A low-carbon, ecological, and efficient rapid in-situ restoration method for chromium-containing soil

Abstract

The present invention provides an in-situ low-carbon, ecological, and efficient rapid remediation technology for chromium-containing soil. This technology improves upon the traditional method of treating chromium-containing soil in situ using biogas residue in conjunction with ferrous sulfate. By introducing an alternating stirring-and-standing curing method, unexpected results were achieved, which not only effectively and efficiently repaired the chromium-containing soil in a green, low-carbon manner, but also greatly accelerated the reaction progress. During the process, a biochemical agent is first injected into an injection well for infiltration, followed by short-term construction with a coupling mixing pile, forming an alternating stirring-and-standing curing method, thereby achieving in-situ efficient treatment. The in-situ technology employed in this process promotes the diverse synergy of anaerobic, facultative, and aerobic microorganisms, avoids the toxic effects of Cr(VI) on microorganisms, and significantly reduces energy consumption.

Description

Technical Field

[0001] The present invention belongs to the field of soil ecological restoration, and specifically relates to a green and low-carbon ecological restoration method for chromium-containing soil. Background Art

[0002] In-situ soil remediation is cost-effective because it doesn't involve excavation. However, achieving compliance is challenging due to the lack of effective contact between the reagents and the contaminated soil, and successful cases are rare. Application No. 201610370419.5 proposes a method for the in-situ treatment of chromium-containing soils using biogas residues and ferrous sulfate. This technology involves pressurizing waste biogas residues, a carbon source, and ferrous sulfate into the soil at the site through an injection well. While this method has some ecological benefits and is effective for treating Cr(VI) in groundwater, achieving compliance with soil Cr(VI) standards remains challenging, and the remediation cycle is extremely long, requiring months or even a year. This is because pressurized injection makes it difficult to fully mix the reagents with the chromium-containing soil, and the dense soil particles easily trap the microbial flora of the biogas residues. Meeting compliance often requires continuous overdose of reagents, and excessive addition of reagents like biogas residues and carbon sources can lead to organic contamination. Therefore, current technologies cannot balance Cr(VI) treatment with organic secondary contamination. Furthermore, the presence of poorly soluble Cr(VI) in soil makes in-situ removal even more difficult.

[0003] In order to improve the mass transfer of reagents, the document "Study on the Synergistic Effect of Ferrous Sulfate and Biogas Residue Co-treatment of Chromium-Containing Soils" attempted to promote biochemical reactions through stirring, but the results showed that the effect was worse. The reason is that improving mass transfer aggravates the toxic effect of Cr(VI) on biogas residue-reducing microorganisms. At the same time, stirring easily brings in air, making it impossible to maintain an anaerobic environment suitable for Cr(VI) reduction. In addition, the contaminated site is large, Cr(VI) is soluble in water, and the contamination depth can reach tens of meters, making stirring extremely difficult in engineering. Therefore, technologies related to the field of Cr(VI) microbial reduction usually do not consider stirring, and all are static anaerobic cultivation. Summary of the Invention

[0004] In response to the shortcomings of the existing technology, the present invention provides a green, low-carbon, efficient and rapid in-situ remediation method for chromium-containing soil. It improves on the traditional method of in-situ treatment of chromium-containing soil with biogas residue and ferrous sulfate. By introducing an alternating stirring-static curing method, it produces unexpected results. It not only repairs the chromium-containing soil in a green, low-carbon and efficient manner, but also greatly accelerates the reaction progress.

[0005] The present invention provides a method for rapid, low-carbon, ecological, and efficient in-situ restoration of chromium-containing soil, comprising the following steps:

[0006] (1) Ferrous sulfate, biogas residue, and biological nutrient agent are mixed with water in a certain proportion to prepare a liquid biochemical agent.

[0007] (2) Several injection wells are laid out and drilled at the contaminated site. Liquid biochemical agents are continuously injected into the injection wells until they are full or above a certain water level. The wells then enter a stable maintenance period, during which the liquid level is maintained through intermittent injection to allow the biochemical agents to penetrate into the soil.

