Method for removing selenate and recycling elemental selenium

Abstract

The invention discloses a method for removing selenate and recovering elemental selenium, and belongs to the technical field of wastewater treatment and resource recovery. According to the method, an anaerobic dynamic biofilm reactor (AnDMBR) is used for treating the selenium-containing wastewater, selenate is reduced into elemental selenium through anaerobic microorganisms, and the elemental selenium is intercepted on a dynamic biofilm. After the operation of the reactor is finished, carrying out ultrasonic cell disruption treatment on the dynamic biological membrane, removing residual biomass on the surface of the selenium elementary substance by using alkali liquor, and then washing by using ultrapure water until the pH is neutral, thereby purifying and recovering the selenium elementary substance. The AnDMBR used in the invention can effectively remove selenate in wastewater, does not accumulate selenite with higher toxicity, and does not cause secondary pollution; the selenium elementary substance separation method is simple, and the separation and purification efficiency is high.

Description

Technical Field

[0001] The invention belongs to the technical field of wastewater treatment and resource recovery, and specifically relates to a method for removing selenate and recovering selenium. Background Art

[0002] Selenium is a natural trace element essential to humans and animals, and its safe range for human use is narrow. Selenium deficiency can cause diseases such as Keshan disease and Kashin-Beck disease, while excessive intake can cause selenium poisoning, which manifests as hair loss, nail abnormalities, skin diseases, and neurological disorders, and can be fatal in severe cases. In selenium-containing wastewater, selenium exists in the form of selenate and selenite, which are extremely toxic and bioaccumulative. Direct discharge into natural water bodies can easily cause selenium pollution, threatening human and ecological health. The reduction product of high-valent selenium, single selenium, is low in toxicity and insoluble in water, and is widely used in the medical, semiconductor and other industries. Therefore, reducing selenate to single selenium and recycling it is a very promising technology for treating selenium-containing wastewater.

[0003] The main treatment methods for selenium-containing wastewater include adsorption, coagulation and sedimentation, membrane separation, chemical reduction, electrochemical reduction and biological treatment. Adsorption and coagulation and sedimentation methods are prone to generate selenite, resulting in secondary pollution; membrane separation methods are relatively expensive; chemical reduction and electrochemical reduction methods require the addition of catalysts or external electric fields. With the help of microbial metabolism, biological treatment methods can efficiently and cost-effectively convert highly toxic soluble selenate into selenium elements, and realize product recycling, showing good application prospects. However, how to effectively regulate and separate and purify the generated selenium elements remains a major challenge.

[0004] Current research focuses on improving the efficiency and rate of selenium removal, but not enough attention is paid to the low-cost separation and recovery of selenium. The separation and recovery of selenium faces many challenges, such as the influence of dispersion on separation efficiency, high separation technology cost, and insufficient purity of recovered products. Therefore, it is of great significance to develop an efficient, environmentally friendly method to simultaneously remove selenate and recover selenium.

[0005] The anaerobic dynamic biofilm reactor (AnDMBR) is a new type of sewage treatment system developed based on the traditional anaerobic membrane bioreactor. It uses cheap coarse-porous materials such as nylon mesh and non-woven fabrics to replace microfiltration / ultrafiltration membrane components, combining the advantages of membrane separation and biological treatment technology. By forming a dynamic biofilm (DM), this system not only makes up for the problem of insufficient retention capacity of the supporting material, but also has a solid-liquid separation effect comparable to that of microfiltration / ultrafiltration membranes. When treating wastewater, AnDMBR can alleviate membrane pollution and reduce operating costs. However, further research and optimization are still needed in the treatment of selenium-containing wastewater and the retention and separation and recovery of selenium elements.

[0006] The present invention aims to provide a method for treating selenium-containing wastewater using an anaerobic dynamic biofilm reactor, reducing selenate to elemental selenium, intercepting the selenium element through a biofilm, and then separating and purifying the selenium element using an ultrasonic crusher, so as to solve the deficiencies in the prior art and achieve efficient and environmentally friendly selenate removal and selenium element recovery. Summary of the invention

[0007] The present invention aims to provide a method for removing selenate and recovering selenium, namely, using an anaerobic dynamic biofilm reactor to treat selenium-containing wastewater, reducing selenate to selenium, and intercepting selenium through DM, and then using an ultrasonic disruptor to separate the selenium on the biofilm, and then using alkaline solution to remove biomass to purify selenium, so as to solve the shortcomings of the existing technology and achieve efficient and environmentally friendly selenate removal and selenium recovery.

