Red mud and ardealite composite improved material and preparation method thereof

Abstract

The invention discloses a red mud and ardealite composite improved material and a preparation method thereof, and belongs to the technical field of industrial solid waste resource utilization. The red mud and ardealite composite improved material is prepared by modifying red mud with a ferric chloride solution, modifying ardealite with a sodium silicate solution, compounding the modified red mud (FRMb) and the modified ardealite (NPG) according to a mass ratio of 7: 3, adding 10% of pure water, and mixing. The improved material can effectively adjust the pH value of soil to a neutral range, reduce the activity of heavy metals, increase the content of available potassium, and maintain stable supply of a large number of nutrients such as nitrogen, phosphorus, potassium, calcium, magnesium and sulfur. Experiments show that the material can significantly promote plant growth (for example, the plant height of ryegrass and the chlorophyll content are increased), reduce leaching of soluble phosphorus, fluorine and other pollutants and improve the diversity of soil microbial communities. According to the method, efficient utilization of'treating waste with waste 'of the red mud and the ardealite is achieved, the method is suitable for ecological restoration of mines and improvement of heavy metal polluted soil, and environmental benefits and economic benefits are both achieved.

Description

Technical Field

[0001] This invention relates to the field of industrial solid waste resource treatment technology, and in particular to a red mud phosphogypsum composite modified material and its preparation method. Background Technology

[0002] Red mud is a highly alkaline industrial waste generated during the alkaline refining process of alumina, with global reserves exceeding 4 billion tons. Red mud possesses a porous structure and high specific surface area, enabling it to adsorb heavy metal ions; however, its strong alkalinity and high salt content limit its direct utilization. The resource utilization process of red mud primarily considers its high salt content and EC (with Na+ as a key component). + (Mainly). When red mud is applied to soil, the exchangeable sodium content in the soil increases significantly, leading to potential high salinity problems in the remediated soil. Therefore, the main use of red mud is for dealkalization and the solidification and stabilization of harmful heavy metal elements. Phosphogypsum is a byproduct of the phosphate fertilizer industry, rich in plant nutrients such as sulfur and phosphorus. However, the strong acidity of phosphogypsum and the soluble phosphorus and fluorine it carries limit its diversified resource utilization pathways.

[0003] In existing technologies, the alkalinity of red mud is mainly controlled through acid neutralization or iron salt treatment, while the impurity treatment of phosphogypsum uses solidifying agents to stabilize its harmful components. The combined remediation technology of red mud and phosphogypsum has shown potential in heavy metal solidification, soil structure improvement, and plant growth promotion, but further optimization of the blending ratio, reduction of environmental risks, and exploration of its long-term ecological effects are still needed.

[0004] Therefore, this invention proposes a highly efficient "waste-to-waste" utilization method. By innovatively using a sodium-iron salt composite conditioner to neutralize phosphogypsum and red mud with acid and alkali, as well as to perform synergistic treatment of dealkali removal and solidification stabilization, waste modification and composite material preparation are achieved. Summary of the Invention

[0005] This invention aims to provide a red mud phosphogypsum composite modified material and its preparation method. By providing a highly efficient "waste-to-waste" utilization method, this invention innovatively employs a sodium-iron salt composite conditioner to perform acid-base neutralization and synergistic treatment of phosphogypsum and red mud, including dealkalization and solidification stabilization, thereby achieving waste modification and composite material preparation.

[0006] To achieve the above objectives, the present invention provides the following technical solution:

[0007] A method for preparing a red mud-phosphogypsum composite modified material involves spraying red mud with ferric chloride solution to obtain modified red mud (FRMb), spraying phosphogypsum with sodium silicate solution to obtain modified phosphogypsum (NPG), mixing the obtained FRMb and NPG at a mass ratio of 7:3, adding 10% of their total mass of pure water during the mixing process, and obtaining the red mud-phosphogypsum composite modified material after mixing.

[0008] Further, red mud was sprayed with a 1 mol / L ferric chloride solution to obtain 15% FRMb by mass, and phosphogypsum was sprayed with a 1.4 mol / L sodium silicate solution to obtain 10% NPG by mass. The prepared FRMb and NPG were mixed at a mass ratio of 7:3, and 10% of their total mass of pure water was added during the mixing process. After the mixing was completed, the red mud phosphogypsum composite modified material was obtained.

[0009] Furthermore, the prepared red mud phosphogypsum composite modified material is used as a plant growth substrate.

[0010] Furthermore, the prepared red mud phosphogypsum composite modified material is applied to the control of heavy metal activity.

[0011] Furthermore, the prepared red mud phosphogypsum composite amendment was applied to improve the diversity and abundance of soil microbial communities.

[0012] The principle and beneficial effects of this technical solution:

[0013] (1) The composite improved material prepared by the present invention can effectively promote plant growth and shows excellent performance in terms of plant height, chlorophyll content and fresh weight. At the same time, the material prepared by the present invention can also adjust the soil pH value to the neutral range, improve the plant growth environment, and can be used as a plant growth substrate.

[0014] (2) The composite improvement material prepared by the present invention can increase the content of available potassium in the soil and maintain a stable supply of available nitrogen and available phosphorus. At the same time, the material of the present invention exhibits excellent heavy metal solidification and stabilization effect, effectively reducing the absorption of heavy metals by plants and keeping the heavy metal content of plants at a safe level.

[0015] (3) The material prepared by the present invention can effectively improve the diversity and abundance of soil microbial communities and promote the decomposition of soil nutrients and the absorption of nutrients by plants.

[0016] (4) The raw materials used in the preparation method of the present invention are red mud and phosphogypsum, two kinds of "waste" that are not easy to handle. This not only provides a solution for the two kinds of "waste", but also obtains beneficial products. Attached Figure Description

[0017] Figure 1 The effect of FeCl3 dosage on pH and EC of red mud in Experiment 1;

[0018] Figure 2 The effect of sodium salt content on pH, EC, and PO4 of phosphogypsum in Experiment 1. 3- and F - The effect of concentration;

[0019] Figure 3 The effect of different ratios on pH and EC of the composite material in Experiment 1;

[0020] Figure 4 For different proportions of PO4 admixtures in Experiment 1 3- With F - The effect of concentration;

[0021] Figure 5 The XRD patterns of the modified composite materials in Experiment 1 are shown in comparison.

