Ex-situ green low-carbon ecological rapid remediation method for chromium-containing soil

Abstract

The application provides a green, low-carbon and rapid chromium-containing soil remediation method, which is improved on the basis of traditional biogas residue and ferrous sulfate for treating chromium-containing soil, and produces unexpected results, that is, the chromium-containing soil is remediated in a green and low-carbon manner, and the reaction progress is greatly accelerated. In the process, the chromium-containing soil is first mixed with biogas residue, ferrous sulfate and the like, and then is allowed to stand and react, and is stirred at irregular time intervals, so that the reduction of Cr(VI) is greatly accelerated. The process avoids the uniform environment formed by traditional continuous stirring, which causes the microorganisms in the biogas residue to be poisoned by Cr(VI), and the introduction of air continuously makes it impossible to maintain the suitable anaerobic environment of anaerobic microorganisms. The standing-alternating environment adopted in the process promotes the diversity of anaerobic, facultative and aerobic microorganisms, and at the same time, the sulfite and sulfide are consumed in time, and the microbial activity is promoted.

Description

Technical Field

[0001] This 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 Technology

[0002] The remediation strategy for chromium-containing soils involves reducing highly toxic Cr(VI) to Cr(III). The traditional process is the ferrous sulfate reduction method. Patent number 201410101586.0 describes a method for in-situ remediation of Cr(VI)-containing sites using ferrous sulfate. This method requires a large amount of ferrous sulfate, leading to soil acidification and secondary sulfate pollution. Furthermore, Cr(VI) residues remain, and the Cr(VI) content cannot meet current national soil management standards. Patent number 201610370386.4 provides a method for the ecological treatment of chromium-containing soils using biogas residue in conjunction with ferrous sulfate. This method mixes waste biogas residue, a carbon source, and ferrous sulfate, then allows it to stand for curing. This effectively removes Cr(VI) while reducing the amount of ferrous sulfate required, significantly alleviating soil acidification and secondary sulfate pollution, resulting in significant ecological benefits. However, this method has a long curing time, increasing the reaction cycle to generally more than 20 days, and it is prone to producing sulfide odors. The literature "Study on the Synergistic Effect of Ferrous Sulfate and Biogas Sludge Co-treatment of Chromium-Containing Soil" attempted to promote the reaction by increasing mass transfer through stirring. However, the results showed that stirring exacerbated the toxicity of Cr(VI) to the biogas sludge inoculant. Furthermore, stirring easily introduced air, failing to achieve an anaerobic environment, resulting in a worse effect. Currently, a green, low-carbon, efficient, and rapid remediation method for chromium-containing soil has not yet been found. Summary of the Invention

[0003] To address the shortcomings of existing technologies, this invention provides a green, low-carbon, and rapid method for remediating chromium-containing soil. It improves upon the traditional method of treating chromium-containing soil with biogas residue and ferrous sulfate, producing unexpected results that not only remediate chromium-containing soil in a green and low-carbon manner but also significantly accelerate the reaction process.

[0004] This application provides a green and low-carbon ecological restoration method for chromium-containing soil, comprising the following steps:

[0005] (1) Mix ferrous sulfate, biogas residue, biological nutrients, chromium-containing soil and water in a certain proportion, and then mix them unevenly.

[0006] (2) Cover the surface of the mixture from step (1) with a shielding material to isolate it from air and retain moisture. After standing and reacting for a period of time, stir briefly and then stand.

[0007] (3) Repeat step (2) until the Cr(VI) content of the mixture meets the standard. Air isolation measures are not required during the process, but moisture must be maintained. At the same time, biological nutrients can be added during the stirring process to maintain microbial activity as needed.

[0008] The mixture of ferrous sulfate, biogas residue, biological nutrient, chromium-containing soil and water in step (1) meets the following requirements: moisture content 25-65%, ferrous sulfate accounting for 0.05-5% of the dry weight of chromium-containing soil, biogas residue accounting for 0.01-5% of the dry weight of chromium-containing soil, biological nutrient accounting for 0.01-3% of the dry weight of chromium-containing soil, and Fe(II):Cr(VI) in the range of 0.01-4.

[0009] The main component of the biological nutrient in step (1) is a carbon source, and it also contains nitrogen source, phosphorus source and trace elements. The carbon source is generally glucose or acetate, but can also be molasses, kitchen waste, biogas slurry and other organic solid waste.

