Geopolymer material for solidifying heavy metal contaminated soil and its preparation method

Abstract

The present invention discloses a geopolymer material for solidifying heavy metal-contaminated soil and a preparation method thereof, belonging to the field of soil pollution treatment. The geopolymer material comprises the following components in parts by weight: 100 parts of a geopolymer precursor; 2-5 parts of sodium alginate; 13-15 parts of an alkali activator; 60-62 parts of water; the alkali activator is a water glass solution with a modulus of 1.4; the geopolymer precursor comprises the following components in weight percentage: 45wt.% - 55wt.% of red mud; 5wt.% - 15wt.% of desulfurized gypsum; 30wt.% - 50wt.% of fly ash, which solves the problems of low compressive strength, high and unstable metal leaching concentration existing in the prior art.

Description

Technical Field

[0001] The present invention relates to the field of soil pollution treatment, and particularly relates to a geopolymer material for solidifying heavy metal contaminated soil and a preparation method thereof. Background Art

[0002] With the extensive exploitation of mineral resources, the large generation of waste tailings has become a global environmental problem. The toxic heavy metals contained in these tailings, such as lead (Pb 2+ ), zinc (Zn 2+ ), copper (Cu 2+ ), and cadmium (Cd 2+ ), etc., are prone to migrate and diffuse from the soil during the stacking process, posing a threat to the surrounding ecological environment and human health. Therefore, effective stabilization treatment of these tailings to prevent further environmental pollution by heavy metals has become an urgent technical problem to be solved.

[0003] As an effective contaminated soil treatment technology, the solidification / stabilization technology adds a solidifying agent to the contaminated soil, making it physically and chemically react with the heavy metals in the soil to form a solidification / stabilization product with a dense structure, strong compressive strength, and low permeability, thereby reducing the mobility of heavy metals in the soil and achieving the safety level specified in the remediation target. Chinese Patent with Publication No. CN106477985A discloses a red mud geopolymer material formula and a preparation method thereof, using red mud and fly ash as basic raw materials, and water glass solution and alkali solution as activators. The material ratio is by mass ratio, red mud: fly ash: water glass solution: alkali solution = 1: 1: 0.7: (0.25 - 0.3). A red mud-based geopolymer material is prepared at room temperature, and the highest strength can reach more than 40 MPa after 28 days. Chinese Patent with Publication No. CN114988728A discloses a method for fixing arsenic in neutralization slag using fly ash-red mud-based geopolymers. Fly ash, red mud, and desulfurized gypsum are mixed evenly to obtain a fly ash-red mud mixture. Sodium hydroxide is dissolved in calcium hydroxide solution and left standing for 5 - 10 h to obtain an alkali activator; the alkali activator is added to the fly ash-red mud mixture, and then neutralization slag is added and mixed evenly to obtain a slurry; the slurry is injected into a mold and sealed and left standing at room temperature for 30 - 33 h, and after demolding, it is cured at room temperature and a humidity of 80 - 90% for 7 - 28 days or more to obtain an arsenic-containing solidified product. However, the above technical solutions have the disadvantages of a large amount of solidifying agent added, low compressive strength, high heavy metal leaching concentration, and instability. Summary of the Invention

[0004] The present invention provides a geopolymer material for solidifying heavy metal contaminated soil and a preparation method thereof, which solves the problems of low compressive strength, high heavy metal leaching concentration, and instability existing in the prior art.

[0005] To solve this technical problem, the present invention provides the following technical solutions:

[0006] A geopolymer material for solidifying heavy metal contaminated soil, comprising the following components in parts by weight:

[0007] Geopolymer precursor: 100 parts;

[0008] Sodium alginate: 2 - 5 parts;

[0009] Alkali activator: 13 - 15 parts;

[0010] Water: 60 - 62 parts;

[0011] The alkali activator is a water glass solution with a modulus of 1.4;

[0012] The geopolymer precursor comprises the following components in weight percentage:

[0013] Red mud: 45wt.% - 55wt.%;

[0014] Desulfurized gypsum: 5wt.% - 15wt.%;

[0015] Fly ash: 30wt.% - 50wt.%.

