In-situ remediation method for contaminated sediments

Abstract

The present invention discloses an in-situ remediation method for contaminated sediments, comprising: enriching and culturing PAHs-degrading bacteria; selecting Fe₃O₄ colloids or Fe₃O₄ solid particles as electron acceptors based on the total sulfate and nitrate concentrations in the sediment to be remediated; and in-situ addition of the electron acceptor and inoculation of the degrading bacteria, followed by degradation and remediation under natural conditions. The present invention utilizes magnetite mineral as an electron-conducting substance to mediate electron transfer between microorganisms, promoting their growth and metabolism. This can reduce the bioavailability of organic pollutants in the sediments, enabling efficient remediation of organic pollutants and significantly shortening the remediation process. The conversion process and products exhibit low biotoxicity, are environmentally and microbially friendly, and enable effective in-situ remediation.

Description

Technical Field

[0001] The present invention belongs to the technical field of water environment ecological restoration, and specifically relates to a method for in-situ restoration of contaminated sediments, and more particularly to a method for promoting microbial degradation of organic pollutants by utilizing magnetite as an electron acceptor / donor for microbial extracellular respiration or an electron transfer medium. Background Art

[0002] Currently, the primary methods for controlling complex pollution in aquatic sediments are dredging, in-situ covering, and enhanced bioremediation. While dredging is effective, it is costly and carries the risk of secondary pollution, such as the subsequent off-site disposal of contaminated sediments. In-situ covering involves depositing inert or active materials onto the surface of contaminated sediments, which can quickly control pollution. However, it carries the risk of pollutants being released into the water as environmental conditions change.

[0003] Iron oxides, nitrates, and sulfates are the three most commonly used electron acceptors for anaerobic respiration and metabolism in aquatic sediments. Nitrates and sulfates, as electron acceptors, are often added to contaminated sediments in dissolved form. Previous studies have shown that the addition of nitrates or sulfates as terminal electron acceptors can promote the anaerobic biodegradation of PAHs, playing an important role in the treatment and remediation of organic pollutants in sediments. However, as dissolved electron acceptors, nitrates and sulfates are easily affected by sediment pore water flow, and their effectiveness decreases as they are lost through pore water flow, potentially having a negative impact on aquatic ecosystems.

[0004] Iron oxides come in various forms, with amorphous iron oxide (FeOOH) and magnetite (Fe3O4) being the most commonly used. Previous studies have shown that iron-reducing bacteria such as Geobacter metallireducens and Shewanella oneidensis can use Fe(III) in iron oxides as a terminal electron acceptor for extracellular respiration. More importantly, iron oxides with mixed valence states, such as Fe3O4, can act as both electron acceptors and electron donors in microbial extracellular respiration, playing an important role in regulating the environmental redox potential. However, Shen et al. found that Fe3O4 particles inhibited the biodegradation of naphthalene. Summary of the Invention

[0005] To address the shortcomings or deficiencies of existing technologies, the present invention provides a method for remediating contaminated sediments. This method specifically uses iron oxide minerals to enhance mineral-microbial interactions within the sediments, promoting the microbial degradation of organic pollutants, particularly PAHs, through direct electron transfer at the mineral-microbe interface.

[0006] To achieve the above objectives, the present invention is specifically implemented through the following technical solutions:

[0007] A method for in-situ remediation of contaminated sediments comprises the following steps:

[0008] (1) Enrichment and cultivation of PAHs-degrading bacteria: fresh wet sediment was cultured in water under constant temperature shaking, and then the concentrated culture solution was transferred to a liquid culture medium, and a phenanthrene solution was added under pH 7.2-7.4, and anaerobically shaken culture was carried out for 5-7 days; the obtained culture solution was enriched and the transfer and anaerobic shaking culture operations were repeated, and a phenanthrene solution with a higher concentration than the previous one was added after each transfer, and the transfer and anaerobically culture were carried out three times in total to finally obtain a suspended bacterial solution;

[0009] (2) Selection of electron acceptor: Investigate the total concentration of sulfate and nitrate in the sediment to be repaired. When the total concentration is above 20 mmol / L, select Fe3O4 colloid as the electron acceptor. When the total concentration is below 20 mmol / L, select Fe3O4 solid particles as the electron acceptor.

