Method for resourcefully treating tailing wastewater and synchronously recovering valuable metals

Abstract

The invention provides a method for resourcefully treating tailing wastewater and synchronously recovering valuable metals, and belongs to the technical field of mine wastewater treatment. The method comprises the following steps: establishing a reaction system, wherein the reaction system comprises a transparent reactor with a built-in working electrode, a photocell circuit system without a power supply, a photoelectrolysis circuit system with a power supply, a light source, and a photocell circuit system and a photoelectrolysis circuit system which are switched in parallel; the method comprises the following steps: introducing tailing wastewater into a transparent reactor, sealing, turning on a light source and only turning on a photocell circuit system, oxidizing and converting ammoniacal nitrogen into nitrogen and nitrate nitrogen, and recovering part of manganese metal; a light source is turned on, only a photoelectrolysis circuit system is started, manganese ions are reduced, manganese metal is recycled, and nitrate nitrogen is reduced and converted into nitrogen. The invention provides a method for cleanly and efficiently oxidizing ammoniacal nitrogen in mine tailing wastewater and synchronously recovering valuable metals in the tailing wastewater. Ammonia-containing pollutants in the tailing wastewater are efficiently degraded and manganese metal is recycled by utilizing a photocell reaction system and a photoelectrolysis reaction system which can be mutually converted.

Description

Technical Field

[0001] The present invention belongs to the technical field of mine wastewater treatment, and particularly relates to a method for resource treatment of tailing wastewater and synchronous recovery of valuable metals. Background Art

[0002] A large amount of wastewater is generated during the processes of ore dressing, leaching, and ore washing in metal mine resources. The ore dressing wastewater includes ore dressing process drainage, tailings pond overflow water, and mine field drainage. At present, in addition to pre-factory treatment and reuse, part of the ore dressing wastewater enters the tailings pond in the form of tailings slurry for natural precipitation, forming mine tailing wastewater.

[0003] In actual engineering, the clarified water in the mine tailings pond is pumped back for further recycling, but its overflow water or the surface water in the surrounding waters of the mining area contains a certain amount of ammonia nitrogen and metal ions. Therefore, before recycling, the mine tailing wastewater is usually pumped to the tail water treatment station for denitrification treatment such as nitrification / denitrification or anaerobic ammonium oxidation. In view of the problem that a large amount of organic carbon sources need to be added for the treatment of mine tailing wastewater, there are deficiencies such as high treatment cost, high energy consumption, and ineffective treatment of metal ions. At present, the treatment methods for mine tailing wastewater mainly include chemical precipitation methods, chemical / physical / biological comprehensive treatment methods, etc.

[0004] Taking chemical precipitation methods such as lime-iron salt-coagulation precipitation as an example, although heavy metal ions can be effectively removed, there are problems such as incomplete metal treatment, high cost, and secondary pollution. In addition, in terms of the ore washing process, washing the mine with clear water or chemical eluents (such as potassium chloride, ferrous sulfate, etc.) or cascade washing can remove part of the ammonia nitrogen, but there are still problems such as residual ammonium salts and slow release of metal ions with rainfall and pollution of the surrounding environment after ore washing;

[0005] Chemical / physical / biological comprehensive treatment methods generally combine chemical methods with processes such as activated carbon adsorption, electrooxidation, Fenton oxidation, biological regulation, flocculation precipitation, and ecological restoration. Although these process methods can treat tailing wastewater to a certain extent and reduce the dosage of chemical eluents, there are still common problems such as long treatment cycle, low efficiency, high energy consumption, long process flow, cumbersome process, and high cost.

[0006] Therefore, in view of the reality of the coexistence of a certain amount of inorganic nitrogen (ammonia nitrogen and nitrate nitrogen) and metal ions in the tailing wastewater after mine exploitation and leaching, finding a clean, efficient, short-process and low / no-energy-consuming method to deeply remove inorganic nitrogen in mine tailing wastewater and synchronously recover metals is an important requirement for the sustainable, healthy and green development of mines.

[0007] Methods such as photocatalysis and photoelectrolysis are widely used in the field of domestic and industrial wastewater treatment due to their advantages such as simple installation and less addition of chemical agents.

[0008] However, these photovoltaic cells or photoelectrolysis cell treatment processes still have many shortcomings, such as long process flow, low removal rate and efficiency, incomplete removal of inorganic nitrogen, high or unsustainable electrode material costs, and low light efficiency utilization. Summary of the invention

[0009] In view of this, the purpose of the present invention is to provide a method for resource-based treatment of tailings wastewater and simultaneous recovery of valuable metals, aiming to solve at least one technical problem in the background technology.

