A method for enhancing the anaerobic microbial degradation of polycyclic aromatic hydrocarbons in soil after oxidation by persulfate using mixed surfactants

Abstract

The present application belongs to the field of polycyclic aromatic hydrocarbons (PAHs) contaminated soil remediation technology, and particularly relates to a method for enhancing PAHs anaerobic microbial degradation in soil after persulfate (PS) oxidation by using mixed surfactants. The present application adopts a combined remediation technology of PS chemical oxidation-anaerobic biodegradation-mixed surfactant enhancement, is suitable for PAHs contaminated soil under anaerobic conditions, and can enhance the desorption capacity of residual PAHs in the soil after PS oxidation, improve the bioavailability of the residual PAHs, and enhance the anaerobic degradation of indigenous microorganisms in the soil, so that the contaminated soil can meet the environmental standards.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of PAHs contaminated soil remediation, and particularly relates to a method for enhancing the anaerobic microbial degradation of polycyclic aromatic hydrocarbons in soil after persulfate oxidation by using mixed surfactants. BACKGROUND

[0002] PAHs (Polycyclic aromatic hydrocarbons, English: PAHs, Chinese: polycyclic aromatic hydrocarbons) are carbon hydrocarbons containing two or more benzene rings in the molecule, which are common persistent organic pollutants with carcinogenic, teratogenic and genotoxic properties in industrial sites. Chemical oxidation (such as persulfate oxidation) is a commonly used PAHs contaminated soil remediation technology, which has the advantages of short reaction period, high efficiency and the ability to degrade a variety of common organic pollutants. Persulfate (PS) is widely used in the field of organic contaminated soil remediation due to its long environmental existence time, wide applicable pH range, strong oxidation and other advantages. However, PAHs often undergo an aging process in soil for several years or even decades. PAHs in soil usually exist in the form of "fast", "slow" and "extremely slow" desorption components. Since the easily desorbed components are easy to oxidize, PAHs in the soil after oxidation usually exist in the form of "slow" and "extremely slow" desorption components. Therefore, the use of PS oxidation alone often has the "tail" phenomenon of PAHs oxidation, which cannot reduce the content of PAHs in soil to below the remediation standard. Therefore, the combined application of chemical oxidation and other technologies is increasingly valued.

[0003] Chemical oxidation combined with microorganisms is a combined remediation technology that can more efficiently remove PAHs in soil. Previous studies have shown that organic pollutants can be degraded by microorganisms under aerobic and anaerobic conditions. However, due to the lower degradation rate of PAHs under anaerobic conditions than under aerobic conditions, more research has been conducted on the combined degradation of PAHs by chemical oxidation and aerobic microorganisms than on the combined degradation by anaerobic microorganisms. However, PAHs not only pollute the surface soil during migration and transformation in the soil environment, but also gradually penetrate and accumulate in deep soil under anaerobic conditions. Therefore, the combined application of chemical oxidation and anaerobic biodegradation to deep PAHs contaminated soil remediation has important practical significance and application value.

[0004] While the combined chemical oxidation and anoxic biodegradation technology is used to further improve the degradation efficiency of PAHs, the low bioavailability of PAHs is a problem encountered during the remediation process due to the high lipophilicity-hydrophobicity of PAHs. Moreover, the bioavailability of PHC residues in the soil after PS oxidation is even lower due to the existence of "slow" and "very slow" desorption components, which limits further biodegradation. In the technical solution to solve the low bioavailability of pollutants, a very promising method is to use surfactants to "mobilize" pollutants. Surfactants are substances with surface activity, with hydrophilic-lipophilic properties, composed of hydrophilic polar groups (head) and hydrophobic non-polar groups (tail), and can be classified as synthetic surfactants or biosurfactants according to their source. According to the type of hydrophilic head group, synthetic surfactants are also classified as non-ionic, anionic, cationic and zwitterionic surfactants.