[0022] Figure 6 SEM comparison images of the modified and unmodified composite materials in Experiment 1;

[0023] Figure 7 The effect of different proportions of compound admixtures on heavy metal concentration in Experiment 1;

[0024] Figure 8 The growth effect of ryegrass at 40 days in Experiment 1;

[0025] Figure 9 The effects of different vegetation substrates on a) pH, b) EC and c) TDS of potted plant leachate in Experiment 2;

[0026] Figure 10 The effects of different vegetation substrates on the concentrations of a) phosphorus, b) fluorine and c) ammonia nitrogen in potted plant leachate in Experiment 2;

[0027] Figure 11 The effect of different plant substrates on the nutrient content of potted substrate in Experiment 2;

[0028] Figure 12 This is a graph showing the relative abundance of fungi at the phylum (a), genus (b), and bacteria at the phylum (c), genus (d) levels in different vegetation substrates during Experiment 2. Detailed Implementation

[0029] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments:

[0030] To verify the effectiveness of the material prepared according to this invention, the following experimental procedure was used for verification and explanation:

[0031] Experiment 1

[0032] Sample collection and preparation

[0033] The red mud was taken from a red mud stockpile in Kaili City, Guizhou Province. It appeared as reddish-brown lumps, with a pH of 10.76 and a water content of 28%. The phosphogypsum was taken from a phosphogypsum stockpile in Wengfu Phosphate Mine, Guizhou Province. It appeared as grayish-black lumps and had a water content of 17%. Both materials were air-dried in the dark under natural conditions, ground, and sieved (60 mesh) for later use. Ultrapure water was used for both experiments.

[0034] Experimental Design

[0035] Red mud was modified by spraying it with 0.5 mol / L and 1 mol / L FeCl3 reagent (FC) at different mass ratios (5%, 10%, and 15%), and phosphogypsum was modified by spraying it with 1.4 mol / L Na2SiO3 reagent (NS). After mixing and aging for 24 hours, FRMb and NPG were obtained. The specific modification test scheme is shown in Table 1.

[0036] Table 1. Experimental Design for FRM and NPG

[0037]

[0038] Spraying 10% (by total mass) of ultrapure water onto the admixtures kept the materials moist and ensured uniform mixing. The materials were then spread out and aged at room temperature for 24 hours to obtain the admixtures. Admixture slag was prepared by uniformly mixing at different mass ratios; the specific experimental design is shown in Table 2.

[0039] Table 2 Experimental Design Scheme for Modified Red Mud Phosphogypsum Composite Materials

[0040]

[0041] FRMb (15%) and NPG (10%) were mixed at mass ratios of 7:3, 6:4, and 5:5, respectively, and 10% pure water was added to each mixture to ensure thorough and uniform mixing. Based on the mass ratios used, the three treatment groups were named FN1, FN2, and FN3, respectively, with untreated fresh soil used as a control group (CK). Ryegrass pot experiments were conducted using these treatment groups as the planting substrate.

[0042] The pot experiment design for ryegrass was as follows: 500g of a well-mixed and aged compound was placed in a free-draining plastic pot with a diameter of 10cm and a height of 9cm. One hundred ryegrass seeds were evenly sown, and the mixture was watered to maintain a field water content of 60%-70%. Each treatment group was repeated three times. To verify the vegetation restoration potential of the modified compound during long-term treatment, a 45-day planting experiment was conducted on ryegrass. Germination rate was calculated, and plant height and chlorophyll content were measured at 20 and 40 days. At 40 days, the leachate from the pots of the control and treatment groups was collected for pH, EC, and PO4 determination. 3- and F - Content analysis. After 45 days, the above-ground parts of the potted plants were collected for dry weight determination.

[0043] Experimental Results and Analysis

[0044] 1. The regulatory effect of ferric chloride on the pH and conductivity of red mud

[0045] like Figure 1 The figure shows a comparison of pH and EC of FRM obtained after modification with 0.5 mol / L and 1 mol / L FeCl3 solutions of different mass percentages with that of raw red mud. The figure shows that the pH of FRM decreased from 10.67 to 8.51–10.05, indicating that the spraying treatment with FeCl3 solution effectively reduced the alkalinity of the red mud, and this reduction further increased with increasing iron salt solution concentration and dosage. Simultaneously, due to the presence of Fe in the modifier… 3+ Cl - The incorporation of ions increases the electrical conductivity of FRM materials from 0.74 mS / cm to 1.23–4.81 mS / cm, showing a continuous increasing trend.

[0046] The interaction between iron(III) and basic anions in red mud (RM) in an aqueous environment, and the process of its dehydrogenation to form colloids, are expressed by formulas (1)-(5). The hydrolysis rate is subject to the concentration of Fe(III) and OH in the solution. - Mole ratio.

[0047] Fe 3+ +OH - →Fe(OH)3↓ (1)

[0048]

[0049] Under the action of iron salts, the strong alkalinity of red mud (RM) was effectively regulated. The pH of the treated groups (FRMa with 15% FeCl3, FRMb with 10% FeCl3, and FRMb with 15% FeCl3) decreased from 10.76 to 8.51–8.72, confirming that iron salt spraying on red mud has a dealkalizing effect. Iron salts promote the reduction of Ca in RM. 2+ Na + Dissolved, by Fe 3+ The substitution of iron salts into the solution leads to an increase in the conductivity of the leachate with increasing FC concentration and dosage. EC is a key indicator of salt content; excessive addition of iron salts should be avoided to prevent soil salinization during red mud dealkalization. Under single ferric chloride treatment, the pH value of the aforementioned treatment group met the requirements of the wastewater discharge standard (GB8978-1996).

[0050] 2. The regulatory effect of sodium silicate on phosphate and fluoride ions in phosphogypsum

[0051] Phosphogypsum provides essential nutrients for vegetation growth, and traditional phosphogypsum-based red mud dealkalization methods utilize its strong acid components to neutralize the lattice and crystalline alkalis exposed on the surface of red mud particles, thereby adjusting the system's alkalinity. Adding a small amount of phosphogypsum (PG) is sufficient to regulate the alkalinity of the ferrous metal matrix (FRM), but the composite material still exhibits an overall nutrient-deficient state. Therefore, sodium silicate-modified phosphogypsum aims to inhibit the leaching of harmful phosphorus and fluorine, activate the effective components of PG, thereby increasing the PG content and enriching the nutrients in the composite material.

