A method for synergistically treating environmental cadmium pollution using aquatic plant root secretions and microalgae

Abstract

The present invention discloses a method for synergistically treating environmental cadmium pollution by using root secretions of aquatic plants and microalgae, comprising the following steps: (1) preparation of root secretions of aquatic plants: washing the aquatic plants, placing them in a container filled with pure water, fixing the roots with clean sand and gravel, and culturing the roots; then taking out the water in the container, filtering the roots, and obtaining a solution of root secretions of aquatic plants; (2) adsorption treatment of cadmium pollutants: adding the root secretion solution of aquatic plants and microalgae to the cadmium pollutants, and carrying out culturing and adsorption treatment. The present invention selects root secretions of four aquatic plants, namely, Vallisneria, Water Cabbage, Water Lily, and Water Hyacinth, to synergistically treat environmental cadmium pollution with Microcystis aeruginosa, Chlorella, Chrococcus, and Scenedesmus, which can effectively improve the absorption of Cd by microalgae. 2+ The adsorption capacity of the product is of great significance to the treatment of environmental pollution.

Description

Technical Field

[0001] The present invention relates to the technical field of cadmium pollution treatment, and in particular to a method for collaboratively treating environmental cadmium pollution by using aquatic plant root secretions and microalgae. Background Art

[0002] Cadmium (Cd) enters the soil and water environment through natural weathering, erosion and human activities, and is absorbed by plants through agricultural activities such as crop planting, farmland irrigation and phosphate fertilizer application. It enters the bodies of animals and even humans through diet. Because it is difficult to biodegrade, long-term accumulation will cause harm to human health.

[0003] Adsorption is a common method for heavy metal removal. Biosorption is a complex process in which microorganisms and plants adsorb, absorb and utilize heavy metals during their growth and metabolism, thereby purifying soil and water. Compared with conventional physical and chemical adsorption methods, biosorption has the advantages of being clean, environmentally friendly and not introducing new pollutants.

[0004] Microalgae refer to algae that are tiny in size and can usually only be seen clearly under a microscope. They include cyanobacteria (Microcystis aeruginosa, Spirulina, Chromococcus, etc.), green algae (Chlorella, Scenedesmus, etc.) and red algae, which are widely present in water and soil environments. Current studies have found that most microalgae have the ability to adsorb Cd(Ⅱ), but their adsorption capacity is weak and their feasibility is low. Therefore, how to improve the adsorption capacity of microalgae for cadmium is of great significance for treating cadmium pollution in soil and water environments. Summary of the Invention

[0005] The main purpose of the present invention is to provide a method for improving the 2+ A method for synergistically treating environmental cadmium pollution by using the adsorption capacity of aquatic plant root exudates and microalgae.

[0006] To achieve the above objectives, the present invention provides a method for synergistically treating environmental cadmium pollution using aquatic plant root exudates and microalgae, comprising the following steps:

[0007] (1) Preparation of aquatic plant root exudates

[0008] The aquatic plants are washed and placed in a container filled with pure water. The roots are fixed with clean sand and gravel, and the culture treatment is carried out. After that, the water in the container is taken out and filtered to obtain a solution of aquatic plant root secretions.

[0009] (2) Adsorption treatment of cadmium pollutants

[0010] Aquatic plant root exudate solution and microalgae are added to cadmium pollutants for cultivation and adsorption treatment.

[0011] Furthermore, the aquatic plant is any one of Vallisneria, Water Cabbage, Water Lily and Water Hyacinth, or a combination of two or more thereof; and the microalgae is any one of Microcystis aeruginosa, Chlorella, Chrococcus and Scenedesmus, or a combination of two or more thereof.

[0012] Furthermore, the aquatic plant is water hyacinth, and the microalgae is Chlorella.

[0013] Furthermore, the pH value of the aquatic plant root exudate solution is 7.0-7.4, and the total organic carbon concentration is 11-52 mg / L.

[0014] Furthermore, in step (1), aquatic plant seedlings are selected and cultured under indoor ventilated conditions for 150 days, and are exposed to simulated sunlight for 8 hours per day.

[0015] Furthermore, in step (1), the filtration treatment adopts 0.45 μm water filter membrane filtration.

[0016] Furthermore, in step (2), the cadmium pollutant is cadmium-contaminated water, the initial cadmium concentration of the mixed system of aquatic plant root exudate solution, microalgae and cadmium-contaminated water is 0.5-1.5 mg / L, the OD680 value of the microalgae is above 1.00 Abs, and the total organic carbon concentration is 10 mg / L.

[0017] Furthermore, in step (2), the cadmium pollutant is cadmium-contaminated soil. The microalgae and the cadmium-contaminated soil are first mixed evenly, with a mass ratio of the microalgae to the cadmium-contaminated soil being 1:5000 to 20000, and then the aquatic plant root secretion solution is added to submerge the soil. During the cultivation and adsorption treatment process, the aquatic plant root secretion solution is supplemented to keep the soil submerged.

[0018] Furthermore, in step (2), the temperature condition for the culture adsorption treatment is 25 to 35°C.

[0019] Furthermore, in step (2), the culture adsorption treatment is carried out under the conditions of a light-dark ratio of 12h:12h and a light intensity of 2000lux.

[0020] The beneficial effects of the present invention are embodied in:

[0021] The present invention selects the root secretions of four aquatic plants, namely, Vallisneria, Water Cabbage, Water Lily and Water Hyacinth, to cooperate with Microcystis aeruginosa, Chlorella, Chrococcus and Scenedesmus to treat environmental cadmium pollution, which can effectively improve the microalgae's ability to absorb Cd. 2+ The adsorption capacity of the product is of great significance to the treatment of environmental pollution. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 This is the UV-visible absorption spectrum of aquatic plant root secretions.

[0023] Figure 2 Three-dimensional fluorescence atlas of root secretions of four aquatic plants: (a) Vallisneria sinensis; (b) Water cabbage; (c) Water lily; (d) Water hyacinth.

[0024] Figure 3 Fluorescence component analysis spectrum of water hyacinth root exudates, (a) component 1; (b) component 2.

[0025] Figure 4 The distribution of fluorescence intensity of components 1 and 2 in water hyacinth root exudates as a function of wavelength.

[0026] Figure 5 The graph shows the change of Cd(Ⅱ) adsorption rate over time by four microalgae in different aquatic plant root secretions: (a) Microcystis aeruginosa; (b) Chlorella vulgaris; (c) Chromococcus; (d) Scenedesmus.

[0027] Figure 6 This is a graph showing the Cd(Ⅱ) adsorption rate of four microalgae under the influence of different plant root secretions.

[0028] Figure 7 The graph shows the change of Cd(Ⅱ) adsorption rate of four microalgae over time at different initial Cd(Ⅱ) concentrations: (a) Microcystis aeruginosa; (b) Chlorella vulgaris; (c) Chromococcus; (d) Scenedesmus.

[0029] Figure 8 The adsorption diagram of Cd(Ⅱ) by four microalgae at different algal biomasses: (a) adsorption rate; (b) adsorption amount.

[0030] Figure 9 The adsorption diagram of Cd(Ⅱ) by four microalgae at different temperatures: (a) adsorption rate; (b) adsorption amount.

[0031] Figure 10 The kinetic fitting curves of Cd(Ⅱ) adsorption by four microalgae are shown.

[0032] Figure 11 Fitting the Langmuir and Freundlich isotherm adsorption models for Cd(Ⅱ) adsorption by four microalgae

[0033] Figure 12 Three-dimensional fluorescence spectra of Microcystis aeruginosa before and after adsorption on four plant root secretions. Before adsorption: (a) Vallisneria sinensis; (b) Water cabbage; (c) Water lily; (d) Water hyacinth; after adsorption: (a') Vallisneria sinensis; (b') Water cabbage; (c') Water lily; (d') Water hyacinth.

[0034] Figure 13Three-dimensional fluorescence spectra of Chlorella vulgaris before and after adsorption on four plant root secretions. Before adsorption: (a) Vallisneria sinensis; (b) Water cabbage; (c) Water lily; (d) Water hyacinth; after adsorption: (a') Vallisneria sinensis; (b') Water cabbage; (c') Water lily; (d') Water hyacinth.

[0035] Figure 14 Three-dimensional fluorescence spectra of Chromococcus before and after adsorption on four plant root secretions. Before adsorption: (a) Vallisneria sinensis; (b) Water cabbage; (c) Water lily; (d) Water hyacinth; after adsorption: (a') Vallisneria sinensis; (b') Water cabbage; (c') Water lily; (d') Water hyacinth.

[0036] Figure 15 These are the three-dimensional fluorescence spectra of Scenedesmus before and after adsorption on four plant root secretions. Before adsorption: (a) Vallisneria sinensis; (b) Water cabbage; (c) Water lily; (d) Water hyacinth; after adsorption: (a') Vallisneria sinensis; (b') Water cabbage; (c') Water lily; (d') Water hyacinth.

[0037] Figure 16 Fourier transform infrared spectra of four microalgae before and after adsorption of Cd(Ⅱ) in the presence of Vallisneria sinensis root secretions. In the figure: (a) before adsorption; (b) after adsorption in pure water; (c) after adsorption in Vallisneria sinensis root secretions.