[0005] The two main mechanisms of surfactants removing PAHs pollutants in soil are mobilization at low concentrations and solubilization when the concentration exceeds the critical micelle concentration (CMC). The former improves the migration ability of pollutants in porous media, and the latter increases the apparent solubility of pollutants. At low concentrations, organic pollutants are surrounded by surfactant monomers, and the aggregation of monomers reduces the oil-water interfacial tension, so the pollutants tend to disperse in the aqueous phase to form an "oil-in-water" emulsion. Surfactant mobilization at low concentrations reduces the resistance of pollutant movement and desorption, ultimately promoting the detachment of pollutants from the soil. With the increase of surfactant concentration, when the concentration exceeds the CMC, surfactant monomers begin to aggregate to form self-assembled bodies called micelles. At this time, surfactants exist in the form of monomers and micelles in the solution. In surfactant micelles, the hydrophobic group is in the interior of the micelle to form a hydrophobic core, and the hydrophobic organic pollutants in the soil can enter the hydrophobic center and disperse into the liquid phase, thereby improving the efficiency of pollutant desorption from the soil. Therefore, the hydrophilic-lipophilic properties of surfactants can effectively increase the solubility of pollutants, thereby achieving the purpose of removing pollutants. While surfactants are solubilized, they are also adsorbed by soil matrix, which enhances the adsorption of pollutants on the fixed adsorbed surfactants.

[0006] Surfactants are classified as synthetic surfactants or biosurfactants according to their source. According to the type of hydrophilic head group, synthetic surfactants are also classified as nonionic, anionic, cationic and zwitterionic surfactants. The hydrophilic group of nonionic surfactants is usually polyoxyethylene, and the hydrophobic group is a straight-chain or branched-chain alkane of various structures. Common nonionic surfactants include Tween 80, Triton X-100, Brij 35, etc. The hydrophilic group of anionic surfactants can ionize in water to form cations and amphiphilic anions, i.e. hydrophobic tails and negatively charged hydrophilic heads. The cation is usually sodium ion, and the anion is usually sulfate, sulfonate, carboxylate and phosphate, etc. Common anionic surfactants include SDS, SDBS, AES, etc. The mixed use of anionic and nonionic surfactants has the following advantages: 1) Anionic surfactants can reduce the adsorption of nonionic surfactants on soil, and nonionic surfactants can effectively make up for the shortcomings of anionic surfactants, such as small solubilizing capacity and easy precipitation with calcium and magnesium ions; 2) Anionic and nonionic surfactants can form mixed micelles, reducing the electrostatic and steric repulsion between surfactants of the same type, making it easier to form micelles, thereby reducing the CMC value of mixed surfactants to a greater extent than single surfactants. Therefore, it is possible to further improve the bioavailability of organic pollutants by using mixed surfactants, promote the degradation of indigenous microorganisms, and improve the remediation efficiency of organic contaminated soil.

[0007] The advantage of the PS chemical oxidation-anoxic biodegradation-surfactant enhanced combined remediation technology is that the chemical oxidation can remove a large part of the pollutants with high biological availability, reducing the concentration of pollutants and their toxicity to microorganisms. In addition, the sulfate radical will gradually change into sulfate, which can further stimulate the growth and reproduction of microorganisms as an electron acceptor, which is beneficial to subsequent anoxic biodegradation. For the residual PAHs in the form of "slow" and "extremely slow" desorption components in the soil after PS oxidation, the addition of mixed surfactants in the oxidized soil can enhance the desorption capacity of PAHs in the soil and improve their bioavailability. This technology has important practical significance and application value for reducing the construction difficulty and remediation cost of in-situ microbial remediation of contaminated sites. SUMMARY

[0008] The technical problem solved by the present application is to solve the limitation of the PS chemical oxidation combined with anoxic biodegradation repair technology, and to solve the problem of residual PAHs in the form of "slow" and "extremely slow" desorption components in the soil after oxidation. By adding a mixed surfactant to the soil after oxidation, the desorption capacity of PAHs in the soil can be enhanced, and the biological effectiveness can be improved. A method for strengthening the anoxic microbial degradation of PAHs in the soil after persulfate oxidation by using a mixed surfactant is provided. The method of the present application is suitable for contaminated soil under anoxic conditions. On the basis of PS oxidation of PAHs contaminated soil, a mixed surfactant is added to the soil to strengthen the anoxic degradation of indigenous microorganisms in the soil, so that the contaminated soil meets the environmental standards.

