Method for treating Cr(III) in wastewater

Abstract

The application discloses a treatment method of Cr(III) in wastewater, wherein Mg(NO3)2 solution is added into the wastewater under stirring, a mixed solution of Na2CO3 and NaOH is used to adjust the pH to 10-11, and the product is separated; when the Cl ‑ concentration is less than 8-12 mM, the concentration is supplemented, electrolysis is carried out under stirring, a DSA chlorine electrode is used as an anode, and carbon felt is used as a cathode; Mg(NO3)2 solution is introduced after electrolysis for 1-2 min, the introduction amount of Mg(NO3)2 solution per 1 mM of total chromium is 0.025-0.03 mM / min, the pH is adjusted to 10-11 during the process, and the total chromium concentration in the filtrate is lower than 20 mg / L; the solid product is ultrasonically treated in pure water, and then hydrothermally synthesized at constant temperature, the obtained product is calcined in a protective atmosphere, a magnesium-chromium layered double metal oxide with dispersed biochar on the surface is obtained, and the double metal oxide is used to adsorb residual trivalent chromium and un-reduced hexavalent chromium generated by anode oxidation.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of heavy metal wastewater treatment, and particularly relates to a treatment method of Cr(III) in wastewater. BACKGROUND

[0002] Chromium belongs to heavy metal pollutants that need to be controlled, so currently organic industrial wastewater containing chromium needs to be treated separately and discharged at the workshop discharge outlet after reaching the standard. Heavy metal chromium can be pollutant resourceization, but the total chromium content of the external discharge is still huge so far.

[0003] Chromium in organic industrial wastewater usually forms a complex with organic pollutants. Due to the differences in types and molecular weights of organic pollutants, and the structural complexity of these complexes with multi-ligands and high polymerization states, multi-dimensional stability breaking bottlenecks and subsequent wastewater treatment instability are caused. The existing pretreatment method of chromium-containing wastewater is mainly physical and chemical coagulation sedimentation, which brings the secondary pollution problem of physical and chemical sludge, and the complex Cr(III) presents an organic matter wrapped state, which is difficult to be effectively captured and precipitated by coagulation technology. In the oxidation regulation process of complex Cr(III), pollutants are difficult to be completely mineralized, and if rapid separation of Cr(III) cannot be achieved, the oxidation by-products are easy to re-complex with Cr(III) to form stable complex Cr(III).

[0004] Taking the leather industry as an example, after several cycles of chromium-containing wastewater reuse, the total chromium concentration accumulates to 1.0-2.0 g / L, and the proportion of accumulated ineffective complex Cr(III) increases to more than 50%, which greatly restricts the treatment of chromium-containing wastewater. The total chromium concentration in the existing pretreated effluent is still much higher than the discharge standard of ≤1.5 mg / L. SUMMARY

[0005] In view of the problems in the prior art, the application provides a treatment method of Cr(III) in wastewater, which is green, energy-saving and efficient, can control the total chromium concentration in wastewater to be below 0.8 mg / L, which is much lower than the discharge requirement of 1.5 mg / L, and the obtained magnesium-chromium layered double metal oxide with surface dispersed biochar realizes the resource utilization of chromium.

[0006] The application is realized by the following technical scheme:

[0007] A treatment method of Cr(III) in wastewater, comprising the following steps:

[0008] S1, under stirring, Mg(NO3)2 solution is added to the wastewater to be treated, wherein Mg 2+ reacts with Cr 3+molar ratio of (2-3):(1-2) to obtain a mixed solution, then a mixed solution of Na2CO3 and NaOH is added to the mixed solution to make the pH of the obtained mixed system 10-11 to obtain mixed system a, and after the product in the mixed system a is separated, solid a and filtrate a are obtained;

[0009] S2, when the Cl - concentration is less than 8-12 mM, Cl - concentration is ≥8-12 mM to obtain mixed solution a, and the filtrate a or the mixed solution a is electrolyzed under stirring, wherein a DSA chlorine evolution electrode is used as an anode and a carbon felt is used as a cathode, Mg(NO3)2 solution is introduced after electrolysis for 1-2 min, the introduction amount of Mg(NO3)2 per 1 mM of total chromium in the filtrate a or the mixed solution a is 0.025-0.03 mM / min, and the pH is adjusted to 10-11 by using a mixed solution of Na2CO3 and NaOH during electrolysis, so that the total chromium concentration in the filtrate b obtained by separating the product s of the obtained mixed system is less than 20 mg / L;

[0010] S3, the product s and the solid a are ultrasonically treated in pure water and then subjected to a hydrothermal synthesis reaction at a constant temperature, the obtained product m is calcined at 350-450°C for 3.5-4.5 h under a protective atmosphere to obtain a magnesium-chromium layered double metal oxide with dispersed biochar on the surface, and the double metal oxide is used to adsorb residual trivalent chromium in the filtrate b and hexavalent chromium not reduced at the cathode during anodic oxidation in S2, thereby completing the treatment of Cr(III) in wastewater.

