Preparation method and application of environment-friendly stabilizer with high selectivity to copper

Abstract

The application discloses a preparation method and application of an environmentally friendly Cu high-selectivity stabilizer. The preparation method comprises the following steps: under certain conditions, 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC) and N-hydroxysuccinimide (NHS) are used to activate the carboxyl group on L-cysteine; after chitosan powder is added, the pH is adjusted until a light blue purple precipitate appears; and then subsequent treatment is performed to obtain an amino acid modified chitosan porous material. The raw material is easy to obtain, the preparation method is simple, and the reaction condition is mild; the obtained stabilizer can efficiently, quickly and selectively adsorb Cu, can effectively stabilize Cu in soil, can reduce the toxicity of Cu on plants, can cooperate with ryegrass to repair Cu in sludge, and can effectively control the leaching risk of heavy metal Cu in sludge compost soil.

Description

Technical Field

[0001] This invention relates to the field of heavy metal stabilizers in the treatment of heavy metal pollution in soil, specifically to a method for preparing an environmentally friendly, highly selective Cu stabilizer and its application in phytoremediation of heavy metals. Background Technology

[0002] Sludge from aerobic composting is rich in nitrogen, phosphorus, and organic matter, making it a high-quality fertilizer that promotes plant growth and improves soil structure through microbial decomposition. The application of compost sludge in landscaping not only avoids the risk of heavy metals entering the human body through the food chain but also allows plants to extract heavy metals from the soil, reducing their concentration and potential risks. However, compost sludge contains high levels of heavy metals. Although these heavy metals may be passivated during fermentation, reducing their mobility and bioavailability, the total amount of heavy metals is not reduced. Furthermore, due to the limited extraction rate of plants, it may lag behind the activation rate of soil microorganisms and plant root exudates. Therefore, during the rainy season, these reactivated heavy metals may be washed into surrounding water bodies by rainwater, threatening ecosystem safety.

[0003] A survey of sludge from wastewater treatment plants in 40 different regions of my country revealed that the heavy metal content, from highest to lowest, was Zn > Cu > Cr > Pb > Ni > As > Cd, with Cu ranking second. In sludge from southern my country, the highest Cu concentration reached 2867 mg / kg. Currently, my country's "Integrated Wastewater Discharge Standard" (GB8978-1996) clearly classifies copper as a Class II pollutant, with a primary discharge limit of 0.5 mg / L. The "Surface Water Environmental Quality Standard" (GB3838-2002) sets the standard limit for copper at 1.0 mg / L. Given the low tolerance threshold of the environment for copper (Cu) and its sensitive impact on the soil ecological environment, we must pay close attention to the cumulative effect of copper in composted sludge and its potential environmental risks when promoting the resource utilization of sludge, especially its application in landscaping. This requires a detailed assessment of the concentration level and migration activity of Cu in composted sludge to ensure effective control of its migration in the soil environment, thereby reducing its potential harm to the ecological environment beyond the soil. On the other hand, although the overall concentration and content of heavy metals in sludge may be high, ecotoxicological studies have shown that the ecological risk of heavy metals is closely related to their chemical forms in sludge. Different forms of heavy metals determine their mobility, bioavailability, and potential ecotoxicity in the soil environment. Therefore, the focus of research is on the specific forms of heavy metals, rather than just the total amount. Internationally, there are different detection methods for the forms of heavy metals in soil. Currently, the most commonly used method is the BCR continuous extraction method. Heavy metal forms, ranked from lowest to highest stability, are weakly acid-extractable (BCR1), reducible (BCR2), oxidizable (BCR3), and residual (BCR4). Sludge composting treatment increases the stability of Cu, reducing its potential migration rate and bioavailability. Organic matter accounts for 76.50% of the total Cu content. These organically bound Cu forms may transform into more active forms under specific environmental conditions or microbial activity, thereby increasing their mobility and bioavailability in the soil. Therefore, when assessing the environmental behavior of copper in soil and its impact on ecosystems, the dynamic changes of these organically bound copper and the risk of potential release must be taken into account. In particular, the heavy metal Cu exhibits high percolation characteristics under acidic conditions.

