard (0.05 μg / L).

[0007] To achieve the above objectives, the present invention adopts the following technical solution: A coupled method for removing ultra-low concentration mercury from wastewater in metal mines includes the following steps: S1. Coupled chemical precipitation: Adjust the pH of the metal mine wastewater to 8-10, add stannous chloride, then add a suspension of amorphous ferrous sulfide nanoparticles, and stir to react; S2. Flocculation and sedimentation: Add hydrophobic modified cationic polyacrylamide as a flocculant to the wastewater after chemical precipitation and stir to react; S3. Deep adsorption with activated carbon: After flocculation and sedimentation, the supernatant is obtained by filtration, sulfur-loaded activated carbon is added, and the mixture is stirred for adsorption. After the adsorption is completed, the mixture is filtered again. The sulfur-loaded activated carbon is obtained by in-situ pulsed electrochemical reduction and deposition of sulfur-containing electrolyte on the activated carbon electrode.

[0008] First, this invention employs coupled chemical precipitation, adding stannous chloride to generate a hydroxide gel, which synergistically interacts with amorphous FeS nanoparticles to adsorb, capture, and precipitate mercury. The specific effects are as follows: (1) Sn(OH)2 / Sn 2+ It exhibits a "reduction fixation-adsorption co-precipitation" effect. While forming a hydroxide gel, it directly adsorbs some Hg... 2+ Reduced to elemental mercury (Hg) 0 And it adsorbs and encapsulates it in the floc, achieving initial fixation; then Sn 2+ As a reducing agent, it can directly reduce Hg 2+ Restored to (Hg) 0 Furthermore, in an alkaline environment (pH 8-10), Sn 2+ A fluffy, amorphous Sn(OH)2 / Sn(OH)4 mixed gel will be formed, and the Hg generated at this time 0 The nanodroplets are rapidly adsorbed and encapsulated by newly formed Sn(OH)4 colloids or Sn(OH)2 flocs, preventing them from re-oxidizing or volatilizing. Furthermore, this gel has a large specific surface area and abundant hydroxyl sites, enabling the surface complexation and adsorption of Hg. 2+ This allows for the initial transfer and stabilization of mercury from the liquid phase to the solid phase.

[0009] (2) Amorphous FeS nanoparticles have a "highly efficient mass transfer-instantaneous sulfidation" effect, which is beneficial for the unreduced Hg. 2+ Achieving ultra-high-speed and thorough sulfide removal overcomes the shortcomings of traditional sulfide methods, such as the formation of stable colloids, high supersaturation, and slow reaction. Amorphous FeS nanoparticles possess higher surface energy and more active sites than their crystalline counterparts, and their surface S... 2- The ions have extremely high reactivity and react with Hg. 2+ The reaction kinetics are extremely fast, almost instantaneous upon contact. This rapid reaction speed avoids excessive local supersaturation, preventing the formation of numerous fine HgS primary particles and thus reducing the formation of stable colloids at the source. Simultaneously, amorphous FeS nanoparticles are adsorbed onto the charged Sn(OH)₂ gel network via electrostatic interactions or van der Waals forces, interacting with HgS enriched on the gel surface. 2+ The reaction produces HgS. Because Hg... 2+ The reaction with FeS occurs inside the Sn(OH)2 gel, which is a short-distance, high-efficiency "solid-solid" or "surface-surface" mass transfer, rather than the slow diffusion in a homogeneous phase. Therefore, the reaction efficiency and thoroughness are greatly improved.

[0010] (3) The two precipitants work synergistically to form a three-in-one process of "reduction-adsorption-sulfidation". First, reduction and initial enrichment occur, and Sn... 2+ After addition, some Hg2+ It was directly reduced to Hg 0 It is initially immobilized, and the Sn(OH)2 gel network formed with the alkali acts like a large net, adsorbing and enriching the remaining Hg. 2+ Then, after deep sulfurization and fixation, the amorphous FeS nanoparticles, once added, are captured by the Sn(OH)2 gel network and react with the already enriched Hg. 2+ A rapid and complete sulfidation reaction occurs, producing HgS; finally, a composite floc is formed, and the final solid product is a composite floc, the core of which is Hg. 0 The sludge consists of droplets and HgS particles encased in layers of Sn(OH)2 and FeS / Fe(OH)2. This structure effectively blocks the leaching pathways of mercury, resulting in extremely high sludge stability.

