Treatment of Pesticide Wastewater by Electrocatalytic Oxidation Technology

1 Introduction

Dimethocarb has the characteristics of broad spectrum, high efficiency, low toxicity and systemic absorption. It belongs to carbamate insecticides and is a low toxicity variety of methomyl. The drug is particularly effective for the control of larvae of Lepidoptera, Homoptera, Hymenoptera, Diptera, Coleoptera and other pests, and is currently an excellent drug for the control of resistant cotton bollworm in China.

Due to the high control effect of thiodimethovil against the resistant cotton bollworm, and easy biodegradation, it has no phytotoxicity to crops, so it has become an urgently needed product at home and abroad. The wastewater produced by the pesticide contains pyridine and pyridine substances, and has poor biodegradability, which has always been a difficult problem in the treatment of pesticide wastewater.

Because pyridine is highly toxic and very stable, it is difficult to destroy. At present, there is no good treatment method for this water at home and abroad, and it can only be incinerated. Therefore, it is inevitable to explore an economical and feasible pretreatment method.

Electrocatalytic oxidation technology is an emerging wastewater pretreatment technology with the following characteristics:

1) No need to add reagents to avoid secondary pollution;

2) carry out under normal temperature and pressure, and the reaction conditions are easy to meet;

3) The reaction device is simple and flexible, and easy to be industrialized. At present, electrocatalytic oxidation technology has achieved little in the wastewater treatment of oil refining, printing and dyeing, tanning and other fields.

Electrocatalytic oxidation mechanism

Electrocatalytic oxidation is divided into two types: direct anodic oxidation and indirect anodic oxidation:

1) Direct anodic oxidation: electrochemical reaction occurs directly on the anode, selective oxidation and degradation of organic matter, accompanied by oxygen evolution, and at the same time a large amount of electrical energy is converted into heat energy, high energy consumption, should be avoided as much as possible;

2) Indirect anodic oxidation: It generates strong oxidant hydroxyl radicals through the anodic oxidation reaction, and the hydroxyl radicals indirectly oxidize the organic matter in the water to achieve the purpose of degradation. Since the indirect oxidation makes full use of the generated strong oxidant hydroxyl radicals, the oxidation efficiency is greatly improved. How to avoid direct oxidation, the key to indirect oxidation is to find suitable anode materials.

Experimental part

3.1 Source and quality of wastewater

There is only one thiodimethocarb wastewater, which is derived from the water used for reaction synthesis and washing products. It is the residual kettle residue after flashing pyridine.

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3.2 Instruments, devices and reagents

Instruments: Agilent1100SeriesLC/MSD liquid phase mass spectrometer, Shimadzu LC20AT/SPD-M20A liquid phase analyzer.

  Reagents: sodium hydroxide, concentrated hydrochloric acid (the above reagents are AR).

3.3 Experimental methods and analytical methods

Experimental method: Adjust the dimethocarb wastewater to a certain pH value, pour it into an electrolytic cell, conduct electrolysis for several hours under a certain current density and plate spacing, and measure the CODCr of the wastewater after neutralization.

Analysis method: The external standard method was used for the analysis of wastewater components; the potassium dichromate method was used for the determination of CODCr.

Results and discussion

4.1 Selection of anode materials and electrocatalytic oxidation conditions

Commonly used materials prone to indirect oxidation: Ti/Sb2O5, Ti/SnO2, Ti/IrO-RuO, graphite as anode, stainless steel as cathode, electrocatalytic oxidation for 3h, current density 30mA·cm-2, initial pH value 6~9 conduct experiment. The experimental results are shown in Table 2.

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It can be seen from Table 2 that the catalytic activity of the coating Ti/IrO-RuO is better. The following experiments are carried out with Ti/IrO-RuO as the anode and stainless steel as the cathode.

4.2 Relationship between current density and CODCr removal rate

The relationship between current density and removal rate is shown in Figure 1.

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It can be seen from Figure 1 that the removal rate of CODCr increases with the increase of the current density for a certain electrolysis time; however, the removal rate of CODCr does not increase after it increases to 30 mA·cm-2. From the electrocatalytic oxidation mechanism, it can be seen that the current density increases.

