CN113415857B

CN113415857B - Method for harmless treatment of hexavalent Cr wastewater by adsorption and electroreduction through carbon paste electrode

Info

Publication number: CN113415857B
Application number: CN202110653224.2A
Authority: CN
Inventors: 张王兵; 蒋璐; 杨新安; 宫艺旋
Original assignee: Anhui University of Technology AHUT
Current assignee: Anhui University of Technology AHUT
Priority date: 2021-06-11
Filing date: 2021-06-11
Publication date: 2023-02-10
Anticipated expiration: 2041-06-11
Also published as: CN113415857A

Abstract

The invention relates to the technical field of wastewater treatment, in particular to a method for harmlessly treating hexavalent Cr wastewater by using a carbon paste electrode through adsorption and electroreduction, wherein polyaniline and D-sodium gluconate or EDTA are modified to the surface of the carbon paste electrode, and the polyaniline can improve the specific surface area of the electrode and the adsorption capacity of the electrode; the complexing agent can provide special functional groups, change an electron transfer path and improve the reduction capability of the electrode, and the holes on the surface of the electrode can be found to be increased through scanning of an electron microscope; in addition, the electrochemical representation shows that electrons can be effectively transferred on the electrode, the electrochemical reaction is promoted, the electrochemical stability is satisfactory, even if Cr (VI) wastewater is treated for many times, the regenerated electrode still keeps high efficiency, and the invention has the advantages of simple synthesis of electrode materials, low cost and short process flow; under the optimal condition, the reduction rate of Cr (VI) reaches 88.3 percent, the total removal rate reaches 95 percent, and the method has practical industrial application prospect.

Description

Method for harmless treatment of hexavalent Cr wastewater by adsorption and electroreduction through carbon paste electrode

Technical Field

The invention relates to the technical field of wastewater treatment, in particular to a method for harmlessly treating hexavalent Cr wastewater by using a carbon paste electrode to adsorb and electrically reduce.

Background

In the fields of electroplating, leather tanning, textile, photography and the like, a large amount of Cr (VI) -containing wastewater is generated. Cr (VI) has strong toxicity, and if excessive Cr (VI) exists in a human body, the Cr (VI) can cause skin diseases, liver and kidney injuries, deformity and weakening of an immune system. There are many methods for processing Cr (VI), and electrochemical processing is divided into electro-reduction and electro-adsorption, and electro-reduction is to reduce Cr (VI) into non-toxic Cr (III); the electro-adsorption is to adsorb Cr (VI) to the surface of the electrode by using a porous electrode, so as to achieve the effects of adsorption and enrichment. In industrial production, the Cr (VI) is generally precipitated or reduced by flocculation and microbial treatment; however, the principles used are difficult to regenerate and the treatment times are too long, which process places a great cost pressure on the plant. And the actual wastewater has complex components, and if a complex process is used for aiming at a certain pollutant, the process is a huge waste for a treatment device and related raw materials. In conclusion, the process flow is simplified, the raw material cost is reduced, and the use of renewable materials is the future development direction of the targeted water treatment process.

The carbon paste electrode is very sensitive to heavy metal ions and is commonly used for detecting the concentration of the heavy metal ions in sewage. The chemically modified carbon paste electrode has different special functional groups on the surface of the electrode, so that the electron conduction mode is changed, and the electroreduction and electroadsorption capacities are greatly improved. And the carbon paste electrode is stable in property of the used raw materials, and is not easy to fall off or denature, and the carbon paste electrode can be regenerated by exchanging the cathode and the anode or soaking the carbon paste electrode in dilute hydrochloric acid. Therefore, the chemically modified carbon paste electrode is an excellent electrode material for treating Cr (VI) wastewater. However, the application of the chemically modified carbon paste electrode to the treatment of Cr (VI) wastewater has not been developed and researched.

The Chinese patent ZL201610026028.1 discloses a water purification system and a water purification method for treating chromium plating wastewater, wherein the device is divided into a three-layer structure, the bottom layer is an anaerobic biological filter layer, the middle layer is a resin type particle electrode layer, and the upper layer is an aerobic biological filter layer. The device has excellent wastewater treatment effect, can reduce the COD of the chromium plating wastewater to the discharge standard, and can completely treat the heavy metal ions in the wastewater. However, this apparatus is complicated and expensive, and it is difficult to regenerate raw materials such as a biological filter layer, and thus it is difficult to industrially apply the apparatus.