[0011] Secondly, considering the characteristics of coupled chemical precipitation, a special design was made for subsequent flocculation and precipitation, using hydrophobically modified cationic polyacrylamide flocculant (HM-CPAM) to further improve the adsorption and precipitation effect of trace mercury. The main functions are: (1) Cationic neutralization: The cationic groups on the CPAM chain effectively neutralize the negative charge on the surface of the composite colloid, reducing electrostatic repulsion and promoting collision aggregation. (2) Hydrophobic association (core function): The flocculant molecular chain has hydrophobic groups, which can effectively neutralize the negative charge on the surface of the composite colloid, reducing electrostatic repulsion and promoting collision aggregation. 0 Droplets and HgS particles are strongly adsorbed onto the hydrophobic surface of the flocs. This adsorption is based on hydrophobic interactions, which are stronger and more stable than hydrogen bonds or electrostatic interactions. Moreover, multiple HM-CPAM molecular chains are "anchored" to different hydrophobic particles through their hydrophobic groups, thereby forming an extremely strong "hydrophobic bridge" between the particles. The flocs formed by this bridging effect are larger in size, denser in structure, and have higher mechanical strength, which can effectively resist hydraulic shear and will not break during transportation or settling. (3) Enhanced complexation: The flocculant has a cationic structure such as quaternary ammonium salt, which can also form a weak complex with certain mercury species, further enhancing the capture ability.

[0012] Finally, deep adsorption with sulfur-loaded activated carbon is performed to specifically adsorb mercury ions and remove residual mercury, ensuring stable effluent quality. This invention utilizes improved sulfur-loaded activated carbon, obtained by in-situ pulsed electrochemical reduction and deposition of sulfur-containing precursors on an activated carbon electrode. Its adsorption mechanism differs from the traditional binary model of "physical adsorption + chemical conversion," instead being based on "electron-mediated surface catalysis," thus exhibiting highly efficient adsorption of low concentrations of mercury. Specifically: (1) The active site is essentially a “sulfur-carbon covalent bridge”, which is a short-chain sulfur species in a metastable state that is firmly anchored to the carbon skeleton by covalent bonds, such as low-coordination sulfur chain (-SS-) or single sulfur bridge (-S-). This structure is achieved in two steps: “violent” interruption by pulse electrochemical reduction and “mild” anchoring during relaxation period, rather than traditional sulfur species (physically adsorbed S8 molecules or small sulfur molecules that fill the gaps).

[0013] (2) For Hg 0 The adsorption mechanism is manifested in the preconstruction of electron transfer channels. The first step involves chemisorption and electron injection of Hg. 0 Atomic physical adsorption occurs near the -SS- site. Since the -SS- bond is fixed and polarized by the carbon framework, it is an electron-deficient center, Hg... 0 6s 2 The lone pair of electrons rapidly transfers to the antibonding orbital of -SS-. In the sulfur-loaded activated carbon of this invention, the -SS- bond itself is in a stretched and activated state; after electron transfer and injection, it can undergo highly efficient homolytic cleavage, generating two ·S-Hg-S· intermediates. In contrast, the traditional S8 and Hg... 0 The reaction requires first breaking the SS bond, which has a high energy barrier. The second step involves surface-mediated electron rearrangement and stabilization. The homolytically cleaved ·S-Hg-S· is still unstable, while the improved sulfur-loaded activated carbon possesses a pre-constructed "carbonyl-enol" anchoring site (C=O---C-OH), which acts as an "electron buffer" or "mediating channel." This localized electron microcycle rapidly promotes electron rearrangement in ·S-Hg-S·, instantly generating stable HgS molecules bonded to the carbon surface. The entire process resembles a surface-catalyzed cycle, where the carbon skeleton near the active site actively participates in electron transfer and rearrangement, rather than passively acting as a carrier. For traditional sulfur-loaded activated carbon, stabilization requires another sulfur species or relies on the spontaneous nucleation of HgS.

[0014] (3) The synergistic effect on the adsorption advantages of trace mercury is manifested in high activity and selectivity, no pore blockage, excellent stability and kinetic advantages. The covalently bonded short-chain sulfur species have much higher activity than S8, and their adsorption of Hg... 0 The capture is "electron starvation" type, targeting trace concentrations of Hg. 0 It exhibits extremely high affinity and selectivity. Sulfur is covalently anchored to the pore surface in monolayer or oligolayer form, maximizing the preservation of the specific surface area and pore volume of activated carbon, thus providing Hg... 0 The covalent bonding provides a pathway for sulfur transport and enrichment. It fundamentally solves the problem of sulfur loss, ensuring that the active component is not lost even under high gas velocities or temperature fluctuations. The highly dispersed sulfur sites in pulse deposition, coupled with the "electron-mediated" mechanism, result in extremely rapid HgS formation, making it suitable for applications requiring fast reactions.

[0015] Preferably, the amorphous ferrous sulfide nanoparticles are added in the form of a suspension, with the interval between preparation and use of the suspension being less than 30 minutes. To ensure the treatment effect of the amorphous ferrous sulfide nanoparticles, the suspension is prepared and added to the wastewater to be treated, and the interval between preparation and use of the suspension is generally controlled within 30 minutes to prevent crystallization or agglomeration.

