Method for repairing high-arsenic groundwater by using magnesium-iron hydrotalcite with nitrate intercalation and iron-oxidizing bacteria

Abstract

The present invention discloses a method for repairing high-arsenic groundwater by using nitrate-intercalated ferrous hydrotalcite in combination with iron-oxidizing bacteria. The method is to add nitrate-intercalated ferrous hydrotalcite and anaerobic nitrate-reducing iron-oxidizing bacteria to a HCO3-rich water. ‑ and Fe 2+ The As(III) and As(V) removal reaction is completed at room temperature in high arsenic groundwater. The bicarbonate widely present in high arsenic groundwater can exchange nitrate between hydrotalcite-like layers. The released nitrate will stimulate the metabolism of anaerobic nitrate-reducing iron-oxidizing bacteria. Nitrate-reducing iron-oxidizing bacteria cooperate with hydrotalcite-like to remove Fe 2+ Oxidized and eventually magnetite is generated. During the microbial-mediated nitrate-reduced iron oxidation process, As(III) and As(V) are fixed into the magnetite, achieving the purpose of enhanced arsenic fixation. At the same time, the arsenic concentration in arsenic-contaminated water sources is reduced to meet the drinking water standard (10µg / L), with high removal efficiency and environmental friendliness.

Description

Technical Field

[0001] The invention belongs to the field of groundwater arsenic pollution treatment and relates to a method for repairing high-arsenic groundwater by utilizing nitrate-intercalated ferrous hydrotalcite in collaboration with iron-oxidizing bacteria. Background Art

[0002] Arsenic (As) is a toxic, carcinogenic metalloid that is widely distributed in groundwater worldwide. The highest detected concentration of arsenic in natural groundwater in my country ranges from 25 μg / L to 2.16 mg / L, far exceeding the World Health Organization's limit of 10 μg / L. In my country, arsenic contamination of groundwater is particularly prominent in arid / semi-arid regions and river deltas in northern China. These groundwaters are primarily reductive, neutral, or weakly alkaline, oxygen-free, and rich in bicarbonate and ferrous ions. Nitrate content is extremely low, and As primarily exists in the form of inorganic arsenic. Currently, As has been listed as a key regulated pollutant, and the exploration of an efficient and cost-effective method for remediating high-arsenic groundwater is urgent.

[0003] Based on research into the matrix characteristics of high-arsenic groundwater in my country, in-situ remediation of iron minerals has great potential for reducing free arsenic levels in groundwater. However, most iron oxides formed by the oxidation of ferrous ions are easily reduced by iron-reducing bacteria, leading to the re-release of As into the water. Previous studies have reported that anaerobic nitrate-reducing iron-oxidizing bacteria can promote the conversion of green rust to magnetite. Magnetite, due to its remarkable stability in reducing environments, is considered to have the potential to achieve long-term arsenic fixation, achieving efficient and stable arsenic removal. However, due to the low abundance of green rust in the natural environment and its susceptibility to oxidation, its application is limited. Therefore, an iron-based precursor of green rust is urgently needed to cooperate with nitrate-reducing iron-oxidizing bacteria to explore an environmentally friendly, efficient arsenic removal method while enhancing arsenic fixation. Summary of the Invention

[0004] Purpose of the invention: In order to solve the problem of inorganic arsenic pollution in water bodies, a material that can not only slowly release nitrates but also serve as a green rust precursor is used to avoid the strong disturbance of the groundwater ecological environment caused by direct injection of oxidants. The technical problem to be solved by the present invention is to provide a method for repairing high-arsenic groundwater by using ferromagnesian hydrotalcite in the nitrate layer in collaboration with iron-oxidizing bacteria.

[0005] Technical solution: In order to solve the above technical problems, the present invention provides a method for repairing high-arsenic groundwater by using nitrate-intercalated magnesium-iron hydrotalcite in conjunction with iron-oxidizing bacteria. The method is to add HCO3 to the groundwater under anaerobic conditions. - 、Fe 2+ Nitrate-intercalated ferromagnesian hydrotalcite and iron-oxidizing bacteria were added to high-arsenic groundwater to complete the As(III) or As(V) removal reaction at room temperature.

