A method for recovering selenium from a selenium-containing wastewater

Abstract

The application provides a method for recovering and extracting selenium from selenium-containing wastewater. By adding lactic acid and a complex bacterial population, Shewanella efficiently reduces selenium oxyanions by lactic acid, and Thiobacillus ferrooxidans dynamically controls the pH of the system to a suitable range through metabolic activity, so that the product elemental selenium is enriched in a solid phase form. High-purity elemental selenium products can be obtained through simple solid-liquid separation and purification. No harmful chemical reagents are added in the whole process, and the method is environmentally friendly.

Description

Technical Field

[0001] This application relates to the field of elemental selenium recovery and preparation technology, specifically to a method for recovering elemental selenium. Background Technology

[0002] Selenium, known as a "metalloid" element, is an element that combines environmental toxicity with resource value. It can be used to manufacture photoelectric tubes, photographic exposure agents, and metallurgical additives. It is also used as a stabilizer in glass, plastics, paints, ceramics, enamel, lubricants, and as a pigment in inks. High-purity selenium can be used in high-efficiency rectifiers. Similarly, selenium is an essential element for plants, animals, and humans. It participates in the formation of many enzymes in the human body and plays an important role in human health. However, because the threshold between nutritional value and toxicity is extremely small, excessively high levels in the environment can easily have adverse effects on human health.

[0003] Selenium is typically recovered from copper electrolysis anode sludge, acid sludge from sulfuric acid plants, and flue dust, or from various selenium-containing wastewaters through enrichment and recovery. Currently, research is focused on selenium-containing oxygen anions (such as SeO4). 2- SeO3 2- Wastewater treatment methods mainly include chemical reduction, adsorption, and biological treatment. Chemical reduction typically uses reducing agents such as sodium sulfite and ferrous sulfate to reduce selenium oxide anions to elemental selenium. However, this method suffers from problems such as large reagent dosages, harsh reaction conditions, and the potential for secondary pollution. Adsorption methods often utilize the adsorption properties of materials such as activated carbon, zeolite, and ion exchange resins to remove selenium. However, these materials generally have limited adsorption capacity and poor adsorption selectivity, and the adsorbed selenium is difficult to effectively desorb and recover, easily causing secondary pollution of the adsorbent.

[0004] Bioleaching technology utilizes the metabolic activity of microorganisms to reduce selenium oxide anions, offering advantages such as low cost, environmental friendliness, and high selectivity. However, single-stage bioleaching processes have significant drawbacks: firstly, microbial growth and metabolism require a continuous supply of carbon sources, and the single carbon source added in traditional processes can only meet the nutritional needs of the microorganisms, failing to enhance extracellular electron transfer efficiency; secondly, to improve the reduction rate, existing technologies often require the addition of electron shuttles (such as humic acid or sodium anthraquinone-2-sulfonate). This dual-reagent addition mode of "carbon source + electron shuttle" not only increases process operating costs and complexity but also easily leads to excessively high organic load in the water body due to reagent imbalances, and may even result in electron shuttles competing with selenium oxide anions for electrons, ultimately limiting the reduction efficiency of selenium oxide anions and the purity of elemental selenium recovery.

[0005] Therefore, how to simultaneously improve the reduction efficiency and recovery purity of selenium in the process of bioleaching for the recovery of elemental selenium remains a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0006] This application provides a method for recovering and extracting selenium from selenium-containing wastewater. Through the synergistic effect of a functional microbial system and a lactic acid medium, the conversion efficiency of selenium oxide anions is significantly improved, and the recovery rate of elemental selenium reaches over 92%.

[0007] In a first aspect, this application provides a method for recovering and extracting selenium from selenium-containing wastewater, comprising the following steps: S1. Pretreatment: Adjust the selenium-containing wastewater to a selenium concentration of 50~200 mg / L, a solid content of less than 5%, and an initial pH value of 5.0~6.0; S2. Selenium reduction: Lactic acid and inoculated functional bacteria are added to the pretreated wastewater, and a bioleaching reduction reaction is carried out under aeration and stirring conditions to reduce selenium oxide anions. The functional microbial community includes dissimilar metal-reducing bacteria that are capable of reducing selenium oxide anions to elemental selenium. S3. Elemental Selenium Recovery: After the S3 reaction is completed, the system is subjected to solid-liquid separation. The liquid phase is treated wastewater that meets the standards, and the solid phase is the elemental selenium enrichment product. The elemental selenium enrichment product is washed, dried, and purified to obtain a high-purity elemental selenium product.

