A method for immobilizing water-soluble cd(ii) in an aerobic environment

Abstract

The application provides a method for fixing water-soluble Cd(II) in an aerobic environment, and belongs to the fields of heavy metal pollution remediation and water environment treatment. The specific method is as follows: under aerobic conditions, FeS material is put into water body containing water-soluble Cd(II) to be treated, a part of the water-soluble Cd(II) is reacted with S(-II) in the FeS to generate CdS precipitate through a displacement reaction; meanwhile, Fe(II) released into the water body is oxidized to generate lepidocrocite, so that the remaining part of the water-soluble Cd(II) is adsorbed to the surface of the lepidocrocite or embedded into the crystal structure of the lepidocrocite; finally, the Cd(II) in the water body to be treated exists in the form of CdS precipitate and lepidocrocite-adsorbed Cd(II). The water-soluble Cd(II) in the aerobic environment is fixed by the FeS material through chemical precipitation and surface adsorption, the preparation method of the FeS material is simple, the FeS material has strong fixing capacity for the water-soluble Cd(II), the fixed Cd(II) has good stability, and the FeS material has potential application prospect in the field of water treatment.

Description

TECHNICAL FIELD

[0001] The application relates to the field of heavy metal pollution remediation and the field of water environment treatment, and in particular to a method for fixing water-soluble Cd(II) in an aerobic environment. BACKGROUND

[0002] Cadmium (Cd) is one of the toxic heavy metals widely existing in nature and is listed as a global priority pollutant. Cd contaminated wastewater from natural sources (erosion of rocks and natural sediments) and anthropogenic sources (mining, electroplating, battery production, dyes) has led to a large amount of land contaminated by Cd. Elevated levels of Cd exposure can cause damage to the lungs, kidneys, bones, and children's brains, causing cardiovascular disease, obstructive pulmonary disease, and even breast cancer. Various technologies have been applied to treat Cd contaminated water, mainly including adsorption, chemical precipitation, ion exchange, membrane separation, and bioremediation. However, these technologies have the disadvantages of high cost, low efficiency, and generation of secondary pollutants. Therefore, it is necessary to explore more Cd remediation technologies.

[0003] Iron (Fe) element has redox activity and is one of the most abundant elements in the subsurface environment. Under reducing conditions, sulfate is bioreduced to sulfide, and sulfide continuously reacts with oxidized iron / hydroxide minerals to form ferrous sulfide (FeS), which is widely distributed in reducing environments, including sediments of urban streams, lakes, marine environments, and aquaculture ponds. Under anaerobic conditions, FeS has excellent reducing capacity. For example, artificially synthesized FeS is used to treat heavy metals, antibiotics, and chlorinated organic compounds in groundwater and soil.

[0004] FeS can effectively complex divalent heavy metal cations. Stabilized FeS can effectively immobilize Hg(II) in reducing environments through surface complexation and chemical precipitation, and the immobilized mercury remains stable for 2.5 years under conditions above neutral pH. FeS can adsorb divalent cationic heavy metals such as Mn(II), Co(II), Ni(II), Cu(II), and Pb(II) by forming surface complexes, insoluble metal sulfides, and solid solutions. However, current research on the removal of heavy metals from water environments by FeS has focused on anaerobic conditions, ignoring the disturbance of mining, groundwater recharge, water level fluctuation, dredging, strong storms, tidal currents, and cave organisms, which causes the anoxic aquifer, lake, and marine sediments to be in contact with the oxidizing environment, resulting in changes in the environmental behavior of heavy metals.

[0005] In recent years, research on divalent heavy metals in aerobic environments has begun to be concerned. For example, oxygen reacts with structural Fe(II) to generate ·OH, which can oxidize CdS and release dissolved Cd(II). Therefore, the method for effectively fixing Cd(II) in aerobic environments is a challenging research direction. SUMMARY

[0006] In view of the above problems existing in the prior art, the application provides a method for fixing water-soluble Cd(II) in an aerobic environment. The method uses FeS material to fix water-soluble Cd(II) in an aerobic environment through chemical precipitation and surface adsorption, the FeS material has a simple preparation method and a strong fixing capacity for water-soluble Cd(II), the fixed Cd(II) has good stability, and has a potential application prospect in the field of water treatment.

[0007] The technical scheme of the application is as follows:

[0008] A method for fixing water-soluble Cd(II) in an aerobic environment, under aerobic conditions, FeS material is put into water body containing water-soluble Cd(II) to be treated, a part of water-soluble Cd(II) reacts with S(-II) in FeS to generate CdS precipitate through displacement reaction; meanwhile, Fe(II) released into the water body is oxidized to generate lepidocrocite, so that the remaining part of water-soluble Cd(II) is adsorbed to the surface of lepidocrocite or embedded in the crystal structure thereof; finally, Cd(II) in the water body to be treated exists in the form of CdS precipitate and lepidocrocite-adsorbed Cd(II).

