Method for oxidation removal and heavy metal toxicity inhibition of complex-state chromium in wastewater by iron-based secondary mineral mediated biological Fenton reaction and application of method for oxidation removal and heavy metal toxicity inhibition of complex-state chromium in wastewater by iron-based secondary mineral mediated biological Fenton reaction

Abstract

The invention discloses a method for oxidation removal and heavy metal toxicity inhibition of complex chromium in wastewater through iron-based secondary mineral mediated biological Fenton reaction and application. According to the method, Fenton reaction is induced through atmosphere regulation and control biological / mineral action, and oxidative degradation of complex-state chromium and heavy metal toxicity inhibition are achieved. The method comprises the following steps: reducing iron by facultative dissimilatory iron reducing bacteria and iron-based secondary minerals under anaerobic conditions; aeration is carried out, and accumulated ferrous iron and oxygen are utilized to initiate Fenton reaction so as to oxidize the organic complexing agent to realize complex breaking; converting to an anaerobic state, and reducing Cr (VI) generated by excessive oxidation; and circularly executing the aerobic / anaerobic operation until the wastewater reaches the standard. The method realizes advanced treatment of the complex chromium wastewater through a single reactor, and has the advantages of simple process, easily available materials and low operation cost.

Description

Technical Field

[0001] This invention relates to the field of complexed heavy metal wastewater treatment technology, and in particular to a method and application of oxidative removal of complexed chromium and inhibition of heavy metal toxicity in wastewater by a bio-Fenton reaction mediated by iron-based secondary minerals. Background Technology

[0002] Chromium, a heavy metal, is widely used in industries such as leather tanning and electroplating. Its biotoxicity is closely related to its chemical form and valence state. Hexavalent chromium is strictly restricted from emission due to its strong carcinogenicity, while trivalent chromium, although less toxic, is easily re-oxidized to hexavalent chromium due to its high fluidity, posing a secondary risk. In industrial chromium-containing wastewater, chromium often exists in an organic complex form, forming highly stable complexes with oxalic acid, citric acid, and ethylenediaminetetraacetic acid. Traditional acid-base precipitation processes are difficult to remove effectively, resulting in total chromium concentrations in the effluent often exceeding 10 mg / L, far exceeding the emission limit of 1.5 mg / L.

[0003] Existing treatment technologies suffer from the following problems: while advanced oxidation can break down complexes, it easily leads to over-oxidation and the formation of hexavalent chromium, requiring an additional reduction stage; adsorption methods suffer from poor selectivity and difficulty in separating adsorbents; and the electro-Fenton method is costly and involves numerous side reactions, remaining in the laboratory research stage. Therefore, there is an urgent need for a process that can simultaneously achieve deep oxidation of complexed chromium and inhibition of hexavalent chromium formation.

[0004] This invention presents a method for the oxidation and removal of complexed chromium and the inhibition of heavy metal toxicity in wastewater via a bio-Fenton reaction mediated by iron-based secondary minerals. The invention utilizes the efficient anaerobic iron metabolism of facultative anaerobic dissimilar iron-reducing bacteria to reduce iron in secondary minerals. Further, by changing the environmental atmosphere, the Fenton reaction between high-concentration reduced ferrous iron and oxygen achieves the oxidative degradation of organic ligands in the wastewater containing organically complexed chromium. A further adjustment of the environmental atmosphere further promotes anaerobic dissimilar iron reduction and bio-organic metabolism to jointly inhibit the accumulation of hexavalent chromium toxicity caused by excessive oxidation. This cyclical process achieves simultaneous oxidation and removal of complexed chromium and inhibition of heavy metal toxicity in industrial wastewater. The advantages of this method lie in its dual effect of deep oxidation of complexed chromium and inhibition of heavy metal toxicity; simple process equipment requirements; a straightforward operation flow; and ease of large-scale application for treating large volumes of chromium-containing wastewater, providing an efficient and economical solution for the treatment of industrial complexed chromium wastewater. Summary of the Invention

