Cigarette Tar Wastewater Treatment by Zero Valent Iron Reduction

 

Treatment of Cigarette Tar Wastewater by Zero Valent Iron Reduction

 

1. Introduction

With the rapid development of my country's economy, the tobacco industry has developed rapidly. The wastewater produced in the cigarette production process contains a large amount of high-concentration harmful substances, which will cause serious environmental pollution if it is directly discharged into nature. 

According to the survey and statistics of wastewater discharge status of cigarette factories in my country, the wastewater discharge volume of cigarette factories is 0.35-0.60m3 per box of cigarettes. 

The most difficult to treat in tobacco wastewater is tar wastewater, which contains a large amount of organic matter, high COD and chromaticity, and poor biodegradability. At present, the treatment methods adopted by domestic cigarette factories in sewage treatment mainly include physicochemical treatment, biochemical treatment and the combination of the two. 

In this study, zero-valent iron was used to reduce cigarette tar wastewater, and the influence of different factors on the treatment effect was investigated, which provided a new way for the discharge of this type of wastewater up to the standard.

 

2. Experimental methods

 

The raw water sample of cigarette tar wastewater is dark in color, dark brown, smelly and viscous, and the CODCr content in the water sample is high. The original water sample was diluted about 1300 times (experimental verification was carried out after the dilution, and the result was feasible). 

The diluted water sample is earthy in color, the chromaticity is smaller than that of the original water sample, and the CODCr value in the water sample is greatly reduced. 

Through multiple sets of untargeted experiments, the approximate dosage of experimental drugs and experimental conditions can be obtained. 

The mass concentration of NaCl in the wastewater is about 200 mg/L, the amount of iron powder is about 20 g/L, the pH value is about 3, and the stirring time is about 200 mg/L. Around 30 minutes. 

The reaction was fully carried out by stirring with a mechanical stirrer. After the reaction, the pH value of the water sample was adjusted to about 10 with sodium hydroxide, and the CODCr value of the treated water sample and the original water sample was measured to obtain the CODCr removal rate.

 

3. Results Analysis

 

3.1 Influence of the amount of iron powder

 

Take 60 mL of diluted water samples into five beakers, add the same amount of NaCl 0.012 g (200 mg/L), and add 0.5 g (8.3 g/L) and 0.8 g/L to each beaker respectively. g (13.3 g/L), 1.2 g (20 g/L), 1.5 g (25 g/L), 1.8 g (30 g/L) iron powder, adjust the pH value of the wastewater to be 3. After stirring for 30 minutes, measure the CODCr value of each water sample. 

 

The optimal amount of iron powder is 1.2 g (20 g/L). Different amounts of zero-valent iron react with the same amount of hydrochloric acid, and the degree of reaction will be different. If the zero-valent iron is too small, the reaction with hydrochloric acid is insufficient, and there is excess hydrochloric acid; if the zero-valent iron is too much, the reaction with hydrochloric acid is excessive, and the zero-valent iron is left. Therefore, when the reaction between zero-valent iron and hydrochloric acid is relatively complete, the The remaining amount of either side is the lowest, the chemical reaction is the best, and the reaction effect is the best.

 

3.2 Effect of pH value on CODCr removal rate

 

Take 60 mL of diluted water samples into five beakers, add the same amount of NaCl 0.012 g (200 mg/L) and iron powder 1.2 g (20 g/L) respectively. Adjust the pH of the solution with hydrochloric acid. The pH values ​​of the solutions in the five beakers were set to 1, 2, 3, 4, and 5 in turn, and the CODCr value of each water sample was measured after stirring for 30 minutes. 

 

The optimum pH value is 2. If the hydrochloric acid is too little, the reaction with zerovalent iron is insufficient, and there is residual zerovalent iron.

If the hydrochloric acid is too much, the reaction with zerovalent iron is excessive, and there is residual hydrochloric acid, so when the reaction between zerovalent iron and hydrochloric acid is relatively complete, the difference between the two is when the remaining amount of either side is the lowest, the chemical reaction is the best, and the reaction effect is the best. 

In the above experiments, it can be seen from the removal rate of CODCr that the pH value of the acid is 2 as the best value, that is, the reaction with zero-valent iron is the most sufficient and complete.

 

3.3 Effect of sodium chloride dosage on CODCr removal rate

 

Take 60 mL of diluted water samples into five beakers, add 0.010 g (166.7 mg/L), 0.012 g (200 mg/L), 0.015 g (250 mg/L), 0.018 g (300 mg/L), 0.020 g (333.3 mg/L) of sodium chloride, add the same amount of iron powder 1.2 g (20 g/L), adjust the pH value of the solution The CODCr value of each water sample was measured after stirring for 30 min. The results are shown in Figure 3.

 

 

As can be seen from the above figure, the optimal amount of sodium chloride is 0.018 g (300 mg/L). There are two reasons for adding sodium chloride. The first is that sodium chloride acts as an electrolyte during the stirring process, which can increase the conductivity of the solution.

Under certain conditions, sodium chloride can react with iron oxide, remove the oxide film on the surface of iron filings, prevent its passivation, and make the reaction more sufficient. However, the addition of sodium chloride also has adverse effects on the experiment. 