[0009] To solve the above technical problems, the technical solution adopted by the present application is as follows:

[0010] A method for strengthening the anoxic microbial degradation of polycyclic aromatic hydrocarbons in the soil after persulfate oxidation by using a mixed surfactant, characterized in that it comprises the following steps:

[0011] 1) For the contaminated deep soil in anoxic environment, add persulfate solution to the deep soil to make the deep soil saturated, and activate the persulfate to oxidize the deep soil for a certain period of time;

[0012] 2) Add a mixed surfactant solution of anionic surfactant and nonionic surfactant to the oxidized soil to make the deep soil saturated, enhance the desorption capacity of residual PAHs in the soil after PS oxidation, and improve the biological effectiveness to strengthen the degradation of PAHs in the deep soil by indigenous microorganisms.

[0013] In the above technical solution, the mixed surfactant solution of anionic surfactant and nonionic surfactant can significantly strengthen the degradation of polycyclic aromatic hydrocarbons in the deep soil by indigenous microorganisms after oxidation.

[0014] Specifically, in step 1):

[0015] The contaminated soil is the soil of gas stations, oil storage depots or oil chemical enterprises;

[0016] The deep soil is soil with a depth of more than 2 meters, or soil under the hardened layer of the contaminated soil, and the oxygen content is less than 1%.

[0017] Specifically, in step 1), the persulfate solution is an aqueous solution of persulfate, and the addition amount of persulfate is not higher than 1% of the mass of the soil.

[0018] In the above technical solution, too high a dosage of persulfate salt will not only result in less obvious improvement of PAHs removal rate, but also a great decrease in the number of indigenous microorganisms.

[0019] Specifically, in step 1), the activator of persulfate salt is indigenous iron in the soil.

[0020] Specifically, in step 1), the oxidation time is until the persulfate salt is completely converted into sulfate.

[0021] Specifically, in step 2),

[0022] The anionic surfactant is sodium dodecyl benzene sulfonate (SDBS);

[0023] The non-ionic surfactant is sorbitan monooleate polyoxyethylene ether (Tween 80);

[0024] The dosage of the mixed surfactant is not higher than 0.08% of the mass of the soil.

[0025] In the above technical solution,

[0026] The PAHs-contaminated soil under anoxic conditions can be added with a surfactant after PS oxidation of the contaminated soil, because the residual PAHs in the soil after PS oxidation exist in the form of “slow” or “extremely slow” desorption components, which have lower bioavailability and limit further biodegradation. The common surfactant synergistic remediation technology uses a single anionic or non-ionic surfactant. The single anionic surfactant will precipitate, and the non-ionic surfactant is easily adsorbed by the soil. Therefore, the mixed surfactant of the anionic surfactant sodium dodecyl benzene sulfonate (SDBS) and the non-ionic surfactant sorbitan monooleate polyoxyethylene ether (Tween 80) can not only reduce the adsorption of the surfactant on the soil surface and improve the desorption efficiency, but also enhance the desorption capacity of the residual PAHs in the soil and improve the bioavailability thereof, so as to strengthen the anoxic degradation of PAHs by indigenous microorganisms in the soil and make the contaminated soil meet the environmental standards. The principle can be referred to Figure 5 .

[0027] Too high a dosage of the mixed surfactant will result in a significant decrease in the number of microorganisms, and too low a dosage will result in less obvious effect of solubilization of the pollutants.

[0028] Too high a concentration of the mixed surfactant will inhibit the anoxic biodegradation of PAHs of all ring numbers, and too low a concentration will result in insufficient improvement of the anoxic biodegradation of PAHs of all ring numbers.

[0029] Specifically, the mass ratio of the anionic surfactant sodium dodecyl benzene sulfonate to the nonionic surfactant sorbitan monooleate polyoxyethylene ether is (1:3) to (1:1).

[0030] In the technical solution, the PAHs anaerobic biodegradation is insufficient when the ratio is too high or too low.

