BDD anodes were selected for quinoline mineralization and influence of operating parameters, such as current density, initial quinoline concentration, supporting electrolyte, and initial pH was investigated. Based on the consideration of quinoline removal efficiency and average current efficiency, at initial quinoline concentration of 50 mg L−1 and pH of 7, the optimal condition was confirmed as current density of 75 mA cm−2, electrolysis time of 1.5 h, and Na2SO4 concentration of 0.05 mol L−1 by orthogonal test. At different electrolysis time, its effluent characteristics were focused on. The biodegradability (the ratio between BOD5 and COD) was enhanced from initial 0.02 to 0.57 at 90 min. The specific oxygen uptake rate was used to assess effluent toxicity, and the value gradually reduced with decreasing effluent organic concentration with mean value of 5.51, 4.19, and 2.20 mgO2 g−1MLSS at electrolysis time of 15, 30, and 45 min, respectively. Compared with control sample (prepared with glucose), the effluent of quinoline mineralization showed obvious inhibition effect on microorganisms at electrolysis time of 15 min, and then it was significantly faded at 30 min and 45 min.
Quinoline, as a typical heterocyclic aromatic hydrocarbon, is mainly derived from coal coking and coal gasification. Because of its pollution persistence and endocrine disturbance, it has been listed as priority pollutant by US Environmental Protection Agency (EPA) [
In this paper, one of the aims was to determine main variable effects and their optimal conditions for quinoline mineralization using BDD anodes. However, the difference from previous research was that variable effects considered not only the pollutant removal efficiency but also the average current efficiency. More importantly, its effluent characteristics including the biodegradability and microbial toxicity were focused on at different electrolysis time. And the results will help to gain basic reference for selecting advanced treatment technology for quinoline wastewater.
The electrochemical oxidation of quinoline was carried out in a 3L electrolysis cell equipped with a magnetic stirrer and direct current (DC) power supply. The BDD anodes were purchased from the company of CONDIAS GmbH with electrode size of
Methanol and acetic acid used as mobile phase in HPLC were purchased from Dikma. Quinoline used was purchased from Sigma. All other chemicals used for the preparation of quinoline wastewater and other studies were of analytical grade with more than 99% purity. And water used for solution preparation was ultrapure water with resistivity of 18.2 MΩcm.
The pH of solution was determined using a pH meter (HQ30d-PHC101, HACH, USA). COD was measured by a COD meter (ET99722, Hanna Corporation, Italy). Total organic carbon (TOC) was monitored with TOC/TN meter (multi N/C 2100, Analytik Jena AG Corporation, Germany). BOD5 was determined employing the dilution inoculation method [
Current density is an important factor affecting the electrolysis kinetics and process economics. It corresponds to the ratio between the applied current and the surface of the working electrode [
Effect of current density on quinoline degradation (a) and TOC removal (b).
The influence of initial quinoline concentration on its mineralization process was also investigated. To do this, solutions containing 50 mg L−1, 80 mg L−1, and 100 mg L−1 of this compound were comparatively degraded by BDD anodes at Na2SO4 of 0.05 mol L−1, initial pH of 7, and current density of 75 mA cm−2. Figure
Effect of initial mass concentration on quinoline degradation (a) and TOC removal (b).
Under the conditions of initial quinoline concentration of 50 mg L−1, current density of 75 mA cm−2, and initial pH of 7, different kinds of electrolytes with same molar concentration of 0.05 mol L−1, including Na2SO4, NaCl, Na2CO3, and NaHCO3, were added into BDD system, and the influence was observed from Figure
Effect of electrolyte type on quinoline degradation (a) and TOC removal (b).
As shown in the figure, the highest quinoline removal efficiency was obtained at electrolytes of Na2SO4 with its value of 96.6%; the reason for this phenomenon is that Na2SO4 will not involve oxidizing organic substances in the process of electrolysis. However, NaCl participates in the degrading reaction and produces Cl2 at anode, which further reacts with H2O and forms HClO. So NaCl selected as electrolyte is helpful for ammonia removal [
Due to the fact that initial pH can exert a significant influence on the generation of hydroxyl radicals, a solution with 50 mg L−1 of quinoline was electrolyzed at initial pH 5.0, 6.0, 7.0, 8.0, and 9.0, with Na2SO4 concentration of 0.05 mol L−1, current density of 75 mA cm−2, and electrolysis time of two hours. As shown in Figure
Effect of pH on quinoline degradation (a) and TOC removal (b).
In addition, at different pH values, the dissociation constant of quinoline also varies [
The above factors’ experimental results showed that, at specific initial quinoline concentration, electrolysis time, current density, electrolyte type, and initial pH, all factors had influence on quinoline removal, in which Na2SO4 was confirmed as optimal electrolyte and pH showed better quinoline removal in the slight alkaline medium. However, compared with the results at pH of 9 and pH of 7, it was found that pH had slightly influence on quinoline removal at higher pH. Therefore, at initial quinoline concentration of 50 mg L−1 and pH of 7, electrolysis time, current density, and Na2SO4 concentration would play major roles for quinoline removal. Thus the orthogonal test
Factors and levels of orthogonal test.
