Method for solidifying dredged sludge with excessive heavy metal content by modified zero-valent iron combined with fly ash and slag micro-powder

Abstract

The present application belongs to the field of dredged sludge solidification, and particularly relates to a method for solidifying dredged sludge with excessive heavy metal content by using modified zero-valent iron combined with fly ash and slag powder. The method comprises the following steps: preparing modified zero-valent iron; modifying the initial dredged sludge by using the modified zero-valent iron; and solidifying the dredged sludge. The present application has the advantages that: in the solidification process, fly ash and slag powder are used to replace cement, which effectively utilizes solid waste and reduces carbon emissions. The zero-valent iron is modified by adding a small amount of fly ash and biochar. The biochar and zero-valent iron form an iron-carbon microcell in the water-containing dredged sludge. The fly ash moderately coats and fixes the zero-valent iron and provides heavy metal adsorption sites. Both of them have a synergistic effect, preventing the passivation of zero-valent iron while improving the reduction efficiency of zero-valent iron.

Description

Technical Field

[0001] This invention belongs to the field of dredged sludge solidification, specifically involving a method for solidifying dredged sludge with excessive heavy metal content by combining modified zero-valent iron with fly ash and slag micro powder. Background Technology

[0002] In recent years, ecological restoration projects for various inland waterways have generated large amounts of dredged sludge. This sludge is difficult to utilize directly as a resource, and its transportation and storage costs are high. Traditional methods for treating dredged sludge, such as dumping and landfilling, are inefficient, costly, and environmentally unfriendly. Therefore, the engineering performance and treatment efficiency of sludge need further improvement. The mainstream treatment methods include vacuum preloading, mechanical dewatering, and solidification. Currently, the commonly used solidification method in engineering involves using cement as a solidifying agent to treat soft soil foundations, and using cement to treat dredged sludge is a common chemical method that has been applied in many construction projects. However, cement production emits approximately 2 billion tons of carbon dioxide annually, accounting for about 8% to 10% of global emissions, and contributes to the greenhouse effect.

[0003] Fly ash (FA) is a solid waste generated by power plants and is a major waste product of coal-fired power plants, but it has not yet been fully utilized. Slag powder is an industrial waste, typically referring to solid waste generated during metallurgical and steelmaking processes. Granulated blast furnace slag possesses certain pozzolanic activity due to the characteristics of aluminosilicate materials. Proper treatment of dredged sludge helps improve water quality and the ecological environment, while materials solidified from solid waste can be used in landfill and foundation engineering, reducing the negative environmental impact of waste.

[0004] Industrial wastewater containing heavy metal ions can accumulate in the environment due to improper treatment during discharge and processing, leading to heavy metal pollution in some areas caused by dredged sludge. How to treat this heavy metal-contaminated sludge is crucial for the ecological environment. Solidification can play a role, but during and after solidification, heavy metals can leach out again in complex environments, easily causing secondary pollution. Zero-valent iron (ZVFe) can fix heavy metal ions, but its use in dredged sludge faces challenges such as passivation due to the complex composition of the sludge, low solidification efficiency, and insufficient stability of heavy metals, especially under long-term harsh environments. Summary of the Invention

[0005] The purpose of this invention is to provide a method for dredging sludge with excessive heavy metal content by solidifying modified zero-valent iron combined with fly ash and slag micro powder.

[0006] To achieve the above objectives, the technical solution adopted by this invention is as follows: a method for solidifying dredged sludge with excessive heavy metal content by combining modified zero-valent iron with fly ash and slag powder, comprising the following steps:

[0007] 1) Preparation of modified zero-valent iron:

[0008] Modified zero-valent iron was obtained by ball milling a mixture of zero-valent iron, fly ash, and biochar; the ratio of the total mass of fly ash and biochar to the total mass of zero-valent iron was 1.5~2.5:1.

[0009] 2) Modification of initial dredged sludge using modified zero-valent iron:

[0010] The modified zero-valent iron is thoroughly mixed with the initial dredged sludge for 0.5 to 1 hour to obtain the modified dredged sludge.

[0011] 3) Dredging and solidifying silt:

[0012] Modified dredged sludge, fly ash, and slag powder are mixed and then solidified; the mass ratio of the dry weight of the modified dredged sludge, fly ash, and slag powder is 100:8.5~9.5:9~10.

[0013] During the solidification process, fly ash and slag powder were used to replace cement, effectively utilizing solid waste and reducing carbon emissions. Furthermore, experiments showed that both fly ash and slag powder could jointly improve the strength and shear resistance of the solidified dredged sludge; however, if only one type of fly ash or slag powder was used, both strength and shear resistance would significantly decrease.

