Effect of pH on textile wastewater treatment by electrochemical coagulation

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1^th International Conference on Water, Environment and Sustainable Development, 72-72 September, 2016

University of Mohaghegh Ardabili, Ardabil, Iran

Effect of pH on textile wastewater treatment by electrochemical coagulation

Toktam Shahriari

^*1

, Bahareh Saeb

²

1Asst. Prof., Faculty of Environment, University of Tehran, Tehran, Iran

2M.S. Student, Faculty of Environment, University of Tehran, Tehran, Iran Present Address: No.22, Azin Ave., Qods Street., Enghelab Street., Tehran, Iran

*(Corresponding Author, E-mail: [email protected])

Abstract

In this study, the dyeing unit's wastewater of a textile plant was studied in order to remove color and chemical oxygen demand (COD) by electrocoagulation method and examine the effect of pH.

The utilized pilot was made of Plexiglas with ten 21ⅹ8 cm¹ iron and graphite electrodes with a thickness of 1 mm. A monopolar electrode array was used. The initial pH of the wastewater was about 7. The experiments were conducted at different pHs. According to the results, at the initial pH of 7, the optimal removal of the color and COD was 99.99 and 95.759, respectively.

Keywords: Color, Electrocoagulation, Electrode, Graphite, Wastewater

1 . Introduction

The textile industry is one of the largest consumers of water, dye and various complex chemical compounds [1-3]. The wastewater produced by this industry is classified as high-toxic wastewater due to high COD, various organic and inorganic contaminants and non-biodegradable polymeric compounds [7, 4]. Therefore, the textile industry's wastewater discharge into the environment should be avoided before proper treatment and harmful substance disinfection. In recent years, electrochemical processes such as electrocoagulation have been among the most effective methods of treating wastewater due to simple system design, lack of need for specific chemicals and little byproducts. Electrochemical processes are widely used for the treatment of industrial wastewater [5].

In electrocoagulation, by applying an electric current to two or more metal electrodes floating in the sample, metal ions are produced by electric dissolution of metal electrodes and metal hydroxide clots are thereby formed. These hydroxide clots are able to adsorb pollutants leading to precipitation and treatment of the aqueous phase. Metal ions are produced at the anode and hydrogen gas is released at the cathode.

Hydrogen gas may facilitate floatation of bulk particles to be removed from water [7]. Electrocoagulation is used for the treatment of wastewater in pharmaceutical industry [6], landfill leachate [2], slaughterhouses [8], refineries [2], electroplating effluent [11] and pulp and paper industry [11].

In this study, the treatment of textile wastewater by iron and graphite electrodes is studied.

2. Material and Methods

The samples were taken from the dyeing unit's wastewater of the Abhar textile plant.

A 17ⅹ17ⅹ74 (cm³) pilot was made of Plexiglas. Ten 17ⅹ8 (cm⁷) iron and graphite electrodes with a thickness of 7 mm were placed in the pilot with a distance of 7 cm. A monopolar electrodes array were used.

In a typical experiment, 7.5 liters of wastewater was poured in the pilot and treated by the electrocoagulation

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process. The experiments were conducted at different pHs. Then, the effluent was sampled and COD and color were analyzed according to the standard methods for water and wastewater experiments. To obtain more accurate results, the experiments were repeated 3 times and results were reported in the form of mean scores.

A magnetic stirrer (RAZI, T.A.115, Iran) was used to mix wastewater samples. A power supply (HUA, HY311F-3) was used. COD was measured by DR5111, HACH. The color of samples was analysed using a spectrophotometer (DR7111, HACH). pH was measured using a pH meter (Metrohm 621, Switzerland). Sulfuric acid and sodium hydroxide (1 N) were used to adjust the pH.

3 . Results and Discussion

In this study, COD and color removal from a textile plant's dyeing unit wastewater by electrocoagulation using graphite and iron electrodes were studied and the effect of pH was investigated. The experiments were conducted at different pHs. Figure 2 and Table 2 show the COD removal results. In these tests, temperature, magnetic stirrer speed and settling time were 15°C, 299 rpm and 09 min, respectively. As can be seen, the optimal removal of COD occurred at pH=7, i.e. the initial pH of the wastewater.

65 70 75 80 85 90 95 100

1 3 5 7 9 11

pH

COD removal (%)

Initial COD (3340 mg/L)

Figure 1. COD removal percentages after electrocoagulation at different pHs

Table 1. Comparison of COD removal at different pHs by electrocoagulation Final COD

(mg/L) Initial COD

(mg/L) Final

pH Initial

pH Electric current

(A) Time

(min)

311 3341

5.24 3.15

3 181

765 3341

8.11 5.1

3 181

147 3341

2.17 2.12

3 181

286 3341

11.73 2.17

3 181

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Figure 1 and Table 1 show the color removal at different pHs. The optimal color removal was obtained at pH =7.

75 80 85 90 95 100 105

1 3 5 7 9 11

pH

Color removal (%)

Initial Color (4980 mg/L)

Figure 2. Color removal percentages after electrocoagulation at different pHs

Table 2. Comparison of Color removal at different pHs by electrocoagulation Final Color

(mg/L) Initial Color

(mg/L) Final

pH Initial

pH Electric current

(A) Time

(min)

117 4281

5.24 3.15

3 181

64 4281

8.11 5.1

3 181

37 4281

2.17 2.12

3 181

561 4281

11.73 2.17

3 181

At the neutral pH (7), the graphite anode electrodes neutralize the negatively charged contaminants in the wastewater while the iron anodes produce ferrous ions. The ferrous ions react with hydroxide ions in the wastewater and produce a ferric hydroxide gel that traps pollutants. On the other hand, the hydrogen cations from water hydrolysis move toward the iron and graphite cathodes and produce hydrogen gas leading to flotation of clots produced by the electrocoagulation process. Therefore, the maximum removal of pollutants occurred at pH=7. At alkaline pHs, there is a competition between the hydroxide ions and negatively charged pollutants which can reduce the removal of contaminants. Studies by Hariz et al., show that pH plays an important role in the removal of pollutants by electrocoagulation process [9].

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4 . Conclusion

Graphite electrodes are both positively and negatively charged via connection to a battery. This mechanism neutralizes the wastewater's pollutants. On the other hand, the iron anode is corroded and released in the wastewater as ferrous ions. The ferrous ions and hydroxide ions from water hydrolysis form a network of hydroxide ions. This leads to neutralization and adsorption of contaminants in the wastewater. Hydrogen ions from water hydrolysis move toward the negatively charged cathode and hydrogen gas is produced. The hydrogen gas improves flotation and removal of contaminants. This mechanism completely occurs at the neutral pH (pH=7).

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