[0009] Further, the water body to be treated is a flowing phase water body.

[0010] Further, the amount of FeS material put into the water body is 0.1-5 g / L; the concentration of water-soluble Cd(II) in the water body to be treated is 1-50 mM.

[0011] Further, the preparation method of the FeS material comprises the following steps:

[0012] (1) under the condition of being filled with ultra-pure N2 and being stirred intensively, 90-110 mL of Na2S·9H2O solution with a concentration of 0.28-0.31 M is added dropwise into 90-110 mL of FeSO4·7H2O solution with a concentration of 0.28-0.31 M; after the dropwise addition is completed, the stirring is continued for 10-30 min under the N2 environment;

[0013] (2) the obtained suspension is centrifuged and washed with anaerobic water for 2-4 times, so that the FeS material is obtained.

[0014] Preferably, the concentration of the ultra-pure N2 in step (1) is 99.999%.

[0015] Preferably, the stirring speed in step (1) is 200-500 rpm, and the dropwise addition speed is 200-300 mL / h.

[0016] Preferably, the speed of centrifugation in step (2) is 10000-12000 rpm, the centrifugation time is 5-15 min, and the temperature is controlled at 20-30℃ during centrifugation.

[0017] Preferably, the anaerobic water in step (2) is nitrogen bubbling stripped ultrapure water for 20-40 min.

[0018] Preferably, the FeS obtained in step (2) is prepared into a stock solution, used immediately, and sealed under N2 protection for no more than 2 h.

[0019] The beneficial technical effects of the present application are:

[0020] 1. The present application synthesizes FeS material in N2 environment by simple co-precipitation method. FeS can release two kinds of electron donors Fe(II) and S(-II), which can fix water-soluble Cd(II) through surface adsorption and chemical precipitation. FeS material, water-soluble Cd(II) and O2 interact, including the following four basic processes:

[0021] ① Part of water-soluble Cd(II) will react with S(-II) in FeS to generate CdS precipitate through displacement reaction;

[0022] ② The Fe(II) released into the aqueous solution will be oxidized by oxygen to generate lepidocrocite;

[0023] ③ The remaining part of water-soluble Cd(II) is adsorbed onto the surface of lepidocrocite or embedded into the crystal structure;

[0024] ④ As the reaction gradually proceeds, structural Fe(II) (from FeS or Fe(II) adsorbed on lepidocrocite) will be activated by molecular oxygen to generate a large amount of ·OH, which can stimulate the CdS generated in step ① to release a small amount of Cd(II) into the solution. Although the ·OH produced by the oxidation of FeS can cause the fixed Cd(II) to be released back into the environment, it soon reaches equilibrium and exhibits strong fixation ability, with a final fixation rate of 99.99%. The original water-soluble Cd(II) in the final reaction product mainly exists in the form of CdS and lepidocrocite-adsorbed Cd(II), of which 75-85% is CdS and 15-25% is adsorbed on lepidocrocite.

[0025] 2. This invention studies the fixation effect of FeS on water-soluble Cd(II) in an aerobic environment. Previous studies have found that FeS has a strong fixation effect on water-soluble Cd(II) in an anaerobic environment, but the fixation effect weakens when the reactor is placed in an aerobic environment. However, in this invention, FeS can still effectively fix Cd(II) in an aerobic environment. The Cd(II) concentration first decreases rapidly and then slowly rises to a stable level, exhibiting a phenomenon of initial fixation followed by release. This research breaks the technical bias of anaerobic fixation of heavy metal ions and makes up for the deficiencies of existing technologies. Attached Figure Description

[0026] Figure 1 Here is a photograph of the FeS stock solution prepared in Example 1;

[0027] Figure 2 The images show the XRD patterns of the FeS material prepared in Example 1 and the process of immobilizing Cd(II) under aerobic conditions in Example 3.

[0028] Figure 3 XPS image of the FeS material prepared in Example 1;

[0029] Figure 4 This is a schematic diagram illustrating the removal effect of 0.5 g / L FeS suspension on 5 mM Cd(II) under aerobic conditions in Example 2.

[0030] Figure 5 This refers to the change in Cd valence state during the fixation of 5 mM Cd(II) in a 0.5 g / L FeS suspension under aerobic conditions in Example 4.