[0008] Furthermore, the AnDMBR is composed of an anaerobic bioreactor and a built-in membrane assembly, and a stirring device is provided in the reactor.

[0009] Furthermore, the membrane assembly is a flat membrane in the form of two double-sided membranes, which are formed by fixing a coarse-porous material to a plate frame. The material of the plate frame includes but is not limited to organic plastics, the coarse-porous material includes but is not limited to nylon mesh and non-woven fabric, the pore size of the coarse-porous material can be 25-50 μm, and the fixing method includes but is not limited to fixing with screws.

[0010] Furthermore, in the process of removing selenate by AnDMBR, the selenium-containing wastewater is pumped into the reactor using an inlet pump. After being treated by anaerobic microorganisms, it is pumped out from the outlet of the membrane module. A pressure gauge is connected between the outlet and the outlet pump to record the transmembrane pressure (TMP). When the TMP reaches 30 kPa, the reactor operation ends.

[0011] Furthermore, the concentration of selenate in the selenium-containing wastewater is 1-5 mg Se / L.

[0012] Furthermore, after the reaction is completed, the biofilm that has retained the selenium element is taken out and cut into small pieces, placed in ultrapure water for ultrasonication, and then the obtained suspension is centrifuged and the precipitate is washed with ultrapure water; ultrasonic cell disruptor is used for ultrasonic disruption, and then the disrupted product is centrifuged, and after centrifugation, alkaline solution is added to the precipitate and stirred to remove the biomass remaining on the surface of the selenium element; centrifugation is performed again, the supernatant is removed, and the selenium element is repeatedly washed with ultrapure water until the pH is neutral; the precipitate is suspended in ultrapure water, and the selenium element is obtained after freeze-drying.

[0013] Furthermore, the centrifugation preferably uses a high-speed refrigerated centrifuge, and the conditions are preferably as follows: a rotation speed of 10,000×g, a temperature of 4° C., and a centrifugation time of 10 to 30 min, preferably 10 min.

[0014] Furthermore, the power of the ultrasonic fragmentation can be 100-450 W, preferably 450 W; the ultrasonication is performed for 10 s, with an interval of 5 s, and the total time is 20-40 min, preferably 40 min.

[0015] Furthermore, the alkali solution is preferably a sodium hydroxide solution with a concentration of 0.5-2 mol / L, preferably 1 mol / L.

[0016] The present invention also provides the method for removing selenate and recovering selenium as described in the above technical solution, or the application of the method for removing selenate and recovering selenium as described in the above technical solution to treat selenium-containing wastewater and recover selenium.

[0017] Compared with the prior art, the method of the present invention has the following advantages: the membrane assembly used in the present invention uses cheap coarse-porous materials as the support layer of the dynamic biofilm, thereby reducing the operating cost. The method of the present invention has a high removal rate for selenate, does not accumulate more toxic selenite, and does not cause secondary pollution. At the same time, the dynamic biofilm can intercept selenium, and simplify the steps of separating and purifying selenium by ultrasonic crushing, thereby improving the recovery rate and purity of selenium. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 Schematic diagram of the AnDMBR experimental device of the embodiment.

[0019] Figure 2 Figure 1 is a physical picture of the membrane assembly before and after the membrane formation in the embodiment, wherein Figure A is the membrane assembly before the membrane formation, and Figure B is the membrane assembly after the membrane formation.

[0020] Figure 3 The figure shows the removal of selenate and total selenium in the selenium-containing wastewater in the example.

[0021] Figure 4 The following are the SEM images and EDS spectra of the selenium element on the surface of the biofilm in the example.

[0022] Figure 5 These are the SEM image of the selenium element after separation and purification in the example, the mapping image of the selenium element and the EDS energy spectrum. Specific implementation methods

[0023] The present invention provides a method for removing selenate and recovering selenium, comprising the following steps: The method of the present invention uses an anaerobic dynamic biofilm reactor (AnDMBR) to treat selenium-containing wastewater, and anaerobic microorganisms are used to reduce selenate in the wastewater to selenium element, and the selenium element is retained on the biofilm. Then, the biofilm containing the selenium element is subjected to ultrasonic cell disruption treatment, and the biomass remaining on the surface of the selenium element is removed by alkaline solution, and washed with ultrapure water until the pH is neutral, thereby purifying and recovering the selenium element.

