Method for reinforcing phytoremediation of heavy metal contaminated soil by using nitrogen and sulfur doped carbon quantum dots

Abstract

The invention discloses a method for repairing heavy metal contaminated soil by using nitrogen and sulfur doped carbon quantum dot reinforced plants, which comprises the following steps: planting plants in the heavy metal contaminated soil, applying nitrogen and sulfur doped carbon quantum dots, and culturing to realize the repairing of the heavy metal contaminated soil. Wherein the nitrogen and sulfur doped carbon quantum dots are prepared by taking citric acid and L-cysteine as raw materials through hydrothermal reaction and purification. The nitrogen and sulfur doped carbon quantum dots have good biocompatibility and water solubility, can promote plant growth under the stress of heavy metal cadmium, and improve the photosynthetic rate and antioxidant capacity of plants; meanwhile, the uptake and accumulation of the plants on the heavy metal cadmium can be enhanced, the transfer of the heavy metal cadmium in the plants to the overground part is promoted, the remediation efficiency of the plants on the heavy metal cadmium in the soil is improved, the remediation period is shortened, and the mobility of the cadmium in the soil and the environmental risk are reduced.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of environment, and particularly relates to a method for reinforcing phytoremediation of heavy metal contaminated soil by using nitrogen and sulfur doped carbon quantum dots. BACKGROUND

[0002] With the rapid development of urbanization and industrialization, a large amount of pollutants such as heavy metals in gas, liquid, solid and other emissions are discharged into the natural environment, leading to increasingly prominent soil heavy metal pollution. Soil heavy metal pollution has the characteristics of wide range, non-degradable, long duration, concealment, accumulation, long-term and irreversibility; moreover, heavy metals are not easy to leach with water in soil, but are easy to be absorbed and accumulated by organisms, and even can be accumulated in the human body through the food chain, seriously endangering human health. Therefore, it is necessary to repair the heavy metal contaminated soil.

[0003] Compared with traditional physical (tillage and landfill), chemical (soil flushing and solidification) and electrical (electric extraction and vitrification) soil remediation technologies, phytoremediation has the advantages of eliminating secondary pollution, low cost and being suitable for large-scale use, and is the most economical and environmentally friendly remediation method. As a solar-driven in-situ remediation technology, phytoremediation includes the following key components: (1) plant absorption: refers to the accumulation of pollutants including heavy metals in various parts of plants such as roots, stems, leaves, flowers and fruits; (2) plant stabilization: refers to the process of stabilizing pollutants in the root system of plants by root structure, root exudates and related microbial communities; (3) plant filtration: absorbing or adsorbing heavy metals into plant tissues to isolate and repair them; (4) plant volatilization: refers to enhancing the volatility of pollutants through the root zone, or absorbing and transporting pollutants through plant transpiration; (5) plant degradation: refers to using the root system, root exudates and related microbial communities of plants to convert pollutants into non-toxic or less toxic forms.

[0004] However, single phytoremediation has unstable effect, has problems of long repair time and being easily affected by the environment, and the types of hyperaccumulators of heavy metals are few, and the efficiency of non-hyperaccumulators of heavy metals is low, the biomass is small, and the tolerance is poor, which will significantly affect the efficiency of phytoremediation. At present, adding exogenous substances to strengthen phytoremediation of heavy metal pollution is a powerful measure, and nanomaterials as a new exogenous substance will have great application potential for improving soil quality, reducing the accumulation of heavy metals in plant bodies and ensuring the environmental safety of crops.

[0005] Carbon quantum dots are a kind of nanomaterials with a particle size of less than 10 nm. Compared with traditional quantum dots, carbon quantum dots have more abundant raw material sources, simpler preparation methods, smaller toxicity, better water solubility and biocompatibility. So far, there is no report on the use of nitrogen and sulfur doped carbon quantum dots to enhance the efficiency of phytoremediation of heavy metal contaminated soil. Therefore, it is particularly important to develop a nitrogen and sulfur doped carbon quantum dots with suitable morphology and properties, and a simple preparation method, which is of great significance for effectively improving the phytoremediation effect of plants on heavy metal contaminated soil by using carbon quantum dots, and comprehensively improving the practical application and popularization of phytoremediation technology. SUMMARY

[0006] The technical problem to be solved by the present application is to overcome the shortcomings of the prior art and provide a method for using nitrogen and sulfur doped carbon quantum dots to enhance the phytoremediation of heavy metal contaminated soil.

[0007] To solve the above technical problems, the present application adopts the following technical solutions.

[0008] A method for using nitrogen and sulfur doped carbon quantum dots to enhance the phytoremediation of heavy metal contaminated soil, the method is to plant plants in heavy metal contaminated soil, apply nitrogen and sulfur doped carbon quantum dots, and culture to realize the remediation of heavy metal contaminated soil, the nitrogen and sulfur doped carbon quantum dots are prepared from citric acid and L-cysteine as raw materials by hydrothermal reaction and purification.

[0009] The above method is further improved, the preparation method of the nitrogen and sulfur doped carbon quantum dots comprises the following steps:

[0010] S1, citric acid, L-cysteine and water are mixed and hydrothermally reacted to obtain a nitrogen and sulfur doped carbon quantum dot crude product;

[0011] S2, the nitrogen and sulfur doped carbon quantum dot crude product is ultrasonically treated, filtered, purified, frozen and dried to obtain a nitrogen and sulfur doped carbon quantum dot.

[0012] The above method is further improved, in step S1, the mass ratio of citric acid to L-cysteine is 0.64:1.08, and the ratio of citric acid to water is 0.64g:60mL.

[0013] The above method is further improved, in step S1, the hydrothermal reaction temperature is 180℃, and the hydrothermal reaction time is 6h.