[0038] Figure 17 Fourier transform infrared spectra of four microalgae before and after adsorption of Cd(Ⅱ) in the presence of water cabbage root secretions. In the figure: (a) before adsorption; (b) after adsorption in pure water; (c) after adsorption in water cabbage root secretions.

[0039] Figure 18 Fourier transform infrared spectra of four microalgae before and after adsorption of Cd(Ⅱ) in water lily root secretions: (a) before adsorption; (b) after adsorption in pure water; (c) after adsorption in water lily root secretions.

[0040] Figure 19 Fourier transform infrared spectra of four microalgae before and after adsorption of Cd(Ⅱ) in the presence of water hyacinth root secretions: (a) before adsorption; (b) after adsorption in pure water; (c) after adsorption in water hyacinth root secretions.

[0041] Figure 20 The XPS spectra are shown before adsorption of Chlorella and after adsorption in pure water and water hyacinth root secretions.

[0042] Figure 21 These are the XPS (C1s) spectra of Chlorella before adsorption and after adsorption in pure water and water hyacinth root secretions, (a) before adsorption; (b) after adsorption in pure water; (c) after adsorption in water hyacinth root secretions.

[0043] Figure 22These are the XPS (N 1s) spectra of Chlorella before adsorption and after adsorption in pure water and water hyacinth root secretions. (a) Before adsorption; (b) After adsorption in pure water; (c) After adsorption in water hyacinth root secretions.

[0044] Figure 23 These are the XPS (Cd 3d) spectra of Chlorella after adsorption in pure water and water hyacinth root secretions, (a) after adsorption in pure water; (b) after adsorption in water hyacinth root secretions.

[0045] Figure 24 Figure 3. Changes of available Cd content in soil over time, (a) in pure water; (b) in water hyacinth root exudates.

[0046] Figure 25 The proportion of Cd(Ⅱ) forms in soils with different algae additions after 28 days of adsorption: (a) in pure water; (b) in water hyacinth root secretions. DETAILED DESCRIPTION

[0047] In order to make the technical solution of the present invention more clearly understood by those skilled in the art, the following examples are given for illustration. It should be noted that the following examples do not limit the scope of protection claimed by the present invention.

[0048] Unless otherwise specified, the raw materials, reagents, or devices used in the following examples can be obtained from conventional commercial sources or by existing known methods.

[0049] Example 1

[0050] Preparation and detection of aquatic plant root exudates

[0051] Purchase seedlings of Vallisneria, Pakchoy, Water Lily, and Water Hyacinth with intact root systems in good condition. First, rinse with tap water until there is no impurity on the plant surface, and then rinse with ultrapure water several times. Place 20 plants each of Vallisneria, Pakchoy, Water Lily, and Water Hyacinth in a plexiglass tank filled with 30L ultrapure water for hydroponics. Fix the plant roots with clean quartz sand and place them in a ventilated indoor environment. Supplement with 100W simulated sunlight (xenon lamp) for 8 hours every day. Keep the water level at the original scale during the cultivation process. The plant cultivation device is as follows: Figure 1 As shown, after 150 days of cultivation, the culture water in the tank was taken out and filtered using a 0.45 μm water filter membrane to obtain root exudate solutions of Vallisneria lappa, water cabbage, water lily and water hyacinth, which were then stored in brown reagent bottles and placed in a 4°C refrigerator for use.

[0052] The total organic carbon concentration (TOC) of the root exudate solution of each plant was measured using a total organic carbon analyzer, and the pH value was measured using a pH meter. The results are shown in Table 1.

[0053] Table 1 TOC and pH of plant root exudate solution

[0054]

[0055] The root exudates of aquatic plants contain some inorganic ions, and their concentrations are generally low. Ion chromatography and inductively coupled plasma mass spectrometry were used to determine the concentrations of anions and metal ions in the root exudates of four plants. The results are shown in Table 2.

[0056] Table 2 Ion concentrations in plant root exudate solutions (mM)

[0057]

[0058] *Note: - means not detected.

[0059] Characteristic Analysis of Root Exudates of Aquatic Plants

[0060] 1. UV-visible absorption spectroscopy analysis

[0061] UV-visible absorption spectroscopy can characterize the molecular weight and structural information of organic matter. Figure 1 Specific optical parameters can help analyze the characteristics of aquatic plant root secretions. The optical index parameters of four aquatic plant root secretions were calculated and shown in Table 3.

[0062] Table 3 Optical index parameters of UV-visible spectra of aquatic plant root secretions

[0063]

[0064] It can be found that the aromaticity of water hyacinth root secretions is the strongest, indicating that its relative content of aromatic and unsaturated bond compounds is the highest. The aromatic substance content of water cabbage root secretions is much lower than that of the other three plant root secretions. Liu Shichao et al. found that the π bond conjugated structure of aromatic compounds can form a more stable M-π structure with more heavy metal cations (M), indicating that the high aromatic content may be one of the reasons why water hyacinth root secretions promote the best adsorption of microalgae. 260 The values ​​show that the hydrophobicity of water hyacinth root secretions is the strongest. Verrillo et al. found that hydrophobic organic matter is more likely to approach the surface of microbial cells, which may be the role of water hyacinth root secretions in the adsorption of Cd by microalgae. 2+One of the reasons for the better promotion effect. The study by Voets et al. showed that high molecular weight organic matter has a better effect on the adsorption of Cd by zebra mussels than low molecular weight organic matter. 2+ This may be one of the reasons why the root secretions of water cabbage have the smallest promoting effect on the adsorption of the four microalgae.

[0065] 2. Fluorescence spectroscopy analysis

[0066] The three-dimensional fluorescence spectrum (EEMs) is a three-dimensional coordinate matrix consisting of excitation light, emission light and fluorescence intensity. It represents the emission wavelength and fluorescence intensity of the substance under the irradiation of excitation light in a certain wavelength range, and can characterize the relative amount of fluorescent substances. The fluorescence spectra of the root secretions of four aquatic plants are as follows: Figure 2 Three-dimensional fluorescence optical indicators can be used to analyze the source and characteristics of DOM. The optical indicator parameters of the root secretions of four aquatic plants calculated based on experimental data are shown in Table 4.

[0067] Table 4 Optical parameters of water of four aquatic plants

[0068]

[0069] The fluorescence indices of root exudates from Vallisneria, water cabbage, water lily, and water hyacinth are 1.918, 1.819, 1.780, and 1.828, respectively. Studies have shown that when the FI is greater than 1.9, DOM produced by microbial metabolism is the primary source. However, when the FI is between 1.4 and 1.9, the DOM content from terrestrial and microbial sources is comparable, indicating that Vallisneria root exudates primarily derive from microbial metabolites. The biological index (BIX), also known as the autogenous index, was calculated to be 0.926, 0.823, 0.777, and 0.704 for Vallisneria, water cabbage, water lily, and water hyacinth, respectively. The protein content in Vallisneria is ranked > Water cabbage > Water lily > Water hyacinth. The humification index (HIX) indicates the degree of humification of DOM. Higher HIX values ​​indicate a higher degree of humification and more stable organic matter. According to the formula in Table 4, the HIX index values ​​of the root secretions of Vallisneria, Water Cabbage, Water Lily, and Water Hyacinth were 4.459, 4.663, 6.325, and 9.659, respectively. The humification degree of DOM was Water Hyacinth> Water Lily> Water Cabbage> Vallisneria. According to the calculation, the freshness index of the root secretions of Vallisneria, Water Cabbage, Water Lily, and Water Hyacinth were 0.102, 0.090, 0.086, and 0.078, respectively. The newly generated DOM in the root secretions of Vallisneria was the largest, and the DOM freshness in the root secretions of Water Hyacinth was the lowest. According to the experimental data, the Fn(355) values ​​of the root secretions of Vallisneria, Water Cabbage, Water Lily, and Water Hyacinth were 554.218, 259.982, 625.952, and 935.207, respectively, indicating that the relative content of humus-like substances was Water Hyacinth> Water Lily> Vallisneria> Water Cabbage, which was consistent with the Cd 2+ Effects of root secretions of four plants on Cd in Microcystis aeruginosa and Chlorella vulgaris at an initial concentration of 0.2 mg / L 2+ The promotion degree of adsorption rate is consistent in order, indicating that humic substances may promote the adsorption of Cd 2+ It plays an important role in adsorption on the surface of microalgae.

[0070] Also, see Figure 3 and Figure 4 After PARAFAC analysis and comparison with the Openfluor database, it was found that the main components of water hyacinth root secretions were humus-like substances with a relatively high content, which was consistent with the results of the optical indicators calculated above. Figure 3 Component 1 in (a) is basically consistent with the components found in water-extractable organic matter in soil by Sharma et al. (TCC>0.95); component 2 is a biodegradable humus-like substance, including large-sized hydrophobic compounds, which is consistent with the results of UV-visible absorption spectroscopy. It can be seen that the highly hydrophobic humus-like substances may promote the adsorption of Cd by microalgae. 2+key factors.

[0071] Example 2

[0072] Synergistic adsorption treatment of cadmium-contaminated water by aquatic plant root exudates and microalgae and effect verification

[0073] The microalgae used in this example are freshwater microalgae Microcystis aeruginosa, Chlorella, Chroococcus, and Scenedesmus. The algae species were purchased from the Freshwater Algae Culture Collection at the Institute of Hydrobiology (FACHB) of the Chinese Academy of Sciences and are numbered FACHB-912, FACHB-9, FACHB-193, and FACHB-489, respectively.