[0010] (3) In situ remediation of contaminated sediments: The electron acceptor selected in step (2) is added to the sediment to be remediated at a final Fe concentration of 10 to 20 mmol / L, and the bacterial suspension obtained in step (1) is inoculated to degrade and remediate the sediment under natural conditions.

[0011] Preferably, the constant temperature shaking culture conditions in step (1) are: 25° C., 120 rpm, and culture for 12 hours.

[0012] Preferably, the water in step (1) is sterile distilled water.

[0013] Preferably, the ratio of the fresh wet sediment to water in step (1) is 10:200 (g / mL).

[0014] Preferably, the concentrated culture solution in step (1) is obtained by allowing the culture solution to stand and taking out the lower concentrated solution.

[0015] Preferably, the concentration of the phenanthrene solution initially added in step (1) is 2 mg / L.

[0016] Preferably, in step (1), each 1 L of the liquid culture medium contains the following components:

[0017] K2HPO4 0.2-0.3 g, KH2PO4 0.3-0.4 g, NH4Cl 1.0-1.5 g, CaCl2·2H2O 0.05-0.10 g, MgCl2·6H2O 0.05-0.12 g, trace element solution 1 mL, and the balance is water.

[0018] Preferably, each 1L of the trace element solution contains the following components:

[0019] CoCl2·6H2O 0.19~0.2g, MnCl2·4H2O 0.05~0.08g, ZnCl2 0.05~0.08g, NiCl2·6H2O 0.02~0.03g, Na2MoO4·H2O 0.02~0.03g, H3BO3 0.003~0.008g, CuCl2·2H2O 0.001~0.003g, FeCl2·4H2O 1.2~1.5g, and the balance is water.

[0020] Preferably, the transfer in step (1) is to transfer the culture fluid into a liquid culture medium with a volume 20 times that of the liquid culture medium.

[0021] Preferably, the Fe3O4 colloid is prepared by the following method:

[0022] Fe 2+ :Fe 3+ :OH - =1:1:6 under anaerobic conditions at 30-40°C and quickly mix the iron salt solution and the alkaline solution, stir for 1-1.5 hours, let it stand to obtain a precipitate, wash it, and then redisperse it in water to obtain the product.

[0023] Preferably, the Fe3O4 solid particles are prepared by the following method: 3+ :Fe 2+ =1.75:1 iron salt, and add alkali to make the solution pH 11, and react at 80-85°C for 1.5-2h to obtain the product.

[0024] Preferably, the iron salt is sulfate or chloride.

[0025] Preferably, the base is sodium hydroxide.

[0026] Preferably, in step (3), if the electron acceptor is Fe3O4 colloid, the degradation and repair time is 7 to 15 days; if the electron acceptor is Fe3O4 solid particles, the degradation and repair time is 20 to 30 days.

[0027] The beneficial effects of the present invention are as follows: the present invention uses magnetite mineral as a remediation raw material, which is abundant in source. Magnetite mineral acts as an electron-conducting substance to mediate electron transfer between microorganisms, thereby promoting the growth and metabolism of microorganisms. The method of the present invention can reduce the bioavailability of organic pollutants in sediments, can efficiently degrade organic pollutants, greatly shorten the degradation time, and significantly shorten the remediation process of pollutants. The electron acceptor used in the present invention has low biotoxicity in its conversion process and products, is friendly to the environment and microorganisms, and can effectively carry out in situ remediation. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 These are the characterization images of the two forms of Fe3O4 described in the present invention, where A is the TEM image of colloidal nano-Fe3O4; B is the SEM image of solid particle Fe3O4; C is the particle size distribution diagram of solid particle Fe3O4; D is the XRD image of solid particle Fe3O4 and colloidal nano-Fe3O4.

[0029] Figure 2 This is the in-situ remediation result of the contaminated sediment in Example 1.

[0030] Figure 3 The content of polycyclic aromatic hydrocarbons in the sediment under different addition concentration conditions in Example 2 (calculated based on Fe content) is shown.

[0031] Figure 4 This is a comparison chart of the degradation performance of phenanthrene by Fe3O4 solid particles and Fe3O4 colloids during anaerobic culture under different concentrations of electron acceptors (sulfate and nitrate) in Example 4. DETAILED DESCRIPTION

[0032] The technical solution of the present invention is further described below with reference to specific drawings and embodiments.

