Remediation Agents and Methods for Remediating Hexavalent Chromium-Contaminated Soil

Abstract

This invention discloses a remediation agent and method for remediating hexavalent chromium contaminated soil. The remediation agent consists of wood vinegar and ferrous salt, wherein the amount of ferrous salt added per milliliter of wood vinegar is 0.006–0.08 g, and the concentration of the wood vinegar is 50–100%. In this invention, the remediation agent is added to chromium-contaminated soil, along with deionized water. The soil is stirred thoroughly, its moisture content is maintained, and the soil is allowed to stand for curing. This process reduces hexavalent chromium in the soil to trivalent chromium, achieving the reduction and removal of hexavalent chromium from the contaminated soil. This method has high efficiency in removing hexavalent chromium, is highly operable, and has excellent practicality, especially in strongly alkaline soils.

Description

Technical Field

[0001] This invention relates to the field of soil pollution remediation, specifically to a remediation agent and method for remediating hexavalent chromium-contaminated soil. Background Technology

[0002] Chromium salts are important industrial raw materials and chemical products, widely used in metallurgy, electroplating, building materials, leather, corrosion protection, and dyeing industries. my country is a major producer of chromium salts, but due to the relatively backward production processes in early chromium salt plants, large amounts of chromium slag were generated during production. Continuous leaching of this slag after it was piled up caused the chromium concentration in the surrounding soil to exceed standards. Furthermore, the high alkalinity of the slag also caused an increase in soil pH. Cr(VI) in the soil is highly toxic and highly mobile, posing a significant threat to the environment and human health.

[0003] Currently, common remediation technologies for removing Cr(VI) pollutants from soil mainly include chemical reduction remediation, solidification / stabilization, electrokinetic remediation, leaching remediation, and bioremediation. Chemical reduction technology is the most widely used method for treating chromium-contaminated soil due to its wide applicability and low cost. Commonly used reducing agents include iron-based reducing agents (ferrous sulfate, ferrous chloride, etc.), sulfur-based reducing agents (calcium polysulfide and sodium thiosulfate, etc.), and organic reducing agents (organic acids, biochar, etc.). However, because chromium slag is strongly alkaline, chromium-contaminated soil affected by chromium slag is generally an alkaline medium system dominated by Cr(VI), which prevents Cr(VI) in the solid phase from fully dissolving and reacting with the reducing agent. Furthermore, Cr(VI) reduction is a process that consumes H₂O. + The remediation effect of reducing agents such as ferrous sulfate and sodium sulfite in the reaction process is limited by soil pH. Patent CN109796984A discloses a reducing and stabilizing agent for chromium-contaminated soil, which directly reduces hexavalent chromium to trivalent chromium using calcium polysulfide and ferrous sulfate. However, the aqueous solution of calcium polysulfide is highly alkaline, which damages the soil structure and its physicochemical properties, and the generated hydrogen sulfide gas is toxic. Patent CN104307868A discloses a method for removing hexavalent chromium from chromium-contaminated soil, which adds ferrous sulfate to reduce hexavalent chromium to trivalent chromium. However, Cr(VI) cannot be fully dissolved and released to react with the agent, and the reducing effect of ferrous sulfate is inhibited under highly alkaline conditions.

[0004] For weakly alkaline hexavalent chromium-contaminated soil, the soil is generally neutralized first with acids (sulfuric acid, hydrochloric acid, etc.), followed by reduction with common reducing agents. However, for highly alkaline chromium-contaminated soil, not only is the alkalinity strong, but the alkalinity is also high, requiring a large amount of acid for neutralization, which greatly increases the remediation cost. Patent CN103071670A discloses a method and equipment for detoxifying chromium slag-contaminated soil. It involves adding an acidic agent (98% concentrated sulfuric acid) and an auxiliary agent (ferrous sulfide) to dissolve hexavalent chromium, and then adding 16%–24% ferrous sulfate to reduce the hexavalent chromium to trivalent chromium under pH 5–6.5 conditions. However, this method requires a large amount of sulfuric acid for large-area soil remediation, and the sulfuric acid reacts with calcium oxide in the alkaline soil to form slightly soluble calcium sulfate, which crystallizes on the surface of soil particles, preventing further reaction between the acid and calcium oxide, significantly increasing the remediation cost. Patent CN105950165A discloses a soil remediation modifier and method for chromium contaminated soil. The modifier is composed of 5%–25% ferrous sulfate, 5%–25% calcium polysulfide, 5%–30% humic acid resin, 1%–25% cow manure, and 1%–25% fungal residue to remediate chromium in the soil. However, calcium polysulfide is highly alkaline and requires more acid to neutralize it, which further increases the remediation cost and poses potential safety issues.