[0008] (3) After a certain period of stable curing, use a mixing pile to stir the site for a short period of time. During the stirring process, an appropriate amount of biochemical agent can be added, and then curing can be carried out. The mixing pile can be set near the midpoint of the line connecting two adjacent injection wells so that the injection well agent can better penetrate into the area during the stirring and extrusion process of the mixing pile.

[0009] (4) After a certain period of maintenance, continue to carry out short-term mixing of the site. During the mixing process of the mixing piles, high-pressure water can be sprayed according to the situation.

[0010] (5) Repeat step (4) a certain number of times to form an alternating pattern of static curing and stirring. Then, several monitoring wells are laid out and drilled in the site remediation area to collect soil and groundwater for testing.

[0011] (6) For monitoring wells where the soil and groundwater do not meet the standards, continue to inject the agent. The amount of the injected agent is added according to the organic content and sulfate content in the groundwater. Then repeat step (4) continuously. In the later stage of the biochemical reaction, high-pressure air can be injected to promote aerobic or aerobic reactions of microorganisms.

[0012] (7) Continue to deploy monitoring wells and repeat step (6) for monitoring wells that do not meet the standards until all monitoring wells meet the standards.

[0013] (8) The site is then maintained for a long time. During the process, earthworms can be spread according to the restoration goals to consume the sludge and loosen the soil. The earthworms continue to promote the reaction between the agent and the soil Cr(VI) during the underground creeping and loosening process, and at the same time promote the reaction between the unreacted secondary reducing agent sulfite and the soil Cr(VI).

[0014] The liquid biochemical agent in step (1) is formulated as follows: ferrous sulfate: biogas residue (dry basis): biological nutrient (dry basis) is (1-5): (1-5): (1-5), and is dissolved in water. The Fe(II) content in the liquid biochemical agent does not exceed 10%, and the organic content does not exceed 5% to avoid clogging the soil.

[0015] The main component of the biological nutrient in step (1) is a carbon source, and it also contains a nitrogen source, a phosphorus source and trace elements. The carbon source is generally glucose, acetate, and can also be organic solid waste such as molasses and kitchen waste.

[0016] The biogas residue in step (1) can be replaced by sludge, or by a bacterial agent prepared mainly from biogas residue, or by chromium-reducing microorganisms such as sulfate-reducing bacteria and iron-reducing bacteria.

[0017] The method of injecting the agent in steps (2) and (6) can be pressurized injection.

[0018] In the step (3), the water level is maintained until the stirring time using the stirring pile is controlled to be 8-120 hours, and the short-term stirring time of the stirring pile is controlled within 10 minutes.

[0019] The interval between curing and stirring with a mixing pile in step (4) is controlled to be 8-240 hours. During the process, water sedimentation needs to be considered to form a dry-wet alternating environment, which is conducive to microbial diversity and the dissolution of Cr(VI) deep in the soil through soil capillary action.

[0020] The short-term mixing time of the mixing pile is controlled within 10 minutes.

[0021] The amount of the reagent injected in step (6) is added according to the organic content and sulfate content in the groundwater. If both the organic content and the sulfate content are too high, no reagent is added and only short-term stirring is performed; if the organic content is high but the sulfate content is low, only ferrous sulfate is added; if the organic content is low but the sulfate content is high, only an organic carbon source is added; if both the organic content and the sulfate content are low, ferrous sulfate and an organic carbon source are added at the same time.

[0022] The innovation of this technology lies in its combination of osmotic mixing, agitation mixing, and static curing. Experiments have demonstrated that an optimized alternation of static and mixing can significantly outperform static alone, and even more so than continuous agitation. The results are surprising, defying the conventional wisdom that agitation is detrimental to the bioreduction of Cr(VI). The alternating cycle of short-term agitation and static curing has surprisingly achieved superior results.