[0010] The present invention is achieved in that:

[0011] A method for resource-based treatment of tailings wastewater and simultaneous recovery of valuable metals, the method comprising the following steps:

[0012] A reaction system is constructed, wherein the reaction system includes a transparent reactor with a built-in working electrode, a photocell circuit system and a photoelectrolysis circuit system respectively connected to the working electrode, and a light source external to the transparent reactor; the photocell circuit system and the photoelectrolysis circuit system are switched in parallel, the photocell circuit system has no power supply, and the photoelectrolysis circuit system has a power supply;

[0013] The tailings wastewater is introduced into the transparent reactor and sealed; the light source is turned on, and only the photocell circuit system is turned on to perform a photocell reaction, which is used to oxidize the ammonia nitrogen into nitrogen gas and nitrate nitrogen and recover part of the manganese metal;

[0014] The light source is turned on, and only the photoelectric circuit system is started. The photocell reaction is converted into a photoelectric reaction, which is used to directly or indirectly reduce manganese ions to recover manganese metal and reduce nitrate nitrogen to nitrogen gas.

[0015] Preferably, the working electrode comprises a photoanode and a photocathode; the material of the photoanode and the photocathode is a porous carbon felt with a support layer disposed on the surface;

[0016] Wherein, the supporting layer on the surface of the photocathode is a composite of metal oxide and graphite-phase carbon nitride; and the supporting layer on the surface of the photoanode is a composite of carbon dots and graphite-phase carbon nitride.

[0017] Preferably, the photocell circuit system comprises a first switch, a first ammeter and a first external resistor, and the photocell circuit system is electrically connected to the photoanode and the photocathode; the light source, the transparent reactor, the photocell circuit system, the photoanode and the photocathode constitute a photocell reaction system;

[0018] The photoelectrolysis circuit system includes a second switch, a second ammeter, a power supply and a second external resistor. The photoelectrolysis circuit system is electrically connected to the photoanode and the photocathode. The light source, the transparent reactor, the photoelectrolysis circuit system, the photoanode and the photocathode constitute a photoelectrolysis reaction system.

[0019] Preferably, the metal oxide is selected from at least one of tungsten molybdenum oxide, iron manganese oxide, zinc ferrite, and silver phosphate.

[0020] Preferably, the loading amount of the loading layer on the surface of the photoanode and the photocathode is 0.1 mg / cm 2 ~20 mg / cm 2 .

[0021] Preferably, in the loading layer of the photoanode, the mass ratio of the metal oxide to graphitic carbon nitride is 10:1 to 1:10;

[0022] In the loading layer of the photocathode, the mass ratio of the carbon dots to graphitic carbon nitride is 10:1 to 1:10.

[0023] Preferably, the first external resistance is 100 Ω to 2000 Ω;

[0024] When the photovoltaic cell circuit system operates, the potential of the photoanode is -0.4 V to -0.3 V, and the potential of the photocathode is 0.2 V to 0.3 V;

[0025] The operating time of the photovoltaic cell reaction is 1.5 h to 2.5 h.

[0026] Preferably, the second external resistance is 10 Ω to 100 Ω;

[0027] The voltage applied by the power supply is 0.3 V to 5.0 V;

[0028] When the photoelectrolysis circuit system operates, the potential of the photoanode is 0.1 V to 2.5 V, and the potential of the photocathode is -2.5 V to -0.4 V;

[0029] The operating time of the photoelectrolysis reaction is 9 h to 11 h.

[0030] Preferably, the reaction system further includes a reference electrode and a gas collecting pipe for collecting gas.

[0031] Preferably, the light source intensity is greater than 5.0 mW / cm 2 .

[0032] Compared with the prior art, the present invention has the following beneficial effects:

[0033] 1. The present invention provides a method for cleaning and efficiently removing inorganic nitrogen in mine tailing wastewater and simultaneously recovering valuable metals (such as Mn(II), etc.) in the tailing wastewater; the photoanode of the photovoltaic cell oxidizes and converts ammonia nitrogen in the mine tailing wastewater into nitrogen gas and nitrate nitrogen, and the photocathode recovers part of the manganese metal with the help of the H2O2 mediator; then the photovoltaic cell is switched to a photoelectrolytic cell to reduce nitrate nitrogen in the mine tailing wastewater to nitrogen gas while further recovering manganese metal.

[0034] 2. The present invention constructs a photoelectric cell reaction system and a photoelectrolysis reaction system that can be mutually converted. On the basis of efficiently degrading pollutants and recovering valuable metals, it can cleanly use sunlight and ensure the light energy utilization efficiency, thereby realizing the process of clean, efficient, short-process deep treatment of mine tailing wastewater and synchronous resource recovery of metals.

[0035] 3. The present invention uses a porous carbon felt with a special loading layer as the working electrode. The carbon-based carbon felt material is more conducive to the two-electron reaction of O2 to generate H2O2; while graphitic carbon nitride (g-C3N4) is loaded on different electrode materials constructed by porous carbon felt (such as MnFe2O4 / g-C3N4, WO3 / MoO3 / g-C3N4, ZnFe2O4 / g-C3N4, carbon dots / g-C3N4, etc.), and has specific photogenerated electron and photogenerated hole separation as well as conduction band and valence band properties after appropriate modification and pretreatment. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 It is a schematic structural diagram of the reaction system of the present invention;

[0037] Illustration: 1 - light source; 2 - photoanode; 3 - transparent reaction vessel; 4 - photocathode, 5 - cathode loading layer, 6 - reference electrode, 7 - carbon rod; 8 - first external resistor, 9 - second external resistor, 10 - first ammeter, 11 - second ammeter, 12 - power supply, 13 - second switch, 14 - first switch, 15 - gas collecting pipe, 16 - anode loading layer.