[0052] like Figure 2 The figure shows the effect of sodium salt content on pH, EC, and PO4 of phosphogypsum. 3- and F - Effect of concentration. The figure shows the effect of different sodium silicate dosages on the pH, conductivity, and phosphorus and fluorine leaching of phosphogypsum. After treatment, pH and EC increased linearly with sodium silicate dosage, with pH increasing from 3.38 to 4.89-9.54 and EC increasing from 1.37 mS / cm to 2.09-3.49 mS / cm; phosphorus and fluorine leaching decreased steadily with increasing sodium silicate dosage, while PO4... 3- The content decreased from 205.7 mg / L to 0.45-150.9 mg / L, F - The content decreased from 48.49 mg / L to 1.69-23.73 mg / L. Compared to the original phosphogypsum residue, the PO4 content in the NPG treatment group with 10% Na2SiO3 usage was significantly lower. 3- and F - The content decreased by 86% and 92%, and the pH value increased from 3.38 to 6.67. NS, as a modifier, effectively enhanced the alkalinity of PG and effectively controlled the release of phosphorus and fluorine. Therefore, the NPG formulation with 10% Na2SiO3 is the preferred option for preparing composite materials.

[0053] 3. Leaching toxicity of modified red mud phosphogypsum composite materials

[0054] Based on the preliminary analysis of FRM and NPG results, NPG with 10% Na2SiO3 and FRMb with 10% FeCl3 were selected for the preparation of the subsequent composite material. FRMb with 15% FeCl3 and FRMa were also selected. The two materials were mixed at different ratios, and the toxicity of the composite material was analyzed using the material leachate.

[0055] Acidity and alkalinity affect the surface charge of composite materials, which is an important variable related to the adsorption and removal of harmful pollutants at the solid-water interface. pH value also affects the availability of nutrients in composite slag materials.

[0056] like Figure 3The figure shows the effect of different proportions on the pH and EC of the composite material. As can be seen from the figure, with the increase of FRM content, the pH value of the FRM-NPG composite material slightly increases between 7.5 and 8.2. This pH change is attributed to the increase in red mud content, where the alkaline substances it carries react with the acid radicals in phosphogypsum to form neutral salts. Furthermore, the free CaO in phosphogypsum... 2+ It can inhibit the dissolution of mixed alkalis in red mud, thereby achieving effective control of the pH of the composite admixture. (The text abruptly shifts to a seemingly unrelated topic about Ca in phosphogypsum.) 2+ It can react with CO3 in red mud 2- HCO 3- OH - Al(OH) 4- The reaction produces calcite (CaCO3), calcium hydroxide, and tricalcium aluminate (Ca3Al2(OH)2). 12 Precipitated substances such as ) affect the pH and Al of the composite material. 3+ The content is maintained at a level that promotes plant growth, thereby stabilizing the pH of the modified slag material at near-neutral conditions.

[0057] Electrical conductivity (EC), as a key indicator of substrate salt concentration, directly affects the efficiency of plant nutrient and water absorption during ecological restoration. For example... Figure 4 As shown in (a), different proportions of compound admixtures affect PO4. 3- As shown in the figure, with the increase of FRM dosage, the phosphorus content in the leachate steadily decreased. At a dosage of 70%, only the T5 treatment group, with 15% FeCl3 using FRMb, was able to control the TP concentration at 0.36 mg / L, meeting the wastewater discharge requirements for PO4. 3- The required content is Class I standard (PO43- ≤ 0.5 mg / L). The phosphorus removal mechanism of FRM mainly relies on three components: Ca, Fe, and Al. In an aqueous medium, these components combine with phosphates to produce insoluble or low-soluble salts such as calcium phosphate and aluminum phosphate. The phosphates are fixed in this solid-phase precipitation process, thus controlling the phosphorus content in the extract.

[0058] The fluoride content in phosphogypsum originates from phosphate rock, accounting for 20%-40%. Given that fluoride leaching is a major factor in environmental fluoride pollution and endemic fluorosis, the fluoride ion concentration must be strictly monitored during the preparation of phosphogypsum-red mud composites. Figure 4 As shown in (b), different proportions of compound admixtures affect F -The concentration remained relatively stable at approximately 1.2-2.2 mg / L, meeting the requirement that F- content in wastewater discharge be ≤10 mg / L. Notably, when the FRM content reached 70%, the F- concentration dropped to a minimum of approximately 1.2 mg / L. This phenomenon can be attributed to the electrostatic adsorption of fluorides by the positive charge on the FRM surface. The adsorption process follows a pseudo-second-order kinetic model, and the ion exchange process on the material surface also contributes to fluoride removal. Considering pH, EC, and PO4... 3- With F - The content of FeCl3 in the T5 treatment group and NPG in the treatment group with 10% Na2SiO3 content of FRMb were used to achieve the modification and optimization of slag material.

[0059] 4. Microscopic analysis of modified red mud phosphogypsum composite material

[0060] Specific surface area, pore volume, and pore size are important parameters for characterizing materials, and their changes have a crucial impact on the adsorption capacity and reactivity of composite materials. Table 3 shows the changes in specific surface area, pore size, and pore volume of red mud, phosphogypsum, and composite materials under different treatment conditions.

[0061] Table 3. Specific surface area, pore size, and pore volume of red mud, phosphogypsum, and composite material under different treatment conditions.

[0062]

[0063] Note: Data are mean ± standard deviation; different lowercase letters indicate significant differences between different treatment groups for the same indicator (P<0.05).