[0004] Heavy metal stabilization is the most effective way to control heavy metal leaching. Common stabilizers include bentonite, lime, and biochar. Yang Lan et al. studied the effect of natural zeolite on Pb leaching. 2 The adsorption effects of natural zeolite on heavy metal ions such as Cu2+, Mn2+, and Zn2+ were observed, revealing its effectiveness against Pb. 2The adsorption capacity for Cu²⁺ was the strongest, with a maximum adsorption capacity of 105.26 mg / g. The adsorption capacity for Cu²⁺, Mn²⁺, and Zn²⁺ decreased in that order, with maximum adsorption capacities of 14.22 mg / g, 12.36 mg / g, and 7.30 mg / g, respectively. Chen Weiqin et al. studied the adsorption capacity of bentonite for Cu²⁺ in water. 2+ Zn 2+ The adsorption potential of bentonite was demonstrated, and the results showed that under suitable reaction conditions, bentonite adsorbed Cu... 2+ and Zn 2+ The maximum adsorption capacity can reach 60 mg / g and 55 mg / g. Zhou Wendi discovered that modified hemp stalk biochar can efficiently remove Cu from water. 2+ and Ni 2+ The maximum adsorption capacities of these adsorbents can reach 25.25 mg / g and 57.57 mg / g. The above adsorbents exhibit good adsorption for Cu... 2+ The maximum adsorption capacity is less than 60 mg / g, and there is an affinity for Cu. 2+ The drawbacks of conventional stabilizers include weak selectivity and even preferential adsorption of other metal ions. Because these conventional stabilizers lack selectivity for Cu, adsorption sites are often occupied by other heavy metals during application. To achieve the desired Cu adsorption effect, a sufficient amount of stabilizer needs to be added, which not only increases costs but also affects soil structure and physicochemical properties, inhibits plant extraction of nutrients and other heavy metals, and significantly reduces ecological and environmental benefits. Therefore, this invention aims to provide a stabilizer with high selective Cu2+ adsorption, which can be used in conjunction with phytoremediation technology. Through the implementation of this invention, the risk of Cu leaching can be effectively controlled, while ensuring that plant extraction of other heavy metals from the soil is not affected, thereby achieving precise treatment and ecological restoration of soil heavy metal pollution. The development of this stabilizer is of significant practical importance and urgency in promoting the development of environmentally friendly soil remediation technologies and meeting current environmental protection needs.

[0005] Chitosan, a natural polysaccharide, is obtained by deacetylation of chitin, a process involving boiling the chitin in potassium hydroxide. Chitin, primarily derived from the shells of insects and marine crustaceans, is the second most abundant biopolymer in nature. Chitosan is not only biodegradable and inexpensive, but its numerous active functional groups on its molecular backbone, including amino (-NH2) and hydroxyl (-OH) groups, provide abundant active adsorption sites, resulting in excellent adsorption capacity for heavy metals. Modification techniques such as cross-linking, grafting, combining with other adsorbent materials, and ion-templating can further enhance its adsorption performance and selectivity for Cu. Summary of the Invention

[0006] This invention aims to address the challenges of controlling the bioavailability of copper (Cu) during the utilization of sludge in landscaping to prevent excessive levels from harming plant growth, and to avoid the difficulty in controlling the risk of copper leaching during heavy or acidic rainfall. To this end, this invention provides a selective Cu adsorption soil heavy metal stabilizer with a simple synthesis method and low cost, along with its application. Furthermore, this invention investigates the role of the stabilizer in co-phytoremediation processes. Simultaneously, the bioavailability and leaching risk of the stabilizer in soil heavy metals were evaluated in practical applications, providing data support and technical guidance for the resource utilization of sludge. The objectives of this invention are achieved based on the following technical solutions:

[0007] The first aspect of the present invention is to provide a method for preparing a stabilizer with high selectivity for Cu.

[0008] Preferably, the preparation method specifically includes the following steps:

[0009] S1: Weigh 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC), N-hydroxysuccinimide (NHS) and L-cysteine ​​in sequence and add them to a round-bottom flask. Then add a certain volume of water and stir magnetically at a constant temperature to activate the reactivity of the carboxyl group on L-cysteine, making it easier for it to react with molecules containing amine groups to form a stable amide bond.

[0010] S2: Add chitosan powder to the mixture in step S1, then add hydrochloric acid aqueous solution to dissolve the chitosan, and continue stirring the reaction for 8-12 hours (more preferably 10 hours);

[0011] S3: Slowly add NaOH aqueous solution to the mixture in step S2 at a rate of about 2 to 3 drops per second, and measure the pH value in real time with precise pH test paper. Adjust the pH of the mixture to 7 to 8. When a light blue-purple precipitate appears, continue stirring for 2 to 4 hours (preferably 3 hours) to allow the precipitate to be evenly distributed.

[0012] S4: Take out the mixed solution from step S3, centrifuge to collect the solid, and the resulting precipitate is post-processed to obtain amino acid-modified chitosan porous material.

[0013] Preferably, in step S1, the masses of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC), N-hydroxysuccinimide (NHS), and L-cysteine ​​are M1 0.51-0.65 parts, M2 0.25-0.35 parts, and M3 0.9-1.1 parts, respectively; and in step S1, the mass of water added is 40-60 times that of the added powder (M1+M2+M3).

[0014] Preferably, in step S1, the conditions for constant temperature stirring are: temperature 20-30°C, time 0.5-2h; more preferably, temperature 25°C, constant temperature stirring time 1h.

[0015] Preferably, in step S2, the mass of chitosan powder added is 0.8-1.2 g. That is, the mass ratio of chitosan powder added to 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride is 0.8-1.2:0.51-0.65.