The Chinese invention patent ZL201410404014.X discloses a chemical reduction auxiliary electrochemical method for removing hexavalent chromium in water, which is divided into three steps, and Cr (VI) in wastewater is completely removed through the steps of chemical reduction, electrochemical reduction, flocculation precipitation and the like. Each step in the process flow is relatively simple, and raw materials in the process flow are not wasted. However, the whole treatment process is still complicated, the cost is high, and the electrode material cannot be recycled, so that the consumption is high.

In view of the above-mentioned drawbacks, the inventors of the present invention have finally obtained the present invention through a long period of research and practice.

Disclosure of Invention

The invention aims to solve the problems that an electrode used in the existing electrochemical treatment method for treating Cr (VI) wastewater is difficult to regenerate or low in reduction efficiency, overlong in treatment time and complex in process, and provides a method for treating hexavalent Cr wastewater in an adsorption and electroreduction harmless manner by using a carbon paste electrode.

In order to achieve the aim, the invention discloses a method for harmless treatment of hexavalent Cr wastewater by using a carbon paste electrode to adsorb electro-reduction, which comprises the following steps:

s1: modifying a carbon paste electrode by adopting D-sodium gluconate (DCP for short) or EDTA (ECP for short);

s2: taking the modified carbon paste electrode obtained in the step S1 as a cathode and graphite as an anode, wherein the size and the shape of the graphite are the same as those of the carbon paste electrode, carrying out an electric reaction on the carbon paste electrode and the graphite electrode in wastewater containing hexavalent Cr, and carrying out adsorption and electric reduction on the hexavalent Cr by the carbon paste electrode;

s3: and regenerating the carbon paste electrode.

The carbon paste electrode modified by D-sodium gluconate or EDTA in the step S1 comprises the following steps:

s11: polishing the graphite sheet;

s12: washing the polished graphite flakes with a nitric acid solution, absolute ethyl alcohol and deionized water;

s13: putting the graphite flake cleaned in the step S12 into a sulfuric acid solution, and performing cyclic voltammetry scanning at a scanning speed of 100mV/S within a potential range of-0.4-1.4V until a curve is stable;

s14: washing the graphite flake obtained in the step S13 with deionized water, and drying for 2h at 60 ℃;

s15: adding polyaniline powder and polyvinylidene fluoride into 1-methyl-2-pyrrolidone, fully stirring until the polyaniline powder and the polyvinylidene fluoride are completely dissolved, and sealing to serve as a binder for later use;

s16: mixing a coordination agent, conductive carbon powder and the binder obtained in the step S5, and stirring for more than 4 hours to obtain carbon paste;

s17: and (4) uniformly coating the carbon paste obtained in the step (S16) on a graphite sheet, and drying at 60 ℃ for 3h to obtain a carbon paste electrode.

The volume ratio of nitric acid to water in the nitric acid solution in the step S12 is 1.

The concentration of the sulfuric acid solution in the step S13 is 0.5mol/L.

In the step S15, the dosage ratio of the polyaniline to the polyvinylidene fluoride to the 1-methyl-2-pyrrolidone is 1g:2g:40mL. .

In the step S16, the dosage ratio of the coordination agent to the conductive carbon powder to the binder is 1g: 2-8 g:2mL.

And the complexing agent in the step S16 is D-sodium gluconate or EDTA.

In the step S2, the volume of the hexavalent Cr-containing wastewater is 50mL.

The conditions of the electric reaction in the step S2 are as follows: the working voltage is-1.2V, the working pH value is 3.0, the distance between polar plates is 1.0cm, the working temperature is 313K, and the concentration of NaCl of electrolyte is 3%.

The regeneration treatment process of the carbon paste electrode in the step S3 comprises the following steps: and soaking the carbon paste electrode after the electric reaction and the counter electrode in 0.5mol/L hydrochloric acid solution for 180min to remove Cr adsorbed on the surface of the electrode and regenerate the electrode material.