[0008] According to this application, lactic acid is the core functional medium, playing a dual role as both a carbon source and an electron shuttle in the reaction system. As a carbon source, lactic acid provides energy and material for the growth and metabolism of the functional microbial community, maintaining its activity and population density. As an electron shuttle, lactic acid and its metabolic intermediates participate in extracellular electron transfer, accelerating the rate of electron transfer from the functional microbial community to selenium oxide anions, thereby improving the reduction efficiency of selenium oxide anions. The dissimilar metal-reducing bacteria included in the functional microbial community can transfer SeO4 through extracellular electron transfer. 2- and SeO3 2- Selenium oxide anions are reduced to elemental selenium, achieving a directional conversion of selenium from a dissolved state to a solid state. The pretreatment step adjusts the wastewater to a suitable selenium concentration range and pH conditions, providing a favorable reaction environment for the subsequent bioreduction reaction; solid content control avoids interference from suspended solids on microbial activity and mass transfer processes.

[0009] This application utilizes the dual-functional design of lactic acid to simultaneously meet the nutritional and electron transport enhancement needs of functional microbial communities in a single medium. This eliminates the need for additional carbon sources and electron shuttles, significantly reducing reagent costs and operational complexity, while also avoiding the potential imbalances and competitive inhibition issues that can arise from adding two reagents. The synergistic effect of lactic acid and functional microbial communities significantly enhances the reduction efficiency of selenium oxide anions. After the reaction, a solid-phase product enriched with elemental selenium can be obtained through simple solid-liquid separation. Following washing, drying, and purification, a high-purity elemental selenium product is obtained, achieving the dual goals of harmless treatment and resource recovery of selenium.

[0010] During the research process, this application discovered that traditional biological treatment processes for selenium-containing wastewater typically require the separate addition of carbon sources (such as glucose, sodium lactate, etc.) and electron shuttles (such as humic acid, sodium anthraquinone-2-sulfonate, etc.). This not only increases operating costs but also makes it difficult to precisely control the dosage ratio of the two types of reagents: insufficient carbon sources limit bacterial growth, while insufficient electron shuttles reduce electron transfer efficiency; and excessive electron shuttles may compete with selenium oxide anions for electrons, thus inhibiting reduction efficiency. To address these issues, this application, through screening, discovered that lactic acid, in the presence of dissimilar metal-reducing bacteria, can simultaneously function as both a carbon source and an electron shuttle: on the one hand, as a small-molecule organic acid, lactic acid can be directly absorbed and utilized by dissimilar metal-reducing bacteria into the tricarboxylic acid cycle, providing energy and a carbon skeleton for bacterial growth; on the other hand, lactic acid can promote the generation of intracellular reducing power during microbial metabolism and efficiently transfer electrons to selenium oxide anions through the extracellular electron transport chain of the bacteria. Through the aforementioned dual-function mechanism, lactic acid simultaneously satisfies the needs of bacterial growth and electron transfer in the same reaction system, ensuring bacterial activity and enhancing reduction efficiency, ultimately achieving efficient treatment of selenium-containing wastewater and high-purity recovery of selenium resources.

[0011] In some embodiments, the dissimilar metal-reducing bacteria include Shewanella strains, preferably Lake Oneida Shewanella MR-1.

[0012] In some of the aforementioned methods, the dissimilar metal-reducing bacteria include Shewanella strains. Lake Oneida Shewanella MR-1, as a typical dissimilar metal-reducing bacterium, possesses a complete extracellular electron transport chain. Its cell membrane contains cytochrome c and conductive nanowires, which act as electron transport mediators, enabling efficient transfer of electrons generated intracellularly to extracellular receptors. In the presence of lactic acid, Shewanella can not only directly utilize lactic acid as a carbon source to maintain growth and metabolism, but also synergistically enhance the function of its extracellular electron transport system with lactic acid as an electron shuttle, allowing electrons to be rapidly transferred from the cell via lactic acid and its metabolic intermediates to selenium oxide anions, significantly increasing SeO4 ion concentration. 2- and SeO3 2- This improves the reduction rate and conversion efficiency, thereby achieving efficient and directional conversion of selenium oxide anions into elemental selenium.

[0013] In some embodiments, the functional microbial community further includes bioleaching functional bacteria, which are chemolithoautotrophic bacteria capable of producing an acidic environment and lowering the pH of the reaction system through metabolism.