[0010] The biogas residue mentioned in step (1) can be replaced by sludge, by a microbial agent mainly prepared from biogas residue, or by chromium-reducing bacteria such as sulfate-reducing bacteria.

[0011] In step (2), the static reaction time is controlled within 8-48 hours, and the short-term stirring time is controlled within 10s-10min.

[0012] The measure to retain moisture in step (3) is to spray water on the surface of the mixture.

[0013] In step (3), the biological nutrients are slow-release carbon sources, generally molasses or kitchen waste.

[0014] Traditional theory has long held that stirring introduces air, affecting anaerobic efficiency. Furthermore, the improved mass transfer leads to close contact between microorganisms and Cr(VI) in the biogas residue, enhancing toxicity. Additionally, soil stirring is energy-intensive. Therefore, it is generally believed that stirring should be avoided during the microbial reduction of Cr(VI), maintaining a heterogeneous environment. This invention combines stirring with settling, alternating between the two. Experiments have shown that this method is significantly more effective than settling alone, and even more effective than continuous stirring, yielding unexpected results.

[0015] Analysis of the above phenomena suggests that the microbial reduction of chromium-containing soil is a heterogeneous reaction. In some areas with lower Cr(VI) content, microorganisms in the biogas residue rapidly reduce Cr(VI) and then continue reducing sulfate to sulfite or sulfide. Due to limited mass transfer during the static solid-phase reaction, sulfite or sulfide cannot readily react with Cr(VI) from other areas, gradually accumulating. Once accumulated to a certain level, sulfite or sulfide inhibits the reducing microorganisms in the biogas residue. However, if stirring is performed, sulfite or sulfide reacts with Cr(VI), causing a rapid decrease in Cr(VI). Simultaneously, the timely elimination of sulfite or sulfide removes the inhibitory effect on the reducing microorganisms, restoring their activity and allowing them to continue reducing Cr(VI) and sulfate. This alternating process of static settling and stirring ultimately achieves the rapid reduction of Cr(VI) in the soil.

[0016] The biochemical reactions during the process are as follows:

[0017] C2H4O (organic carbon source) + SO4 2- →SO3 2- +CO2+H2O (Stationary process)

[0018] C2H4O (organic carbon source) + SO3 2- →S 2- +CO2+H2O (Stationary process)

[0019] CrO4 2- +SO3 2- +H + →Cr 3+ +SO4 2- +H2O After short-term stirring

[0020] CrO4 2- +S 2- +H + →Cr 3+ +S+H2O After short-term stirring

[0021] The initial mixing requirement of this technology is incomplete mixing to alleviate the toxicity of high concentrations of Cr(VI) to the microorganisms in the biogas residue. Later, as the microorganisms gradually evolved from anaerobic microorganisms to facultative chromium-reducing microorganisms, the emphasis on absolutely anaerobic operation is no longer required.

[0022] This technology is a novel biochemical synergistic treatment technology for chromium-containing soil, which has the following advantages over previous technologies:

[0023] (1) It greatly accelerated the reaction rate and reduced the engineering cycle;

[0024] (2) The generated sulfites and sulfides react with Cr(VI) in a timely manner to prevent the accumulation of sulfides from producing hydrogen sulfide and forming a foul odor;

[0025] (3) Short-term stirring reduces the inhibition of microorganisms by sulfides, promotes biological activity, and can consume carbon sources as humus in a timely manner, thus avoiding the formation of organic pollution.

[0026] (4) Compared with continuous soil mixing, short-term mixing consumes less energy and is green and low-carbon;

[0027] (5) Make full use of the sulfites and sulfides produced by biochemistry as reducing agents, reduce the amount of chemical reagent ferrous sulfate used, reduce resource consumption, and be green and low-carbon.

[0028] (6) Short-term stirring promotes the production of facultative bacteria, avoids harsh anaerobic environments, and helps reduce management costs. Detailed Implementation

[0029] The present application will be further described below with reference to specific embodiments.

[0030] Example 1

[0031] The chromium-containing soil prepared from kaolin had a Cr(VI) content of 3000 mg / kg and a water-soluble Cr(VI) content of 2900 mg / kg.