[0024] AnDMBR is composed of a rectangular anaerobic bioreactor and a membrane assembly. The membrane assembly fixes the coarse-porous material on both sides of the plate frame by screws. The material of the plate frame is preferably organic plastic, and the coarse-porous material is preferably nylon mesh; the pore size of the nylon mesh is 25-50 μm, preferably 50 μm.

[0025] The anaerobic sludge was washed three times with physiological saline, and concentrated sludge was obtained after removing the supernatant. Simulated selenium-containing wastewater with a selenate concentration of 5 mg Se / L was added to the concentrated sludge. The obtained mud-water mixture was mixed evenly and then sieved to remove sludge particles with larger diameters. The suspension was added to the reactor, and nitrogen was introduced to keep the dissolved oxygen in the suspension below 0.2 mg / L. The reactor was internally circulated at a membrane flux of 2.3 LMH and stirred for 12 h by a stirring device to form a dynamic biofilm on the coarse-pore material. When the effluent turbidity is stable, it indicates that the biofilm is successfully formed.

[0026] In the treatment of selenium-containing wastewater by AnDMBR, a peristaltic pump is used to pump the selenium-containing wastewater into AnDMBR and discharge it through the membrane module. A pressure gauge is connected between the outlet of the membrane module and the outlet pump to read the TMP. When the TMP reaches 30 kPa, the reactor operation ends.

[0027] After the operation of the reactor is completed, the biofilm that intercepted the selenium element is taken out and cut into small pieces, and then placed in ultrapure water for ultrasonication. The obtained suspension is then centrifuged at a speed of 10000×g and 4°C for 10 min. The precipitate is washed with ultrapure water 3 times, and then ultrasonically disrupted using an ultrasonic cell disruptor, with a power of 100-450 W, preferably 450 W, ultrasonication for 10 s, an interval of 5 s, and a total time of 20-40 min, preferably 40 min; after centrifugation at a speed of 10000×g and 4°C for 10 min, NaOH is added to the precipitate at a concentration of 0.5-2 mol / L, preferably 1 mol / L, and stirred at room temperature for 30 min to allow NaOH to fully contact the remaining biomass on the selenium element and remove the biomass remaining on the surface of the selenium element; then centrifuged at a speed of 10000×g and 4°C for 10 min, the supernatant is removed, the precipitate is repeatedly washed with ultrapure water until the pH is neutral, and the precipitate is freeze-dried to obtain selenium element.

[0028] The following is a detailed description of a method for removing selenate and recovering selenium provided by the present invention in conjunction with the embodiments, but they should not be construed as limiting the scope of protection of the present invention. Example

[0029] Removal of selenate: The anaerobic dynamic biofilm reactor (AnDMBR) used in this example is composed of a rectangular anaerobic bioreactor and a flat membrane assembly with a nylon mesh with a pore size of 50 μm as a support layer. The schematic diagram of the experimental device is shown in FIG. Figure 1 The reactor is 12 cm long, 11.2 cm wide, and 30 cm high. The total volume is about 4.03 L, and the effective volume is 3.09 L. The membrane assembly is located in the middle of the reactor, and the nylon mesh is fixed to the organic plastic frame by screws. The membrane assembly is 8 cm long, 8 cm high, and the effective membrane area is 0.0256 m 2 .

