Method for reducing heavy metals by coordination dissolution induction synergistic high-activity crystal face goethite

Abstract

The application provides a method for reducing heavy metals by coordination dissolution induction and synergistic high-activity crystal face goethite, and belongs to the technical field of heavy metal pollution treatment.The method provided by the application comprises the following steps: (1) preparation of high-activity crystal face goethite; (2) mixing a heavy metal-containing to-be-treated system with the high-activity crystal face goethite; (3) adding a small-molecule organic acid coordination agent to the mixture of step (2) and regulating the pH of the system to induce the surface of the goethite to release Fe(II) and Fe(III) ions through coordination dissolution; (4) under light irradiation, Fe(II) / Fe(III) forms an oxidation-reduction cycle in a coordination environment to synergistically realize reduction and fixation of high-valence heavy metal ions.The application is based on a synergistic mechanism of photocatalysis, crystal face regulation, coordination dissolution and iron cycle, significantly improves the electron transfer efficiency and reduction capacity of the goethite, does not need external energy or a strong reducing agent, has mild conditions, simple operation and low cost, and is suitable for efficient detoxification and stabilization treatment of heavy metals in water bodies and soil.

Description

Technical Field

[0001] This invention belongs to the field of heavy metal pollution control and environmental remediation technology, specifically relating to a method for the reduction and fixation of heavy metals based on the coordination chemistry of iron oxide interfaces and the synergistic mechanism of redox reactions. Furthermore, this invention is particularly applicable to the reduction, detoxification, and stabilization treatment of highly toxic and highly mobile heavy metals (such as chromium, uranium, and technetium) in water, soil, or sediments, and belongs to the field of environmental functional materials application and pollution control chemistry technology. Background Technology

[0002] With the acceleration of industrialization and modernization in human society, the unreasonable exploitation of mineral resources and their smelting emissions, the use of sewage for irrigation and sludge application, and atmospheric deposition caused by human activities, soil pollution has become increasingly serious, urgently requiring efficient and green environmental protection technologies. Among environmental pollution control technologies, photocatalysis technology has attracted much attention due to its green, pollution-free, and low-energy consumption, and its ability to synergistically achieve energy conversion and environmental pollution control (Christoforidis KC, Fornasiero P. Chemcatchem, 2017, (9), 1523-1544; Wang HL. Chemical Society Reviews, 2014, (43), 5234-5244.), becoming one of the most active research directions in the field of energy and environmental function applications in recent years.

[0003] Iron (water) oxides are widely distributed in soil, sediments, rocks, and water bodies. Due to their wide distribution, they play a crucial role in geochemical cycles and environmental processes. They can serve as important adsorbents for adsorbing and fixing many environmental pollutants, such as heavy metal ions and organic pollutants. The dissolution process of iron oxides in nature involves two processes: biological dissolution and abiotic dissolution. Abiotic dissolution refers to the process of structural decomposition caused by physical or chemical actions without the participation of organisms. For example, organic acids promote dissolution through coordination (Zhan G. Environmental Science: Nano, 2022, 9(5), 1770-1779; Liang L. Geochimica et Cosmochimica Acta, 2000, 64(12), 2027-2037). In addition, light exposure can promote the dissolution process by photo-dissolving and destroying the crystal structure of iron oxides. The dissolution process of iron oxides is directly related to the migration and transformation of pollutants in the Earth's environment. Therefore, they show great potential in the field of environmental pollution remediation.

[0004] Goethite (α-FeOOH) is one of the most stable iron (hydride) oxides in the natural environment. It possesses advantages such as a large specific surface area, abundant surface active sites, and good environmental compatibility, making it a common adsorbent for heavy metals. Furthermore, its narrow band gap allows it to absorb visible solar radiation, generating photocatalytic activity. However, iron in goethite mainly exists in the Fe(III) form, resulting in weak inherent reducing power and low efficiency when directly used to reduce high-valence heavy metals. Existing research often relies on external reducing agents (such as sodium dithionite and zero-valent iron) or microbial activity to enhance the reducing performance of goethite, which suffers from high costs, significant secondary pollution risks, and stringent reaction conditions.

