Chromium-containing wastewater treatment method

Abstract

The invention relates to the technical field of water treatment, and discloses a chromium-containing wastewater treatment method. The treatment method comprises the following steps: adding a ferric salt aqueous solution into the chromium-containing wastewater, adjusting the pH value of the system to 7.0-8.0 to obtain ferrihydrite, and adsorbing Cr (III) to obtain a Cr-ferrihydrite suspension; and heating the Cr-ferrihydrite turbid liquid in a water bath to obtain a Cr-hematite turbid liquid, thereby finishing the treatment of the chromium-containing wastewater. According to the method for treating the chromium-containing wastewater, the synthesized Cr (III)-containing hematite nanoparticles are stable in property, low in solubility in water environments with different pH values and capable of keeping stable for a long time and preventing Cr (III) from being released again, so that rapid, efficient, stable and durable fixation of Cr (III) in a water body is achieved, the method is simple in step, reaction raw materials are easy to obtain, cost is low, and the method is suitable for industrial production. The product hematite nanoparticles are low in toxicity, free of secondary pollution risk, environment-friendly and suitable for large-scale popularization and application.

Description

Technical Field

[0001] The present invention relates to the technical field of water treatment, and particularly relates to a method for treating chromium-containing wastewater. Background Art

[0002] Trivalent chromium (Cr(III)) is a common form of chromium in the water environment. Although its toxicity is lower than that of hexavalent chromium (Cr(VI)), its long-term cumulative effect still poses a potential threat to human health and the ecosystem. First, industrial activities (such as metallurgy, tanning, and electroplating, etc.) and natural geological weathering cause a large amount of Cr(III) to enter the water body. Its enrichment in sediments will damage the metabolic functions of aquatic organisms, inhibit microbial activity, and amplify the toxic effect step by step through the food chain, ultimately affecting human health. Second, Cr(III) may be oxidized into highly toxic Cr(VI) under specific environmental conditions (such as light or microorganisms), exacerbating the pollution risk. Third, the excessive concentration of Cr(III) in water bodies will interfere with the photosynthesis of aquatic plants, leading to water eutrophication or ecological imbalance, further threatening biodiversity.

[0003] From a public health perspective, long-term exposure to drinking water or irrigation water containing Cr(III) may cause skin allergies, respiratory diseases, and DNA damage in humans. With the tightening of environmental protection regulations, industrial wastewater discharge standards (such as the Discharge Standard of Pollutants for Electroplating, etc.) require enterprises to achieve the standard treatment of Cr(III) through efficient technologies. Therefore, removing Cr(III) from water bodies is not only an inevitable requirement for maintaining ecological balance and ensuring public health, but also a core link in promoting green industrial development and fulfilling environmental responsibilities.

[0004] As one of the heavy metal pollutants in water bodies, the main removal methods of Cr(III) include chemical precipitation method and adsorption method. Among them, the chemical precipitation method refers to adjusting the pH of the water body to 8-10 to make Cr(III) combine with hydroxide ions to form Cr(OH)3 precipitate, or adding precipitants such as sulfates or phosphates to form insoluble compounds. This method is simple to operate and has low cost, but there are problems such as the need for subsequent treatment of the precipitate and being easily interfered by coexisting ions; the adsorption method refers to using materials such as activated carbon, clay minerals (such as montmorillonite, kaolinite), and biochar (such as straw and coconut shell charcoal), and through surface functional groups (-OH and -COOH) to coordinate or ion-exchange adsorb Cr(III). This method has a high Cr(III) removal efficiency, but the preparation process of the adsorption material is complex. In recent years, nanomaterials (such as iron oxide and titanium dioxide) have been widely studied due to their advantages of high specific surface area and active sites, but their synthesis cost is high and it is difficult to be applied on a large scale.

[0005] Current research on materials for removing Cr(III) from water bodies mostly focuses on the modification of nano-iron-based materials, and the methods are diverse, but there are still significant deficiencies. For example, CN 222151663 U discloses an impurity removal device for a zinc-plated trivalent chromium color passivation solution. By combining a passivation solution impurity removal device with a sieve and stirring design, Cr(III) in water can be effectively removed. Although this method simplifies the sediment cleaning process, it requires frequent pH adjustment and addition of a precipitant, which is prone to generating sludge; CN 113173608 A discloses a loaded microscale zero-valent iron biochar for removing hexavalent chromium, its preparation method and application. Although the adsorption capacity of microscale iron for Cr(III) is improved through a biochar carrier, the activity of microscale iron is low, and a high dosage is required to effectively remove Cr(III) from water bodies; CN 105903453 A discloses a preparation method of an EDTA-modified magnetic adsorbent and a method for removing trivalent chromium in water. The EDTA-modified magnetic adsorbent combines the advantages of magnetic separation and complexation adsorption, and has a significant removal rate for low-concentration Cr(III). However, the synthesis of this material requires multiple steps of modification, such as methanol dispersion and treatment with a silane coupling agent, etc., with a high cost, and EDTA may introduce secondary pollution; CN 103418335 A discloses a composite nano-adsorbent for removing trivalent chromium ions in wastewater and a removal method. Using a composite adsorbent formed by Fe3O4 and metal nanoparticles, it can have a high removal rate for Cr(III) under ultrasonic assistance. However, this nano-material is prone to agglomeration, reducing the removal rate of Cr(III).