[0016] Preferably, in step S2, the concentration of the hydrochloric acid aqueous solution added is 0.10-0.15 mol / L, and the volume is 6-10 ml. That is, the ratio of the hydrochloric acid aqueous solution to the chitosan powder is 6-10 mL: 0.8-1.2 g.

[0017] Preferably, in step S2, the reaction is continued at 20°C to 30°C for 8 to 12 hours. More preferably, the temperature of the reaction is 25°C.

[0018] Preferably, in step S3, the concentration of the NaOH aqueous solution is 0.3-1.2 mol / L.

[0019] Preferably, in step S4, the post-treatment includes: washing twice with anhydrous ethanol and twice with deionized water (to neutrality), then freezing in an ultra-low temperature freezer for 0.5 to 2 hours (more preferably 1 hour), and after the precipitate is completely frozen, placing it in a freeze dryer for freeze drying for 68 to 76 hours (more preferably 72 hours).

[0020] A second aspect of the present invention is to provide an environmentally friendly, highly selective Cu stabilizer, prepared according to the above-described preparation method.

[0021] A third aspect of the present invention provides an environmentally friendly, highly selective Cu-chitosan-based stabilizer for selectively adsorbing Cu and controlling the risk of Cu leaching from sludge, while not affecting the enrichment and extraction of other heavy metals besides Cu by plants.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] 1. This invention synthesizes Cys-CS by grafting cysteine ​​onto chitosan. The chitosan-modified heavy metal adsorbent is used as a soil heavy metal stabilizer. The selective adsorption of Cys-CS in five mixed heavy metal solutions was compared. Infrared spectroscopy was used to analyze the main adsorption groups in the material that selectively adsorb heavy metals, thus inferring the selective adsorption relationship between the groups and the heavy metals. Furthermore, the adsorption behavior and adsorption kinetics of the material in single-metal and mixed-metal solutions were compared to infer the adsorption mechanism. In this work, through adsorption kinetics and isothermal adsorption studies, the adsorption capacity of Cys-CS was found to be 97.8 mg / g in the presence of Zn. 2+ Pb 2+ Cd 2+ Ni 2+ In a multi-component system where interfering ions are present, Cys-CS adsorbent for Cu 2+ It exhibits excellent selective adsorption performance. The partition coefficient (k...) is used... d As an indicator of the retention efficiency of adsorbents for heavy metal ions, k is used to measure the efficiency of adsorbents for heavy metal ions. d A direct comparative analysis of the values ​​was conducted to evaluate and compare the adsorption performance of different adsorbents for target heavy metal ions. The partition coefficients of Cys-CS for Cu were 11.09–118 times that of Pb, 12.17–84 times that of Zn, 8.64–32.97 times that of Cd, and 32.64–46.40 times that of Ni, indicating that the stabilizer of the present invention can tightly bind to Cu(II) in terms of spatial structure and binding sites, exhibiting good spatial matching and specific selective recognition performance. Compared with existing metal stabilizers, the chitosan-based selective stabilizer prepared in this invention can efficiently, rapidly, and selectively adsorb Cu, effectively stabilize Cu in soil, reduce toxicity to plants, synergistically remediate Cu in sludge with ryegrass, and effectively control the leaching risk of heavy metal Cu in sludge compost.

[0024] 2. The present invention provides an environmentally friendly and highly selective chitosan-based stabilizer preparation method with simple steps, mild and non-harsh reaction conditions, safe process, and no need for specific reaction equipment. Attached Figure Description

[0025] Figure 1 The images shown are SEM and EDS images of the stabilizer of this invention.

[0026] Figure 2 This is the chemical reaction formula for the stabilizer of the present invention.

[0027] Figure 3 This is the XRD pattern of the stabilizer of the present invention.

[0028] Figure 4This is the FTIR spectrum of the stabilizer of the present invention.

[0029] Figure 5 This diagram illustrates the adsorption of five heavy metals by the stabilizer of this invention at different pH values.

[0030] Figure 6 This describes the adsorption behavior of the stabilizer of the present invention in a mixed system.

[0031] Figure 7 The figures are fitting curves of the Langmuir and Freundlich adsorption isotherms of the stabilizer of this invention.

[0032] Figure 8 The stabilizer of Cu in this invention 2+ The adsorption kinetics curve.

[0033] Figure 9 The change in Cu content in ryegrass after 60 days of applying the stabilizer of the present invention.

[0034] Figure 10 The changes in Cu speciation in soil 60 days after the application of the stabilizer of the present invention were studied.

[0035] Figure 11 A graph showing the concentration of heavy metals in the leaching solution after applying the stabilizer of this invention.

[0036] Figure 12 The enrichment factor (BCF) of ryegrass after 60 days of application of the stabilizer of the present invention.

[0037] Figure 13 The translocation factor (TF) of ryegrass after 60 days of applying the stabilizer of the present invention. Detailed Implementation

[0038] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. 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 skilled in the art without creative effort are within the scope of protection of the present invention.