[0074] OD 680 The OD value is the absorbance value of the algae solution at 680nm, which has a good linear relationship with the dry weight of algae biomass. The OD values ​​of Microcystis aeruginosa, Chlorella, Chrococcus, and Scenedesmus used in this example are 680 The relationship between the values ​​and the dry weight of algae is y = 0.0049x, y = 0.0045x, y = 0.0044x, and y = 0.0042x, respectively. Where: y is the dry weight of 20 mL of algae solution, g; x is the OD680 value of the algae solution, Abs.

[0075] The treatment method is as follows: take the algae-containing liquid, centrifuge it, remove the supernatant, add pure water and mix it thoroughly with a cyclone stirrer, centrifuge it again, remove the supernatant, then resuspend it with pure water, add cadmium-contaminated water and the aquatic plant root exudate solution prepared in Example 1, and place it in a constant temperature water bath shaker for cultivation and adsorption treatment.

[0076] The following verifies the influence of factors such as the types of aquatic plant root secretions, microalgae species, and culture adsorption temperature on the adsorption effect:

[0077] 1. Verification of the effect of aquatic plant root secretions on cadmium adsorption by microalgae

[0078] Experimental method: Take 5 20 mL portions of the same algae-containing solution, centrifuge at 4500 r / min for 5 min, remove the supernatant, add pure water and mix thoroughly with a cyclone mixer, centrifuge under the same conditions for 5 min, remove the supernatant, resuspend in pure water in a 50 mL centrifuge tube, and add Cd 2+ The solution was prepared by adding the root secretion solution of Vallisneria, Water Lily, Water Cabbage and Water Hyacinthus prepared in Example 1 to the other 4 parts, and the Cd 2+The initial concentration was 0.2 mg / L, and the OD 680 The value is 1.00Abs, and the total organic carbon concentration is 10mg / L. The centrifuge tube was placed in a constant temperature water bath shaker, set at 25℃ and 200r / min, and samples were taken with a 5mL sterile syringe at 0, 5, 10, 30, and 60min, filtered with a 0.45μm water filter membrane and stored in a 5mL centrifuge tube. Cd was measured by flame atomic spectrophotometry. 2+ The experiment was repeated three times for four different microalgae to calculate the concentration of Cd 2+ adsorption rate.

[0079] Experimental results: Figure 5 (a) Cd concentrations of Microcystis aeruginosa in the presence of different aquatic plant root secretions 2+ The curve of adsorption rate changing with time. It can be seen that compared with pure water, after adding four root secretions of Vallisneria, Water Cabbage, Water Lily and Water Hyacinth, Cd 2+ The adsorption rate increased significantly, indicating that it was effective in the adsorption of Cd by Microcystis aeruginosa. 2+ There is a promoting effect, and the effect size is water hyacinth > water lily > water cabbage > water grass; under the root secretions of water hyacinth, the equilibrium adsorption rate of Microcystis aeruginosa reaches 92.1%, which is a significant increase compared with the adsorption rate of 65.7% under pure water.

[0080] Figure 5 (b) Cd content of Chlorella vulgaris under different root exudates 2+ The curve of adsorption rate changes with time shows that similar to Microcystis aeruginosa, all four root secretions can promote the absorption of Cd by Chlorella vulgaris. 2+ The adsorption rates were 87.65%, 90.52%, 91.96% and 99.58% respectively, and the promotion effect was in the order of water hyacinth>water lily>water cabbage>vallisneria.

[0081] Figure 5 (c) is the Cd content of Chromococcus in different root secretions 2+ The curve of adsorption rate change over time shows that after adding four root secretions of Vallisneria, Water Cabbage, Water Lily, and Water Hyacinth, the adsorption rates at equilibrium were 75.91%, 77.23%, 76.51%, and 78.24%, respectively. Similar to Microcystis aeruginosa and Chlorella, the four root secretions all promoted the adsorption of Cd by Chromococcus. 2+ The effect of the four root secretions was the best, and the effect of water hyacinth was the best. The difference was that the promoting effect of the four root secretions was water hyacinth > water cabbage > water lily > Vallisneria, but it was still not much different from the adsorption rate of 73.20% under pure water, which showed that the root secretions had a significant effect on the adsorption of Cd by Chromococcus. 2+ The promoting effect is not obvious.

[0082] Figure 5(d) Cd content of Scenedesmus in different root exudates 2+ The curve of adsorption rate changing with time shows that after adding four root secretions of Vallisneria, water cabbage, water lily and water hyacinth, the adsorption rates at equilibrium were 79.54%, 66.01%, 85.53% and 54.28% respectively. Compared with the adsorption rate of 64.80% in pure water, the root secretions of Vallisneria and water lily have a greater effect on the adsorption of Cd by Scenedesmus. 2+ It has a significant promoting effect, while water cabbage has almost no effect. Unlike the above three types of microalgae, the root secretions of water hyacinth have a significant inhibitory effect on the adsorption of Scenedesmus.

[0083] Depend on Figure 6 It can be seen that the root secretions of Vallisneria, Pakchoy and Water Lily have an impact on the Cd 2+ The root secretions of water hyacinth promoted the adsorption of Cd by Microcystis aeruginosa, Chlorella and Chrococcus, but had no significant effect on the adsorption of Cd by Scenedesmus. 2+ Shows an inhibitory effect.

[0084] In addition, by analyzing the changes in microalgae adsorption kinetics and isothermal adsorption characteristics before and after the addition of aquatic plant root secretions, the mechanism of the influence of aquatic plant root secretions on microalgae adsorption was studied, as follows:

[0085] Effects of aquatic plant root exudates on the adsorption kinetics of microalgae:

[0086] Four microalgae under initial Cd 2+ The concentration was 0.2 mg / L, the algal biomass OD 680 The adsorption kinetic model fitting curve under the conditions of 1.00Abs and temperature of 25℃ is shown in Figure 10 It can be seen that the pseudo-first-order kinetics and pseudo-second-order kinetics models have a high degree of fitting for the adsorption process of the four microalgae.

[0087] Table 5 lists the fitting parameters of the two models for the adsorption process of four microalgae. It can be seen from Table 5 that the pseudo-first-order kinetics predicted the adsorption of Cd by Microcystis aeruginosa, Chlorella, Chrococcus, and Scenedesmus. 2+ The equilibrium adsorption capacities of Microcystis aeruginosa, Chlorella vulgaris, Chrococcus, and Scenedesmus were 0.531, 0.491, 0.654, and 0.608 mg / g, respectively. The pseudo-second-order kinetics predicted the adsorption of Cd by Microcystis aeruginosa, Chlorella, Chrococcus, and Scenedesmus. 2+The equilibrium adsorption amounts were 0.539, 0.511, 0.670, and 0.628 mg / g, respectively, which were not much different from the actual adsorption amounts of 0.536, 0.510, 0.665, and 0.617 mg / g. For Microcystis aeruginosa, Chlorella, and Chrococcus, the adsorption amount predicted by pseudo-second-order kinetics was closer to the actual adsorption amount, while for Scenedesmus, the adsorption amount predicted by pseudo-first-order kinetics was closer to the actual adsorption amount. Comparing the correlation coefficients of the two kinetic models, it can be found that the pseudo-second-order kinetic model is more suitable for describing the adsorption of Cd by Microcystis aeruginosa, Chlorella, and Chrococcus. 2+ The adsorption process of the three microalgae indicated that chemical adsorption was the main mechanism for the adsorption of Cd 2+ Therefore, Scenedesmus may have a relatively smaller number of adsorption sites compared with the other three microalgae.

[0088] Table 5 Kinetic fitting parameters of Cd(Ⅱ) adsorption by four microalgae

[0089]

[0090] After adding aquatic plant root secretions, Table 6 lists the initial Cd 2+ The concentration was 0.2 mg / L, algal biomass OD 680 Under the conditions of 1.00 Abs, total organic carbon concentration of 10 mg / L and temperature of 25℃, the adsorption of Cd by four microalgae in root secretions of different plants 2+ The pseudo-first-order and pseudo-second-order kinetic fitting parameters.

[0091] Table 6 shows that the two kinetic models have a significant impact on the adsorption of Cd by Microcystis aeruginosa. 2+ The processes all have a high degree of fitting. Compared with the adsorption in pure water environment, the adsorption amount increased after adding root secretions, and the order was water hyacinth> water lily> water cabbage> Vallisneria. The adsorption process of Microcystis aeruginosa in pure water and water cabbage root secretion environments was more consistent with pseudo-second-order kinetics. After adding root secretions of Vallisneria, water lily and water hyacinth, pseudo-first-order kinetics was more suitable for the description of the adsorption process, that is, extracellular diffusion was the rate-limiting step of the adsorption process, and the predicted adsorption amount was also closer to the actual adsorption amount.

[0092] Similar to Microcystis aeruginosa, the Cd content of Chlorella vulgaris in root secretions of different plants is 2+ The adsorption amount also increased significantly, and the order was Water Hyacinthus> Water Lily> Water Cabbage> Vallisneria. The pseudo-first-order kinetics and pseudo-second-order kinetics fitted the adsorption process of Chlorella very well, and the pseudo-second-order kinetic adsorption constant showed that Chlorella had a higher adsorption rate for Cd after adding root secretions. 2+The adsorption of Chlorella was accelerated. As in pure water, the adsorption of Chlorella after adding water cabbage and water lily root exudates was more consistent with pseudo-second-order kinetics, that is, chemical binding was the rate-limiting step of the process, while the adsorption process of Chlorella after adding Vallisneria and water hyacinth root exudates was more consistent with the pseudo-first-order kinetic model.