[0064] As shown in the table, after treatment with ferric chloride, the specific surface area of ​​FRMb (15%) increased by 8.71%, the pore size decreased by 55.06%, and the pore volume decreased by 42.57%. This indicates that ferric chloride treatment significantly enhanced the surface active sites of red mud, thereby strengthening its adsorption capacity for pollutants. The smaller pore size means that the modified red mud is more suitable for adsorbing small molecule pollutants, but the lower pore volume may limit its adsorption capacity for large molecule pollutants. In contrast, after treatment with sodium silicate, the specific surface area and pore volume of NPG (10%) increased by 7.44% and 5.62%, respectively, while the pore size decreased by 1.73%. This indicates that sodium silicate treatment further optimized the microstructure of phosphogypsum, increased its surface activity, and had little effect on its pore structure, while slightly increasing the pore volume of phosphogypsum. Compared with the T5 blank test group, the specific surface area and pore volume of the T5 (FRMb 15%) treatment increased by 48.9% and 6.5%, respectively, while the pore size decreased by 28.6%. This indicates that the modification treatment of the composite material significantly increased its specific surface area and optimized its pore structure, resulting in a better balance in adsorbing pollutants of different molecular sizes. Furthermore, the modification treatment significantly increased the pore volume of the composite material, enhancing its adsorption capacity. In summary, T5 (FRMb 15%) exhibits a better balance in adsorbing pollutants of different molecular sizes due to its high specific surface area and moderate pore size. Its maximum pore volume allows it to accommodate more adsorbate, thereby significantly improving the material's adsorption capacity.

[0065] like Figure 5 The image shows the XRD patterns of the modified composite materials. Figure 5 The XRD patterns of the composite materials from the T5 blank group (RM(70%) + PG(30%)) and the T5 treated group (FRMb(15%)) were compared. Observing the T5 blank group's spectrum, diffraction peaks of gypsum and calcium phosphate were clearly visible, indicating that these two minerals are the main components in the unmodified composite material. Meanwhile, although the diffraction peaks were weak, small amounts of calcium magnesium carbonate minerals and borate were still present. In the modified composite material, the diffraction peaks of gypsum and calcium phosphate remained, but their intensity may have changed. Furthermore, diffraction peaks of iron di(2,3,4-trihydroxybutyric acid) hydrate and iron-molybdenum fluoride were also observed, indicating that new mineral phases were introduced during the modification process. Simultaneously, the diffraction peaks of some minerals may have weakened or even disappeared after modification, further demonstrating the alteration of the mineral composition of the composite material by the modification treatment. Combined with the BET analysis results in Table 3, it can be concluded that the modification treatment not only optimized the pore structure of the composite material but also improved its surface properties, thereby significantly enhancing its adsorption capacity and reactivity.

[0066] like Figure 6 The image shown is a SEM comparison of the compound admixture before and after modification. Figure 6The SEM images of the T5 blank experimental group (composed of 70% RM and 30% PG) and the T5 treatment group (FRMb (15%)) are shown in comparison. In the blank experimental group ( Figure 6 In (a)), the particles exhibit irregular shapes, rough surfaces, large pores, and a relatively loose overall packing. In contrast, the modified composite material ( Figure 6 (b) The modified particles exhibit a more regular crystal grain shape, improved surface smoothness, and a denser pore structure. The regularization of particle shape and the smoothing of the surface after modification are likely attributed to the modification treatment promoting the recrystallization process and surface modification. Simultaneously, the reduction in pore size and the increase in packing density indicate that the modification treatment optimized the pore structure of the composite, potentially enhancing its adsorption capacity and reactivity. Furthermore, the modification treatment resulted in a smoother surface and tighter interparticle connections, which may improve the surface properties of the composite, thereby enhancing its stability and mechanical strength. These changes presented in the SEM images are consistent with the changes in specific surface area and pore volume shown in Table 3, further verifying that the modification treatment made the material's microstructure more compact and promoted interparticle aggregation.

[0067] 5. Analysis of Heavy Metal Characteristics in Modified Red Mud Phosphogypsum Composites

[0068] like Figure 7 The figure shows the effect of different proportions of admixtures on heavy metal concentrations. As can be seen from the figure, the heavy metal concentrations of the leachates from both types of unmodified slag meet the maximum allowable discharge concentration requirements of the "Integrated Wastewater Discharge Standard" (GB8978-1996). The heavy metal concentrations in the treated groups were effectively reduced compared to the original slag, without increasing due to admixture. The concentrations of Ni, Mn, and As increased with increasing NPG dosage, while the concentrations of Cd, Cu, and Zn were less affected by the NPG ratio. The effective reduction of heavy metal content in each treatment group is mainly due to the activation of active substances such as SiO2, Al2O3, and CaO in the red mud by sulfates in phosphogypsum, which promotes the formation of CSH gel and AFt, thereby accelerating the dissolution of red mud and phosphogypsum and increasing the concentrations of aluminosilicates and Ca. 2+ Release. The physical encapsulation, adsorption, isomorphic displacement, and precipitation solidification mechanisms of calcium silicate gel and calcium vanadate stone achieve the stabilization of heavy metals in the composite admixture.

[0069] 6. Results and Analysis of Pot Experiment

[0070] Based on the above conclusions regarding the modified composite material, vegetation substrates were prepared using FRMb (15%) and NPG (10%) in different ratios (7:3, 6:4, 5:5), designated as FN1, FN2, and FN3. Fresh soil was used as a blank control group (CK) to study the vegetation restoration potential of the modified red mud-phosphogypsum composite material. A 45-day pot experiment with ryegrass was conducted. At 40 days, the growth status of the plants in the CK, FN1, FN2, and FN3 substrates was recorded, and the results are as follows: Figure 7 As shown in Table 4, the growth parameters of ryegrass and the physicochemical properties of CK, FN1, FN2 and FN3 are listed.

[0071] Table 4. Growth parameters and physicochemical properties of ryegrass samples

[0072]

[0073] Note: Data are mean ± standard deviation; different lowercase letters indicate significant differences (P<0.05) between different treatment groups for the same indicator. "-" in the table indicates below the detection line.

[0074] As shown in the table, the pH values ​​of FN1, FN2, and FN3 remained stable between 8.27 and 8.41, meeting the requirements for plant growth. The electrical conductivity, in descending order, was: FN1 > FN2 > FN3 > CK, with all groups maintaining a conductivity below 4 mS / cm, satisfying the salinity threshold for safe plant growth. However, only FN1 met the Class I requirements for wastewater discharge in terms of phosphorus and fluoride concentrations in the potted plant leachate.