[0006] Preferably, the initial concentration of As(III) or As(V) in the high-arsenic groundwater is 0.2-2 mg / L.

[0007] Preferably, the final concentration of the nitrate-intercalated magnesium-iron hydrotalcite material in water is 0.25-1 g / L.

[0008] Preferably, HCO3 in the high arsenic groundwater - Derived from sodium bicarbonate, the initial concentration of sodium bicarbonate is 200-500 mg / L.

[0009] As a preference, Fe in the high arsenic groundwater 2+ The initial concentration is 28-112 mg / L.

[0010] Preferably, the iron-oxidizing bacteria are anaerobic nitrate-reducing iron-oxidizing bacteria (Acidovorax sp. strainBoFeN1).

[0011] Preferably, the reaction temperature is 28±2° C., and the reaction time under anaerobic conditions is 48 to 96 hours.

[0012] Preferably, anaerobic nitrate-reducing iron-oxidizing bacteria are added to the water body at a bacterial concentration of about 8×10 6 ~2×10 7 cells / mL.

[0013] Preferably, the preparation method of the nitrate-intercalated magnesium-iron hydrotalcite comprises the following steps: pre-dissolving magnesium nitrate hexahydrate, ferric nitrate nonahydrate and sodium nitrate, adding sodium hydroxide to adjust the pH of the suspension to 9-10, and reacting for 24-48 hours in a nitrogen atmosphere at 30-70° C. to form the nitrate-intercalated magnesium-iron hydrotalcite.

[0014] Preferably, the molar ratio of magnesium nitrate hexahydrate, ferric nitrate nonahydrate and sodium nitrate is: 1.5-2.5:0.8-1.3:2-4. Preferably, the method further comprises centrifuging the product after the reaction and repeatedly washing it with pure water 5-6 times, and drying it in a freeze dryer for 24-48 hours to obtain nitrate-intercalated magnesium-iron hydrotalcite.

[0015] The mechanism of the present invention is as follows: the nitrate-intercalated magnesium-iron hydrotalcite prepared by the present invention can be used as a precursor of green rust; the characteristic of groundwater being rich in bicarbonate ions is then utilized to exchange with nitrates between the hydrotalcite layers, thereby achieving the purpose of slow release of nitrate and preventing excessive local nitrate concentration from affecting the groundwater ecological environment; since the release of nitrate ions stimulates the metabolism of nitrate-reducing iron-oxidizing bacteria, driving the oxidation of ferrous ions, the final precipitate formed is mainly magnetite, accompanied by lepidocrocite. Compared with other iron oxides, magnetite has a more long-term arsenic fixation ability, and can better achieve the removal and long-term fixation of arsenic in groundwater.

[0016] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages: The present invention proposes for the first time to utilize the nitrate-intercalated magnesium-iron hydrotalcite's nitrate-release capacity to couple with nitrate-reducing iron-oxidizing bacteria to remove HCO3-rich - and Fe 2+ A method for treating arsenic in groundwater can effectively improve the removal and fixation of As(III) and As(V), and the method is simple to operate and environmentally friendly. The method has a long-lasting treatment effect, is economical, and minimizes secondary pollution to the environment during the groundwater arsenic treatment process. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 , XRD spectra of nitrate-intercalated MgFe-type hydrotalcite before and after nitrate release;

[0018] Figure 2 , the nitrate sustained-release effect of Example 2;

[0019] Figure 3 , As concentration changes in Example 3 and Comparative Example 1;

[0020] Figure 4 , X-ray diffraction pattern of the final product of Example 3;

[0021] Figure 5 , Fe(II) concentration changes in Example 3;

[0022] Figure 6 , Fe(II) concentration changes of Comparative Example 2 and Comparative Example 3;

[0023] Figure 7 , the removal effect of Comparative Example 2 and Comparative Example 3 on As;

[0024] Figure 8 , X-ray diffraction pattern of the final product of Comparative Example 3;

[0025] Figure 9 , X-ray diffraction pattern of the final product of Comparative Example 4;

[0026] Figure 10 , Example 3, Comparative Example 1 and Comparative Example 3 products' arsenic extraction results. DETAILED DESCRIPTION

[0027] The technical solution of the present invention is described in detail below with reference to examples.