[0016] Further preferred, the preparation process of the amorphous ferrous sulfide nanoparticle suspension is as follows: Na2S solution is rapidly poured into the FeSO4 solution under stirring, and stirring is continued for 10-15 minutes to obtain the nanoparticle suspension.

[0017] Preferably, the hydrophobically modified cationic polyacrylamide is prepared by aqueous solution copolymerization or reverse emulsion polymerization, and the reactive monomers include acrylamide (AM), cationic monomers and hydrophobic monomers.

[0018] More preferably, the cationic monomer is dimethyl diallyl ammonium chloride (DADMAC) or acryloyloxyethyl trimethyl ammonium chloride (DAC), and the hydrophobic monomer is octadecyl methacrylate (SMA).

[0019] Preferred cationic monomers with quaternary ammonium salt structures provide charge to neutralize the negative charge on the colloidal surface, and their quaternary ammonium salt structure also enables complexation, improving the capture of mercury species. Preferred long-chain alkyl hydrophobic monomers enhance hydrophobic interactions, allowing Hg to be captured more effectively. 0 Droplets and HgS particles are better adsorbed onto the hydrophobic surface of the floc.

[0020] Preferably, the sulfur-loaded activated carbon is prepared as follows: (1) Activated carbon pretreatment: The activated carbon electrode was cleaned and dried, and cyclic voltammetric electrochemical activation was carried out in H2SO4 solution in a three-electrode system; (2) Pulsed current electrochemical deposition: Using the pretreated activated carbon electrode as the working electrode, a pulsed current is applied in the sulfur-containing electrolyte to perform electrochemical deposition; (3) Post-processing: Remove the working electrode, rinse and dry to obtain sulfur-loaded activated carbon.

[0021] The sulfur-loaded activated carbon used in this invention is obtained through pulsed current electrochemical deposition. The adsorption mechanism is based on "electron-mediated surface catalysis," which can exhibit high efficiency in adsorbing low concentrations of mercury. Sulfur in a sulfur-containing electrolyte is reduced and deposited in situ on a specially pretreated activated carbon electrode using a precisely controlled pulsed current mode, while the final form of sulfur is regulated by the interfacial microenvironment.

[0022] In terms of preparation, firstly, in a three-electrode system, a cleaned activated carbon electrode is used as the working electrode and undergoes cyclic voltammetric electrochemical activation pretreatment in H2SO4 solution to introduce oxygen-containing functional groups and defect sites. Furthermore, using an activated carbon electrode with a high specific surface area is more conducive to subsequent sulfur species loading. Then, using the pretreated activated carbon electrode as the working electrode, a platinum sheet as the counter electrode, and Ag / AgCl as the reference electrode, a pulsed current is applied in the three-electrode system to electrochemically reduce and deposit sulfur-loaded activated carbon. Maintaining constant temperature and pH during deposition improves the quality stability of the sulfur-loaded activated carbon. Finally, the deposited sulfur-loaded activated carbon is rinsed and dried. This preparation process uses a three-electrode system as the preparation environment, with continuous activation and deposition processes requiring only solution replacement, making it simple to operate.

[0023] Further preferred, the specific preparation process of the sulfur-containing electrolyte is as follows: sodium thiosulfate and Na2SO4 are dissolved in water, the pH is adjusted to 4.0~5.5 with dilute H2SO4, and nitrogen gas is passed through for deoxygenation. This sulfur-containing electrolyte uses sodium thiosulfate as the main salt, Na2SO4 as the supporting electrolyte, and is adjusted to a slightly acidic environment with dilute sulfuric acid. The microenvironment of the ionic liquid is key to preventing sulfur chain growth and stabilizing short-chain species (-SS-, -S-).

[0024] As a preferred option, the reaction time for step (1) is 60-90 min; the reaction time for step (2) is 20-60 min; and the reaction time for step (3) is 40-80 min.

[0025] Preferably, the mercury content in the treated metal mine wastewater is between 0.2 and 1.5 μg / L, and after treatment, the mercury content is reduced to below 0.05 μg / L. The coupling method provided by this invention has a high removal rate for low-concentration mercury, and the treated effluent has a concentration below 0.05 μg / L, which meets the national Class II surface water standard.

[0026] Preferably, the dosage of stannous chloride (SnCl2·2H2O) is 0.2~1.1 mg / L, the dosage of amorphous ferrous sulfide nanoparticles is 0.1~0.4 mg / L, the dosage of hydrophobically modified cationic polyacrylamide is 2~8 mg / L, and the dosage of sulfur-loaded activated carbon is 1.5~6 g / L.