[0014] The method is further improved, and the ultrasonic time in the step S2 is 30min-60min; the pore size of the filter membrane used in the filtration is 0.22μm; the purification time is 48h-72h, the purification is performed by using a dialysis bag, the molecular weight cut-off of the dialysis bag is 500Da; the freezing temperature is-150℃--80℃, and the freezing time is 12h-48h; and the drying is vacuum freeze drying.

[0015] The method is further improved, the plant comprises at least one of Medicago sativa and Saxifraga fortunei, the heavy metal in the heavy metal contaminated soil is cadmium, and the application mode comprises at least one of soil infiltration and foliar spraying.

[0016] The method is further improved, when the application mode is soil infiltration, the addition amount of the nitrogen-sulfur doped carbon quantum dots is 100mg-5000mg per kilogram of heavy metal contaminated soil.

[0017] The method is further improved, when the application mode is foliar spraying, the concentration of the foliar spraying of the nitrogen-sulfur doped carbon quantum dots is 10mg / L-500mg / L, the single dose of the nitrogen-sulfur doped carbon quantum dots is 1.5mL-5mL, and the foliar spraying cycle of the nitrogen-sulfur doped carbon quantum dots is 1time / week-3times / week.

[0018] The method is further improved, and the initial concentration of the heavy metal in the heavy metal contaminated soil is ≤100mg / kg.

[0019] The method is further improved, and the culture time is ≥2weeks.

[0020] Compared with the prior art, the method has the following advantages:

[0021] The application provides a method for reinforcing phytoremediation of heavy metal contaminated soil by using nitrogen-sulfur doped carbon quantum dots, which are prepared from citric acid and L-cysteine as raw materials through a hydrothermal reaction and purification. BRIEF DESCRIPTION OF DRAWINGS

[0022] Figure 1 TEM and HRTEM images of the nitrogen-sulfur doped carbon quantum dots prepared in Example 1 of the application.

[0023] Figure 2 Particle size distribution diagram of the nitrogen-sulfur doped carbon quantum dots prepared in Example 1 of the application.

[0024] Figure 3 Fourier infrared spectroscopy characterization diagram of the nitrogen-sulfur doped carbon quantum dots prepared in Example 1 of the application.

[0025] Figure 4 XPS full spectrum diagram of the nitrogen-sulfur doped carbon quantum dots prepared in Example 1 of the application.

[0026] Figure 5 High-resolution XPS C 1s spectrum diagram of the nitrogen-sulfur doped carbon quantum dots prepared in Example 1 of the application.

[0027] Figure 6 High-resolution XPS O 1s spectrum diagram of the nitrogen-sulfur doped carbon quantum dots prepared in Example 1 of the application.

[0028] Figure 7 High-resolution XPS N 1s spectrum diagram of the nitrogen-sulfur doped carbon quantum dots prepared in Example 1 of the application.

[0029] Figure 8 High-resolution XPS S 2p spectrum diagram of the nitrogen-sulfur doped carbon quantum dots prepared in Example 1 of the application. P ​

[0030] Figure 9 The UV absorption spectrum of the nitrogen-sulfur doped carbon quantum dots prepared in Example 1 of the present application.

[0031] Figure 10 The fluorescence emission spectrum of the nitrogen-sulfur doped carbon quantum dots prepared in Example 1 of the present application.

[0032] Figure 11 The effect of foliar spraying of nitrogen-sulfur doped carbon quantum dots on the plant height of Medicago polymorpha under cadmium stress in Example 1 of the present application.

[0033] Figure 12 The effect of foliar spraying of nitrogen-sulfur doped carbon quantum dots on the aboveground dry weight and root dry weight of Medicago polymorpha under cadmium stress in Example 1 of the present application.

[0034] Figure 13 The effect of foliar spraying of nitrogen-sulfur doped carbon quantum dots on the aboveground cadmium content and root cadmium content of Medicago polymorpha under cadmium stress in Example 1 of the present application.

[0035] Figure 14 The effect of soil incorporation of nitrogen-sulfur doped carbon quantum dots on the plant height of Medicago polymorpha at different periods under cadmium stress in Example 2 of the present application.

[0036] Figure 15 The effect of soil incorporation of nitrogen-sulfur doped carbon quantum dots on the aboveground fresh weight and root fresh weight of Medicago polymorpha at different periods under cadmium stress in Example 2 of the present application.

[0037] Figure 16 The effect of soil incorporation of nitrogen-sulfur doped carbon quantum dots on the aboveground cadmium content and root cadmium content of Medicago polymorpha at different periods under cadmium stress in Example 2 of the present application.

[0038] Figure 17 The effect of soil incorporation of nitrogen-sulfur doped carbon quantum dots on the cadmium forms in cadmium contaminated soil at different periods in Example 2 of the present application.

[0039] Figure 18 The effect of foliar spraying and soil incorporation of nitrogen-sulfur doped carbon quantum dots on the plant height of Sedum plumbizincicola under cadmium stress in Example 3 of the present application.

[0040] Figure 19 The effect of foliar spraying and soil incorporation of nitrogen-sulfur doped carbon quantum dots on the aboveground fresh weight and root fresh weight of Sedum plumbizincicola under cadmium stress in Example 3 of the present application.

[0041] Figure 20 The effect of foliar spraying and soil incorporation of nitrogen-sulfur doped carbon quantum dots on the root, stem and leaf cadmium content of Sedum plumbizincicola under cadmium stress in Example 3 of the present application. DETAILED DESCRIPTION

[0042] The application will be further described in conjunction with the accompanying drawings and specific preferred embodiments, but the protection scope of the application is not limited thereby. The materials and instruments used in the following examples are commercially available.

