CN111892932A

CN111892932A - Curing agent for high-concentration heavy metal cadmium polluted soil and preparation method thereof

Info

Publication number: CN111892932A
Application number: CN202010659606.1A
Authority: CN
Inventors: 朱志铎; 王海荣; 浦少云; 霍王文; 万瑜; 赵文政; 姚凯; 王朋; 胡乐乐
Original assignee: Southeast University
Current assignee: Southeast University
Priority date: 2020-07-09
Filing date: 2020-07-09
Publication date: 2020-11-06

Abstract

The invention discloses a curing agent for high-concentration heavy metal cadmium polluted soil and a preparation method thereof, wherein the curing agent comprises an alkali activator solution and gelled powder, the mass ratio of the alkali activator solution to the gelled powder is 0.2-0.7, the alkali activator solution comprises a sodium silicate solution and sodium hydroxide, and the gelled powder comprises fly ash, granulated blast furnace slag and nano SiO₂And nano Al₂O₃. The preparation method comprises the following steps: step one, fly ash, granulated blast furnace slag and nano SiO₂And nano Al₂O₃Mixing and stirring uniformly to prepare gelled powder; secondly, after the polluted soil is passivated, adding the gelled powder and uniformly mixing; adding NaOH into the sodium silicate solution to adjust the modulus to obtain an alkali activator solution; and step four, adding distilled water and alkali activator solution into the polluted soil treated in the step two, and uniformly stirring. The invention is to heavy goldThe cadmium curing agent has good cadmium curing effect, effectively utilizes industrial byproducts, is an environment-friendly curing agent, and has high strength of the repaired polluted soil.

Description

Curing agent for high-concentration heavy metal cadmium polluted soil and preparation method thereof

Technical Field

The invention relates to a soil remediation curing agent and a preparation method thereof, in particular to a curing agent for high-concentration heavy metal cadmium polluted soil and a preparation method thereof.

Background

In the production process of enterprises such as chemical plants, metal smelting plants, electroplating plants and the like, a large amount of pollutants such as toxic and harmful heavy metals and the like are generated to enter soil and underground water, so that the original site of the enterprise becomes an industrial pollution site, and potential health risks or ecological risks exist in the pollution sites during land function conversion and secondary development. Therefore, remediation of such heavy metal contaminated soil is necessary.

Cadmium (Cd) is the first pollutant in industrial and agricultural land and is the first pollutant in the overproof rate of inorganic pollutants in various heavy metal pollutants. Cadmium (Cd) pollution in industry mainly comes from nickel-cadmium electroplating, nickel-cadmium alloy production and metal mine mining and smelting, and cadmium (Cd) pollution in non-ferrous metal peripheral areas is more serious.

At present, the technology for repairing the heavy metal polluted soil mainly comprises solidification stabilization, soil leaching, vitrification and the like, wherein the solidification stabilization technology is a technology for repairing the heavy metal polluted soil, which has the advantages of short time consumption, low economic cost and relatively obvious repairing effect, and is widely applied to the treatment of solid wastes and polluted sites. The research on the solidification and stabilization of cadmium (Cd) polluted soil mainly focuses on traditional solidification materials such as cement, lime and phosphate, and still faces many problems, for example, the lime and the phosphate are non-renewable natural materialsMineral and high cost, and the large amount of phosphate applied can seriously change the soil structure and further cause phosphorus pollution of underground water and surface water. As the most commonly used curing agent, the cement has the advantages of simple operation, mature technology, low price and the like, but CO also exists in the production process₂Large discharge, inhibition of high-concentration heavy metal to cement hydration, limitation under the action of acid and erosion and the like.

The existing curing agent has poor treatment effect when repairing cadmium polluted soil with the concentration higher than 5000 mg/Kg. Therefore, the search for a novel curing agent which can cure and stabilize high-content heavy metals, reduce the use of cement and phosphate materials, and has the advantages of high curing and stabilizing efficiency, low cost, stable performance, wide material source and environmental friendliness becomes the focus of attention of environmental protection science and technology workers.