The invention applies a certain voltage on the polar plate, and the surfaces of the anode plate and the cathode plate can carry opposite charges. When the wastewater passes through the electrolytic cell, the charged particles move to the polar plate with opposite charges under the action of the electrostatic field and are gathered on the surface of the polar plate until the wastewater is saturated. As shown in FIG. 1, the electro-adsorption is to concentrate the charged pollutants on the surface of the electrode, and the surface of the electrode plate and the wastewater in the center of the electrolytic cell form an electric double layer. When the waste water flows out from the other end, the concentration of the pollutants is reduced, and the purpose of purifying the water quality is achieved.

The saturation can be increased by increasing the specific surface area of the electrode, and the treatment effect is improved. On the other hand, the charged particles adsorbed on the surface of the electrode are subjected to electrocatalytic reduction, so that the concentration of the charged particles on the surface of the electrode can be reduced, and pollutants can be reduced into non-toxic or low-toxic particles.

The principle of electro-reduction is as follows:

the oxidized mediator species on the electrode surface is reduced to a reduced mediator (e.g., H), cr (VI) is reduced by the reduced mediator, and the oxidized mediator can be regenerated by electrochemical reduction.

Modifying the electrode surface with the coordination agent can change the electron transfer path and improve the electroreduction efficiency. Taking an EDTA modified carbon paste (ECP) electrode as an example, FIG. 2 shows a mechanism diagram of Cr (VI) wastewater treatment by an ECP electrode electrochemical method.

Two carboxyl groups of EDTA can accept H under acidic condition ⁺ Formation of hexabasic acids, in solutionThe Cr (VI) migrates to the surface of the ECP electrode under continuous stirring to interact with the positive charge group of EDTA, and then electron reduction to Cr (III) is obtained on the cathode through EDTA transfer.

Compared with the prior art, the invention has the beneficial effects that: according to the invention, polyaniline and D-sodium gluconate or EDTA are modified to the surface of the carbon paste electrode, so that the polyaniline can improve the specific surface area of the electrode and improve the electric adsorption capacity of the electrode; the complexing agent can provide special functional groups, change an electron transfer path and improve the electroreduction capability of the electrode, and holes on the surface of the electrode can be found to be increased through scanning of an electron microscope; in addition, the electrochemical characterization is utilized to discover that electrons can be effectively transferred on the electrode, the electrochemical reaction is promoted to occur, the satisfactory electrochemical stability is realized, and even if Cr (VI) wastewater is treated for many times, the regenerated electrode still maintains high efficiency, so that the method has the advantages of simple electrode material synthesis, low cost and short process flow; under the optimal condition, the removal rate of Cr (VI) reaches 95 percent, and the method has practical industrial application prospect.

Drawings

FIG. 1 is a schematic diagram of the principle of electro-adsorption;

FIG. 2 is a schematic diagram of the mechanism of the electro-reduction reaction of the ECP electrode;

FIG. 3 is a scanning electron micrograph of the electrode before the electric reaction, wherein A is a graphite sheet, B is a DCP electrode, C is an ECP electrode, and D is an ECP (N-ECP) electrode without polyaniline modification;

FIG. 4 is the cycling performance of the electrode, bare is the graphite sheet;

FIG. 5 is a plot of cyclic voltammograms, A for 1 cycle of three electrodes, B for 25 cycles of graphite electrode, C for 25 cycles of DCP electrode, and D for 25 cycles of ECP electrode;

FIG. 6 is a graph showing the effect of the carbon paste electrode composition on the Cr (VI) residual ratio in example 5, A being a DCP electrode and B being an ECP electrode;

FIG. 7 is a graph showing the effect of the plate spacing on the Cr (VI) residual ratio in example 6, wherein A is a graphite electrode, B is a DCP electrode, and C is an ECP electrode;

FIG. 8 influence of operating voltage on Cr (VI) residual ratio in example 7, A is graphite electrode, B is DCP electrode, and C is ECP electrode;

FIG. 9 the change with time of the concentrations of Cr (VI), adsorbed Cr (VI) and reduced Cr (VI) in the solution in example 8, A is a graphite electrode, B is a DCP electrode, C is an ECP electrode, a is the Cr (VI) concentration in the solution, B is the adsorbed Cr (VI) concentration, and C is the reduced Cr (VI) concentration.