[0014] Among the aforementioned methods, the introduction of chemoautotrophic bacteria enables the reaction system to dynamically regulate pH. During their metabolism, these bacteria oxidize inorganic substances such as sulfur or iron, producing acidic metabolites that gradually reduce the system pH from the initial 5.0-6.0 to 3.0-4.0. This gradual decrease in pH promotes the protonation of selenium oxide anions, lowering the activation energy of their reduction reaction and thus further improving reduction efficiency. The symbiotic relationship between bioleaching functional bacteria and Shewanella enables the reaction system to have self-optimizing environmental regulation capabilities, eliminating the need for external acid-base regulators, simplifying process operations, and improving system stability and environmental adaptability.

[0015] It should also be understood that the optimal growth and metabolic environment for Shewanella from Lake Oneida is a neutral pH condition. Although the overall treatment system is acidic, the acidic environment will inhibit the basic physiological activity of the strain to a certain extent. However, combined with the supporting process, bacterial community compatibility and reaction condition design of this application, the negative impact of acid stress can be weakened, and the strain can still maintain an effective ability to reduce dissimilar metals, thus ensuring the overall treatment effect.

[0016] In some embodiments, the chemoautotrophic bacteria include *Thiobacillus ferrooxidans*.

[0017] In some of the aforementioned methods, *Thiobacillus ferrooxidans*, as a typical bioleaching functional bacterium, can produce sulfuric acid through ferrous oxide or reduced sulfur, continuously lowering the pH of the reaction system during metabolism. This bacterium grows well in acidic environments, and its working pH window overlaps well with that of *Shewanella*. In this invention, lactic acid is not directly used as a carbon or energy source for *Thiobacillus ferrooxidans*, but rather through multiple mechanisms to create a more favorable metabolic environment: First, lactic acid is preferentially utilized by heterotrophic bacteria such as *Shewanella*, and the CO2 produced by their metabolism can serve as a carbon source for *Thiobacillus ferrooxidans*, satisfying its chemoautotrophic needs; second, the lactate-lactate buffer pair (pKa≈3.86) can alleviate drastic pH fluctuations in the early stages of the reaction, providing a more stable starting environment for the acid-producing bacteria; third, lactic acid participates in the electron transport chain of *Shewanella*, causing the system's ORP to drop to a negative value. The lower redox potential helps maintain the activity of *Thiobacillus ferrooxidans* reductase, indirectly promoting its acid-producing metabolism. Under the synergistic effect of the above mechanisms, *Thiobacillus ferrooxidans* and *Shewanella* form a stable symbiotic relationship in a lactic acid medium: *Thiobacillus ferrooxidans* regulates the pH of the system from the initial 5.5 to a suitable range of 3.0-4.0 through acid production, promoting the protonation of selenoxy anions and thus lowering the reduction activation energy; while *Shewanella* utilizes lactic acid as a carbon source and electron shuttle to efficiently reduce selenoxy anions. The two mutually benefit each other, synergistically enhancing each other's effects, and jointly achieving the efficient and targeted conversion of selenoxy anions.

[0018] In some embodiments, the *Thiobacillus ferrooxidans* and *Shewanella oneidae* are mixed in a volume ratio of 1:0.5-1 in the composite bacterial community, and the bacterial density of both *Thiobacillus ferrooxidans* and *Shewanella oneidae* MR-1 is controlled at 10. 7 ~10 8 CFU / mL.

[0019] In some of the methods mentioned above, the ratio and density design are based on the functional synergistic requirements of the two types of bacteria: *Thiobacillus ferrooxidans*, as a pH-regulating bacterium, needs to be matched in quantity with *Shewanella*, which is the main force in selenium reduction, while the density of both is controlled at 10. 7 ~10 8 Within the range of CFU / mL, it ensures that the bacterial community can quickly enter the logarithmic growth phase after inoculation, and form a stable synergistic working state in the early stage of reaction initiation.

[0020] In some embodiments, in step S2, the amount of lactic acid added is 0.5 to 2.0 g per liter of wastewater.

[0021] In some of the above methods, the range of addition amount is optimized based on the dual functional balance requirements of lactic acid. Within the range of 0.5~2.0 g / L, the dual functions of lactic acid are coordinated, which not only ensures the activity and population density of the microbial community, but also achieves efficient extracellular electron transfer.

[0022] In some embodiments, in step S2, the amount of lactic acid added is 1.0 to 1.5 g per liter of wastewater.

[0023] Among the methods mentioned above, the carbon source supply and electron shuttle transfer efficiency can be more precisely balanced within this range, ensuring sufficient growth of the microbial community while avoiding excessive organic load, maximizing the dual function of lactic acid, and further improving the reduction efficiency of selenium oxide anions and the stability of system operation.