[0011] Preferably, in S1, 0.75 mol / L Mg(NO3)2 solution is added to the wastewater to be treated under stirring at 300-500 rpm, and the addition rate is 5 mL / min.

[0012] Preferably, in S1 and S2, the molar ratio of Na2CO3 to NaOH in the mixed solution of Na2CO3 and NaOH is 1:8, and both are added dropwise at a dropwise addition rate of 0.5 mL / min.

[0013] Preferably, in S1, the mixed system a is stirred for 5-15 min, then left to stand for 25-35 min, and then qualitative filter paper is used for suction filtration to obtain the filtrate a, and the obtained product is washed with pure water and suction filtered 3-4 times to obtain the solid a.

[0014] Preferably, the electrolysis in S2 is carried out in a cuboid electrolytic cell with a rotor placed in the middle of the bottom, and the anodes and cathodes are in the form of sheets with the same shape and size, with 3 anodes and 4 cathodes, the anodes and cathodes being placed alternately and vertically, each anode being placed in the middle of two cathodes, the two outermost cathodes being symmetrical along the length of the electrolytic cell, the bottoms of all the electrodes being flush and leaving a gap with the bottom of the electrolytic cell, the front and back ends of all the electrodes being flush, and the upper ends of all the electrodes being flush with the liquid level, the ratio of the volume of the filtrate a or the mixed liquid a in the electrolytic cell to the volume of the cuboid surrounded by all the electrodes being ≤1.5.

[0015] Preferably, the inter-electrode current density for every 1 mM of total chromium in the filtrate a or the mixed liquid a during the electrolysis in S2 is 1-2 mA / cm 2 .

[0016] Further, when the total chromium concentration in the filtrate a is greater than 100 mg / L and less than 2000 mg / L, the electrolysis is continued for 55-65 min to obtain the mixed system b1, and when the total chromium concentration in the filtrate a is less than 100 mg / L, the electrolysis is continued for 15-25 min to obtain the mixed system b2, and after the products in the mixed system b1 and the mixed system b2 are separated, the solid b1, the filtrate b1, the solid b2 and the filtrate b2 are obtained.

[0017] If the total chromium concentration in the filtrate b2 is greater than 20 mg / L, the filtrate b2 is treated according to the process of the mixed system b2 until the total chromium concentration is less than 20 mg / L to obtain the solid b3 and the filtrate b3, and the filtrate b1 is treated according to the case where the total chromium concentration in the filtrate a is less than 100 mg / L to obtain the solid b4 and the filtrate b4.

[0018] In S3, the solid a, the solid b1 and the solid b4 are ultrasonically treated in pure water and then subjected to a hydrothermal synthesis reaction at a constant temperature, or the solid a, the solid b2 and the solid b3 are ultrasonically treated in pure water and then subjected to a hydrothermal synthesis reaction at a constant temperature.

[0019] Preferably, the ultrasonic treatment in S2 is carried out at 700-800 W for 15-25 min, followed by a hydrothermal synthesis reaction at 115-125 °C under a sealed condition.

[0020] Further, the hydrothermal synthesis reaction is carried out for 22-26 h, the solid is washed, dried and then ground to obtain the product m.

[0021] Preferably, after the bicomponent oxide in S3 adsorbs the residual trivalent chromium in the filtrate b and the hexavalent chromium not reduced in the cathode in the anodic oxidation in S2, the bicomponent oxide is separated, cleaned, and finally calcined under N2 atmosphere at 350-450 DEG C for 3.5-4.5 h to obtain the magnesium-chromium layered bicomponent oxide with the surface dispersed with biochar, which can continue to be adsorbed according to the corresponding process in S3, and the process is repeated, and the magnesium-chromium layered bicomponent oxide with the surface dispersed with biochar is recycled for 6-8 times.