Detailed Description

The above and further features and advantages of the present invention are described in more detail below with reference to the accompanying drawings.

The relevant characterization methods of the examples are as follows:

observing the microscopic morphology of the sample by using a scanning electron microscope (ZeissEVOLS-15), analyzing Cr (VI) and total chromium by using an ultraviolet-visible spectrophotometer (TU-1810), and finally analyzing and characterizing the electrochemical properties of the electrode by using a multichannel potentiostat (CHI 1000).

Example 1

1) The graphite flakes (5 x 1cm) were polished clean with 360, 600 mesh sandpaper.

2) And (3) washing the polished graphite flakes with nitric acid (1.

3) The cleaned graphite flake is put into a 0.5mg/L sulfuric acid solution, and cyclic voltammetry scanning is carried out at a scanning speed of 100mV/s in a potential range of-0.4-1.4V until the curve is stable.

4) Washing the graphite flakes in the step 3) with deionized water, baking for 2 hours at the temperature of 60 ℃, and taking out for later use.

5) 0.5g of polyaniline powder with a particle size less than 80 meshes and 1g of polyvinylidene fluoride are weighed and added into 20mL of 1-methyl-2-pyrrolidone, and the mixture is fully stirred until the polyaniline powder and the polyvinylidene fluoride are completely dissolved and then sealed to be used as a binder for later use.

6) Weighing 2g of D-sodium gluconate/EDTA, 4g of conductive carbon powder and 4mL of 5) of the binder, mixing, and stirring for 4h to ensure uniformity. And (2) uniformly coating the carbon paste prepared in the step on a graphite sheet by using a scraping blade, and drying at 60 ℃ for three hours to obtain electrodes, namely a polyaniline-D-sodium gluconate modified carbon paste electrode (DCP) and a polyaniline-EDTA modified carbon paste Electrode (ECP).

7) And adding 1g of polyvinylidene fluoride into 20mL of 1-methyl-2-pyrrolidone, fully stirring until the polyvinylidene fluoride is completely dissolved, and sealing to serve as a binder for later use.

8) Weighing 2g of EDTA, 4g of conductive carbon powder and 4mL of 7) of the binder, mixing, and stirring for 4h to ensure uniformity. And (3) uniformly coating the carbon paste prepared in the step on a graphite sheet by using a scraping blade, and drying the graphite sheet at the temperature of 60 ℃ for three hours. The obtained electrode is ECP (N-ECP) without polyaniline modification

And scanning the DCP, the ECP and the N-ECP by an electron microscope, and observing the surface morphology of the DCP, the ECP and the N-ECP. Taking the pretreated graphite sheet in the step 3) as a contrast, and carrying out electron microscope scanning. The results are shown in FIG. 3.

As can be seen from an electron microscope scanning image, compared with graphite flakes and N-ECP, the surfaces of DCP and ECP are more irregular, compared with the graphite flakes and the N-ECP, the pores and the holes are obviously increased, the specific surface area is larger, and the adsorption effect is better.

Example 2

Wastewater treatment experiments were conducted using DCP, ECP, N-ECP and graphite flakes described in example 1 as cathode electrodes and graphite flakes of the same size and shape as anodes.

The volume of the wastewater to be treated is 50mL, and the contact area between the electrode and the wastewater is 3cm ² . Under the conditions that the pH value is 3.0, the working voltage is set to be-1.2V, the distance between the polar plates is 1.0cm, the temperature is 313K, and the concentration of NaCl of electrolyte is 3%, the experimental electrode performs electric reaction for 3h in a water sample with the initial concentration of 50mg/L Cr (VI), and the residual rates of Cr (VI) are 65.1% (graphite sheet electrode), 53.7% (N-ECP), 33.5% (DCP) and 4.8% (ECP) respectively.

Example 3

The working electrode and the counter electrode after the electric reaction in example 2 were immersed in 0.5mol/L hydrochloric acid solution to remove Cr (VI) adsorbed on the surface of the electrode for 180min.