It directly leads to the increase of hydroxyl radical·OH concentration in the solution, the reaction is enhanced, and the treatment effect is improved; but the current density exceeds 30mA·cm-2, and the excessive current aggravates the side reaction of oxygen evolution, thereby weakening the removal rate of CODCr, Therefore, it is appropriate to use 30mA·cm-2 for the treatment of this wastewater.

4.3 Relationship between electrolysis time and CODCr removal rate

The relationship between electrolysis time and removal rate is shown in Figure 2. It can be seen from Figure 2 that the CODCr removal rate increases with increasing electrolysis time. The CODCr removal rate decreased at about 2h because the pyridine ring was not destroyed at the beginning, so it did not account for CODCr.

However, with the gradual opening of the pyridine ring, its CODCr changed from recessive to dominant, increasing the CODCr in water, so the CODCr removal rate did not increase but decreased.

However, the electrolysis time was prolonged after 3h, and the CODCr removal rate no longer increased. This is because: in the first 3 hours of the reaction, the oxidatively decomposed organic matter in the wastewater tends to be completed, so the removal rate of CODCr does not increase even if the reaction time is prolonged.

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4.4 Relationship between plate spacing and CODCr removal rate

Under the conditions of current density of 30mA·cm-2, initial pH value of 6~9, and electrolysis for 3h, the plate spacing was changed to observe the change of COD removal rate. The results are shown in Figure 3. It can be seen from Figure 3 that the smaller the distance between the plates, the greater the electric field strength and the higher the COD removal rate. But the board spacing is too small to be processed. Comprehensive consideration, the use of 2cm board spacing is appropriate.

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4.5 Relationship between pH value of influent water and CODCr removal rate

The relationship between influent pH value and CODCr removal rate is shown in Figure 4. It can be seen from Figure 4 that the initial pH value has a great influence on the removal effect of CODCr. With the increase of pH value, the removal rate of CODCr first increased and then decreased, and the removal rate of CODCr reached 80% when the pH value was about 8.0.

Due to the high sulfur content in wastewater, the stronger the acidity, the less conducive to the mineralization of sulfides to sulfuric acid, and OH is also difficult to generate. The stronger the alkalinity is, although it is beneficial to the generation of OH, it is more conducive to the side reaction of oxygen evolution. When the pH value is 6~9, OH is generated along with the electrode reaction.

At the same time, the product H+ and OH undergo a neutralization reaction. At this time, the oxidation reaction state is better and the reaction rate is higher. Therefore, the pH value of 6~9 is presumed to be the optimum point for the generation of hydroxyl radicals.

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In summary, the optimal conditions for electrocatalysis of sulfur double wastewater are: Ti/IrO-RuO as anode plate, stainless steel as cathode plate, current density 30mA·cm-2, initial pH value 6~9, reaction 3h, CODCr removal rate up to 80%.

Changes in wastewater before and after electrolysis

5.1 B/C comparison before and after electrolysis

From the comparison of B/C before and after electrolysis in Table 3, it can be seen that after electrocatalytic oxidation treatment, the wastewater changed from non-biochemical to biochemical wastewater.

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5.2 Color change before and after wastewater treatment

The thiosuncarb wastewater was electrocatalytically oxidized under the above conditions, and the wastewater changed from black to light yellow in the raw water, and the color changed significantly.

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5.3 Analysis results

5.3.1 Spectrogram of wastewater

The water-liquid spectra before and after electrolysis are shown in Figure 6.

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5.3.2 Removal of main characteristic pollutants in wastewater

The external standard analysis of characteristic pollutants was carried out by liquid chromatography, and the characteristic pollutants were significantly removed by comparison before and after electrocatalytic oxidation.

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6 Conclusion

The self-made electrocatalytic oxidation device effectively treats dimethocarb wastewater.

Ti/IrO-RuO plate is used as anode, stainless steel plate is used as cathode, the current density is 30mA·cm-3, the oxidation time is 3h, the plate spacing is 2cm, the pH value of the wastewater is 6~9, the CODCr removal rate is 80%, and the wastewater is difficult to biochemically convert. In order to be easy to biochemically, the characteristic pollutants in the water are greatly reduced, and this technology has a good development prospect in the treatment of pesticide wastewater.

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