[0043] Example 1:

[0044] A method for enhancing phytoremediation of heavy metal contaminated soil by nitrogen and sulfur doped carbon quantum dots according to the present application, comprising the following steps:

[0045] (1) Test soil

[0046] The soil used in the study was collected from an uncontaminated farmland soil in Qingjiangpu District, Huai'an City, Jiangsu Province (119°01'53"E, 33°61'04"N). The collected soil was naturally air-dried and sieved through a 100-mesh sieve.

[0047] (2) Soil contamination

[0048] The sieved soil was divided into 2.5 kg aliquots and artificially added with 4 mg / kg of cadmium. The soil was added with water to 70% of the maximum water-holding capacity of the soil, mixed with the same volume of washed quartz sand (v:v = 1:1), and allowed to stand at a constant temperature (22-24°C) for 2 months to achieve equilibrium and promote the adsorption of added cadmium.

[0049] (3) Medicago sativa culture

[0050] Intact and uniform Medicago sativa seeds were selected, soaked in 5% NaClO4 for 30 minutes, then rinsed three times with deionized water, and the seeds were placed in a conical flask containing deionized water, and germinated in the dark on a shaker at a temperature of 23°C and a speed of 180 rpm. Uniformly germinated seeds were selected and cultivated in substrate soil. The experiment was conducted in a greenhouse with a temperature of 24 / 18°C (day / night), a light cycle of 16 / 8 h (light / dark), and a light intensity of 500 μmol m -2 s -2 After two weeks of cultivation, Medicago sativa seeds with consistent growth conditions were randomly selected and transplanted into experimental pots.

[0051] (4) Experimental grouping and treatment

[0052] Leaf spraying group (F1-F4): using cadmium contaminated soil with a concentration of 4 mg / kg, each group of alfalfa was sprayed with 5 mL of nitrogen and sulfur doped carbon quantum dots with a concentration of 10, 50, 100 and 500 mg / L respectively, and each group was marked as F1, F2, F3, F4. Each group had three replicates, a total of 18 pots, and each pot had 3 alfalfa plants. Spray the same volume of nitrogen and sulfur doped carbon quantum dots once a week, and water the alfalfa plants irregularly according to the soil conditions to maintain their normal growth. Harvest after eight weeks to repair the heavy metal contaminated soil.

[0053] Cadmium pollution control group (CKK): using cadmium-free contaminated soil, each alfalfa plant was sprayed with 5 mL of deionized water, and other conditions were the same.

[0054] Cadmium pollution control group (CK): using cadmium contaminated soil with a concentration of 4 mg / kg, each alfalfa plant was sprayed with 5 mL of deionized water, and other conditions were the same.

[0055] In this embodiment, the preparation method of nitrogen and sulfur doped carbon quantum dots includes the following steps:

[0056] S1, 0.64 g of citric acid, 1.08 g of L-cysteine and 60 mL of deionized water were placed in a 100 mL polytetrafluoroethylene lined high temperature reaction kettle, and reacted in an oven at 180℃ for 6 h, then cooled to room temperature to obtain a crude product of nitrogen and sulfur doped carbon quantum dots.

[0057] S2, the above-mentioned crude product of nitrogen and sulfur doped carbon quantum dots was ultrasonically dispersed in water for 30 min; then, it was filtered with a 0.22 μm filter membrane, and the filtrate was placed in a dialysis bag with a molecular weight cut-off of 500 Da for purification for 48 hours; the purified solution was placed in a -80℃ refrigerator overnight (i.e. 24 h); finally, vacuum freeze-drying was performed to obtain nitrogen and sulfur doped carbon quantum dots.

[0058] In this embodiment, the yield of the prepared nitrogen and sulfur doped carbon quantum dots is 50%.

[0059] The above-prepared nitrogen and sulfur doped carbon quantum dots were dissolved in ethanol as a solvent, ultrasonicated for 10 min, then transferred to an ultrathin carbon film, and then the copper mesh was placed under light to dry. The morphology and size of the nitrogen and sulfur doped carbon quantum dots were observed using a JEM 2100F transmission electron microscope produced by JEOL Co., Ltd. in Tokyo, Japan, with an acceleration voltage of 200 kV, and the results are shown in Figure 1 . Subsequently, the particle size of 100 random nanoparticles was statistically analyzed, and a Gaussian distribution was used for fitting, revealing a normal distribution of particle size distribution, and the results are shown in Figure 2 .

[0060] Figure 1These are TEM and HRTEM images of the nitrogen-sulfur-doped carbon quantum dots prepared in Example 1 of this invention. From... Figure 1 It can be seen that nitrogen-sulfur-doped carbon quantum dots are uniformly dispersed in water, exhibiting a spherical shape; high-resolution transmission electron microscopy (HRTEM) images show obvious lattice stripes, indicating that nitrogen-sulfur-doped carbon quantum dots have a certain degree of crystallinity, with a lattice spacing of 0.21 nm, corresponding to the graphite (100) plane.

[0061] Figure 2 This is a particle size distribution diagram of the nitrogen-sulfur-doped carbon quantum dots prepared in Example 1 of the present invention. From... Figure 2 It can be seen that the average diameter of nitrogen-sulfur-doped carbon quantum dots is 3.92±0.8nm.

[0062] The molecular structure and chemical composition of nitrogen-sulfur-doped carbon quantum dots were studied using Fourier transform infrared spectroscopy (FTIR). The steps were as follows: A Nicolet iS 5 Fourier transform infrared spectrometer (Thermo Fisher Scientific) was used. 1–2 mg of powdered sample and 200 mg of pure KBr were finely ground and uniformly mixed. The mixture was placed in a mold and pressed into a transparent sheet using a hydraulic press. KBr powder was used as a blank reference. The sample sheet was then placed in the infrared spectrometer for testing in the wavenumber range of 4000–4000 cm⁻¹. -1 32 scans, 4cm resolution -1 The result is as follows Figure 3 As shown.