[0005] Given the high stability and solubility of organically complexed chromium in industrial wastewater, industrial acid-base precipitation processes struggle to achieve compliant discharge of total chromium in biochemical effluents. Addressing the shortcomings of conventional treatment processes, this invention provides a method for the oxidation and removal of complexed chromium and the inhibition of heavy metal toxicity in wastewater via a bio-Fenton reaction mediated by iron-based secondary minerals. This method utilizes anaerobic iron-reducing bacteria to induce iron reduction in secondary minerals through anaerobic iron metabolism, generating high concentrations of ferrous iron. The microbial-driven bio-Fenton process in an oxygen-rich atmosphere achieves the oxidation and removal of organically complexed chromium. In the next anaerobic atmosphere, a secondary reduction of iron and reduction of Cr(VI) toxicity formed by excessive oxidation are achieved. This simple, cyclical process enables the deep treatment of wastewater containing complexed chromium.

[0006] To address the aforementioned technical problems and simultaneously achieve the deep oxidation and removal of organically complexed chromium from industrial wastewater while inhibiting the toxicity of associated heavy metal Cr(VI), this invention adopts the following technical solution: According to a first aspect of the present invention, a method for the oxidative removal of complexed chromium and the inhibition of heavy metal toxicity in wastewater by a bio-Fenton reaction mediated by iron-based secondary minerals is provided. This method induces microbial dissimilar iron metabolism and reactions with iron-based minerals by regulating oxygen concentration, thereby achieving a microbially driven Fenton reaction cycle to oxidatively degrade organically complexed trivalent chromium in wastewater and inhibit hexavalent chromium toxicity. The method includes the following steps: S1. Facultative anaerobic dissimilar iron-reducing bacteria were activated and expanded in Luria-Bertani medium to obtain a high-concentration bacterial suspension. This suspension was then added together with iron-based secondary minerals to wastewater containing organically complexed trivalent chromium. The suspension was then statically or with slight stirring under strictly anaerobic conditions for 6 to 12 hours. Through the iron reduction metabolism of the dissimilar iron-reducing bacteria, the iron in the secondary minerals was used as the terminal electron acceptor to reduce and dissolve the iron in the minerals, releasing a high concentration of ferrous ions into the aqueous phase. S2. After completing step S1, immediately perform mechanical stirring and aeration or ventilation treatment on the wastewater to maintain an aerobic state for 0.5 to 2 hours. Utilize the ferrous iron accumulated in the anaerobic stage to initiate a homogeneous or heterogeneous Fenton reaction with oxygen to generate highly oxidizing active oxygen species, which oxidize the organic complexing agent coordinated with trivalent chromium, thereby breaking the complex and degrading it. Free trivalent chromium forms chromium hydroxide precipitate under weakly alkaline or neutral conditions. Some trivalent chromium may be oxidized to hexavalent chromium under a strong oxidizing environment. S3. After completing step S2, stop aeration and seal the reactor to switch to a strictly anaerobic state. Repeat the cultivation conditions of step S1 to reduce the hexavalent chromium generated in step S2 to trivalent chromium, stabilize it in the form of chromium hydroxide precipitate, and eliminate its ecotoxicity. S4. Based on the initial pollution load and treatment effect of the wastewater, repeat steps S2 and S3 for several cycles until the concentrations of total chromium and hexavalent chromium drop below the emission standards, thereby achieving complexed chromium mineralization and heavy metal toxicity inhibition.

[0007] Optionally, the facultative anaerobic dissimilar iron-reducing bacteria mentioned in step S1 may be one or more of the genera *Shewanella*, *Geobacterium*, or *Enterobacter aerogenes*.

[0008] Optionally, the iron-based secondary mineral in step S1 is one or more of the following: Schieleite, japotassium ferruginous, ferrihydrite, goethite, lepidocrocite, or hematite, with a specific surface area of ​​not less than 30 m² / g, and an addition amount of 0.2 g / L to 20 g / L of wastewater.

[0009] Optionally, in step S1, the initial pH of the wastewater is adjusted to 6.0 to 9.0, and strict anaerobic conditions are achieved by introducing nitrogen or argon gas, controlling the dissolved oxygen concentration to be below 0.1 mg / L.

[0010] Optionally, the completion of anaerobic cultivation in step S1 can be determined by monitoring the concentration of ferrous ions in the aqueous phase. The process is considered complete when the concentration of ferrous ions reaches 400 mg / L to 600 mg / L.