[0093] Through the Cd 2+ The fitting results of the adsorption amount over time showed that the adsorption process after adding root exudates still conformed to the pseudo-second-order kinetic model, and the adsorption amount increased compared with that in pure water. The pseudo-second-order kinetic adsorption constants showed that after adding root exudates of Vallisneria, Water Cabbage and Water Lily, the adsorption of Cd by Chrococcus was more stable than that of Chrococcus. 2+ The adsorption rate of water hyacinth root secretions increased, while in the water hyacinth root secretions, although the adsorption amount of Chromococcus increased, the adsorption rate decreased.

[0094] By comparison, the adsorption process of Scenedesmus in pure water is more consistent with the pseudo-first-order kinetic model, but in the presence of aquatic plant root secretions, the correlation coefficient of the pseudo-second-order kinetic model fitting is higher. It is possible that the addition of plant root secretions changes the adsorption of Cd by Scenedesmus. 2+ After the addition of root exudates of Vallisneria and Water Lily, Cd 2+ The adsorption of Cd increased significantly, while the adsorption of Scenedesmus remained almost unchanged under the root secretions of water cabbage, while the adsorption of Cd by water hyacinth increased significantly under the root secretions of water hyacinth. 2+ The adsorption capacity is reduced.

[0095] The attached constants show that after adding root exudates, the Cd 2+ The adsorption process was accelerated.

[0096] Table 6 Adsorption kinetic parameters of four microalgae under different plant root exudates

[0097]

[0098] Effects of aquatic plant root exudates on isothermal adsorption characteristics of microalgae:

[0099] Depend on Figure 11 It can be seen that with the initial Cd 2+ With the increase of Cd concentration, the four microalgae 2+ The adsorption capacity of Cd increases rapidly. 2+ When the concentration increases to a certain level, Cd 2+ The adsorption amount tends to be stable and no longer changes significantly. Compared with the Freundlich model, the Langmuir fitting curve has a better fitting degree.

[0100] As shown in Table 7, the correlation coefficient of the Langmuir model is higher than that of the Freundlich model. The Langmuir model is more suitable for the adsorption of Cd by the four microalgae. 2+ The description is more accurate, and experimental data show that Microcystis aeruginosa, Chlorella, Chrococcus, and Scenedesmus are sensitive to Cd 2+ The maximum adsorption capacities of the four microalgae were 112.00, 125.16, 96.54, and 156.48 mg / g, respectively. 2+ All of them have strong adsorption capacity, the adsorption capacity is Scenedesmus>Chlorella>Microcystis aeruginosa>Chrococcus, and the maximum adsorption capacity predicted by the Langmuir model is not much different from 113.93, 121.29, 96.89, and 167.65 mg / g, indicating that the adsorption of Cd2+ by microalgae is more consistent with monolayer adsorption. The difference is that the correlation coefficients of the Langmuir model and the Freundlich model of Scenedesmus are both above 0.9, indicating that the adsorption of Cd2+ by Scenedesmus is more consistent with the adsorption of Cd2+ by microalgae. 2+ The process shows some characteristics of the Freundlich model. According to the SEM analysis of Scenedesmus, this may be because Scenedesmus is spindle-shaped and presents an uneven surface in pure water, so it also shows some characteristics of heterogeneous adsorption.

[0101] Table 7 Fitting parameters of the isothermal adsorption model for Cd(Ⅱ) by four microalgae

[0102]

[0103]

[0104] After adding aquatic plant root secretions, the adsorption of Cd by microalgae under the influence of four aquatic plant root secretions is listed in Table 8. 2+ The fitting parameters of Langmuir model and Freundlich model. Adsorption capacity Q e It is the maximum amount of adsorbate that a unit mass of adsorbent can accommodate and is an important indicator for evaluating the adsorption capacity of adsorbents. 2+ The adsorption capacity has changed, and in most cases it has increased compared to the pure water environment.

[0105] Table 8 Isothermal adsorption model fitting parameters of four microalgae under plant root exudates

[0106]

[0107] Table 8 shows that the Cd content of Microcystis aeruginosa in the root exudates of four plants 2+The adsorption capacity increased. The adsorption under the root secretions of Vallisneria, Water Cabbage and Water Hyacinth still conforms to the uniform monolayer adsorption described by the Langmuir model, while the adsorption after adding the root secretions of Water Lily is more consistent with the Freundlich model. The parameter n can represent the adsorption of Cd by the adsorbent. 2+ The affinity of Microcystis aeruginosa in water lily root secretions is 0.32, which is higher than the n value of 0.21 obtained in pure water in 3.6.2. This shows that in water lily root secretions, Microcystis aeruginosa has a strong affinity for heavy metal Cd. 2+ The affinity of the adsorption isotherm of Microcystis aeruginosa in pure water described in 3.6.2 is more consistent with the Langmuir model, with a correlation coefficient exceeding 0.99. However, this section found that the Langmuir correlation coefficient in the presence of plant root exudates was relatively low and not much different from the correlation coefficient of the Freundlich model. This suggests that the addition of root exudates shifted the original homogeneous monolayer adsorption to a more complex heterogeneous adsorption pattern, exhibiting heterogeneous adsorption characteristics. This may be due to the binding of root exudates to the microalgae cell surface, altering its surface structure.

[0108] Compared to the adsorption capacity of 121.29 mg / g in pure water, the adsorption capacity of Chlorella in the four plant root secretions increased, and the process was more consistent with the uniform monolayer adsorption described by the Langmuir model, but the correlation coefficient was lower than that in pure water, indicating that the presence of the four plant root secretions changed the surface properties of Chlorella. In addition, the correlation coefficients of the Langmuir model and the Freundlich model in the water lily root secretions were relatively close, indicating that Chlorella adsorbed Cd in the water lily root secretions. 2+ The process is between uniform monolayer adsorption and heterogeneous adsorption. According to experimental data and the Langmuir model's prediction of the adsorption capacity of Chlorella, the promoting effects of the four plant root secretions on Chlorella adsorption are as follows: Water hyacinth > Water lily > Vallisneria > Water cabbage.

[0109] As shown in Table 8, like Microcystis aeruginosa, Chrococcus has the same effect on Cd in the root secretions of Vallisneria salsa, Pakchoy pakchoy and Eupatorium truncatum. 2+ The adsorption process of water lily root secretions is more consistent with the uniform monolayer adsorption described by the Langmuir model, while the adsorption under water lily root secretions is more consistent with the heterogeneous adsorption described by the Freundlich model. The adsorption capacity under the four plant root secretions is higher than that in pure water. Like Microcystis aeruginosa and Chlorella, the greatest promoting effect is from the water hyacinth root secretions, followed by water lily, Vallisneria and water cabbage. The correlation coefficient of the Langmuir model fitting is reduced, while the correlation coefficient of the Freundlich model fitting is increased, and the two are relatively close, indicating that the adsorption of Cd by Chrococcus under plant root secretions is better. 2+The adsorption is also between uniform monolayer adsorption and heterogeneous adsorption.

[0110] The isothermal adsorption model fitting parameters of Scenedesmus in Table 8 show that the adsorption of Cd by Scenedesmus is the same as that in pure water. 2+ The adsorption of Cd still conforms to the uniform monolayer adsorption described by the Langmuir model. However, unlike the above three microalgae, the Cd adsorption of Scenedesmus under the root secretions of water hyacinth is 2+ The adsorption capacity was lower than that in pure water, which is consistent with the adsorption of Cd by water hyacinth root secretions on Scenedesmus in 4.2.1. 2+ The inhibitory effect was consistent, and for Scenedesmus, the promoting effect of adding plant root secretions on the adsorption capacity was water lily > Vallisneria > water cabbage.

[0111] In addition, preliminary research on the mechanism by which aquatic plant root secretions affect cadmium adsorption by microalgae was conducted using three-dimensional fluorescence spectroscopy and Fourier transform infrared spectroscopy. The results are as follows:

[0112] Three-dimensional fluorescence spectroscopy analysis:

[0113] Figure 12 The three-dimensional fluorescence spectra of Microcystis aeruginosa before and after adsorption of four plant root secretions are compared. It is found that the tryptophan-like fluorescence peak (Ex / Em=230, 280nm / 340nm) and the humic acid-like peak (Ex / Em=340nm / 436nm) in the plant water after adsorption are significantly enhanced, which may be related to the extracellular secretions released by Microcystis aeruginosa. 2+ The water samples were subjected to three-dimensional fluorescence detection to prove the adsorption of Cd 2+ Microcystis aeruginosa did release both substances.

[0114] Depend on Figure 13 It can be seen that after Chlorella adsorbed the root secretions of the four plants, the humic acid-like peaks (Ex / Em=340nm / 436nm) all decreased, indicating that the humic acid-like peaks were adsorbed by Chlorella. In addition, it was observed that the tryptophan-like fluorescence peaks (Ex / Em=230, 280nm / 340nm) in the root secretions of water cabbage and water lily were significantly weakened after adsorption, indicating that they also participated in the absorption of Cd by Chlorella. 2+ adsorption.