[0006] In summary, among the existing water body Cr(III) removal technologies, the chemical method has a low cost but prominent sludge problems, the adsorption materials are efficient but the preparation process is complex, and the biological coupling technology is environmentally friendly but condition-sensitive. Therefore, it is of great significance to develop a simple, efficient, stable, low-cost and non-secondary pollution treatment method for Cr(III)-containing wastewater. Summary of the Invention

[0007] The present invention aims to at least solve one of the above technical problems existing in the prior art. For this reason, the purpose of the present invention is to provide a treatment method for chromium-containing wastewater.

[0008] In order to achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0009] The present invention provides a treatment method for chromium-containing wastewater, comprising the following steps:

[0010] S1. Add an iron salt aqueous solution to the chromium-containing wastewater, adjust the pH of the system to 7.0 - 8.0 to obtain ferrihydrite, adsorb Cr(III) to obtain a Cr-ferrihydrite suspension;

[0011] S2. Heat the Cr-ferrihydrite suspension in a water bath to obtain a Cr-hematite suspension, completing the treatment of the chromium-containing wastewater.

[0012] In some embodiments of the present invention, the water quality parameters of the chromium-containing wastewater include at least one of the following:

[0013] 1) The main pollutant is Cr(III), and the concentration of Cr(III) is 100 - 300 mg / L;

[0014] 2) The water contains Cl - and SO4 2- ;

[0015] 3) pH = 4 - 6.

[0016] In some specific embodiments of the present invention, the water quality parameters of the chromium-containing wastewater include at least one of the following:

[0017] 1) The chromium in the chromium-containing wastewater is mainly Cr(III), and the concentration of Cr(III) is 150 - 250 mg / L;

[0018] 2) The water contains Cl - and SO4 2- ;

[0019] 3) pH = 4 - 5.

[0020] In some embodiments of the present invention, in the chromium-containing wastewater, the content of Cl - is 0.5 - 1.5 mg / L; the content of SO4 2- is 0.5 - 1.5 mg / L.

[0021] In some specific embodiments of the present invention, the chromium-containing wastewater includes tannery tanning wastewater.

[0022] In some specific embodiments of the present invention, the chromium-containing wastewater includes tannery tanning wastewater filtered by a 0.2 - 0.25 μm polyethersulfone membrane.

[0023] In some embodiments of the present invention, the concentration of the iron salt aqueous solution in step S1 is 0.4 - 0.6 mol / L.

[0024] In some specific embodiments of the present invention, the concentration of the iron salt aqueous solution in step S1 is 0.4 - 0.5 mol / L.

[0025] In some embodiments of the present invention, the iron salt is selected from at least one of ferric chloride, ferric sulfate, ferric nitrate, and ferric acetate.

[0026] In some embodiments of the present invention, the volume ratio of the iron salt aqueous solution to the chromium-containing wastewater in step S1 is 100:(10 - 15).

[0027] In some specific embodiments of the present invention, the volume ratio of the iron salt aqueous solution to the chromium-containing wastewater in step S1 is 100:(10 - 12).

[0028] In some embodiments of the present invention, the process of adding the iron salt aqueous solution in step S1 is assisted by stirring, and the stirring time is 25 - 35 min.

[0029] In some embodiments of the present invention, the pH is adjusted by dropping an alkali solution in step S1, the concentration of the alkali solution is 3 - 7 mol / L; the dropping rate is 2 - 4 mL / min.

[0030] In some specific embodiments of the present invention, the pH is adjusted by dropping an alkali solution in step S1, the concentration of the alkali solution is 4 - 6 mol / L; the dropping rate is 2.5 - 3.5 mL / min.

[0031] In some embodiments of the present invention, the alkali source of the alkali solution includes alkali metal hydroxides.

[0032] In some specific embodiments of the present invention, the alkali source of the alkali solution is selected from at least one of sodium hydroxide and potassium hydroxide.

[0033] In some embodiments of the present invention, the process of adjusting the pH in step S1 is assisted by stirring; the stirring speed is 260 - 300 rpm.

[0034] In some embodiments of the present invention, in the reaction system of step S1, the content of Cr(III) is less than or equal to 1.5% of the total mass of Cr(III) and ferrihydrite.

[0035] In some specific embodiments of the present invention, in the reaction system of step S1, the content of Cr(III) is 1% - 1.5% of the total mass of Cr(III) and ferrihydrite.

