Heavy metal contaminated soil leaching remediation method

Abstract

The invention provides a heavy metal contaminated soil leaching remediation method, and relates to the technical field of soil remediation. The leaching remediation method for the heavy metal contaminated soil comprises the following steps that the heavy metal contaminated soil to be remedied is air-dried, ground and then sieved through a 2 mm sieve, leacheate is added in proportion, oscillation leaching and separation are conducted, leaching and separation operation is repeatedly conducted, the leacheate is recycled and regenerated to obtain regenerated leacheate, the soil is washed with deionized water to remove residual leacheate, air drying is conducted, and the soil is obtained. And repairing is completed. The leacheate is prepared from compound bacteria fermentation liquor, rhamnolipid, sodium humate and water; the compound bacteria fermentation liquor is compound fermentation liquor of bacillus seed liquor and aspergillus niger seed liquor. According to the method, an organic acid complexing dissolution-surfactant solubilizing leaching promoting-humic acid complexing stable synergistic mechanism is constructed, efficient leaching remediation of the soil compositely polluted by multiple heavy metals such as lead, cadmium, copper and zinc is achieved, meanwhile, the leaching liquid can be regenerated and recycled after being subjected to pH adjustment and precipitation treatment, and the method has a good application prospect.

Description

Technical Field

[0001] This invention relates to the field of soil remediation technology, and in particular to a method for leaching and remediation of heavy metal contaminated soil. Background Technology

[0002] Heavy metal contamination of soil is one of the major environmental problems facing the world. Heavy metals such as lead, cadmium, copper, and zinc enter soil in large quantities through mining, smelting, electroplating, and agricultural inputs. Because they are non-degradable and can accumulate in organisms, they pose a long-term threat to agricultural product safety, ecosystem function, and human health. Soil leaching remediation technology involves applying a leaching solution to contaminated soil, transferring solid heavy metals to the liquid phase for separation. It has advantages such as short remediation cycles and applicability to a wide range of heavy metals, making it one of the more mature ex-situ remediation methods currently used in engineering applications.

[0003] Existing leaching agents are mainly divided into two major systems: chemically synthesized and natural organic. Chelating chemical leaching agents, represented by EDTA, have high leaching efficiency for heavy metals, but their biodegradability in soil is extremely poor. After application, they easily cause vertical migration pollution of heavy metals and long-term residues of EDTA itself in the soil. Waste liquid requires special treatment, resulting in high treatment costs and significant secondary environmental risks. Single organic acid leaching agents, represented by citric acid and oxalic acid, have good biodegradability, but their complexing ability for different forms of heavy metals is limited. In particular, their desorption efficiency for heavy metals bound to iron and manganese oxides and organic matter is low. Furthermore, acidic single organic acid systems have insufficient stable carrying capacity for dissolved heavy metals, making them prone to re-adsorption, resulting in overall removal rates that are difficult to meet engineering requirements.

[0004] Biosurfactants have attracted attention due to their ability to significantly reduce soil-water interfacial tension and enhance the migration activity of heavy metals. However, studies have shown that biosurfactants, when used alone, have very limited ability to remove ionic heavy metals from soil. They only exert effective solubilizing and leaching-promoting effects when used in synergistic effects with complexing systems such as organic acids. In addition, existing bio-based leaching systems mostly use metabolites from single bacterial strains, resulting in relatively low organic acid yields and high purification costs, which restricts their large-scale application.

[0005] Therefore, it is of great significance to develop an efficient, low-cost, biodegradable, and recyclable waste liquid method for the leaching remediation of heavy metal contaminated soil. Summary of the Invention

[0006] In view of this, the present invention provides a method for leaching and remediation of heavy metal contaminated soil. The present invention prepares a complex bacterial fermentation broth rich in various organic acids through staged carbon source switching co-fermentation of Bacillus and Aspergillus niger. This broth is then compounded with rhamnolipids and sodium humate to construct a synergistic mechanism of organic acid complexation and dissolution—surfactant solubilization and leaching—humic acid complexation and stabilization. This achieves highly efficient leaching and remediation of soils contaminated with multiple heavy metals such as lead, cadmium, copper, and zinc. Furthermore, the leachate can be recycled after pH adjustment and precipitation treatment, demonstrating good prospects for engineering application and large-scale promotion.