[0039] Example 1

[0040] Weigh 0.534 g of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC), 0.320 g of N-hydroxysuccinimide (NHS), and 1.018 g of L-cysteine ​​and add them to a round-bottom flask. Then add 100 ml of water and stir magnetically at 25 °C for 1 hour to activate the carboxyl group on L-cysteine.

[0041] Add 1g of chitosan powder to the mixture obtained above, then add 10ml of 0.1mol / L hydrochloric acid aqueous solution to dissolve the chitosan, and continue stirring at 25℃ for 10 hours.

[0042] The pH of the mixed solution was adjusted to 7-8 by adding 1 mol / L NaOH aqueous solution, resulting in a light blue-purple precipitate. The solution was then stirred at 25°C for 3 hours to allow the precipitate to distribute evenly.

[0043] The mixed solution was collected by centrifugation and washed twice with anhydrous ethanol and twice with deionized water (until neutral). Then it was placed in an ultra-low temperature freezer and frozen at -80°C for 1 hour. After the precipitate was completely frozen, it was placed in a freeze dryer and freeze-dried at -40°C for 72 hours to obtain amino acid modified chitosan porous material, which is the final product Cys-CS.

[0044] Example 2

[0045] Weigh out 0.1599 g, 0.4550 g, 0.3802 g, 0.4955 g, and 0.2744 g of lead nitrate, zinc nitrate hexahydrate, copper nitrate trihydrate, nickel nitrate hexahydrate, and cadmium nitrate tetrahydrate, respectively. Add them to ultrapure water and dissolve them completely in an ultrasonic processor. Transfer the solutions to 1000 ml volumetric flasks and rinse them three times with ultrapure water from a wash bottle. Finally, make up to volume to prepare five 100 mg / L single metal solutions.

[0046] The pH was adjusted using 0.1 mol / L nitric acid and 0.1 mol / L sodium hydroxide solutions, resulting in five pH values: 3.5, 4.5, 5.5, 6.0, and 6.5. Three replicates were prepared for each pH value. 25 mL of the heavy metal solution was placed in an Erlenmeyer flask, followed by the addition of 15 mg of the material, and immediately transferred to a constant-temperature shaker (25°C, 150 rpm) for 2 hours. The supernatant was filtered through a 0.45 μm filter membrane, diluted, and then Cu was measured. 2+ Zn 2+ Pb 2+ Cd 2+ Ni 2+ Concentration, calculate Cu at different pH values 2+ Zn 2+ Pb 2+ Cd 2+ Ni 2+ Adsorption capacity. The adsorption capacity of the stabilizer for the five metals was plotted. Figure 5 .

[0047] Example 3

[0048] Weigh out 0.1599g, 0.4550g, 0.3802g, 0.4955g, and 0.2744g of lead nitrate, zinc nitrate hexahydrate, copper nitrate trihydrate, nickel nitrate hexahydrate, and cadmium nitrate tetrahydrate, respectively, and place them in a 500ml Erlenmeyer flask. Add ultrapure water and place the flask in an ultrasonic processor until completely dissolved. Transfer the solution to a 1000ml volumetric flask and rinse three times with ultrapure water from a wash bottle. Finally, make up to volume to prepare a 100mg / L mixed metal solution. Similarly, prepare mixed heavy metal solutions of 50mg / L, 150mg / L, and 200mg / L.

[0049] All mixed solutions were adjusted to pH 6.0, and three replicates were prepared for each concentration. 25 mL of the heavy metal solution was placed in an Erlenmeyer flask, followed by the addition of 15 mg of the material, and immediately transferred to a constant-temperature shaker (25°C, 150 rpm) for 2 hours. The supernatant was filtered through a 0.45 μm filter membrane, diluted, and then Cu was measured. 2+ Zn 2+ Pb 2+ Cd 2+ Ni 2+ Concentration, calculate Cu at different concentrations. 2+ Zn 2+ Pb 2+ Cd 2+ Ni 2+ Adsorption capacity. The adsorption capacity of the stabilizer for the five metals was plotted. Figure 6 .

[0050] To more intuitively describe the high selectivity of Cys-CS for Cu, the partition coefficient K is used. d The adsorption effect of the stabilizer on heavy metals in the mixed system was analyzed. The results are shown in Table 1.

[0051] Table 1. Distribution coefficients of Cys-CS (L / g)

[0052]

[0053] The experimental results show that the stabilizer in this invention has the highest partition coefficient for Cu in the concentration range of 50-200 mg / L of mixed heavy metals, which is much higher than the partition coefficients of other heavy metals, namely 11.09-118 times that of Pb, 8.64-32.97 times that of Cd, 12.17-84 times that of Zn, and 32.64-46.40 times that of Ni.