Disclosure of Invention

The purpose of the invention is as follows: in order to overcome the defects in the prior art, the invention aims to provide the curing agent for the high-concentration heavy metal cadmium polluted soil, which has a good curing effect, is low-carbon and environment-friendly, and the invention also aims to provide the preparation method for the curing agent for the high-concentration heavy metal cadmium polluted soil, which is simple, convenient, low in cost and stable in performance.

The technical scheme is as follows: the curing agent for the high-concentration heavy metal cadmium polluted soil comprises an alkali activator solution and gelled powder, wherein the mass ratio of the alkali activator solution to the gelled powder is 0.2-0.7, the alkali activator solution comprises a sodium silicate solution and sodium hydroxide, and the gelled powder comprises fly ash, granulated blast furnace slag and nano SiO₂And nano Al₂O₃。

Wherein the modulus of the alkali activator solution is 1-2. The gelled powder comprises the following substances in parts by weight: 60-70 parts of fly ash, 30-40 parts of granulated blast furnace slag and nano SiO₂2-3 parts of nano Al₂O₃2-3 parts.

The mass ratio of the alkali activator solution to the gelled powder is 0.2-0.7. The fly ash is 1250-1400 meshes. The granulated blast furnace slag is grade S95. Fly ash and granulated blast furnace slag (GGBS) are two industrial wastes that are produced in large quantities worldwide by coal-fired power generation and steel smelting, requiring large-scale land accumulation, which has a non-negligible impact on the environment and land utilization. By alkali excitation of the fly ash and the GGBS, the heavy metal in the polluted soil is effectively solidified, the waste is changed into valuable, the waste is treated by the waste, and the requirement of green cycle low-carbon economic development is met.

Preferably, the mass ratio of the alkali activator solution to the gelled powder is 0.2-0.4.

The preparation method of the curing agent for the high-concentration heavy metal cadmium polluted soil comprises the following steps:

step one, fly ash, granulated blast furnace slag and nano SiO₂And nano Al₂O₃Mixing and stirring uniformly to prepare gelled powder;

step two, after the polluted soil is passivated for 4 days, adding the gelled powder and uniformly mixing;

adding NaOH into the sodium silicate solution to adjust the modulus to obtain an alkali activator solution;

and step four, in order to ensure the hydration reaction of the curing agent, adding distilled water and an alkali activator solution into the polluted soil treated in the step two, wherein the adding amount of the distilled water is 5-7 wt% of the mass of the polluted soil, and uniformly stirring to ensure that the soil body has consistent color and can not have blocky particles.

The preparation principle is as follows: under the excitation of sodium silicate, the fly ash fills the gaps of the particles to form N-A-S-H gel with good mechanical behavior, and in addition, the addition of the granulated blast furnace slag can provide more C- (A) -S-H gel for A cementing material system, and the C- (A) -S-H gel and the N-A-S-H gel form A denser microstructure to provide strength. Heavy metal can be fixed in the gel pores through physical adsorption and can also be wrapped by gel in the process of forming gel, in addition, the chemical adsorption and chemical precipitation effect also make great contribution to the solidification and stabilization of the heavy metal, and the heavy metal ions can partially replace Na⁺And Ca² ⁺Bonded to the gel structure of the geopolymer, and can also form insoluble silicate precipitates or hydroxide precipitates.

Mixing nano-scale SiO₂With Al₂O₃Added into curing agent, because of its high activity, it can accelerate the hydration reaction to generate more N-A-S-H and C-A-S-H gels, which are filled in the pores of soil particles to make the structure more compact, besides, the nano SiO₂With Al₂O₃And micro pores in the soil particles can be filled, so that the microstructure of the soil body is refined, and the leaching of heavy metal ions is reduced.

Has the advantages that: compared with the prior art, the invention has the following remarkable characteristics:

l, the heavy metal cadmium has good curing effect, and after the soil body polluted by the high-concentration heavy metal cadmium of 5000mg/Kg is repaired, the leaching toxicity result of the heavy metal cadmium is less than 0.01 mg/L;

2. the industrial byproduct is effectively utilized, the curing agent is an environment-friendly curing agent, heavy metals in polluted soil are effectively cured by alkali excitation of the fly ash and the granulated blast furnace slag, waste is changed into valuable, waste is treated by waste, and the curing agent meets the requirement of green cycle low-carbon economic development;

3. the repaired polluted soil has high strength, and the unconfined compressive strength of the soil body 28d is higher and can reach 1414kPa at most after being repaired by the curing agent, so that the remediation method is favorable for secondary development and application of polluted sites, such as use as shallow foundation or road subgrade filler.