[0031] Preferably, the mass ratio of the anionic surfactant sodium dodecyl benzene sulfonate to the nonionic surfactant sorbitan monooleate polyoxyethylene ether is 1:3. The ratio is the optimal ratio. BRIEF DESCRIPTION OF DRAWINGS

[0032] Figure 1 is the residual content of PAHs in the soil after oxidation of the persulfate solution. US is the original soil, SS is the sterilized soil, 1% is the experimental group with 1% (wt / wt) PS added, and CK is the control group with only deionized water added;

[0033] Figure 2 is the solubilization effect of different concentrations and ratios of surfactants on PAHs in the soil;

[0034] Figure 3 is the quantitative analysis result of the total amount of bacteria in the soil treated with different contents of surfactants. Note: US represents the original contaminated soil, CK, CS, CY, and YS represent the treatment groups with only deionized water, electron acceptor, nutrient substance, and electron acceptor and nutrient substance added, respectively;

[0035] Figure 4 is the retention rate of PAHs with different ring numbers in the soil after adding mixed surfactants;

[0036] Figure 5 is the schematic diagram of the present application. DETAILED DESCRIPTION

[0037] The principles and features of the present application are described below, and the examples are used to explain the present application and not to limit the scope of the present application.

[0038] The present application is suitable for repairing the soil in anoxic environment, a material adding well vertically downward is established in the contaminated site to be repaired, the depth is to the PAHs contaminated soil region in anoxic environment, the PS solution is added to the contaminated soil through the material adding well until the solution reaches saturation state, and the PAHs pollutants in the soil are oxidized. The PAHs and persulfate in the soil are monitored by regular sampling, and after the persulfate ion in the soil is activated by indigenous iron to complete the oxidation of PAHs and is completely converted into sulfate ion, the oxidation stage of PS is completed. After the oxidation of PS is completed, the mixed surfactant solution is added to the contaminated soil through the material adding well until the solution reaches saturation state, and the effect of the mixed surfactant is used to strengthen the degradation effect of indigenous microorganisms on PAHs in the contaminated soil until the concentration of PAHs in the contaminated soil reaches the environmental standard through detection.

[0039] Further, the present application is not only suitable for repairing deep soil in anoxic environment, but also suitable for soil under the cement hardening layer of gas station, oil depot, petrochemical enterprise and other plots covered by artificial hardening layer.

[0040] The oxidant used in the present application is persulfate activated by indigenous iron, and the activated persulfate has good stability, half-life far exceeding other medicaments, wide pH adaptation range and other advantages. The addition amount of persulfate is not more than 1% of the weight of PAHs contaminated soil, and the oxidant is added in the form of solution to saturation state. The sign of completion of persulfate oxidation in the present application is that the persulfate ion in the area to be repaired is completely converted into sulfate ion. The practical effect of the above technical means has been proved by the prior art (Removal of polycyclic aromatic hydrocarbons (PAHs) and the response of indigenous bacteria in highly contaminated aged soil after persulfate oxidation, Yaling Gou, Qianyun Zhao, Sucai Yang, Hongqi Wang, Pengwei Qiao, Yun Song, Yanjun Cheng, Peizhong Li).

[0041] The mixed surfactant of the present application is: the anionic surfactant is sodium dodecyl benzene sulfonate (SDBS), and the non-ionic surfactant is sorbitan monooleate polyoxyethylene ether (Tween 80); the application concentration of the mixed surfactant is 0-3000 mg·L -1 , for example Figure 2The mass ratio of the two surfactants is SDBS:Tween 80 = 1:3 ~ 1:1, preferably 1:3. The effect of mixed surfactants on the enhancement of the anaerobic degradation of PAHs in soil by indigenous microorganisms is shown in the following table Figure 3 . .

[0042] 1. Test of residual PAHs content in soil after persulfate oxidation

[0043] The test results are shown in the following table Figure 1 . Figure 1 is the effect of PS oxidation on the anaerobic degradation of PAHs with different ring numbers and ΣPAHs (excluding naphthalene and acenaphthylene) in soil, wherein US is the data of the original contaminated soil after 30 days of anaerobic culture, SS is the data of the original contaminated soil after 30 days of anaerobic culture after sterilization, CK in the original soil and the sterilized group is the data of the control group after 30 days of anaerobic culture by adding only deionized water, and 1% is the data of the experimental group after 30 days of anaerobic culture by adding 1% mass concentration of PS solution. As can be seen from the figure, after 30 days of anaerobic culture, the degradation rate of ΣPAHs in the original soil control group is 7.63% (CK / US), and the degradation rate of ΣPAHs in the sterilized soil control group is 0.76% (CK / SS), which proves that the use of anaerobic microorganisms alone plays a certain degree of degradation effect; the degradation rate of ΣPAHs in the sterilized soil experimental group is 22.22% (1% / SS), which proves that the degradation effect of chemical oxidation alone is better than that of anaerobic microorganisms alone; the degradation rate of ΣPAHs in the original soil experimental group is 25.64% (1% / US), which is higher than that of the sterilized soil experimental group, which proves that the effect of chemical oxidation combined with anaerobic microbial degradation is better.