[0022] Compared with the prior art, the present application has the following beneficial technical effects:

[0023] The present application is a method for treating Cr(III) in wastewater, which realizes the removal of trivalent chromium in chromium-containing wastewater by the orderly combination of Mg 2+ As a source of divalent metal ions, Cr 3+ The precipitate is obtained under the environment with pH of 10-11, realizing the complete removal of trivalent chromium in chromium-containing wastewater. The DSA chlorine evolution electrode as the anode material is beneficial to promote the generation of active chlorine oxide, and has long service life. Through anodic oxidation, Cl - The substitution caused by the loss of electrons and the oxidation of chlorine-containing oxide promote the oxidation of complex trivalent chromium in the filtrate or mixed solution to form hexavalent chromium anion of chromate or dichromate which cannot be complexed with organic matter; the carbon felt as the cathode material can maximize the inhibition of cathodic hydrogen evolution reaction, and can reduce the hexavalent chromium generated by anodic oxidation to ionic trivalent chromium, and convert more than 95% of the complex trivalent chromium into ionic trivalent chromium. The trivalent chromium is combined with dynamically added Mg(NO3)2, and the excess trivalent chromium is complexed to form organic complex Cr(III). The solid product is magnesium and chromium hydroxide, which is further synthesized into functional material magnesium-chromium layered bicomponent hydroxide (MgCr-LDHs) through ultrasonic and hydrothermal synthesis, and is carbonized into BC-MgCr-LDO with the surface dispersed with nano or micron particles of biochar, which can adsorb the residual trivalent chromium and a small amount of hexavalent chromium not reduced in the cathode generated by anodic oxidation, realizing the efficient removal of heavy metal Cr in wastewater and the reduction of hazardous waste of heavy metal Cr. In other wastewater, BC-MgCr-LDO can also adsorb hexavalent chromium and a part of organic matter such as phenol, antibiotics or protein, realizing the resource utilization and recovery of heavy metal chromium, and avoiding the generation of chromium-containing hazardous waste.

[0024] Further, the BC-MgCr-LDO after adsorption can be recycled through calcination and carbonization under N2 atmosphere, and the highest utilization efficiency can reach 8 times. Subsequently, the biochar and magnesium ions and chromium metal ions can be separated through acid leaching, and the metal magnesium ions and chromium ions can be further purified and separated. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 Flow chart for the process described in Example 1 of the present invention;

[0026] Figure 2a Schematic diagram of the arrangement of the electrodes described in the present invention in an electrolytic cell;

[0027] Figure 2b Schematic diagram of the electrode shape described in the present invention;

[0028] Figure 3 Scanning electron microscope image of the magnesium chromium layered double hydroxide (MgCr-LDHs) obtained in Example 1 of the present invention;

[0029] Figure 4 Scanning electron microscope image of the composite material of biochar and magnesium chromium layered double hydroxide (BC-MgCr-LDO) obtained in Example 1 of the present invention;

[0030] Figure 5 Cycle application total chromium concentration data graph of the BC-MgCr-LDO obtained in Example 1 of the present invention.

[0031] Figure: 1-electrode, 2-electrode wire interface, 3-rotor, 4-liquid inlet pipe, 5-water inlet pipe, 6-water outlet pipe, 7-electrolytic cell. DETAILED DESCRIPTION

[0032] The present invention will be further described in detail below with reference to the accompanying drawings and specific examples, which are explanatory of the present invention rather than limiting.

[0033] The present invention is a method for treating and resource utilization of complex trivalent chromium in wastewater, see Figure 1 , comprising the following steps:

[0034] (1) First take 200 mL of organic industrial wastewater, determine the total chromium concentration and hexavalent chromium concentration therein, and obtain the trivalent chromium concentration. Subsequently, adjust the pH of the remaining wastewater to 8.5-9.0 by NaOH solution, stir for 2 min, then stand for 30 min, and again take the supernatant to determine the residual total chromium concentration and hexavalent chromium concentration therein, and obtain the residual trivalent chromium concentration. By the difference between the two trivalent chromium concentrations, the concentrations of ionic trivalent chromium and complex trivalent chromium in the wastewater are obtained by difference.

[0035] (2) Take another 1 L of the above organic industrial wastewater in beaker R-1, according to the concentration of ionic Cr(III) obtained in (1), add 0.75 mol / L Mg(NO3)2·6H2O aqueous solution to R-1 at a rate of 5 mL / min under stirring conditions of 300-500 rpm, to ensure that the final Mg 2+ and Cr 3+The molar ratio of Na2CO3 and NaOH is (2-3):(1-2). Subsequently, the pH of R-1 is adjusted to 10-11 by continuously adding mixed solution A of Na2CO3 and NaOH under stirring at a rate of 0.5 mL per minute using a rubber bulb dropper. The molar ratio of Na2CO3 and NaOH is 1:8. The pH should be adjusted slowly and uniformly to ensure the proper combination of magnesium and chromium hydroxide. After adjusting the pH, continue stirring for 10 minutes and standing for 30 minutes. Finally, use qualitative filter paper to filter, take the filtrate as W-1, and wash the precipitate on the filter paper with pure water three times. Then, use 10 mL of pure water to transfer the precipitate to a beaker using the force of the water, and label it as S-1.