[0011] Optionally, in step S2, mechanical aeration or ventilation is achieved through microporous aeration discs, jet aerators, or mechanical agitator blades to shear oxygenation, thereby raising the dissolved oxygen concentration to 3 mg / L to 5 mg / L and maintaining it stably.

[0012] Optionally, the completion of the aerobic stage in step S2 can be determined by monitoring the concentration of ferrous ions in the aqueous phase. The stage is considered complete when the concentration of ferrous ions is below 20 mg / L.

[0013] Optionally, the strongly oxidizing reactive oxygen species mentioned in step S2 include one or more of hydroxyl radicals, superoxide radicals, singlet oxygen, and hydrogen peroxide.

[0014] Optionally, in step S3, the reduction of hexavalent chromium to trivalent chromium is achieved by facultative anaerobic dissimilar iron-reducing bacteria using ferrous iron or organic carbon sources as electron donors, through enzymatic catalysis or indirect electron transfer; the endpoint of anaerobic reduction detoxification is when the concentration of hexavalent chromium is below 0.1 mg / L.

[0015] Optionally, the number of cycles and running time in step S4 can be dynamically optimized and adjusted based on the initial organic carbon concentration, total chromium concentration, and hexavalent chromium residue in the wastewater.

[0016] According to a second aspect of the present invention, the above method is provided for use in treating wastewater containing organically complexed trivalent chromium, wherein the organic complexing agent is one or more of ethylenediaminetetraacetic acid, citric acid, oxalic acid, tartaric acid, or aminotriacetic acid, and the wastewater type includes one or more of electroplating and electronics industry wastewater, leather tanning wastewater, mining and metallurgical wastewater, or printing and dyeing industry wastewater.

[0017] The advantages of this invention are as follows: It addresses the problems of existing wastewater treatment technologies containing complexed chromium, such as difficulty in achieving compliant discharge of heavy metal chromium, high dependence on chemical reagents, and the accumulation of Cr(VI) toxicity due to excessive oxidation. This invention utilizes the interaction between the metabolism of facultative anaerobic dissimilar iron-reducing bacteria and iron-based secondary minerals, and adjusts the ambient oxygen atmosphere to cyclically prepare and initiate the Fenton reaction process. Ultimately, it simultaneously achieves deep oxidation removal of complexed chromium in wastewater and inhibition of heavy metal toxicity, realizing the conversion of free organic complexed chromium into mineral-phase solid chromium hydroxide. In summary, this invention has three significant advantages: First, the materials required for the process are simple and readily available. Facultative anaerobic dissimilar iron-reducing bacteria can rapidly proliferate under aerobic conditions using conventional nutrient culture media, and iron-based secondary minerals are common, readily available, and inexpensive. Second, the process involves fewer steps and requires simpler equipment. The cyclic process only requires a single wastewater treatment reactor and does not require additional reagents. Third, it effectively solves the problems of deep oxidation removal of complexed chromium and the risk of Cr(VI) discharge due to excessive oxidation, providing an effective solution for the treatment of corresponding industrial wastewater. Attached Figure Description

[0018] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0019] Figure 1 This is a scanning electron microscope image of Schiele minerals; Figure 2 This is a scanning electron microscope image of goethite. Figure 3 X-ray diffraction patterns of Schieleinite and goethite; Figure 4 Scanning electron microscope image of Shewanella; Figure 5 The effect of different systems on the removal of complexed chromium (change in total Cr); Figure 6 The effect of different systems on the removal of complexed chromium (Cr(VI) changes); Figure 7 The effect of different systems on the removal of complexed chromium (Fe(II) changes); Figure 8 The effect of different systems on the removal of complexed chromium (variation of hydroxyl radical concentration); Figure 9 The generation of hydroxyl radicals in different systems; Figure 10 The generation of superoxide radicals in different systems; Figure 11 The generation of singlet oxygen in different systems; Figure 12 The X-ray photoelectron spectrum of Cr on the surface of the precipitate is shown. Figure 13 A comparison of the removal efficiency of complexed chromium under continuous aerobic / anaerobic conditions; Figure 14 The removal efficiency of complexed chromium at different initial concentrations; Figure 15 The removal efficiency of chromium in complexed state under different organic ligands; Figure 16 The effect of removing complexed chromium from actual industrial wastewater. Detailed Implementation