[0033] Example 1

[0034] (1) Preparation of Fe3O4 colloid: Control the total concentration of ferrous sulfate and ferric chloride to 0.5 mol / L, dissolve FeSO4·7H2O and FeCl3·6H2O in 100 mL of oxygen-free deionized water, and place in a water bath at 30°C with constant stirring. Quickly add 0.25 mol / L NaOH solution to make the molar ratio of Fe 2+ :Fe 3+ :OH - =1:1:6. After stirring for 1 hour, the mixture was allowed to settle and the supernatant was removed. Deoxygenated deionized water was added to the remaining precipitate, which was repeatedly washed until neutral. The precipitate was then dispersed in a beaker filled with deionized water to obtain a stable Fe₃O₄ colloidal solution. Inductively coupled plasma emission spectrometry (ICP) analysis revealed an Fe concentration of 6.2 g / L. The resulting colloidal solution remained well dispersed even after three months.

[0035] (2) Preparation of Fe3O4 solid particles: Under high-speed stirring, 1 mol / L NaOH was quickly added to the solution of FeSO4·7H2O and FeCl3·6H2O to make the molar ratio of Fe 3+ :Fe 2+=1.75:1, the solution pH reached 11. The beaker was placed in an 80°C water bath for 1.5 hours. The resulting precipitate was repeatedly washed until neutral and then dried naturally. The Fe₃O₄ product obtained under these conditions had the highest purity and largest particle size.

[0036] (3) The two forms of magnetite prepared by co-precipitation were characterized to understand their surface morphology. The TEM image of Fe3O4 colloid is shown in Figure 2. Figure 1 As shown in Figure A, it can be observed that the colloidal nano-Fe3O4 synthesized by this method is approximately spherical in shape, has uniform particle size and good dispersion, and has a particle size of about 10 to 20 nm.

[0037] SEM micrograph of Fe3O4 solid particles Figure 1 As shown in B, it can be seen that the Fe3O4 solid particles are granular or blocky. The Fe3O4 solid particle size distribution measured by the laser particle size analyzer is as follows Figure 1 C, the diameter corresponding to 50% in the cumulative distribution of Fe3O4 solid particle size is 26.337 μm. Figure 1 D is the XRD pattern of two different forms of Fe3O4. Five obvious diffraction peaks appeared at diffraction angles 2θ = 30.2°, 35.4°, 43.2°, 57.2° and 62.7°, which are consistent with the XRD standard card of Fe3O4 (JCPDS no.19-0629), corresponding to the (220), (311), (400), (511) and (440) crystal planes, respectively. At the same time, the diffraction peaks are sharp, indicating that the synthesized Fe3O4 is of high purity, small crystals and good crystallinity. The above results show that the successfully prepared colloidal nano-Fe3O4 has different size and morphological characteristics from the solid particulate Fe3O4.

[0038] (4) Enrichment and culture of PAHs-degrading bacteria: 10 g of fresh wet sediment sample was placed in 200 mL of sterile distilled water, stirred, and placed in a constant temperature shaking incubator (25°C, 120 rpm) for 12 hours. The suspended culture was allowed to stand for 1 hour, and 10 mL of the concentrated culture was transferred to a conical flask containing 200 mL of liquid culture medium (both the culture medium and the conical flask were sterilized). Each 1 L of the liquid culture medium contained: K2HPO4 0.2-0.3 g, KH2PO4 0.3-0.4 g, NH4Cl 1.0-1.5 g, CaCl2·2H2O 0.05-0.10 g, MgCl2·6H2O 0.05-0.12 g, trace element solution 1 mL, and the balance was water. Each 1L of the trace element solution contains: CoCl2·6H2O 0.19-0.2g, MnCl2·4H2O 0.05-0.08g, ZnCl2 0.05-0.08g, NiCl2·6H2O 0.02-0.03g, Na2MoO4·H2O 0.02-0.03g, H3BO3 0.003-0.008g, CuCl2·2H2O 0.001-0.003g, FeCl2·4H2O 1.2-1.5g, and the balance is water.