[0023] The above reasons are analyzed to be due to the fact that the microbial reduction of chromium-containing soil is a heterogeneous reaction. The Cr(VI) content in some areas is low. After the microorganisms in the sludge quickly reduce Cr(VI), they continue to reduce sulfate to sulfite or sulfide. Due to the limited mass transfer in the solid-phase static reaction, it is difficult for sulfite or sulfide to contact and react with Cr(VI) in other areas in time, and gradually accumulate. When sulfite or sulfide accumulates to a certain level, it will inhibit the reducing microorganisms in the sludge. If stirring is performed at this time, sulfite or sulfide will contact and react with Cr(VI). In this case, Cr(VI) drops rapidly. At the same time, after sulfite or sulfide is eliminated in time, the inhibitory effect on the reducing microorganisms is eliminated, the activity is restored, and Cr(VI) and sulfate are continued to be reduced. The coordinated alternating operation of static and stirring ultimately achieves the rapid reduction of Cr(VI) in the soil.

[0024] The biochemical reactions during the process are as follows:

[0025] C2H4O (organic carbon source) +SO4 2-→SO3 2- +CO2+H2O static curing

[0026] C2H4O (organic carbon source) + SO3 2- →S 2- +CO2+H2O static curing

[0027] CrO4 2- +SO3 2- +H + →Cr 3+ +SO4 2- +H2O after short-term stirring

[0028] CrO4 2- +S 2- +H + →Cr 3+ +S+H2O after short-term stirring

[0029] This in-situ agent injection method utilizes soil infiltration injection and mixing with a mixing pile to effectively mix the soil. In-situ injection uses low energy consumption, but the infiltration mass transfer and mixing effect is poor. Initially, the biochemical reaction using injection creates a heterogeneous reaction environment that helps eliminate the toxic effects of Cr(VI) in localized areas. It also facilitates the growth of reducing microorganisms, while consuming carbon sources to reduce sulfate to sulfite and sulfide. Subsequent in-situ mixing with a mixing pile further promotes mass transfer, consumes sulfite and sulfide, and prevents the toxic effects of Cr(VI) on microorganisms. The large pores created by the injection well facilitate mixing with the mixing pile, reducing energy consumption and facilitating shear mixing of the injection well agent with the surrounding soil. After mixing with the mixing pile, the soil becomes loose, making it easier to construct monitoring wells and inject the agent. Furthermore, high-pressure water jets from the mixing piles promote the dissolution of Cr(VI) under the high hydraulic shear force. This also promotes alternating dry and wet conditions vertically across the site, creating conditions for microbial growth and the precipitation of deep-seated Cr(VI) through soil capillaries.

[0030] Furthermore, the present invention considers that although the soil and groundwater in the monitoring wells meet standards, soil is, after all, a heterogeneous system. It cannot be guaranteed that all areas adjacent to the monitoring wells meet standards, and the monitoring results may not fully reflect the soil's overall picture. Therefore, the final method is to introduce earthworms. This not only consumes biogas residue, preventing secondary contamination, but also promotes the dissolution of Cr(VI) and its reaction with reducing substances generated by microbial biochemical reactions through soil peristalsis, ensuring that the soil as a whole meets standards.

[0031] Furthermore, traditional Cr(VI) microbial reduction technologies generally require strict, continuous anaerobic conditions. However, this technology innovatively introduces a facultative or aerobic environment at a later stage. This is because the bacterial species on the site gradually evolve into chromium-reducing bacteria, which are facultative or aerobic. Furthermore, the alternating operation of facultative or aerobic and anaerobic conditions is more conducive to the degradation of organic pollutants and further promotes Cr(VI) reduction. A facultative or aerobic environment helps alleviate the inhibitory effects of sulfide on chromium-reducing microorganisms.