[0007] The method for remediation of heavy metal contaminated soil by leaching according to the present invention includes the following steps: The heavy metal contaminated soil to be remediated is air-dried, ground, and sieved through a 2 mm sieve. Leaching solution is added in proportion, and the soil is shaken and leached. The leaching and separation operations are repeated. The leachate is recovered and regenerated to obtain regenerated leachate. The soil is washed with deionized water to remove residual leachate, and then air-dried to complete the remediation. The eluent, by mass percentage, has the following composition: The compound bacterial fermentation broth contains 40%-70% rhamnolipin, 0.5%-2.0% sodium humate, and the remainder is deionized water.

[0008] Preferably, the rinsing solution is added at a liquid-to-soil ratio of 4:1-5:1 (L / kg); the shaking rinsing is carried out at 20-30℃ and 150-200rpm for 6-10 hours; the rinsing and separation operations are carried out in 2-4 rounds, with fresh or regenerated rinsing solution used in each round; Preferably, the regeneration method of the eluent is as follows: collect the eluent, add NaOH to adjust the pH to 8.0-9.0, stir thoroughly for 20-40 min to precipitate, filter, and collect the metal hydroxide precipitate; adjust the pH of the obtained supernatant back to 3.5-4.5, and add rhamnolipid and sodium humate at 20%-35% of the initial addition amount to obtain the regenerated eluent.

[0009] Preferably, the soil is washed with deionized water, air-dried, and then tested for heavy metal content. Only after confirming that the target remediation value has been reached can it be backfilled or utilized as a resource.

[0010] Preferably, the preparation method of the compound bacterial fermentation broth is as follows: Bacillus seed culture and Aspergillus niger seed culture were inoculated into the culture medium, fermented in stages, and centrifuged to remove the bacterial cells. The resulting supernatant was the compound bacterial fermentation broth.

[0011] Preferably, the preparation method of the Bacillus seed culture is as follows: Bacillus is inoculated into LB liquid medium and cultured at 37°C and 180 rpm for 24 h to obtain Bacillus seed culture; the Bacillus is Bacillus subtilis ATCC 6051, and the OD of the Bacillus seed culture is... 600 It is 2.0-3.0.

[0012] Preferably, the preparation method of the Aspergillus niger seed solution is as follows: Aspergillus niger (CBS554.65) is inoculated into potato dextrose liquid medium (PDB) and cultured at 28°C and 150 rpm for 48 h to obtain the Aspergillus niger seed solution; the spore concentration of the Aspergillus niger seed solution is ≥1×10⁻⁶. 7 CFU / mL.

[0013] Preferably, the volume ratio of Bacillus seed liquid to Aspergillus seed liquid is 2:1, and the inoculation amount is 5% of the total volume of the culture medium.

[0014] Preferably, the culture medium is prepared as follows: an inorganic salt culture medium based on (NH4)2SO4 3.0 g / L, KH2PO4 1.0 g / L, MgSO4·7H2O 0.5 g / L, FeSO4·7H2O 0.01 g / L, and ZnSO4·7H2O 0.01 g / L is obtained after sterilization (121℃, 20 min).

[0015] The staged fermentation method is as follows: Phase 1 (0-48 h): The carbon source is glucose (initial concentration 30 g / L), the temperature is 30℃, the pH fluctuates naturally, the rotation speed is 150 rpm, and the dissolved oxygen is ≥20%. The main goal of this phase is to promote the rapid proliferation of the two bacterial species and establish a dominant bacterial community. Bacillus will simultaneously produce surfactant. The dissolved oxygen concentration is controlled by the aeration rate. The second stage (48-96 h): The carbon source was switched to industrial molasses waste liquid (total sugar content ≥45%, the amount added was based on 40 g / L of glucose equivalent), the rotation speed was reduced to 120 rpm, and the temperature was adjusted to 28℃. The rich sucrose and trace mineral elements in the molasses waste liquid induced Aspergillus niger to start the citric acid synthesis metabolic pathway on a large scale. At the same time, the surfactant produced by Bacillus continuously solubilized the bacterial metabolites, synergistically improving the total yield of organic acids. The third stage (96-144 h): KH2PO4 is added (final concentration increased to 2.0 g / L) to promote the synthesis of secondary metabolites such as oxalic acid, and fermentation continues until the total yield of organic acids plateaus; After fermentation, centrifuge at 5000 rpm for 15 min to remove the bacterial cells. The resulting supernatant is the compound bacterial fermentation broth (containing citric acid, oxalic acid, gluconic acid, malic acid and other organic acids).

[0016] The preparation method of the eluent is as follows: Take the compound bacterial fermentation broth, add sodium humate (dissolved in a small amount of warm deionized water first, and added after full swelling) and rhamnolipid (added directly and stirred to dissolve) in sequence according to the formula ratio, make up the volume with deionized water, stir evenly, and detect the pH. If the pH is lower than 3.5, adjust it with NaOH solution; if the pH is higher than 4.5, adjust it with citric acid to obtain the eluent.