Drawings

FIG. 1 is a scanning electron microscope image of the solidified contaminated soil cured for 28 days by using the curing agent of the present invention, wherein a is 2000 times and b is 10000 times.

Detailed Description

In the following examples, the properties of the raw materials were as follows:

test soil sample: the clay used in the test is taken from 1m underground of a correction area of the great-east Jiulong lake, large-particle-size soil blocks in the clay are smashed by a hammer and dried in the sun, and then smashed by a smashing machine and sieved by a 1mm sieve for later use. The main physical mechanical indexes of the clay are shown in table 1.

TABLE 1 main physical and mechanical indexes of test soil sample

Exogenous contaminant forms: sulfate radical and acetate radical can generate acid excitation on GGBS, nitrate radical has small interference on cement hydration reaction, and cadmium nitrate tetrahydrate heavy metal cadmium (Cd) (NO) is comprehensively considered₃)₂·4H₂O as an external doping agent for heavy metal cadmium (Cd) pollutants²⁺) The designed concentration of the mixing amount is 0.5 percent (5000mg/Kg) of the mass of the dry soil.

Preparing cadmium (Cd) polluted soil: drying and crushing clay, sieving the clay by a 1mm sieve, weighing the mass of the tetrahydrate cadmium nitrate according to the designed concentration of heavy metal, weighing a proper amount of distilled water according to the optimal water content of the soil, adding the distilled water into a beaker, stirring to completely dissolve the tetrahydrate cadmium nitrate, adding the dissolved cadmium nitrate into the soil, and stirring uniformly to passivate for 4 days so that the heavy metal is uniformly dispersed in soil particles.

Fly ash: the fly ash is 1250-1400 meshes and is purchased from platinum casting materials, Inc. in Henan province, and the main chemical components and the content are shown in Table 2.

TABLE 2 main chemical composition and content of fly ash

Main chemical composition	Al₂O₃	SiO₂	Fe₂O₃	CaO	TiO₂	MgO	Na₂O	P₂O₅	SO₃	ZnO
											Content (%)	32.3	49.1	8.0	3.4	1.5	0.6	0.06	1.0	1.6	0.23

Granulated blast furnace slag: granulated blast furnace slag (GGBS) is grade S95, and is purchased from Baifeng mineral processing factory in Lingshou county, Hebei, and the main chemical components and contents are shown in Table 3.

TABLE 3 GGBS main chemical composition and content

Main chemical composition	CaO	Al₂O₃	SiO₂	Fe₂O₃	K₂O	MgO	Na₂O	P₂O₅	SO₃	MnO
											Content (%)	43.0	14.0	28.3	0.5	0.5	6.7	0.3	0.2	4.0	0.7

Na₂SiO₃Solution: liquid sodium silicate (modulus M ═ n (SiO)₂)/n(Na₂O)＝3.3，Na₂O(％)＝8.3，SiO₂(％)＝26.5)。

Sodium hydroxide: sodium hydroxide pellets were analytically pure.

Nano SiO₂And nano Al₂O₃: the nanomaterial silicon dioxide (SiO) used in the test₂) Nano aluminium oxide (Al)₂O₃) All produced by Nanjing Haitai Nano materials Co., Ltd, the basic parameters are as follows:

TABLE 4 nanometer SiO₂Parameter(s)

Appearance of the product	Whiteness degree	Average particle diameter (nm)	Bulk Density (g/cm)³)	Specific surface area (m)²/g)	SiO₂Content (wt.)
						White powder	94.7	20	0.05	160	99.0％

TABLE 5 nanometer Al₂O₃Parameter(s)

Appearance of the product	Whiteness degree	Average particle diameter (nm)	Bulk Density (g/cm)³)	Specific surface area (m)²/g)	Al₂O₃Content (wt.)
						White powder	96.6	300	0.52	5.0	99.0％

Example 1

A preparation method of a curing agent for high-concentration heavy metal cadmium polluted soil comprises the following steps:

(1) 60 parts of fly ash, 30 parts of granulated blast furnace slag and 2 parts of nano SiO₂With 2 parts of nano Al₂O₃Mixing and stirring uniformly to prepare gelled powder;

(2) after the polluted soil is passivated for 4 days, adding gelled powder and uniformly mixing, wherein the mixing amount of the gelled powder is 8 wt% of the mass of the polluted soil;

(3) adding NaOH into the sodium silicate solution to adjust the modulus to be 1 to obtain an alkali activator solution;

(4) in order to ensure the hydration reaction of the curing agent, distilled water and alkali activator solution which account for 5 wt.% of the mass of the polluted soil are added into the polluted soil treated in the step (2), the mixture is uniformly stirred, the color of the soil body is consistent, blocky particles cannot exist, and the mass ratio of the alkali activator solution to the gelled powder is 0.2.

Maintaining 28d as shown in figure 1, after the curing agent is added, coral-shaped (rectangular frame), rod-shaped (oval frame) and sheet-shaped (hexagonal frame) gels are generated in the polluted soil, and the gels fill the pores between the soil particles and the spherical fly ash particles, so that the internal structure of the soil body is more complete and compact.

Example 2

(1) 70 parts of fly ash, 40 parts of granulated blast furnace slag and 3 parts of nano SiO₂With 3 parts of nano Al₂O₃Mixing and stirring uniformly to prepare gelled powder;

(3) adding NaOH into the sodium silicate solution to adjust the modulus to be 2, and obtaining an alkali activator solution;

(4) in order to ensure the hydration reaction of the curing agent, distilled water and alkali activator solution which account for 7 wt.% of the contaminated soil are added into the contaminated soil treated in the step (2), the mixture is uniformly stirred, the color of the soil body is consistent, blocky particles cannot exist, and the mass ratio of the alkali activator solution to the gelled powder is 0.7.

Example 3

(1) 65 parts of fly ash, 35 parts of granulated blast furnace slag and 2.5 parts of nano SiO₂With 2.5 parts of nano Al₂O₃Mixing and stirring uniformly to prepare gelled powder;

(3) adding NaOH into the sodium silicate solution to adjust the modulus to be 1.5, and obtaining an alkali activator solution;

(4) in order to ensure the hydration reaction of the curing agent, distilled water and alkali activator solution which account for 6 wt.% of the mass of the polluted soil are added into the polluted soil treated in the step (2), the mixture is uniformly stirred, the color of the soil body is consistent, blocky particles cannot exist, and the mass ratio of the alkali activator solution to the gelled powder is 0.4.

Comparative example 1

(1) mixing 65 parts of fly ash and 35 parts of granulated blast furnace slag, and uniformly stirring to prepare gelled powder;

(4) in order to ensure the hydration reaction of the curing agent, distilled water and alkali activator solution which account for 6 wt.% of the mass of the polluted soil are added into the polluted soil treated in the step (2), the mixture is stirred and stirred uniformly, the color of the soil body is consistent, blocky particles cannot exist, and the mass ratio of the alkali activator solution to the gelled powder is 0.4.

Example 4

(1) 64 parts of fly ash, 34 parts of granulated blast furnace slag and 2.5 parts of nano SiO₂With 3 parts of nano Al₂O₃Mixing and stirring uniformly to prepare gelled powder;

(4) in order to ensure the hydration reaction of the curing agent, distilled water and alkali activator solution which account for 6 wt.% of the mass of the polluted soil are added into the polluted soil treated in the step (2), the mixture is uniformly stirred, the color of the soil body is consistent, blocky particles cannot exist, and the mass ratio of the alkali activator solution to the gelled powder is 0.2.

Example 5

This example was the same as example 4 except that the mass ratio of the alkali-activator solution to the gelled powder material was 0.3.

Example 6

This example was the same as example 4 except that the mass ratio of the alkali-activator solution to the gelled powder material was 0.4.

Example 7

This example was the same as example 4 except that the mass ratio of the alkali-activator solution to the gelled powder material was 0.5.

Example 8

This example was the same as example 4 except that the mass ratio of the alkali-activator solution to the gelled powder material was 0.6.