[0020] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] Example 1 In this embodiment, representative iron-based secondary minerals, namely Schieleite and goethite, were selected as materials for mediating the removal of complexed chromium from wastewater by dissimilar iron-reducing bacteria. Schieleite was synthesized via a conventional chemical oxidation method, specifically using ferrous sulfate heptahydrate as the iron source. 30% hydrogen peroxide was added while stirring at 200 rpm, utilizing the strong oxidizing properties of hydrogen peroxide to rapidly oxidize the ferrous iron in the solution. The solution was then maintained at pH 2.5 and room temperature with shaking for 24 hours to allow the generated Fe2+ to hydrolyze. 3+ The precipitate, formed by the combination of sulfate ions, is a yellowish-brown solid. The precipitate is collected by filtration, repeatedly washed with distilled water, dried at low temperature, ground, and sieved to obtain the Scherstein mineral sample. Goethite is synthesized via a co-precipitation-aging method. Specifically, ferric nitrate nonahydrate is used as the iron source. Potassium hydroxide solution is added dropwise under stirring at 200 rpm until the pH stabilizes at 12.0 to form a hydroxide precursor. The reaction solution is then aged at a constant temperature of 70°C for 48 hours to fully convert the amorphous precipitate into well-crystallized goethite. The precipitate is repeatedly washed with distilled water until neutral, dried at low temperature, ground, and sieved to obtain the goethite sample. The two selected iron-based secondary minerals were analyzed using scanning electron microscopy and X-ray crystal diffraction to determine the correctness of the synthesized mineral samples through morphology and structure. The Scherstein mineral sample was characterized under scanning electron microscopy as follows: Figure 1 As shown, its structure conforms to the typical urchin-like spherical structure of Schöndorfite minerals, with individual particles ranging from 2 μm to 3 μm; the goethite sample was characterized under a scanning electron microscope as follows: Figure 2 As shown, its structure conforms to the needle-like structure of common α-goethite, with individual plate-like particles ranging from 1 μm to 4 μm in length. X-ray crystallography characterization is as follows: Figure 3 As shown, analysis using Jade 6.0 software revealed that the mineral samples contained Schwertmannite and Goethite, indicating that the prepared iron-based secondary minerals were Schwertmannite and goethite, respectively.

[0022] Example 2 The facultative anaerobic dissimilar iron-reducing bacterium used in this embodiment is Shewanella, which will serve as a biological agent for the removal of complexed chromium from wastewater by bio-Fenton. The specific method for activating and propagating the agent involves removing the glycerol storage tube containing Shewanella from its cryopreservation and thawing it at room temperature. Under aseptic conditions, a 2% inoculum is inoculated into Luria-Bertani medium (10.0 g / L tryptone, 10.0 g / L sodium chloride, 5.00 g / L yeast extract, pH=7.0) at 38°C and 180 rpm for 8 hours. The propagated bacterial solution is then collected by centrifugation and resuspended in physiological saline. The structural characterization of Shewanella cells using scanning electron microscopy is beneficial for determining the strain's characteristics and purity. Figure 4 Scanning electron microscopy revealed that the bacteria were all rod-shaped structures, with individual bacteria ranging in length from 0.8 μm to 3 μm, consistent with the typical bacterial structure characteristics of Shewanella.