[0017] Preferably, the citric acid equivalent concentration of the compound bacterial fermentation broth is ≥45 mmol / L; the molecular weight of the sodium humate is 5000-20000 Da, and the humic acid content is ≥70%; the pH of the leaching solution is 3.5-4.5, and the leaching solution is stored at 4°C for use and has a shelf life of 90 days.

[0018] Compared with the prior art, the beneficial technical effects of the present invention are as follows: This invention fully utilizes the complementary advantages of Bacillus and Aspergillus niger in organic acid synthesis and metabolism, resulting in a fermentation broth rich in multiple organic acids such as citric acid, oxalic acid, gluconic acid, and malic acid. This broth exhibits a synergistic complexing and dissolution effect on heavy metals in different forms (carbonate-bound, iron-manganese oxide-bound, and organic matter-bound), overcoming the limitations of single organic acid-based methods in terms of limited selectivity and low overall removal rate. Furthermore, this invention introduces low-cost industrial molasses wastewater as a carbon source, significantly reducing raw material costs while increasing the total yield of organic acids.

[0019] This invention constructs a triple synergistic mechanism of organic acid complexation and dissolution, surfactant solubilization and leaching promotion, and humic acid complexation and stabilization. All components used have good biodegradability and no long-term toxic residues to the soil micro-ecology. The waste liquid can be treated simply to recover heavy metals and regenerate the leaching liquid, resulting in high recycling rate and low waste liquid treatment cost. This avoids the problems of complex waste liquid disposal and high risk of secondary pollution in EDTA system.

[0020] The method of this invention demonstrates excellent remediation effects on soils contaminated with multiple heavy metals, including lead, cadmium, copper, and zinc, and has a wide range of applications. The leachate can be recycled for subsequent batches of soil leaching after regeneration treatment, further reducing overall remediation costs and showing promising application prospects. Detailed Implementation

[0021] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0022] Unless otherwise specified, all experiments were repeated three times, and the results are expressed as averages.

[0023] Example 1 Test soil: The soil (top 0-30 cm) in agricultural land surrounding a lead-zinc smelter is contaminated. The soil texture is clay loam, pH 6.8, organic matter content 3.2%, CEC 18.5 cmol / kg. Aqua regia digestion-ICP-OES analysis revealed the following initial heavy metal concentrations: Pb 820 mg / kg, Cd 58 mg / kg, Cu 312 mg / kg, Zn 524 mg / kg.

[0024] The soil is air-dried, ground, and sieved through a 2 mm sieve before use.

[0025] Preparation of compound bacterial fermentation broth: (1) Bacillus subtilis ATCC 6051 was inoculated into LB liquid medium and cultured at 37°C and 180 rpm for 24 h to obtain Bacillus seed culture (OD). 600 =2.7); Aspergillus niger (CBS 554.65) was inoculated into potato dextrose liquid medium (PDB) and cultured at 28°C and 150 rpm for 48 h to obtain Aspergillus niger seed culture (spore concentration of 1.1 × 10⁻⁶). 7 (CFU / mL) (2) An inorganic salt culture medium based on (NH4)2SO4 3.0 g / L, KH2PO4 1.0 g / L, MgSO4·7H2O 0.5 g / L, FeSO4·7H2O 0.01 g / L, and ZnSO4·7H2O 0.01 g / L was sterilized (121℃, 20 min) and then cooled for later use. (3) Inoculate the Bacillus seed solution and Aspergillus niger seed solution into the culture medium obtained in step S2 at a volume ratio of 2:1, with the inoculation amount being 5% of the total volume; Phase 1 (0-48 h): Carbon source: glucose (initial concentration 30 g / L), temperature: 30℃, pH fluctuates naturally, rotation speed: 150 rpm, dissolved oxygen: ≥20%; The second stage (48-96 h): the carbon source is switched to industrial molasses waste liquid (total sugar content is 48%, the amount added is based on 40 g / L of glucose equivalent), the rotation speed is reduced to 120 rpm, and the temperature is adjusted to 28℃. Phase 3 (96-144 h): KH2PO4 supplementation (final concentration increased to 2.0 g / L); The fermenter has a capacity of 10 L and a total fermentation volume of 8 L. After fermentation, the cells are removed by centrifugation at 5000 rpm for 15 min. The resulting supernatant is the compound bacterial fermentation broth.