[0014] Zero-valent iron (ZVFe) was modified by adding a small amount of fly ash and biochar. The biochar and ZVFe formed an iron-carbon micro-battery in the moist dredged sludge. The fly ash moderately coated and immobilized the ZVFe, providing adsorption sites for heavy metals, preventing passivation of ZVFe while improving its reduction efficiency. Simultaneously, this modification method, including the selection of biochar raw materials (calamus and peanut shells), improved the stability of heavy metals under long-term harsh environments, preventing their re-precipitation under such conditions.

[0015] The reason for first modifying the initial dredged sludge with modified zero-valent iron and then solidifying the dredged sludge is that experiments have shown that if the dredged sludge is solidified at the same time, heavy metals are more likely to precipitate again under long-term harsh conditions.

[0016] In a preferred embodiment, in step 2), the mass ratio of modified zero-valent iron to the initial dry weight of dredged sludge is 0.5~1:100. This ratio is also quite important. If the doping ratio of modified zero-valent iron is too high, it will increase the cost and affect the final strength; if the doping ratio of modified zero-valent iron is too low, heavy metals will be more likely to precipitate again under long-term harsh environments.

[0017] Furthermore, in step 2), the initial dredged sludge has a moisture content of 10-40%.

[0018] In a preferred embodiment, in step 1), ball milling modification is carried out in a ball mill at a speed of 350~450 r / min for 2-3 h.

[0019] In a preferred embodiment, in step 1), the mass ratio of fly ash to biochar is 2~4:1.

[0020] In a preferred embodiment, in step 1), the biochar is obtained by treating a mixture of calamus and peanut shells at 400-600°C for 1-2 hours; the mass ratio of calamus to peanut shells is 3-5:1.

[0021] The biochar prepared by mixing calamus and peanut shells has a stronger ability to stabilize and solidify heavy metal ions compared to using peanut shells alone.

[0022] Another objective of this invention is to disclose a modified zero-valent iron combined with fly ash and slag powder curing agent. Modified zero-valent iron is prepared using the aforementioned method, and the modified zero-valent iron, fly ash, and slag powder are mixed in a ratio of 0.5~1:8.5~9.5:9~10 to form the curing agent.

[0023] Another object of the present invention is to provide the application of the aforementioned method or the aforementioned curing agent in curing dredged sludge with excessive heavy metal content, or in improving the strength of the cured dredged sludge.

[0024] In a preferred embodiment, it is used to reduce the amount of heavy metals leached out during the solidification process of dredged sludge with excessive heavy metal content.

[0025] The advantages of this invention are:

[0026] During the solidification process, fly ash and slag powder are used to replace cement, which effectively utilizes solid waste and reduces carbon emissions.

[0027] Zero-valent iron (ZVFe) was modified by adding a small amount of fly ash and biochar. The biochar and ZVFe formed an iron-carbon micro-battery in the moist dredged sludge. The fly ash moderately coated and immobilized the ZVFe, providing adsorption sites for heavy metals. The two components worked synergistically, preventing ZVFe passivation while improving its reduction efficiency. Furthermore, this modification method, including the selection of biochar raw materials (calamus and peanut shells), improved the stability of heavy metals under long-term harsh environments, preventing their re-precipitation. Attached Figure Description

[0028] Figure 1 This is a step-by-step diagram of a method for dredging sludge with excessive heavy metal content by combining modified zero-valent iron with fly ash and slag micro powder for solidification. Detailed Implementation

[0029] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0030] Unless otherwise specified, all reagents and materials used in the following examples are commercially available.

[0031] Example 1

[0032] A method for solidifying dredged sludge with modified zero-valent iron combined with fly ash and slag powder to remove excessive heavy metal content includes the following steps:

[0033] 1) Preparation of modified zero-valent iron:

[0034] Zero-valent iron, fly ash, and biochar were mixed and ball-milled (400 r / min) for 2.5 h to obtain modified zero-valent iron. The total mass ratio of fly ash and biochar to the total mass of zero-valent iron was 2:1, with the fly ash to biochar ratio being 3:1. The biochar was obtained by treating a mixture of calamus leaf fragments (1-2 cm in diameter) and peanut shell powder at 500°C for 1.5 h, cooling, and then crushing it into powder. The mass ratio of calamus to peanut shells was 4:1.