[0024] In some embodiments, in step S2, an ammonium sulfate solution is also added, wherein the concentration of the ammonium sulfate solution is 18-25 wt%, and the amount of ammonium sulfate solution added is 5-12 g per kilogram of wastewater.

[0025] Among the methods mentioned above, the addition of ammonium sulfate provides nitrogen nutrition for the growth and metabolism of functional microbial communities. Microorganisms need nitrogen sources to synthesize life substances such as proteins and nucleic acids, and the original nitrogen content in wastewater may not be sufficient to support the growth needs of high-density microbial communities. As a common inorganic nitrogen source, ammonium sulfate has good solubility and is easily absorbed and utilized by microorganisms. Its addition can promote the rapid proliferation and activity maintenance of functional microbial communities, thereby ensuring the continuous and efficient progress of selenium reduction reaction. At the same time, the ammonium ions in ammonium sulfate can also serve as energy metabolism substrates for some chemoautotrophic bacteria (such as ferrooxidase), further enhancing the activity of bioleaching functional bacteria and indirectly promoting the pH regulation effect of the system.

[0026] In some embodiments, in step S2, the amount of the functional microbial community added is 8% to 12% of the wastewater volume.

[0027] In some of the above methods, the inoculation amount range is obtained based on the optimization of reaction initiation efficiency and system stability. An inoculation amount of 8% to 12% can achieve a balance between reaction initiation speed, processing efficiency and economic cost, ensuring that the functional flora quickly enters the logarithmic growth phase after inoculation and maintains high activity for a period of 6 to 8 days, giving full play to the dual functions of lactic acid and the synergistic effect of the flora.

[0028] In some embodiments, the temperature range of the reduction reaction system is 28~35℃, the stirring speed range is 120~180rpm, and the aeration rate is 0.5~1.0 L / h.

[0029] In some of the above methods, the temperature range matches the optimal growth temperature of *Thiobacillus ferrooxidans* and *Shewanella*, ensuring the activity and metabolic efficiency of key enzymes; the stirring speed provides thorough mixing while avoiding excessive shear damage to the cells, promoting the uniform distribution of lactic acid, dissolved oxygen, and the bacterial community in the system, and enhancing the mass transfer process; the aeration rate is controlled to meet the oxygen requirements of aerobic and facultative bacteria. Within this parameter range, the reaction system maintains a stable mass and heat transfer state, allowing the dual function of lactic acid and the synergistic effect of the bacterial community to be fully utilized.

[0030] In some implementations, the recovery of elemental selenium in step S3 specifically includes: After the reaction is completed, allow the mixture to stand for 30-60 minutes to allow the elemental selenium enrichment product to settle naturally to the bottom of the reactor. Separate the solid and liquid components of the settled bottom slurry by passing it through a microporous membrane or plate and frame filter press to obtain an elemental selenium filter cake with a water content of less than 30%. Wash the filter cake 1-2 times with dilute hydrochloric acid with a pH of 2.0-3.0 to remove impurity ions adsorbed on the surface, and then wash it with deionized water until the pH of the washing solution is close to neutral. The washed elemental selenium product is then vacuum dried at 50-60℃ for 12-24 hours to obtain an elemental selenium powder product.

[0031] Among the methods mentioned above, a recovery process combining static precipitation, solid-liquid separation, acid washing, water washing, and vacuum drying was used to achieve efficient enrichment and purification of elemental selenium products.

[0032] Secondly, this application provides an application of the method for recovering and extracting selenium from selenium-containing wastewater as described in any embodiment of the first aspect in the field of recovering elemental selenium from selenium-containing wastewater.

[0033] According to this application, the directional conversion of selenium anions is achieved through the dual-functional design of lactic acid and the synergistic effect of functional bacterial communities. In the field of elemental selenium recovery, dissolved selenium is converted into elemental selenium products through biological reduction, avoiding the problems of high acid and alkali consumption, numerous byproducts, and difficult purification in traditional chemical reduction methods. It also overcomes the shortcomings of traditional biological methods, such as the need for dual reagent addition, high cost, and susceptibility to secondary pollution. This method is highly efficient, economical, and environmentally friendly, and is suitable for the industrial treatment and selenium resource recovery of selenium-containing wastewater from industries such as electronics, printing, and metallurgy.