[0036] (3) Take 5 mL of W-1 obtained in step (2) to determine the total chromium concentration (less than 2000 mg / L). All the trivalent chromium in it is complexed trivalent chromium. Then, transfer the remaining W-1 through the water inlet pipe 5 to the rectangular electrolytic cell R-2 (electrolytic cell 7 in the figure). The electrolytic cell R-2 is 22 cm long and 9 cm high. A rotor 3 is placed in the middle of the bottom. The water inlet pipe 5 is opened at the bottom of one side of the electrolytic cell R-2. The water outlet pipe 6 is opened at the upper end of the other side. The lower end of the water outlet pipe 6 is flush with the liquid level. The Cl - concentration in the wastewater should be ≥10 mM. If not, add a certain amount of NaCl to supplement it. The electrode 1 is in the shape of a rectangular sheet structure with a thickness of 1.5 mm. One corner extends vertically upward to form a rectangular protrusion. The protrusion has an electrode wire interface 2. The electrode 1 is 7.5 cm long (excluding the protrusion) and 5 cm wide. Figure 2b Figure 2a ​As shown, the anode 3 blocks, cathode 4 blocks, anode and cathode are placed alternately, the spacing is 3 cm, the cathode is more than one block, so the three anodes are respectively located in the middle position of the two cathodes, the bottom of all electrodes 1 is flush and has a spacing with the bottom of the electrolytic cell R-2, the upper end of all electrodes 1 (not including the protrusion) is flush with the liquid level, the front and rear ends of all electrodes 1 are also flush, the bottom of the protrusion is also flush with the liquid level, the outer wall of one of the two cathodes on the outermost side is symmetrical along the length direction of the electrolytic cell, and the spacing between the inner wall of the electrolytic cell R-2 and the outer wall of the cathode is 1.5 cm, the ratio of the volume V1 of the electrolytic cell (the volume of W-1) to the volume V2 of the electrode working area (the cuboid area formed by the 7 electrodes 1 as a whole, excluding the protrusion) is ≤1.5, the DSA chlorine evolution electrode is used as the anode material, specifically the DSA electrode (chlorine evolution: ruthenium dioxide and iridium dioxide), which is beneficial to promote the generation of active chlorine oxide and has a long electrode life, and the carbon felt is used as the cathode material, which can maximize the inhibition of the cathode hydrogen evolution reaction, and at the same time, the hexavalent chromium generated by the anode oxidation is reduced to ionic trivalent chromium, and at the same time, according to the total chromium concentration in W-1, the W-1 is kept in a mixed state under the condition of 800 rpm stirring, the power supply is connected to the three anodes and four cathodes as a whole, respectively, under the condition of power supply, the electrode current density applied to each 1 mM total chromium (TCr) is 1-2 mA / cm 2 The specific current can be determined according to the total chromium concentration and the cross-sectional area of the electrode working area, the pH change is monitored every 2-3 min, and the mixed solution A is used to continue to control the pH range to be 10-11, and the electrolysis is continued until the end. After power on for 1-2 min, a peristaltic pump is used to add 0.75 mol / L Mg(NO3)2·6H2O solution to the electrolytic cell at a constant speed, a liquid inlet pipe 4 is installed above the water inlet pipe 5, and the water inlet pipe 5 is also at the bottom of the electrolytic cell R-2, the dosage flow of 1 mM total chromium (TCr) Mg(NO3)2·6H2O is 0.025-0.03 mM / min, and the specific dosage can be determined according to the total chromium concentration, and the electrolysis is continued for 60 min. After power off, continue to stir for 10 min, and stand for 30 min, finally use qualitative filter paper to filter, take the filtered liquid as W-2, wash the precipitate on the filter paper with pure water for three times, then use 10 ml of pure water to transfer to a beaker by force, and mark it as magnesium and chromium hydroxide compound S-2.