[0036] In some embodiments of the present invention, the adsorption time in step S1 is 45 - 55 h.

[0037] In some specific embodiments of the present invention, the adsorption time in step S1 is 45 - 50 h.

[0038] In some embodiments of the present invention, the water bath heating temperature in step S2 is ≥70 °C, and the time is 100 - 120 h.

[0039] In some specific embodiments of the present invention, the water bath heating temperature in step S2 is 70 - 80 °C, and the time is 110 - 120 h.

[0040] In some embodiments of the present invention, the pH of the Cr-ferrihydrite suspension in step S2 is ≥10.0.

[0041] In some specific embodiments of the present invention, the pH of the Cr-ferrihydrite suspension in step S2 is 10.0 - 12.0.

[0042] In some embodiments of the present invention, the solid-phase mineral composition of the Cr-hematite in step S2 includes 1 wt% - 2 wt% ferrihydrite, 2 wt% - 4 wt% goethite, and 94 wt% - 96 wt% hematite.

[0043] In some specific embodiments of the present invention, the solid-phase mineral composition of the Cr-hematite in step S2 includes 1 wt% - 2 wt% ferrihydrite, 2 wt% - 3 wt% goethite, and 95 wt% - 96 wt% hematite.

[0044] In some embodiments of the present invention, the particle size of the Cr-hematite in step S2 is less than 100 nm.

[0045] The basic principle of the present invention is described as follows:

[0046] 1) First, an aqueous solution of ferric ions is added to the chromium (Cr(III))-containing wastewater in the present invention. By dropping an alkaline solution, the hydrolysis rate of ferric ions is controlled to form ferrihydrite. Ferrihydrite has a loose and porous structure and a large specific surface area, providing abundant sites for the adsorption of Cr(III). This enables Cr(III) to combine with hydroxide ions in the system and be fixed on the surface of ferrihydrite by coprecipitation. The process of Cr(III) transferring from the water phase to the solid surface of ferrihydrite realizes the preliminary removal of Cr(III) in the water body.

[0047] 2) By subjecting the Cr-ferrihydrite suspension to water bath heating, using dissolution-recrystallization, ion migration and lattice reconstruction, as well as thermodynamic and kinetic factors, the mineral phase transformation of ferrihydrite is promoted to form hematite. During this process, Cr(III) changes from an unstable adsorbed state and solution state to a stable lattice doping state, enhancing the stability of the combination of Cr(III) with the mineral and reducing its mobility, thus realizing the effective and stable fixation of Cr(III) in the water body, and thereby achieving the treatment of Cr(III)-containing wastewater.

[0048] Among them, the principle of the mineral phase transformation process is specifically as follows:

[0049] ① Dissolution-recrystallization: Under the condition of water bath heating, the temperature of the environment where ferrihydrite is located rises. Ferrihydrite has a certain solubility in water. As the temperature rises, part of the ferrihydrite dissolves, and the dissolved iron ions and other related ions are redistributed in the solution. When the solution reaches a supersaturated state, these ions will recrystallize and precipitate out according to the crystal structure of hematite, gradually forming hematite.

[0050] ②Ion migration and lattice reconstruction: The energy provided by heating enables the ions in the goethite crystal structure to acquire sufficient kinetic energy, enhancing the mobility of the ions. The ions can migrate within the crystal structure. During the migration process, the crystal structure of goethite gradually disintegrates, and iron ions, etc., rearrange and combine at new positions to form a more stable hematite lattice structure. Lattice reconstruction is a key step in the transformation of goethite to hematite, which changes the properties of the crystal;

[0051] ③Thermodynamic and kinetic factors: Thermodynamically, when performing water bath heating at 55 - 85 °C, the transformation of goethite to hematite is a process with a decrease in Gibbs free energy, and the reaction proceeds spontaneously. As the temperature increases, the energy of the system increases, and the reaction proceeds towards the more stable hematite to reach a lower energy state; Kinetically, heating increases the reaction rate and accelerates the process of goethite transforming into hematite. As the temperature increases, the molecular motion intensifies, and the reaction frequency between ions increases. The originally slow transformation process can be basically completed within 120 h under the condition of water bath heating, achieving the effective conversion of goethite to hematite.