[0034] Compared with the prior art, the beneficial effects of this application are at least as follows: 1. This application achieves simultaneous carbon source supply and enhanced electron transfer in the same reaction system by adding lactic acid, eliminating the need for additional electron shuttles and significantly reducing reagent costs and operational complexity. Compared to the traditional dual-reagent addition mode of "carbon source + electron shuttle," it reduces operating costs while avoiding electron competition and secondary pollution problems caused by reagent imbalance.

[0035] 2. The complex microbial community of *Shewanella osmosis* and *Thiobacillus ferrooxidans* constructed in this application forms a synergistic effect with the lactic acid medium. *Shewanella osmosis* efficiently reduces selenium oxide anions using lactic acid, while *Thiobacillus ferrooxidans* dynamically regulates the pH of the system to a suitable range through metabolic activities. This mutually beneficial symbiosis enables the reaction system to possess self-optimization capabilities.

[0036] 3. The reaction product elemental selenium of this application is enriched in solid phase. High-purity elemental selenium products can be obtained through simple solid-liquid separation and purification. No harmful chemical reagents are added throughout the process, making it environmentally friendly. Detailed Implementation

[0037] The various embodiments or implementation schemes in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments.

[0038] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "illustrative embodiment," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with an embodiment or example is included in at least one embodiment or example of this application. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0039] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this application, "multiple" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0040] In this specification, unless otherwise specified, "parts" refers to "parts by weight".

[0041] The following describes embodiments of this application. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0042] Lactic acid: purity ≥ 85%; Dissimilar metal-reducing bacteria: Shewanella oneidensis MR-1 (bio-097658), prepared at a concentration of 5.0 × 10⁻⁶. 7 CFU / mL of Shewanella MR-1 bacterial suspension from Lake Oneida; Chemoautotrophic bacteria: Acidithiobacillus ferrooxidans LX5 (CCTCCZB 2008081), prepared at a concentration of 5.0 × 10⁻⁶. 7 CFU / mL of Thiobacillus ferrooxidans LX5 bacterial suspension; Wastewater: Wastewater from a power plant was used, with a selenium concentration of 100 mg / L, mainly in the form of SeO4. 2- It exists in form.

[0043] Example 1 Methods for recovering and extracting selenium from selenium-containing wastewater (bioleaching method) Pretreatment: 100 L of actual selenium-containing wastewater discharged from a power plant was taken. First, the solids content was adjusted by natural sedimentation to remove large suspended solids, controlling the solids content (suspended solids) in the wastewater to be below 3%. The initial pH of the wastewater was adjusted to 5.5 using dilute sulfuric acid (1 mol / L). After adjustment, the total selenium concentration in the wastewater was measured to be 100 mg / L. Selenium reduction: 120g (1.2g / L) of lactic acid was added to the pretreated wastewater, and functional bacteria were inoculated at 10% (10L) of the wastewater volume. At the same time, to supplement the microbial nutrition, 1kg of 20wt% ammonium sulfate solution was added. The reaction was carried out at 32℃ with aeration and stirring (150 rpm, 0.8 L / h) for 7 days to reduce selenium oxide anions. The functional bacteria were obtained by mixing Bacillus ferrooxidans LX5 and Shewanella onedae MR-1 at a volume ratio of 1:0.75. Elemental selenium recovery: After the reduction reaction is completed, stop stirring and let it stand for 1 hour to settle. At this time, reddish-brown precipitate can be seen at the bottom of the reactor. The concentrated slurry at the bottom is filtered through a plate and frame filter press to obtain an elemental selenium filter cake with a water content of about 28%. The filtrate is clear and transparent. The filter cake was first washed twice with a dilute hydrochloric acid solution with a pH of 2.5 to remove impurity ions such as iron and calcium adsorbed on the surface. Then it was repeatedly washed with deionized water until the pH of the washing solution was close to neutral. The washed elemental selenium product was placed in a vacuum drying oven and dried for 18 hours at 55°C and a vacuum of -0.08 MPa to obtain elemental selenium powder product.

[0044] Example 2 Methods for recovering and extracting selenium from selenium-containing wastewater (bioleaching method) Similar to Example 1, except that the functional bacterial group was obtained by mixing Bacillus ferrooxidans LX5 bacterial suspension and Shewanella Oneda Lake bacterial suspension in a volume ratio of 1:1.

[0045] Example 3 Methods for recovering and extracting selenium from selenium-containing wastewater (bioleaching method) The results are largely the same as in Example 1, except that the functional bacterial group is a mixture of *Thiobacillus ferrooxidans* LX5 and *Shewanella oneda* MR-1 in a volume ratio of 1:0.5.