[0038] Figure 2 It is a graph showing the change of each pollutant in the mine tailing wastewater with the running time in Example 1;

[0039] Figure 3 It is a graph showing the change of the pH and conductivity of the effluent of the mine tailing wastewater with the running time in Example 1;

[0040] Figure 4 It is a graph showing the change of the electrode potential and loop current with the running time in Example 1;

[0041] Figure 5 It is a cyclic voltammogram of the three-electrode system electrode in Example 1;

[0042] Figure 6 It is the E-pH relationship of different inorganic nitrogen species and manganese species in the mine tailing wastewater in Example 1;

[0043] Figure 7 It is a comparison graph of the removal rates of each pollutant in the mine tailing wastewater of the photovoltaic cell group, the open-circuit control group and the non-light control group in the photovoltaic cell stage;

[0044] Figure 8Cyclic voltammograms of photoanodes and photocathodes with and without light illumination and with and without a load layer

[0045] Figure 9 Impedance analysis diagrams of photoanodes and photocathodes with and without light and with and without a load layer

[0046] Figure 10 Scanning electron microscope morphology diagrams of photoanodes and photocathodes after running five cycles according to Example 1

[0047] Figure 11 X-ray energy spectrum analysis diagrams of photoanodes and photocathodes after running five cycles according to Example 1

[0048] Figure 12 Impedance analysis and cyclic voltammograms during the process of running five cycles according to Example 1 Detailed implementation manners

[0049] In order to make the objectives, technical solutions and advantages of the present invention clearer and more understandable, the present invention will be further described in detail below in conjunction with embodiments. It should be understood that the specific implementation cases described herein are only used to explain the present invention and are not used to limit the present invention.

[0050] The reaction system adopted by the present invention for resource treatment of tailing wastewater based on the dual-photoelectric synergistic method is as Figure 1 shown.

[0051] The reaction system includes: a transparent reactor 3 with a built-in working electrode, a photovoltaic cell circuit system connected to the working electrode, a photoelectrolysis circuit system connected to the working electrode, and a light source 1 externally disposed on the transparent reactor 3; the photovoltaic cell circuit system and the photoelectrolysis circuit system are switched in parallel, the photovoltaic cell circuit system has no power supply, and the photoelectrolysis circuit system is provided with a power supply; the photovoltaic cell circuit system and the photoelectrolysis circuit system are externally disposed on the transparent reactor 3. In specific implementation, the reaction system further includes a gas collecting pipe 15 and a reference electrode 6 disposed on the transparent reactor 3.

[0052] The working electrode includes a photoanode 2 and a photocathode 4. The material of the photoanode 2 is a porous carbon felt with an anodic load layer 16 on its surface; the material of the photocathode 4 is a porous carbon felt with a cathodic load layer 5 on its surface. Among them, the cathodic load layer 5 is a composite of metal oxide and graphitic carbon nitride, and the metal oxide is selected from at least one of tungsten molybdenum oxide (WO3 / MoO3), iron manganese oxide (MnFe2O4), zinc ferrite (ZnFe2O 4) , silver phosphate (Ag3PO4); the anodic load layer 16 is a composite of carbon dots and graphitic carbon nitride. The loading amount of the respective load layers on the surfaces of the photoanode and the photocathode is 0.1 mg / cm 2 ~20 mg / cm 2。The mass ratio of metal oxide to graphitic carbon nitride in the cathode loading layer 5 is 10:1 to 1:10; the mass ratio of carbon dots to graphitic carbon nitride in the anode loading layer 16 is 10:1 to 1:10. The substrate material of the working electrode is the key to controlling the in-situ production yield of H2O2. In this invention, it is more beneficial for the photoanode 2 and the photocathode 4 to use a carbon-based carbon felt material as the substrate for the two-electron reaction of O2 to generate H2O2; while graphitic carbon nitride (g-C3N4) is loaded on different electrode materials constructed by porous carbon felt (such as MnFe2O4 / g-C3N4, WO3 / MoO3 / g-C3N4, ZnFe2O4 / g-C3N4, carbon dots / g-C3N4, etc.), and after appropriate modification and pretreatment, it has specific photogenerated electron and photogenerated hole separation as well as conduction band and valence band properties.

[0053] The photovoltaic cell circuit system includes a first switch 14, a first ammeter 10 and a first external resistor 8, and the photovoltaic cell circuit system is electrically connected to the photoanode 2 and the photocathode 4; the photoelectrolysis circuit system includes a second switch 13, a second ammeter 11, a power supply 12 and a second external resistor 9, and the photoelectrolysis circuit system is electrically connected to the photoanode 2 and the photocathode 4; in a specific implementation, both the photoanode 2 and the photocathode 4 are circuit-connected to the photovoltaic cell circuit system and the photoelectrolysis circuit system through carbon rods 7. Among them, the first external resistor 8 is 100Ω to 2000Ω, the second external resistor 9 is 10Ω to 100Ω, and the voltage applied by the power supply 12 is 0.3V to 5.0V.