[0063] Figure 3 This is a Fourier transform infrared (FTIR) spectrum characterization of the nitrogen-sulfur-doped carbon quantum dots prepared in Example 1 of this invention. From... Figure 3 It can be seen that in the range of 3280-3650cm -1 The broad vibrational bands within the range can be attributed to the stretching vibrations of the NH and OH bonds; at 1450 cm⁻¹ -1 There is a distinct absorption peak at 880 cm⁻¹, corresponding to the stretching vibrations of CO, CN, and S=O bonds; -1 A sharp absorption peak appears at 700 cm⁻¹, corresponding to the torsional vibration related to the NH bond; at 700 cm⁻¹... -1 The absorption peak at that point can be attributed to the deformation vibration of the CH bond and the stretching vibration of the CS bond.

[0064] The chemical composition, elemental binding energies, and valence states of nitrogen-sulfur-doped carbon quantum dots were characterized using X-ray photoelectron spectroscopy (XPS). The steps were as follows: Measurements were performed using a Thermo Fisher Scientific ESCALAB 250XI XPS. The elements and their relative concentrations within the nitrogen-sulfur-doped carbon quantum dots were characterized by Al target excitation (1486.6 eV) with a monochromatic X-ray source. The results are as follows: Figures 4-8The C-C bond in C1s (284.8 eV) was used as calibration data in the characterization data.

[0065] From Figure 4 It can be seen that the nitrogen and sulfur doped carbon quantum dots prepared in the embodiment are mainly composed of C, N, S and O elements. From Figure 5 It can be seen that the XPS spectrum of C 1s can be fitted into four main binding energy peaks, which are located at 284.8, 286.5, 288.2 and 289.5 eV, corresponding to C-C / C=C, C-N / C-S, C=O / C=N and O-C=C. From Figure 6 It can be seen that the O 1s XPS spectrum has three well-fitted peaks, which are located at 531.4, 533.1 and 535.5 eV, corresponding to C-O, C-OH / C-O-C and O-C=O. From Figure 7 It can be seen that the N 1s XPS spectrum has two peaks at 399.45 and 397.9 eV, respectively, corresponding to pyrrole and pyridine N. From Figure 8 It can be seen that the S2p spectrum is well fitted into three peaks, which are located at 163.5, 164.5 and 168.5 eV, representing the 2p3 / 2 and 2p1 / 2 states of thiophene S and oxidized S (S=O). The above results prove that N and S elements are successfully doped into carbon quantum dots, and verify the existence of surface functional groups related to N and S, such as C-N, C=N, C-S and S=O.

[0066] The chemical bonds of nitrogen and sulfur doped carbon quantum dots are further confirmed by ultraviolet-visible absorption spectrum analysis, and the steps are as follows: 3 mL of nitrogen and sulfur doped carbon quantum dots solution with a concentration of 100 mg / L is placed in a UVmini-1280 type ultraviolet spectrophotometer produced by Shimadzu Corporation, and the absorption spectrum under 200-800 nm is tested, and the result is shown in Figure 9 .

[0067] Figure 9 The ultraviolet absorption spectrum of the nitrogen and sulfur doped carbon quantum dots prepared in Example 1 of the present application is shown. From Figure 9 It can be seen that the nitrogen and sulfur doped carbon quantum dots show typical absorption at 225 nm, which usually belongs to π→π* transition of aromatic sp2 region, and the absorption of nitrogen and sulfur doped carbon quantum dots at about 340 nm comes from n→π* edge transition, which is related to the core edge of nitrogen and sulfur doped carbon quantum dots, responsible for the bright light of the main light center.

[0068] The fluorescence of the nitrogen-sulfur doped carbon quantum dots is characterized by fluorescence spectrum, and the steps are as follows: 3mL of nitrogen-sulfur doped carbon quantum dot solution with a concentration of 100mg / L is taken and placed in a F-2500 fluorescence spectrometer produced by Japan Hitachi Company, and the fluorescence spectrum under 310-400nm is tested. The signal range is-5-300, the slit width is 5nm, the detection speed is "Super", and the detection sensitivity is "Low". The test solution is loaded into a special four-side light-transmitting quartz colorimetric cell, the outer wall of the colorimetric cell is wiped with a mirror paper, and the colorimetric cell is placed in a sample groove. The results are shown in Figure 10 .

[0069] Figure 10 The fluorescence emission spectrum of the nitrogen-sulfur doped carbon quantum dots prepared in Example 1 is shown in Figure 10 It can be seen that the excitation wavelength range of the nitrogen-sulfur doped carbon quantum dots is 320-410nm, the maximum emission wavelength peak is located at 468nm, and the shoulder peak is located at 544nm.

[0070] In combination with Figures 1-10 the description, the average diameter of the nitrogen-sulfur doped carbon quantum dots prepared in the embodiment is 3.92±0.8nm, has obvious lattice fringes, the surface has the attachment of nitrogen elements and sulfur elements, and has oxygen-containing, nitrogen-containing and sulfur-containing functional groups, such as C-N, C=N, C-S and S=O.

[0071] The growth index of alfalfa is measured, and the height of the alfalfa is measured with a ruler after the alfalfa is harvested, and the results are shown in Figure 11 . After the measurement, the above-ground part and the root part are collected and washed with deionized water, and the surface water is absorbed with a water absorption paper. 2g of the above-ground part and the root part of the alfalfa are accurately weighed and placed in a glass container, and are placed in an electric heating constant temperature drying oven and dried at 70℃ until the weight is constant. Then, the alfalfa is cooled to room temperature, and the dry weight of the above-ground part and the root part of the alfalfa is measured by using an electronic analytical balance, and the results are shown in Figure 12 .