[0052] Compared with the prior art, the beneficial effects of the present invention are:

[0053] 1) The method for treating chromium-containing wastewater provided by the present invention in-situ synthesizes goethite by adjusting the hydrolysis rate of ferric ions. Goethite has a porous structure and a large specific surface area, providing abundant sites for the adsorption of Cr(III), enabling Cr(III) to transfer from the water body to the solid surface of goethite, and achieving the preliminary removal of Cr(III) in the water body; Using water bath heating to transform goethite adsorbed with Cr(III) on the surface into hematite nanoparticles. Due to the transformation of the mineral crystal phase, Cr(III) can be doped into the lattice of hematite nanoparticles in the form of isomorphic substitution, thereby further effectively enhancing the fixation of Cr(III);

[0054] 2) The method for treating chromium-containing wastewater provided by the present invention, the synthesized hematite nanoparticles containing Cr(III) have stable properties and low solubility in water environments with different pH values, can remain stable for a long time, prevent the re-release of Cr(III), and thus achieve the rapid, efficient, stable and persistent fixation of Cr(III) in the water body;

[0055] 3) The method for treating chromium-containing wastewater provided by the present invention has simple steps, easily available reaction raw materials, low cost, low toxicity of the product hematite nanoparticles, no risk of secondary pollution, is environmentally friendly, and is suitable for large-scale popularization and use. Description of the Drawings

[0056] Figure 1 It is the transmission electron microscope images of 1.5 wt% Cr-goethite and 1.5 wt% Cr-hematite in Example 1;

[0057] Figure 2 TEM images of 0 wt% Cr-ferrihydrite and 0 wt% Cr-hematite in Comparative Example 1;

[0058] Figure 3 X-ray diffraction comparison charts of hematite nanoparticles before and after synthesis in Example 1 and Comparative Example 1;

[0059] Figure 4 Comparison charts of changes in mineral components of hematite nanoparticles before and after synthesis in Example 1 and Comparative Example 1;

[0060] Figure 5 Specific surface area comparison charts of hematite nanoparticles before and after synthesis in Example 1 and Comparative Example 1;

[0061] Figure 6 Morphological change diagrams of Cr(III) before and after synthesis of hematite nanoparticles in Example 1;

[0062] Figure 7 Stability test results of hematite nanoparticles in soil extract with pH = 4.3;

[0063] Figure 8 Stability test results of hematite nanoparticles in soil extract with pH = 6.8;

[0064] Figure 9 Stability test results of hematite nanoparticles in soil extract with pH = 8.1. Detailed Description of the Invention

[0065] The content of the present invention will be further described in detail below through specific examples. The raw materials, reagents or devices used in the examples and comparative examples can be obtained from conventional commercial channels or by existing technical methods without special instructions. Unless otherwise specified, the test or measurement methods are conventional methods in the art.

[0066] 1. The chromium-containing wastewater in the example is tannery tanning wastewater. The key water quality parameters are: pH = 4.5, the main pollutant is Cr(III), the concentration is 200 mg / L, and there is a small amount of Cl - and SO4 2- in the water, and the concentrations are both about 1 mg / L. Before use, the chromium-containing wastewater is filtered through a 0.22 μm polyethersulfone membrane and stored in a brown glass bottle and placed in a 4 °C refrigerator for preservation;

[0067] 2. Reagents such as anhydrous ferric chloride, nitric acid, hydrochloric acid, and sodium hydroxide used in the examples, comparative examples, and test examples are all purchased from Sinopharm Reagents and are all of analytical grade.

[0068] Example 1

[0069] This example provides a method for treating chromium-containing wastewater, and the steps are as follows:

[0070] S11. Weigh 13.83 g of anhydrous ferric chloride solid into a 250 mL glass beaker, add 200 mL of deionized water, and dissolve it fully to prepare 200 mL of ferric ion solution with a concentration of 293.31 mmol / L;

[0071] S12. Weigh 40 g of sodium hydroxide solid into a 250 mL beaker, add 200 mL of deionized water, and prepare 200 mL of sodium hydroxide solution with a concentration of 5 mol / L;

[0072] S13. Measure 100 mL of the ferric ion solution and add it to a 250 mL glass beaker. Place it on a magnetic stirrer and stir for 30 min. Add 11.7 mL of chromium-containing wastewater, stir at a speed of 280 rpm for 30 min, and then add the sodium hydroxide solution dropwise at a speed of 2.9 mL / min. The color of the solution gradually turns red and turbid. Keep adding dropwise until the pH of the suspension rises to 7.5, and keep stirring for 48 h. During this period, keep the pH of the suspension constant at 7.5. Ferrihydrite is formed by the reaction and adsorbs Cr(III) in the water to obtain a Cr-ferrihydrite suspension. Among them, the content of Cr(III) in the system is 1.5% of the total mass of Cr(III) and ferrihydrite. The obtained Cr-ferrihydrite is marked as 1.5 wt% Cr-ferrihydrite;

[0073] S21. Place the Cr-ferrihydrite suspension in a polytetrafluoroethylene flask, and then continue to add the sodium hydroxide solution dropwise to the suspension to increase the pH of the suspension to 10.0. Heat it in a water bath at 70 °C and continuously transform for 120 h to obtain a 1.5 wt% Cr-hematite suspension. During the whole heating process, regularly monitor the pH of the suspension to keep the pH of the suspension constant at 10.0.