[0016] In the geopolymer material for solidifying heavy metal contaminated soil provided by this solution, sodium alginate and Ca in desulfurized gypsum 2+ form a coordination polymer in the geopolymer precursor matrix and participate in the formation of N - A - S - H and C - A - S - H gels; these polymers fill the voids left by the geopolymer hydration products, increase the cross - link density, and enhance the compressive strength and heavy metal resistance performance; at the same time, sodium alginate can also complex or cross - link with heavy metal ions and the silicate skeleton, further improving the solidification / stabilization effect on heavy metal contaminated soil.

[0017] Preferably, the geopolymer material for solidifying heavy metal contaminated soil comprises the following components in parts by weight:

[0018] Geopolymer precursor: 100 parts;

[0019] Sodium alginate: 2 parts;

[0020] Alkali activator: 13 parts;

[0021] Water: 62 parts.

[0022] Preferably, the above - mentioned geopolymer precursor comprises the following components in weight percentage:

[0023] Red mud: 50wt.%;

[0024] Desulfurized gypsum: 10 wt.%;

[0025] Fly ash: 40 wt.%.

[0026] Preferably, the method for preparing the sodium silicate solution with a modulus of 1.4 is as follows:

[0027] Use sodium hydroxide to reduce the modulus of the sodium silicate solution with a modulus of 3.2 to 1.4. The Baume degree of the sodium silicate solution with a modulus of 3.2 is 40°Bé.

[0028] In the sodium silicate solution with a modulus of 3.2, n(SiO2) / n(Na2O) = 3.2, where the content of Na2O is 8.54% and the content of SiO2 is 27.3%, and the Baume degree is 40°Bé.

[0029] This solution also provides a preparation method for the geopolymer material for solidifying heavy metal contaminated soil as described above, including the following steps:

[0030] S1. Take solid sodium hydroxide to adjust the modulus of the sodium silicate solution to 1.4, stir and dissolve it, and then let it stand to obtain an alkali activator;

[0031] S2. Mix red mud, desulfurized gypsum and fly ash to obtain a geopolymer precursor;

[0032] S3. Mix the alkali activator, sodium alginate solid and supplementary water in proportion and stir evenly to obtain a composite alkali activator;

[0033] S4. Add the composite alkali activator to the geopolymer precursor and stir evenly to obtain a geopolymer material;

[0034] Steps S2 and S3 can be interchanged.

[0035] Preferably, the standing time in step S1 is ≥ 24 hours.

[0036] Preferably, the stirring time in step S3 is ≥ 5 min.

[0037] When the geopolymer material of the present invention is applied, the prepared material can be directly mixed with the soil contaminated by single ions of Pb(II), Zn(II), Cu(II), and Cd(II), which is suitable for large-scale application. Sodium alginate SA and Ca 2+ Form a coordination polymer in the geopolymer matrix and participate in the formation of N-A-S-H and C-A-S-H gels. These polymers fill the voids left by the geopolymer hydration products, increase the crosslinking density, and enhance the compressive strength and heavy metal resistance performance. At the same time, SA can also complex or crosslink with heavy metal ions and the silicate backbone, further improving the solidification / stabilization effect on heavy metal contaminated soil.

[0038] Compared with the prior art, the present invention has the following advantages:

[0039] The sodium alginate enhanced composite geopolymer material SA-AGP has remarkable effects in solidifying / stabilizing heavy metal contaminated soil. Compared with AGP without sodium alginate, during the preparation of geopolymer, the natural material sodium alginate is introduced to form the enhanced composite geopolymer material SA-AGP prepared from sodium alginate, precursors (red mud, gypsum, fly ash) and sodium silicate. The geopolymer prepared by adding sodium alginate to the alkali activator is used to solidify / stabilize heavy metal contaminated soil, which can significantly reduce the addition amount of the material. And due to the addition of sodium alginate, the geopolymer material SA-AGP can significantly improve the compressive strength of the contaminated soil, reduce the leaching concentration of heavy metals, and convert heavy metals into a more stable form. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] The drawings described herein are used to provide a further understanding of the embodiments of the present invention, form a part of this application, and do not limit the embodiments of the present invention. In the drawings:

[0041] Figure 1 It is a schematic diagram of the preparation method of the geopolymer material of the present invention.