[0054] The light source 1, the transparent reactor 3, the photovoltaic cell circuit system and the photoanode 2 and the photocathode 4 form a photovoltaic cell reaction system. In the photovoltaic cell constructed by the carbon dots / g-C3N4-loaded photoanode 2 and the metal oxide / g-C3N4-loaded photocathode 4, by using the different bias voltages generated by their Fermi levels, an anodic and cathodic spontaneous redox reaction is constructed, where the anodic reaction is to convert ammonia nitrogen into nitrate nitrogen and nitrogen gas, and the cathodic reaction is to convert dissolved oxygen into H2O2 to mediate and indirectly reduce manganese ions.

[0055] The light source 1, the transparent reactor 3, the photoelectrolysis circuit system and the photoanode 2 and the photocathode 4 form a photoelectrolysis reaction system. Due to the input of an external small voltage (0.3V to 5.0V) and the change of the internal resistance, both the loop current and the electrode potential are improved and increased. Furthermore, the electrode current acts together with the photogenerated electrons of the metal oxide / g-C3N4-loaded photocathode 4 and the photogenerated electrons of the carbon dots / g-C3N4-loaded photoanode 2 to directly reduce manganese ions, or recover manganese metal through an indirect reduction pathway that reduces the dissolved oxygen to generate an H2O2 intermediate mediator; at the same time, the photogenerated electrons in the photoelectrolysis cell stage can also reduce and convert nitrate nitrogen into nitrogen gas, thereby realizing the process of clean and efficient treatment of mine tailing wastewater and synchronous resource recovery of valuable metals.

[0056] The present invention constructs a mutually convertible photovoltaic cell reaction system and a photoelectrolysis reaction system. By utilizing the oxidizing property of ammonia nitrogen and the reducing properties of nitrate nitrogen and / or metal ions in mine tailing wastewater, and based on the different bias voltages generated by the Fermi levels of different photoanode materials and photocathode materials, a photovoltaic cell that spontaneously undergoes anodic ammonia nitrogen oxidation and synchronous cathodic two-electron reduction of dissolved oxygen (H2O2) and metal reduction, and a photoelectrolysis cell with an externally applied small voltage are matched and constructed to achieve the process of converting ammonia nitrogen in mine tailing wastewater into nitrogen and simultaneously recovering manganese metal.

[0057] The light source 1 is used to provide visible light in the direction of the working electrode, and the visible light used is simulated sunlight. In a specific implementation, a 100W iodine-tungsten lamp is placed at about 15 cm from each of the photoanode 2 and the photocathode 4 as the light source 1, and the intensity of the light source 1 is greater than 5.0 mW / cm 2 . At the same time, a small fan is set to adjust the temperature change caused by light through air convection heat dissipation.

[0058] Taking the hydrothermal method as an example, the preparation method of the loading layers of the photoanode 2 and the photocathode 4 is briefly described as follows: The porous carbon felt is respectively placed in the corresponding loading solution, soaked and then heat-treated at 100 °C to 150 °C, and after taking out, it is cured (such as high-temperature calcination at 400 °C to 450 °C for about 1 h) to form a loading layer on the surface of the porous carbon felt. The preparation steps of the loading solution of the photocathode 4 are as follows: Metal oxide, graphitic carbon nitride and urea are mixed, the pH is adjusted to 12 with alkali, and then placed in a stainless steel-lined Teflon reaction kettle at about 200 °C for 12 hours, and rinsed with ethanol and deionized water. The preparation steps of the loading solution of the photoanode 2 are as follows: Citric acid and urea are dissolved in deionized water, placed in a stainless steel-lined Teflon reaction kettle at about 180 °C for 5 hours, and the supernatant after centrifugation is the carbon dot solution. This carbon dot solution is mixed with urea and left overnight at about 90 °C. In a specific implementation, the loading layers of the photoanode 2 and the photocathode 4 can adopt any method that can be realized in the art, such as the hydrothermal method, the spraying method, the electrodeposition method, etc., and no specific limitation is made here. The above reaction methods, steps and parameters can be adjusted according to actual needs.

[0059] A method for resource treatment of tailing wastewater and simultaneous recovery of valuable metals includes the following steps:

[0060] S1. Assemble the reaction system as shown in Figure 1 .

[0061] S2. The tailings wastewater is introduced into the transparent reactor 3 and sealed. The light source 1 is turned on, and only the photovoltaic cell circuit system is activated for the photovoltaic cell reaction. At this time, the potential of the photoanode 2 is -0.4V to -0.3V, and the potential of the photocathode 4 is 0.2V to 0.3V. It runs for 1.5h to 2.5h. In this step, the photoanode 2 oxidizes and converts the ammonia nitrogen in the tailings wastewater into nitrogen gas and nitrate nitrogen, and the photocathode 4 recovers part of the manganese metal with the help of the H2O2 mediator. During the operation of the photovoltaic cell, a two-electron reduction reaction of dissolved oxygen occurs at the cathode, and the dissolved oxygen comes from the natural standing of the mine tailings wastewater.