[0046] Example 4 Methods for recovering and extracting selenium from selenium-containing wastewater (bioleaching method) It is largely the same as Example 1, except that ammonium sulfate is not added.

[0047] Comparative Example 1 Methods for recovering and extracting selenium from selenium-containing wastewater (bioleaching method) The results are largely the same as in Example 1, except that the functional bacterial group is a mixture of *Thiobacillus ferrooxidans* LX5 and *Shewanella oneda* MR-1 in a volume ratio of 1:0.2.

[0048] Comparative Example 2 Methods for recovering and extracting selenium from selenium-containing wastewater (bioleaching method) The results are largely the same as in Example 1, except that the functional bacterial group is a mixture of *Thiobacillus ferrooxidans* LX5 and *Shewanella oneda* MR-1 in a volume ratio of 1:1.2.

[0049] Comparative Example 3 Methods for recovering and extracting selenium from selenium-containing wastewater (bioleaching method) Similar to Example 1, except that lactic acid is not added, 100g of glucose is added to the wastewater as a carbon source, and 50g of sodium anthraquinone-2-sulfonate is added as an electron shuttle.

[0050] Comparative Example 4 Methods for recovering and extracting selenium from selenium-containing wastewater (chemical reduction method) Pretreatment: Take 100 L of actual selenium-containing wastewater discharged from an electronics factory. First, adjust the solid content by removing large suspended solids through natural sedimentation and controlling the solid content (suspended solids) in the wastewater to be below 3%. Add concentrated sulfuric acid to adjust the initial pH value of the wastewater to 2.0. Chemical reduction reaction: The acidified wastewater was transferred to a reaction vessel, and the stirring speed was set to 150 rpm, with the temperature controlled at 25℃. 350g of sodium sulfite was added to the wastewater as a reducing agent, at a dosage of 3.5 g / L. Immediately after the addition of sodium sulfite, the reaction solution became light yellow and turbid, indicating that some selenium was reduced. The reaction continued for 6 hours. First solid-liquid separation: The reaction solution was allowed to stand for 30 minutes. A small amount of reddish-brown precipitate (elemental selenium) and a large amount of grayish-white precipitate (unreacted sodium sulfite and by-products) were mixed at the bottom. The mixture was separated by centrifugation (4000 rpm, 10 minutes) to obtain a mixed filter cake. Neutralization and Precipitation: Since the wastewater remains highly acidic after the reaction, it cannot be discharged directly. Sodium hydroxide solid is slowly added to the wastewater for neutralization, adjusting the pH to 7.0. A large amount of white precipitate is generated during neutralization (mainly co-precipitates of calcium sulfate, calcium sulfite, and a small amount of selenium), requiring secondary solid-liquid separation. Secondary solid-liquid separation: The white precipitate produced by neutralization is combined with the filter cake from the first separation and centrifuged again to obtain the final solid mixture. The filtrate is the neutralized wastewater. Washing and drying: The final solid mixture was washed three times with deionized water to remove soluble salts. The washed solid was then placed in a vacuum drying oven and dried at 55°C and a vacuum of -0.08 MPa for 18 hours to obtain elemental selenium.

[0051] Test section pH test: The pH values ​​of the reaction systems of Examples 1-4 and Comparative Examples 1-3 were measured and recorded using a pH meter. The test was conducted once a day, and the results are shown in Table 1.

[0052] Elemental selenium recovery rate: Elemental selenium recovery rate refers to the percentage of the mass of elemental selenium product obtained from the reduction of wastewater relative to the total mass of selenium in the initial wastewater. The mass of elemental selenium product is determined by the reduction pre-separation-sodium thiosulfate titration method to determine the mass of elemental selenium in the recovered product.

[0053] The recovered elemental selenium product was dissolved in nitric acid, and impurities were removed by reduction pre-separation. The released iodine was titrated with a standard sodium thiosulfate solution in the presence of potassium iodide. Using starch solution as an indicator, the selenium content was calculated. The formula for calculating the selenium content is as follows: Where: M se denoted as ν, where ν is the molar mass of selenium (78.96 g / mol); n is the stoichiometric ratio of selenium to sodium thiosulfate in the titration reaction (1:1); c is the concentration of the sodium thiosulfate standard titration solution (0.15 mol / L); and V is the volume consumed in the titration (mL).