[0042] Figure 2 It is the influence of the addition amount of SA-AGP and curing time on the compressive strength of heavy metal contaminated soil; wherein, Figure 2 (a) is the influence of SA-AGP on the compressive strength of zinc contaminated soil; Figure 2 (b) is the influence of SA-AGP on the compressive strength of lead contaminated soil; Figure 2 (c) is the influence of SA-AGP on the compressive strength of copper contaminated soil; Figure 2 (d) is the influence of SA-AGP on the compressive strength of cadmium contaminated soil.

[0043] Figure 3 It is the influence of the addition amount of SA-AGP and curing time on the toxic leaching concentration of contaminated soil; wherein, Figure 3 (a) is the influence of SA-AGP on the toxic leaching of zinc contaminated soil; Figure 3 (b) is the influence of SA-AGP on the toxic leaching of lead contaminated soil; Figure 3 (c) is the influence of SA-AGP on the toxic leaching of copper contaminated soil; Figure 3 (d) is the influence of SA-AGP on the toxic leaching of cadmium contaminated soil.

[0044] Figure 4 It is the occurrence form of heavy metals in soil; wherein, Figure 4 (a) is the influence of SA-AGP on the toxic leaching of zinc contaminated soil; Figure 4(b) shows the effect of SA-AGP on the toxic leaching of lead-contaminated soil; Figure 4 (c) shows the effect of SA-AGP on the toxic leaching of copper-contaminated soil; Figure 4 (d) shows the effect of SA-AGP on the toxic leaching of cadmium-contaminated soil. **Detailed implementation manners**

[0045] To make the objectives, technical solutions and advantages of the present invention clearer and more understandable, the following further elaborates on the present invention in combination with embodiments. The illustrative implementation manners and descriptions of the present invention are only used to explain the present invention and do not limit the present invention.

[0046] **Embodiment 1**

[0047] The geopolymer precursor includes the following components by weight percentage:

[0048] Red mud: 50 wt.%;

[0049] Desulfurized gypsum: 10 wt.%;

[0050] Fly ash: 40 wt.%.

[0051] The geopolymer material for solidifying heavy metal-contaminated soil, which contains the above geopolymer precursor, includes the following components by weight:

[0052] Geopolymer precursor: 100 parts;

[0053] Sodium alginate: 2 parts;

[0054] Alkali activator: 13 parts;

[0055] Water: 62 parts.

[0056] The preparation method of the geopolymer material SA-AGP is as Figure 1 shown. Take 4.8 mol of solid sodium hydroxide to adjust the modulus of 1000 g of water glass solution with a modulus of 3.2 (Baumé: 40°Bé) to 1.4, fully stir and dissolve, and let it stand for at least 24 h to obtain the alkali activator. First, weigh red mud, desulfurized gypsum and fly ash at 50 wt.%, 10 wt.% and 40 wt.% respectively and thoroughly mix them to obtain the geopolymer precursor. Subsequently, the alkali activator, solid sodium alginate and supplementary water are mixed and stirred for 5 min until completely uniform according to the total mass percentages of 2 wt.%, 13 wt.% and 62 wt.% of the geopolymer precursor solid to obtain the composite alkali activator, and then it is added to the geopolymer precursor and stirred evenly to obtain the geopolymer material SA-AGP.

[0057] **Embodiment 2**

[0058] The geopolymer material for solidifying heavy metal-contaminated soil includes the following components by weight:

[0059] 100 parts of geopolymer precursor;

[0060] 5 parts of sodium alginate;

[0061] 15 parts of alkali activator;

[0062] 60 parts of water;

[0063] The above geopolymer precursor includes the following components by weight percentage:

[0064] 45 wt.% of red mud;

[0065] 5 wt.% of desulfurized gypsum;

[0066] 50 wt.% of fly ash;