[0062] S3. Continue to turn on the light source 1, and only activate the photoelectrolysis circuit system. The photovoltaic cell reaction is converted into a photoelectrolysis reaction. The potential of the photoanode 2 is 0.1V to 2.5V, and the potential of the photocathode 4 is -2.5V to -0.4V. It runs for 9h to 11h. In this step, the photocathode 4 and the photoanode 2 directly or indirectly reduce manganese ions to recover manganese metal, and reduce and convert nitrate nitrogen into nitrogen gas.

[0063] In specific implementation, the actual mine tailings wastewater contains low concentrations of ammonia nitrogen (10mg / L to 30mg / L), nitrate nitrogen (5mg / L to 20mg / L), manganese ions (10mg / L to 50mg / L), and a certain amount of dissolved oxygen (7.0mg / L to 9.0mg / L).

[0064] Example 1

[0065] A method for resource treatment of tailings wastewater and synchronous recovery of valuable metals, including the following steps:

[0066] S1. Place the photoanode 2 (a carbon felt electrode with a size of 1.0cm×1.0cm×0.5cm, and its coating is carbon dots / g-C3N4) and the photocathode 4 (a carbon felt electrode with a size of 1.0cm×1.0cm×0.5cm, and its coating is Fe2MnO4 / g-C3N) in the transparent reactor 3 (about 25ml) made of plexiglass, and electrically connect them to the external photovoltaic cell circuit system and photoelectrolysis circuit system through the carbon rod 7. The photovoltaic cell circuit system and the photoelectrolysis circuit system are switched in parallel. The photovoltaic cell circuit system includes a first switch 14, a first ammeter 10, and a first external resistor 8 (set to 1000Ω) connected in parallel; the photoelectrolysis circuit system includes a second switch 13, a second ammeter 11, a power supply 12 (with a set voltage of 3.0V), and a second external resistor 9 (set to 10Ω). Place 100W iodine tungsten lamps as the light source 1 at 15cm away from the photocathode 4 and the photoanode 2 respectively, and set the intensity of the light source 1 to 24.3mW / m 2 , and at the same time set up a small fan to adjust the temperature change caused by light through air convection heat dissipation;

[0067] S2. The tailings wastewater is introduced into the transparent reactor 3 and sealed. The light source 1 is turned on, the second switch 13 is turned off, and the first switch 14 is turned on. The photovoltaic reaction system is started and operated for 2 hours at room temperature (20°C - 25°C).

[0068] S3. Keep the light source 1 on, close the second switch 13, and open the first switch 14. Switch the photovoltaic reaction system to the photoelectrolysis reaction system and operate for about 10 h.

[0069] 1. Analysis of the treatment efficiency of tailings wastewater

[0070] During the reaction process of Example 1, the liquid in the transparent reactor 3 is sampled regularly to detect the concentration of target pollutants in the tailings wastewater (as shown in Figure 2 ), pH, and conductivity (as shown in Figure 3 ).

[0071] Specifically, analyze the concentrations of species such as ammonia nitrogen, nitrite nitrogen, nitrate nitrogen, total nitrogen, manganese ions, and H2O2 in the liquid phase, and calculate the total nitrogen content in the liquid phase, as well as the removal rates of ammonia nitrogen (%), nitrate nitrogen (%), total nitrogen (%), manganese ions (%), and energy utilization rate (%).

[0072]

[0073] Energy utilization rate (%) = Energy utilization rate in S2 + Energy utilization rate in S3 =

[0074]

[0075] In the formula, ΔG is the Gibbs free energy of the corresponding chemical species, Δn 氨态氮 , Δn 硝态氮 , Δn 锰离子 , Δn 氢气 are the net change in the number of moles of ammonia nitrogen, nitrate nitrogen, manganese ions, and gaseous hydrogen in the liquid phase during the operation of the photovoltaic cell and the photoelectrolytic cell, respectively; the energy utilization rate (%) is based on the molar change in the initial and final states of the inorganic nitrogen (ammonia nitrogen or nitrate nitrogen) and manganese element species in the tailings wastewater, as well as the number of moles of hydrogen generated, multiplied by the corresponding Gibbs free energy and summed up, and then is the ratio to the total input energy (the sum of the light source and electric energy).

[0076] 2. Evaluation of the redox performance of the electrode

[0077] In Example 1, a reference electrode 6 and a gas collecting pipe 15 are installed on the transparent reactor 3. In the three - electrode system composed of the working electrodes (photoanode 2, photocathode 4) and the reference electrode 6, the change process of the electrode potential and current over time is as shown in Figure 4As shown; at the same time, a carbon felt without any load layer was used as the counter electrode to conduct a control experiment for Example 1. The cyclic voltammograms of the photoanode, photocathode, and counter electrode in Example 1 are as shown in Figure 5 to evaluate the redox performance of the electrode.

[0078] The E-pH relationships of nitrogen element species and manganese element species are respectively as shown in Figure 6 A and B in.