Example 9

This example was the same as example 4 except that the mass ratio of the alkali-activator solution to the gelled powder material was 0.7.

Respectively preparing samples of the products obtained in the examples 1-9 and the comparative example 1 by a static pressure method, preparing samples with the dry density of about 95% of the maximum dry density of clay, controlling the mass of each sample to be about 230g, filling the weighed solidified soil into a rigid mold with the diameter of 50mm and the height of 55mm in three layers, manually vibrating the mold after filling one layer to remove air bubbles in the samples, carrying out static pressure forming by using a jack, and maintaining the pressure for 1 min. And demolding after molding, filling into a sealed fresh-keeping bag, placing the sample in a standard curing room, curing for 28d (the curing condition is that the temperature is 20 +/-3 ℃, and the relative humidity is more than or equal to 95 percent), and carrying out performance test.

Test example 1

Unconfined compressive strength test

Test standards: ASTM D4219-08

The test process comprises the following steps: the test equipment adopts a CBR-2 type bearing ratio tester, is matched with a pressure sensor and a displacement sensor, controls the axial strain rate to be 1%/min, and tests 3 parallel samples to take an average value. The computer automatically collects the axial force and the vertical displacement every 1 second. The test results are shown in table 6.

TABLE 6 unconfined compressive strength test results (kPa)

The recycling of the solidified heavy metal contaminated soil, such as the backfill soil for urban residences, businesses, schools, municipal roads and greenery lands, requires both engineering stability (strength) and environmental safety of the solidified contaminated soil, and in addition, the advantage of strength is also an important factor affecting the toxic leaching amount of heavy metals, and if the waste is fixed in a form similar to cement, the strength can be significantly increased. In comparative examples 4 to 9, the strength advantage is more remarkable when the amount of the alkali activator used is slightly smaller than in examples 4 and 5 (laser gel ratio: 0.2 and 0.3, respectively).

Test example 2

Toxicity Leaching test

Test standards: the national environmental protection industry Standard "solid waste Leaching toxicity Leaching method sulfuric acid nitric acid method" (HJ/T299-2007).

The test process comprises the following steps: the preparation and curing processes of the samples of examples 1 to 9 and comparative example 1 were the same. And immediately taking the soil blocks in the broken test samples for toxicity leaching test after the standard cured 28d samples are subjected to the strength test. Weighing 20-30 g of soil blocks in a crucible, putting the crucible into a drying oven at 105 ℃ for drying, keeping the weight constant until the error of two weighing values is less than +/-1%, calculating to obtain the water content of a sample, then calculating the volume of a required leaching agent according to the water content of the sample and the liquid-solid ratio of 10: 1(L/Kg), and performing a test according to a method and steps specified in the solid waste leaching toxicity leaching method, namely sulfuric acid-nitric acid method (HJ/T299-2007). The test results are shown in Table 7.

The IV-class standard in the groundwater quality standard (GB/T14848-2017) is adopted as the identification standard of leaching toxicity in the experiment. The leaching toxicity results of heavy metal cadmium (Cd) in the embodiments 5 and 6 are less than 0.01mg/L, the IV-type standard is met, the results of the embodiments 1, 3 and 4 are slightly higher than the limit value of 0.01mg/L, and the laser glue ratio is too large, so that the leaching concentration value of the heavy metal cadmium (Cd) is slightly higher as in the embodiments 2, 7, 8 and 9, which indicates that the laser glue ratio has a very good curing effect at 0.2-0.4, and the curing effect is greatly reduced after the laser glue ratio exceeds the value. The results of comparative example 1 and example 3 show that the addition of nanomaterial can make Cd²⁺The leaching concentration of (2) is reduced.

TABLE 7 toxicity Leaching test results (mg/L)

Test example 3

Test for restoring soil acidity and alkalinity

Test standards: ASTM D4972-01

The test process comprises the following steps: after a sample subjected to standard maintenance for 28d is subjected to a strength test, immediately taking soil blocks inside a crushed sample to perform a pH test, stirring and mixing 10g of air-dried soil passing through a 1mm sieve and 10g of distilled water, standing for 1h, measuring, and testing 3 parallel samples to take an average value. The test results are shown in table 6.