[0054] Example 4

[0055] Weigh the appropriate mass of copper nitrate trihydrate into a beaker and prepare copper nitrate aqueous solutions of 50, 100, 150, 200, 250, 300, 350, and 400 mg / L according to the steps in Example 3, and adjust the pH to 6.0.

[0056] Three parallel samples were prepared for each concentration gradient. 25 mL of solution was transferred to an Erlenmeyer flask, 15 mg of material was added, and the flask was shaken in a constant-temperature shaker (25℃, 200 rpm) for 2 h. The supernatant was filtered through a 0.45 μm filter membrane, diluted, and the Cu content was measured. 2+ Concentration, calculate Cu at different initial concentrations. 2+ Adsorption capacity. The adsorption results were fitted using the Langmuir and Freundlich models, and the fitting results are as follows: Figure 7 As shown.

[0057] Example 5

[0058] Weigh the appropriate mass of copper nitrate trihydrate into a beaker, prepare a 100 mg / L copper nitrate aqueous solution according to the steps in Example 3, and adjust the pH to 6.0. Transfer 25 mL of the metal solution into an Erlenmeyer flask, add 15 mg of the material, and set adsorption times of 1, 2, 5, 10, 20, 50, 70, 90, 120, and 150 min respectively, with three parallel samples prepared for each time point. Place the samples sequentially in a constant temperature shaker (25℃, 200 rpm). Filter the supernatant through a 0.45 μm filter membrane, dilute, and determine the Cu content. 2+ Concentration, calculate Cu at different contact times 2+ Adsorption capacity. Kinetic models were fitted using pseudo-first-order, pseudo-second-order, and Weber-Morris intramolecular diffusion models. The fitting results are as follows: Figure 8 As shown.

[0059] Example 6

[0060] To investigate the application effect of the stabilizer of this invention in soil, we set up pot experiments and leaching experiments. The pot experiments evaluated the stabilizing effect of the stabilizer on copper in the soil environment by studying the changes in copper content in plants and copper speciation in soil.

[0061] The pot experiment was conducted outdoors under natural conditions. Ryegrass was planted in polyethylene pots with a diameter of 15.5 cm and a height of 16.7 cm. Three groups were set up, with three replicates in each group (see Table 2). 1 kg of soil was added to each pot. The control group (CK) received no added materials, while the other two groups received 0.5‰ (by weight) and 2‰ Cys-CS, respectively, and were thoroughly mixed with the soil. A layer of absorbent paper was placed in each petri dish, and tap water was added. Ryegrass seeds were then sown on the paper, and seedlings germinated in about 7 days. 25 seedlings with similar growth were transplanted into each pot. Watering was adjusted according to the actual soil moisture content. The entire process from transplanting to harvest took 60 days.

[0062] Table 2 Pot Experiment

[0063]

[0064] Plant sample collection first requires removing soil and other debris from the surface of the plant roots. The ryegrass was then washed multiple times with tap water and ultrapure water. Next, the ryegrass was placed in a forced-air drying oven and blanched at 105℃ for 30 minutes, followed by drying at 60℃ to constant weight. Finally, the dried roots and stems were ground to 100 mesh and stored for later analysis.

[0065] After digestion and acid removal, the Cu content in the plant samples was measured. The results of changes in Cu content in the plants are shown below. Figure 9 As shown.

[0066] Soil samples need to be air-dried naturally in a well-ventilated indoor area, and then a suitable amount of soil is taken using the quartering method for grinding. First, the sample is coarsely ground by spreading it flat on kraft paper and crushing it with a wooden stick to remove plant debris, stones, and other impurities. A balloon is rubbed against the fabric to electrostatically attract small, broken plant roots. The crushed soil sample is then ground to a 100-mesh sieve for heavy metal analysis.

[0067] The determination of heavy metal speciation in soil was carried out using a modified BCR continuous extraction method. Based on stability, the method classifies heavy metal speciation into four types, from low to high: weakly acid extractable, reducible, oxidizable, and residual. Figure 10 The study showed the changes in the speciation of Cu in the soil under different Cys-CS dosages.

[0068] Example 7

[0069] Leaching is one of the most important pathways by which heavy metals in soil can cause environmental pollution, and leaching experiments can simulate to the greatest extent the release of heavy metals from soil into the surrounding environment when soil comes into contact with precipitation. By comparing the results of leaching experiments with and without the application of the stabilizer of this invention, the ability of the stabilizer to control the leaching risk of heavy metals can be effectively evaluated.