[0031] Figure 6 This is a high-resolution TEM image of the mineral formed after the reaction of FeS and Cd(II) in Example 5. Detailed Implementation

[0032] The present invention will now be described in detail with reference to the accompanying drawings and embodiments. Obviously, the described embodiments are merely some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0033] Example 1:

[0034] The FeS material with an elliptical microstructure was prepared by coprecipitation. The specific steps are as follows:

[0035] (1) Under the condition of super-pure N2(99.999%) and 200 rpm stirring, 100 mL of 0.3 M Na2S·9H2O solution was added into 100 mL of 0.3 M FeSO4·7H2O solution at a flow rate of 200 mL / h. After the addition, the mixture was stirred for another 30 min under N2;

[0036] (2) The obtained suspension was centrifuged at 11000 rpm for 10 min, and the temperature was controlled at about 25°C. Then, the obtained FeS material was washed with anaerobic water for 3 times.

[0037] (3) The black precipitate obtained after the anaerobic water washing was dissolved in anaerobic water to prepare 100 mL of FeS stock solution (as shown in FIG. 1B), which was used immediately and stored under N2 for no more than 2 h. Figure 1

[0038] The obtained FeS material was characterized by XRD and XPS. As shown in FIG. 2A, the XRD (as shown in FIG. 2A) spectrum showed obvious FeS diffraction peaks, which corresponded to PDF #80-1027 card. As shown in FIG. 2B, the XPS spectrum showed that Fe Figure 2 Figure 3 2+ and S 2- were the main species.

[0039] Example 2:

[0040] Under aerobic conditions, the FeS material prepared in Example 1 was used to solidify water-soluble Cd(II). The specific operation steps were as follows:

[0041] The reaction was carried out in a 500 mL five-necked flask containing 250 mL of Cd(II) solution (5 mM CdCl2) at 25±2°C. The reactor was open to the atmosphere, and the surface of the five-necked flask was wrapped with aluminum foil to avoid potential photochemical reactions. In addition, the system included an ORP probe and a DO probe.

[0042] The reaction pH was maintained at 7.0 using 100 mM MOPs buffer solution, and the FeS concentration was set to 0.5 g / L. The experiment was carried out under a 500 rpm magnetic stirrer, and 1.5 ml of suspension was taken at predetermined time intervals (0, 5, 15, 30, 60, 120, 240, 480, and 960 min) for detection of heavy metal concentration in the filtrate after filtration through a 0.45 μm nylon filter. All experimental settings were triplicate.

[0043] The effect of FeS material on the solidification of water-soluble Cd(II) in an aerobic environment was as shown in FIG. 3. Figure 4 ​​​The horizontal axis is the degradation time (Time), and the vertical axis is the Cd(II) fixation rate (C / C0), where C0 is the initial concentration of Cd(II) and C is the real-time concentration. It can be seen from Figure 4 that the Cd(II) concentration first rapidly decreases and then slowly rises to a stable state (960 min), showing a phenomenon of first fixation and then release. In the aerobic FeS suspension, the Cd(II) concentration first rapidly decreases to 0.014 mg / L and then slightly rises to 0.018 mg / L. The fixation rate of FeS for Cd(II) is 99.99% at the reaction equilibrium.

[0044] Example 3:

[0045] XRD analysis was used to analyze the changes in the solid-phase composition of FeS during the fixation of water-soluble Cd(II) under aerobic conditions. The specific operation steps are shown in Example 2. During the removal of Cd(II), the suspension was extracted from the reactor at predetermined time intervals and filtered through a 0.45 μm filter. The resulting slurry was pre-frozen for 12 h under N2 and freeze-dried for 24 h. The obtained solid material was stored under a nitrogen atmosphere.

[0046] Empyrean sharp shadow was selected for XRD phase analysis of the solid-phase material during the oxidation of FeS by dissolved oxygen. The solid sample was placed on the sample stage for testing, and the main phases were analyzed using Jade 6.0 software to obtain data information. The detection conditions were: Cu K target, power 60 kV and 100 mA, test range 5-90°, scanning step 2° / min.

[0047] The test results are shown in Figure 2 , where the horizontal axis is the test range (2theta (degree)), and the vertical axis is the peak intensity (a.u.). It can be seen from Figure 2 that the crystalline FeS gradually decreases to an undetectable level within the first 120 min, and new minerals appear during the reaction process, and the longer the oxidation time, the higher the peak intensity. The final solid for removing Cd(II) by FeS oxidation is mainly lepidocrocite, CdS, and elemental sulfur.

[0048] Example 4:

[0049] XPS analysis was used to analyze the changes in the Cd valence state during the fixation of water-soluble Cd(II) by FeS under aerobic conditions. The specific operation steps are shown in Example 2. During the removal of Cd(II), the suspension was extracted from the reactor at predetermined time intervals and filtered through a 0.45 μm filter. The resulting slurry was pre-frozen for 12 h under N2 and freeze-dried for 24 h. The obtained solid material was stored under a nitrogen atmosphere.