TABLE 8 PH value test results

pH is an important factor affecting the morphological transformation and bioavailability of heavy metal cadmium (Cd) in soil, and in general, lower pH soil metals have higher activity. The change of the pH value of the solidified soil not only affects the solidification and stabilization effect of the heavy metal, but also determines the capability of a solidified body to resist external erosion to a great extent, such as acid rain corrosion, groundwater infiltration and carbonization, and the like, and in addition, the pH value of the solidified soil also affects the growth of plants and microorganisms (the pH value of the environment suitable for the survival of soil microorganisms is 6.5-9.5), and determines the mode and the degree of secondary utilization of a restoration site. As can be seen from Table 8, the pH value of the solidified soil body gradually rises by increasing the gel stimulation ratio, after the examples 4-7 are subjected to standard maintenance for 28d, the pH value of the solidified polluted soil is about 9 and is alkalescent, so that the utilization of a repair site is facilitated, the pH values of the soil bodies are slightly higher due to the large dosage of the alkali activator in the examples 8 and 9, which are respectively 9.49 and 9.62, and if the pH value is continuously increased, a plurality of problems are brought to the later development and utilization of the soil.

Test example 4

Conductivity EC test for restored soil

Test standards: japanese Chassis Industrial Association' basic Manual of soil texture testing

The test process comprises the following steps: after a sample subjected to standard maintenance for 28d is subjected to a strength test, immediately taking soil blocks in a crushed sample for a conductivity test, mixing dry soil and distilled water according to a mass ratio of 1: 5, stirring for 30min, standing for 2h, testing an EC value, testing 3 parallel samples, and taking an average value. The test results are shown in table 9.

TABLE 9 conductivity test results (ms/cm)

The soil pure water leachate contains abundant information such as salt content and ion composition in soil, the conductivity value of the leachate is closely related to activity energy, price, concentration and the like of soil electrolyte ions, the content of soluble salt in the soil can be reliably reflected, and the detection is quick and accurate, so that the soil conductivity (solid-to-liquid ratio of 1: 5) is often used as a quantitative index for measuring the salinization degree of the soil in practice. As can be seen from Table 9, the conductivity of the solidified soil was about 2ms/cm, and the content of the soluble salt was high.

Claims

1. A curing agent for high-concentration heavy metal cadmium polluted soil is characterized in that: the composite material comprises an alkali activator solution and gelled powder, wherein the mass ratio of the alkali activator solution to the gelled powder is 0.2-0.7, the alkali activator solution comprises a sodium silicate solution and sodium hydroxide, and the gelled powder comprises fly ash, granulated blast furnace slag and nano SiO₂And nano Al₂O₃。

2. The curing agent for high-concentration heavy metal cadmium polluted soil according to claim 1, which is characterized in that: the modulus of the alkali activator solution is 1-2.

3. The curing agent for high-concentration heavy metal cadmium polluted soil according to claim 1, which is characterized in that: the gelled powder comprises the following substances in parts by weight: 60-70 parts of fly ash, 30-40 parts of granulated blast furnace slag and nano SiO₂2-3 parts of nano Al₂O₃2 to 3 parts of。

4. The curing agent for high-concentration heavy metal cadmium polluted soil according to claim 1, which is characterized in that: the mass ratio of the alkali activator solution to the gelled powder is 0.2-0.4.

5. The curing agent for high-concentration heavy metal cadmium polluted soil according to claim 1, which is characterized in that: the fly ash is 1250-1400 meshes.

6. The curing agent for high-concentration heavy metal cadmium polluted soil according to claim 1, which is characterized in that: the granulated blast furnace slag is S95 grade.

7. The preparation method of the curing agent for the soil polluted by the high-concentration heavy metal cadmium according to claim 1, characterized by comprising the following steps:

secondly, after the polluted soil is passivated, adding the gelled powder and uniformly mixing;

and step four, adding distilled water and alkali activator solution into the polluted soil treated in the step two, and uniformly stirring.

8. The preparation method of the curing agent for the soil polluted by the high-concentration heavy metal cadmium according to claim 7, wherein the curing agent comprises the following components in percentage by weight: in the fourth step, the addition amount of the distilled water is 5-7 wt% of the mass of the polluted soil.