[0070] The leaching experiment used a 15cm diameter, 65cm high transparent PVC pipe as the leaching column. The column was filled from bottom to top with 5cm of gravel, 40cm of normal soil, and 15cm of composted soil. The control group (CK) received no additional materials, while the other group had 0.5‰ Cys-CS added to the composted soil portion. Simulating natural rainwater from Hangzhou, the solution was prepared using 0.25mg / L H2SO4 and 0.05mg / L HNO3 as SO4. 2- NO3 - A stock solution was prepared at a ratio of 5:1 and diluted with ultrapure water to simulate rainwater at pH 5.0. The leaching water volume was adjusted using a peristaltic pump, simulating the highest hourly rainfall of 135.3 mm in Jiashan, Zhejiang Province during Typhoon Talim. The formula for calculating the volume of rainwater leaching in the column is as follows. Using an average flowerpot radius of 7.5 cm, the calculated simulated rainwater volume was 1.67 L. A 120-mesh filter was fixed at the bottom of the leaching column to prevent impurities from entering the PVC box. After the experiment, the PVC box lid was sealed, and the leached liquid was ready for testing.

[0071] V = α × A × h × 10 -4

[0072] In the formula: V—rainfall volume, L;

[0073] α—Runoff loss coefficient, taken as 0.7;

[0074] A – Cross-sectional area of ​​the leaching column, in cm² 2 ;

[0075] h — Rainfall in 1 hour, in mm.

[0076] The contents of Cu, Pb, Zn, Cd, and Ni in the leachate were determined. Figure 11 The changes in the concentration of various heavy metals in the leachate under different treatments are shown.

[0077] The chitosan-based stabilizer of this invention, exhibiting high selectivity for Cu, uses 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC), N-hydroxysuccinimide (NHS), and L-cysteine ​​as excipients (step S1), chitosan powder as the main material (step S2), and then adjusts the pH with sodium hydroxide aqueous solution and undergoes subsequent treatment to obtain an amino acid-modified chitosan porous material (steps S3 and S4). Therefore: Figure 1 (a)(b) are the morphology of the final product Cys-CS; Figure 1 (c) is the XRD pattern of the product. There is a large peak at 11.9° of 2θ angle, and a narrow and sharp diffraction peak at 20.6°, indicating that Cys-CS has a certain degree of crystallinity.

[0078] like Figure 1 As shown in Figure a, a cross-sectional view of the material reveals that Cys-CS has a coiled, interwoven structure. Magnifying the layered structure to 1 μm, numerous pores are visible on the surface, facilitating the diffusion of heavy metal ions into the hydrogel. Furthermore, these pores possess a large specific surface area and numerous active sites, promoting the adsorption of heavy metal ions. Regarding elemental composition, Figure c shows that the weight percentages of C, N, O, and S in Cys-CS are 51.32%, 10.41%, 35.45%, and 2.82%, respectively, as shown in Table 3. The detection of S indicates that cysteine ​​is grafted onto chitosan. Based on the atomic ratio of C:N:S:O in Table 3 (4.28:0.74:0.09:2.22), its molecular formula is deduced to be C0.09. 97 H 205 N 15 O 56 S2. Based on the chemical reaction formula, deduce the chemical structural formula of the product as follows: Figure 2 As shown.

[0079] Table 3 Cys-CS elemental analysis

[0080]

[0081] like Figure 3 As shown, Cys-CS has a large peak at 11.9° 2θ, while exhibiting a narrow and sharp diffraction peak at 20.6°, indicating that Cys-CS has a certain degree of crystallinity.

[0082] like Figure 4 As shown, after reacting with cysteine, a value of 1652 cm⁻¹ can be observed in the Cys-CS infrared spectrum. -1 The peak shifted to 1656 cm. -1 Furthermore, the reaction is enhanced because an amide bond is formed at the C=O position. At 1153 cm⁻¹ -1 At this point, a new peak appears, caused by the stretching vibration of the sulfur-oxygen double bond in S=O, indicating that some -SH is oxidized to S=O. After Cu adsorption in Cys-CS, the peak is at 2866 cm⁻¹. -1 The nearby wavelength was identified as -NH, which shifted to 2890 cm⁻¹ after Cu adsorption. -1 Furthermore, the absorption peak intensity also weakens. At 1366 cm⁻¹ -1 The nearby wavelength was identified as -NH2, which shifted to 1323 cm⁻¹ after Cu adsorption. -1 At this point, the absorption peak intensity also increases. (1656 cm⁻¹) -1 and 1587cm -1The nearby wavelengths were identified as C=O and -NH. After Cu adsorption, the peak of the C=O group disappeared, and the double peaks merged into a single peak, shifting to 1615 cm⁻¹. -1 This indicates a strong interaction between the -NH2 and amide groups and the Cu metal ion.

[0083] like Figure 5 As shown, Cys-CS exhibits significantly better adsorption for Cu, while its adsorption capacity for the other four heavy metals is very low. It can be observed that within the pH range of 3.5–6, the adsorption capacity of Cys-CS for Cu shows a slow increasing trend with increasing pH, reaching 46.6 mg / g at pH 6. At pH 6.5, its adsorption capacity for Cu reaches 72.7 mg / g, far exceeding the adsorption capacity at any other pH value. This is because during the experiment, it was found that some Cu ions in the solution precipitated at this pH value, resulting in pseudo-adsorption; therefore, the adsorption capacity at this pH value was chosen to be ignored.