[0035] 2) Modification of initial dredged sludge using modified zero-valent iron:

[0036] The project obtained dredged silt (taken from a section of river in Zhenjiang area; the basic physical and mechanical properties of the dredged silt were determined, with a soil particle specific gravity of 2.65, liquid limit and plastic limit of 42.5% and 21.2% respectively, water content of 35%; pH 7.6; heavy metals Cr 1285mg / kg and As 92mg / kg, the same below) which was sieved through a 2mm sieve to obtain initial dredged silt; modified zero-valent iron was thoroughly mixed with the initial dredged silt for 1 hour to obtain modified dredged silt; the mass ratio of modified zero-valent iron to the dry weight of the initial dredged silt was 1:100.

[0037] 3) Dredging and solidifying silt:

[0038] The modified dredged sludge, fly ash, and slag powder were mixed for 5 minutes to prepare a sample, which was then cured for a certain period of time under controlled conditions (temperature 20±2°C, relative humidity ≥90%). The mass ratio of the modified dredged sludge dry weight, fly ash, and slag powder was 100:9.5:10.

[0039] This invention uses Class C fly ash (FA). The slag powder is granulated blast furnace slag powder (GGBS).

[0040] Example 2

[0041] A method for solidifying dredged sludge with modified zero-valent iron combined with fly ash and slag powder to remove excessive heavy metal content includes the following steps:

[0042] 1) Preparation of modified zero-valent iron:

[0043] Zero-valent iron, fly ash, and biochar were mixed and ball-milled (350 r / min) for 3 h to obtain modified zero-valent iron. The total mass ratio of fly ash and biochar to the total mass of zero-valent iron was 1.5:1, with the fly ash to biochar mass ratio being 2:1. The biochar was obtained by treating a mixture of calamus leaf fragments (1-2 cm in diameter) and peanut shell powder at 600°C for 1 h, cooling, and then crushing it into powder. The mass ratio of calamus to peanut shells was 3:1.

[0044] 2) Modification of initial dredged sludge using modified zero-valent iron:

[0045] The project obtained dredged sludge that passed through a 2mm sieve to obtain initial dredged sludge; modified zero-valent iron was thoroughly mixed with the initial dredged sludge for 0.5 hours to obtain modified dredged sludge; the mass ratio of modified zero-valent iron to the dry weight of the initial dredged sludge was 1:100.

[0046] 3) Dredging and solidifying silt:

[0047] The modified dredged sludge, fly ash, and slag powder were mixed for 5 minutes, then molded and cured for a certain period of time under controlled conditions (temperature 20±2°C, relative humidity ≥90%). The mass ratio of the modified dredged sludge dry weight, fly ash, and slag powder was 100:9.5:10.

[0048] Example 3

[0049] A method for solidifying dredged sludge with modified zero-valent iron combined with fly ash and slag powder to remove excessive heavy metal content includes the following steps:

[0050] 1) Preparation of modified zero-valent iron:

[0051] Zero-valent iron, fly ash, and biochar were mixed and ball-milled (450 r / min) for 2 h to obtain modified zero-valent iron. The total mass ratio of fly ash and biochar to the total mass of zero-valent iron was 2:1, with the fly ash to biochar ratio being 4:1. The biochar was obtained by treating a mixture of calamus leaf fragments (1-2 cm in diameter) and peanut shell powder at 400°C for 2 h, cooling, and then crushing it into powder. The mass ratio of calamus to peanut shells was 5:1.

[0052] 2) Modification of initial dredged sludge using modified zero-valent iron:

[0053] The project obtained dredged sludge that passed through a 2mm sieve to obtain initial dredged sludge; modified zero-valent iron was thoroughly mixed with the initial dredged sludge for 1 hour to obtain modified dredged sludge; the mass ratio of modified zero-valent iron to the dry weight of the initial dredged sludge was 1:100.

[0054] 3) Dredging and solidifying silt:

[0055] The modified dredged sludge, fly ash, and slag powder were mixed for 5 minutes, then molded and cured for a certain period of time under controlled conditions (temperature 20±2°C, relative humidity ≥90%). The mass ratio of the modified dredged sludge dry weight, fly ash, and slag powder was 100:9.5:10.

[0056] Comparative Example 1, Step 3) Change the ratio of fly ash to slag powder

[0057] Compared to Example 1, step 3) is modified to: the mass ratio of modified dredged sludge dry weight, fly ash, and slag powder is 100:4.5:15. The rest is the same as in Example 1.

[0058] Comparative Example 2, Step 3) Change the ratio of fly ash to slag powder

[0059] Compared to Example 1, step 3) is modified to: the mass ratio of modified dredged sludge dry weight, fly ash, and slag powder is 100:14.5:5. The rest is the same as in Example 1.