[0072] In combination with Figure 11 and Figure 12It can be seen that the growth of alfalfa was significantly affected under cadmium stress. Compared with CK, the plant height of CK decreased by 27.77%, and the aboveground and underground dry weights decreased by 25.75% and 59.24%, respectively. After spraying different concentrations of nitrogen-sulfur doped carbon quantum dots, compared with CK, the plant height of F1-F4 group increased by 12.57%-17.37%, the aboveground dry weight of F1-F4 group increased by 1.18%-30.27%, and the root dry weight of F1-F4 group increased by 18.19%-83.55%. This shows that foliar spraying of nitrogen-sulfur doped carbon quantum dots can promote the growth of alfalfa under cadmium stress and alleviate the inhibition of cadmium on alfalfa.

[0073] The aboveground and root parts of alfalfa were collected and washed with deionized water. The roots were soaked in a 10 mmol / L Na2EDTA solution for 30 min to remove metal ions adhering to the surface of the roots. After soaking, the roots were rinsed with deionized water. The washed aboveground and root parts were placed in a glass container and placed in an electric heating constant temperature drying oven at 70°C until constant weight. After cooling to room temperature, the aboveground and root parts were ground and sieved through a 200 mesh sieve. 0.1 g of sieved aboveground and root powder was placed in a polytetrafluoroethylene crucible, 3 mL of HClO4 and 10 mL of HNO3 were added, and the sample was digested on an electric heating plate at 180°C. After digestion, the sample was removed and cooled to room temperature, then diluted with 1% HNO3, filtered with a 0.22 μm filter membrane, and stored in a 4°C refrigerator. Finally, the concentration of heavy metal cadmium was determined.

[0074] The concentration of cadmium in the sample was determined by inductively coupled plasma mass spectrometry (ICP-MS): 1) Standard curve preparation: Take cadmium standard solution (1 g / L) and dilute it several times to prepare 1, 2, 3, 4, and 5 mg / L cadmium solutions. The absorbance of the above cadmium solutions was measured by inductively coupled plasma mass spectrometry to obtain the standard curve; 2) Sample detection: The sample to be tested was measured by inductively coupled plasma mass spectrometry. If the measured concentration exceeds the range of the standard curve, the sample is diluted by a certain multiple and then measured again. According to the concentration measured by inductively coupled plasma mass spectrometry, the dilution multiple of the sample, and the sample volume, the concentration of cadmium in the aboveground and root parts of alfalfa was calculated, and the results are shown in Table 2. Figure 13

[0075] From Figure 13 It can be seen that compared with CK, the cadmium concentration in the aboveground part of alfalfa in F1-F4 group increased by 22.9%-48.7%, and the cadmium concentration in the root part of alfalfa in F1-F4 group increased by 2.9%-10.1%. This shows that foliar spraying of nitrogen-sulfur doped carbon quantum dots can promote the uptake and accumulation of cadmium in alfalfa, and mainly promote the transport of cadmium in alfalfa to the aboveground part.

[0076] Example 2:​

[0077] A method for enhancing phytoremediation of heavy metal contaminated soil using nitrogen-sulfur-doped carbon quantum dots according to the present invention includes the following steps:

[0078] (1) Test soil,

[0079] The soil used for testing was the same as in Example 1.

[0080] (2) Soil contamination

[0081] The soil contamination steps were basically the same as those in Example 1, except that the concentration of the soil used in the experiment was 10 mg / kg.

[0082] (3) Alfalfa cultivation

[0083] The alfalfa cultivation steps are the same as in Example 1.

[0084] (4) Experimental grouping

[0085] Foliar spray groups (M0-M4): Cadmium-contaminated soil at a concentration of 10 mg / kg was used. 0, 100, 500, 1000, and 5000 mg / kg of nitrogen-sulfur-doped carbon quantum dots were added to each flowerpot and mixed into the soil. These groups were designated M0, M1, M2, M3, and M4, respectively. Each treatment group had nine replicates. Samples were taken from each treatment group at weeks 2, 4, and 8. During these periods, alfalfa was watered intermittently depending on soil conditions to maintain normal growth and achieve remediation of heavy metal-contaminated soil.

[0086] Cadmium-free control group (NC): Using cadmium-free soil, all other conditions were the same.

[0087] After the alfalfa was harvested, its height was measured with a ruler, and the results were as follows: Figure 14 As shown in the figure. After the measurement was completed, the aboveground parts and roots were collected separately, washed with deionized water, and the surface moisture was absorbed with absorbent paper. The fresh weight of the aboveground parts and roots of alfalfa was determined using an electronic analytical balance. The results are shown in the figure. Figure 15 As shown.

[0088] Combination Figures 14-15It can be seen that compared with the control group (NC), the plant height of M0 was reduced by 24.08%, 9.21% and 22.91% at 2, 4 and 8 weeks, respectively, the fresh weight of aboveground part of M0 was reduced by 26.35%, 5.54% and 14.25% at 2, 4 and 8 weeks, respectively, and the fresh weight of root of M0 was reduced by 39.31%, 25.7% and 17.85% at 2, 4 and 8 weeks, respectively, which indicated that the growth of alfalfa in different periods was affected under cadmium stress. After different concentrations of nitrogen and sulfur doped carbon quantum dots were added into the soil, compared with M0, the plant height of M1-M3 groups was increased by 27.79%-90.74% at 2 weeks, the plant height of M1-M3 groups was increased by 9.82%-25.96% at 4 weeks, and the plant height of M1-M3 groups was increased by 39.36%-55.32% at 8 weeks; but too high concentration of nitrogen and sulfur doped carbon quantum dots inhibited the growth of alfalfa, and compared with M0, the plant height of M4 was decreased by 20.09%, 49.83% and 19.61% at 2, 4 and 8 weeks, respectively. The fresh weight of aboveground part and root of alfalfa was consistent with the trend of plant height, compared with M0, the fresh weight of aboveground part of M1-M3 groups was increased by 123.54%-227.87% at 2 weeks, the fresh weight of aboveground part of M1-M3 groups was increased by 1.46%-47.77% at 4 weeks, and the fresh weight of aboveground part of M1-M3 groups was increased by 33.47%-80.72% at 8 weeks; the fresh weight of root of M1-M3 groups was increased by 18.62%-124.77% at 2 weeks, the fresh weight of root of M1-M3 groups was increased by 19.40%-42.80% at 4 weeks, and the fresh weight of root of M1-M3 groups was increased by 35.35%-58.37% at 8 weeks; while too high concentration of nitrogen and sulfur doped carbon quantum dots inhibited the growth of alfalfa, and compared with M0, the fresh weight of aboveground part of M4 was decreased by 8.97%, 53.59% and 22.35% at 2, 4 and 8 weeks, respectively, and the fresh weight of root of M4 was decreased by 51.59%, 77.59% and 14.01% at 2, 4 and 8 weeks, respectively.