[0084] like Figure 6 As shown in Figure a, the adsorption capacity of Cys-CS for Cu in the mixed solution is significantly higher than that for other heavy metals. This corresponds to the result in the pH experiment where Cys-CS exhibited the highest adsorption capacity for Cu in the single-metal solution. Figure b shows the adsorption order of Cys-CS for the five heavy metals as follows: Cu (0.687–1.718 mmol / g) > Cd (0.016–0.120 mmol / g) > Zn (0.020–0.095 mmol / g) > Pb (0.003–0.058 mmol / g) > Ni (0.026–0.053 mmol / g). Comparing with the pH experiment, at a pH of 6, the adsorption capacity of Cys-CS for Cu in the 100 mg / L mixed system is 77.5 mg / g, which is higher than the adsorption capacity in the 100 mg / L single-metal Cu solution.

[0085] like Figure 7 As shown, this study uses Langmuir and Freundlich adsorption isotherm models to explain the mechanism of heavy metal adsorption by the material. The Langmuir adsorption isotherm model assumes that the adsorbent surface is homogeneous, and the adsorbate forms a uniform monolayer adsorption on its surface. The Freundlich model assumes that the adsorption process is not homogeneous, but rather multilayer adsorption. The Langmuir model is more suitable for describing the adsorption process of heavy metals by the material of this invention. According to the model, the maximum adsorption capacity of Cys-CS for Cu is calculated to be 97.8 mg / g, which is close to the maximum adsorption capacity actually measured.

[0086] like Figure 8As shown in figure a, the adsorption of Cu by Cys-CS is a slow process, reaching adsorption equilibrium at approximately 150 min, with a maximum adsorption capacity of 43.6 mg / g. The fitting coefficient Rs of the pseudo-second-order reaction kinetic model for Cys-CS is... 2 The value is 0.9394, higher than the pseudo-first order, indicating that chemisorption is the dominant adsorption mechanism. For example... Figure 8 As shown in b, the adsorption process of the material is divided into three stages. The first stage is the instantaneous adsorption stage, in which metal ions can rapidly diffuse to the material surface and be captured by the functional groups, presenting a rapid adsorption process. The Kx of Cys-CS... 1id The value is 7.60; the second stage is the internal diffusion adsorption stage, in which heavy metal ions begin to diffuse into the interior of the material, and the adsorption rate slows down. Cys-CS K 2id The value is 3.27: The third stage is the adsorption equilibrium stage. In this stage, the groups on the material surface adsorb heavy metals reach saturation and equilibrium is reached. The Kt of Cys-CS is... 3id The value is 0.35. The K of the stabilizer... 1id Much larger than K 2id The rate of adsorption reached 2.3 times, indicating that intraparticle diffusion is the rate-limiting stage for the adsorption of heavy metals by the stabilizer. Secondly, it can be observed that the fitting curve of the Cys-CS intraparticle diffusion model does not cross the origin, indicating that the adsorption process is controlled by both intraparticle diffusion and surface diffusion. By comparing the adsorption capacity ratio at each stage, it can be found that the adsorption of Cu by Cys-CS mainly occurs in the intraparticle diffusion stage.

[0087] like Figure 9 As shown, the addition of stabilizer reduced the Cu content in both the aboveground and underground parts of ryegrass. At a dosage of 0.5‰, the Cu content in the aboveground and underground parts decreased by 11.64% and 1.86%, respectively; at a dosage of 2‰, the Cu content in the aboveground and underground parts decreased by 16.8% and 15.9%, respectively. This also indicates that Cys-CS has a stronger stabilizing effect on Cu at high dosages.

[0088] like Figure 10As shown, the forms of Cu in each treatment group were mainly BCR4 and BCR3. With the addition of the stabilizer, the distribution of Cu forms in the soil changed significantly. BCR1 and BCR2 showed a decreasing trend, and their proportions were inversely proportional to the dosage; the higher the dosage, the more pronounced the decrease. Specifically, BCR1 decreased from 14% in the control group to 8%–9%, ​​a reduction of 35.7%–42.9%. BCR2 also decreased to some extent, from 6% to 5% and 4%, respectively. Correspondingly, BCR3 and BCR4 increased significantly, with BCR3 increasing more than BCR4. BCR3 increased from 37% to 42%–43%, while BCR4 only increased from 42% to 44%–45%. The reason for this phenomenon is that Cys-CS adsorbed acid-extractable Cu in the soil and converted it into organically bound Cu, thus increasing the proportion of oxidizable Cu. On the other hand, some oxidizable Cu also combined with mineral components in the soil, thus transforming into residual Cu.