[0075] The germination rates of the groups were ranked as follows: CK > FN1 > FN2 > FN3. The growth of ryegrass in FN1 and fresh soil at 40 days was similar, with germination rates of 84% and 92%, respectively. However, the overall germination rates of FN2 and FN3 were not high, at 36% and 34%, respectively. Measurements of plant height and chlorophyll content at 20 and 40 days showed that the plant height of ryegrass in fresh soil and FN1 was slightly lower than that in FN2 and FN3, but the chlorophyll content of both groups was significantly higher in FN2 and FN3. Figure 7 As can be seen from the growth status and the aboveground dry weight in Table 4, the ryegrass planting effect of FN1 and fresh soil is significantly better than that of FN2 and FN3. In conclusion, the pot leachate from the modified composite material FN1 not only meets the treatment requirements of Class I wastewater discharge in GB8978-1996, but also has a similar planting effect to that of ryegrass in fresh soil.

[0076] 7. Summary

[0077] Experiment 1 shows that when the mass ratio of red mud to phosphogypsum in the modified composite material is 7:3, its performance is optimal: the pH value is stably maintained at 8.03, the electrical conductivity (EC) remains at a suitable level of 4.89 mS / cm, and the phosphate (PO4) content is also optimal. 3- ) and fluoride ions (F - The concentrations of slag were reduced to 0.36 mg / L and 1.34 mg / L, respectively, successfully achieving the upgrade from Class II industrial slag to Class I slag.

[0078] After modification, the composite material possesses a high specific surface area, moderate pore size, and maximized pore volume, which significantly enhances its adsorption performance. Iron salt treatment not only introduces iron-based compounds, altering the mineral phase structure of the original admixtures, but also achieves solidification and stabilization of characteristic pollutants, forming a compact and stable structure.

[0079] In the composite material, synergistic interlocking effects and calcium supplementation / alkalinity reduction work together. The strongly alkaline conditions of red mud stimulate the silica-alumina components on the phosphogypsum surface, promoting the formation of CSH gel. Simultaneously, under the combined action of Fe(OH)3 and Al(OH)3, the composite material effectively adsorbs, encapsulates, and co-precipitates toxic pollutants. Furthermore, the suitable alkalinity in the composite material enhances its ability to remove F... - HPO4 2- PO4 3- The electrostatic attraction of negatively charged ions is utilized. Furthermore, the gel structure and the excellent specific surface area and overlapping interlayer structure of AFt provide abundant active sites for the adsorption and stabilization of heavy metals, thus stably solidifying them within the crystal structure. Therefore, the toxicity leaching content of the modified composite material fully meets the standard requirements for Class I solid waste slag.

[0080] Experiment 2

[0081] Experimental protocol

[0082] FRMa (15%), FRMb (10%), and FRMb (15%) and NPG (10%) were mixed at a mass ratio of 7:3, and 10% pure water was added to each mixture to ensure thorough and uniform mixing. Based on the mass ratios used, the three treatment groups were named S1, S2, and S3, respectively. A control group was used, consisting of red mud-phosphogypsum (S4) and fresh soil (CK), and red mud (RM) and phosphogypsum (PG) at a mass ratio of 7:3. A pot experiment was conducted using these treatment groups as the growing substrate for ryegrass. 1000g of the well-mixed and aged mixture was placed in a free-draining plastic pot (10cm in diameter and 9cm in height), and 30 ryegrass seeds were evenly sown. Watering was maintained to keep the mixture at 60%-70% field capacity. Each treatment group was repeated three times. To verify the vegetation restoration potential of the modified composite material during long-term treatment, a 90-day planting experiment was conducted on ryegrass. Germination rate was calculated, and plant height and chlorophyll content were measured at 15, 30, 45, and 60 days. Simultaneously, leachate from potted plants in each treatment group was collected for pH, EC, ammonia nitrogen, and PO4 content analysis. 3- and F - Content analysis. After 90 days, the fresh weight was measured and the heavy metal content was determined after drying at 45℃ to constant weight. The bulk density of the potting substrate was determined using the ring cutter method. The total heavy metal content of the substrate was determined after constant weight treatment with ryegrass, as well as the determination of substrate nutrients (AN, AP, AK and organic matter) and the available form of heavy metals. Fresh potting substrate samples were collected, stored at -80℃ and transported to Shanghai Meiji Biomedical Technology Co., Ltd. to determine the microbial community structure and diversity.

[0083] Experimental Results and Analysis

[0084] 1. The effect of modified admixtures on ryegrass vegetation

[0085] The growth of ryegrass indirectly reflects the quality of the potting substrate. Table 5 shows the growth performance of ryegrass under different modified compound substrate conditions.

[0086] Table 5 Record of ryegrass vegetation in different substrates

[0087]

[0088] Note: Data are mean ± standard deviation; different lowercase letters indicate significant differences between different treatment groups for the same indicator (P<0.05).

[0089] The table shows that in the potted plants using red mud as the substrate, no plants survived, indicating that the untreated red mud, due to its highly alkaline environment, is unsuitable for plant growth. Regarding germination rate, ryegrass in fresh soil had the highest germination rate, followed by phosphogypsum, while the germination rates of ryegrass in treatment groups S1, S2, S3, and S4 fluctuated between 75% and 79%. Germination data indicates that the germination rates of ryegrass were similar when using modified composite materials (S1, S2, and S3) and unmodified composite materials (S4) as potting substrates, suggesting that the modification treatment of the composite materials did not hinder the initial growth of ryegrass.

[0090] Based on the plant height and chlorophyll data collected at 15, 30, 45, and 60 days, it was found that when phosphogypsum was used as the potting substrate, the plant height and chlorophyll content of ryegrass were the best among all treatment groups, and it remained in a stable growth state after 60 days. Furthermore, the results at each sampling stage were significantly higher than those of other treatment groups (P<0.05). In the control group (CK), ryegrass showed leaf yellowing and wilting after 45 days, resulting in a decrease in vegetation data at 60 days compared to 45 days. Among the four treatment groups, S1 and S4 showed the most significant plant height, but at 15 days, the chlorophyll content of the S4 group was significantly lower than that of the S1 group (P<0.05). The chlorophyll changes in the S4 group exhibited slow early growth, smaller leaves, and poor development, a trend that continued until 60 days. Combining the plant height and chlorophyll data, among the modified compound substrate treatments S1, S2, and S3, the S1 treatment showed the best growth status of ryegrass. The aboveground fresh weight data further validated the growth of ryegrass in each treatment group. The fresh weight ranking was: PG>S1>S4>CK>S2>S3, which is consistent with the long-term monitoring data on plant height and chlorophyll content.