[0027] Therefore, the present invention has the following beneficial effects: (1) A process using two precipitant technologies, tin hydroxide and amorphous ferrous sulfide nanoparticles, was adopted. Through the synergistic effect of the two, mercury was adsorbed, captured and precipitated. At the same time, hydrophobic modified flocculants were combined to strongly adsorb and aggregate mercury-containing substances, which effectively improved the adsorption and precipitation effect of trace mercury. (2) The preparation of sulfur-loaded activated carbon was improved by using covalent bonding to produce a high affinity and selective adsorption effect on mercury, thus solving the problem of sulfur loss in ordinary sulfur-loaded activated carbon. (3) Both the coupled chemical precipitation and sulfur-loaded activated carbon adsorption processes can target mercury removal and have a specific removal effect on mercury ions. (4) This combined process is specifically designed for wastewater containing low concentrations of mercury from metal mines, and improves the removal efficiency of low concentrations of mercury to meet the Class II surface water standard. Attached Figure Description

[0028] Figure 1 This is a process flow diagram of Embodiment 1 of the present invention. Detailed Implementation

[0029] The present invention will now be further described with reference to the accompanying drawings and specific embodiments.

[0030] The reagents used in the embodiments and comparative examples of this invention were either self-made or purchased.

[0031] The self-made reagents include amorphous ferrous sulfide nanoparticles, hydrophobically modified cationic polyacrylamide HM-CPAM, and sulfur-loaded activated carbon.

[0032] (1) Preparation process of amorphous ferrous sulfide nanoparticles: I. Solution preparation: Based on a Fe:S molar ratio of 1:1.1, weigh 0.316~1.265mg FeSO4·7H2O (molecular weight 278.01) and 0.301~1.202mg Na2S·9H2O (molecular weight 240.18) for treating 1L of wastewater, and dissolve them separately in 10~20mL of deionized water to prepare solutions; II. Stirring reaction: Under the conditions of 10~25℃ and rapid stirring (300~500rpm), Na2S solution is quickly poured into FeSO4 solution, and stirring is continued for 10~15min to obtain a black amorphous ferrous sulfide nanoparticle suspension.

[0033] It should be noted that if amorphous ferrous sulfide nanoparticle suspension is used in any of the embodiments or comparative examples, it must be added within 30 minutes after preparation.

[0034] (2) The preparation process of hydrophobically modified cationic polyacrylamide is as follows: The aqueous copolymerization method was adopted. 68.5g of acrylamide (AM), 39.7g of dimethyl diallyl ammonium chloride aqueous solution (DADMAC, 60% aqueous solution) (equivalent to 23.8g of pure product, about 13mol%), and 7.7g of octadecyl methacrylate (SMA) (about 2mol%) were weighed and added to deionized water to prepare a monomer aqueous solution with a total mass concentration of 25% (i.e., water was added to a total mass of about 400g). The pH was adjusted to 5-6 with dilute hydrochloric acid, and nitrogen gas was purged for 30min to remove oxygen. Then, 0.3g of ammonium persulfate and 0.15g of sodium bisulfite were added as redox initiators. The reaction was stirred at 45℃ for 5h to obtain a viscous HM-CPAM colloid.

[0035] (3) The improved preparation process of sulfur-loaded activated carbon is as follows: I. Activated carbon pretreatment: High specific surface area activated carbon was selected and ultrasonically cleaned with deionized water and ethanol in sequence, followed by vacuum drying. In a three-electrode system, cyclic voltammetric electrochemical activation was carried out in 0.5M H2SO4 solution (-0.8V~1.2V, 20mV / s, 10~20 cycles) to introduce oxygen-containing functional groups and defect sites. II. Preparation of sulfur-containing electrolyte: Use 0.05~0.2M sodium thiosulfate as the main salt and 0.1~0.5M Na2SO4 as the supporting electrolyte. Adjust the pH to 4.0~5.5 with dilute H2SO4 and deoxygenate with nitrogen for 20 min. III. Pulsed Current Electrochemical Deposition: A three-electrode system was used (working electrode: pretreated activated carbon electrode; counter electrode: platinum sheet; reference electrode: Ag / AgCl), with pulsed current applied (current density -0.5~-2 mA / cm²). 2 The pulse width was 0.1–2 s, the pulse interval was 0.5–5 s, the duty cycle was 10%–50%, and the total deposition time was 30–120 min. During deposition, a constant temperature (25±1℃) and pH were maintained. IV. Post-treatment: After deposition, remove the electrode, rinse it quickly with deionized water, and dry it in a vacuum drying oven at 50~60℃ for 12h.