[0005] Using organic reducing agents such as small-molecule organic acids and biochar does not damage soil structure and is environmentally friendly. However, their main drawback as electron donors is the slow reaction rate, requiring a relatively long time for successful remediation. Therefore, there is an urgent need to develop a treatment method suitable for alkaline soils surrounding chromium slag dumps that has a high efficiency in removing hexavalent chromium. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a remediation agent and method for remediating hexavalent chromium-contaminated soil. This invention utilizes biomass pyrolysis to prepare wood vinegar, which is then formulated with ferrous salts to create a remediation agent for hexavalent chromium-contaminated soil. This agent is applied to highly alkaline soil containing hexavalent chromium, reducing the hexavalent chromium in the soil to trivalent chromium. This invention's remediation agent is low-cost, highly efficient, and easily operable, possessing excellent practicality.

[0007] To achieve the above objectives, the technical solution designed by the present invention is as follows:

[0008] This invention provides a remediation agent for remediating hexavalent chromium contaminated soil. The remediation agent is composed of wood vinegar and ferrous salt, wherein the amount of ferrous salt added per milliliter of wood vinegar is 0.006-0.08 g, and the concentration of wood vinegar is 50-100%.

[0009] Furthermore, the ferrous salt is ferrous sulfate or ferrous sulfate heptahydrate.

[0010] Furthermore, the wood vinegar has a pH of 2.6 and an organic acid content of 12% (the main components of the wood vinegar are acids, phenols, ketones and alcohols, with the acids mainly being acetic acid and propionic acid).

[0011] The present invention also provides a method for preparing the repair agent, comprising the following steps:

[0012] (1) The biomass raw material is washed, dried, and crushed to obtain raw material powder;

[0013] (2) The raw material powder is subjected to high-temperature carbonization under nitrogen or inert gas atmosphere, and the condensate is collected after cooling to obtain crude wood vinegar.

[0014] (3) Let the crude wood vinegar stand until the layers appear, then extract the clear liquid in the middle layer.

[0015] (4) Add activated carbon powder to the clear liquid, mix and stir, let stand, filter, and obtain wood vinegar;

[0016] (5) Mix wood vinegar and ferrous salt to obtain a remediation agent for soil contaminated with hexavalent chromium.

[0017] Furthermore, in step (1), the biomass raw material is bamboo shavings or cotton stalks; the particle size of the raw material powder is 0.5-3 mm.

[0018] Furthermore, in step (2), the heating rate during the high-temperature carbonization process is 10-20℃ / min, the maximum carbonization temperature is 250-350℃, and the maximum carbonization temperature is maintained for 1 hour.

[0019] Furthermore, in step (4), the amount of activated carbon powder added is 3-5%, and the mixture is stirred for 10 minutes and left to stand for 72 hours.

[0020] The present invention also provides a method for remediating chromium-contaminated soil using the remediation agent, comprising the following steps: adding the remediation agent to the chromium-contaminated soil, adding deionized water at the same time, stirring the soil evenly, maintaining the soil moisture content, and allowing it to stand for curing.

[0021] Furthermore, the chromium-contaminated soil is hexavalent chromium-contaminated soil, with a hexavalent chromium content of 20–50 mg / kg.

[0022] Furthermore, the amount of remediation agent added is 20-40% of the soil mass.

[0023] Furthermore, the soil moisture content was 40%, and the static curing time was 4 days.

[0024] The principle of this invention:

[0025] (1) Bamboo shavings and cotton stalks have a high hemicellulose content. The refined wood vinegar produced after high-temperature carbonization is rich in organic acids such as acetic acid and formic acid, as well as phenols, aldehydes, and ketones. The functional groups such as hydroxyl and carboxyl groups result in a lower pH in the wood vinegar, which can reduce soil pH upon addition and also dissolve soil minerals, leading to the release of encapsulated Cr(VI). The functional groups such as hydroxyl, aldehyde, and carbonyl groups can lower soil Eh, giving the wood vinegar the potential to reduce Cr(VI). Furthermore, the wood vinegar contains Fe... 3 + Mn 2+ Plasma has catalytic properties and can promote the reduction of Cr(VI) by reducing substances such as small-molecule organic acids in wood vinegar.