[0028] In the following examples, laboratory-prepared As(III) solution and As(V) solution were used as simulated solutions for high-arsenic groundwater;

[0029] Sodium bicarbonate-rich groundwater uses a high-concentration sodium bicarbonate solution prepared in the laboratory as the simulated fluid;

[0030] Contains HCO3 - 、Fe 2+ The high-arsenic groundwater is treated with a simulated solution prepared in the laboratory using sodium bicarbonate, ferrous sulfate heptahydrate, sodium arsenite or sodium arsenate dodecahydrate.

[0031] Preparation of As(III) and As(V) Stocks:

[0032] As(III) solution was prepared by dissolving sodium arsenite (NaAsO2) in ultrapure water with a concentration of 1 g / L (as As);

[0033] The As(V) solution is prepared by dissolving sodium arsenate dodecahydrate (NaHAsO4·12H2O) in ultrapure water, and its concentration is 1 g / L (in terms of As).

[0034] Preparation of sodium bicarbonate solution: 420 mg / L sodium bicarbonate (adjust pH to 7-7.5 with dilute hydrochloric acid).

[0035] Contains HCO3 - 、Fe 2+ Preparation of high As(III) solution: Pipette As(III) stock solution at a ratio of 0.2% (v / v) into 420 mg / L sodium bicarbonate solution, then add and dissolve ferrous sulfate heptahydrate to make the total Fe(II) concentration 56 mg / L.

[0036] Contains HCO3 - 、Fe 2+ Preparation of high As(V) solution: Pipette As(V) stock solution at a ratio of 0.2% (v / v) into 420 mg / L sodium bicarbonate solution, then add and dissolve ferrous sulfate heptahydrate to make the total Fe(II) concentration 56 mg / L.

[0037] The anaerobic nitrate-reducing iron-oxidizing bacteria (Acidovorax sp. strain BoFeN1) in the examples of the present invention were purchased from the German Collection of Microorganisms.

[0038] Cultivation process;

[0039] (1) Heterotrophic denitrification medium containing 20 mM PIPES buffer (pH 7.0) was used for cultivation. The components of the heterotrophic denitrification medium included 10 mM sodium nitrate, 10 mM sodium acetate, inorganic salts (0.3 g / L NH4Cl, 0.3 g / L NaCl, 0.14 g / L NaH2PO4, 0.42 g / L MgCl2·6H2O, 0.1 g / L CaCl2·2H2O) and trace minerals (12.5 mg / LEDTA, 0.25 mg / L ZnSO4·7H2O, 0.075 mg / L MnCl2·4H2O, 0.75 mg / L H3BO3, 0.5 mg / L CoCl2·6H2O, 0.025 mg / L CuCl2·2H2O, 0.05 mg / L NiCl2·6H2O, 0.075 mg / L Na2MoO4·2H2O). Nitrate-reducing iron-oxidizing bacteria (50% bacterial solution and 50% glycerol) stored in a -80°C freezer were inoculated into the above culture medium at a 4% (v / v) inoculum and incubated in anoxic conditions at 28°C in the dark for 24 hours. The activated bacterial solution was transferred to a fresh culture medium at a 4% (v / v) concentration using the same method and incubated in anoxic conditions at 28°C in the dark for 20 hours to obtain nitrate-reducing iron-oxidizing bacteria in the logarithmic phase.

[0040] (2) Centrifugation of nitrate-reducing iron-oxidizing bacteria: The nitrate-reducing iron-oxidizing bacteria cultured to the logarithmic phase were centrifuged at 6000 rpm and 4°C, and washed twice with 420 mg / L sodium bicarbonate solution. The obtained centrifuged bacteria were placed in sodium bicarbonate solution and the bacterial solution concentration was adjusted to about 5 × 10 7 cells / mL.