[0030] Biofilm formation stage: The inoculum sludge was taken from the anaerobic zone sludge of Qilidian Wastewater Treatment Plant in Guilin, Guangxi Zhuang Autonomous Region, and washed three times with saline. After removing the supernatant, concentrated sludge was obtained. Simulated selenium-containing wastewater was added to the concentrated sludge, and the composition was as follows: 5 mg Se / L Na 2 SeO 4 , 10 mg N / L KNO 3 , 7 mg N / L NH 4 Cl, 3 mg / LK 2 HPO 4 , 3 mg / LNaH 2 PO 4 , 40 mg / L NaCl, 80 mg / L NaHCO 3 ; 1 mL / L trace element solution (trace element composition: 1 mg / LMgCl 2 6H 2 O; 0.8 mg / L CaCl 2 ·2H 2 O; 0.05 mg / L ZnCl 2 ; 0.05 mg / L FeSO 4 7H 2 O; 0.05 mg / L CuSO 4 ·5H 2 O), with 300 mg / L COD Cr Sodium acetate is the only carbon source, the inlet temperature is 25±1℃, and the pH is 7.0±0.1. The obtained mud-water mixture is mixed evenly and sieved to remove the sludge particles with larger diameters. The suspension and nylon mesh are then placed in the reactor, and nitrogen is introduced to keep the dissolved oxygen in the suspension below 0.2 mg / L. Finally, the magnetic stirrer is turned on and stirred for 12 hours to allow the microorganisms in the suspension to grow to a certain extent. The reactor is then circulated internally with a membrane flux of 2.3 LMH. When the effluent turbidity is stable, it indicates that the biofilm is successfully formed.

[0031] Figure 2 Figure 1 is a physical picture of the membrane assembly before and after biofilm formation, wherein Figure A is the membrane assembly before biofilm formation, and Figure B is the membrane assembly after biofilm formation.

[0032] The simulated selenium-containing wastewater was pumped into the AnDMBR by a peristaltic pump. After being treated by anaerobic microorganisms, the water was discharged from the membrane module. The HRT was 24 h, the inlet flow rate was 2.1 mL / min, and the outlet flow rate was 2.08 mL / min. The membrane flux was controlled to be 5.03 LMH by adjusting the flow rate of the peristaltic pump. An agitator was installed inside the reactor to mix the mud-water mixture evenly. Nitrogen was introduced into the reactor to make the dissolved oxygen lower than 0.2 mg / L. A pressure gauge was set between the outlet of the membrane module and the pump to read the TMP. When the TMP reached 30 kPa, the reactor operation ended.

[0033] Figure 3 This is the removal of selenate from selenium-containing wastewater in the example.

[0034] Depend on Figure 3 It can be seen that after 24 days of operation, the TMP of AnDMBR reached 30 kPa, and the operation of the reactor ended. The selenate, selenite and total selenium in the inlet and outlet water were tested. The removal rate of selenate by AnDMBR was as high as 100% from the second day, and the removal rate of total selenium was between 92.2% and 99.4%. Although the intermediate product selenite was generated during the operation, its effluent concentration was low and could meet the secondary standard of my country's "Comprehensive Wastewater Discharge Standard" (GB8978-1996), and it can be discharged safely.

[0035] SEM characterization analysis of selenium on the surface of the biofilm: After the reactor was completed, a 1 cm × 1 cm DM membrane surface was cut from the membrane assembly, washed three times with saline, fixed with 12.5% ​​glutaraldehyde solution for 6-8 h, and then rinsed with phosphate buffer solution three times, each time for 15-30 min; dehydrated with 50%, 70%, 80%, 90%, 95%, and 100% ethanol in a gradient manner, each time for 10-15 min, and then placed in a sterilized culture dish to air dry naturally; the dried sample was ground with an agate mortar and sieved through a 200-mesh sieve to obtain the test sample.

[0036] Figure 4 The following are the SEM images and EDS spectra of the selenium element on the surface of the biofilm in the example.

[0037] The test samples were characterized by SEM-EDS. Figure 4 As shown, the selenium attached to the surface of the biofilm is mainly in the shape of rods with a length of 2-3 μm. The EDS spectrum of selenium nanoparticles shows that the Se content is 56.07%, indicating that the biofilm effectively intercepts the selenium.

[0038] Separation and purification of selenium: The membrane surface (8×8 cm) that retained selenium was cut into small pieces, placed in 40 mL ultrapure water for ultrasonication, and then the obtained suspension was centrifuged at 10000×g for 10 min and washed with ultrapure water three times. Ultrasonic cell disruptor was used for ultrasonic disruption, with the power set to 450 W, ultrasonication for 10 s, intermittent for 5 s, and a total time of 40 min; then the broken product was centrifuged at 10000×g for 10 min, and 40 mL of 1 mol / L NaOH solution was added to the precipitate after centrifugation, and constant temperature magnetic stirring was performed at 25°C for 30 min to allow NaOH to fully contact with the remaining biomass on the selenium element and remove the residual biomass on the surface of the selenium element; then centrifuged at 10000×g for 10 min, the supernatant was removed, and the precipitate was repeatedly washed with ultrapure water until neutral, and freeze-dried to obtain selenium element. The dried sample was ground with an agate mortar and passed through a 200-mesh sieve to obtain the test sample.