Levels | Variables | ||
---|---|---|---|
|
|
|
|
1 | 50 | 1.0 | 0.03 |
2 | 75 | 1.5 | 0.04 |
3 | 100 | 2.0 | 0.05 |
Number |
|
|
|
Quinoline removal (%) | ACE (×10−2) | |
---|---|---|---|---|---|---|
1 | 1 | 1 | 1 | 40.2 | 4.02 | |
2 | 1 | 2 | 2 | 61.8 | 4.22 | |
3 | 1 | 3 | 3 | 76.8 | 4.16 | |
4 | 2 | 1 | 2 | 46.3 | 3.16 | |
5 | 2 | 2 | 3 | 85.9 | 4.17 | |
6 | 2 | 3 | 1 | 91.7 | 3.36 | |
7 | 3 | 1 | 3 | 74.2 | 4.00 | |
8 | 3 | 2 | 1 | 92.7 | 3.40 | |
9 | 3 | 3 | 2 | 99.4 | 2.74 | |
Quinoline removal |
|
178.8 | 160.6 | 224.5 | ||
|
223.8 | 240.3 | 207.4 | |||
|
266.2 | 267.8 | 236.9 | |||
|
59.6 | 53.5 | 74.8 | |||
|
74.6 | 80.1 | 69.1 | |||
|
88.7 | 89.3 | 78.9 | |||
|
29.1 | 35.7 | 9.8 | |||
Factors |
|
|||||
Optimal condition |
|
|||||
ACE |
|
12.40 × 10−2 | 11.18 × 10−2 | 10.78 × 10−2 | ||
|
10.69 × 10−2 | 11.79 × 10−2 | 10.12 × 10−2 | |||
|
10.14 × 10−2 | 10.26 × 10−2 | 12.33 × 10−2 | |||
|
4.13 × 10−2 | 3.73 × 10−2 | 3.59 × 10−2 | |||
|
3.56 × 10−2 | 3.93 × 10−2 | 3.37 × 10−2 | |||
|
3.38 × 10−2 | 3.42 × 10−2 | 4.11 × 10−2 | |||
|
0.75 × 10−2 | 0.51 × 10−2 | 0.74 × 10−2 | |||
Factors |
|
|||||
Optimal condition |
|
Note:
According to Table
The COD and BOD5 of quinoline mineralization by BDD anodes were measured every 15 minutes under the above optimal conditions (Figure
COD, BOD5, and BOD5/COD of quinoline effluent at different electrolysis time.
The specific oxygen uptake rate (SOUR) can be used in a simple static test to assess the composition stability and toxicity of wastewater [
SOUR of effluent at electrolysis time of 15 min, 30 min, and 45 min.
Samples | OUR (mg L−1min−1) | MLSS (g L−1) | SOUR (mgO2 g−1MLSS h−1) |
---|---|---|---|
15 min | |||
Sample 1 | 0.1739 | 1.986 | 5.25 |
Sample 2 | 0.1908 | 1.988 | 5.76 |
Control sample | 0.2307 | 1.975 | 7.01 |
30 min | |||
Sample 1 | 0.1008 | 1.396 | 4.33 |
Sample 2 | 0.0991 | 1.470 | 4.04 |
Control sample | 0.1322 | 1.554 | 5.10 |
45 min | |||
Sample 1 | 0.0865 | 2.642 | 1.96 |
Sample 2 | 0.1028 | 2.528 | 2.44 |
Control sample | 0.0946 | 2.202 | 2.58 |
Note: OUR is oxygen uptake rate.
BDD anode represented an efficient method for mineralization of quinoline, and the effect of electrolyte type showed that the excellent quinoline and TOC removal were achieved at 0.05 mol L−1 Na2SO4 as electrolyte; the effect of pH showed that removal efficiencies of quinoline and TOC were better in the slight alkaline medium. Moreover, at initial quinoline concentration of 50 mg L−1 and initial pH of 7, orthogonal experiment for quinoline removal and average current efficiency showed that the optimal electrolysis condition was current density of 75 mA cm−2, electrolysis time of 1.5 h, and Na2SO4 concentration of 0.05 mol L−1 with quinoline removal efficiency of 85.9% and average current efficiency of 4.17 × 10−2.
At different electrolysis time, the characteristic of quinoline effluent treated by BDD anodes was evaluated from variation of BOD5/COD and toxicity to microorganism. BOD5/COD was enhanced from initial 0.02 to 0.57 at 90 min. The SOUR was used to assess compost stability and toxicity of wastewater, and the value gradually reduced with decreasing effluent organic concentration with mean value of 5.51, 4.19, and 2.20 mgO2 g−1MLSS at electrolysis time of 15, 30, and 45 min, respectively. Meanwhile, at the same COD concentration, the SOUR of control sample (prepared with glucose) was 7.01, 5.10, and 2.58 mgO2 g−1MLSS, respectively. It can be comparatively found that the effluent of quinoline mineralization showed obvious inhibition effect on microorganisms at electrolysis time of 15 min, and then it was significantly faded at 30 min and 45 min. The above results will help to gain basic reference for selecting advanced quinoline wastewater treatment technology.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This research was funded by the Fundamental Research Funds for the Central Universities (2009QH01) and the Special Research Funding for the Public Benefits Sponsored by the Ministry of Environmental Protection of China (2012467025). The anonymous reviewers are also gratefully acknowledged for their very helpful comments and suggestions.