[0115] Cd adsorption by Chromococcus in the presence of root exudates from four plants 2+ Three-dimensional fluorescence contrast before and after Figure 14As shown in the figure, there was no significant change in the DOM in the water before and after adsorption of the root secretions of Vallisneria and Water Cabbage, but a significant weakening of the tryptophan-like fluorescence peak (Ex / Em=280nm / 330nm) was observed before and after adsorption of the root secretions of Water Lily, indicating that the Chrococcus algae was likely to interact with this component; similar to Chlorella, the humus-like substances in the root secretions of Water Hyacinthus were significantly reduced before and after adsorption, indicating that the humus-like substances interacted with the Chrococcus algae during the adsorption process and were filtered and separated after the adsorption.

[0116] Depend on Figure 15 It can be seen that Scenedesmus adsorbs Cd in the root secretions of Vallisneria and Water Lily. 2+ After the adsorption, the humic acid-like peak at Ex / Em=250nm / 400nm weakened, indicating that Scenedesmus had combined with DOM with the fluorescence peak at this position in some way. The humic acid-like peak before and after adsorption under the water hyacinth root secretions was enhanced. The addition of water hyacinth root secretions can reduce the absorption of Cd by Scenedesmus. 2+ The adsorption rate of humic acid is the result of the decrease of humic acid and Cd 2+ Competitive adsorption occurred on the surface of Scenedesmus, which may be related to the complexity and diversity of microalgae cells.

[0117] In general, in the presence of plant root secretions, microalgae may release some secretions, and the DOM in the plant root secretions will also interact with the microalgae. When Chlorella and Chrococcus adsorb on the root secretions of water hyacinth, the root secretions of water hyacinth bind to the surface of the microalgae and are filtered and separated after the adsorption. 2+ When adsorbed, secretions are released, resulting in the enhancement of tryptophan-like peaks (Ex / Em=280nm / 330nm) and humic acid-like peaks (Ex / Em=340nm / 436nm) in the three-dimensional fluorescence spectrum of plant root secretions after adsorption.

[0118] Fourier transform infrared spectroscopy analysis:

[0119] Figure 16 The following is a comparison of the Fourier transform infrared spectra of the four microalgae before and after adsorption with the root secretions of Vallisneria lappa and in pure water (blank control). It can be seen that after adsorption with the addition of Vallisneria lappa root secretions, the aeruginosa microcystis at 3298 cm -1 The NH peak at 3294 cm-1 showed a blue shift, which is consistent with the red shift to 3294 cm-1 in pure water. -1 There are some differences; 1742cm -1 The C=O peak in the ester does not change in pure water, but it blue-shifts to 1747 cm after adsorption in the root secretions of Vallisneria. -1 1655cm -1The amide I band at 1242 cm-1 showed no significant change compared with that in pure water, indicating that the root secretions of Vallisneria salsa promoted the adsorption of Microcystis aeruginosa and did not act on the amide group. -1 The stretching vibration of the CO bond at 100 nm shifted blue after adsorption in pure water, but red in the presence of Vallisneria root secretions. Compared with that in pure water, the PO peak also shifted red in the presence of Vallisneria root secretions, indicating that amino, ester, and phosphate groups may be the factors that promote the adsorption of Cd by Microcystis aeruginosa by Vallisneria root secretions. 2+ The main target of action.

[0120] Compared with the blank control group in pure water, the amide II band that did not change after Chlorella was adsorbed in the root secretions of Vallisneria lappa was red-shifted to 1742 cm -1 The C=O peak in the ester that originally blue-shifted has a larger shift, while the shifts of the NH peak and the CO peak in the ester are reduced, and the C=O peak in the amide does not even change, indicating that the root secretions of Vallisneria may promote the absorption of Cd by Chlorella by promoting the amide II band and the C=O in the ester. 2+ The binding of amide C=O and Cd 2+ combination.

[0121] Like Microcystis aeruginosa, the NH peak of Chrococcus abruptly shifted after adsorption on the root secretions of Vallisneria lappa, and the C=O peak of the ester underwent a more significant blue shift based on adsorption in pure water, which may be related to the fact that they are both cyanobacteria. However, the CO peak of the ester did not change significantly, and the amide I band red shifted. In addition, the 913 cm -1 The peak area of ​​OH in carboxylic acid is significantly reduced in the environment of Vallisneria root exudates, indicating that NH, C=O in amide, C=O in ester and OH in carboxylic acid may be the factors that promote the adsorption of Cd by Chromococcus by Vallisneria root exudates. 2+ The main target of action.

[0122] The addition of Vallisneria root secretions caused a greater blue shift in NH after Scenedesmus adsorption. The CO peak in the ester, which did not shift in pure water, shifted blue, while the shift of the C=O peak in the carboxylic acid decreased. Other peaks did not change significantly.

[0123] Figure 17The following is a comparison of the Fourier transform infrared spectra of four microalgae before and after adsorption on water cabbage root secretions and in pure water (blank control). Compared with the blank control group, after adsorption on water cabbage root secretions, the OH or NH peaks of Microcystis aeruginosa shifted significantly in the opposite direction, the C=O in esters shifted slightly to the blue, the C=O in amides shifted red in the opposite direction, the CH in alkyls did not change much, and the PO in the phosphate group shifted slightly to the blue, indicating that water cabbage root secretions may promote the absorption of Cd by Microcystis aeruginosa by these groups. 2+ Adsorption of Cd by Chlorella vulgaris in the environment of water cabbage root secretions 2+ After adsorption, the C-H bond in the alkyl group undergoes a slight blue shift, the blue shift of the OH or NH peak, the C=O in the ester and the C=O in the amide group is suppressed, and the other peaks do not change significantly. Compared with the infrared spectrum after adsorption in pure water environment, Figure 4-18 The peak area under the condition of water cabbage root secretions increased significantly, indicating that water cabbage root secretions inhibited the chemical binding during the adsorption process of Chlorella. This may be due to the effect of water cabbage root secretions on the adsorption of Cd by Chlorella. 2+ One of the reasons for the minimal promoting effect is that, compared to pure water, after adsorption of Chromococcus on water cabbage root exudates, the OH or NH peaks shifted inversely, while the C=O, CN, and NH in amide groups, as well as COC in carbohydrates, all underwent a certain degree of blue shift, while other peaks showed no significant changes. In contrast, for Scenedesmus, the CO in its esters and COC in its polysaccharides showed a significant blue shift compared to pure water, while other characteristic peaks such as NH, C=O in carboxylic acids, and C=O in amides were suppressed.

[0124] Figure 18 The following is a comparison of the Fourier transform infrared spectra of four microalgae before and after adsorption on water lily root secretions and adsorption on pure water (blank control). Figure 18 It can be seen that the root secretions of water lilies promote the absorption of Cd by Microcystis aeruginosa through the amino groups, amide C=O and PO 2+ For Chlorella vulgaris, water lily root secretions primarily promoted the adsorption of esters, amides, CN, and COC, while inhibiting the effects of amine groups. Similar to Microcystis aeruginosa, Chromococcus exhibited greater shifts in both amine and amide groups, with the CO bond also playing a greater role. For Scenedesmus, water lily root secretions significantly shifted the peaks of groups such as amine NH, ester CO, alcohol CO, and carboxylic acid OH.

[0125] Cd adsorption by four microalgae in the presence of water hyacinth root exudates 2+ The Fourier transform infrared spectra before and after are shown in Figure 19 Comparison of Cd adsorption by Microcystis aeruginosa before and after adsorption in pure water (blank control) and by water hyacinth root secretions2+ FT-IR of the blank control was found to be 1742 cm -1 The C=O in the ester blue shifted to 1746 cm after adding water hyacinth root secretions. -1 There was no shift in the blank control, but there was a shift in the water hyacinth root secretion environment. The CH bond in the alkyl group at 1452 cm -1 and 1393cm -1 The bending vibrations generated at 1455 cm-1 and 1465 cm-2 were shifted to 1455 cm-1 after adsorption in the water hyacinth root secretion environment. -1 and 1396cm -1 The synchronous shift of these two peaks also confirms that the absorption peaks are generated by alkyl groups. The C=O in amide and the PO in phosphate groups show a greater shift after adsorption in the water hyacinth root secretion environment than after adsorption in pure water, shifting from 1657 cm to 2667 cm respectively. -1 and 1031cm -1 Shifted to 1660cm -1 and 1034cm -1 However, the CN and NH groups in the amide II band and the P=O groups in the phosphodiester of peptidoglycan did not change significantly in the blank control and water hyacinth root exudates. This indicates that water hyacinth root exudates promote the adsorption of Cd by Microcystis aeruginosa. 2+ This is likely achieved by promoting C=O in esters, C=O in amide groups, alkyl groups, and PO in phosphate groups. However, compared to pure water, the 3298 cm -1 NH at 1242 cm -1 The adsorption of CO in the ester was inhibited after adding water hyacinth root secretions, indicating that water hyacinth root secretions promoted the adsorption of some groups on Cd 2+ The adsorption of Cd may also inhibit the adsorption of some groups. 2+ Combining ability.