[0036] In addition to the above, the acid-adjusting solution is prepared using commercially available concentrated H2SO4, concentrated HCl, or other acids as raw materials to form a low-concentration dilute acid solution; the alkali-adjusting solution is prepared using commercially available Na2CO3, NaOH, NaHCO3, or other alkalis as raw materials to form a low-concentration alkali solution; the unmodified flocculant is not the commercially available organic flocculant polyacrylamide (CPAM); the crystalline FeS nanoparticles are commercially available FeS nanoparticles; and the traditional sulfur-loaded activated carbon is commercially available sulfur-loaded activated carbon.

[0037] The technical solution of the present invention will be explained and illustrated below with specific embodiments, but the scope of protection of the present invention is not limited thereto. Example 1:

[0038] The treatment target was mine water discharged from a gold mine, which contained various heavy metals, including mercury at a concentration of approximately 0.34 μg / L.

[0039] The coupled method for ultra-low concentration mercury removal is as follows: Figure 1 The details are as follows: (1) Adjust pH: Adjust the pH of the mine water sample to 9; (2) Coupled chemical precipitation: Add 0.5 mg SnCl2·2H2O to 1 L of mine water, then add freshly prepared amorphous FeS nanoparticle suspension (the amount is 0.2 mg based on amorphous FeS nanoparticles), and stir for 80 min. (3) Flocculation and sedimentation: Add 5 mg of hydrophobic modified cationic polyacrylamide HM-CPAM to the water sample after chemical precipitation, stir and react for 30 min, and then filter to clarify; (4) Deep adsorption of activated carbon: Add 3g of improved sulfur-loaded activated carbon to the supernatant obtained by filtration, stir slowly for 60min, and then filter again to effluent.

[0040] The mercury content of the treated water sample was determined. In this embodiment, the mercury concentration of the effluent sample was less than 0.04 μg / L, which is below the instrument detection limit.

[0041] Comparative Example 1: The difference between Comparative Example 1 and Example 1 is that only SnCl2 is added in the coupled chemical precipitation process.

[0042] The specific processing procedure is as follows: (1) Adjust pH: Adjust the pH of the mine water sample to 9; (2) Coupled chemical precipitation: Add 0.5 mg SnCl2·2H2O to 1 L of mine water and stir for 80 min; (3) Flocculation and sedimentation: Add 5 mg of hydrophobic modified cationic polyacrylamide HM-CPAM to the water sample after chemical precipitation, stir and react for 30 min, and then filter to clarify; (4) Deep adsorption of activated carbon: Add 3g of improved sulfur-loaded activated carbon to the supernatant obtained by filtration, stir slowly for 60min, and then filter again to effluent.

[0043] Mercury content was determined in the treated water sample. The mercury concentration in the effluent sample of this comparative example was 0.17 μg / L.

[0044] Comparative Example 2: The difference between Comparative Example 2 and Example 1 is that only a suspension of amorphous FeS nanoparticles was added in the coupled chemical precipitation process.

[0045] The specific processing procedure is as follows: (1) Adjust pH: Adjust the pH of the mine water sample to 9; (2) Coupled chemical precipitation: Add freshly prepared amorphous FeS nanoparticle suspension (0.2 mg based on amorphous FeS nanoparticles) to 1 L of mine water and stir for 80 min; (3) Flocculation and sedimentation: Add 5 mg of hydrophobic modified cationic polyacrylamide HM-CPAM to the water sample after chemical precipitation, stir and react for 30 min, and then filter to clarify; (4) Deep adsorption of activated carbon: Add 3g of improved sulfur-loaded activated carbon to the supernatant obtained by filtration, stir slowly for 60min, and then filter again to effluent.

[0046] Mercury content was determined in the treated water sample. The mercury concentration in the effluent sample of this comparative example was 0.23 μg / L.

[0047] Comparative Example 3: The difference between Comparative Example 3 and Example 1 is that SnCl2 and crystalline FeS nanoparticles were added during the coupled chemical precipitation process.

[0048] The specific processing procedure is as follows: (1) Adjust pH: Adjust the pH of the mine water sample to 9; (2) Coupled chemical precipitation: Add 0.5 mg SnCl2·2H2O to 1 L of mine water, then add 0.2 mg of crystalline FeS nanoparticles, and stir for 80 min; (3) Flocculation and sedimentation: Add 5 mg of hydrophobic modified cationic polyacrylamide HM-CPAM to the water sample after chemical precipitation, stir and react for 30 min, and then filter to clarify; (4) Deep adsorption of activated carbon: Add 3g of improved sulfur-loaded activated carbon to the supernatant obtained by filtration, stir slowly for 60min, and then filter again to effluent.

[0049] Mercury content was determined in the treated water sample. The mercury concentration in the effluent sample of this comparative example was 0.15 μg / L.