The treated working electrode was subjected to wastewater treatment as described above, and the above cycle was repeated 5 times to investigate the regeneration performance of the electrode, and the results are shown in fig. 4.

As can be seen from fig. 4, after the graphite electrode, the DCP electrode and the ECP electrode are used for five cycles, the Cr (VI) residue rates are respectively increased from 60.6% to 70.9%, from 33.5% to 43.9% and from 4.8% to 17.4%. After five times of use, the residual rate of Cr (VI) of the DCP electrode and the ECP electrode is not increased by more than 15.0%, and the electrochemical removal of Cr (VI) is more stable.

Example 4

The graphite electrode, the DCP electrode and the ECP electrode which have the same preparation parameter steps as those of the embodiment 1 are taken for electrochemical characterization. The results are shown in FIG. 5.

A is CV curve of graphite electrode, DCP electrode and ECP electrode at scanning rate of 100mV/s, and electrolyte is 1mol/LNa ₂ SO ₄ And (3) solution. Compared with a graphite electrode, the DCP electrode and the ECP electrode show larger closed area, which indicates that the carbon paste electrodes modified by the two complexing agents have larger specific capacitance. B. C and D are CV curves of a graphite electrode DCP electrode and an ECP electrode, respectively, cycled 25 times at a scan rate of 100mV/s in 1mol/L NaCl electrolyte. As can be seen, the CV curve shape did not change significantly even after 25 cycles, indicating that all three electrodes had satisfactory electrochemical stability.

Example 5

The using amount of the fixed coordination agent 2g and the binder is 4mL, the using amount of the conductive carbon powder is gradually increased from 2g to 10g, other data and steps are the same as those of the embodiment 1, and the experimental result is shown in FIG. 6. As can be seen from fig. 6, the ratio of the complexing agent to the carbon powder is 1:1 or 1: the most obvious decrease of Cr (VI) concentration is shown in case 2.

Example 6

The electrode plate pitch was changed to 0.5, 1.5, and 2.0cm, and other data and procedure were the same as in example 1, and the experimental results are shown in fig. 6. As can be seen from FIG. 7, too large or too small a plate pitch is not favorable for reducing the Cr (VI) concentration.

Example 7

The working voltage was changed to 0, -0.8, -1.0, -1.4V, and other data and procedure were the same as in example 1, and the experimental results are shown in FIG. 7. As can be seen from fig. 8, when the voltage is higher than-1.4V, the residual ratio decreases as the voltage increases. However, too high a potential can promote the progress of side reactions, affecting the overall reduction.

Example 9

The graphite electrode, DCP and ECP electrode obtained in example 1 were used to perform experiments according to the experimental parameters and procedures described in example 2, and the changes of the concentrations of Cr (VI), adsorbed Cr (VI) and Cr (III) in the solution with time during the electrochemical removal of Cr (VI) were measured during the experiments, and the change curves are shown in fig. 9. After 180min of electrolysis, the concentrations of medium Cr (VI), adsorbed Cr (VI) and Cr (III) in the solution are shown in Table 1.

TABLE 1 percentage of Cr (VI), adsorbed Cr (VI) and reduced Cr (VI) concentrations in the solution at 180min of electrical reaction

	Cr (VI)/% in solution	Adsorbed Cr (VI)/%	Reduced Cr (VI)/%
				Graphite electrode	65.1	5.0	29.9
DCP electrode	33.5	6.0	60.5
				ECP electrode	4.8	6.9	88.3

As shown in fig. 9, the amount of Cr (VI) ions in the solution is decreasing with time, and the amount of Cr (VI) adsorbed in the electric double layer and the amount of Cr (III) electro-reduced tend to increase and stabilize. Graphite, DCP and ECP electrodes were reduced to Cr (VI) of 29.9%, 60.5% and 88.3% in solution at 3h of the electro-reaction as shown in Table 1; the adsorbed Cr (VI) was 5.0%, 6.0% and 6.9%. This indicates that the electrochemical removal of Cr (VI) is the result of the combined action of electro-adsorption and electro-reduction. Due to the limitation of adsorption capacity, the electro-reduction of Cr (VI) to Cr (III) in electrochemical reaction is much higher than the amount of Cr (VI) adsorbed between the double electric layers, however, electro-adsorption may cause local Cr (VI) concentration to increase, speeding up the electro-reduction reaction. And the addition of the coordination agent can change an electron transfer path and is beneficial to the electro-reduction reaction.