[0041] Example 1 Preparation of Nitrate-Intercalated Magnesium-Iron Hydrotalcite

[0042] Appropriate amounts of magnesium nitrate hexahydrate and ferric nitrate nonahydrate (molar ratio 2:1) were weighed and dissolved in 100 mL of anoxic deionized water to form a solution of 0.667 mol / L magnesium nitrate hexahydrate and 0.333 mol / L ferric nitrate nonahydrate. Under a nitrogen atmosphere, this solution was added at a rate of 8 mL / min to a 500 mL beaker containing 100 mL of 1.0 mol / L sodium nitrate solution. Simultaneously, an appropriate amount of 1.0 mol / L sodium hydroxide was added to the mixture, maintaining the pH of the suspension between 9 and 10 throughout the synthesis. The addition of sodium hydroxide was stopped when the pH stabilized. The resulting suspension was stirred at 70°C under a stream of nitrogen for 24 hours. The resulting solid was recovered by centrifugation, washed repeatedly with pure water 5-6 times, and dried in a freeze dryer for 24 hours to obtain the nitrate-intercalated magnesium-iron hydrotalcite.

[0043] Example 2 Nitrate-intercalated magnesium-iron hydrotalcite-type nitrate sustained release effect in sodium bicarbonate-rich groundwater

[0044] 50 mg of the nitrate-intercalated magnesium-iron hydrotalcite prepared in Example 1 was weighed using an electronic balance and added to 100 mL of a sodium bicarbonate solution with a sodium bicarbonate concentration of 420 mg / L. The mixture was reacted at room temperature for 24 hours.

[0045] Example 3 Nitrate-intercalated Mg-Fe hydrotalcite cooperates with nitrate-reducing iron-oxidizing bacteria to oxidize Fe 2+ Methods for removing As(III) and As(V)

[0046] 25 mg of nitrate-intercalated magnesium-iron hydrotalcite prepared in Example 1 was weighed using an electronic balance, and 2 mL of nitrate-reducing iron-oxidizing bacteria centrifuge solution (about 5 × 10 7 cells / mL), and added the two into a 50mL anaerobic bottle containing sodium bicarbonate solution, where the sodium bicarbonate concentration was 420mg / L, the As(III) or As(V) concentration was 2mg / L, and the Fe 2+ The concentration is 56 mg / L, the concentration of sodium acetate is 82 mg / L, and the reaction is carried out under anaerobic conditions at 28°C for 48 hours.

[0047] Comparative Example 1 Method for removing As(III) and As(V) based on nitrate intercalated magnesium iron hydrotalcite (without adding iron oxidizing bacteria and Fe 2+ )

[0048] 25 mg of the nitrate-intercalated magnesium-iron hydrotalcite prepared in Example 1 was weighed using an electronic balance and added to a 50 mL anaerobic bottle containing arsenic-containing sodium bicarbonate solution, wherein the sodium bicarbonate concentration was 420 mg / L and the As(III) or As(V) concentration was 2 mg / L. The mixture was reacted under anaerobic conditions at 28° C. for 48 h.

[0049] Comparative Example 2: Method for Removing As(III) and As(V) Based on Nitrate-Reducing Iron-Oxidizing Bacteria (Magnesium-Fe Hydrotalcite without Nitrate Intercalation)

[0050] Pipette 2mL of nitrate-reducing iron-oxidizing bacteria centrifuge solution (about 5×10 7 cells / mL) and added to a 50 mL anaerobic bottle containing sodium bicarbonate solution, where the sodium bicarbonate concentration was 420 mg / L, the As(III) or As(V) concentration was 2 mg / L, and the Fe 2+ The concentration is 56 mg / L, the concentration of sodium acetate is 82 mg / L, and the reaction is carried out under anaerobic conditions at 28°C for 48 hours.

[0051] Comparative Example 3: Oxidation of Fe by Nitrate-Reducing Iron-Oxidizing Bacteria2+ Method for removing As(III) and As(V) (MgFe-Hydrotalcite without nitrate intercalation and addition of sodium nitrate)

[0052] Pipette 2mL of nitrate-reducing iron-oxidizing bacteria centrifuge solution (about 5×10 7 cells / mL) and added to a 50 mL anaerobic bottle containing sodium bicarbonate solution, where the sodium bicarbonate concentration was 420 mg / L, the As(III) or As(V) concentration was 2 mg / L, and the Fe 2+ The concentration is 56 mg / L, the concentration of sodium acetate is 82 mg / L, and the concentration of sodium nitrate is 85 mg / L, and the reaction is carried out under anaerobic conditions at 28°C for 48 hours.