[0005] In recent years, coordination leaching strategies have gradually attracted attention. Certain organic ligands (such as oxalic acid, citric acid, EDTA, etc.) can coordinate with Fe(III) on the surface of goethite, inducing iron ions to dissolve from the mineral lattice and forming Fe(II) / Fe(III) redox pairs under appropriate conditions. However, in existing technologies, coordination leaching is mostly used to promote the dissolution of iron minerals or the release of iron ions. There are no reports on technologies that utilize the coordination leaching process to induce a continuous and efficient Fe(II) / Fe(III) cycle in goethite itself and synergistically couple it with heavy metal reduction processes. Panias et al. (Panias D, et al. Hydrometallurgy, 1996, 42(2): 257-265.) found that dissolved organic matter can promote the reductive dissolution of iron minerals and summarized it into three stages: (1) organic ligands are adsorbed onto Fe(III) on the surface of iron minerals; (2) the complex accepts protons and undergoes non-reductive dissolution; (3) the complex undergoes reductive dissolution through ligand-to-metal charge transfer (LMCT). Dang et al. (Dang Z, et al. EnvironmentalPollution, 2002, 118(3): 419-426.) found that heavy metal ions can be complexed with organic compounds and adsorbed by amorphous ferric hydroxide colloids when diffusing into the environment.

[0006] The photocatalytic activity of goethite is closely related to its exposed crystal facets. Common exposed crystal facets in natural environments include (010), (110), and (021). Different crystal facets affect their interaction with reactants and their reactivity. To effectively increase the proportion of superior exposed crystal facets, specific crystal facets, such as the (021) active crystal facet, are often exposed using crystal facet modifiers to optimize the photocatalyst activity and thus enhance the efficiency of the photocatalytic reaction. Based on this, combining illumination conditions with the coordination-induced dissolution effect of goethite may provide a new solution for the reduction of heavy metals.

[0007] Therefore, there is an urgent need to develop a new method that does not require external reducing agents, operates under mild conditions, and can fully utilize the iron recycling capacity of active acicular iron ore structure to achieve efficient reduction of heavy metals, in order to overcome the shortcomings of existing technologies. Summary of the Invention

[0008] In view of the shortcomings of the prior art, the purpose of this invention is to provide a method for coordination leaching-induced synergistic reduction of heavy metals by goethite, which effectively improves the efficiency of heavy metal reduction.

[0009] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A method for reducing heavy metals using coordination-induced synergistic highly active acicular ferrite, comprising the following steps: (1) The pH of the iron salt solution was adjusted with alkaline solution, a crystal surface regulator was added, and the solution was aged at a certain temperature. After centrifugation, washing and drying, highly active crystal surface nano needle iron ore was obtained. (2) The highly active crystal nano needle iron ore is mixed with the solution system containing heavy metals at a solid-liquid ratio of 1:500~1:5000 g / L; (3) Add small molecule organic acid ligands to the mixed system of step (2) and adjust the pH of the system to induce coordination dissolution on the surface of goethite; (4) Under light conditions, Fe(II) / Fe(III) form a redox cycle in the coordination environment, which synergistically achieves the reduction and fixation of high-valence heavy metal ions (such as Cr(VI)).

[0010] The preparation method of this invention is based on the principle of heterogeneous photocatalysis and coordination dissolution. Using highly active crystalline goethite as raw material, it incorporates small-molecule organic acids to induce the Fe(III) dissolution effect of goethite, coupled with photoinduced photogenerated electrons in the goethite to form a Fe(II) / Fe(III) cycle, synergistically improving the reduction efficiency of high-valence heavy metal ions. Experimental verification shows that the method of this invention can effectively utilize the excellent photocatalytic performance of highly active crystalline goethite nanoparticles, synergistically reducing heavy metals through the coordination-induced dissolution effect of small-molecule organic acids, providing a new approach for the remediation of heavy metal pollution.

[0011] Prior to obtaining the solution of this invention, the inventors conducted extensive research on the reduction of heavy metals (taking a Cr(VI) concentration of 10 ppm as an example), including utilizing the photocatalytic activity of goethite and the reduction performance of small molecule organic acids alone, but failed to obtain effective reduction efficiency. For example, when using goethite and oxalic acid alone as raw materials for heavy metal reduction, the reduction efficiencies were only 23.41% and 3.58%, respectively.

[0012] This invention uses highly active crystalline needle iron ore and small molecule organic acids as raw materials. Through the inductive effect of organic acids, the heavy metal Cr(VI) in wastewater is reduced and fixed by coordination-induced dissolution and photocatalytic reaction, achieving excellent reduction efficiency, which can reach 100%.