After the water sample is processed, when the CODCr value in the water sample is measured by the potassium dichromate method, the chloride ion in the sodium chloride will become an interfering ion, which has no effect on the water sample. The determination of CODCr is affected.

 

3.4 Influence of stirring reaction time on CODCr removal rate

 

Take 60 mL of diluted water samples into five beakers, add the same amount of NaCl 0.018 g (300 mg/L), and then the amount of iron powder 1.2 g (20 g/L), adjust the pH of the solution to 2. 

The stirring time of each beaker is 30, 45, 60, 75, and 90 min, respectively, and the CODCr value of each water sample is measured. The results are shown in Figure 4.

It can be seen from the above figure that the optimal stirring time is 75min. By stirring the Fe and the water sample to fully contact, various reactions in the water sample can also be fully carried out, which is beneficial to remove the organic matter in the water sample. However, if the stirring reaction time is too long, the various reactions taking place in the water sample reach an equilibrium and no longer react. Therefore, after the system reacts for a certain period of time, the removal rate of CODCr hardly changes and tends to be stable.

 

3.5 Orthogonal experimental design

 

The optimal value of each factor has been found through the data of the single factor experiment. Next, it is necessary to use the orthogonal experiment to find out what level of combination is the best and can maximize the removal rate. Orthogonal experiment is a design method to study multiple factors and multiple levels.

 

Based on the above experimental data, the optimal values ​​of the four factors have been obtained. Taking these four factors as variables, taking the optimal value as the center number, and taking a value up and down, an orthogonal experiment with four factors and three levels is performed.

It can be seen from the above table that the optimal combination conditions for this experiment are that the amount of sodium chloride is 0.024 g (400 mg/L), the amount of iron powder is 1.8 g (30 g/L), the pH value is 2, and the stirring The time was 75 min. Under this condition, the removal rate of CODCr reached the maximum value.

 

The extreme value of the amount of sodium chloride is 6.6, the extreme value of the amount of iron powder is 9.5, the extreme value of pH is 3.6, and the extreme value of stirring time is 10.2, so the impact of stirring time on the experiment is extreme.

The amount of iron powder is the largest, followed by the amount of sodium chloride, and the pH value has the smallest effect. There are errors in the experiment, including human error and instrument error. 

The human error is mainly because when adjusting the pH value of the solution, due to the limited experimental conditions, the pH test paper is used instead of the pH meter. During the measurement process, the naked eye is compared with the test paper. 

To determine the pH value, there may be more or less addition of hydrochloric acid at the same pH value. At this time, the influence of stirring time will increase, that is, whether the reaction between iron and hydrochloric acid is sufficient.

 

In the experiment, sodium chloride was added at the same time as the iron powder was added. This is because sodium chloride acts as an electrolyte during the stirring process, which can increase the conductivity of the solution. So under acidic conditions, sodium chloride can react with iron oxide, remove the oxide film on the surface of iron filings, prevent its passivation, and make the reaction more sufficient.

 

Theoretically, when the pH of the solution is adjusted with alkali, the final solution should be reddish-brown because Fe(OH)3 is a reddish-brown precipitate, but the phenomenon in this experiment is that the solution is a gray-green precipitate, because the gray-green precipitate is Fe The mixture of (OH)2 and Fe(OH)3 is an excessive color and is an intermediate product of the reaction. 

The reason why such a transition color occurs is because of excess sodium hydroxide. During the experiment, the pH value of the alkali was adjusted by adding sodium hydroxide. Because the pH value of the alkali was adjusted with pH test paper, the human error caused an excess of sodium hydroxide, so the solution was mostly gray-green flocculent. shape. .

 

From the data analysis of the above orthogonal test, the optimal combination conditions can be obtained as follows: the amount of sodium chloride is 0.024 g (400 mg/L), the amount of iron powder is 1.8 g (30 g/L), and the pH value of the acid is was 2, and the stirring time was 75 min. 

After obtaining the optimal conditions, in order to verify whether this is the optimal condition and whether the CODCr removal rate is the highest under this condition, three sets of parallel verification experiments were performed, as shown in Table 2.

It can be seen from Table 2 that the removal rate of CODCr reaches the maximum value under the optimal combination condition, which is 48.1% (the average value of the three data). Therefore, from the final conclusion, the amount of sodium chloride is 0.024 g (400 mg/L), the amount of iron powder is 1.8 g (30 g/L), the pH of the acid is 2, and the stirring time is 75 min. conditions for the best combination.

 

4. Conclusion

Through the orthogonal test, the optimum conditions for the highest removal rate of CODCr were obtained: the amount of sodium chloride was 0.024 g (400 mg/L), the amount of iron powder was 1.8 g (30 g/L), and the amount of acid The pH value was 2, and the stirring time was 75 min. Under these conditions, the CODCr removal rate of tar wastewater was the highest, reaching 48.1%.

 

After the water sample is reduced with zero-valent iron, the color of the water sample is greatly reduced. Observed with the naked eye, the color of the water sample is quite close to that of distilled water, and there is no odor, precipitation, impurities, etc. From the appearance and the data of the whole experiment, it is successful to pretreat cigarette tar waste by the method of zero-valent iron reduction.