[0023] Example 3 This embodiment involves adding the iron-based secondary minerals prepared in Example 1 and the Shewanella bacterial suspension obtained in Example 2 to a chromium-containing wastewater simulated with citric acid. The reactor atmosphere is adjusted to achieve deep oxidative degradation of the chromium-containing complex and inhibit the accumulation of associated hexavalent chromium toxicity through alternating anaerobic and aerobic processes. The single anaerobic period is set to 8 hours, and the single aerobic period is set to 1 hour. The number of anaerobic-aerobic alternations is determined based on the initial concentration of chromium-containing complex. The system reaction conditions are 10.5 mg / L citric acid-containing chromium complex and 0.5 × 10⁻⁶ mg / L chromium-containing complex. 7 Shewanella cells / ml, 5 g / L secondary mineral material, initial pH set between 7.0 and 7.5, anaerobic conditions protected by nitrogen, aerobic dissolved oxygen concentration maintained between 3.5 mg / L and 4.0 mg / L. Complexed chromium in the wastewater is expressed as total Cr concentration, such as... Figure 5 As shown, the removal of complexed chromium only occurred significantly during the aerobic process. The amount of complexed chromium in the goethite + Shewanella system was significantly higher than that in the Scheres mineral + Shewanella system in a single aerobic treatment. Figure 6The cumulative amount of hexavalent chromium, which is produced only after the aerobic reaction and fully reduced in the next stage of anaerobic treatment, is as low as below 0.1 mg / L, which confirms that the system can effectively inhibit the toxic accumulation of hexavalent chromium. Figure 7 The amount of Fe(II) ions produced in the solution was significantly higher in the goethite + Shewanella system than in the Scheres mineral + Shewanella system. However, the amount of Fe produced in the subsequent aerobic stage was reduced by oxidation and was lower than 20 mg / L. Figure 8 This study investigated the transient concentration of hydroxyl radicals, a typical reactive oxygen species. High concentrations of hydroxyl radicals were observed in both systems during the aerobic Fenton stage, while no significant presence was detected in the anaerobic stage. This confirms that the aerobic stage, induced by the reaction of Fe(II) with oxygen, led to the oxidative degradation of complexed chromium and the formation of some hexavalent chromium. During the aerobic Fenton process, electron paramagnetic resonance spectroscopy and the use of 5,5-dimethyl-1-pyrroline-N-oxide and 2,2,6,6-tetramethylpiperidine-nitrogen-oxide scavengers were employed to detect the generation of reactive oxygen species in the goethite + Shewanella system and the Scheresburg mineral + Shewanella system, respectively. Figures 9-11 As shown, hydroxyl radicals, superoxide radicals, and singlet oxygen were all clearly detected during the aerobic phase. The reactive oxygen species level in the goethite + Shewanella system was significantly higher than that in the schwannite + Shewanella system, confirming that the goethite + Shewanella system was more effective in degrading complexed chromium. X-ray photoelectron spectroscopy analysis was performed on the solid precipitate in the reaction system in the later stages of treatment to detect the valence state distribution of Cr on the precipitate surface, such as... Figure 12 As shown, the distribution of Cr species mainly exists in the form of trivalent chromium, confirming that the complexed chromium undergoes hydrolysis and precipitation after oxidative degradation by organic ligands, and the resulting chromium hydroxide or iron-chromium hydroxide is effectively fixed in iron-based secondary minerals.

[0024] Comparative Example 1 This comparative example will set up a single mineral material or Shewanella bacteria addition condition to verify the effect on the oxidative degradation of complexed chromium in citric acid-simulated complexed chromium wastewater. The single anaerobic period is set to 8 hours, and the single aerobic period to 1 hour. The system reaction conditions are 10.5 mg / L citric acid-complexed chromium, 0.5 × 10⁻⁶ mg / L chromium complex, and 0.5 × 10⁻⁶ mg / L citric acid. 7 Shewanella cells / ml, 5 g / L secondary mineral material, initial pH set between 7.0 and 7.5, anaerobic conditions protected by nitrogen, and aerobic dissolved oxygen concentration maintained between 3.5 mg / L and 4.0 mg / L. Under conditions of single mineral material or Shewanella cell addition, no removal of complexed chromium, generation of hexavalent chromium, generation of Fe(II) reduction, or generation of hydroxyl radicals was observed.

[0025] Comparative Example 2 This comparative example will use a goethite + Shewanella system under continuous aerobic or continuous anaerobic conditions to verify the effect of alternating anaerobic and aerobic conditions on the oxidative degradation of complexed chromium in citric acid-simulated complexed chromium wastewater. The continuous aerobic or continuous anaerobic time is set to 26 hours. The system reaction conditions are 10.5 mg / L citric acid complexed chromium, 0.5 × 10⁻⁶ mg / L chromium, and 0.5 × 10⁻⁶ mg / L citric acid. 7 Shewanella cells / ml, 5 g / L goethite, initial pH set between 7.0 and 7.5, anaerobic conditions protected by nitrogen, aerobic dissolved oxygen concentration maintained between 3.5 mg / L and 4.0 mg / L. Comparative results are shown below. Figure 13 As shown, under continuous aerobic or continuous anaerobic conditions, the goethite + Shewanella system could not achieve the oxidative degradation of complexed chromium, confirming that alternating anaerobic and aerobic conditions under controlled atmosphere are the key control conditions for the degradation of organic complexed chromium.