[0050] Based on the low-concentration mercury treatment effect of mine water discharged from a gold mine in Example 1 and Comparative Examples 1-3, the combination of SnCl2 and amorphous FeS nanoparticles of this invention for coupled chemical precipitation, followed by flocculation and precipitation using the hydrophobically modified flocculant HM-CPAM, and finally deep adsorption using improved sulfur-loaded activated carbon, resulted in an effluent mercury concentration of less than 0.04 μg / L, which fully meets the Class II surface water standard of 0.05 μg / L (Surface Water Environmental Quality Standard GB3838-2002). Comparative Examples 1 and 2, using only one precipitant, produced effluent mercury concentrations of 0.17 and 0.23 μg / L, respectively, with removal rates of only 50% and 32%, failing to meet the Class III surface water standard (0.1 μg / L). Comparative Example 3, using a combination of two precipitants—SnCl2 and crystalline FeS nanoparticles—achieved an effluent mercury concentration of 0.15 μg / L and a removal rate of 56%, slightly higher than Comparative Examples 1 and 2, but still failing to meet the Class III surface water standard. Therefore, the use of specific precipitants for sulfide precipitation plays an indispensable role in the coupled methods for treating ultra-low concentration mercury.

[0051] This indicates that SnCl2 and amorphous FeS nanoparticles, as precipitants, exhibit a synergistic effect, forming a three-in-one "reduction-adsorption-sulfidation" process. Specifically, Sn... 2+ After addition, some Hg 2+ It was directly reduced to Hg 0 It is initially fixed, while another part forms a loose, amorphous Sn(OH)2 / Sn(OH)4 mixed gel network, which adsorbs and enriches the remaining Hg. 2+ This process achieves reduction and initial enrichment; then, amorphous FeS nanoparticles are added, which are captured by the Sn(OH)2 gel network due to electrostatic interactions or van der Waals forces, and react with the already enriched Hg. 2+ A rapid and complete sulfidation reaction occurs, generating HgS, thus achieving deep sulfidation fixation; under the action of reduction, adsorption, and sulfidation, a core of HgS is formed. 0 The droplets and HgS particles are a composite flocculent whose outer shell is a coating layer of Sn(OH)2 and FeS / Fe(OH)2, which blocks the leaching of mercury.

[0052] Furthermore, Sn 2+ It has strong reducing properties and participates in the formation of Hg. 0 Thermodynamically, the reaction is spontaneous. The resulting Sn(OH)₂ / Sn(OH)₄ mixed gel has a huge specific surface area and abundant hydroxyl sites, enabling it to adsorb Hg through surface complexation. 2+ and Hg 0 It can also provide a huge substrate for subsequent FeS nanoparticles to adhere to.

[0053] Similarly, amorphous FeS nanoparticles have the dual functions of instantaneous sulfidation and efficient directional mass transfer. They have higher surface energy and more active sites than their crystalline form, and are less likely to form stable colloids due to high supersaturation. They interact with Sn(OH)2 gel networks to form short-distance, high-efficiency "solid-solid" or "surface-surface" mass transfer within the gel, which greatly improves reaction efficiency and thoroughness.

[0054] Comparative Example 4: The difference between Comparative Example 4 and Example 1 is that the flocculant added during the flocculation and sedimentation process was not modified.

[0055] The specific processing procedure is as follows: (1) Adjust pH: Adjust the pH of the mine water sample to 9; (2) Coupled chemical precipitation: Add 0.5 mg SnCl2·2H2O to 1 L of mine water, then add freshly prepared amorphous FeS nanoparticle suspension (the amount is 0.2 mg based on amorphous FeS nanoparticles), and stir for 80 min. (3) Flocculation and sedimentation: Add 5 mg of polyacrylamide (CPAM) to the water sample after chemical precipitation, stir and react for 30 min, then clarify and filter; (4) Deep adsorption of activated carbon: Add 3g of improved sulfur-loaded activated carbon to the supernatant obtained by filtration, stir slowly for 60min, and then filter again to effluent.

[0056] Mercury content was determined in the treated water sample. The mercury concentration in the effluent sample of this comparative example was 0.11 μg / L.

[0057] Based on the effluent conditions of Example 1 and Comparative Example 4, it can be seen that when using the unmodified hydrophobic flocculant, the mercury concentration in the effluent was 0.11 μg / L, and the removal rate was 68%, which still failed to meet the Class III surface water standard, a significant difference compared to Example 1. The main reason is that the hydrophobically modified flocculant can exert hydrophobic and bridging effects. Specifically, the long-chain alkyl hydrophobic groups on the flocculant molecular chain strongly adsorb Hg. 0 For droplets and HgS particles, this interaction is far stronger and more stable than hydrogen bonding or electrostatic interactions. Furthermore, multiple HM-CPAM molecular chains can be "anchored" to different hydrophobic particles through their hydrophobic groups, forming extremely robust hydrophobic bridges between particles. The resulting flocs are larger, denser, and have higher mechanical strength, effectively resisting hydraulic shear and preventing breakage during transport or settling. Additionally, hydrophobic modification using quaternary ammonium salt monomers allows the quaternary ammonium structure to exert a certain complexing effect, which is beneficial for mercury removal. Therefore, the use of hydrophobically modified cationic polyacrylamide flocculants plays a crucial role in the entire coupling method.