[0037] Through anode oxidation, combined with Cl - Substitution caused by loss of electrons and oxidation of chlorine-containing oxides, which promotes the oxidation of complex trivalent chromium in W-1 to form hexavalent chromium anion of chromate or dichromate that cannot be complexed with organic matter; hexavalent chromium anion is reduced to form trivalent chromium near the inert cathode, which can convert more than 95% of complex trivalent chromium to ionic trivalent chromium, trivalent chromium combines with dynamically added Mg(NO3)2·6H2O, and the excess trivalent chromium is complexed again to form organic complex Cr(III).

[0038] It should be noted that if the total chromium concentration of W-2 needs to be lower than 100 mg / L at this time, this step can be directly skipped and (4) is directly performed. In order to illustrate all cases clearly, the case of being higher than 100 mg / L is illustrated here.

[0039] (4) Take 5 ml of W-2 obtained in step (3) to determine the total chromium concentration (lower than 100 mg / L), in which all the trivalent chromium is complex trivalent chromium, and then transfer all the W-2 to the electrolytic cell R-3, and the specific process is the same as step (3), only the power-on time is reduced to 20 min. After power-off, continue to stir for 10 min to convert more than 90% of the complex trivalent chromium into ionic trivalent chromium, and stand for 30 min. Finally, use qualitative filter paper to filter, take the filtrate and mark it as W-3, and wash the precipitate on the filter paper with pure water for three times, then use 10 ml of pure water to transfer it to a beaker by force and mark it as magnesium and chromium precipitate S-3. It should be noted that when (6) is performed later, the total chromium concentration needs to be lower than 20 mg / L, therefore, if the total chromium concentration is still greater than 20 mg / L after this step, it needs to be performed several times according to the total chromium concentration until the total chromium concentration is lower than 20 mg / L, and the last filtrate is taken and the magnesium and chromium precipitate is combined. For the sake of convenience, one performance is illustrated here.

[0040] (5) Mix S-1, S-2, and S-3 obtained in steps (2), (3), and (4) in a beaker and perform ultrasonic treatment for 20 min under a power of 750 W. Then transfer to a polytetrafluoroethylene reaction kettle and hydrothermally synthesize at 120°C for 24 h under sealed conditions. After completion, wash the solid with anhydrous ethanol and deionized water alternately for three times, and place it in a 80°C vacuum drying box for drying for 12 h. Take it out and grind to obtain magnesium-chromium layered double hydroxide (MgCr-LDHs). As shown in Figure 3 , the obtained MgCr-LDHs has a significant sheet structure and good crystallinity, and the surface is wrapped with a layer of partial organic matter due to adsorption, which is the pollutant in the original wastewater. Therefore, the obtained hydrotalcite MgCr-LDHs is calcined and carbonized in a tube furnace under N2 atmosphere at 400±50°C for 4±0.5 h to obtain a composite material of biochar and magnesium-chromium layered double metal oxide (BC-MgCr-LDO), as shown in Figure 4 , the biochar is in the form of nano or micrometer particles and is distributed in a dispersed state on the surface of the magnesium-chromium layered double metal oxide.

[0041] (6) Take 5 ml of W-3 obtained in step (4) to measure the total chromium concentration (less than 20 mg / L), in which all the trivalent chromium is complex trivalent chromium, and then transfer all of W-3 to beaker R-4. Add BC-MgCr-LDO material (the concentration of the added is 5 g / L) to R-4, BC-MgCr-LDO is restored to BC-MgCr-LDHs, and is stirred at a speed of 200 rpm for 4 h, further adsorbing the residual trivalent chromium in W-3 and a small amount of hexavalent chromium generated by anode oxidation which is not reduced in the cathode, at this time the total chromium concentration of W-4 is less than 0.8 mg / L, and the BC-MgCr-LDHs after adsorption saturation are collected by suction filtration and washing, and then BC-MgCr-LDO can be formed again by carbonization under N2 atmosphere at 400±50℃ for 4±0.5h, which can be recycled. In addition, BC-MgCr-LDO can also be used as an adsorption material for hexavalent chromium generated in other processes, as well as trace amounts of phenol, antibiotics and protein pollutants; at the same time, BC-MgCr-LDO can also be used to obtain a solution containing magnesium ions and chromium ions by hydrochloric acid leaching, and the separation and recovery of magnesium ions and chromium ions can be realized by further purification process.

[0042] Example 1

[0043] Treatment of chromium-containing wastewater from a tanning enterprise

[0044] The tanning enterprise mainly processes fur and leather integrated products, and needs to use heavy metal chromium for tanning in the processing process. In addition to heavy metal chromium, polyphenol, polyacrylic acid, citric acid and other chemicals are also added. The mechanism of the tanning process is to connect the skin collagen and chemicals through covalent and non-covalent bonds by the hydroxyl bridge coordination of chromium, to give the leather product specific properties. The hydrolyzed collagen shed in the process is easily crosslinked with the chromium and other chemicals that have not been completely combined to form stable complex Cr(III) wastewater.