[0091] 2. Effects of modified admixtures on the physicochemical properties of potted substrate leachate

[0092] pH is a key indicator in the modification process of compound admixtures. pH value often affects the surface charge changes of soil clay minerals and organic matter. Simultaneously, the pH value of the soil environment alters the desorption and solidification process of heavy metals in the soil. For example... Figure 8The figure shows the effects of different planting substrates on the pH (a), EC (b), and TDS (c) of potted plant leachate. As can be seen from the figure, the leachate from red mud and phosphogypsum substrates remained in a strongly alkaline and strongly acidic environment, respectively, after 60 days. Treatments S1-S4 maintained the pH of the leachate within the neutral range (7-9), which was significantly different from the RM (pH 9.8-10.9) and PG (pH 2.8-4.3) control groups (P<0.05). Furthermore, the pH of the leachate in each group remained relatively stable during the experiment. The initial EC and TDS of treatments S1-S3 were significantly higher than the blank control group, indicating that the FRM-NPG composite substrate was used in a high salinity state, which inhibited seed germination in the early stages. However, the overall values ​​decreased as the experimental time increased. High TDS values ​​led to salt accumulation, causing salt stress, resulting in yellowing and even death of ryegrass, which is consistent with the observed plant growth trend: chlorophyll content at 60 days was S1>S2>S3. However, the chlorophyll content of the S1 and S2 treatment groups was much higher than that of the S4 group during the experiment (P<0.05), indicating that the modified treatment was more beneficial for RM-PG compound as a plant substrate.

[0093] Phosphorus in phosphogypsum is an important nutrient element for plant growth and can be applied to the soil as a phosphate fertilizer. However, excessive phosphorus and fluoride leaching from phosphogypsum is a significant environmental impact factor that needs to be considered during its utilization. Therefore, the phosphorus and fluoride content in the leachate should be a key monitoring target during plant growth. Figure 9 The effects of different growing substrates on the concentrations of phosphorus (a), fluorine (b), and ammonia nitrogen (c) in the leachate from potted plants are shown.

[0094] from Figure 9 (a) It can be seen that the total phosphorus concentration in the leachate of each treatment group remained almost zero during the growth process, except for the phosphogypsum-based leachate, where the phosphorus concentration decreased from the initial 749.54 mg / L to 147.2 mg / L. However, at day 60, the phosphorus content in the phosphogypsum-based leachate still far exceeded the phosphorus concentration requirement for Class V water bodies (<0.4 mg / L) in the Surface Water Environmental Quality Standard. If phosphogypsum matrix is ​​used in practical applications, it may lead to regional soil degradation and eutrophication of water bodies. In contrast, the composite admixtures S1, S2, S3, and S4 were able to control the phosphorus concentration in the leachate within the Class II water body standard range (<0.1 mg / L) and maintain stability throughout the growth process.

[0095] from Figure 9(b) It can be seen that the concentration of soluble fluoride in the leachate remained as high as 254.01 mg / L after 60 days. In the compound admixture treatment group, the fluoride ion concentration in S4 was significantly higher than that in the modified compound admixture groups S1, S2, and S3 before 45 days (P<0.05). In contrast, the fluoride ion concentration in the leachate from the modified compound admixture was relatively stable, fluctuating between 8.22 and 11.86 mg / L. This indicates that the modified compound admixture can stably control the release of soluble phosphorus and fluoride from phosphogypsum and did not leach out again with prolonged planting time.

[0096] Nitrogen enhances a plant's disease resistance and photosynthetic efficiency, playing a crucial role in its growth and development. However, excessive nitrogen can inhibit root development, making the plant more susceptible to lodging. Figure 9 (c) It can be seen that the ammonia nitrogen concentration in the phosphogypsum-based leachate was high, and the ammonia nitrogen values ​​at all four sampling time points were significantly higher than those in other treatment groups (P<0.05). Compared with the control group, each treatment group reduced the ammonia nitrogen concentration in the leachate to varying degrees. Combined with the analysis of ryegrass growth data, phosphogypsum, as a plant growth substrate, can provide essential nutrients, improve leaf quality, and enhance photosynthetic efficiency. However, as an industrial solid waste, phosphogypsum is unstable. Long-term use as a single plant substrate leads to the continuous leaching of excessive soluble impurities (such as P, F, and NH3-N) during irrigation, resulting in soil and water pollution. The co-blending of red mud and phosphogypsum (S4 treatment) reduced the leaching of soluble phosphorus and ammonia nitrogen in the substrate leachate, but the fluoride ion content was still significantly higher than that of the modified co-blending materials (S1, S2, and S3 treatments). Therefore, the modified co-blending materials showed a more significant effect in controlling phosphorus, fluoride, and ammonia nitrogen in the leachate. In addition, considering the vegetation conditions, the modified compound material of treatment group S1 is more suitable as a potting substrate.

[0097] 3. Effects of modified compound admixtures on the nutrient and heavy metal content of potted substrates

[0098] like Figure 10The figure shows the effect of different growing substrates on the nutrient content of the potting substrate. As can be seen from the figure, the available nitrogen and available phosphorus contents in the phosphogypsum substrate were significantly higher than those in other treatment groups (P<0.05), but its available potassium content was only 12.6 mg / kg, far lower than that in the red mud substrate (643.80 mg / kg). In the four compound substrate treatment groups, the changes in available nitrogen and available phosphorus were relatively small, while the changes in available potassium were particularly significant, especially in the S1 and S2 treatment groups. The phosphogypsum substrate mainly provided nitrogen and phosphorus, the two key nutrients for ryegrass, while the available nitrogen and available phosphorus in the compound substrate remained at a stable level and were even higher than those in the unmodified compound substrate. Furthermore, the modified admixtures (S1, S2, and S3) effectively released potassium from the red mud, stabilizing the available potassium content between 430.47 and 481.23 mg / kg, significantly higher than the 354.2 mg / kg in the unmodified admixture S4 treatment group (P<0.05). This indicates that modification not only helps improve the potassium release efficiency from the admixtures but also optimizes the overall nutrient supply of the substrate, thereby promoting plant growth and soil health.