[0089] like Figure 11 As shown, after applying Cys-CS, the contents of Zn, Cd, and Ni in the leachate did not change significantly. The Cu concentration in the CK group was 0.143 mg / L, meeting the Class III groundwater standard. The application of the stabilizer significantly reduced the Cu concentration to 0.045 mg / L, a reduction of 68.53%, bringing the leachate to the Class II standard. The Pb concentration in the CK group was 0.189 mg / L, which decreased to 0.106 mg / L after adding Cys-CS, a reduction of only 43.92%, and the leachate still did not meet the Class IV standard. Therefore, in soil remediation practice, the material of this invention exhibits significant selective adsorption characteristics, helping to reduce the leaching risk of target heavy metals and showing good application prospects in the field of soil remediation.

[0090] like Figure 12As shown, the order of ryegrass's ability to accumulate various metals is Cd > Cu > Zn > Ni > Pb. The phytoaccumulation factor (BCF) represents a plant's ability to accumulate heavy metals; it is the ratio of the heavy metal concentration in plant roots to the heavy metal concentration in the soil. A higher BCF value indicates a stronger ability of the plant to stabilize heavy metals. Cd had the highest BCF value, reaching 1.57. This is mainly because the Cd content in the sludge compost soil was much lower than the other four heavy metals, and since the BCF is calculated by dividing the heavy metal concentration in the roots by the heavy metal concentration in the soil, Cd's BCF value was much higher than the other heavy metals. Pb had the lowest BCF value, only 0.337, because Pb has a strong stress-inhibiting effect on plants, and the Pb content in the experimental soil was high. After adding the stabilizer, the BCF values ​​of Ni and Cd did not change significantly, indicating that the addition of the stabilizer had almost no effect on the accumulation of Ni and Cd by ryegrass. The addition of Cys-CS reduced the BCF value of Cu by 8.7% to 15.4%, indicating that the stabilizer of this invention has high selectivity and excellent adsorption capacity for Cu in soil. After the stabilizer of this invention is added to the soil, Cu in the soil is first fixed, which not only reduces the mobility and bioavailability of Cu, but also increases the resistance to Cu absorption by roots due to the influence of the rhizosphere environment, and this resistance increases with the increase of the addition ratio.

[0091] like Figure 13 As shown, in the CK group, the plant translocation coefficients (TF) of the five heavy metals were in the order of Cd > Zn > Pb > Ni > Cu. The translocation coefficients of ryegrass for Cu, Zn, Pb, Cd, and Ni were 0.061, 0.423, 0.105, 0.738, and 0.189, respectively. The addition of the stabilizer had no significant effect on the TF values ​​of the five heavy metals, indicating that the addition of the stabilizer does not affect the plant's ability to translocate heavy metals. Therefore, Cys-CS stabilizes Cu without affecting the ryegrass's ability to extract this heavy metal.

Claims

1. The application of an environmentally friendly, highly selective Cu stabilizer in assisting the extraction of heavy metals from plants, characterized in that, The preparation method of the environmentally friendly, highly selective Cu stabilizer includes the following steps: S1: Weigh 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride, N-hydroxysuccinimide and L-cysteine ​​and add them to the reactor. Then add water and stir magnetically at a constant temperature to obtain a mixture. The mass ratio of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride, N-hydroxysuccinimide, and L-cysteine ​​is 0.51-0.65 : 0.25-0.35 : 0.9-1.

1. The conditions for constant temperature magnetic stirring are: temperature 20~30℃, time 0.5~2h; S2: Add chitosan to the mixture in step S1, then add hydrochloric acid aqueous solution to dissolve the chitosan, and continue to stir the reaction at 20℃~30℃ for 8~12h to form a reaction system; S3: Add NaOH aqueous solution dropwise to the reaction system obtained in step S2 to adjust the pH of the solution to 7-8. A light blue-purple precipitate will appear. Continue stirring at 20-30℃ for 2-4 hours to allow the precipitate to be evenly distributed and obtain a mixed solution. S4: Take out the mixed solution from step S3, centrifuge to collect the solid, and the resulting precipitate is post-processed to obtain amino acid-modified chitosan porous material, i.e., an environmentally friendly, highly selective stabilizer for Cu. The post-processing includes: washing with anhydrous ethanol, washing with deionized water, then freezing in a refrigerator for 0.5 to 2 hours, and after the precipitate is completely frozen, placing it in a freeze dryer for freeze drying for 68 to 76 hours.

2. The application according to claim 1, characterized in that, In step S1, the mass of water added is 40-60 times the total mass of the added 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride, N-hydroxysuccinimide and L-cysteine.

3. The application according to claim 1, characterized in that, In step S2, the mass ratio of chitosan powder to 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride is 0.8-1.2:0.51-0.65; In step S2, the concentration of the hydrochloric acid aqueous solution added is 0.10-0.15 mol / L, and the ratio of the hydrochloric acid aqueous solution to the chitosan powder is 6-10 mL: 0.8-1.2 g.

4. The application according to claim 1, characterized in that, In step S3, the concentration of the NaOH aqueous solution is 0.3-1.2 mol / L.

Images