[0074] Comparative Example 1

[0075] This comparative example provides a method for treating chromium-containing water body, and the steps are as follows:

[0076] S11. Weigh 13.83 g of anhydrous ferric chloride solid into a 250 mL glass beaker, add 200 mL of deionized water, and dissolve it fully to prepare 200 mL of ferric ion solution with a concentration of 293.31 mmol / L;

[0077] S12. Weigh 40 g of sodium hydroxide solid into a 250 mL beaker, add 200 mL of deionized water, and prepare 200 mL of sodium hydroxide solution with a concentration of 5 mol / L;

[0078] S13. Measure 100 mL of ferric ion solution and add it to a 250 mL glass beaker. Place it on a magnetic stirrer and stir for 30 min. Then, dropwise add sodium hydroxide solution at a rate of 2.9 mL / min. The color of the solution gradually turns red and becomes turbid. Continue to dropwise add until the pH of the suspension increases to 7.5. Keep stirring for 48 h, maintaining the pH of the suspension constant at 7.5 during this period. Ferrihydrite is formed by the reaction to obtain a ferrihydrite suspension. Among them, there is no Cr(III) in the system, and the obtained Cr-ferrihydrite is labeled as 0 wt% Cr-ferrihydrite;

[0079] S21. Place the ferrihydrite suspension in a polytetrafluoroethylene flask, and then continue to dropwise add sodium hydroxide solution to the suspension to increase the pH of the suspension to 10.0. Heat it in a water bath at 70 °C for continuous conversion for 120 h to obtain a 0 wt% Cr - hematite suspension. During the entire heating process, regularly monitor the pH of the suspension to keep the pH of the suspension constant at 10.0.

[0080] Experimental Example 1

[0081] For the treatment of chromium-containing water bodies in Example 1 and Comparative Example 1, three parallel sets were set up. Samples were taken at the 0 h, 24 h, 48 h, and 120 h during the water bath heating of the ferrihydrite suspension. The suspension samples were centrifuged at a centrifugal force of 10052 g for 10 min, the supernatant was removed, the same volume of deionized water was added, and after thorough mixing, centrifugal separation was carried out again. Repeat this step three times to remove other impurities remaining on the solid surface. The wet solid was freeze-dried for 48 h and then used for characterization. The characterization methods and results are as follows:

[0082] 1. Transmission electron microscopy characterization:

[0083] Take 10 μL of the suspension of different treatment groups before and after conversion and disperse it in an ethanol solution. To prevent changes in the solid phase components during the ultrasonic process, ultrasonicate it at room temperature (25 °C) for 15 min to uniformly disperse the suspension in the ethanol solution. The obtained liquid is used for transmission electron microscopy analysis and characterization; use a pipette to drop the ultrasonified suspension onto an ultra-thin C film supported by a 200-mesh molybdenum grid, and then place it at room temperature to dry naturally; use a transmission electron microscope (FEI Tecnai F20) to analyze and characterize the mineral morphology; to ensure reliable results, select multiple regions of the sample for analysis.

[0084] Figure 1 For the transmission electron microscopy images of 1.5 wt% Cr-ferrihydrite and 1.5 wt% Cr-hematite in Example 1, among them, Figure 1 (a) in Figure 1 (b) in Figure 1 and (c) in are the transmission electron microscopy images of 1.5 wt% Cr-ferrihydrite at different magnifications, Figure 1 (d) inFigure 1 (e) in Figure 1 and (f) in Figure 1 are transmission electron microscope images of 1.5 wt% Cr - goethite at different magnifications. Figure 2 are transmission electron microscope images of 0 wt% Cr - ferrihydrite and 0 wt% Cr - hematite in Comparative Example 1, where Figure 2 (a) in Figure 2 (b) in Figure 2 and (c) in Figure 2 are transmission electron microscope images of 0 wt% Cr - ferrihydrite at different magnifications, Figure 2 (d) in Figure 2 (e) in Figure 2 and (f) in Figure 2 are transmission electron microscope images of 0 wt% Cr - hematite at different magnifications. From Figure 1 and Figure 2 it can be seen that both 1.5 wt% Cr - ferrihydrite in Example 1 and 0 wt% Cr - ferrihydrite in Comparative Example 1 show a cloudy appearance, are porous, have good dispersibility and a good pore structure, indicating that the addition of Cr(III) does not affect the formation of the hematite precursor (ferrihydrite); 1.5 wt% Cr - hematite in Example 1 is ellipsoidal, 0 wt% Cr - hematite in Comparative Example 1 is strip-shaped, the particle size of 1.5 wt% Cr - hematite is less than 100 nm, while the particle size of 0 wt% Cr - hematite is larger than that of 1.5 wt% Cr - hematite, indicating that the addition of Cr(III) can inhibit the growth of hematite and promote the formation of hematite nanoparticles.