[0099] The available form of heavy metals is the form in which plants can absorb and utilize them, playing a crucial role in plant growth and development. Table 6 shows the effects of different vegetation substrates on the content of available heavy metals.

[0100] Table 6. Effects of different vegetation substrates on the bioavailable content of heavy metals.

[0101]

[0102] Note: Data are mean ± standard deviation; different lowercase letters indicate significant differences (P<0.05) between different treatment groups for the same indicator. "-" in the table indicates below the detection line.

[0103] The table shows that different composite matrices exhibit varying effects in controlling the available content of heavy metals. The S1 matrix demonstrates excellent performance in effectively suppressing cadmium and arsenic, while the S2 matrix has a more significant impact on copper enrichment. The S1 and S2 matrices perform well in controlling most heavy metals, particularly showing significant reductions in the available content of arsenic, chromium, and lead. Specifically, the available contents of arsenic, cadmium, and lead in the S1 treatment group decreased by approximately 58.1%, 66.67%, and 31%, respectively, while in the S2 treatment group, the reductions for arsenic and lead were approximately 62.8% and 59.2%, respectively. However, both the S1 and S2 groups showed an increase in the available content of zinc, at 41.9% and 49.4%, respectively, which may be related to the different adsorption and release characteristics of zinc by the different matrices. The S3 group showed a large reduction in the control of copper and lead, at 55.7% and 55.3%, respectively, but showed little change in the absorption of other heavy metals. Although the S4 group shows a decrease in the bioavailable content of some heavy metals, it shows an increase in zinc and arsenic. Therefore, special attention should be paid to its potential environmental impact when using it.

[0104] In summary, modified admixture matrices generally exhibit low levels of available heavy metals, which has a minimal impact on soil microorganisms and plant growth and metabolism, thus helping to reduce the negative effects of heavy metals. Therefore, the rational selection and optimization of admixture matrices are of great significance for soil health and plant growth.

[0105] 4. Impact of modified admixtures on heavy metal absorption by ryegrass and risk assessment

[0106] Table 7 shows the effects of different growing substrates on the absorption of heavy metals by ryegrass.

[0107] Table 7. Effects of different planting substrates on heavy metal absorption by ryegrass

[0108]

[0109] Note: Data are mean ± standard deviation; different lowercase letters indicate significant differences between different treatment groups for the same indicator (P<0.05).

[0110] As shown in the table, the PG group exhibited good control over most heavy metals (such as Cr, Ni, Cu, and Zn), especially excelling in the absorption of chromium and nickel, effectively limiting their absorption. The S1, S2, and S3 treatments showed a relatively balanced performance in heavy metal absorption, moderately controlling their accumulation, with the S2 group showing the lowest absorption of certain elements (such as arsenic). In contrast, the S4 treatment significantly improved the absorption of zinc, lead, and arsenic, but some issues remained, such as cadmium and lead absorption approaching or exceeding the limits set in the "Feed Hygiene Standard," requiring careful consideration during application. Overall, the effects of different substrates on heavy metal absorption by ryegrass varied considerably. Ryegrass grown on phosphogypsum substrate effectively controlled the absorption of most heavy metals, while the S1, S2, and S3 treatments showed a relatively balanced approach to heavy metal control, meeting the requirements of the limit standards, indicating that planting ryegrass on these modified compound substrates is relatively safe. However, S4 substrate can enhance plants' ability to absorb certain heavy metals, which may have adverse effects on the environment. Therefore, it should be selected according to specific circumstances in practical applications.

[0111] Table 8 shows the evaluation results of the single-factor index of heavy metals and the Nemerow comprehensive pollution index for different groups.

[0112] Table 8. Evaluation results of heavy metal single-factor index and Nemerow comprehensive pollution index.

[0113]

[0114] From the evaluation, the ryegrass in the CK, S1, and S4 groups was at a relatively clean (alert limit) level, while the other treatment groups reached a safe level. Using modified composite substrates as the growing medium not only effectively controlled heavy metal pollution in the substrate but also prevented heavy metals from migrating and accumulating within the ryegrass, thus preventing toxicity to the plant growth process. The absorption capacity of ryegrass for heavy metals varied significantly under different substrate treatments. Treatments S1, S2, and S3 showed good control effects, maintaining pollution levels within the safe range, but treatment S1 approached the alert limit. In contrast, while treatment S4 showed an increase in the absorption of some heavy metals, it did not exceed the alert limit, and the overall pollution level remained at a "relatively clean" state. However, the long-term environmental impact of heavy metals in the ryegrass treated with S4 still requires special attention.

[0115] Based on the evaluation results, treatments S2 and S3 were the most ideal, effectively controlling heavy metal pollution and maintaining clean and safe levels. Although treatments CK and S1 showed relatively balanced performance overall, the absorption of some elements approached the warning limits, thus requiring attention to their long-term safety. Particular attention should be paid to treatment S4, as it has relatively high absorption of some heavy metals, and its selection in practical applications should be approached with caution. Overall, ryegrass grown on modified composite substrates of S2 and S3 performed excellently in controlling heavy metal pollution. However, for treatments CK and S1, their long-term environmental impact still needs close monitoring to ensure their safety in practical applications.

[0116] 5. Microbial community structure in the modified composite matrix

[0117] Microbial communities are an important component of soil ecosystems. Their structure directly affects soil fertility and nutrient cycling, and influences soil structure and water stability.

[0118] like Figure 11 As shown, there are significant differences in the composition and abundance percentage of bacterial and fungal communities at the phylum and genus levels in different matrices. Due to the low bacterial count in red mud treatment, which did not meet sequencing requirements, relevant data are lacking.