[0013] It should be noted that the order of each implementation step and the addition of the small molecule organic acid ligand in step (3) are crucial in the above method of the present invention.

[0014] This invention uses highly active crystalline goethite as raw material, adds small molecule organic acids to induce Fe(III) dissolution of goethite through coordination, and couples photocatalytic effects to form Fe(II) / Fe(III) cycle, thereby improving the reduction efficiency of heavy metal ions and providing a new approach and idea for the remediation of heavy metal pollution.

[0015] Furthermore, the iron salt mentioned in step (1) is FeCl3•6H2O, and its concentration is 0.05~0.10 M.

[0016] Furthermore, the alkaline solution used to adjust the pH in step (1) is a NaOH solution with a concentration of 0.1~1 M and a pH adjustment range of 10~12.

[0017] Furthermore, the crystal plane regulator added in step (1) is one of ethanol, ethylene glycol, or glycerol, and the added volume is 1 to 5 mL.

[0018] Furthermore, the temperature in step (1) is 60~80℃ and the aging time is 24~48 h.

[0019] Furthermore, the washing method described in step (1) is to wash three times each with deionized water and anhydrous ethanol.

[0020] Furthermore, the drying method described in step (1) is vacuum drying or freeze drying.

[0021] Furthermore, the microstructure of the goethite with highly active crystal faces described in step (1) is short rod-shaped with a length of about 500~600 nm and a width of 200 nm, and the proportion of (021) / (110) crystal faces is not less than 28.57%.

[0022] Furthermore, the solid-liquid ratio of the goethite and the heavy metal-containing solution system in step (1) is 1:500~1:5000.

[0023] Furthermore, the small molecule organic acid ligand mentioned in step (2) is any one of formic acid, acetic acid, oxalic acid, citric acid, ascorbic acid or tartaric acid.

[0024] Furthermore, the concentration of the small molecule organic acid in step (2) is 1~10 mM.

[0025] Furthermore, the pH condition of the system described in step (3) is in the range of 2 to 6.

[0026] Furthermore, step (4) involves placing the mixed system under natural or artificial light conditions.

[0027] Furthermore, the step (4) of placing the mixed system under natural or artificial light conditions requires the addition of shaking or stirring.

[0028] Furthermore, the reaction time of placing the mixed system under natural light or artificial light conditions in step (4) is 1 to 8 hours.

[0029] The beneficial effects of this invention are as follows: The method of this invention has the advantages of simple process, strong controllability, good stability, mild reaction conditions, wide availability of raw materials, and low cost. The overall process is conducive to large-scale industrial processing.

[0030] This invention has the combined advantages of high reduction efficiency and low cost. Attached Figure Description

[0031] Figure 1 FESEM images of goethite with different active crystal facet ratios; (021) / (110) crystal facet ratios are 28.57% (a, b) and 33.33% (c, d). Detailed Implementation

[0032] The present invention will be specifically described below through embodiments. It should be noted that the following embodiments are only used to further illustrate the present invention and should not be construed as limiting the scope of protection of the present invention. Some non-essential improvements and adjustments made by those skilled in the art based on the above-described invention are still within the scope of protection of the present invention.

[0033] Example 1

[0034] The pH of the system was adjusted to 12 by slowly adding 1M NaOH to a 0.1 M FeCl3 solution. 1.0 mL of ethylene glycol, a crystal facet regulator, was added. The solution was aged in an 80℃ water bath for 24 h. After centrifugation, washing, and freeze-drying, the highly active crystal facet nano-needle iron ore was obtained, with a (021) / (110) crystal facet ratio of 28.57%. Figure 1a) and b) 15.00 mg of this highly active crystalline needle-like iron ore was added to 30.00 mL of a solution containing oxalic acid (5 mM) and Cr(VI) (10 ppm). The pH of the solution was adjusted to 4.00 using 0.1 M HCl and 0.1 M NaOH. A photocatalytic reduction experiment was then conducted in a 250 mL beaker. The solution was first sonicated in the dark for 30 min to reach adsorption equilibrium, and then placed in a constant-temperature shaker at 25℃ and a shaking rate of 120 rpm under a 32 W light intensity of 5 mW / cm². 2 The mixture was vibrated under LED light for 4 hours. The upper liquid was then analyzed to determine the Cr(VI) content and the reduction rate was calculated to be 85.23%.