[0039] The pH of the culture medium was adjusted to 7.2–7.4. A phenanthrene solution was added to the culture medium by sterilization through filtration using a 0.22 μm filter, resulting in a phenanthrene concentration of approximately 2 mg / L. Nitrogen was then introduced into the culture medium for 30 minutes to create an anaerobic environment. The conical flask was placed in a constant-temperature shaking incubator (25°C, 200 rpm) and incubated for 5 days. To obtain a stable bacterial community, 20 mL of the enriched culture medium was transferred to a fresh 200 mL anaerobic medium, increasing the phenanthrene concentration to approximately 4 mg / L. Seven days later, the enriched culture medium was transferred again to the fresh anaerobic medium. This was repeated three times, adding a higher concentration of phenanthrene solution each time. After substantial degradation of the phenanthrene, the suspended bacterial suspension was used as the enrichment medium for the phenanthrene-degrading bacteria in the sediment.

[0040] (5) Selection of electron acceptors: Dashuigang is one of the rivers entering East Taihu Lake and is the shipbuilding and ship repair base of Dongshan Town, Suzhou. Its water body is polluted by motor oil, diesel and other pollutants all year round. Using the sediment samples of Dashuigang piled up after dredging, the investigation found that the total PAHs concentration in its sediments reached 31.30 mg / kg, of which the total concentration of sulfate and nitrate was 22.5 mmol / L. Fe3O4 colloid and Fe3O4 solid particles were added to the surface of the sediment at the estuary of the lake, respectively, at a rate of 80 g per square meter of sediment, and inoculated with the enrichment culture of degrading bacteria. The in-situ repair was carried out under natural conditions for 45 days, and the sediments without magnetite and degrading bacteria were used as the control group. Figure 2It can be seen that in this area, Fe3O4 colloid is most efficient in removing pollutants from sediments, with the highest removal rate reaching 84%.

[0041] Example 2

[0042] Dashuigang is one of the rivers flowing into East Taihu Lake and is the shipbuilding and ship repair base of Dongshan Town, Suzhou. Its water body is polluted by motor oil, diesel and other pollutants all year round. Sediment samples from Dashuigang were collected and microcosm culture experiments were carried out indoors. The colloidal nano-Fe3O4 or solid particle Fe3O4 prepared in steps (1) and (2) of Example 1 and the enriched culture solution of phenanthrene-degrading bacteria obtained in step (4) were added according to the Fe content (0, 10mmol / L, 20mmol / L and 25mmol / L) to study the degradation rate of polycyclic aromatic hydrocarbons in sediments under different concentrations of electron acceptors (calculated as Fe content). The results showed that after the microcosm culture experiment, the group treated with 20mmol / LFe3O4 (calculated as Fe content) had a significantly (p<0.05) higher degradation rate than other treatment conditions, and the degradation rate of polycyclic aromatic hydrocarbons reached 44% ( Figure 3 ).

[0043] Example 3

[0044] Anaerobic degradation experiments of the polycyclic aromatic hydrocarbon phenanthrene were carried out under different electron acceptor conditions using phenanthrene as the target compound. The Fe3O4 solid particles and Fe3O4 colloids prepared in steps (1) and (2) of Example 1 and the enriched culture of phenanthrene-degrading bacteria obtained in step (4) were used.

[0045] Sediment was treated with phenanthrene to an initial phenanthrene concentration of 500 μg / L. The degradation rates of phenanthrene during anaerobic incubation were compared under natural degradation conditions (containing 2.5 mmol / L sulfate and 0.01 mmol / L nitrate), a sulfate group (using a sulfate concentration of 22.5 mmol / L), a nitrate group (using a nitrate concentration of 22.5 mmol / L), and a sulfate and nitrate coexistence group (using a total sulfate and nitrate concentration of 22.5 mmol / L) (Table 1). After 72 hours of anaerobic incubation, it was found that under natural degradation conditions (control group), the degradation efficiency of phenanthrene with the addition of solid particulate Fe3O4 was significantly higher than that with colloidal nano-Fe3O4. However, under sulfate reduction, nitrate reduction, and sulfate and nitrate coexistence conditions, the degradation efficiency of phenanthrene with the addition of colloidal nano-Fe3O4 was significantly higher than that with solid particulate Fe3O4.

[0046] Table 1 Residual concentration and degradation rate of phenanthrene in the system with different forms of Fe3O4 added under different anaerobic culture conditions

[0047]

[0048] Example 4

[0049] Anaerobic degradation experiments of the polycyclic aromatic hydrocarbon phenanthrene were carried out under different electron acceptor conditions using phenanthrene as the target compound. The Fe3O4 solid particles and Fe3O4 colloids prepared in steps (1) and (2) of Example 1 and the enriched culture of phenanthrene-degrading bacteria obtained in step (4) were used.