[0067] The preparation method of the geopolymer material is as follows: Take 4.8 mol of solid sodium hydroxide to adjust the modulus of 1000 g of water glass solution with a modulus of 3.2 (Baumé: 40°Bé) to 1.4, fully stir and dissolve, and let it stand for at least 24 h to obtain the alkali activator. Weigh red mud, desulfurized gypsum and fly ash at 45 wt.%, 5 wt.% and 50 wt.% respectively and mix them thoroughly to obtain the geopolymer precursor. Then, the alkali activator, solid sodium alginate and supplementary water are mixed and stirred for more than 5 min according to 5 wt.%, 15 wt.% and 60 wt.% of the total mass percentage of the geopolymer precursor solid until it is completely uniform to obtain the composite alkali activator, and then add it to the geopolymer precursor and stir evenly to obtain the geopolymer material.

[0068] Example 3

[0069] The geopolymer material for solidifying heavy metal contaminated soil includes the following components by weight:

[0070] 100 parts of geopolymer precursor;

[0071] 3 parts of sodium alginate;

[0072] 14 parts of alkali activator;

[0073] 61 parts of water.

[0074] The above geopolymer precursor includes the following components by weight percentage:

[0075] 55 wt.% of red mud;

[0076] 15 wt.% of desulfurized gypsum;

[0077] 30 wt.% of fly ash;

[0078] The preparation method of the geopolymer material is as follows: 4.8 mol of sodium hydroxide solid is used to adjust the modulus of 1000 g of water glass solution (Baume: 40°Bé) with a modulus of 3.2 to 1.4, and the solution is fully stirred and dissolved and allowed to stand for at least 24 hours to obtain an alkali activator. First, red mud, desulfurized gypsum and fly ash are weighed at 55wt.%, 15wt.%, and 30wt.% respectively and thoroughly mixed to obtain a geopolymer precursor, and then the alkali activator, sodium alginate solid and supplemented water are mixed and stirred for more than 5 minutes at 3wt.%, 14wt.% and 61wt.% of the total mass percentage of the geopolymer precursor solid respectively until completely uniform to obtain a composite alkali activator, which is then added to the geopolymer precursor and stirred to obtain a geopolymer material.

[0079] Comparative Example 1

[0080] The only difference between this comparative example and Example 1 is that sodium alginate is not contained; the geopolymer material comprises the following components in parts by weight:

[0081] 100 parts of geopolymer precursor;

[0082] 13 parts of alkali activator;

[0083] 62 parts of water;

[0084] The geopolymer precursor comprises the following components in percentage by weight:

[0085] Red mud 50wt.%;

[0086] Desulfurized gypsum 10wt.%;

[0087] Fly ash 40wt.%;

[0088] The preparation method of geopolymer material AGP is as follows: take 4.8 mol of sodium hydroxide solid to adjust the modulus of 1000g of water glass solution (Baume: 40°Bé) with a modulus of 3.2 to 1.4, fully stir to dissolve and stand for at least 24 hours to obtain an alkali activator. First, red mud, desulfurized gypsum and fly ash are weighed at 50wt.%, 10wt.%, and 40wt.% respectively and thoroughly mixed to obtain a geopolymer precursor, and then the alkali activator and the supplemented water are mixed and stirred for 5 minutes at 13wt.%, 62wt.% of the total mass percentage of the geopolymer precursor solids, respectively, until completely uniform to obtain a composite alkali activator, which is then added to the geopolymer precursor and stirred to obtain AGP.

[0089] According to CJJ / T 286-2018, the addition amount of soil stabilizer accounts for 3-15% of the dry soil mass percentage. To verify the influence of sodium alginate on the material properties and the properties of the material, by controlling variables, SA-AGP prepared in Example 1 and AGP prepared in Comparative Example 1 were added to the soil contaminated with single ions of Pb(II), Zn(II), Cu(II), and Cd(II) at 3%, 6%, 12%, and 15% of the dry soil mass percentage respectively. After stirring the neat paste at 80 rad / s, it was loaded into a mold (diameter 50 mm, height 50 mm), and the cured samples were cured at room temperature (temperature 25°C, humidity 60%) for (7-28 days).