[0044] 2. Solubilization experiment of PAHs in soil by different concentrations and proportions of surfactants

[0045] The solubilization experiment of PAHs in soil by different concentrations and proportions of surfactants was carried out using the soil sample after 30 days of anaerobic oxidation by PS. Under the condition of water-soil ratio 1:1, the solubilization effect of PAHs in soil by different concentrations and proportions of surfactants is shown in the following table Figure 2 .

[0046] The control group has very little ΣPAHs solubilized, and the content of ΣPAHs in the supernatant is 1.59 mg·L -1 . When low concentration (800 mg·L -1 ) of SDBS:Tween 80 = 1:3, 1:2, 1:1, 2:1, 3:1, 1:0, 0:1 (w / w) surfactants are used for solubilization, the concentration of ΣPAHs in the supernatant is 11.12 mg·L -1 , 9.19 mg·L -1 , 7.10 mg·L -1 , 5.40 mg·L -1 , 4.56 mg·L-1 3.47 mg·L -1 and 3.86 mg·L -1 When adding a high concentration (3000 mg·L⁻¹) -1 When SDBS:Tween80 was used for solubilization at ratios of 1:3, 1:2, 1:1, 2:1, 3:1, 1:0, and 0:1 (w / w), the concentration of ΣPAHs in the supernatant was 12.99 mg·L⁻¹. -1 10.82 mg·L -1 10.14 mg·L -1 9.32 mg·L -1 9.13 mg·L -1 7.53 mg·L -1 and 8.87 mg·L -1 Therefore, it can be seen that, under the same mixing ratio, the solubilizing effect of high-concentration surfactants is better than that of low-concentration surfactants; for surfactants of the same concentration, the solubilizing effect increases with the increase of the proportion of nonionic surfactant Tween80 in the mixed system, but the solubilizing effect of adding a single surfactant is lower than that of the mixed system.

[0047] 3. Experiment on the effect of surfactants on soil microbial abundance under anaerobic conditions

[0048] After 150 days of anaerobic incubation, the total bacterial count in soil samples from different surfactant treatment groups was as follows: Figure 3 As shown in the figure. On day 0, the Log copy number of bacteria in the soil was 7.14. After 150 days, the total bacterial count in the soil of each control group (CK, CY, CS, YS) increased by orders of magnitude of 0.40, 0.61, 0.57, and 0.75, respectively. Compared with the YS control group, the total bacterial count in the soil treated with low-dose (0.08%) surfactant (Tween80, SDBS) increased by orders of magnitude of 0.41 and 0.01, respectively; the total bacterial count in the soil treated with 0.30% surfactant decreased by orders of magnitude of 0.05 and 1.03, respectively; and the total bacterial count in the soil treated with 0.80% surfactant decreased by orders of magnitude of 0.49 and 1.09, respectively. Therefore, the addition of an appropriate amount of surfactant helps to increase the number of microorganisms, while excessive surfactant addition will lead to a significant decrease in the number of microorganisms. The amount of mixed surfactant added should not exceed 0.08% of the soil mass.

[0049] 4. Experiment on the anaerobic biodegradation of PAHs enhanced by surfactants after PS oxidation: Retention rate (%) of PAHs with different ring numbers in soil.