[0045] The method of the application comprises the following processes:

[0046] (1) First, the total chromium concentration in the tanning chromium-containing wastewater is measured to be 2273 mg / L, in which 964 mg / L is ionic trivalent chromium, 1309 mg / L is complex trivalent chromium, and no hexavalent chromium is detected.

[0047] (2) 1 L of tannage wastewater containing chromium was placed in beaker R-1, 50 mL of 0.75 mol / L Mg(NO3)2·6H2O aqueous solution was slowly added to R-1 at a rate of 5 mL / min under a stirring rate of 500 rpm, then the mixed solution A of Na2CO3 and NaOH was slowly added dropwise to adjust the pH to 10 at a rate of 10 drops (0.5 mL) per minute using a rubber bulb dropper, stirring was continued for 10 min, and standing for 30 min, then the supernatant was extracted by suction filtration using qualitative filter paper and was recorded as TW-1, and the extraction was repeated three times by washing with pure water, and the precipitate was taken as TS-1. The residual total chromium concentration in TW-1 was 1147 mg / L.

[0048] (3) TW-1 was taken and placed in electrolytic cell R-2, 3 anode electrodes and 4 cathode electrodes were used, W-1 was kept in a mixed state under a stirring condition of 800 rpm, the mixed solution A of Na2CO3 and NaOH was used to continue to control the pH to 10, and a current of 6.5 A was applied. After 2 min of power-on, 0.75 mol / L Mg(NO3)2·6H2O aqueous solution was slowly added to R-2 at a rate of 1 mL / min by using a peristaltic pump. After 60 min of reaction, the power was turned off, stirring was continued for 10 min, and standing for 30 min, finally suction filtration was performed using qualitative filter paper, and the total chromium concentration in the supernatant TW-2 was measured to be 99.72 mg / L, and the extraction was repeated three times by washing with pure water, and the precipitate was taken as TS-2.

[0049] (4) The electrolytic cell R-3 had the same size and electrode number as R-2, and a current of 0.6 A was applied. After 2 min of power-on, 0.075 mol / L Mg(NO3)2·6H2O aqueous solution was slowly added to R-3 at a rate of 2.5 mL / min by using a peristaltic pump. After 20 min of reaction, the power was turned off, stirring was continued for 10 min, and standing for 30 min, finally suction filtration was performed using qualitative filter paper, and the total chromium concentration in the supernatant TW-3 was measured to be 6.45 mg / L, and the extraction was repeated three times by washing with pure water, and the precipitate was taken as TS-3.

[0050] (5) TS-1, TS-2, and TS-3 were mixed uniformly in a beaker, and ultrasonic treatment was performed at a power of 750 W for 20 min. Then, under the condition of airtightness in a polytetrafluoroethylene reaction kettle, hydrothermal synthesis was performed at 120°C for 24 h, then the solid was washed three times alternately with anhydrous ethanol and deionized water, and was placed in a vacuum drying box at 80°C for drying for 12 h, and then was taken out and ground to obtain MgCr-LDHs, wherein the surface was wrapped with a layer of partially organic matter due to adsorption, and MgCr-LDHs were calcined and carbonized at 400°C for 4 h under N2 atmosphere to obtain 12.6 g of BC-MgCr-LDO composite material.

[0051] (6) Put 100 mL of TW-3 solution into beaker R-4, add 0.5 g of BC-MgCr-LDO composite material, stir at a speed of 200 rpm for 4 h, and the total chromium concentration in the effluent TW-4 is 0.24 mg / L at the end. The participating material in the stirring R-4 is collected by suction filtration and washing, and then carbonized at 400 ℃ for 4 h under N2 atmosphere, and repeatedly applied to 100 mL of TW-3 wastewater, and the cycle is applied for 8 times, and the results are shown in Figure 5 As shown in the table, the total chromium concentration in the effluent TW-4 is still less than 1.5 mg / L.

[0052] Example 2

[0053] Treatment of chromium-containing dyeing wastewater of a tannery

[0054] The tannery mainly processes cowhide products, and the leather needs to be dyed after the tanning section. Part of the chromium added in the tanning process is not combined stably, and the unstable chromium falls off into the dyeing wastewater together with part of the dyes.