[0054] The recovery rate R of elemental selenium is calculated using the following formula:

[0055] Where: m se m represents the mass (g) of selenium in the recovered product determined by titration. total The mass (g) of total selenium in the initial wastewater is calculated by multiplying the selenium concentration (mg / L) of the wastewater after pretreatment by the total volume (L). The selenium concentration in the wastewater was determined by inductively coupled plasma optical emission spectrometry (ICP-OES). The recovery rate of elemental selenium is shown in Table 2.

[0056] Selenium content test in effluent: The selenium content of the wastewater treated liquid of each example and comparative example was determined by inductively coupled plasma optical emission spectrometry (ICP-OES) according to the method described in the national standard "HJ 776-2015 Determination of 32 elements in water quality by inductively coupled plasma optical emission spectrometry". The results are shown in Table 2.

[0057] Table 1

[0058] Table 2

[0059] Examples 1, 2, and 3 show that the bacterial community ratio has a significant impact on the treatment effect. Under identical conditions (lactate addition and other parameters), Example 1 exhibited the highest elemental selenium recovery rate (95.2%) and the lowest selenium concentration in the effluent. This result indicates that a good synergistic treatment effect can be achieved when the volume ratio of *Thiobacillus ferrooxidans* to *Shewanella* is within the range of 1:0.5 to 1, but 1:0.75 is a more optimal ratio—this ratio provides a more balanced distribution of acid-producing and reducing bacteria, ensuring sufficient acid production to regulate the system pH to a suitable range while also ensuring a sufficient number of reducing bacteria to efficiently convert selenium oxide anions, thus maximizing the synergistic effect.

[0060] Examples 1 and 4 demonstrate that the addition of ammonium sulfate as a nitrogen source significantly improves the treatment effect. Under identical conditions (bacterial community ratio, lactic acid addition, and other parameters), Example 1 with added ammonium sulfate achieved a final pH of 3.2, a selenium recovery rate of 95.2%, and an effluent selenium concentration of 4.8 mg / L. In contrast, Example 4 without added ammonium sulfate achieved a final pH of 3.7, with a selenium recovery rate of 91.5%. These results indicate that the addition of ammonium sulfate supplemented the functional bacterial community with nitrogen, promoting the growth and metabolism of *Thiobacillus ferrooxidans* and *Shewanella*. Increased activity of *Thiobacillus ferrooxidans* enhanced its acid-producing capacity, resulting in a more thorough pH decrease and a state closer to the optimal acidic environment for selenium oxide anion reduction. Increased activity of *Shewanella* directly improved the reduction efficiency of selenium oxide anions. Although no ammonium sulfate was added in Example 4, a recovery rate of 91.5% was still achieved, indicating that the core scheme of lactic acid and bacterial community synergy itself has good processing capabilities. The recovery rate was further increased to 95.2% after adding ammonium sulfate, which confirms the promoting effect of nitrogen source supplementation on maintaining bacterial community activity and enhancing synergistic effect.

[0061] As can be seen from Example 1 and Comparative Example 1, deviations from the optimal bacterial ratio have a significant negative impact on the treatment effect. Both were conducted under conditions of added lactic acid and ammonium sulfate, with the only difference being the bacterial ratio: in Comparative Example 1, the endpoint pH was as low as 2.7, the elemental selenium recovery rate was only 82.0%, and the effluent selenium concentration rose to 18 mg / L. This result indicates that when the Shewanella ratio is too low (1:0.2), even though lactic acid fully utilizes its dual functions as a carbon source and electron shuttle, and the acid production by *Thiobacillus ferrooxidans* lowers the pH to 2.7, the reduction efficiency of selenium oxide anions is still limited due to the severely insufficient number of reducing bacteria; at the same time, the excessively low pH exceeds the optimal working range of Shewanella, further inhibiting its reducing activity.

[0062] As can be seen from Example 1 and Comparative Example 2, an excessively high proportion of Shewanella can also affect the treatment effect. Both were conducted under conditions of added lactic acid and ammonium sulfate, the difference being the bacterial community ratio: Comparative Example 2 had an endpoint pH of 4.1, an elemental selenium recovery rate of 89.5%, and an effluent selenium concentration of 10.5 mg / L. This result indicates that when the proportion of Shewanella is too high (1:1.2), there is a relative deficiency of *Thiobacillus ferrooxidans*, resulting in weakened acid production capacity and preventing the system pH from being sufficiently reduced to the optimal range, thus inhibiting the reduction efficiency of selenium oxide anions to some extent. Although Comparative Example 2 still achieved a recovery rate of 89.5% due to the fortification effect of lactic acid and a sufficient number of Shewanella, it is still lower than the 95.2% of Example 1, confirming that an acidic environment is crucial for the reduction of selenium by Shewanella.