[0026] Compared with single Bacillus fermentation (total citric acid equivalent concentration of 38.5 mmol / L) and single Aspergillus niger fermentation (total citric acid equivalent concentration of 55.1 mmol / L), the total yield of organic acids in the fermentation broth of the compound bacteria increased by 134% and 64%, respectively.

[0027] Preparation of rinsing solution: Take the fermentation broth of the compound bacteria (60 wt.%) and add the following in sequence: sodium humate: 2.0 g / 100 mL (2.0 wt.%), rhamnolipin: 1.0 g / 100 mL (1.0 wt.%), and deionized water to 100 mL. After mixing, the pH is measured to be 3.9. No adjustment is required. Set aside for later use.

[0028] Batch rinsing experiment: Weigh 50 g of sieved soil into a 250 mL Erlenmeyer flask, add 250 mL of compound leaching solution (liquid-to-soil ratio 5:1, L / kg), place in a constant temperature shaker, shake at 25℃ and 200 rpm for 8 h, centrifuge at 4000 rpm for 10 min to separate the leaching solution from the soil. Wash the soil once with deionized water, air dry, digest, and then test for residual heavy metal content. Determine the heavy metal concentration in the leaching solution. Repeat the above operation for a total of three rounds of leaching (using fresh leaching solution in each round).

[0029] The residual heavy metal content and total removal rate in the soil after three rounds of leaching are as follows:

[0030] Leachate recycling: Collect and combine the leachate from the three batches, adjust the pH to 8.5 with NaOH, stir thoroughly for 30 min to allow precipitation, filter to recover the metal hydroxide precipitate (which can be used as a smelting raw material), adjust the pH of the supernatant back to 3.8-4.0, replenish the lost rhamnolipin (approximately 30% of the initial amount) and sodium humate (approximately 25% of the initial amount), and it can be recycled for the second batch of soil leaching, with a recycling rate of ≥72%.

[0031] Example 2 Using the same soil and composite leaching solution as in Example 1, with a fixed single-round leaching time of 8 hours, and liquid-to-soil ratio gradients of 2:1, 3:1, 5:1, 8:1, and 10:1 (L / kg), a single-round leaching experiment was conducted, and the results are as follows:

[0032] Example 3 Test soil: Soil A (industrial origin, predominantly carbonate-bound): taken from an abandoned chemical plant, Cd 42 mg / kg, speciation analysis (BCR continuous extraction method) showed that carbonate-bound form accounted for 62%; Soil B (mine source, mainly iron-manganese oxide bound state): taken from a copper mine tailings area, Cu 580 mg / kg, iron-manganese oxide bound state accounted for 57%; Soil C (agricultural source, with a high proportion of organic matter bound): taken from a farmland that has been using sludge for a long time, Cd 18 mg / kg, organic matter bound 35%.

[0033] Leaching conditions: liquid-to-soil ratio 5:1, shaking for 8 hours, three rounds of leaching, conditions same as in Example 1.

[0034] Experimental results:

[0035] Example 4 Using the same test soil and compound bacterial fermentation broth (60 wt.%) as in Example 1, with a fixed liquid-to-soil ratio of 5:1, shaking conditions of 25℃ / 200 rpm / 8 h, and three rounds of rinsing, the effects of the dosage of rhamnolipin and sodium humate on the removal rates of Cd and Pb were investigated.

[0036] (1) Gradient experiment of rhamnolipin dosage (fixed sodium humate 2.0 wt.%):

[0037] (2) Experiment on the gradient of sodium humate dosage (fixed rhamnolipin 1.0 wt.%):

[0038] Comparative Example 1 Experimental Design:

[0039] After fermentation, each group prepared a leaching solution with the same solid-liquid ratio (60%), the same rhamnolipin (1.0%), and the same sodium humate (2.0%). This solution was then used to leach the same batch of contaminated soil in a single round (liquid-to-soil ratio 5:1, 8 h, initial Cd content 58 mg / kg). The results are as follows:

[0040] Comparative Example 2 Experimental design: (Each group was replaced or omitted with the corresponding component, liquid-to-soil ratio 5:1, 8 h, three rounds of rinsing)

[0041] The total removal rates of Cd and Pb after three rounds of rinsing are as follows:

[0042] When saponins were used to replace rhamnolipin, the Cd removal rate decreased because saponins are easily hydrolyzed and deactivated at pH < 4.5, and their stability is significantly weaker than that of rhamnolipin. Furthermore, their surface tension reduction effect (34.8 mN / m) is not as good as that of rhamnolipin (29.4 mN / m). The decrease in Cd removal rate after the absence of sodium humate indicates that the dissolved Cd... 2+ Significant re-adsorption occurs without sodium humate protection, and the complexing and stabilizing effect of sodium humate cannot be ignored. At the same organic acid concentration, the compound bacterial fermentation broth (containing multiple organic acids) of the present invention has a higher removal rate of Cd and Pb than that of a single equal volume purified citric acid solution, and the compound bacterial fermentation broth can be used directly to avoid the high-cost purification process.