[0079] 3. Influence of different conditions on the photocell stage

[0080] The conditions of the photocell stage were adjusted, and the specific grouping was as follows:

[0081] Photocell group: Step S2 in Example 1, which was a reaction condition of closed circuit and with light illumination;

[0082] Open-circuit control group: That is, the first switch was disconnected, the light source was started, and a reaction condition of open circuit and with light illumination was formed.

[0083] Dark control group: That is, the first switch was closed and it was shielded from light. Shielding from light means turning off the iodine tungsten lamp and wrapping the transparent reactor with tin foil to form a reaction condition of closed circuit and without light illumination.

[0084] After the photocell stage ended, the concentrations of target pollutants in the tailings wastewater under different grouping conditions were detected, and the removal rates were calculated. The results are as shown in Figure 7 shown.

[0085] 4. Influence of working electrodes under different conditions on the reaction

[0086] According to whether light illumination was applied to the working electrode and whether a corresponding load layer was formed on the surface of the working electrode, cyclic voltammograms and impedance analyses were plotted, as shown in Figure 8 and Figure 9 shown respectively.

[0087] 5. Taking one operation of the photocell + one operation of the photoelectrolytic cell as a cycle, after intermittent operation for five cycles, the total concentrations of ammonia nitrogen, nitrite nitrogen, nitrate nitrogen, total nitrogen, manganese ions, Fe(III), and Fe(II) in the liquid phase in the reaction system were analyzed and determined; the surface morphologies and element compositions of the photoanode and photocathode were observed and analyzed by SEM-EDS. Among them, the scanning electron microscopes SEM of the photoanode and photocathode are respectively as shown in Figure 10 A and B in, and the X-ray energy spectrum analyses EDS of the photoanode and photocathode are respectively as shown in Figure 11 A and B in; the impedance analyses and cyclic voltammograms for five cycles are respectively as shown in Figure 12 A and B in, which were used to evaluate the multi-cycle and long-term operation performance of the carbon dots / g-C3N4 carbon felt electrode (photoanode) and the Fe2MnO4 / g-C3N4 carbon felt electrode (photocathode).

[0088] Combined with Figures 2 to 7 it can be seen that under the photovoltaic cell reaction mode of S2, after reacting for 2 hours, the ammonia nitrogen content is only 0.12 mg / L ± 0.01 mg / L( Figure 2 ), meeting the Class III surface water discharge standard of GB / T 14848-2017 (ammonia nitrogen not exceeding 0.50 mg / L); at this time, the ammonia nitrogen removal rate reaches 99.3%( Figure 7 ), significantly superior to the open-circuit control with only photocatalysis (18.3% ± 2.8%) and the dark control with only current (4.3% ± 1.3%)( Figure 7 ), indicating the significant role of the circuit electrons and photocatalysis of the photovoltaic cell in removing ammonia nitrogen; in addition, the nitrite nitrogen in the system is almost negligible, while the nitrate nitrogen slightly increases from the initial 11.6 mg / L to 12.7 mg / L ± 0.2 mg / L( Figure 2 ), and the total nitrogen concentration drops from the initial 28.6 mg / L to 12.8 mg / L ± 0.8 mg / L( Figure 2 ), and the total nitrogen removal rate is 55.3% ± 0.7%( Figure 7 ), significantly higher than that of the open-circuit control (7.1% ± 2.1%) and the dark control (0.7% ± 1.0%)( Figure 7 ), indicating that while the photovoltaic cell efficiently removes and converts ammonia nitrogen in mine tailing wastewater, a small amount of nitrate nitrogen is added to the system; the nitrogen element mass balance results show that the main destination of the removed total nitrogen is gaseous N2. Combined with Figure 6 the E-pH relationship curve showing the conversion of ammonia nitrogen to nitrogen gas as shown, the photoanode potential of the photovoltaic cell is -0.352 V( Figure 4 ), which is a necessary condition for the system to achieve the conversion of ammonia nitrogen to nitrogen gas. At the same time, the manganese ion concentration drops from the initial 24.7 mg / L to 22.2 mg / L ± 0.6 mg / L( Figure 2 ), and the removal rate of manganese ions is 10.1% ± 2.4%( Figure 7 ), higher than that of the open-circuit control (5.9% ± 1.2%) and the dark control (0.8% ± 1.2%)( Figure 7 ), indicating the combined promotion effect of the circuit electrons and photocatalysis on the removal of manganese ions by the photovoltaic cell. The amount of H2O2 in the liquid phase drops from 650 μg / L at 0.5 hours of operation to 540 μg / L at 2 hours of operation, indicating that the H2O2 and OH - generated from the dissolved oxygen in the liquid phase Figure 3 may be the main reasons for the conversion of manganese ions and the increase in the liquid phase pH(