[0126] The main groups that changed after adsorption of Cd in pure water by Chlorella included amino groups, esters, amides, and CO in polysaccharides. Compared with pure water, after adsorption under water hyacinth root secretions, the C=O in esters, C=O in amides, and CO in alcohols on the surface of Chlorella cells underwent more significant shifts, indicating that water hyacinth root secretions promoted the adsorption of Cd by Chlorella. 2+ It is possible that by promoting the Cd 2+ Combination to achieve.

[0127] Amide groups adsorb Cd in Chromococcus 2+ However, after adsorption in the water hyacinth root secretion environment, the C=O in the amide group shifted significantly, indicating that the water hyacinth root secretion may promote the adsorption of amide groups on Cd2+ In addition, the carbohydrate COC of the pure water control group was at 1035 cm -1 The absorption peak at 1039 cm shifted to 1039 cm after the addition of water hyacinth root secretions. -1 This is consistent with what was observed for Microcystis aeruginosa and Chlorella vulgaris.

[0128] In pure water, carboxyl and amine groups participate in the adsorption of Cd by Scenedesmus 2+ process, and after adsorption in water hyacinth root secretions, the CO peak in carbohydrates changed from 1041 cm -1 Offset to 1037cm -1 The other groups did not change significantly, indicating that the effect of water hyacinth root secretions on the adsorption of Scenedesmus was mainly on cellulose and other polysaccharides. Combined with the phenomenon that water hyacinth root secretions inhibited Scenedesmus, it may be that the root secretions and Cd 2+ Competitive adsorption was formed on the surface of Scenedesmus cells.

[0129] X-ray photoelectron spectroscopy analysis:

[0130] The water hyacinth root secretions were used to absorb Cd 2+ The types of elements and their content changes before and after adsorption were determined on Chlorella vulgaris.

[0131] Figure 20 The XPS spectra before adsorption of Chlorella and after adsorption in pure water and water hyacinth root secretions are shown. Two obvious Cd 3d binding energy peaks can be seen, indicating that it is adsorbed on the surface of Chlorella. In addition, the Ca 2p3 peak at 347.09 eV before adsorption decreases in turn after adsorption in pure water and water hyacinth root secretions until it basically disappears, indicating that Cd 2+ With Ca 2+ Ion replacement occurs on the cell surface. Studies have shown that Cd 2+ It has a complete d-shell of electrons and is therefore capable of forming ionic bonds.

[0132] As can be seen from Table 9, after adsorption in pure water or water hyacinth root secretions, Cd elements appeared on the surface of Chlorella cells, proving that Cd 2+ Adsorbed on the cell surface, the proportion of Cd adsorbed in water hyacinth root secretions was higher, consistent with the experimental results. Furthermore, after adsorption in pure water, the proportions of C, O, and N elements remained largely unchanged. However, after adsorption in water hyacinth root secretions, the proportion of C significantly increased, while the proportion of N significantly decreased. This may be due to the low proportion of N in the humus-like substances in water hyacinth root secretions, which then adsorbed on the surface of Chlorella, altering its elemental content.

[0133] Table 9 The percentage of element contents before Chlorella adsorption and after adsorption in pure water and water hyacinth root secretions

[0134]

[0135] To further understand the effect of water hyacinth root secretions on the adsorption of Cd by Chlorella vulgaris 2+ The promotion mechanism of β-catenin was used to perform peak fitting on each fine spectrum.

[0136] (1) C 1s fine spectrum

[0137] Depend on Figure 21 It can be seen that the C1s binding energy peaks of Chlorella before adsorption include 284.80eV, 286.48eV and 288.68eV, corresponding to CC / CH, CO / CN and C=O, respectively. CC / CH corresponds to the carbon skeleton, CO / CN comes from hydroxyl, ether and amino groups, and C=O comes from carboxyl, ester and amide groups. This is consistent with the results obtained by FT-IR in 3.6.4. After adsorption in pure water, the C1s binding energy peaks are 284.80eV, 286.53eV and 288.80eV, respectively. Among them, CO and C=O have shifted. The reason may be that O atoms provide electrons to heavy metal cations to form covalent bonds. The electron density of O atoms and their adjacent C atoms decreases, and the binding energy increases, indicating that CO and C=O participate in the adsorption of Cd by Chlorella. 2+ After adsorption in the root secretions of water hyacinth, the C 1s binding energy peaks were 284.80eV, 286.50eV and 288.91eV, respectively, and CO and C=O showed more shifts. According to the results of three-dimensional fluorescence spectrum analysis, the main component of the root secretions of water hyacinth is humus-like substances, which contain carboxyl groups that can combine with the carboxyl groups on the surface of Chlorella cells in the form of hydrogen bonds. The O atoms provide electrons to the bonding H atoms, thereby reducing the electron density of the adjacent C atoms and shifting towards the direction of increasing the binding energy. This shows that the root secretions of water hyacinth bind to the microalgae cells and Cd through hydrogen bonds. 2+ bridges between the 2+ The adsorption of water hyacinth was consistent with the results of FT-IR that the root secretions of water hyacinth acted on C=O and other groups to promote the adsorption of Chlorella.

[0138] (2) N 1s fine spectrum

[0139] Studies have shown that when N combines with divalent metal cations, the metal cations provide empty orbitals, while N provides electrons to form complexes with the metal cations in a coordinated form. The electron density is reduced, and the binding energy is reduced. Figure 22It can be seen that the binding energy peak of Chlorella N1s is 399.99eV, and after adsorption in pure water, the N1s binding energy peak shifts to 400.08eV, indicating that the CN, NH in the amide and amine groups bind to the Cd 2+ This is consistent with the FT-IR results in 3.5.4 and 4.5.3. In the water hyacinth root secretion environment, the N 1s binding energy peak area is significantly reduced, indicating that more CN and NH are involved in the Cd 2+ combination.

[0140] (3) Cd 3d fine spectrum

[0141] Depend on Figure 23 It can be found that Cd 3d has two binding energy peaks, namely Cd 3d 3 / 2 (411.07 eV) and Cd 3d 5 / 2 (404.33 eV), the appearance of double peaks is caused by the spin-orbit splitting of Cd 3d, and the peak areas generally have a fixed ratio.

[0142] 2. Verification of the effect of initial cadmium concentration on cadmium adsorption by microalgae

[0143] Experimental method: Take 5 20 mL portions of the same algae-containing solution, centrifuge at 4500 r / min for 5 min, remove the supernatant, add pure water and mix thoroughly with a cyclone mixer, centrifuge under the same conditions for 5 min, remove the supernatant, resuspend in pure water in a 50 mL centrifuge tube, and add Cd 2+ solution, respectively, Cd 2+ The initial concentrations were 0, 0.2, 0.5, 1.0, and 1.5 mg / L, and the OD 680 The value was 1.00Abs. The centrifuge tube was placed in a constant temperature water bath shaker, the temperature was set to 25°C, the speed was set to 200r / min, and samples were taken with a 5mL sterile syringe at 0, 5, 10, 30, and 60min, respectively. The samples were filtered with a 0.45μm water filter membrane and stored in a 5mL centrifuge tube. Cd was measured by flame atomic spectrophotometry. 2+ The experiments on four different microalgae were repeated three times.

[0144] Experimental results: At the initial concentrations of 0.2, 0.5, 1.0, and 1.5 mg / L, the effects of Microcystis aeruginosa, Chlorella vulgaris, Chrococcus, and Scenedesmus on Cd 2+ The adsorption rate changes with time. Figure 7 .Depend on Figure 7 It can be seen that the four microalgae have the best response to Cd at 0-5min. 2+ All showed a rapid adsorption process, which was basically completed in about 5 to 10 minutes, and Cd 2+ The adsorption rate tends to be stable; in addition, as the initial Cd2+ As the concentration increases, the adsorption rate also increases accordingly, which may be due to the fact that 2+ The concentration is low, resulting in Cd 2+ The extracellular diffusion of Cd is restricted, and the appropriate increase of Cd 2+ The initial concentration is conducive to increasing the microalgae and Cd 2 + Opportunity to contact.

[0145] Microcystis aeruginosa under initial Cd 2+ The adsorption rates at equilibrium at concentrations of 0.2, 0.5, 1.0, and 1.5 mg / L were 65.7%, 85.1%, 92.8%, and 95.4%, respectively. Figure 7 (a) It can be seen that when the initial cadmium concentration is 0.2-0.5 mg / L, the cadmium adsorption rate of Microcystis aeruginosa increases significantly with the increase of concentration, but when the concentration rises to 1.0 and 1.5 mg / L, the adsorption rate does not increase significantly. Figure 7 (b) and Figure 7 (c) shows that Chlorella vulgaris 2+ The adsorption rates at equilibrium when the concentrations were 0.2, 0.5, 1.0, and 1.5 mg / L were 57.4%, 80.8%, 90.8%, and 93.0%, respectively. 2+ The adsorption rates at equilibrium at concentrations of 0.2, 0.5, 1.0, and 1.5 mg / L were 73.2%, 85.6%, 92.3%, and 92.5%, respectively. It can be found that Chlorella and Chrococcus had good adsorption performance at 0.2-1.5 mg / L Cd. 2+ Within the concentration range, the pattern of adsorption rate changing with initial concentration is basically consistent with that of Microcystis aeruginosa. Figure 7 (d) is the growth of Scenedesmus at different initial Cd 2+ The change of adsorption rate under the concentration of initial Cd 2+ Average Cd at concentrations of 0.2, 0.5, 1.0, and 1.5 mg / L 2+ The adsorption rates were 64.8%, 71.8%, 72.4% and 77.1% respectively. As with Microcystis aeruginosa, Chlorella and Chrococcus, the adsorption rates of Cd 2+ As the concentration increased, the adsorption rate of Scenedesmus also gradually increased, but its growth rate was obviously smaller than that of the above three microalgae. 2+ The adsorption rate reached 64.8% at an initial Cd concentration of 1.5 mg / L. 2 + At the concentration, the adsorption rate was only 77.1%. Xiao Wanlu et al. studied the adsorption of Scenedesmus quadricauda under initial Cd 2+ When the concentration is 0.5~10mg / L, Cd 2+ The change of adsorption rate is as the initial Cd 2+The content of β-catenin increased with the increase of concentration, but only increased from 66.67% to 72.93%, with a small change.