[0085] 2. X - ray diffraction characterization and mineral composition quantification:

[0086] The freeze - dried powder sample was ground in an agate mortar, and the ground powder sample was collected and placed in a 3.0 mL polyethylene centrifuge tube. The solid powder was characterized using an X - ray diffractometer (XRD, Rigaku SmartLab), which was equipped with Cu Kα, and the diffraction wavelength λ was 0.154 nm; during the acquisition of the diffraction patterns of all solid powder samples, the tube current of the instrument was set to 50 mA, the tube voltage was set to 40 kV, the step speed was set to 1° / min, the step size was set to 0.02°, and the collection range of the diffraction spectra of all solid powder samples was 10° - 80°;

[0087] To determine the composition of the phases during the formation of hematite nanoparticles, qualitative analysis of the mixed phases in the samples was carried out using Jade 6.0 in combination with the standard cards of ferrihydrite (ICSD#46-1315), goethite (ICSD#29-0713) and hematite (ICSD#33-0664); to quantify the composition of each phase in the samples, Rietveld refinement was performed on the obtained diffraction patterns using TOPAS 5.0, and this calculation mainly uses the integrated intensity of the diffraction peaks and the crystal structure to calculate the relative mass of each phase in the sample.

[0088] Figure 3 Figure for comparison of X-ray diffraction before and after the synthesis of hematite nanoparticles in Example 1 and Comparative Example 1. It can be seen from Figure 3 that both 1.5wt% Cr-ferrihydrite in Example 1 and 0wt% Cr-ferrihydrite in Comparative Example 1 are weakly crystalline iron minerals. After transformation, the crystallinity of both 1.5wt% Cr-hematite and 0wt% Cr-hematite generated becomes higher, and no other minerals are found before and after the transformation, indicating that both the synthesized 1.5wt% Cr-hematite and 0wt% Cr-hematite nanoparticles have high purity.

[0089] Figure 4 Figure for comparison of the changes in mineral components before and after the synthesis of hematite nanoparticles in Example 1 and Comparative Example 1. It can be seen from Figure 4 that the products in the solid phase of 1.5wt% Cr-ferrihydrite in Example 1 and 0wt% Cr-ferrihydrite in Comparative Example 1 are almost 100% ferrihydrite, and no other minerals are found. After the transformation reaction, the solid phase of 1.5wt% Cr-hematite in Example 1 contains 2.1wt% ferrihydrite, 5.7wt% goethite and 92.2wt% hematite, and the solid phase of 0wt% Cr-ferrihydrite in Comparative Example 1 contains 1.5wt% ferrihydrite, 2.9wt% goethite and 95.6wt% hematite, indicating that heating in a water bath at 70°C for 120h can promote the basic complete transformation of ferrihydrite, and the transformation products are mainly hematite, and the addition of Cr(III) has almost no effect on the formation of hematite nanoparticles, indicating that using water bath heating to promote the mineral phase transformation of ferrihydrite and then fix Cr(III) has strong applicability for the removal of Cr(III) in water.

[0090] 3. Specific surface area characterization:

[0091] To characterize the specific surface areas of minerals in different treatment groups before and after transformation, 35 mg of solid-phase powder was weighed and placed in a glass tube. All samples were degassed under vacuum conditions at 298 K for 96 h to remove adsorbed gases, moisture, and impurities on the surface of solid particles. Then, in an N2 atmosphere at 77 K, a specific surface area analyzer (Micromeritics ASAP 2020) was used to analyze the samples. The adsorption-desorption isotherms of the samples were collected in the partial pressure region of 0.995 to 0.01 (P / P0), and the BET equation was used to analyze the desorption and adsorption curves to obtain the specific surface areas of the samples.

[0092] Figure 5 Figure for comparing the specific surface areas of hematite nanoparticles before and after synthesis in Example 1 and Comparative Example 1. From Figure 5 it can be seen that both 1.5 wt% Cr-ferrihydrite in Example 1 and 0 wt% Cr-ferrihydrite in Comparative Example 1 have relatively high specific surface areas, and the specific surface area of 0 wt% Cr-ferrihydrite in Comparative Example 1 is slightly higher than that of 1.5 wt% Cr-ferrihydrite in Example 1, indicating that ferrihydrite can provide sufficient adsorption sites for the fixation of Cr(III), enabling Cr(III) to be fixed on the surface of ferrihydrite through coprecipitation, thus achieving the preliminary removal of Cr(III) in water. After transformation by water bath heating, the specific surface areas of 1.5 wt% Cr-hematite and 0 wt% Cr-hematite decreased significantly, but their crystallinity was higher and their stability was stronger.