[0050] The test results are as follows Figure 4The residual rates of three-, four-, and five- to six-ring PAHs in CK (control group with only deionized water added) were 75.32%, 76.91%, and 95.60%, respectively, after PS oxidation and continued incubation with the addition of a surfactant. When a low concentration (800 mg·L -1 When the mass ratio of SDBS to Tween 80 was 1:3, 1:2, 1:1, 2:1, 3:1, 1:0, and 0:1, the residual rates of three-ring PAHs in the experimental group were 52.53%, 63.22%, 61.55%, 67.71%, 69.84%, 69.84%, and 61.76%, respectively; the residual rates of four-ring PAHs were 56.21%, 62.05%, 64.86%, 66.70%, 74.00%, 69.37%, and 63.80%, respectively; and the residual rates of five- to six-ring PAHs were 89.35%, 90.13%, 92.24%, 91.73%, 93.56%, 94.83%, and 92.21%, respectively. It can be seen that the degradation effect of low-concentration mixed surfactants on five- to six-ring PAH monomers was the lowest, while the degradation effect on three- to four-ring monomers was better, and the degradation effect of SDBS to Tween 80 at a mass ratio of 1:3 was higher than that of other mixing ratios.

[0051] When the concentration of the added surfactant was 3000 mg·L -1 When the mass ratio of SDBS to Tween 80 was 1:3, 1:2, 1:1, 2:1, 3:1, 1:0, and 0:1, the residual rates of three-ring PAHs in the experimental group were 81.49%, 85.95%, 82.43%, 88.16%, 87.82%, 84.80%, and 77.33%, respectively; the residual rates of four-ring PAHs were 84.53%, 86.88%, 80.06%, 85.49%, 90.63%, 86.25%, and 83.27%, respectively; and the residual rates of five- to six-ring PAHs were 95.63%, 97.12%, 97.73%, 97.28%, 98.03%, 98.20%, and 96.80%, respectively. It can be seen that the addition of high-concentration surfactants inhibited the anaerobic biodegradation of PAHs of all ring numbers, and the inhibition of three-ring PAHs was the most obvious.

[0052] In summary, the degradation effect of low-concentration mixed surfactants on PAHs was higher than that of high-concentration mixed surfactants, and the degradation effect of SDBS to Tween 80 at a mass ratio of 1:3, 1:2, and 1:1 was good, but the best degradation effect was achieved when the mass ratio of SDBS to Tween 80 was 1:3.

[0053] The above merely describes preferred embodiments of the present application, and is not used to limit the present application, and any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A method for enhancing the anaerobic microbial degradation of PAHs in soil after persulfate oxidation by using mixed surfactants, characterized in that, The method comprises the following steps: 1) for PAHs contaminated deep soil in anoxic environment, adding persulfate solution to the deep soil to saturate the deep soil, and using activated persulfate to oxidize the deep soil for a certain time; 2) adding a mixed surfactant solution of anionic surfactant and nonionic surfactant to the oxidized soil to saturate the deep soil, enhancing the desorption capacity of residual PAHs in the PS-oxidized soil, and strengthening the degradation of PAHs in the deep soil by indigenous microorganisms; The contaminated soil is the soil of a gas station, an oil storage depot or an oil chemical enterprise, the deep soil is soil with a depth of more than 2 meters or soil under a cement hardening layer of an industrial site, and the oxygen content is less than 1% (V / V); In step 1), the persulfate solution is an aqueous solution of sodium persulfate, and the addition amount of sodium persulfate is not higher than 1% of the mass of the soil; In step 2), The anionic surfactant is sodium dodecyl benzene sulfonate; The nonionic surfactant is sorbitan monooleate polyoxyethylene ether; The addition amount of the mixed surfactant is not higher than 0.08% of the mass of the soil; The mass ratio of the anionic surfactant sodium dodecyl benzene sulfonate to the nonionic surfactant sorbitan monooleate polyoxyethylene ether is (1:3) to (1:1).

2. The method for enhancing PAHs anaerobic microbial degradation in soil after persulfate oxidation by using mixed surfactants according to claim 1, characterized in that, In step 1), the activator of the persulfate is indigenous iron in the soil.

3. The method for enhancing PAHs anaerobic microbial degradation in soil after persulfate oxidation by using mixed surfactants according to claim 1, characterized in that, In step 1), the oxidation time is until the persulfate is completely converted into sulfate.

4. The method for enhancing PAHs anaerobic microbial degradation in soil after persulfate oxidation by using mixed surfactants according to claim 1, characterized in that: The mass ratio of the anionic surfactant sodium dodecyl benzene sulfonate to the nonionic surfactant sorbitan monooleate polyoxyethylene ether is 1:3.

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