[0035] Example 2

[0036] The pH of the system was adjusted to 12 by slowly adding 1M NaOH to a 0.1 M FeCl3 solution. 2.5 mL of ethylene glycol, a crystal facet modifier, was added. The solution was aged in an 80℃ water bath for 24 h. After centrifugation, washing, and freeze-drying, the highly active crystal facet nano-needle iron ore was obtained, with a (021) / (110) crystal facet ratio of 33.33%. Figure 1 c, d). 15.00 mg of this highly active crystalline needle-like iron ore was added to 30.00 mL of a solution containing oxalic acid (5 mM) and Cr(VI) (10 ppm). The pH of the solution was adjusted to 2.50 using 0.1 M HCl and 0.1 M NaOH. A photocatalytic reduction experiment was then conducted in a 250 mL beaker. The solution was first sonicated in the dark for 30 min to reach adsorption equilibrium, and then placed in a constant-temperature shaker at 25℃ and a shaking rate of 120 rpm under a 32 W light intensity of 5 mW / cm². 2 The mixture was vibrated under LED light for 4 hours. The upper liquid was then analyzed to determine the Cr(VI) content and the reduction rate was calculated to be 100%.

[0037] Example 3

[0038] The pH of the system was adjusted to 12 by slowly adding 1M NaOH to a 0.1 M FeCl3 solution. 2.5 mL of ethylene glycol, a crystal facet modifier, was added. The solution was aged in an 80℃ water bath for 24 hours. After centrifugation, washing, and freeze-drying, the highly active crystal facet nano-needle iron ore was obtained, with a (021) / (110) crystal facet ratio of 33.33%. Figure 1c, d). 15.00 mg of this highly active crystalline needle-like iron ore was added to 30.00 mL of a solution containing oxalic acid (5 mM) and Cr(VI) (10 ppm). The pH of the solution was adjusted to 3.50 using 0.1 M HCl and 0.1 M NaOH. A photocatalytic reduction experiment was then conducted in a 250 mL beaker. The solution was first sonicated in the dark for 30 min to reach adsorption equilibrium, and then placed in a constant-temperature shaker at 25℃ and a shaking rate of 120 rpm under a 32 W light intensity of 5 mW / cm². 2 The mixture was vibrated under LED light for 4 hours. The upper liquid was then analyzed to determine the Cr(VI) content and the reduction rate was calculated to be 95.23%.

[0039] Comparative Example 1 The pH of the system was adjusted to 12 by slowly adding 1M NaOH to a 0.1 M FeCl3 solution. The solution was aged in an 80℃ water bath for 24 h, and after centrifugation and washing, it was freeze-dried to obtain nano-needle iron ore with a (021) / (110) crystal plane ratio of 8.69%. 15.00 mg of this needle iron ore was added to 30.00 mL of a solution containing oxalic acid (5 mM) and Cr(VI) (10 ppm). The pH of the solution was adjusted to 4.00 using 0.1 M HCl and 0.1 M NaOH. The photocatalytic reduction experiment was carried out in a 250 mL beaker. The solution was first sonicated in the dark for 30 min to reach adsorption equilibrium, and then placed in a constant temperature shaker with a temperature of 25℃ and a shaking rate of 120 rpm under a light intensity of 5 mW / cm². 2 The mixture was vibrated under LED light for 4 hours. The upper liquid was then analyzed to determine the Cr(VI) content and the reduction rate was calculated to be 23.41%.

[0040] Comparative Example 2 The pH of the system was adjusted to 12 by slowly adding 1M NaOH to a 0.1 M FeCl3 solution. 1.0 mL of ethylene glycol, a crystal facet regulator, was added. The solution was aged in an 80℃ water bath for 24 h. After centrifugation, washing, and freeze-drying, nano-needle iron ore was obtained, with a (021) / (110) crystal facet ratio of 28.57%. 15.00 mg of this highly active crystalline facet goethite was added to 30.00 mL of a solution containing Cr(VI) (10 ppm). The pH of the solution was adjusted to 4.00 using 0.1 M HCl and 0.1 M NaOH. A reduction experiment was conducted in a 250 mL beaker. The solution was first sonicated in the dark for 30 min to reach adsorption equilibrium. Then, it was placed in a constant temperature shaker at 25℃ and a shaking rate of 120 rpm for 4 h. The Cr(VI) content of the supernatant was analyzed, and the reduction rate was calculated to be 31.54%.