[0058] Comparative Example 5: The difference between Comparative Example 5 and Example 1 is that traditional sulfur-loaded activated carbon is added during the deep adsorption process of activated carbon.

[0059] The specific processing procedure is as follows: (1) Adjust pH: Adjust the pH of the mine water sample to 9; (2) Coupled chemical precipitation: Add 0.5 mg SnCl2·2H2O to 1 L of mine water, then add freshly prepared amorphous FeS nanoparticle suspension (the amount is 0.2 mg based on amorphous FeS nanoparticles), and stir for 80 min. (3) Flocculation and sedimentation: Add 5 mg of hydrophobic modified cationic polyacrylamide HM-CPAM to the water sample after chemical precipitation, stir and react for 30 min, and then filter to clarify; (4) Deep adsorption of activated carbon: Add 3g of traditional sulfur-loaded activated carbon to the supernatant obtained by filtration, stir slowly for 60min, and then filter again to effluent.

[0060] Mercury content was determined in the treated water sample. The mercury concentration in the effluent sample of this comparative example was 0.12 μg / L.

[0061] Combining Example 1 and Comparative Example 5, it can be seen that when traditional sulfur-loaded activated carbon is used, the mercury concentration in the effluent is 0.12 μg / L, which cannot achieve the mercury removal effect of Example 1 and cannot meet the Class III surface water standard. Therefore, this process serves as a final safeguard in the treatment process, using improved sulfur-loaded activated carbon to specifically adsorb mercury ions and remove residual mercury, ensuring stable effluent compliance with standards.

[0062] Compared to traditional sulfur-loaded activated carbon, whose adsorption mechanism is based on a binary model of "physical adsorption + chemical conversion," the improved sulfur-loaded activated carbon of this invention is based on "electron-mediated surface catalysis." Its active sites are metastable, low-coordinated sulfur chains (-SS-) or monosulfide bridges (-S-) firmly anchored to the carbon skeleton by covalent bonds, exhibiting activity far exceeding that of S8. Its activity towards Hg... 0 The capture is "electron starvation" type, targeting trace concentrations of Hg. 0 It exhibits extremely high affinity and selectivity; and sulfur is covalently anchored to the pore surface in monolayer or oligolayer form, maximizing the preservation of the specific surface area and pore volume of activated carbon, providing Hg 0 The sulfur transport and enrichment pathways are provided, and the highly dispersed sulfur sites make it suitable for applications requiring rapid reactions. Furthermore, the improved sulfur-loaded activated carbon of this invention exhibits superior stability, fundamentally solving the problem of sulfur loss. Example 2:

[0063] The wastewater being treated was generated from the mining of a gold and silver mine, with a mercury content of approximately 1.2 μg / L.

[0064] The specific processing procedure is as follows: (1) Adjust pH: Adjust the pH of the mining wastewater sample to 9; (2) Coupled chemical precipitation: Add 0.5 mg SnCl2·2H2O to 1 L of mine water, then add freshly prepared amorphous FeS nanoparticle suspension (the amount is 0.2 mg based on amorphous FeS nanoparticles), and stir for 80 min. (3) Flocculation and sedimentation: Add 5 mg of hydrophobic modified cationic polyacrylamide HM-CPAM to the water sample after chemical precipitation, stir and react for 30 min, and then filter to clarify; (4) Deep adsorption of activated carbon: Add 3g of improved sulfur-loaded activated carbon to the supernatant obtained by filtration, stir slowly for 60min, and then filter again to effluent.

[0065] Mercury content was measured in the treated water sample. In this embodiment, the mercury concentration of the effluent sample was less than 0.04 μg / L, meeting the Class II surface water standard. Example 2:

[0066] The object of treatment was mine water generated during the mining process of a lead-zinc mine, which contained approximately 0.75 μg / L of mercury.

[0067] The specific processing procedure is as follows: (1) Adjust pH: Adjust the pH of the mine water sample to 9; (2) Coupled chemical precipitation: Add 0.5 mg SnCl2·2H2O to 1 L of mine water, then add freshly prepared amorphous FeS nanoparticle suspension (the amount is 0.2 mg based on amorphous FeS nanoparticles), and stir for 80 min. (3) Flocculation and sedimentation: Add 5 mg of hydrophobic modified cationic polyacrylamide HM-CPAM to the water sample after chemical precipitation, stir and react for 30 min, and then filter to clarify; (4) Deep adsorption of activated carbon: Add 3g of improved sulfur-loaded activated carbon to the supernatant obtained by filtration, stir slowly for 60min, and then filter again to effluent.