[0060] Comparative Example 3: Omitted Biochar

[0061] Compared to Example 1, step 1) omits biochar, and the ratio of fly ash mass to zero-valent iron mass remains 2:1. The rest is the same as in Example 1.

[0062] Comparative Example 4: Changing the ratio of fly ash to biochar

[0063] Compared to Example 1, the mass ratio of fly ash to biochar in step 1) was adjusted to 1:3, while the rest were the same as in Example 1.

[0064] Comparative Example 5: Simultaneous Modification and Solidification of Dredged Silt

[0065] Compared to Example 1, steps 2) and 3) are adjusted as follows: the dredged sludge obtained from the project is sieved through a 2mm sieve to obtain initial dredged sludge; modified zero-valent iron, fly ash, slag powder, and initial dredged sludge are mixed for 5 minutes, then shaped, and cured for a certain period under controlled conditions (temperature 20±2°C, relative humidity ≥90%); the mass ratio of modified zero-valent iron to the dry weight of initial dredged sludge is 1:100; the mass ratio of the dry weight of initial dredged sludge, fly ash, and slag powder is 100:9.5:10. The rest is the same as in Example 1.

[0066] Comparative Example 6: Omitting Calamus

[0067] Biochar was obtained by treating peanut shell powder at 500°C for 1.5 hours, cooling it, and then crushing it into powder. The rest was the same as in Example 1.

[0068] Comparative Example 7 used chitosan

[0069] The project obtained initial dredged sludge by passing it through a 2mm sieve. The initial dredged sludge, chitosan, fly ash, and slag powder were mixed for 5 minutes, then shaped and cured for a certain period of time under controlled conditions (temperature 20±2°C, relative humidity ≥90%). The mass ratio of the initial dredged sludge dry weight, fly ash, and slag powder was 100:9.5:10; the mass ratio of chitosan to the initial dredged sludge dry weight was 1:100.

[0070] Performance testing:

[0071] Direct shear test: A strain-controlled direct shear apparatus was used, and the test was conducted strictly in accordance with the operating procedures specified in the "Standard for Geotechnical Testing Methods" (GB / T50123-2019). The test was conducted under normal stresses of 50 kPa, 100 kPa, and 200 kPa to cover different stress ranges and simulate different pressure conditions that soil may experience in actual engineering projects. The operating procedure was as follows: the specimen was placed in the apparatus and loaded; after the settlement per hour was measured to be ≤0.005 mm using a dial gauge, the equipment was started and the test was conducted at a shear rate of 0.8 mm / min, with relevant data recorded simultaneously. The cohesion (c), internal friction angle (φ), and shear strength τ under 200 kPa pressure of the 28-day cured specimens of Examples 1-3 and Comparative Examples 1-2 were measured. f As shown in Table 1.

[0072] Unconfined compressive strength test: The test was conducted strictly in accordance with the UCS Test procedure in the "Standard for Geotechnical Testing Methods" (GB / T 50123-2019), as follows: Sample preparation: After preparation, the sample was placed in a curing chamber for 28 days. After reaching the corresponding curing age, the sample was removed and placed on the unconfined compressive strength tester, and the instrument was adjusted to the test preparation state. Finally, the instrument was run, and the axial strain of the pressure plate was increased uniformly at a rate of 1%-3% per minute until the sample failed. Data was collected to measure the unconfined compressive strength value of the sample. The unconfined compressive strengths of the 28-day cured samples measured in Examples 1-3 and Comparative Examples 1-4 are shown in Table 2.

[0073] Table 1 Cohesion, Angle of Internal Friction, and Shear Strength

[0074]

[0075] As shown in Table 1, the ratio of fly ash to slag powder has a significant impact on shear resistance.

[0076] Table 2 Unconfined compressive strength

[0077] Table 2 shows that the ratio of fly ash to slag powder has a significant impact on unconfined compressive strength. Meanwhile, biochar has no effect on unconfined compressive strength in the system of this embodiment. However, excessive biochar has a certain adverse effect on strength.

[0078] Heavy metal leaching test: The test was conducted according to the "Method for Determination of Leaching Toxicity of Solid Waste" (HJ / T299-2007), on samples cured for 90 days under complex conditions. The heavy metal leaching concentrations of Examples 1-3 and Comparative Examples 3-7 are shown in Table 3. "Complex environment" refers to the samples being naturally cured for the first 28 days, followed by daily immersion in a mixed solution of nitric acid and sulfuric acid (pH 4.5, molar ratio 1:1) for 2 hours, then naturally cured again, continuing until day 90. To prevent premature leaching of heavy metals and the resulting difficulty in calculation, the heavy metal content in the immersion liquid after 90 days was measured and added to the heavy metal leaching concentration determined by the solid waste leaching toxicity test method.