[0032] (1) Mix ferrous sulfate, biogas residue, glucose, chromium-containing soil and water in a certain proportion, and then mix them unevenly. The moisture content is controlled at 45%, ferrous sulfate accounts for 2% of the dry weight of chromium-containing soil, biogas residue accounts for 3% of the dry weight of chromium-containing soil, glucose, a biological nutrient, accounts for 2% of the dry weight of chromium-containing soil, and Fe(II):Cr(VI) is about 1-1.5. Uneven mixing is achieved by short-term stirring, and the stirring time does not exceed 1 minute.

[0033] (2) Cover the mixture from step (1) with a black film to isolate air and retain moisture. After standing for 24 hours, stir for 1 minute and then stand.

[0034] (3) Repeat step (2) continuously.

[0035] The following table compares the changes in soil Cr(VI) content over reaction days after continuous stirring and settling:

[0036] Table 1. Effects of the operation, settling, and continuous stirring operations of this invention on soil Cr(VI) content.

[0037]

[0038] As shown in Table 1, overall, the present invention is significantly more effective than static and continuous stirring in remediating chromium-containing soils, with continuous stirring showing the worst effect. The present invention achieved undetectable Cr(VI) content in the soil after 7 days, while static and continuous stirring failed to achieve this. The static process showed a slow decrease in Cr(VI) content after 3 days of reaction time, presumably due to the formation of sulfides that inhibit microbial activity.

[0039] Example 2

[0040] The chromium-containing soil sample was taken from a real chromium slag contaminated area. The Cr(VI) content was 2600 mg / kg, of which the soluble Cr(VI) content was 1800 mg / kg and the insoluble Cr(VI) content was 600 mg / kg.

[0041] (1) Mix ferrous sulfate, biogas residue, glucose, chromium-containing soil and water in a certain proportion, and then mix them unevenly. The moisture content is controlled at 50%, the dry weight of ferrous sulfate is 3%, the dry weight of biogas residue is 5%, the dry weight of the biological nutrient glucose is 1%, the total dry weight of nitrogen source, phosphorus source and trace primary color does not exceed 0.1%, and the Fe(II):Cr(VI) is about 2.2. The uneven mixing is achieved by short-term stirring. Use an excavator to stir for no more than 1 minute. Then place it in a seepage-proof pit with seepage prevention function.

[0042] (2) Cover the mixture in the seepage prevention pit in step (1) with a black film to isolate the air and retain the moisture. After standing for 12 hours, use an excavator to stir for 5 minutes.

[0043] (3) Repeat step (2) twice;

[0044] (4) After standing for 12 hours, add 1% dry weight of molasses to the mixture in the seepage prevention pit as a slow-release carbon source. Then use an excavator to stir for 5 minutes, adding water during the process to keep the moisture content in the range of 55-60%.

[0045] (5) After standing for 24 hours, add 1% dry weight of molasses to the mixture in the seepage prevention pit as a slow-release carbon source. Then use an excavator to stir for 5 minutes, adding water during the process to keep the moisture content in the range of 55-60%.

[0046] (6) After standing for 24 hours, use an excavator to stir the mixture in the seepage prevention pit for 5 minutes. During the process, add water to keep the moisture content in the range of 55-60%.

[0047] (7) Repeat step (6) 6 times, and then enter the long-term static stage.

[0048] The high content of insoluble Cr(VI) in the soil of Example 2 posed a challenge for treatment. After the above operations, the Cr(VI) content in the chromium-containing soil decreased to 350 mg / kg after 2 days, with insoluble Cr(VI) at 250 mg / kg. After 4 days, it decreased to 30 mg / kg, with insoluble Cr(VI) at 26 mg / kg. After 10 days, the Cr(VI) content decreased to 5.0 mg / kg, with insoluble Cr(VI) at 4.8 mg / kg. After 30 days, the Cr(VI) content decreased to 1.5 mg / kg, meeting the residential land standard in the "Soil Environmental Quality Standard for Construction Land Soil Pollution Risk Control (Trial)".

[0049] Compared with the traditional continuous static process, the Cr(VI) content was still 150 mg / kg after 10 days, and decreased to 50 mg / kg after 30 days. Subsequently, the degradation was extremely slow, and it took 240 days to reach the standard for residential land in the "Soil Environmental Quality Standard for Construction Land Soil Pollution Risk Control (Trial)".