[0039] Figure 5 These are the SEM image of the selenium element obtained by separation and purification in the example, the mapping image of the selenium element and the EDS spectrum.

[0040] from Figure 5 Clearly separated particles of selenium can be seen, indicating that selenium has been well separated. Selenium is mainly rod-shaped and spherical in structure. The length of the rods is between 0.45-2.4 μm, and the particle size of the spheres is 0.08-0.89 μm, mostly 0.22-0.27 μm, and a small part is between 0.35-0.39 μm. The mapping image of selenium shows that selenium is widely distributed, indicating that selenium has been well purified. The EDS spectrum shows that the content of selenium is 75.94%, which is 19.87% higher than that of the biofilm surface, indicating that this method can improve the purity of selenium.

[0041] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A method for removing selenate and recovering selenium, characterized in that: The steps are as follows: use an anaerobic dynamic biofilm reactor (AnDMBR) to treat selenium-containing wastewater, reduce selenate in the wastewater to elemental selenium through anaerobic microorganisms, and retain the elemental selenium on the biofilm; after the reactor operation is completed, the biofilm is subjected to ultrasonic cell disruption treatment, and alkaline solution is used to remove the biomass remaining on the surface of the elemental selenium, and then it is washed with ultrapure water until the pH is neutral, thereby purifying and recovering the elemental selenium.

2. The method according to claim 1, characterized in that The AnDMBR is composed of an anaerobic bioreactor and a built-in membrane assembly, and a stirring device is arranged in the reactor.

3. The method according to claim 1, characterized in that The membrane assembly is a flat membrane in the form of two double-sided membranes, which are fixed to a plate frame by a coarse-porous material; the material of the plate frame includes but is not limited to organic plastic, the coarse-porous material includes but is not limited to nylon mesh, and the fixing method includes but is not limited to fixing with screws.

4. The method according to claim 1, characterized in that After the AnDMBR is assembled, a mud-water mixture is added and the reactor is operated in an internal circulation form to form a dynamic biofilm on the coarse-porous material; the mud-water mixture refers to a mixture of selenium-containing wastewater to be treated and anaerobic sludge, and the selenium concentration in the selenium-containing wastewater is 1~5 mg Se / L.

5. The method according to claim 1, characterized in that In the process of selenate removal, the selenium-containing wastewater is pumped into the AnDMBR using an inlet pump. After being treated by anaerobic microorganisms, it is pumped out from the outlet of the membrane module. The transmembrane pressure (TMP) is recorded using a pressure gauge. When the TMP reaches 30 kPa, the reactor operation ends.

6. The method according to claim 1, characterized in that After the reaction is completed, the dynamic biofilm that has retained selenium is taken out, placed in ultrapure water for ultrasonication, the obtained suspension is centrifuged, and the precipitate is washed with ultrapure water; then, an ultrasonic cell disruptor is used for ultrasonic disruption, the product is centrifuged, the supernatant is removed, and alkaline solution is added to the precipitate and stirred to remove the biomass remaining on the surface of the selenium; Centrifuge again, wash the precipitate repeatedly with ultrapure water until the pH is neutral, and then freeze-dry to obtain selenium element.

7. The method according to claim 1, characterized in that The centrifugation was performed using a high-speed refrigerated centrifuge at a speed of 10,000×g, a temperature of 4° C., and a duration of 10 to 30 min.

8. The method according to claim 1, characterized in that The power of ultrasonic disruption is 100~450 W, ultrasonication is 10 s, and the interval is 5 s, and the total time is 20~40 min.

9. The method according to claim 1, characterized in that The alkali solution is preferably a sodium hydroxide solution with a concentration of 0.5-2 mol / L.

10. Use of the method for removing selenate and recovering selenium as claimed in claims 1 to 9 in treating selenium-containing wastewater and recovering selenium.

Images