[0063] As can be seen from Examples 1 and 3, the endpoint pH of Comparative Example 3 was 4.6, the elemental selenium recovery rate was only 78.5%, and the effluent selenium content was 21.5 mg / L, showing a significant increase. The use of lactic acid as a composite functional medium in this invention has significant advantages over the traditional dual-reagent dosing process of "carbon source + electron shuttle". These results indicate that: First, the dual-functional design of lactic acid allows a single medium to simultaneously meet the needs of carbon source supply and electron transfer enhancement, avoiding a series of problems caused by reagent imbalance in the traditional dual-reagent addition mode. In contrast, the addition ratio of glucose to anthraquinone-2-sulfonate in Comparative Example 3 is difficult to control precisely. If the electron shuttle is relatively excessive, it will compete with selenium oxide anions for electrons, thus inhibiting the reduction efficiency. If the carbon source is relatively excessive, it will lead to excessive organic load in the water body, increasing the risk of secondary pollution. Second, lactic acid is more conducive to the acid production metabolism of *Thiobacillus ferrooxidans*, allowing the system pH to be sufficiently reduced to the optimal range, while glucose is difficult for the bacteria to utilize effectively, resulting in insufficient pH reduction. Third, lactic acid can be completely metabolized and utilized by microorganisms, making it more environmentally friendly. In contrast, anthraquinone-2-sulfonate in Comparative Example 3, as a non-biomass electron shuttle, is prone to residues in the system, and the dual-reagent addition significantly increases operating costs and brings the risk of organic pollution.

[0064] As can be seen from Example 1 and Comparative Example 4, the chemical reduction method is far inferior to the biological leaching method in terms of the recovery rate of elemental selenium and the selenium content in the effluent. Moreover, as can be seen from the implementation process of Comparative Example 4, the chemical reduction method requires a large amount of acid-base adjustment agents, generates a large amount of acidic wastewater, and requires additional investment in neutralization costs for subsequent treatment. It is inferior to the method of this application in terms of both environmental protection and economic efficiency.

[0065] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application.

Claims

1. A method for recovering and extracting selenium from selenium-containing wastewater, characterized in that, Includes the following steps: S1. Pretreatment: Adjust the selenium-containing wastewater to a selenium concentration of 50~200 mg / L, a solid content of less than 5%, and an initial pH value of 5.0~6.0; S2. Selenium reduction: Lactic acid and inoculated functional bacteria are added to the pretreated wastewater, and a bioleaching reduction reaction is carried out under aeration and stirring conditions to reduce selenium oxide anions. The functional bacterial community includes *Shewanella oneidensis* and *Acidithiobacillus ferrooxidans*, with the inoculation ratio of *Acidithiobacillus ferrooxidans* to *Shewanella oneidensis* being 1:0.5-1. The bacterial concentrations of both *Acidithiobacillus ferrooxidans* and *Shewanella oneidensis* were controlled at 10⁻⁶. 7 ~10 8 CFU / mL, the inoculation amount of the functional bacterial group is 8%~12% of the volume of selenium-containing wastewater; the Shewanella from Lake Oneida is Shewanella from Lake Oneida MR-1, and the ferrooxidizobacterium is ferrooxidizobacterium LX5; S3. Elemental selenium recovery: After the reaction is completed, the system is subjected to solid-liquid separation. The liquid phase is treated wastewater that meets the standards, and the solid phase is the elemental selenium enrichment product. The elemental selenium enrichment product is washed, dried and purified to obtain a high-purity elemental selenium product.

2. The method for recovering and extracting selenium from selenium-containing wastewater according to claim 1, characterized in that, In S2, the amount of lactic acid added is 0.5~2.0 g per liter of wastewater.

3. The method for recovering and extracting selenium from selenium-containing wastewater according to claim 2, characterized in that, In step S2, the amount of lactic acid added is 1.0~1.5g per liter of wastewater, the temperature range of the reduction reaction system is 28~35℃, the stirring speed range is 120~180 rpm, and the aeration rate is 0.5~1.0 L / h.

4. The method for recovering and extracting selenium from selenium-containing wastewater according to claim 1, characterized in that, In step S2, an ammonium sulfate solution is also added, the concentration of which is 18-25 wt%, and the amount of which is added is 5-12 g per kilogram of wastewater.

5. The application of the method for recovering and extracting selenium from selenium-containing wastewater as described in any one of claims 1 to 4 in the field of elemental selenium recovery from selenium-containing wastewater.

Images