[0050] Stirring introduces air into the system, and alternating between static and stirring allows the biochemical reactions to proceed alternately between anaerobic and aerobic processes. This promotes soil microbial diversity, fosters the growth of facultative and aerobic bacteria, and the CO2 produced by microbial respiration accelerates the dissolution of insoluble Cr(VI), thus speeding up the reactions. Furthermore, the presence of facultative and aerobic bacteria promotes the humification of organic matter in the system, contributing to soil ecological balance.

[0051] Example 3

[0052] The chromium-containing soil prepared from kaolin had a Cr(VI) content of 2000 mg / kg and a water-soluble Cr(VI) content of 1900 mg / kg.

[0053] (1) Ferrous sulfate, Cr(VI) reducing microorganisms, glucose, chromium-containing soil and water are mixed in a certain proportion and then mixed unevenly. The moisture content is controlled at 45%, ferrous sulfate accounts for 2% of the dry weight of chromium-containing soil, Cr(VI) reducing microorganisms account for 5% of the dry weight of chromium-containing soil, glucose, a biological nutrient, accounts for 2% of the dry weight of chromium-containing soil, and the Fe(II):Cr(VI) ratio is about 1-1.5. Uneven mixing is achieved by short-term stirring, and the stirring time does not exceed 1 minute.

[0054] (2) Cover the mixture from step (1) with a black film to isolate air and retain moisture. After standing for 24 hours, stir for 1 minute and then stand.

[0055] (3) Repeat step (2) continuously.

[0056] The Cr(VI)-reducing microorganisms were mainly sulfate-reducing bacteria. The results showed that the soil Cr(VI) content changed with the number of reaction days after continuous stirring and settling, as shown in the table below:

[0057] Table 2. Effects of the operation, settling, and continuous stirring of this invention on soil Cr(VI) content.

[0058]

[0059] As shown in Table 2, overall, the present invention is significantly more effective than static and continuous stirring in remediating chromium-containing soils, with continuous stirring showing the worst effect. Same as Example 1. On the sixth day, the present invention achieved undetectable Cr(VI) content.

[0060] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions claimed by the present invention.

Claims

1. A method for ex-situ green low-carbon ecological rapid remediation of chromium-containing soil, characterized in that, The method comprises the following steps: (1) mixing ferrous sulfate, biogas residue, biological nutrient, chromium-containing soil and water in a certain proportion, and then carrying out uneven mixing by short-term stirring; (2) covering the mixture formed in step (1) with a barrier to isolate air and keep moisture, and then carrying out short-term stirring after standing for a period of time; the standing time is controlled to be 8-48 hours, and the short-term stirring time is controlled to be less than 10 minutes; (3) continuously repeating step (2), and then stacking after standing; in the process, absolute anaerobic operation is not emphasized, but moisture is kept, and biological nutrients can be added during stirring according to the actual situation to maintain microbial activity; short-term stirring reduces the inhibition of sulfides on microorganisms, and short-term stirring promotes the generation of facultative bacteria.

2. The method according to claim 1, wherein the method is characterized by, The mixture of ferrous sulfate, biogas residue, biological nutrient, chromium-containing soil and water after mixing meets the following conditions: the moisture content is 25-65%, the ferrous sulfate accounts for 0.05-5% of the dry weight of the chromium-containing soil, the biogas residue accounts for 0.01-5% of the dry weight of the chromium-containing soil, the biological nutrient accounts for 0.01-3% of the dry weight of the chromium-containing soil, and the Fe(II):Cr(VI) is in the range of 0.01-4.

3. The method according to claim 1, wherein the method is characterized by, The biological nutrient mainly contains carbon sources, and also contains nitrogen sources, phosphorus sources and trace elements; the carbon sources are glucose, acetate or organic solid waste.

4. The method according to claim 1, wherein the method is characterized by, The biogas residue refers to the solid residue in the anaerobic reaction process, which can be replaced by sludge, or replaced by a microbial agent mainly prepared from biogas residue, or replaced by microorganisms capable of reducing Cr(VI) such as sulfate-reducing bacteria.

5. The method according to claim 1, wherein the method is characterized by, The step (3) keeps moisture by spraying water on the surface of the mixture.

6. The method according to claim 1, wherein the method is characterized by, The biological nutrient in step (3) is a slow-release carbon source, and the slow-release carbon source is molasses or kitchen waste.

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