[0090] Figure 2 It shows the influence of the addition amount of SA-AGP (or AGP) (3%-15%) and the curing time (7-28 days) on the unconfined compressive strength of heavy metal contaminated soil. With the increase of the SA-AGP addition amount, the unconfined compressive strength of the soil mass increases significantly. As Figure 2 (a), Figure 2 (b), Figure 2 (c), Figure 2 (d) shows, when the SA-AGP addition amount is 3 wt.%, after curing for 28 days, the unconfined compressive strengths of the soil contaminated with Pb 2+ , Zn 2+ , Cu 2+ and Cd 2+ are 1.45, 1.60, 1.54, and 1.79 Mpa respectively, which are 103.0%, 113.9%, 50.9%, and 95.83% higher than those of the control example AGP with 3 wt.% respectively. At the same time, when the AGP addition amount is 6 wt.%, it is basically the same as the unconfined compressive strength of 3 wt.% SA-AGP, indicating that introducing SA in the preparation process of geopolymers can significantly improve the mechanical properties of the cured soil mass. When the addition amount increases to 15 wt.%, after curing for 28 days, the unconfined compressive strengths of the soil contaminated with Pb 2+ , Zn 2+ , Cu 2+ and Cd 2+ by SA-AGP are 2.44, 3.62, 1.54, and 1.79 Mpa respectively.

[0091] Figure 3 It shows the influence of the addition amount of SA-AGP (or AGP) (3%-15%) and the curing time (7-28 days) on the toxicity leaching concentration of the contaminated soil. As Figure 3 (a), Figure 3 (b), Figure 3 (c), Figure 3 (d) shows, after adding 3 wt.% SA-AGP and curing for 28 days, the Pb 2+ , Zn 2+, Cu 2+ and Cd 2+ The leaching concentrations were 1.101, 2.012, 1.569, and 1.855 mg / L respectively, and their solidification efficiencies were 83.3%, 70.7%, 74.9%, and 72.7% respectively. The leaching concentration of AGP was basically the same as that of 3wt.% SA-AGP only when 6wt.% was added, indicating that the introduction of SA during the preparation of geopolymers can significantly improve the retardation performance of heavy metals. After curing the soil with 15wt.% SA-AGP for 28 days, the leaching concentrations of Pb 2+ , Zn 2+ , Cu 2+ and Cd 2+ were 0.015, 0.712, 0.230, and 0.524 mg / L respectively. Compared with the untreated contaminated soil, when the addition amount was 15wt.%, the leaching concentrations of the four heavy metals could be reduced by more than 90%. The solidification efficiency of SA-AGP for heavy metals from high to low was Pb 2+ > Cu 2+ > Cd 2+ > Zn 2+ .

[0092] Figure 4 For the occurrence forms of heavy metals in soil, SA-AGP can effectively convert heavy metals into more stable residual forms. As shown in Figure 4 (a), Figure 4 (b), Figure 4 (c), Figure 4 (d), after curing the contaminated soil of Pb 2+ , Zn 2+ , Cu 2+ and Cd 2+ for 28 days, the contents of the residual form (F4) of the solidified soil with 3wt.% SA-AGP added were 46%, 17%, 30%, and 35% respectively. Compared with 3wt.% AGP, the acid-soluble form (F1) that was more easily migratory decreased by 30%, 8%, 10%, and 11% respectively, and the content of the residual form (F4) increased by 38%, 20%, 14%, and 9% respectively. When the addition amount of SA-AGP was increased to 15wt.%, the contents of the heavy metal residual form could be increased to 72%, 46%, 53%, and 49%. After treatment with SA-AGP, most of the heavy metals in the soil were converted into stable residual forms (F4), greatly reducing the migration ability of heavy metals. At the same time, it was again shown that the solidification efficiency order of SA-AGP for heavy metals was Pb 2+ > Cu 2+ > Cd 2+ > Zn 2+ .