[0079] Table 3. Heavy metal leaching concentrations after 90 days of curing under complex conditions

[0080]

[0081] Comparing Comparative Example 3 and Example 1, it can be seen that biochar has a significant impact on the leaching concentration of heavy metals after 90 days of curing under complex conditions. This is because the peanut shell and calamus composite biochar can improve the ability to leach zero-valent iron. At the same time, the peanut shell and calamus composite biochar, together with fly ash and the overall structure of the final sample, can significantly enhance the stability of the two heavy metal ions.

[0082] Comparing Comparative Example 4 and Example 1, it can be seen that the mass ratio of fly ash to biochar is crucial for forming an overall structure that promotes the stability of heavy metal ions.

[0083] Comparing Comparative Example 5 and Example 1, it can be seen that if dredging and solidification of silt are carried out simultaneously, heavy metals are more likely to precipitate again under long-term harsh conditions.

[0084] Comparative Example 6 and Example 1 show that calamus biochar plays a role in promoting the stability of heavy metal ions under harsh conditions.

[0085] As can be seen from Comparative Example 7 and Example 1, the method and materials of the present invention exhibit higher stability of heavy metals under long-term harsh environments compared to the use of chitosan.

[0086] The above embodiments are only used to illustrate the technical solutions of the present invention. Those skilled in the art should understand that the above embodiments do not limit the present invention in any way. All technical solutions obtained by equivalent substitution or equivalent transformation fall within the protection scope of the present invention.

Claims

1. A method for solidifying dredged sludge with excessive heavy metal content by combining modified zero-valent iron with fly ash and slag powder, characterized in that... Includes the following steps: 1) Preparation of modified zero-valent iron: Modified zero-valent iron was obtained by ball milling a mixture of zero-valent iron, fly ash, and biochar; the ratio of the total mass of fly ash and biochar to the total mass of zero-valent iron was 1.5~2.5:

1. 2) Modification of initial dredged sludge using modified zero-valent iron: The modified zero-valent iron is thoroughly mixed with the initial dredged sludge for 0.5 to 1 hour to obtain the modified dredged sludge. 3) Dredging and solidifying silt: Modified dredged sludge, fly ash, and slag powder are mixed and then solidified; the mass ratio of the dry weight of the modified dredged sludge, fly ash, and slag powder is 100:8.5~9.5:9~10.

2. The method for solidifying dredged sludge with excessive heavy metal content using modified zero-valent iron combined with fly ash and slag powder according to claim 1, characterized in that, In step 2), the mass ratio of modified zero-valent iron to the initial dry weight of dredged sludge is 0.5~1:

100.

3. The method for solidifying dredged sludge with excessive heavy metal content using modified zero-valent iron combined with fly ash and slag micro-powder according to claim 1, characterized in that... In step 2), the initial dredged silt has a water content of 10-40%.

4. The method for solidifying dredged sludge with excessive heavy metal content using modified zero-valent iron combined with fly ash and slag micro-powder according to claim 1, characterized in that... In step 1), ball milling modification is carried out in a ball mill at a speed of 350~450 r / min for 2-3 h.

5. The method for solidifying dredged sludge with excessive heavy metal content using modified zero-valent iron combined with fly ash and slag powder according to claim 1, characterized in that... In step 1), the mass ratio of fly ash to biochar is 2~4:

1.

6. The method for solidifying dredged sludge with excessive heavy metal content using modified zero-valent iron combined with fly ash and slag powder according to claim 1, characterized in that... In step 1), the biochar is obtained by treating a mixture of calamus and peanut shells at 400-600°C for 1-2 hours; the mass ratio of calamus to peanut shells is 3-5:

1.

7. A modified zero-valent iron combined with fly ash and slag micro-powder curing agent, characterized in that, Modified zero-valent iron is prepared by the method described in claim 1. Modified zero-valent iron, fly ash, and slag powder are mixed in a ratio of 0.5~1:8.5~9.5:9~10 to form a curing agent.

8. The method according to any one of claims 1-6 or the curing agent according to claim 7 in the application of curing dredged sludge with excessive heavy metal content or improving the strength of dredged sludge curing.

9. The application according to claim 8, characterized in that, The method according to any one of claims 1-6 or the curing agent according to claim 7 reduces the amount of heavy metal leaching during the solidification process of dredged sludge with excessive heavy metal content.

10. The application of the method according to any one of claims 1-6 or the curing agent according to claim 7 in improving the strength of solidified dredged sludge.

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