[0146] 3. Verification of the effect of algal biomass on cadmium adsorption by microalgae

[0147] Experimental method: Take three 20 mL volumes of the same algae-containing solution, centrifuge at 4500 r / min for 5 minutes, remove the supernatant, add pure water and mix thoroughly with a cyclone mixer, centrifuge under the same conditions for 5 minutes, remove the supernatant, resuspend in pure water in a 50 mL centrifuge tube, and add Cd 2+ solution, respectively, to make the OD of algae 680 The values ​​are 0.20, 0.50, 1.00 Abs, Cd 2+ The initial concentration was 0.2 mg / L. The centrifuge tube was placed in a constant temperature water bath shaker, the temperature was set to 25°C, the speed was 200 r / min, and a 5 mL sterile syringe was used to sample at 60 min. The sample was filtered with a 0.45 μm water filter membrane and stored in a 5 mL centrifuge tube. The Cd content was measured by flame atomic spectrophotometry. 2+ The experiments on four different microalgae were repeated three times.

[0148] Experimental results: When the temperature is 25℃ and the initial Cd 2+ The OD 680 The four microalgae showed the same Cd absorption when the values ​​were 0.20, 0.50 and 1.00 Abs. 2+ The changes in adsorption rate, the experimental results are as follows Figure 8 As shown. When the algal biomass is as low as OD 680 When the Abs value was 0.20, the sensitivity of Microcystis aeruginosa, Chlorella, Chrococcus, and Scenedesmus to Cd 2+ The adsorption rates at equilibrium were 32.9%, 32.5%, 36.8% and 37.5% respectively; at OD 680 When the Abs value was 0.50, the sensitivity of Microcystis aeruginosa, Chlorella, Chrococcus, and Scenedesmus to Cd 2+ The adsorption rates increased to 41.8%, 46.4%, 46.8% and 42.8% respectively; and when the algae biomass increased to OD 680 When the Abs value was 1.00, the equilibrium adsorption rates of Microcystis aeruginosa, Chlorella vulgaris, Chromococcus, and Scenedesmus increased significantly, reaching 65.7%, 57.4%, 73.2%, and 64.8%, respectively. 2+ The adsorption rates increased significantly with the increase of algal biomass.

[0149] 4. Verification of the effect of temperature on cadmium adsorption by microalgae

[0150] Experimental method: Take three 20 mL volumes of the same algae-containing solution, centrifuge at 4500 r / min for 5 minutes, remove the supernatant, add pure water and mix thoroughly with a cyclone mixer, centrifuge under the same conditions for 5 minutes, remove the supernatant, resuspend in pure water in a 50 mL centrifuge tube, and add Cd 2+ solution, so that the OD of algae 680 The value is 1.00Abs, Cd 2+ The initial concentration was 0.2 mg / L. The centrifuge tube was placed in a constant temperature water bath shaker, and the temperature was set to 15, 25, and 35 °C, and the speed was 200 r / min. After 60 minutes, a 5 mL sterile syringe was used to take a sample, which was filtered with a 0.45 μm water filter membrane and stored in a 5 mL centrifuge tube. The Cd content was measured by flame atomic spectrophotometry. 2+ The experiments on four different microalgae were repeated three times.

[0151] Experimental results: In OD 680 The value is 1.00Abs, the initial Cd 2+ Under the conditions of Cd concentration of 0.2 mg / L and adsorption time of 60 min, the four microalgae were subjected to Cd adsorption at three temperatures: 15, 25, and 35 °C. 2+ The change of adsorption rate is as follows Figure 9 As shown in the figure, at 15℃, the sensitivity of Microcystis aeruginosa, Chlorella vulgaris, Chrococcus and Scenedesmus to Cd 2+ The adsorption rates of the four microalgae were 64.3%, 59.2%, 68.1% and 64.6%, respectively, which were not much different from 65.7%, 57.4%, 73.2% and 64.8% at 25°C. As the experimental temperature increased to 35°C, the adsorption rates of the four microalgae increased slightly, namely 73.3%, 62.4%, 78.4% and 71.9%, respectively. The adsorption rates of the four microalgae did not change significantly with temperature. Figure 9 The effects of four microalgae on Cd at different temperatures 2+ The change of adsorption amount shows an upward trend with the increase of temperature. For example, the adsorption amount of Cd 2+ The adsorption amounts were 64.33, 65.66, and 73.25 mg / g, respectively. That is, when the temperature changed from 15°C to 25°C, the adsorption amount increased slightly. When the temperature was raised to 35°C, the adsorption amount increased more significantly. The increase in temperature accelerated the diffusion rate of the adsorbate on the surface boundary and in the pores of the adsorbent, indicating that the microalgae adsorbed Cd. 2+ The process may involve chemical binding and physical adsorption.

[0152] Example 3

[0153] Synergistic adsorption treatment of cadmium-contaminated soil by aquatic plant root exudates and microalgae and effect verification

[0154] The cadmium-contaminated soil in this example was collected from a farmland in Yueyang City, Hunan Province. The soil pH was 7.22 and the total cadmium content was 3.31 mg / kg. The microalgae used were the same as those in Example 2.

[0155] The treatment method is as follows: the collected soil samples are first dried at 50° C., weeds, branches and other impurities are picked out, ground and mixed evenly, and then passed through a 2 mm stainless steel sieve, placed in a sterilized sample bag and stored for future use. Four 50 g portions of pretreated soil are accurately weighed, one portion is used as a blank control, and three portions are added with different dry weights of Chlorella at a microalgae to soil mass ratio of 1:5000, 1:10000, and 1:20000, respectively. The water hyacinth root exudate solution prepared in Example 1 is added to immerse the soil, and culture is carried out in a constant temperature shaking incubator (model BS-1E, Changzhou Guohua Electric Co., Ltd.). During the experiment, the water hyacinth root exudate solution is kept immersed in the soil. The culture light-dark ratio is 12h:12h, and the light intensity is 2000 lux (xenon lamp). Appropriate amounts of soil samples are weighed on days 0, 3, 7, 14, 21, and 28, and fully dried at 50° C. The available cadmium concentration in the soil samples is measured according to the method of 0.

[0156] Processing result: Figure 24 It can be seen that the available Cd in the soil decreased rapidly within 0 to 7 days, but remained basically unchanged after the 21st day, indicating that the adsorption of available Cd in the soil by Chlorella includes a rapid adsorption process and a biological transformation process. On the 28th day, the available Cd content in the soil of the experimental group with a Chlorella to soil mass ratio of 1:10000 was reduced to 1.44 mg / kg, and the adsorption effect was the best among the three algae addition amounts. The changes in the available Cd content in the soil of the experimental group with the addition of water hyacinth root secretions are shown in the figure below. Figure 4-25 (b) Similar to the pure water group, the available Cd content also showed a rapid decrease from 0 to 7 days. From 7 to 21 days, the available Cd content continued to decrease, with a greater decrease than in the pure water group. This indicates that the addition of water hyacinth root secretions enhanced the sustained adsorption of Cd by Chlorella. Among the experimental groups with different addition doses, the 1:10,000 addition dose showed the best removal of available Cd, with the available Cd content decreasing from 2.13 mg / kg on day 0 to 1.12 mg / kg on day 28.

[0157] To further explore the transformation of Cd in soil, the BCR extraction method was used to detect the changes in the content of various Cd forms in the soil after 28 days of treatment. Figure 25It can be seen that after the Cd in the contaminated soil was fixed by Chlorella, the proportion of weakly acidic and reducible Cd contents at the addition of 1:5000 and 1:10000 algae decreased, while the proportion of oxidizable Cd and residual Cd contents increased. Residual Cd refers to the part of the soil that is difficult to be utilized by organisms. It generally exists in the form of metal sulfides. The addition of Chlorella adsorbs a part of the Cd while increasing the surface negative charge in the soil solid phase, thereby enhancing the binding of the soil to heavy metals. In addition, the soil is in a reduced state under flooding conditions, and SO4 2- Gaining electrons and being reduced to S 2- , combines more Cd, converts it into a more stable phase, and thus reduces the risk of cadmium pollution in the soil.

[0158] After adding water hyacinth root secretions, the changes in the proportion of Cd forms in the soil of the experimental groups with different algae addition amounts are as follows: Figure 25 As shown in (b), the experimental soils with three different algae addition amounts showed a decrease in the proportion of weakly acidic Cd and reducible Cd, a slight increase in the proportion of oxidizable Cd, and a significant increase in the proportion of residual Cd, indicating that the addition of water hyacinth root secretions promoted the fixation of Cd in the soil by Chlorella. The reasons include: the strong aromaticity and hydrophobicity of water hyacinth root secretions, and the addition of organic matter may increase the negative charge on the surface of soil particles, thereby enhancing the binding of soil and Cd.