[0041] Comparative Example 3 The pH of the system was adjusted to 12 by slowly adding 1M NaOH to a 0.1 M FeCl3 solution. 1.0 mL of ethylene glycol, a crystal facet regulator, was added. The solution was aged in an 80℃ water bath for 24 h. After centrifugation, washing, and freeze-drying, nano-needle iron ore was obtained, with a (021) / (110) crystal facet ratio of 28.57%. 15.00 mg of this highly active crystalline facet goethite was added to 30.00 mL of a solution containing EDTA (5 mM) and Cr(VI) (10 ppm). The pH of the solution was adjusted to 4.00 using 0.1 M HCl and 0.1 M NaOH. A reduction experiment was conducted in a 250 mL beaker. The solution was first sonicated in the dark for 30 min to reach adsorption equilibrium. Then, it was placed in a constant-temperature shaker at 25℃ and a shaking rate of 120 rpm for 4 h. The Cr(VI) content of the supernatant was analyzed, and the reduction rate was calculated to be 36.85%.

[0042] Comparative Example 4 A 30.00 mL solution containing oxalic acid (5 mM) and Cr(VI) (10 ppm) was prepared. The pH of the solution was adjusted to 4.00 using 0.1 M HCl and 0.1 M NaOH. A photocatalytic reduction experiment was then conducted in a 250 mL beaker. The solution was first sonicated in the dark for 30 min to reach adsorption equilibrium. Then, it was placed in a constant-temperature shaker at 25°C and a shaking rate of 120 rpm under a 32 W light intensity of 5 mW / cm². 2 The mixture was vibrated under LED light for 4 hours. The upper liquid was then analyzed to determine the Cr(VI) content and the reduction rate was calculated to be 3.58%.

Claims

1. A method for the synergistic reduction of heavy metals by coordination-induced dissolution of highly active acicular ferruginous iron ore, characterized in that, The method includes the following steps: (1) The pH of the iron salt solution was adjusted with alkaline solution, a crystal surface regulator was added, and the solution was aged at a certain temperature. After centrifugation, washing and drying, highly active crystal surface nano needle iron ore was obtained. (2) The highly active crystal nano needle iron ore is mixed with the system to be treated containing heavy metals at a solid-liquid ratio of 1:500~1:5000 g / L; (3) Add small molecule organic acid ligands to the mixture in step (2) and adjust the pH of the system to induce coordination dissolution on the surface of goethite; (4) Under light conditions, Fe(II) / Fe(III) form a redox cycle in the coordination environment, which synergistically achieves the reduction and fixation of high-valence heavy metal ions.

2. The method according to claim 1, characterized in that, The iron salt mentioned in step (1) is FeCl3•6H2O; preferably, the alkaline solution is NaOH solution with a concentration of 0.1~1 M and a pH adjustment range of 10~12.

3. The method according to claim 1, characterized in that, The crystal plane regulator mentioned in step (1) is one of ethanol, ethylene glycol, and glycerol, and the added volume is 1~5 mL.

4. The method according to claim 1, characterized in that, The temperature in step (1) is 60~80℃ and the aging time is 24~48 h.

5. The method according to claim 1, characterized in that, The washing method described in step (1) is to wash three times each with deionized water and anhydrous ethanol; preferably, the drying method is vacuum drying or freeze drying.

6. The method according to claim 1, characterized in that, The microstructure of the highly active crystal nano needle iron ore described in step (1) is a coarse rod shape, and the proportion of (021) / (110) crystal planes is not less than 28.57%.

7. The method according to claim 1, characterized in that, The small molecule organic acid ligand mentioned in step (3) is any one of formic acid, acetic acid, oxalic acid, citric acid, ascorbic acid or tartaric acid, and the concentration of the added small molecule organic acid ligand is 1 ~ 10 mM.

8. The method according to claim 1, characterized in that, The pH adjustment in step (3) is based on adjusting the pH to a range of 2 to 6 using different small molecule acids.

9. The method according to claim 1, characterized in that, The lighting conditions mentioned in step (4) are natural lighting or artificial lighting conditions; preferably, the mixing system needs to be shaken or stirred when placed under natural lighting or artificial lighting conditions.

10. The method according to claim 1, characterized in that, The reaction time of the redox cycle in step (4) is 1 to 8 hours.

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