[0026] Example 4 This example investigates the effect of initial concentrations of complexed chromium in simulated citric acid-contaminated wastewater on the oxidative degradation of complexed chromium using a goethite + Shewanella system. It explores the required number of anaerobic and aerobic cycles to treat different concentrations of complexed chromium, with each anaerobic cycle lasting 8 hours and each aerobic cycle lasting 1 hour. The system reaction conditions are 0.5 × 10⁻⁶. 7 Shewanella cells / ml, 5 g / L goethite, initial pH set between 7.0 and 7.5, anaerobic conditions protected by nitrogen, aerobic dissolved oxygen concentration maintained between 3.5 mg / L and 4.0 mg / L. Figure 14 As shown, the higher the initial concentration of complexed chromium, the more anaerobic and aerobic cycles are required. Specifically, one cycle is required when the initial concentration is 5.25 mg / L, two cycles are required when the initial concentration is 10.5 mg / L, three cycles are required when the initial concentration is 21.0 mg / L, and five cycles are required when the initial concentration is 42.0 mg / L.

[0027] Example 5 This embodiment investigates the effects of different organic acid ligands (oxalic acid, citric acid, tartaric acid, and ethylenediaminetetraacetic acid) on the oxidative degradation of complexed chromium in wastewater simulated by a goethite + Shewanella system. The anaerobic period was set to 8 hours, and the aerobic period to 1 hour. The system reaction conditions were 10.5 mg / L complexed chromium and 0.5 × 10⁻⁶ mg / L chromium. 7 Shewanella cells / ml, 5 g / L goethite, initial pH set between 7.0 and 7.5, anaerobic conditions protected by nitrogen, aerobic dissolved oxygen concentration maintained between 3.5 mg / L and 4.0 mg / L. Figure 15As shown, all 10.5 mg / L complexed chromium wastewater can be completely degraded in two cycles. The system is more likely to degrade chromium complexed with oxalic acid or citric acid, but more difficult to degrade chromium complexed with ethylenediaminetetraacetic acid.

[0028] Example 6 This embodiment investigates the oxidative degradation effect of a goethite + Shewanella system on complexed chromium-containing wastewater from different industrial sources. The wastewater included tanneries, electroplating wastewater, and dyeing and printing wastewater after biochemical treatment. The concentration of complexed chromium in the wastewater was adjusted to 10.0 mg / L by simple dilution with deionized water. The anaerobic period was set to 8 hours, and the aerobic period to 1 hour. The system reaction conditions were 10.0 mg / L complexed chromium and 0.5 × 10⁻⁶ mg / L chromium. 7 Shewanella cells / ml, 5 g / L goethite, initial pH set between 7.0 and 7.5, anaerobic conditions protected by nitrogen, aerobic dissolved oxygen concentration maintained between 3.5 mg / L and 4.0 mg / L. Figure 16 As shown, all the actual complexed chromium wastewater can be completely degraded in two cycles, which confirms that the goethite + Shewanella system can effectively degrade complexed chromium in wastewater under alternating anaerobic and aerobic conditions.