[0159] The present invention studies the effects of Microcystis aeruginosa, Chlorella vulgaris, Chrococcus and Scenedesmus on Cd in the root secretions of four aquatic plants: Vallisneria truncatula, water cabbage, water lily and water hyacinth. 2+ The composition and characteristics of root exudates and their effects on the adsorption of Cd by four microalgae were investigated by EEMs, FT-IR and XPS. 2+ The main conclusions are as follows:

[0160] (1) At the initial Cd 2+ When the concentration was 0.2 mg / L, the root secretions of the four plants promoted the adsorption of Microcystis aeruginosa, Chlorella vulgaris and Chrococcus. For Microcystis aeruginosa and Chlorella vulgaris, the four plant waters had a greater effect on the adsorption of Cd. 2+ The promoting effect of adsorption rate is water hyacinth>water lily>vallisneria>water cabbage, and the adsorption of Cd by Chromococcus is promoted. 2+ The promoting effect of water lily>vallisneria>water cabbage was not obvious, while the promoting effect of water hyacinth root secretion on the adsorption of Cd by scenedesmus was as follows: water lily>vallisneria>water cabbage, while the root secretion of water hyacinth had an obvious promoting effect on the adsorption of Cd by scenedesmus. 2+ Shows an inhibitory effect.

[0161] (2) At the initial Cd 2+ When the concentration was 0.2 mg / L, the effects of four root exudates on the adsorption of Cd by microalgae 2+The kinetic characteristics of Cd adsorption were affected in different ways. The root secretions of Vallisneria, Nymphaea and Water Hyacinth changed the adsorption kinetic characteristics of Microcystis aeruginosa, and the adsorption process was more consistent with the pseudo-first-order kinetic model. After adding Vallisneria and Nymphaea root secretions, the adsorption of Cd by Chlorella was improved. 2+ The process of Cd adsorption by water cabbage and water hyacinth root secretions was more consistent with the pseudo-first-order kinetic model, while the adsorption by water cabbage and water hyacinth root secretions was still more consistent with the pseudo-second-order kinetic model; the four plant root secretions had little effect on the adsorption kinetics of Chromococcus; for Scenedesmus, its adsorption of Cd in pure water was 2+ The adsorption of Cd by the four plant root secretions was more in line with the pseudo-first-order kinetic model, while after adding the four plant root secretions, it was more inclined to the pseudo-second-order kinetic model, indicating that the four plant root secretions enhanced the adsorption of Cd by Scenedesmus. 2+ Chemical bonding during the process.

[0162] (3) The adsorption of the four microalgae in pure water is more consistent with the Langmuir model, and R 2 All of them reached above 0.98, among which Microcystis aeruginosa, Chlorella and Chrococcus reached above 0.99. Among the root secretions of four aquatic plants, the R 2 Both decreased, and the R of the Freundlich model 2 The results showed that the addition of root secretions changed the uniformity of the microalgae surface, making the microalgae show more heterogeneous adsorption characteristics. Among them, the adsorption of Microcystis aeruginosa and Chrococcus under the root secretions of water lilies was more consistent with the Freundlich model. Plant root secretions increased the adsorption capacity of microalgae, among which the root secretions of water hyacinth had a significant promoting effect on Microcystis aeruginosa and Chlorella. 2+ The adsorption capacities reached 330.10 mg / g and 311.03 mg / g respectively.

[0163] (4) The UV-vis and EEMs optical index parameters of the root secretions of the four aquatic plants showed that the aromaticity and hydrophobicity of DOM in the root secretions of water hyacinth were relatively strong. These two characteristics were conducive to the adsorption of heavy metal cations by biological cells. The relative content of humus-like substances in the root secretions was water hyacinth > water lily > water grass > water cabbage, which was consistent with the degree of promotion of the adsorption effect on Microcystis aeruginosa and Chlorella vulgaris. Humus-like substances may promote the adsorption of Cd by microalgae. 2+ key factors.

[0164] (5) The root secretions of the four aquatic plants had different effects on the position and peak area of ​​the characteristic peaks of the functional groups on the surface of microalgae. Among them, the root secretions of Vallisneria lappa mainly promoted the ester, amide, carbohydrate (COC) and phosphate groups to Cd 2+The root secretions of water lilies mainly acted on the amide groups, esters and phosphate groups on the cell surface of Microcystis aeruginosa. For Chromococcus, it increased the displacement of amide groups, OH in carboxyl groups and COC in carbohydrates, while for Chlorella and Scenedesmus, it mainly acted on CO in esters. The root secretions of water lilies promoted the adsorption of Cd by the four microalgae. 2+ This is mainly achieved by promoting NH in amine groups, C=O in esters, CN and NH in amides, PO in phosphate groups, and CO in polysaccharides; water hyacinth root exudates affect the adsorption of Cd by four microalgae 2+ It is mainly achieved by acting on amide groups, esters and COC in carbohydrates.

[0165] (6) X-ray photoelectron spectroscopy showed that compared with pure water, Cd 2+ There are two main ways for Cd to increase its adsorption on the surface of Chlorella cells: 2+ By 2+ On the other hand, the root secretions of water hyacinth are adsorbed on the surface of Chlorella cells through hydrogen bonds between Chlorella and Cd 2+ Bridges are formed between them to increase the adsorption sites on the cell surface.

[0166] (7) Whether in pure water or water hyacinth root secretions, Chlorella can reduce the content of available Cd in soil, and the effect is even better after adding water hyacinth root secretions. The increase in the removal rate of available Cd in soil at different algae addition amounts reaches 8% to 15%. After 28 days of treatment with Chlorella, the content of Cd in the contaminated soil changed. Whether in pure water or water hyacinth root secretions, the proportion of weakly acidic and reducible Cd increased, and the proportion of residual Cd increased significantly. That is, some of the Cd in the soil that is easily desorbed was converted into a more stable residual Cd, and the bioavailability of Cd in the soil decreased.

[0167] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for synergistically treating environmental cadmium pollution by using aquatic plant root secretions and microalgae, characterized in that: The following steps are involved: (1) Preparation of aquatic plant root exudates The aquatic plants are washed and placed in a container filled with pure water. The roots are fixed with clean sand and gravel, and the culture treatment is carried out. After that, the water in the container is taken out and filtered to obtain a solution of aquatic plant root secretions. The aquatic plant is any one of Vallisneria, Water Cabbage, Water Lily and Water Hyacinth, or a combination of two or more thereof, and the microalgae is Chlorella vulgaris; (2) Adsorption treatment of cadmium pollutants Aquatic plant root exudate solution and microalgae are added to cadmium pollutants for cultivation and adsorption treatment.

2. The method for synergistically treating environmental cadmium pollution by using aquatic plant root secretions and microalgae as claimed in claim 1, characterized in that: The aquatic plant is water hyacinth.

3. The method for synergistically treating environmental cadmium pollution by using aquatic plant root secretions and microalgae as claimed in claim 1 or 2, characterized in that: The pH value of the aquatic plant root exudate solution is 7.0-7.4, and the total organic carbon concentration is 11-52 mg / L.

4. The method for synergistically treating environmental cadmium pollution by using aquatic plant root secretions and microalgae as claimed in claim 1 or 2, characterized in that: In step (1), aquatic plant seedlings are selected and cultured under indoor ventilation conditions for 150 days, and are irradiated with simulated sunlight for 8 hours per day.

5. The method for synergistically treating environmental cadmium pollution by using aquatic plant root secretions and microalgae as claimed in claim 1 or 2, characterized in that: In step (1), the filtration treatment adopts 0.45 μm water filter membrane filtration.

6. The method for synergistically treating environmental cadmium pollution using aquatic plant root secretions and microalgae as claimed in claim 1 or 2, characterized in that: In step (2), the cadmium pollutant is cadmium-contaminated water, the initial cadmium concentration of the mixed system of aquatic plant root exudate solution, microalgae and cadmium-contaminated water is 0.5-1.5 mg / L, and the OD of microalgae is 0.5-1.5 mg / L. 680 The value is above 1.00Abs and the total organic carbon concentration is 10mg / L.

7. The method for synergistically treating environmental cadmium pollution using aquatic plant root secretions and microalgae as claimed in claim 1 or 2, characterized in that: In step (2), the cadmium pollutant is cadmium-contaminated soil. The microalgae and the cadmium-contaminated soil are first mixed evenly, with the mass ratio of the microalgae to the cadmium-contaminated soil being 1:5000 to 20000. Then, a solution of aquatic plant root secretions is added to submerge the soil. During the cultivation and adsorption treatment process, the soil is submerged by supplementing the solution of aquatic plant root secretions.

8. The method for synergistically treating environmental cadmium pollution using aquatic plant root secretions and microalgae as claimed in claim 1 or 2, characterized in that: In step (2), the temperature condition for the culture adsorption treatment is 25 to 35°C.

9. The method for synergistically treating environmental cadmium pollution using aquatic plant root secretions and microalgae as claimed in claim 1 or 2, characterized in that: In step (2), the adsorption culture was carried out under the conditions of a light-dark ratio of 12h:12h and a light intensity of 2000lux.

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