[0089] It can be seen that the addition of nitrogen and sulfur doped carbon quantum dots into the soil can promote the growth of alfalfa under cadmium stress and alleviate the inhibition of cadmium on alfalfa, but too high concentration of nitrogen and sulfur doped carbon quantum dots will inhibit the growth of alfalfa, so the appropriate concentration of nitrogen and sulfur doped carbon quantum dots should be selected for application in practical application.

[0090] The method for determining the content of heavy metal cadmium in alfalfa is the same as that in Example 1, and the results are shown in Table 2. Figure 16

[0091] From the above experimental results, it can be seen that the addition of nitrogen and sulfur doped carbon quantum dots into the soil can promote the growth of alfalfa under cadmium stress and alleviate the inhibition of cadmium on alfalfa, but too high concentration of nitrogen and sulfur doped carbon quantum dots will inhibit the growth of alfalfa, so the appropriate concentration of nitrogen and sulfur doped carbon quantum dots should be selected for application in practical application. Figure 16 ​It can be seen that the accumulation of cadmium in the roots of alfalfa was significantly increased after the soil was mixed with nitrogen-sulfur doped carbon quantum dots at different time periods. Compared with M0, the cadmium content in the roots of M1-M4 increased by 17.23%-57.90% at the 2nd week, the cadmium content in the roots of M1-M4 increased by 16.96%-63.35% at the 4th week, and the cadmium content in the roots of M1-M4 increased by 32.24%-174.92% at the 8th week. At the same time, with the increase of the concentration of nitrogen-sulfur doped carbon quantum dots, the cadmium content in the roots of alfalfa also increased. As for the aboveground part of alfalfa, the accumulation of cadmium in M1 and M2 groups was not significantly affected, and even decreased the accumulation of cadmium, which indicated that the soil mixed with nitrogen-sulfur doped carbon quantum dots inhibited the transfer of cadmium from the roots to the aboveground part. In summary, the soil mixed with nitrogen-sulfur doped carbon quantum dots significantly enhanced the uptake and accumulation of cadmium in the roots of alfalfa, and limited the transport of cadmium from the roots to the aboveground part of alfalfa, reducing the accumulation of cadmium in the aboveground part of alfalfa.

[0092] The effect of soil mixed with nitrogen-sulfur doped carbon quantum dots on the forms of cadmium in cadmium contaminated soil at different periods was determined by BCR (European Community Bureau of Reference) sequential extraction method. The results are as follows Figure 17The specific steps are as follows: (1) weak acid extraction state: 0.5 g of air-dried soil sample passing through a 100-mesh sieve is taken into a 50-mL centrifuge tube, and 20 mL of HOAc with a concentration of 0.11 mol / L is added. Then, under the condition of 22±5℃ and 250 rpm, oscillation is performed for 16 h. After the extraction is completed, centrifugation is performed at a speed of 10000 g for 10 min. The supernatant is taken through a 0.22-μm filter membrane, and then stored in a 4℃ refrigerator for testing. The solution extracted this time is recorded as T1. After 10 mL of deionized water is added to the residue, washing is performed under the same oscillation condition for 20 min, and then centrifugation is performed at a speed of 10000 g for 10 min. The supernatant is discarded, but any solid residue cannot be discarded; (2) reducible state: 20 mL of freshly prepared NH2OH·HCl solution with a concentration of 0.5 mol / L is added to the solid residue, and then oscillation is performed at 22±5℃ and 250 rpm for 16 h. After the extraction is completed, centrifugation is performed at a speed of 10000 g for 10 min. The supernatant is taken through a 0.22-μm filter membrane, and then stored in a 4℃ refrigerator for testing. The solution extracted this time is recorded as T2. After 10 mL of deionized water is added to the residue, washing is performed for 20 min, and then centrifugation is performed at a speed of 10000 g for 15 min. The supernatant is discarded, but any solid residue cannot be discarded; (3) oxidizable state: 5 mL of 30% H2O2 is added to the solid residue in 2-3 times, so as to avoid causing too intense reaction. The lid is covered, but not tightened, and then placed into a constant-temperature oscillation box for digestion at 25℃ and 180 rpm for 1 h. Then, placed into a water bath at 85±2℃ for digestion for 1 h, and then intermittently shaken by hand in the first half hour. Then, the lid is opened, and the solution is continuously heated until the solution is reduced to below 1.5 mL. Then, 5 mL of solution C is added, the lid is covered, and then intermittently shaken by hand in the first half hour. Then, the lid is opened, and the solution is continuously heated until the solution is reduced to about 1 mL. 25 ml of 1 mol / L NH4OAc solution is added, and then placed into an oscillator for oscillation at 22±5℃ and 250 rpm for 16 h. After the extraction is completed, centrifugation is performed at a speed of 10000 g for 10 min. The supernatant is taken through a 0.22-μm filter membrane, and then stored in a 4℃ refrigerator for testing. The solution extracted this time is recorded as T3; (4) residual state: the container containing the solid residue is placed into an oven at about 60℃ for drying to constant weight, and then the solid residue is ground. 0.2 g of the solid residue is taken, and then determined by using the four-acid digestion method. The solution after digestion is diluted with 2% nitric acid solution, and then stored in a 4℃ refrigerator for testing after being taken through a 0.22-μm filter membrane. The solution extracted this time is recorded as T4.