The foregoing is illustrative of the preferred embodiments of the present invention, which is set forth only, and not to be taken as limiting the invention. It will be appreciated by those skilled in the art that many variations, modifications, and equivalents may be made thereto without departing from the spirit and scope of the invention as defined in the claims.

Claims

1. A method for harmlessly treating hexavalent Cr wastewater by using a carbon paste electrode through adsorption and electroreduction is characterized by comprising the following steps of:

s1: modifying the carbon paste electrode by adopting D-sodium gluconate or EDTA;

s2: taking the modified carbon paste electrode obtained in the step S1 as a cathode and graphite as an anode, wherein the size and the shape of the graphite are the same as those of the carbon paste electrode, carrying out an electric reaction on the carbon paste electrode and the graphite electrode in wastewater containing hexavalent Cr, and adsorbing and carrying out an electric reduction on the hexavalent Cr by the carbon paste electrode;

s3: regenerating the carbon paste electrode;

s11: polishing the graphite flake;

s13: putting the graphite flake cleaned in the step S12 into a sulfuric acid solution, and performing cyclic voltammetry scanning at a scanning speed of 100mV/S in a potential range of-0.4 to 1.4V until a curve is stable;

s14: washing the graphite flake obtained in the step S13 with deionized water, and drying for 2 hours at the temperature of 60 ℃;

s15: adding polyaniline and polyvinylidene fluoride into 1-methyl-2-pyrrolidone, fully stirring until the polyaniline and the polyvinylidene fluoride are completely dissolved, and sealing to serve as a binder for later use;

s16: mixing a coordination agent, conductive carbon powder and the binder obtained in the step S15, and stirring for more than 4 hours to obtain carbon paste, wherein the coordination agent is D-sodium gluconate or EDTA;

2. The method for the adsorptive electroreduction innocent treatment of hexavalent Cr wastewater by using a carbon paste electrode according to claim 1, wherein the volume ratio of nitric acid to water in the nitric acid solution in the step S12 is 1.

3. The method for harmlessly treating hexavalent Cr wastewater by adsorption and electro-reduction using a carbon paste electrode according to claim 1, wherein the concentration of the sulfuric acid solution in the step S13 is 0.5mol/L.

4. The method for harmless treatment of hexavalent Cr wastewater by adsorption and electro-reduction using a carbon paste electrode according to claim 1, wherein the polyaniline, the polyvinylidene fluoride, and the 1-methyl-2-pyrrolidone are used in a ratio of 1g in the step S15: 2g:40mL.

5. The method for harmlessly treating hexavalent Cr wastewater by adsorption and electro-reduction using a carbon paste electrode according to claim 1, wherein the amount ratio of the complexing agent, the conductive carbon powder and the binder in step S16 is 1g:2 to 8g:2mL.

6. The method for harmlessly treating hexavalent Cr wastewater by adsorption and electro-reduction using a carbon paste electrode according to claim 1, wherein the volume of the hexavalent Cr-containing wastewater treated in the step S2 is 50mL.

7. The method for harmlessly treating hexavalent Cr wastewater by adsorption and electro-reduction using a carbon paste electrode according to claim 1, wherein the conditions of the electro-reaction in the step S2 are as follows: the working voltage is-1.2V, the working pH value is 3.0, the distance between polar plates is 1.0cm, the working temperature is 313K, and the concentration of NaCl in electrolyte is 3%.

8. The method for the adsorptive electroreduction innocent treatment of hexavalent Cr wastewater by using a carbon paste electrode according to claim 1, wherein the regeneration treatment process of the carbon paste electrode in the step S3 is as follows: and soaking the carbon paste electrode after the electric reaction and the counter electrode in 0.5mol/L hydrochloric acid solution for 180min to remove a small amount of hexavalent Cr remained on the surface of the electrode and regenerate the electrode material.