[0026] (2) Ferrous salts can reduce hexavalent chromium to trivalent chromium under acidic and neutral conditions with high reduction efficiency. The reaction equation is: Cr 6+ +3Fe 2+ +8H + =Cr 3+ +3Fe 3+ +4H2O.

[0027] (3) After wood vinegar is mixed with ferrous salt, the small molecule organic acids in the wood vinegar can form complexes with Fe(III), thereby forming Fe(II) / Fe(III)-organic acid complexes with low redox potential, which promotes the reduction of Cr(VI) by Fe(II).

[0028] The beneficial effects of this invention are:

[0029] This invention utilizes a remediation agent prepared from a combination of wood vinegar and ferrous salts to synergistically remediate hexavalent chromium-contaminated soil. Through the complexation and reduction effects of the wood vinegar and ferrous salts, hexavalent chromium in the soil is reduced to trivalent chromium, exhibiting high reduction efficiency and strong applicability in strongly alkaline soils. The use of wood vinegar as the primary treatment agent reduces reagent costs and expands the resource utilization pathways for agricultural waste such as bamboo shavings. Detailed Implementation

[0030] The present invention will now be described in further detail with reference to specific embodiments, so that those skilled in the art can understand it.

[0031] Example 1

[0032] Preparation of wood vinegar:

[0033] (1) Wash and dry the biomass raw material bamboo chips, and then crush them to 0.5-3mm;

[0034] (2) Bamboo chips are subjected to high-temperature carbonization under nitrogen or inert gas atmosphere. The heating rate during the high-temperature carbonization process is 10-20℃ / min, the maximum carbonization temperature is 250-350℃, the maximum carbonization temperature is maintained for 1h, and the condensate is collected to obtain crude wood vinegar.

[0035] (3) Let the crude wood vinegar stand until it separates into layers, then extract the clear brown liquid in the middle layer.

[0036] (4) Add 3-5% activated carbon powder to the clear brown liquid and mix. Stir for 10 minutes, let stand for 72 hours, filter, and obtain wood vinegar.

[0037] The above-mentioned wood vinegar has a pH of 2.6 and an organic acid content of 12%. Its main components are acids (mainly acetic acid and propionic acid), phenols, ketones and alcohols.

[0038] Example 2

[0039] Preparation of Remediation Agent 1 for Hexavalent Chromium Contaminated Soil:

[0040] (1) Weigh 0.04g FeSO4·7H2O and 4mL of the wood vinegar prepared in Example 1;

[0041] (2) Mix wood vinegar and FeSO4·7H2O to obtain repair agent 1.

[0042] Example 3

[0043] Preparation of Remediation Agent 2 for Hexavalent Chromium Contaminated Soil:

[0044] (1) Weigh 0.1g FeSO4·7H2O and 4mL of the wood vinegar prepared in Example 1;

[0045] (2) Mix wood vinegar and FeSO4·7H2O to obtain repair agent 2.

[0046] Example 4

[0047] Preparation of Remediation Agent 3 for Hexavalent Chromium Contaminated Soil:

[0048] (1) Weigh 0.04g FeSO4·7H2O and 2mL of the wood vinegar prepared in Example 1;

[0049] (2) Mix wood vinegar and FeSO4·7H2O to obtain repair agent 3.

[0050] Example 5

[0051] Preparation of Remediation Agent 4 for Hexavalent Chromium Contaminated Soil:

[0052] (1) Weigh 0.1g FeSO4·7H2O and 2mL of the wood vinegar prepared in Example 1;

[0053] (2) Mix wood vinegar and FeSO4·7H2O to obtain repair agent 4.

[0054] Comparative Example 1

[0055] Preparation of Common Repair Agent 1:

[0056] The standard repair agent 1 is only 2 mL of wood vinegar.

[0057] Comparative Example 2

[0058] Preparation of Common Repair Agent 2:

[0059] The standard repair agent 2 is only 4 mL of wood vinegar.

[0060] Comparative Example 3

[0061] Preparation of Common Repair Agent 3:

[0062] Ordinary repair agent 3 contains only 0.04g of FeSO4·7H2O.

[0063] Comparative Example 4

[0064] Preparation of Common Repair Agent 4:

[0065] Ordinary repair agent 4 contains only 0.1g of FeSO4·7H2O.