[0093] Test Example 2

[0094] The concentration of Cr(III) adsorbed on the mineral surface in 1.5 wt% Cr-ferrihydrite in Example 1 (defined as adsorbed Cr) was measured, and the concentration of Cr(III) doped into the hematite lattice in 1.5 wt% Cr-hematite (defined as doped Cr) was calculated. The specific steps are as follows:

[0095] 1) In Example 1, the treatment of chromium-containing water bodies was set up in three groups in parallel. Sampling was carried out at the 0th h, 24th h, 48th h, and 120th h when the ferrihydrite suspension was heated in a water bath. The suspension samples were centrifuged at a centrifugal force of 10052 g for 10 min. The supernatant was used to determine the content of Cr(III) (solution-state Cr). 0.1 g of the solid-phase product was collected respectively, mixed with 15 mL of nitric acid with a concentration of 1.0 mmol / L, and then placed in a shaking box and shaken for extraction for 12 h. 0.1 mol / L nitric acid was used to control the pH of the whole extraction reaction to 3.0 (nitric acid was selected as the extractant because under acidic conditions, Cr(III) can be desorbed from the mineral surface); the suspension after the above shaking was placed in a 50 mL centrifuge tube, and then centrifuged in a centrifuge. The rotation speed of the centrifuge was set to 10052 g and the time was 20 min; then the supernatant was filtered through a 0.45 μm polyethersulfone membrane, the filtrate was collected, and then acidified and stored in a 4 °C refrigerator for determining the concentration of surface-adsorbed Cr(III) (adsorbed-state Cr);

[0096] 2) After the filtrates of all samples were collected, the concentration of Cr(III) was measured by inductively coupled plasma. The actual concentration of Cr(III) in the filtrate was obtained by conversion according to the dilution factor. Based on the total added Cr(III) concentration (total Cr concentration) and the concentrations of the supernatant (solution-state Cr) and filtrate (adsorbed-state Cr) measured by the instrument, the concentration of Cr(III) (doped-state Cr) doped into the iron mineral lattice was obtained by calculation. The calculation formula is:

[0097] Percentage content of doped-state Cr (%) = (total Cr concentration - solution-state Cr concentration - adsorbed-state Cr concentration) / total Cr concentration * 100%.

[0098] Figure 6 Figure Figure 6 shows the morphological changes of Cr(III) before and after the synthesis of hematite nanoparticles in Example 1. As can be seen, within 0 - 120 h, with the promotion of the gradual formation of hematite nanoparticles by water bath heating, the content of solution-state Cr(III) in the system decreased from the initial 4.81% to 0.85%, the content of adsorbed-state Cr(III) decreased from the initial 91.92% to 0.16%, while the content of doped-state Cr(III) increased from the initial 3.27% to 93.72%. The quantitative results show that with the gradual formation of hematite nanoparticles, more adsorbed-state Cr(III) is converted into doped-state Cr(III), and Cr(III) is gradually doped into the hematite nanoparticle lattice, significantly reducing the mobility of Cr(III) and promoting the effective and stable fixation of Cr(III).

[0099] In addition, the present invention further verified through experiments the optimal content of Cr(III) in the system during the generation of Cr-ferrihydrite, as well as the optimal conditions for promoting the transformation of Cr-ferrihydrite into Cr-hematite by water bath heating. The results showed that when an iron salt aqueous solution was mixed with chromium-containing wastewater, the content of Cr(III) in the system was 1.5% of the total mass of Cr(III) and ferrihydrite, and under the conditions of water bath heating at 70 °C and pH = 10.0 for 120 h to promote the mineral phase transformation of ferrihydrite, the treatment effect of chromium-containing wastewater was the best. If the content of Cr(III) was greater than 1.5 wt%, the conversion temperature was lower than 70 °C, and the pH was less than 10.0, it would lead to insufficient conversion of ferrihydrite, and the purity of the synthesized hematite nanoparticles was relatively low, ultimately reducing the fixation amount of hematite nanoparticles to Cr(III) and affecting the fixation of Cr(III) and subsequent stability.

[0100] Test Example 3

[0101] Soils with three different pH values were collected, located in Yingtan City, Jiangxi Province, Heshan City, Guangdong Province, and Tongren City, Guizhou Province, respectively. The pH values of the three soils were 4.3, 6.8, and 8.1. The stability evaluation experiment of Cr(III) removal by hematite nanoparticles was carried out using the leaching solutions of the three soils.

[0102] 1) Weigh 20 g of each of the above three soils into glass conical flasks, add 200 mL of sterile deionized water, place them in a shaker with a rotation speed of 280 rpm and shake for 10 h, then let them stand for 1 h, and take 180 mL of the upper-layer solution in the conical flask, which is the soil leaching solution.

[0103] 2) Sterilize the obtained leaching solution at a high temperature for 30 min, suck 40 mL of the sterilized soil leaching solution into a 100 mL serum bottle, and then add 0.20 g of the dried 1.5 wt% Cr-hematite nanoparticles in Example 1 to the treatment groups respectively. Samples were taken from different treatment groups on the 0th day, 5th day, 10th day, and 100th day.