[0032] This technology, through a series of engineering combinations, enhances the feasibility of biological-chemical co-treatment of chromium-containing soils. Compared with previous technologies, it has the following advantages:

[0033] Highly efficient reduction of Cr(VI) accelerates the reaction rate and reduces the engineering cycle;

[0034] Compared with continuous stirring, alternating short-term stirring and static curing not only greatly reduces energy consumption, but also unexpectedly promotes biochemical effects;

[0035] The soil infiltration injection and mixing pile technology are organically combined and constructed at different locations. While reducing engineering energy consumption, it can better cultivate chromium-reducing microorganisms and effectively promote mass transfer.

[0036] Alternating soil drying and wetting is conducive to the dissolution of insoluble Cr(VI) by soil capillary action;

[0037] Make full use of biochemically produced sulfites and sulfides as reducing agents, reducing the use of chemical ferrous sulfate, reducing resource consumption, and being green and low-carbon;

[0038] Alternating dry and wet conditions and high-pressure air injection form a diverse synergy of anaerobic, facultative and aerobic microorganisms, which is beneficial to the treatment of organic pollution and avoids secondary pollution;

[0039] Adding earthworms to the site can eliminate the secondary organic pollution of the sludge and further promote remediation. DETAILED DESCRIPTION

[0040] The present application will be further described below in conjunction with specific implementation methods.

[0041] At a chromium-contaminated site, the first underground aquiclude is located approximately 15 meters below ground level, and the groundwater is approximately 8 meters below ground level. The average Cr(VI) content in the contaminated area is 1000 mg / kg, and the Cr(VI) concentration in the groundwater is 120 mg / L. The remediation plan is as follows:

[0042] Ferrous sulfate, biogas residue and biological nutrient are mixed with water to form a liquid biochemical agent, with the ratio of ferrous sulfate: biogas residue (dry basis): biological nutrient (dry basis) being 5:5:1. Water is added to dissolve the mixture. The Fe(II) content in the liquid biochemical agent does not exceed 10%, and the organic content does not exceed 5%. The biological nutrient is molasses.

[0043] (2) A water-stop curtain is constructed around the contaminated area of ​​the contaminated site, and then several injection wells are laid out and drilled in the contaminated site. The drilling depth is 9 meters, reaching the groundwater level. The distance between adjacent injection wells is no more than 3 meters, and the borehole diameter is not less than 20 cm. Then, liquid biochemical agents are continuously injected into the injection wells until the water level is around 1 meter underground. Then, a stable maintenance period is entered, and the liquid surface water level is maintained at 1 meter underground through intermittent injection.

[0044] (3) After 24 hours of stable maintenance, use a mixing pile. The mixing pile is located near the midpoint of the line connecting two adjacent injection wells. Drill the mixing pile into the soil until it is about 2 meters above the first aquiclude. Then stir it for 5 minutes. During the process, biochemical agents are sprayed through the mixing pile. Finally, high-pressure water is used for spraying. The spraying device is cleaned and then maintenance is carried out.

[0045] (4) After 48 hours of curing, continue to carry out short-term mixing of the site, with the mixing time not exceeding 5 minutes, and spray high-pressure water during the mixing process of the mixing piles.

[0046] (5) Repeat step (4) once, then lay out and drill several monitoring wells in the site remediation area, with the distance between adjacent monitoring wells not exceeding 3 meters, and take soil and groundwater samples for testing.

[0047] (6) For monitoring wells where soil and groundwater do not meet the standards, continue to inject chemicals. If the organic content is >1000mg / L and the sulfate content is >1000mg / L, no chemicals are added and only short-term stirring is performed. If the organic content is >1000mg / L and the sulfate content is <1000mg / L, only ferrous sulfate is added. If the organic content is <1000mg / L and the sulfate content is >1000mg / L, only an organic carbon source is added. If the organic content is <1000mg / L and the sulfate content is <1000mg / L, ferrous sulfate and an organic carbon source are added simultaneously. Then repeat step (4) twice. After the second repetition, high-pressure air is sprayed into the mixing pile to promote facultative aerobic or aerobic reactions of microorganisms.