[0089] Switch the system to the photoelectrolysis reaction mode of S3 (3.0 V) and continue to operate for 12 hours. Only 0.48 mg / L ± 0.68 mg / L of manganese ions remain in the liquid phase( Figure 2), so that the total removal rate of manganese ions reaches 98.1%, indicating that the operation of the photoelectrolytic cell mainly removes manganese ions; combined with the E-pH relationship curve of manganese ions ( Figure 6 in B), the relatively negative photocathode potential (-1.093V to -0.621V) of this photoelectrolytic cell ( Figure 4 ) may be the main reason for the (partial) reduction and deposition of manganese ions on the photocathode. At the same time, the total nitrogen content of the reaction system is reduced to 7.3 mg / L ± 0.3 mg / L ( Figure 2 ), so that the total nitrogen removal rate of the effluent reaches 74.5% ± 1.0%; considering that the nitrite nitrogen hardly changes, while the nitrate nitrogen drops from 12.7 mg / L ± 0.2 mg / L at the initial stage (2 h) of the photoelectrolytic cell to 7.3 mg / L ± 0.9 mg / L at 12 h ( Figure 2 ), it shows that the total nitrogen removal in this stage is mainly attributed to the reduction and conversion of nitrate nitrogen to N2. The cyclic voltammetry analysis results show that compared with the carbon felt control, the carbon dots / g-C3N4 electrode has better oxidation performance, while the Fe2MnO4 / g-C3N4 electrode has better reduction performance ( Figure 5 ), therefore, the carbon dots / g-C3N4 anode electrode undergoes the oxidation reaction of ammonia nitrogen species, while the Fe2MnO4 / g-C3N4 cathode electrode undergoes the reduction of manganese ions, hydrogen evolution, and the reduction of dissolved oxygen to H2O2. The pH in this stage further increases to 6.56 ± 0.23 ( Figure 3 ), indicating that the deposition of metallic manganese is accompanied by the occurrence of hydrogen evolution reaction, and the hydrogen production of the system is 0.0028 m 3 / m 2 / d ± 0.0005 m 3 / m 2 / d, and the energy utilization rate of the system is about 15% - 25%. The solution conductivity of the system decreases from the initial 0.472 mS / cm to 0.435 mS / cm ( Figure 3 ). In fact, the removal of manganese ions reduces the solution conductivity, while the partial detachment of the photocathode material (such as about 0.26% of iron in the first cycle of operation) increases the conductivity. The net result of the two is the main reason for the slight decrease in the liquid phase conductivity.

[0090] Figure 8 Cyclic voltammetry analysis is shown under light and dark conditions, with and without a load layer on the photoanode and photocathode, and the results corroborate the redox performance superior to the dark control under light illumination. Figure 9The impedance analysis of the photoanode in A shows that, compared with the photoanode without light and the photoanode without a loading layer, the photoanode of the present invention (carbon dots / g-C3N4 carbon felt electrode) has a smaller ohmic internal resistance (109 Ω vs. 162 Ω and 165 Ω) and diffusion internal resistance (634 Ω vs. 734 Ω and 1040 Ω), but a larger charge transfer internal resistance (173 Ω vs. 116 Ω and 145 Ω). Similarly, Figure 9 The impedance analysis of the photocathode in B shows that, compared with the photocathode without light and the photocathode without a catalyst, the photocathode of the present invention (Fe2MnO4 / g-C3N4 carbon felt electrode) has a smaller ohmic internal resistance (122 Ω vs. 144 Ω and 165 Ω), charge transfer internal resistance (112 Ω vs. 195 Ω and 145 Ω), and diffusion internal resistance (400 Ω vs. 602 Ω and 1040 Ω).

[0091] The results of continuous operation for five cycles show that there is no significant difference in the removal of ammonia nitrogen, nitrate nitrogen, total nitrogen, and manganese ions compared with the results of one cycle of operation (p > 0.05), but the iron element shed in the liquid phase is only 0.15%, lower than 0.26% in the first cycle, and the system pH increases with the extension of the operation time ( Figure 3 ) which may be the main reason for the gradual decrease in the shedding of iron elements. Scanning electron microscope observations of the photoanode and photocathode show that there are a large number of spherical particles on the surface of the photocathode loaded with Fe2MnO4 / g-C3N4 (as Figure 10 shown in B), while the photoanode loaded with carbon dots / g-C3N4 on the surface is only covered with a thin layer of precipitate (as Figure 10 shown in A); X-ray energy spectrum analysis further corroborates that more manganese elements are generated on the surface of the photocathode loaded with Fe2MnO4 / g-C3N4 (as Figure 11 shown in B) than on the carbon dots / g-C3N4 photoanode (as Figure 11 shown in A). This result is consistent with the usual theoretical explanation that the reductive cathode recovers oxidizing manganese ions and the anode adsorbs a small amount of manganese elements. And the continuously accumulating manganese elements on the surface of the Fe2MnO4 / g-C3N4 photocathode with the extension of the operation time may prevent and reduce the shedding of the catalyst, which is also the main reason for the stable operation in the fifth cycle. As Figure 12 shown in the impedance analysis of A, the internal resistance of each part of the photoanode after five cycles of operation decreases (ohmic internal resistance: 55 Ω vs. 109 Ω; charge transfer internal resistance: 53 Ω vs. 173 Ω; diffusion internal resistance: 442 Ω vs. 634 Ω); while the internal resistance of each part of the photocathode increases to varying degrees. As Figure 12Cyclic voltammetry analysis of the photocathode, photoanode, and reference electrode shown in B indicates that the capacitances of the photoanode with carbon dots / g-C3N4 loaded on the surface and the photocathode with Fe2MnO4 / g-C3N4 loaded on the surface are improved to varying degrees after long-term operation. In summary, by switching the photoelectric cell constructed by the photoanode and photocathode to a photoelectrolytic cell, the synchronous recovery of manganese metal from mine tailing wastewater can be achieved through short-range, highly efficient, and clean deep treatment. The process is clean and pollution-free, with multiple benefits in terms of environment and ecology, society, and economy.