[0104] 3) Pipette 5 mL of the suspension into a 50 mL polyethylene centrifuge tube, centrifuge and filter, collect the supernatant for the determination of the concentration of dissolved Cr(III) (dissolved Cr), and use the wet solid remaining in the centrifuge tube for the extraction experiment. Add 20 mL of 1 mmol / L HNO3 to the 50 mL centrifuge tube, and at the same time adjust the pH of the mixture to 3.0. Place it in a shaking incubator and shake for 12 h at a rotation speed of 280 rpm. Place all centrifuge tubes in a centrifuge and centrifuge at a speed of 10052 g for 20 min. Then filter the supernatant with a 0.45 μm polyethersulfone membrane, collect the filtrate, acidify it and store it in a refrigerator at 4 °C. After the filtrates of all samples are collected, determine the concentration of Cr(III) (adsorbed Cr) by inductively coupled plasma, and calculate the percentage content of doped Cr(III) (doped Cr) by mass conservation. The calculation method is the same as that in Test Example 2.

[0105] Figure 7 This is the stability test result of hematite nanoparticles in the soil extract with pH = 4.3. As can be seen from Figure 7 it, in the slightly acidic soil extract in Yingtan City, Jiangxi Province, after 100 days of cultivation, 92.99% of Cr(III) is still fixed in the lattice of the synthesized hematite nanoparticles in the form of doped state, 2.41% of Cr(III) exists in the form of adsorbed state, and only 4.60% of dissolved Cr(III) is released into the solution.

[0106] Figure 8 This is the stability test result of hematite nanoparticles in the soil extract with pH = 6.8. As can be seen from Figure 8 it, in the slightly neutral soil extract in Heshan City, Guangdong Province, after 100 days of cultivation, the content of Cr(III) doped into the hematite lattice accounts for 93.89% of the total fixed amount, 5.87% of Cr(III) exists in the form of adsorbed state, and only 0.23% of dissolved Cr(III) is detected in the solution.

[0107] Figure 9 This is the stability test result of hematite nanoparticles in the soil extract with pH = 8.1. As can be seen from Figure 9 it, in the slightly alkaline soil extract in Tongren City, Guizhou Province, after 100 days of cultivation, the content of Cr(III) fixed in the hematite nanoparticles accounts for 93.76% of the total fixed amount, 6.10% of Cr(III) exists in the form of adsorbed state, and only 0.15% of dissolved Cr(III) is detected in the solution.

[0108] The stability test in Test Example 3 shows that for the treatment method of chromium-containing water provided by the present invention, after fixing Cr(III) inside the hematite lattice, the formed Cr-hematite nanoparticles have good stability in soil extracts with different pH values. In weakly acidic, neutral, and weakly alkaline soil extracts, after culturing for 100 days, the content of dissolved Cr(III) is less than 5%, indicating that the method provided by the present invention can achieve efficient and stable removal of Cr(III) in water, and the Cr-hematite nanoparticles can maintain long-term stability in water environments with different pH values and are not likely to cause secondary pollution by releasing the fixed Cr(III) again.

Claims

1. A method for treating chromium-containing wastewater, characterized in that It includes the following steps: S1. Add an iron salt aqueous solution to the chromium-containing wastewater, adjust the pH of the system to 7.0 - 8.0 to obtain ferrihydrite, adsorb Cr(III) to obtain a Cr-ferrihydrite suspension; S2. Heat the Cr-ferrihydrite suspension in a water bath to obtain a Cr-hematite suspension, thus completing the treatment of the chromium-containing wastewater.

2. The processing method according to claim 1, wherein The water quality parameters of the chromium-containing wastewater in step S1 include at least one of the following: 1) The chromium in the chromium-containing wastewater is mainly Cr(III), and the concentration of Cr(III) is 100 - 300 mg / L; 2) Cl in water - and SO4 2- ; 3) pH = 4 - 6.

3. The processing method according to claim 2, wherein The concentration of the iron salt aqueous solution in step S1 is 0.4 - 0.6 mol / L.

4. The processing method according to claim 3, wherein The volume ratio of the iron salt aqueous solution to the chromium-containing wastewater in step S1 is 100:(10 - 15).

5. The processing method according to claim 1, characterized in that, In step S1, add an alkali solution to adjust the pH. The concentration of the alkali solution is 3 - 7 mol / L; the dropping rate is 2 - 4 mL / min.

6. The processing method according to claim 1, wherein, In the reaction system of step S1, the content of Cr(III) is less than or equal to 1.5% of the total mass of Cr(III) and ferrihydrite.

7. The processing method according to claim 1, wherein The adsorption time in step S1 is 45 - 55 h.

8. The processing method according to claim 1, characterized in that The water bath heating temperature in step S2 is ≥70 °C, and the time is 100 - 120 h.

9. The processing method according to claim 1, characterized in that, The pH of the Cr-ferrihydrite suspension in step S2 is ≥10.

0.

10. The processing method according to claim 1, characterized in that, The solid-phase mineral composition of the Cr-hematite in step S2 includes 1 wt% - 2 wt% ferrihydrite, 2 wt% - 4 wt% goethite, and 94 wt% - 96 wt% hematite.

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