[0050] Following the steps of the phenanthrene degradation experiment in Example 3, the degradation performance of Fe3O4 solid particles and Fe3O4 colloids on phenanthrene during anaerobic culture under different electron acceptor (sulfate and nitrate) concentrations was studied. The degradation kinetic curve of Fe3O4 solid particles was obtained: y = 86.623*e -0.036x , R 2 =0.93; Degradation kinetic curve of Fe3O4 colloid: y=38.435*e 0.0564x , R 2 =0.90, where y represents the degradation rate of phenanthrene and x represents the concentration of electron acceptor. By calculating the critical point of Fe3O4 solid particles and Fe3O4 colloid through the phenanthrene degradation kinetic curve, it can be obtained that the critical point of addition is about 20mmol / L ( Figure 4 ).

Claims

1. A method for in-situ remediation of contaminated sediments, characterized in that: The steps include: (1) Enrichment and cultivation of PAHs-degrading bacteria: fresh wet sediment was cultured in water under constant temperature oscillation, and then the concentrated culture solution was transferred to liquid culture medium, and phenanthrene solution was added under pH 7.2-7.4, and anaerobically cultured for 5-7 days; the obtained culture solution was enriched and the transfer and anaerobically cultured were repeated, and a phenanthrene solution with a higher concentration than the previous one was added after each transfer, and the transfer and anaerobically cultured were repeated three times to finally obtain a suspended bacterial solution; (2) Selection of electron acceptor: Investigate the total concentration of sulfate and nitrate in the sediment to be repaired. When the total concentration is above 20 mmol / L, select Fe3O4 colloid as the electron acceptor. When the total concentration is below 20 mmol / L, select Fe3O4 solid particles as the electron acceptor. The particle size of the Fe3O4 colloid is 10 to 20 nm, and the diameter corresponding to 50% of the cumulative distribution of the Fe3O4 solid particle size is 26.337 μm; The Fe3O4 colloid is prepared by the following method: Fe 2+ : Fe 3+ : OH - =1:1:6 under anaerobic conditions at 30-40°C, quickly mix the iron salt solution and the alkaline solution, stir for 1-1.5 hours, let it stand to obtain a precipitate, wash it and redisperse it in water to obtain the product; The Fe3O4 solid particles are prepared by the following method: Fe 3+ :Fe 2+ =1.75:1 iron salt, add alkali to make the solution pH 11, react at 80-85℃ for 1.5-2 h to obtain the product; (3) In-situ remediation of contaminated sediments: the electron acceptor selected in step (2) is added to the sediment to be remediated at a final Fe concentration of 10 to 20 mmol / L, and the bacterial suspension obtained in step (1) is inoculated to degrade and remediate the sediment under natural conditions; If the electron acceptor is Fe3O4 colloid, the degradation and repair time is 7 to 15 days; if the electron acceptor is Fe3O4 solid particles, the degradation and repair time is 20 to 30 days.

2. The method according to claim 1, wherein The constant temperature shaking culture conditions in step (1) are: 25°C, 120 rpm, and 12 hours.

3. The method according to claim 1, wherein The ratio of fresh wet sediment to water in step (1) was 10:200 g / mL.

4. The method according to claim 1, wherein The concentration of the phenanthrene solution initially added in step (1) is 2 mg / L.

5. The method according to claim 1, wherein In step (1), each 1 L of the liquid culture medium contains the following components: K2HPO4 0.2~0.3g, KH2PO4 0.3~0.4g, NH4Cl 1.0~1.5g, CaCl2•2H2O 0.05~0.10g, MgCl2•6H2O 0.05~0.12g, trace element solution 1 mL, the balance is water.

6. The method according to claim 5, characterized in that Each 1L of the trace element solution contains the following components: CoCl2•6H2O 0.19~0.2 g, MnCl2•4H2O 0.05~0.08 g, ZnCl2 0.05~0.08 g, NiCl2•6H2O 0.02~0.03 g, Na2MoO4•H2O 0.02~0.03 g, H3BO3 0.003~0.008 g, CuCl2•2H2O 0.001~0.003g, FeCl2•4H2O 1.2~1.5g, and the balance is water.

7. The method according to claim 1, characterized in that The transfer described in step (1) is to transfer the culture solution into a liquid culture medium with a volume of 20 times.

8. The method according to claim 1, characterized in that The iron salt is sulfate or chloride.

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