[0043] Comparative Example 3 Experimental Design:

[0044] Using the soil tested in Example 1 (Pb 820 mg / kg, Cd 58 mg / kg), with a liquid-to-soil ratio of 5:1, shaking for 8 hours, and three rounds of leaching, the results are as follows:

[0045] In summary, this invention achieves a balance in terms of removal efficiency, biodegradability, reagent cost, and ease of wastewater treatment, and has good application prospects.

[0046] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. A method for leaching and remediation of heavy metal contaminated soil, characterized in that, Includes the following steps: The heavy metal contaminated soil to be remediated is air-dried, ground, and sieved through a 2 mm sieve. Leaching solution is added in proportion, and the soil is shaken and leached. The leaching and separation operations are repeated. The leachate is recovered and regenerated to obtain regenerated leachate. The soil is washed with deionized water to remove residual leachate, and then air-dried to complete the remediation. The eluent, by mass percentage, has the following composition: The compound bacterial fermentation broth contains 40%-70% rhamnolipin, 0.5%-2.0% sodium humate, and the remainder is deionized water. The preparation method of the compound bacterial fermentation broth is as follows: Bacillus seed culture and Aspergillus niger seed culture were inoculated into the culture medium, fermented in stages, and centrifuged to remove the bacterial cells. The resulting supernatant was the compound bacterial fermentation broth.

2. The method for leaching and remediation of heavy metal contaminated soil according to claim 1, characterized in that, The leaching solution is added at a liquid-to-soil ratio of 4:1-5:1 (L / kg).

3. The method for leaching and remediation of heavy metal contaminated soil according to claim 1, characterized in that, The oscillating rinsing is carried out at 20-30℃ and 150-200 rpm for 6-10 h; the rinsing and separation operations are carried out in 2-4 rounds, with fresh or regenerated rinsing solution used in each round.

4. The method for leaching and remediation of heavy metal contaminated soil according to claim 3, characterized in that, The regenerated leaching solution is prepared as follows: collect the leaching solution, add NaOH to adjust the pH to 8.0-9.0, stir thoroughly for 20-40 min to precipitate, filter, and collect the metal hydroxide precipitate; adjust the pH of the obtained supernatant back to 3.5-4.5, and supplement rhamnolipid and sodium humate at 20%-35% of the initial addition amount to obtain the regenerated leaching solution.

5. The method for leaching and remediation of heavy metal contaminated soil according to claim 1, characterized in that, The OD of the Bacillus seed liquid 600 It is 2.0-3.

0.

6. The method for leaching and remediation of heavy metal contaminated soil according to claim 1, characterized in that, The spore concentration of the Aspergillus niger seed liquid is ≥1×10⁻⁶. 7 CFU / mL.

7. The method for leaching and remediation of heavy metal contaminated soil according to claim 1, characterized in that, The volume ratio of Bacillus seed liquid to Aspergillus seed liquid is 2:1, and the inoculation amount is 5% of the total volume of the culture medium.

8. The method for leaching and remediation of heavy metal contaminated soil according to claim 1, characterized in that, The staged fermentation method is as follows: The first stage lasts 0-48 hours, using glucose as the carbon source; the second stage lasts 48-96 hours, using industrial molasses waste liquid as the carbon source; and the third stage lasts 96-144 hours, during which KH2PO4 is added to a final concentration of 2.0 g / L and fermentation continues for 48 hours. After fermentation, the bacterial cells are removed by centrifugation, and the resulting supernatant is the compound bacterial fermentation broth.

9. The method for leaching and remediation of heavy metal contaminated soil according to claim 1, characterized in that, The preparation method of the leaching solution is as follows: Take the compound bacterial fermentation broth, add sodium humate and rhamnolipin in sequence according to the formula ratio, make up the volume with deionized water, stir evenly, and obtain the leaching solution.

10. The method for leaching and remediation of heavy metal contaminated soil according to claim 1, characterized in that, The citric acid equivalent concentration of the compound bacterial fermentation broth is ≥45 mmol / L; the molecular weight of the sodium humate is 5000-20000 Da, and the humic acid content is ≥70%; the pH of the leaching solution is 3.5-4.5.

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