[0066] Example 5

[0067] Methods for remediating hexavalent chromium contaminated soil:

[0068] The total chromium content of the chromium slag stockpile of a ferroalloy plant in Hunan Province was 2360 mg / kg, the hexavalent chromium content was 29.58 mg / kg, and the leaching concentration of hexavalent chromium was 1.039 mg / L.

[0069] The remediation agents 1-4 of Examples 2-5 for remediating hexavalent chromium-contaminated soil and the ordinary remediation agents 1-4 of Comparative Examples 1-4 were used to remediate hexavalent chromium-contaminated soil.

[0070] (1) Take 10g of hexavalent chromium contaminated soil and add compound agents (i.e., remediation agents 1-4 for remediating hexavalent chromium contaminated soil and ordinary remediation agents 1-4), and add deionized water to control the moisture content to 40%.

[0071] (2) After mixing evenly, the soil was naturally cured for 4 days, and the leaching concentration of Cr(VI) was measured.

[0072] Table 1 shows the Cr(VI) leaching of hexavalent chromium contaminated soil remediated by remediation agents 1-4 and ordinary remediation agents 1-4.

[0073] Table 1. Soil Cr(VI) leaching status

[0074] deal with Cr(VI) leaching concentration (mg / L) Percentage decrease in Cr(VI) leaching concentration (%) Example 1 0.031 97.0 Example 2 0.027 97.4 Example 3 0.107 89.7 Example 4 0.210 79.8 Comparative Example 1 0.805 22.5 Comparative Example 2 0.752 27.6 Comparative Example 3 0.747 28.1 Comparative Example 4 0.141 86.4

[0075] The conclusion drawn from Table 1 is that Example 2 had the lowest Cr(VI) leaching concentration, at 0.027 mg / L, and the Cr(VI) leaching concentration was reduced by 97.4%. Therefore, Example 2 is the optimal soil remediation solution.

[0076] All other parts not described in detail are existing technologies. Although the above embodiments have provided a detailed description of the present invention, they are only some embodiments of the present invention, not all embodiments. People can obtain other embodiments based on these embodiments without creative effort, and these embodiments all fall within the protection scope of the present invention.

Claims

1. A remediation agent for soil contaminated with hexavalent chromium, characterized in that: The repair agent is composed of wood vinegar and ferrous salt, wherein the amount of ferrous salt added per milliliter of wood vinegar is 0.006~0.08 g, and the concentration of wood vinegar is 50~100%; the ferrous salt is ferrous sulfate or ferrous sulfate heptahydrate; the pH of the wood vinegar is 2.6, and the organic acid content is 12%.

2. A method for preparing the repair agent as described in claim 1, characterized in that: Includes the following steps: (1) The biomass raw materials are washed, dried, and crushed to obtain raw material powder; (2) The raw material powder is subjected to high-temperature carbonization under nitrogen or inert gas atmosphere, and the condensate is collected after cooling to obtain crude wood vinegar. (3) Let the crude wood vinegar solution stand until it separates into layers, then extract the clear liquid in the middle layer. (4) Add activated carbon powder to the clear liquid, mix and stir, let stand, filter, and obtain wood vinegar; (5) Mix wood vinegar and ferrous salt to obtain a remediation agent for soil contaminated with hexavalent chromium.

3. The preparation method according to claim 2, characterized in that: In step (1), the biomass raw material is bamboo shavings or cotton stalks; the particle size of the raw material powder is 0.5~3 mm.

4. The preparation method according to claim 2, characterized in that: In step (2), the heating rate during high-temperature carbonization is 10~20℃ / min, the maximum carbonization temperature is 250~350℃, and the maximum carbonization temperature is maintained for 1 h.

5. The preparation method according to claim 2, characterized in that: In step (4), the amount of activated carbon powder added is 3-5%, and the mixture is stirred for 10 min and left to stand for 72 h.

6. A method for remediating chromium-contaminated soil using the remediation agent as described in claim 1, characterized in that: Includes the following steps: Add a remediation agent to the chromium-contaminated soil, along with deionized water, mix the soil thoroughly, maintain soil moisture content, and allow it to stand for curing.

7. The method according to claim 6, characterized in that: The chromium-contaminated soil is hexavalent chromium-contaminated soil, with a hexavalent chromium content of 20-50 mg / kg.

8. The method according to claim 6, characterized in that: The amount of remediation agent added is 20-40% of the soil mass; the soil moisture content is 40%, and the static curing time is 4 days.