[0055] The method of the application comprises the following processes:

[0056] (1) The total chromium concentration in the chromium-containing dyeing wastewater is measured to be 152.6 mg / L, of which 35.8 mg / L is ionic trivalent chromium, 116.8 mg / L is complex trivalent chromium, and no hexavalent chromium is detected.

[0057] (2) Put 1 L of chromium-containing dyeing wastewater into beaker R-1, and add 1.9 mL of 0.75 mol / L Mg(NO3)2·6H2O aqueous solution at a rate of 5 mL / min under a stirring rate of 500 rpm, then slowly drop the mixed solution of Na2CO3 and NaOH at a rate of 10 drops per minute using a rubber bulb dropper to adjust the pH to 10, continue to stir for 10 min, stand for 30 min, then extract the supernatant RW-1 by suction filtration using qualitative filter paper, wash the suction filtration three times with pure water, take the precipitate RS-1, and the residual total chromium concentration in RW-1 is 97.2 mg / L, which has been reduced to below 100 mg / L, so it is directly put into (4).

[0058] (4) Put RW-1 into the electrolytic cell R-3, use 3 anodes and 4 cathodes, keep W-1 in a mixed state under the condition of stirring at 800 rpm, and continue to control the pH to be 11 with the mixed solution A of Na2CO3 and NaOH, and apply a current of 0.6 A. After 2 min of power-on, gradually add 0.075 mol / L aqueous Mg(NO3)2·6H2O solution to R-2 at a rate of 2.5 ml / min through a peristaltic pump. After 20 min of reaction, extract the supernatant by qualitative filter paper suction filtration, and measure the colority to be reduced from the initial 2000 of RW-1 to 25 by the platinum-cobalt standard colorimetric method. The total chromium concentration in the supernatant is 28.9 mg / L, which exceeds 20 mg / L. Then continue the above process in the electrolytic cell R-3 under the same conditions, and after 20 min of continuous reaction, extract the supernatant by qualitative filter paper suction filtration, and measure the total chromium concentration in the supernatant to be 16.7 mg / L. The supernatant is recorded as RW-2, and the precipitate on the filter paper is repeatedly washed with pure water and suction filtered three times. The combined precipitate is recorded as RS-2.

[0059] (5) Mix RS-1 and RS-2 uniformly in a beaker, and perform ultrasonic treatment under a power of 750 W for 20 min. Then perform hydrothermal synthesis at 120℃ under constant temperature and sealed conditions in a polytetrafluoroethylene reaction kettle for 24 h. Then wash the solid with anhydrous ethanol and deionized water alternately for three times, and place it in a vacuum drying box at 80℃ for drying for 12 h. Take it out and grind to obtain MgCr-LDHs, wherein the surface is wrapped with a layer of partially organic matter due to adsorption. Carbonize MgCr-LDHs at 400℃ under N2 atmosphere for 4 h to obtain 0.9 g of BC-MgCr-LDO composite material.

[0060] (6) Put 100 mL of RW-2 solution into beaker R-4, add 0.5 g of BC-MgCr-LDO composite material, and stir at a speed of 200 rpm for 4 h. At the end of the process, the total chromium concentration in the water RW-3 is 0.76 mg / L.

Claims

1. A method for treating Cr(III) in wastewater, characterized by, The method comprises the following steps: S1, under stirring, adding Mg(NO3)2 solution into the wastewater to be treated, wherein the molar ratio of Mg 2+ to Cr 3+ in the wastewater is (2-3):(1-2), to obtain a mixed solution, then adding a mixed solution of Na2CO3 and NaOH into the mixed solution, adjusting the pH of the obtained mixed system to 10-11 to obtain a mixed system a, separating the product in the mixed system a to obtain a solid a and a filtrate a; S2, when the filtrate a Cl - concentration of less than 8-12 mM, supplement Cl - concentration of 8-12 mM, get mixed liquid a, filtrate a or mixed liquid a in the state of stirring electrolysis, wherein the DSA chlorine electrode as anode, carbon felt as cathode, electrolysis 1-2 min after the introduction of Mg(NO3)2 solution, filtrate a or mixed liquid a in each 1 mM total chromium Mg(NO3)2 introduction amount of 0.025-0.03 mM / min, electrolysis process using Na2CO3 and NaOH mixed solution to adjust pH to 10-11, make the product s separation after the mixed system of filtrate b in the total chromium concentration is less than 20 mg / L; S3, the product s and solid a are ultrasonically treated in pure water, and then subjected to hydrothermal synthesis reaction at constant temperature, and the obtained product m is calcined at 350-450 ℃ for 3.5-4.5 h under a protective atmosphere to obtain a magnesium-chromium layered double metal oxide with surface-dispersed biochar, and the double metal oxide is used to adsorb residual trivalent chromium in the filtrate b and hexavalent chromium not reduced in the cathode in S2, thereby completing the treatment of Cr(III) in the wastewater.