[0093] In summary, the sodium alginate (SA)-enhanced composite geopolymer (SA-AGP) provided by this solution is used for solidifying / stabilizing heavy metal-contaminated soil. During the preparation of the geopolymer, natural material sodium alginate is introduced to form an enhanced composite geopolymer (SA-AGP) prepared from sodium alginate, precursors (red mud, gypsum, fly ash), and sodium silicate. The effects of the addition amount of SA-AGP and the curing time on the unconfined compressive strength of the soil mass, the leaching concentration of heavy metals, and the occurrence forms of heavy metals are analyzed: Pb with a concentration of 500 mg / kg each 2+ , Zn 2+ , Cu 2+ and Cd 2+ After 28 days of curing the contaminated soil, the compressive strengths of the solidified soil masses with 3% SA-AGP added are 1.45, 1.60, 1.54, and 1.79 MPa respectively, and the leaching concentrations are 1.101, 2.012, 1.569, and 1.855 mg / L respectively. Compared with the leaching concentrations of the untreated contaminated soil, they are reduced by 83.3%, 70.7%, 74.9%, and 72.7% respectively, and the contents of the residual state (F4) are 46%, 17%, 30%, and 35% respectively. When the addition amount is gradually increased to 15 wt.%, the contents of the heavy metal residual state can be increased to 72%, 46%, 53%, and 49%. The order of the solidification / stabilization efficiency of the geopolymer for heavy metals is Pb 2+ > Cu 2+ > Cd 2+ > Zn 2+ . When sodium alginate is added to the alkali activator and then the prepared geopolymer is used for solidifying / stabilizing heavy metal-contaminated soil, the addition amount of the material can be significantly reduced.

[0094] The specific implementation manners described above have further detailed the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above are only the specific implementation manners of the present invention and are not used to limit the protection scope of the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A geopolymer material for solidifying heavy metal contaminated soil, characterized in that, It comprises components in the following parts by weight: Geopolymer precursor: 100 parts; Sodium alginate: 2 - 5 parts; Alkali activator: 13 - 15 parts; Water: 60 - 62 parts; The alkali activator is a sodium silicate solution with a modulus of 1.4; The geopolymer precursor comprises components in the following weight percentages: Red mud: 45wt.% - 55wt.%; Desulfurized gypsum: 5wt.% - 15wt.%; Fly ash: 30wt.% - 50wt.%; The addition amount of the geopolymer material for solidifying heavy metal - contaminated soil accounts for 3 - 15% of the dry soil mass percentage.

2. The geopolymer material for solidifying heavy metal contaminated soil according to claim 1, characterized in that, It comprises components in the following parts by weight: Geopolymer precursor: 100 parts; Sodium alginate: 2 parts; Alkali activator: 13 parts; Water: 62 parts.

3. The geopolymer material for solidifying heavy metal contaminated soil according to claim 2, characterized in that, The geopolymer precursor comprises components in the following weight percentages: Red mud: 50wt.%; Desulfurized gypsum: 10wt.%; Fly ash: 40wt.%.

4. The geopolymer material for solidifying heavy metal contaminated soil according to any one of claims 1-3, characterized in that, The preparation method of the sodium silicate solution with a modulus of 1.4 is as follows: Using sodium hydroxide to reduce the modulus of the sodium silicate solution with a modulus of 3.2 to 1.4, and the Baume degree of the sodium silicate solution with a modulus of 3.2 is 40°Bé.

5. The preparation method of the geopolymer material for solidifying heavy metal contaminated soil according to any one of claims 1-4, characterized in that, It comprises the following steps: S1. Take solid sodium hydroxide to adjust the modulus of the sodium silicate solution to 1.4, stir to dissolve and let it stand to obtain the alkali activator; S2. Mix red mud, desulfurized gypsum and fly ash to obtain the geopolymer precursor; S3. Mix the alkali activator, solid sodium alginate and supplementary water in proportion and stir evenly to obtain the composite alkali activator; S4. Add the composite alkali activator to the geopolymer precursor and stir evenly to obtain the geopolymer material; Steps S2 and S3 can be interchanged.

6. The preparation method of the geopolymer material for solidifying heavy metal contaminated soil according to claim 5, characterized in that, The standing time in step S1 is ≥ 24 hours.

7. The preparation method of the geopolymers material for solidifying heavy metal contaminated soil according to claim 5, characterized in that, The stirring time in step S3 is ≥ 5 min.

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