[0119] At the fungal level, Ascomycota accounted for over 60% in all treatment groups. Particularly in treatments S1-S3 prepared with FRM-NPG admixture, the proportion of Ascomycota further increased to 67%-85%. These saprophytic fungi can produce extracellular enzymes, effectively decomposing substances in the soil that are difficult to degrade, and promoting soil nutrient cycling. In the CK treatment group, Ascomycota and Basidiomycota were the dominant phyla, both of which contribute to the decomposition, absorption, and cycling of soil nutrients. Basidiomycota can also form symbiotic relationships during vegetation growth, helping plants absorb water and minerals, and improving their resistance to harsh environments. In the microbial community analysis of S1-S3 and PG treatment groups, Mucoromycota also participated in nutrient transformation and material decomposition processes. Combined with the fungal abundance analysis of treatment S4, under the same conditions, the modified vegetation substrate was more conducive to the development of microbial diversity. At the fungal level, *Morchella* is a common beneficial microorganism among root microbes, contributing to plant growth and enhancing resistance; it was detected only in PG and S1-S3. In contrast, the S4 treatment contained up to 56% of the pathogenic fungus *Stachybotrys*.

[0120] For bacterial communities, those with a phylum abundance of less than 1% were grouped as "others". Actinobacteriota, Proteobacteria, Acidobacteriota, Firmicutes, Bacteroidota, and Chloroflexi were the major bacterial phyla. In treatment S1, Proteobacteria had the highest proportion, reaching 62.20%; Acidobacteriota had a higher relative abundance in nutrient-poor environments, and its relative abundance was significantly negatively correlated with soil organic matter content. Among all treatments, S1 had the lowest relative abundance of Acidobacteriota, indicating the highest organic matter content. At the genus level, *Burkholderia*, which can form a symbiotic relationship with nitrogen fixation in plant roots, had the highest proportion in treatment S1, reaching 10.58%; *Cellvibrio*, which participates in root organic matter decomposition and nutrient cycling, accounted for 10.41%.

[0121] Different treatment groups of the vegetation substrate led to significant differences in the composition and relative abundance of the microbial community. Overall, the S1 treatment showed an increased proportion of Ascomycota and a decreased relative abundance of Acidobacteria, which was beneficial to plant growth. However, the S4 treatment exhibited a single fungal phylum and contained a high proportion of pathogens such as *Botrytis* and *Cladosporium*, which was detrimental to nutrient transformation and the growth of beneficial bacteria, consistent with observed ryegrass growth. Therefore, FRMa (15%)-NPG (10%) is the optimal modified admixture ratio.

[0122] 6. Summary

[0123] Experiment 2 shows that the modified compound admixture significantly promoted the growth of ryegrass, particularly in terms of plant height, chlorophyll content, and fresh weight. The S1 treatment group exhibited the best growth, with significantly higher plant height and chlorophyll content compared to other treatment groups (P<0.05). By adjusting the soil pH to the neutral range (7-9), the modified compound admixture significantly improved the growth environment of ryegrass and reduced the negative impact of salt stress on plant growth.

[0124] The modified admixture effectively regulated the pH of the leachate, maintaining it within the neutral range, which was significantly better than the strongly alkaline and strongly acidic environments of red mud (pH 9.8-10.9) and phosphogypsum (pH 2.8-4.3). Simultaneously, the modified admixture significantly reduced the electrical conductivity (EC) and total dissolved solids (TDS) in the leachate, alleviating soil salinity accumulation and creating a more suitable environment for plant growth.

[0125] The modified admixtures significantly increased the available potassium content in the soil while maintaining a stable supply of available nitrogen and phosphorus. Regarding heavy metal pollution control, the S1 treatment group reduced the available arsenic, lead, and cadmium content by 58.1%, 66.67%, and 31%, respectively, demonstrating excellent heavy metal immobilization effects.

[0126] The modified admixture significantly improved the diversity and abundance of the soil microbial community. The proportion of Ascomycota in the S1 treatment group was significantly increased, reaching 67%-85%, while the abundance of Proteobacteria and Burkholderia also increased, promoting the decomposition of soil nutrients and their uptake by plants. In contrast, the microbial community structure in the unmodified admixture (S4) treatment group was relatively simple and contained a high proportion of pathogens, which was detrimental to plant growth.

[0127] In summary, Experiments 1 and 2 demonstrate that the red mud phosphogypsum composite amendment prepared in this invention can serve as a plant growth substrate to help plants grow effectively, help fix heavy metals to keep them at safe levels, and improve the diversity and abundance of soil biological communities.

[0128] The above descriptions are merely embodiments of the present invention, and common technical solutions or characteristics known in the schemes are not described in detail here. For those skilled in the art, various modifications and improvements can be made without departing from the technical solutions of the present invention, and these should also be considered within the scope of protection of the present invention. These modifications and improvements will not affect the effectiveness of the implementation of the present invention or the practicality of the patent. The scope of protection claimed in this application shall be determined by the content of its claims, and the specific embodiments described in the specification can be used to interpret the content of the claims.

Claims

1. A method for preparing a red mud-phosphogypsum composite modified material, characterized in that: FRMb was obtained by spraying red mud with ferric chloride solution, and NPG was obtained by spraying phosphogypsum with sodium silicate solution. The prepared FRMb and NPG were mixed at a mass ratio of 7:

3. During the mixing process, 10% of the total mass of pure water was added. After the mixing was completed, the red mud phosphogypsum composite modified material was obtained.

2. The preparation method of the red mud phosphogypsum composite modified material according to claim 1, characterized in that: Red mud was sprayed with a 1 mol / L ferric chloride solution to obtain 15% FRMb by mass, and phosphogypsum was sprayed with a 1.4 mol / L sodium silicate solution to obtain 10% NPG by mass. The obtained FRMb and NPG were mixed at a mass ratio of 7:

3. During the mixing process, 10% of the total mass of pure water was added. After the mixing was completed, the red mud-phosphogypsum composite modified material was obtained.

3. A red mud-phosphogypsum composite modified material prepared according to any one of claims 1-2, characterized in that: The prepared red mud phosphogypsum composite modified material was used as a plant growth substrate.

4. A red mud-phosphogypsum composite modified material prepared according to any one of claims 1-2, characterized in that: The prepared red mud phosphogypsum composite modified material is applied to the control of heavy metal activity.

5. A red mud-phosphogypsum composite modified material prepared according to any one of claims 1-2, characterized in that: The prepared red mud phosphogypsum composite amendment was applied to improve the diversity and abundance of soil microbial communities.

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