2. The method for treating Cr(III) in wastewater according to claim 1, characterized in that, S1, under stirring at 300-500 rpm, 0.75 mol / L Mg(NO3)2 solution is added to the wastewater to be treated at a rate of 5 mL / min.

3. The method of claim 1, wherein the method is characterized by, In the mixed solution of Na2CO3 and NaOH in S1 and S2, the molar ratio of Na2CO3 to NaOH is 1:8, and both are added dropwise at a rate of 0.5 mL per minute.

4. The method for treating Cr(III) in wastewater according to claim 1, wherein S1, the mixed system a is stirred for 5-15 min, then is left standing for 25-35 min, and then is filtered with qualitative filter paper to obtain the filtrate a, and the obtained product is washed with pure water and filtered 3-4 times to obtain the solid a.

5. The method for treating Cr(III) in wastewater according to claim 1, wherein The electrolysis in S2 is carried out in a cuboid electrolytic cell, a rotor is placed in the middle of the bottom of the electrolytic cell, the anode and the cathode are in the form of sheets and have the same shape and size, there are 3 anodes and 4 cathodes, the anodes and the cathodes are alternately and vertically placed, each anode is located in the middle position between two cathodes, the outermost two cathodes are symmetrical along the length direction of the electrolytic cell, the bottoms of all the electrodes are flush and have a spacing with the bottom of the electrolytic cell, the front and back ends of all the electrodes are flush, and the upper ends of all the electrodes are flush with the liquid level, and the ratio of the volume of the filtrate a or the mixed solution a in the electrolytic cell to the volume of the cuboid surrounded by all the electrodes is ≤1.

5.

6. The method of claim 1, wherein the wastewater is a wastewater from a steelmaking process. The electrode intercurrent density of each 1 mM total chromium in filtrate a or mixed solution a is 1-2 mA / cm2 during electrolysis in S2. 2 .

7. The method of claim 6, wherein the method is characterized by, In S2, when the total chromium concentration in the filtrate a is greater than 100 mg / L and less than 2000 mg / L, the electrolysis is continuously carried out for 55-65 min to obtain the mixed system b1, and when the total chromium in the filtrate a is less than 100 mg / L, the electrolysis is continuously carried out for 15-25 min to obtain the mixed system b2, and after the products in the mixed system b1 and the mixed system b2 are separated, the solid b1, the filtrate b1, the solid b2 and the filtrate b2 are obtained. If the total chromium concentration in the filtrate b2 is greater than 20 mg / L, the filtrate b2 is treated according to the process of the mixed system b2 until the total chromium concentration is less than 20 mg / L to obtain the solid b3 and the filtrate b3, and the filtrate b1 is treated according to the case that the total chromium in the filtrate a is less than 100 mg / L to obtain the solid b4 and the filtrate b4. In S3, the solid a, the solid b1 and the solid b4 are ultrasonically treated in pure water, and then subjected to hydrothermal synthesis reaction at constant temperature, or the solid a, the solid b2 and the solid b3 are ultrasonically treated in pure water, and then subjected to hydrothermal synthesis reaction at constant temperature.

8. The method for treating Cr(III) in wastewater according to claim 1, wherein The ultrasonic treatment in S2 is carried out at 700-800 W for 15-25 min, and then subjected to hydrothermal synthesis reaction at 115-125 ℃ under a closed condition.

9. The method of claim 8, wherein the method is characterized by, The hydrothermal synthesis reaction is carried out for 22-26 h, then the solid is washed, dried and ground to obtain the product m.

10. The method of claim 1, wherein the wastewater is a wastewater from a steelmaking process. The bimetallic oxide in S3 adsorbs the residual trivalent chromium in the filtrate b and the hexavalent chromium not reduced in the cathode generated in the anodic oxidation in S2, is separated again, is cleaned, and finally is calcined under N2 atmosphere at 350-450℃ for 3.5-4.5h, to obtain the magnesium-chromium layered bimetallic oxide with surface dispersion of biochar, which can continue to be adsorbed according to the corresponding process in S3, and the process is repeated, and the magnesium-chromium layered bimetallic oxide with surface dispersion of biochar is recycled for 6-8 times.

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