[0029] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for the oxidation and removal of complexed chromium and the inhibition of heavy metal toxicity in wastewater by a bio-Fenton reaction mediated by iron-based secondary minerals, characterized in that, By regulating oxygen concentration to induce microbial catabolism of iron metabolism and reactions with iron-based minerals, a microbially driven Fenton reaction cycle is achieved to oxidatively degrade organically complexed trivalent chromium in wastewater and inhibit the toxicity of hexavalent chromium. This includes the following steps: S1. Facultative anaerobic dissimilar iron-reducing bacteria were activated and expanded in Luria-Bertani medium to obtain a high-concentration bacterial suspension. This suspension was then added together with iron-based secondary minerals to wastewater containing organically complexed trivalent chromium. The suspension was then statically or with slight stirring under strictly anaerobic conditions for 6 to 12 hours. Through the iron reduction metabolism of the dissimilar iron-reducing bacteria, the iron in the secondary minerals was used as the terminal electron acceptor to reduce and dissolve the iron in the minerals, releasing a high concentration of ferrous ions into the aqueous phase. S2. After completing step S1, immediately perform mechanical stirring and aeration or ventilation treatment on the wastewater to maintain an aerobic state for 0.5 to 2 hours. Utilize the ferrous iron accumulated in the anaerobic stage to initiate a homogeneous or heterogeneous Fenton reaction with oxygen to generate highly oxidizing active oxygen species, which oxidize the organic complexing agent coordinated with trivalent chromium, thereby breaking the complex and degrading it. Free trivalent chromium forms chromium hydroxide precipitate under weakly alkaline or neutral conditions. Some trivalent chromium may be oxidized to hexavalent chromium under a strong oxidizing environment. S3. After completing step S2, stop aeration and seal the reactor to switch to a strictly anaerobic state. Repeat the cultivation conditions of step S1 to reduce the hexavalent chromium generated in step S2 to trivalent chromium, stabilize it in the form of chromium hydroxide precipitate, and eliminate its ecotoxicity. S4. Based on the initial pollution load and treatment effect of the wastewater, repeat steps S2 and S3 for several cycles until the concentrations of total chromium and hexavalent chromium drop below the emission standards, thereby achieving complexed chromium mineralization and heavy metal toxicity inhibition.

2. The method as described in claim 1, characterized in that, The facultative anaerobic dissimilar iron-reducing bacteria mentioned in step S1 are one or more of the genera *Shewanella*, *Geobacterium*, or *Enterobacter aerogenes*.

3. The method as described in claim 1, characterized in that, The iron-based secondary minerals mentioned in step S1 are one or more of the following: Schieleite, japotassium ferruginous, ferrihydrite, goethite, lepidocrocite, or hematite, with a specific surface area of ​​not less than 30 m² / g, and an addition amount of 0.2 g / L to 20 g / L of wastewater.

4. The method as described in claim 1, characterized in that, In step S1, the initial pH value of the wastewater is adjusted to 6.0 to 9.0, and strict anaerobic conditions are achieved by introducing nitrogen or argon gas to control the dissolved oxygen concentration below 0.1 mg / L.

5. The method as described in claim 1, characterized in that, The completion of anaerobic cultivation in step S1 is determined by monitoring the concentration of ferrous ions in the aqueous phase. The process is considered complete when the concentration of ferrous ions reaches 400 mg / L to 600 mg / L.

6. The method as described in claim 1, characterized in that, In step S2, mechanical aeration or ventilation is achieved through microporous aeration discs, jet aerators, or mechanical agitator blades to shear oxygenation, thereby raising the dissolved oxygen concentration to 3 mg / L to 5 mg / L and maintaining it stably.

7. The method as described in claim 1, characterized in that, The completion of the aerobic stage in step S2 is determined by monitoring the concentration of ferrous ions in the aqueous phase. The stage is considered complete when the concentration of ferrous ions is below 20 mg / L.

8. The method as described in claim 1, characterized in that, The highly oxidizing reactive oxygen species mentioned in step S2 include one or more of hydroxyl radicals, superoxide radicals, singlet oxygen, and hydrogen peroxide.

9. The method as described in claim 1, characterized in that, In step S3, the reduction of hexavalent chromium to trivalent chromium is achieved by facultative anaerobic dissimilar iron-reducing bacteria using ferrous iron or organic carbon sources as electron donors, through enzyme catalysis or indirect electron transfer; the endpoint of anaerobic reduction detoxification is when the concentration of hexavalent chromium is below 0.1 mg / L.

10. The method as described in claim 1, characterized in that, In step S4, the number of cycles and the running time are dynamically optimized and adjusted based on the initial organic carbon concentration, total chromium concentration and hexavalent chromium residue in the wastewater.

11. The application of the method according to any one of claims 1 to 10 in treating wastewater containing organically complexed trivalent chromium, characterized in that, The organic complexing agent is one or more of ethylenediaminetetraacetic acid, citric acid, oxalic acid, tartaric acid, or aminotriacetic acid, and the wastewater type includes one or more of electroplating and electronics industry wastewater, leather tanning wastewater, mining and metallurgical wastewater, or printing and dyeing industry wastewater.

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