[0093] The form of cadmium in soil will affect its bioavailability and biological toxicity in soil. The absorption of cadmium in soil by alfalfa does not depend on the total content of cadmium in soil, but the content of cadmium that can be utilized, which is called bioavailable cadmium. In this case, the four-step BCR sequential extraction method is used to analyze the form of cadmium in soil during the process of phytoremediation. According to the method, heavy metal cadmium can be divided into four different forms, and the mobility of the four forms of heavy metals is weak acid extraction state > reducible state > oxidizable state > residual state, and the biological toxicity is opposite. That is, the bioavailability of heavy metal cadmium is inversely proportional to its toxicity, and the higher the bioavailability of the form, the greater the biological toxicity.

[0094] From Figure 17 It can be seen that, from the time point of view, with the increase of the repair time, cadmium is absorbed and accumulated by alfalfa into the plant body, and the proportion of acid extractable cadmium in each group is decreasing, while the proportion of residual cadmium is increasing. In the second week, the weak acid extractable cadmium of M1-M4 group is higher than that of M0, because the introduction of nitrogen-sulfur doped carbon quantum dots releases a large amount of H + from the dissociation of the surface -COOH and -OH, which reduces the pH and improves the bioavailability of cadmium; and by the eighth week, with the absorption of plants and the consumption of nitrogen-sulfur doped carbon quantum dots, the proportion of weak acid extractable cadmium in soil is decreased, and the proportion of residual cadmium is increased. This shows that the planting of alfalfa can reduce the biological effectiveness of cadmium in soil, and with the increase of the concentration of nitrogen-sulfur doped carbon quantum dots, the proportion of weak acid extractable cadmium decreases while the proportion of residual cadmium increases.

[0095] Example 3:

[0096] A method for strengthening phytoremediation of heavy metal contaminated soil by using nitrogen-sulfur doped carbon quantum dots according to the application, comprising the following steps:

[0097] (1) Test soil,

[0098] The test soil is the same as in Example 1.

[0099] (2) Soil contamination

[0100] The soil contamination step is basically the same as that of Example 1, the difference is that the concentration of the soil used in the experiment is 10 mg / kg.

[0101] (3) Saxifraga cuttage

[0102] The saxifraga is washed with tap water, cut into branches of the same size, only two leaves are left at the top, the rest of the leaves are removed, and transplanted into the experimental pots for culture. The experiment is carried out in a greenhouse, the temperature is 24 / 18℃ (day / night), the light cycle is 16 / 8h (light / dark), and the light intensity is 500μmol m -2 s-2 .

[0103] (4) Experimental grouping

[0104] Foliar spray group (FT): Using cadmium-contaminated soil at a concentration of 10 mg / kg, each pot of *Sedum aizoon* was sprayed with 5 mL of nitrogen-sulfur-doped carbon quantum dots at a concentration of 100 mg / L. Soil incorporation group (MT): Using cadmium-contaminated soil at a concentration of 10 mg / kg, 500 mg / kg of nitrogen-sulfur-doped carbon quantum dots were added to each pot and mixed into the soil. Each pot of *Sedum aizoon* was sprayed with 5 mL of deionized water. Each group had five replicates, for a total of 15 pots, with 3 *Sedum aizoon* plants per pot. The same volume of nitrogen-sulfur-doped carbon quantum dots or deionized water was sprayed three times a week. The *Sedum aizoon* was watered intermittently depending on soil conditions to maintain normal growth. Harvesting was carried out after eight weeks.

[0105] Cadmium pollution control group (CK): 10 mg / kg cadmium-contaminated soil was used, and 5 mL of deionized water was sprayed on each pot of Sedum sarmentosum. All other conditions were the same.

[0106] After harvesting *Sedum aizoon*, the plant height was measured with a ruler. The above-ground parts and roots were collected separately, washed with deionized water, and dried with absorbent paper. The fresh weight of the above-ground parts and roots of *Sedum aizoon* was then determined using an electronic analytical balance. The results are as follows: Figure 18 , Figure 19 As shown.

[0107] Combination Figure 18 , Figure 19 It can be seen that, compared with CK, the plant height of MT and FT increased by 14.1% and 51.7%, respectively; the aboveground fresh weight of MT and FT increased by 58.3% and 337.7%, respectively; and the root fresh weight of MT and FT increased by 25.7% and 242.9%, respectively. This indicates that foliar spraying and soil incorporation of nitrogen-sulfur-doped carbon quantum dots can promote the growth of Sedum aizoon under cadmium stress and alleviate the inhibitory effect of cadmium on the growth of Sedum aizoon.

[0108] The method for determining the cadmium content in *Sedum morganianum* is the same as in Example 1, and the results are as follows: Figure 20 As shown.

[0109] from Figure 20 It can be seen that, compared with the control (CK), the cadmium concentration in the leaves of *Sedum morganianum* MT and FT increased by 143.7% and 161.7%, respectively; the cadmium concentration in the stems of *Sedum morganianum* MT and FT increased by 9.3% and 51.0%, respectively; and the cadmium concentration in the roots of *Sedum morganianum* MT and FT increased by 41.5% and 43.8%, respectively. This indicates that foliar spraying and soil incorporation of nitrogen-sulfur-doped carbon quantum dots can promote the uptake and accumulation of cadmium in *Sedum morganianum*, and mainly promote the translocation of cadmium from *Sedum morganianum* to the leaves.