[0092] The above-described embodiments merely represent several implementation manners of the present invention, and the description thereof is relatively specific and detailed. However, it should not be construed as a limitation to the scope of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can still be made, and these all fall within the protection scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the appended claims.

Claims

1. A method for resource treatment of tailings wastewater and simultaneous recovery of valuable metals, characterized in that, The method includes the following steps: Construct a reaction system, which includes a transparent reactor with a built-in working electrode, a photovoltaic cell circuit system and a photoelectrolysis circuit system respectively connected to the working electrode, and a light source externally disposed on the transparent reactor; the photovoltaic cell circuit system and the photoelectrolysis circuit system are switched in parallel, the photovoltaic cell circuit system has no power supply, and the photoelectrolysis circuit system is provided with a power supply; Tailings wastewater is introduced into the transparent reactor and sealed; the light source is turned on, and only the photovoltaic cell circuit system is turned on for photovoltaic cell reaction, which is used to oxidize and convert ammonia nitrogen into nitrogen and nitrate nitrogen and recover part of the manganese metal; The light source is turned on, and only the photoelectrolysis circuit system is started, and the photovoltaic cell reaction is converted into a photoelectrolysis reaction, which is used to directly or indirectly reduce manganese ions to recover manganese metal and reduce and convert nitrate nitrogen into nitrogen.

2. The method for resource treatment of tailings wastewater and simultaneous recovery of valuable metals according to claim 1, wherein The working electrode includes a photoanode and a photocathode; the materials of the photoanode and the photocathode are porous carbon felts with load layers respectively provided on the surfaces; Among them, the load layer on the surface of the photocathode is a metal oxide and graphitic carbon nitride composite; the load layer on the surface of the photoanode is a carbon dot and graphitic carbon nitride composite.

3. The method for resource treatment of tailings wastewater and simultaneous recovery of valuable metals according to claim 2, wherein The photovoltaic cell circuit system includes a first switch, a first ammeter and a first external resistor, and the photovoltaic cell circuit system is electrically connected to the photoanode and the photocathode; the light source, the transparent reactor, the photovoltaic cell circuit system and the photoanode and the photocathode form a photovoltaic cell reaction system; The photoelectrolysis circuit system includes a second switch, a second ammeter, a power supply and a second external resistor, and the photoelectrolysis circuit system is electrically connected to the photoanode and the photocathode; the light source, the transparent reactor, the photoelectrolysis circuit system and the photoanode and the photocathode form a photoelectrolysis reaction system.

4. The method for resource treatment of tailings wastewater and simultaneous recovery of valuable metals according to claim 2, characterized in that, The metal oxide is selected from at least one of tungsten molybdenum oxide, iron manganese oxide, zinc ferrite, and silver phosphate.

5. The method for resource treatment of tailings wastewater and simultaneous recovery of valuable metals according to claim 2, characterized in that, The loading amount of the loading layer on the surface of the photoanode and photocathode is 0.1 mg / cm 2 ~20 mg / cm 2 .

6. The method for resource treatment of tailings wastewater and simultaneous recovery of valuable metals according to claim 2, wherein In the load layer of the photoanode, the mass ratio of the metal oxide to graphitic carbon nitride is 10:1 to 1:10; In the load layer of the photocathode, the mass ratio of the carbon dots to graphitic carbon nitride is 10:1 to 1:

10.

7. The method for resource treatment of tailings wastewater and simultaneous recovery of valuable metals according to claim 3, characterized in that, The first external resistor is 100Ω to 2000Ω; When the photovoltaic cell circuit system operates, the potential of the photoanode is -0.4V to -0.3V, and the potential of the photocathode is 0.2V to 0.3V; The operating time of the photovoltaic cell reaction is 1.5h to 2.5h.

8. The method for resource treatment of tailings wastewater and simultaneous recovery of valuable metals according to claim 3, characterized in that, The second external resistor is 10Ω to 100Ω; The voltage applied by the power supply is 0.3V to 5.0V; When the photoelectrolysis circuit system operates, the potential of the photoanode is 0.1V to 2.5V, and the potential of the photocathode is -2.5V to -0.4V; The operating time of the photoelectrolysis reaction is 9h to 11h.

9. The method for resource treatment of tailings wastewater and simultaneous recovery of valuable metals according to claim 1, characterized in that, The reaction system further includes a reference electrode and a gas collecting pipe for collecting gas.

10. The method for resource treatment of tailings wastewater and simultaneous recovery of valuable metals according to claim 1, characterized in that, The light source intensity is greater than 5.0 mW / cm 2 .

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