[0050] The Shimadzu Kratos AXIS SUPRA+ model was selected for XPS analysis of the Cd valence state on the surface of solid materials during heavy metal removal. XPS peak41 software was used to fit the components of the XPS spectrum. The test conditions were: Al Kα (1486.6 eV), voltage and power of 15 kV and 150 W, respectively.

[0051] The measurement results are as follows Figure 5 As shown, the horizontal axis represents binding energy (eV), and the vertical axis represents peak intensity (cunts / s). From... Figure 5 It was observed that no characteristic Cd peaks were detected before the reaction began. At the end of the reaction, the XPS spectrum showed four peaks (405.0, 408.0, 411.8, and 414.8 eV). The binding energies of the Cd 3d5 / 2 and Cd 3d3 / 2 peaks were located at 405.0 and 411.8 eV, respectively, corresponding to the typical energy values ​​of the CdS complex. The binding energy peaks of 408.0 and 414.8 eV are presumably characteristic peaks resulting from the binding of Cd with the hydroxyl groups (-OH) or deprotonated forms (-O-) of ferrimagnesia produced by FeS oxidation via surface complexation.

[0052] Example 5:

[0053] The morphology and crystal structure of the products of the reaction between FeS and Cd(II) were characterized by transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), and selected area electron diffraction microscopy.

[0054] Characterization results as follows Figure 6 The images shown (where: (a) TEM image of the reaction products, (b) high-resolution image of FeOOH, (c) high-resolution image of CdS, (d) HAADF image, (e) Cd Kα, (f) S-Cd superposition, (h) Fe+O+Cd superposition, (g) Fe+O+S+Cd superposition, (i) EDS line outlines of O (yellow), S (green), Fe (red), and Cd (cyan)). Figure 6 It can be seen that the main product of FeS oxidation and fixation of water-soluble Cd(II) is CdS; at the same time, a small amount of Cd is adsorbed or embedded in fibrous ore. Based on the above experiments, it is calculated that 82.6% of the product is CdS, and 17.4% is adsorbed on fibrous ore.

[0055] Although the embodiments of the present invention have been disclosed above, they are not limited to the applications listed in the specification and embodiments. They can be applied to various fields suitable for the present invention. For those skilled in the art, and for those of ordinary skill in the art, various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the present invention. Therefore, the present invention is not limited to the specific details without departing from the general concept defined by the claims and their equivalents.

Claims

1. A method for immobilizing aqueous Cd(II) in an aerobic environment, characterized in that, Under aerobic conditions, the FeS material is put into the water body to be treated containing water-soluble Cd(II), a part of the water-soluble Cd(II) reacts with S(-II) in the FeS to generate CdS precipitate through displacement reaction; meanwhile, the Fe(II) released into the water body is oxidized to generate lepidocrocite, so that the remaining part of the water-soluble Cd(II) is adsorbed onto the surface of the lepidocrocite or embedded in the crystal structure thereof; finally, the Cd(II) in the water body to be treated exists in the form of CdS precipitate and lepidocrocite-adsorbed Cd(II), wherein 75-85% is CdS, 15-25% is adsorbed on the lepidocrocite, and the final fixation rate is 99.99%; The preparation method of the FeS material comprises the following steps: (1) Under the conditions of being filled with ultra-pure N2 and being stirred intensively, 90-110 mL of Na2S·9H2O solution with a concentration of 0.28-0.31 M is added dropwise into 90-110 mL of FeSO4·7H2O solution with a concentration of 0.28-0.31 M; after the dropwise addition is completed, the stirring is continued for 10-30 min under the N2 environment; (2) The obtained suspension is centrifuged, and washed with anaerobic water for 2-4 times, so as to obtain the FeS material.

2. The method of claim 1, wherein, The water body to be treated is a flowing phase water body.

3. The method of claim 1, wherein, The input amount of the FeS material in the water body is 0.1-5 g / L; the concentration of the water-soluble Cd(II) in the water body to be treated is 1-50 mM.

4. The method of claim 1, wherein, The concentration of the ultra-pure N2 in step (1) is 99.999%.

5. The method of claim 1, wherein, The stirring speed in step (1) is 200-500 rpm, and the dropwise addition speed is 200-300 mL / h.

6. The method of claim 1, wherein, The centrifugation speed in step (2) is 10000-12000 rpm, the centrifugation time is 5-15 min, and the temperature is controlled at 20-30 ℃ during the centrifugation.

7. The method of claim 1, wherein, The anaerobic water in step (2) is ultra-pure water bubbled and stripped by nitrogen for 20-40 min.

8. The method of claim 1, wherein, The FeS obtained in step (2) is prepared into a stock solution, used immediately, and sealed and stored under N2 protective atmosphere for no more than 2 h.

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