[0110] In addition, studies have shown that treating melon seedlings with carbon quantum dots can reduce the Cd content in the roots and leaves of melon seedlings 2+ Or adding carbon quantum dots can reduce the Cd absorption and accumulation in ryegrass, that is, carbon quantum dots prepared by the existing preparation method are applied to plants to reduce the absorption of cadmium by plants, and cannot promote the absorption of cadmium by plants. It can be seen that the application of nitrogen and sulfur doped carbon quantum dots prepared by the present application can enhance the uptake and accumulation of cadmium by plants, thereby improving the remediation efficiency of plants on cadmium contaminated soil, and also promoting the growth and adaptability of plants in cadmium contaminated soil.

[0111] In summary, the nitrogen and sulfur doped carbon quantum dots prepared by the present application not only promote the growth and adaptability of plants in heavy metal (such as cadmium) contaminated soil, but also enhance the uptake and accumulation of heavy metals (such as cadmium) by plants, promote the transportation of heavy metals (such as cadmium) in plants to the aboveground part, improve the remediation efficiency of plants on heavy metal (such as cadmium) contaminated soil, and shorten the remediation period. The method for strengthening plant remediation of heavy metal contaminated soil by using nitrogen and sulfur doped carbon quantum dots of the present application not only has no secondary pollution to the environment, but also can realize in-situ remediation, reduce economic loss, and has practical significance for improving the practical application and popularization of plant remediation technology.

[0112] The above is only a preferred embodiment of the present application, and does not limit the present application in any form. Although the present application has been disclosed as above with a preferred embodiment, it is not intended to limit the present application. Any person skilled in the art can make many possible changes and modifications to the technical solutions of the present application, or modify equivalent embodiments, without departing from the spirit and technical solutions of the present application, by using the disclosed methods and technical contents. Therefore, any simple modification, equivalent replacement, equivalent change and modification of the above embodiments made according to the technical essence of the present application, without departing from the technical solutions of the present application, are still within the scope of protection of the present application.

Claims

1. A method for enhancing phytoremediation of heavy metal contaminated soil by using nitrogen-sulfur doped carbon quantum dots, characterized in that, The method is for planting plants in heavy metal contaminated soil, applying nitrogen and sulfur doped carbon quantum dots, and culturing to realize the remediation of the heavy metal contaminated soil; the nitrogen and sulfur doped carbon quantum dots are prepared from citric acid and L-cysteine as raw materials through hydrothermal reaction and purification.

2. The method for enhancing phytoremediation of heavy metal contaminated soil using nitrogen-sulfur-doped carbon quantum dots according to claim 1, characterized in that, The preparation method of the nitrogen and sulfur doped carbon quantum dots comprises the following steps: S1, mixing citric acid, L-cysteine and water, and performing hydrothermal reaction to obtain a nitrogen and sulfur doped carbon quantum dot crude product; S2, performing ultrasonic treatment on the nitrogen and sulfur doped carbon quantum dot crude product, filtering, purifying, freezing and drying to obtain the nitrogen and sulfur doped carbon quantum dots.

3. The method for enhancing phytoremediation of heavy metal contaminated soil using nitrogen-sulfur-doped carbon quantum dots according to claim 2, characterized in that, In step S1, the mass ratio of the citric acid to the L-cysteine is 0.64:1.08, and the ratio of the citric acid to water is 0.64g:60mL.

4. The method according to claim 3, wherein the nitrogen and sulfur doped carbon quantum dots are used to enhance phytoremediation of heavy metal contaminated soil. In step S1, the temperature of the hydrothermal reaction is 180℃, and the time of the hydrothermal reaction is 6h.

5. The method according to claim 4, wherein the nitrogen and sulfur doped carbon quantum dots are used to enhance phytoremediation of heavy metal contaminated soil. In step S2, the time of the ultrasonic treatment is 30min-60min; the pore size of the filter membrane used for the filtering is 0.22μm; the time of the purifying is 48h-72h, the purifying is performed by using a dialysis bag, the molecular weight cut-off of the dialysis bag is 500Da; the freezing temperature is-150℃--80℃, and the freezing time is 12h-48h; the drying is vacuum freeze drying. 6.The method for enhancing phytoremediation of heavy metal contaminated soil by using nitrogen-sulfur doped carbon quantum dots according to any one of claims 1-5, characterized in that, The plants include at least one of Medicago sativa and Saxifraga fortunei, the heavy metal in the heavy metal contaminated soil is cadmium, and the application mode includes at least one of soil infiltration and foliar spraying.

7. The method according to claim 6, wherein the nitrogen and sulfur doped carbon quantum dots are used to enhance phytoremediation of heavy metal contaminated soil. When the application mode is soil infiltration, the addition amount of the nitrogen and sulfur doped carbon quantum dots is 100mg-5000mg per kilogram of the heavy metal contaminated soil.

8. The method according to claim 6, wherein the nitrogen and sulfur doped carbon quantum dots are used to enhance phytoremediation of heavy metal contaminated soil. When the application mode is foliar spraying, the concentration of the foliar spraying of the nitrogen and sulfur doped carbon quantum dots is 10mg / L-500mg / L, the single dose of the nitrogen and sulfur doped carbon quantum dots is 1.5mL-5mL, and the cycle of the foliar spraying of the nitrogen and sulfur doped carbon quantum dots is 1time / week-3times / week.

9. The method for enhancing phytoremediation of heavy metal contaminated soil using nitrogen-sulfur-doped carbon quantum dots according to claim 6, characterized in that, The initial concentration of the heavy metal in the heavy metal contaminated soil is ≤100mg / kg.

10. The method for enhancing phytoremediation of heavy metal contaminated soil by using nitrogen and sulfur doped carbon quantum dots according to claim 6, characterized in that, The culturing time is ≥2weeks.

Images