[0053] Comparative Example 4 is based on Fe 2+ Transformation of nitrate-intercalated MgFe-Hydrotalcites (without iron-oxidizing bacteria)

[0054] 25 mg of the nitrate-intercalated magnesium-iron hydrotalcite prepared in Example 1 was weighed using an electronic balance and added to a 50 mL anaerobic bottle containing a sodium bicarbonate solution containing arsenic, wherein the sodium bicarbonate concentration was 420 mg / L, the As(III) or As(V) concentration was 2 mg / L, and the Fe 2+ The concentration was 56 mg / L and the reaction was carried out under anaerobic conditions at 28°C for 48 hours.

[0055] The above experimental results and analysis can be found in Figures 1 to 9 .

[0056] Depend on Figure 1 It can be seen that the nitrate-intercalated ferrous hydrotalcite prepared in Example 1 has characteristic peaks of hydrotalcite-like materials, indicating that the material is indeed a nitrate-intercalated ferrous hydrotalcite material.

[0057] Depend on Figure 2 It can be seen that in Example 2, when nitrate-intercalated ferrous hydrotalcite was added to the NaHCO3 solution, the nitrate concentration in the solution continued to increase, reaching 1.21 mM within 12 h, indicating that in the NaHCO3 buffer medium, bicarbonate can replace nitrate from the nitrate-intercalated ferrous hydrotalcite particles. Figure 1 The XRD spectra of the nitrate-intercalated Mg-Fe-type hydrotalcite after nitrate release remain basically unchanged from the original nitrate-intercalated Mg-Fe-type hydrotalcite sample. However, the (003) peak shifted slightly from 10.91° to 11.32°, indicating that the interlayer spacing of the hydrotalcite has changed from Reduce to Since the interlayer spacing of nitrate LDH is larger than that of carbonate LDH, it indicates that carbonate may be inserted into the middle layer of LDH.

[0058] Depend on Figure 3It can be seen that compared with Comparative Example 1 in which only nitrate-intercalated ferrous hydrotalcite was added, Example 3 in which nitrate-reducing iron-oxidizing bacteria were coupled with nitrate-intercalated ferrous hydrotalcite had a better removal effect on As(III) and As(V).

[0059] Depend on Figure 4 It can be seen that no matter in the presence of As(III) or As(V), nitrate-intercalated ferrous hydrotalcite cooperates with iron-oxidizing bacteria to oxidize ferrous iron to oxidize Fe(II), and magnetite is generated after 48 hours, accompanied by lepidocrocite.

[0060] Depend on Figure 5 It can be seen that, in the presence of either As(III) or As(V), nitrate-intercalated MgFe-type hydrotalcites can stimulate iron-oxidizing bacteria to oxidize Fe(II).

[0061] Depend on Figure 6 It can be seen from Comparative Examples 2 and 3 that, in the absence of nitrate, iron-oxidizing bacteria cannot effectively oxidize Fe(II).

[0062] Depend on Figure 7 It can be seen that Figure 6 Correspondingly, a higher degree of ferrous oxidation can contribute to better arsenic removal.

[0063] Depend on Figure 8 It can be seen that in Comparative Example 3, when there is no nitrate-intercalated magnesium-iron hydrotalcite involved, whether in the presence of As(III) or As(V), only nitrate-reducing iron-oxidizing bacteria oxidize ferrous ions, and after 48 hours, goethite and lepidocrocite are generated, and magnetite cannot be generated. Magnetite has the ability to fix arsenic in a long-term manner, while the former two do not have this ability.

[0064] Depend on Figure 9 It can be seen that the presence of inorganic arsenic inhibits the conversion of green rust to magnetite. Unlike Example 3, in the absence of nitrate-reducing iron-oxidizing bacteria, green rust was formed after 48 h in the presence of As(V). In the presence of As(III), the main product was still green rust, accompanied by a small amount of lepidocrocite and magnetite products.

[0065] Depend on Figure 10 As can be seen from the arsenic extraction experiment, the arsenic fixation capacity of the reaction products was measured. In Comparative Example 1, approximately 90% of the arsenic was adsorbed, while in Comparative Example 3, approximately 55% and 30% of the arsenic were adsorbed and bound to crystalline iron oxides, respectively. In Example 3, approximately 70% of the arsenic was bound to recalcitrant iron oxides. This indicates that magnetite, ultimately formed by the nitrate-intercalated magnesian hydrotalcite and the oxidation of ferrous iron by iron-oxidizing bacteria, has a long-term arsenic fixation capacity.