[0048] (7) Continue to deploy monitoring wells and repeat step (6) for monitoring wells that do not meet the standards until all monitoring wells meet the standards.

[0049] (8) The site is then maintained for a long time and earthworms are released.

[0050] After 60 days of long-term curing, the soil Cr(VI) content was less than 3 mg / kg, meeting the residential land standard in the "Soil Environmental Quality Construction Land Soil Pollution Risk Control Standard (Trial)", and the groundwater concentration was less than 0.1 mg / L. Thereafter, the site soil and groundwater were tested every 90 days, and no rebound was found. Compared to the patented technology of 201610370419.5, this technology not only effectively treats Cr(VI) in groundwater, but also effectively treats Cr(VI) in soil. Example 2:

[0051] At a chromium-contaminated site, the first underground aquiclude is located approximately 6 meters below ground level, with no groundwater. The average Cr(VI) content in the contaminated area is 500 mg / kg. The remediation plan is as follows:

[0052] (1) Ferrous sulfate, biogas residue and biological nutrient are mixed with water to prepare a liquid biochemical agent. The ratio of ferrous sulfate: biogas residue (dry basis): biological nutrient (dry basis) is (1-2): (3-4): 1. Add water to dissolve. The Fe(II) content in the liquid biochemical agent shall not exceed 10%, and the organic content shall not exceed 5%. The biological nutrient is glucose.

[0053] (2) A water-stop curtain is constructed around the contaminated area of ​​the contaminated site, and then several injection wells are laid out and drilled in the contaminated site. The drilling depth is 4 meters, and the distance between adjacent injection wells is no more than 2 meters. The borehole diameter is not less than 20 cm. Then, liquid biochemical agents are pressurized and injected into the injection wells until the water level is around 0.5 meters underground. Then, a stable maintenance period is entered, and the liquid water level is maintained at 0.5 meters underground through intermittent injection.

[0054] (3) After 24 hours of stable maintenance, use a mixing pile. The mixing pile is located near the midpoint of the line connecting two adjacent injection wells. Drill the mixing pile into the soil until it reaches the first aquiclude. Then stir the soil for no more than 5 minutes. During the process, biochemical agents are sprayed through the mixing pile. Finally, high-pressure water is used for spraying. The spraying device is cleaned and then maintenance is carried out.

[0055] (4) After 48 hours of curing, continue to stir the site for a short period of time, with the stirring time not exceeding 5 minutes.

[0056] (5) Repeat step (4) once, then lay out and drill several monitoring wells in the site remediation area, with the distance between adjacent monitoring wells not exceeding 3 meters, and take soil and groundwater samples for testing.

[0057] (6) For monitoring wells where soil and groundwater do not meet the standards, continue to inject chemicals. If the organic content is >1000 mg / L and the sulfate content is >1000 mg / L, do not add chemicals and only stir for a short period of time. If the organic content is >1000 mg / L and the sulfate content is <1000 mg / L, only add ferrous sulfate. If the organic content is <1000 mg / L and the sulfate content is >1000 mg / L, only add an organic carbon source. If the organic content is <1000 mg / L and the sulfate content is <1000 mg / L, add ferrous sulfate and an organic carbon source at the same time. Then repeat step (4) twice.

[0058] (7) Continue to deploy monitoring wells and repeat step (6) for monitoring wells that do not meet the standards until all monitoring wells meet the standards.

[0059] (8) The site is then maintained for a long time and earthworms are released.

[0060] After 60 days of long-term curing, the soil Cr(VI) content was less than 3 mg / kg, meeting the residential land standard in the "Soil Environmental Quality Standard for Soil Pollution Risk Control for Construction Land (Trial)." The groundwater concentration was less than 0.1 mg / L. Site soil and groundwater testing has since been conducted every 90 days, with no rebound observed.