[0068] Mercury content was measured in the treated water sample. In this embodiment, the mercury concentration of the effluent sample was less than 0.04 μg / L, meeting the Class II surface water standard.

[0069] The specific treatment processes and effluent conditions of the above embodiments and comparative examples are shown in Table 1: Table 1. Treatment process and effluent conditions of the examples and comparative examples As can be seen from Examples 1-3 in Table 1, the coupling method proposed in this invention is suitable for treating ultra-low concentration mercury-containing wastewater from metal mines (0.2~1.5 μg / L), achieving a high mercury removal rate with effluent mercury levels consistently below 0.04 μg / L, and consistently meeting the Class II surface water discharge standards. This has significant practical implications for the treatment of metal mine wastewater. Furthermore, the coupling method provided by this invention has a short treatment time, and the treatment time for each process—coupling chemical precipitation, flocculation sedimentation, and deep adsorption—can be reasonably adjusted according to the initial conditions of the wastewater. The process proposed in this invention overcomes the limitations of single processes in effectively treating ultra-low concentration mercury-containing wastewater, and is energy-efficient and cost-effective, making it suitable for large-scale application.

Claims

1. A coupled method for removing ultra-low concentration mercury from wastewater in metal mines, characterized in that, Includes the following steps: S1. Coupled chemical precipitation: Adjust the pH of the metal mine wastewater to 8-10, add stannous chloride, then add amorphous ferrous sulfide nanoparticles, and stir to react; S2. Flocculation and sedimentation: Add hydrophobic modified cationic polyacrylamide as a flocculant to the wastewater after chemical precipitation and stir to react; S3. Deep adsorption with activated carbon: After flocculation and sedimentation, the supernatant is obtained by filtration, sulfur-loaded activated carbon is added, and the mixture is stirred for adsorption. After the adsorption is completed, the mixture is filtered again. The sulfur-loaded activated carbon is obtained by in-situ pulsed electrochemical reduction deposition of sulfur-containing electrolyte on the activated carbon electrode.

2. The coupling method according to claim 1, characterized in that, The amorphous ferrous sulfide nanoparticles are added in the form of a suspension, and the time interval between preparation and use of the suspension is less than 30 minutes.

3. The coupling method according to claim 2, characterized in that, The preparation process of the amorphous ferrous sulfide nanoparticle suspension is as follows: Na2S solution is rapidly poured into the FeSO4 solution under stirring, and stirring is continued for 10-15 minutes to obtain the nanoparticle suspension.

4. The coupling method according to claim 1, characterized in that, The hydrophobically modified cationic polyacrylamide is prepared by aqueous solution copolymerization or reverse emulsion polymerization, and the reactant monomers include acrylamide, cationic monomers and hydrophobic monomers.

5. The coupling method according to claim 4, characterized in that, The cationic monomer is dimethyl diallyl ammonium chloride or acryloyloxyethyl trimethyl ammonium chloride, and the hydrophobic monomer is octadecyl methacrylate.

6. The coupling method according to claim 1, characterized in that, The specific preparation process of the sulfur-loaded activated carbon is as follows: (1) Activated carbon pretreatment: The activated carbon electrode was cleaned and dried, and cyclic voltammetric electrochemical activation was carried out in H2SO4 solution in a three-electrode system; (2) Pulsed current electrochemical deposition: Using the pretreated activated carbon electrode as the working electrode, a pulsed current is applied in the sulfur-containing electrolyte to perform electrochemical deposition; (3) Post-processing: Remove the working electrode, rinse and dry to obtain sulfur-loaded activated carbon.

7. The coupling method of claim 1 or 6, wherein, The specific preparation process of the sulfur-containing electrolyte is as follows: sodium thiosulfate and Na2SO4 are dissolved in water, the pH is adjusted to 4.0~5.5 with dilute H2SO4, and nitrogen gas is passed through for deoxygenation.

8. The coupling method of claim 1, wherein, The reaction time for step (1) is 60-90 min; the reaction time for step (2) is 20-60 min; and the reaction time for step (3) is 40-80 min.

9. The coupling method of claim 1, wherein, The mercury content in the treated metal mine wastewater ranged from 0.2 to 1.5 μg / L, and after treatment, the mercury content decreased to below 0.05 μg / L.

10. The coupling method according to claim 9, characterized in that, The dosage of stannous chloride is 0.2~1.1 mg / L, the dosage of amorphous ferrous sulfide nanoparticles is 0.1~0.4 mg / L, the dosage of hydrophobically modified cationic polyacrylamide is 2~8 mg / L, and the dosage of sulfur-loaded activated carbon is 1.5~6 g / L.

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