Claims

1. A method for remediating high-arsenic groundwater using nitrate-intercalated magnesium-iron hydrotalcite in collaboration with iron-oxidizing bacteria, characterized in that: The method is to add HCO3 to the - 、Fe 2+ Nitrate-intercalated ferromagnesian hydrotalcite and iron-oxidizing bacteria were added to high-arsenic groundwater to complete the As(III) or As(V) removal reaction at room temperature.

2. The method for remediating high-arsenic groundwater using nitrate-intercalated ferrous hydrotalcite in collaboration with iron-oxidizing bacteria according to claim 1, characterized in that: The initial concentration of As(III) or As(V) in the high-arsenic groundwater is 0.2-2 mg / L.

3. The method for remediating high-arsenic groundwater using nitrate-intercalated ferrous hydrotalcite in collaboration with iron-oxidizing bacteria according to claim 1, characterized in that: The final concentration of the nitrate-intercalated magnesium-iron hydrotalcite material in water is 0.25-1 g / L.

4. The method for remediating high-arsenic groundwater using nitrate-intercalated ferrous hydrotalcite in collaboration with iron-oxidizing bacteria according to claim 1, characterized in that: HCO3 in the high arsenic groundwater - Derived from sodium bicarbonate, the initial concentration of sodium bicarbonate is 200-500 mg / L.

5. The method for remediating high-arsenic groundwater using nitrate-intercalated ferrous hydrotalcite in collaboration with iron-oxidizing bacteria according to claim 1, characterized in that: The Fe in the high arsenic groundwater 2+ The initial concentration was 28-112 mg / L.

6. The method for remediating high-arsenic groundwater using nitrate-intercalated ferrous hydrotalcite in collaboration with iron-oxidizing bacteria according to claim 1, characterized in that: The iron-oxidizing bacteria are anaerobic nitrate-reducing iron-oxidizing bacteria (Acidovorax sp. strain BoFeN1).

7. The method for remediating high-arsenic groundwater using nitrate-intercalated ferrous hydrotalcite in collaboration with iron-oxidizing bacteria according to claim 1, characterized in that: The reaction temperature is 28±2°C, and the reaction time is 48-96 hours under anaerobic conditions.

8. The method for remediating high-arsenic groundwater using nitrate-intercalated ferrous hydrotalcite in collaboration with iron-oxidizing bacteria according to claim 6, characterized in that: Anaerobic nitrate-reducing iron-oxidizing bacteria were added to the water body at a bacterial concentration of 8×10 6 -2×10 7 cells / mL.

9. The method for remediating high-arsenic groundwater using nitrate-intercalated ferrous hydrotalcite in collaboration with iron-oxidizing bacteria according to claim 3, characterized in that: The preparation method of the nitrate-intercalated magnesium-iron hydrotalcite comprises the following steps: pre-dissolving magnesium nitrate hexahydrate, ferric nitrate nonahydrate and sodium nitrate, adding sodium hydroxide to adjust the pH of the suspension to 9-10, and reacting for 24-48 hours under a nitrogen atmosphere at 30-70° C. to form the nitrate-intercalated magnesium-iron hydrotalcite.

10. The method for remediating high-arsenic groundwater using nitrate-intercalated ferrous hydrotalcite in collaboration with iron-oxidizing bacteria according to claim 9, characterized in that: The molar ratio of the magnesium nitrate hexahydrate, the ferric nitrate nonahydrate and the sodium nitrate is: 1.5-2.5:0.8-1.3:2-4.

11. The method for remediating high-arsenic groundwater using nitrate-intercalated ferrous hydrotalcite in collaboration with iron-oxidizing bacteria according to claim 9, characterized in that: The method further comprises the steps of centrifuging the product after the reaction, repeatedly washing the product with pure water for 5-6 times, and drying the product in a freeze dryer for 24-48 hours to obtain nitrate-intercalated magnesium-iron hydrotalcite.

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