[0061] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, it is still possible for a person skilled in the art to modify the technical solutions described in the aforementioned embodiments, or to replace some of the technical features therein with equivalents. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions claimed to be protected by the present invention.

Claims

1. A low-carbon, ecological, and efficient rapid in-situ restoration method for chromium-containing soil, characterized in that: The following steps are involved: (1) Ferrous sulfate, biogas residue, and biological nutrient agent are mixed with water in a certain proportion to prepare a liquid biochemical agent. (2) Drill and lay out several injection wells at the contaminated site, and continuously inject liquid biochemical agents into the injection wells to allow the biochemical agents to penetrate into the soil, and then enter a stable maintenance period. (3) After a period of stabilization, use a mixing pile to stir the site for a short period of time. During the stirring, an appropriate amount of biochemical agents can be added, and then maintenance is carried out. (4) After a certain period of maintenance, continue to mix the site for a short period of time. During the mixing process of the mixing pile, high-pressure water can be sprayed according to the situation. (5) Repeat step (4) a certain number of times to form a static curing and stirring alternation, then lay out and drill several monitoring wells in the site remediation area, and take soil and groundwater for testing. (6) For monitoring wells where soil and groundwater do not meet the standards, continue to inject the agent. The amount of the agent injected is selectively added according to the groundwater test results. Then repeat step (4) continuously. In the late stage of the biochemical reaction, high-pressure air can be injected to promote microbial aerobic or aerobic reactions. (7) Continue to deploy monitoring wells, and repeat step (6) for monitoring wells that do not meet the standards until all monitoring wells meet the standards. (8) The site is then maintained for a long time. During the process, earthworms can be spread according to the restoration goals to consume the sludge and loosen the soil. The earthworms continue to promote the reaction between the agent and the soil Cr(VI) during the underground creeping and loosening process.

2. The in-situ low-carbon, ecological, and efficient rapid remediation method for chromium-containing soil according to claim 1, characterized in that: Ferrous sulfate: dry biogas residue: dry biological nutrient agent is (1-5): (1-5): (0-5), dissolved in water, the Fe(II) content in the liquid biochemical agent shall not exceed 10%, and the organic content shall not exceed 5%.

3. The in-situ low-carbon, ecological, and efficient rapid remediation method for chromium-containing soil according to claim 1, characterized in that: The main component of the biological nutrient is a carbon source, and it also contains a nitrogen source, a phosphorus source and trace elements. The carbon source is glucose, acetate, or organic solid waste.

4. The in-situ low-carbon, ecological, and efficient rapid remediation method for chromium-containing soil according to claim 1, characterized in that: In the step (1), the biogas residue can be replaced by sludge, or by a bacterial agent prepared mainly from the biogas residue, or by chromium-reducing microorganisms.

5. The in-situ low-carbon, ecological, efficient and rapid remediation method for chromium-containing soil according to claim 1, characterized in that: In the step (3), the stable curing time is controlled within 8-120 hours, the short-term stirring time of the mixing pile is controlled within 10 minutes, and the agent can be intermittently injected during the stable curing period to improve the soil penetration effect of the agent.

6. The in-situ low-carbon, ecological, and efficient rapid remediation method for chromium-containing soil according to claim 5, characterized in that: In the step (4), the interval time from curing to stirring with a mixing pile is controlled to be 8-240 hours, and the short-term stirring time of the mixing pile is controlled to be within 10 minutes.

7. The in-situ low-carbon, ecological, and efficient rapid remediation method for chromium-containing soil according to claim 5, characterized in that: The amount of the reagent injected in step (6) is added according to the organic content and sulfate content in the groundwater. If both the organic content and the sulfate content are too high, no reagent is added and only short-term stirring is performed; if the organic content is high but the sulfate content is low, only ferrous sulfate is added; if the organic content is low but the sulfate content is high, only an organic carbon source is added; if both the organic content and the sulfate content are low, ferrous sulfate and an organic carbon source are added at the same time.