The multiwall carbon nanotube-mollified electrode (MWCNT-ME) was fabricated and its electrocatalytic activity of refractory organic pollutants of coking wastewater was investigated. The surface morphology, absorption properties, and the electrochemical behavior of phenol and aniline at the MWCNT-ME were analyzed. Using ultraviolet-visible adsorption spectroscopy (UV-vis), Gas chromatography mass spectrometry (GC/MS), and chemical oxygen demand (COD) test, the electrochemical oxidation properties of refractory organic pollutants of coking wastewater using the MWCNT-ME and the IrSnSb/Ti electrode were analyzed. Compared with the powder adsorption media, the MWCNT-ME was proved to have weaker adsorption activity, which means electrochemical degradation is the decisive factor of the removal of organic pollutants. The MWCNT-ME shows high electrochemical reactivity with oxidation peaks of 0.18 A and 0.12 A for phenol and aniline, respectively. Under the same working conditions, the MWCNT-ME COD removal rate 51% is higher than IrSnSb/Ti electrode’s rate 35%. The MWCNT-ME has application potential of electrochemical oxidation of refractory organic pollutants of coking wastewater.
The increasing emission of refractory organic pollutants has challenged the conventional biological treatment. These contaminants such as polynuclear aromatic hydrocarbons PAHs, nitrogen-, oxygen-, and sulfur-containing heterocyclic compounds possess high resistance to microbial degradation or utilization. As one of the advanced treatment systems, electrochemical oxidation has drawn significant attention in water treatment research [
With the combination of high aspect ratio, nanometer-sized dimensions, good electrical conductivity [
This paper is focused on the electrochemical degradation characteristics of refractory organic pollutants in coking wastewater on a multiwall carbon nanotube-modified electrode (MWCNT-ME). To the best of our knowledge, few studies on electrocatalytic properties of CNT-modified electrodes for degradation of refractory organic pollutants in actual wastewater have been reported. Coking wastewater after biologically purification was chosen as model reactant, in which constituents are low concentration, refractory, toxic, and carcinogenic. The basic electrode configuration of the MWCNT-ME is randomly dispersed multiwall carbon nanotubes (MWCNTs) in polytetrafluoroethylene (PTFE) dropped on the surface of a Ti conducting support macroelectrode. Firstly, phenol and aniline were chosen as the model reactants to exam the electrocatalytic characteristics of the MWCNT-ME. As priority pollutants, phenol and aniline are ubiquitous in the environment and have been chosen frequently as typical pollutants because many data are available on their removal and destruction, in particular, with respect to wastewater treatments [
MWCNTs were purchased from Shenzhen Nanotech Port Co., Ltd. (Shenzhen, China), a commercial supplier of carbon nanotubes. MWCNTs have tube diameters of 60–100 nm, lengths of 5–15
PTFE is a chemically stable material. The fluoropolymer composite film prepared with PTFE and MWCNTs had a high resistance to solvents, acids, and bases [
The MWCNTs and activated carbon with same quantity of 1 g were fed into the two beakers filled with the wastewater with volume of 1 L, respectively. The adsorption experiment lasted for 1 h and proceeded with magmatic stirring. 5 mL each of treated wastewater before and after adsorption experiment were taken for UV-vis spectrum test. The same experiment was carried out for the MWCNT-ME which contains 1 g MWCNTs.
All the electrochemical experiments were performed at
The degradation experiment of coking wastewater was carried out in an electrochemical cell under magnetic stirring with a volume of 1 L. The performing temperature was at
Figure
The Field-emission SEM (FESEM) top-view image of the MWCNT-ME.
As quantum wires made up of curled graphite layers, MWCNTs have obvious adsorption ability for fluid and gas. UV-vis spectrums of samples before and after degradation experiment were shown in Figure
UV-Vis spectrum of coking wastewater (a) untreated, (b) treated by the MWCNT-ME, (c) treated by active carbon, and (d) treated by MWCNTs powder.
For single-ring aromatic organic pollutants whose absorbency of UV-vis lies in the range from 200 nm to 250 nm, active carbon has better adsorption ability than MWCNTs powder. Basic structure cell of the active carbon is graphite-like microcrystalline with hybrid of sp2, graphite-like microcrystalline cells form the ultrafine particles with nanoscale, and pore structures with different scales can be found in this system in which adsorption can be easy going. For MWCNTs, the adsorption sites lie on inner walls of tubes with open ports and accumulation pores formed among MWCNTs. However, lack of open ports of MWCNTs affects adsorption efficiency of single-ring aromatic organic pollutants with small molecular diameter in coking wastewater, which is consistent with the result in UV-vis spectrum. As the binder of the MWCNT-ME, infiltration of PTFE over the MWCNTs will decrease adsorption sites. In addition, only MWCNTs on the surface of MWCNT-ME can react with pollutants in the solution. Compared with the powder adsorption, media active adsorption sites on the MWCNT-ME were reduced. Adsorption ability is weakened, which makes electrochemical degradation be the determinant of concentration reduction of the pollutants.
Electrochemical oxidation is a common way to abate phenol and Aniline. MWCNT-ME is expected to have good performance when used for electrochemical oxidation of phenol and aniline. Figure
Performance of MWCNT-ME in oxidation of phenol and aniline, scan rate: 0.01 V/s (a) 0.06 g/L phenol in 20 g/L NaSO4 and (b) 0.06 g/L aniline in 20 g/L NaSO4.
The UV-vis spectrum during electrochemical oxidation of coking wastewater by MWCNT-ME was showed in Figure
UV-Vis spectra of coking wastewater oxidized by MWCNT-ME for (a) 0 min, (b) 30 min, (c) 60 min, (d) 120 min.
Figure
COD of coking wastewater treated by IrSnSb/Ti electrode and MWCNT-ME.
IrSnSb/Ti | MWCNT-ME | |
---|---|---|
COD (after tow-hour electrochemical degradation) | 93 | 71 |
Degradation efficiency (%) | 35.9 | 51 |
GC/MS spectra of the coking wastewater samples of (a) untreated, (b) treated by the MWCNT-ME for 120 min, (c) treated by the IrSnSb/Ti electrode for 120 min.
An MWCNT-ME was made. Basic electrode configuration of the MWCNT-ME is randomly dispersed MWCNTs in PTFE dropped on the surface of a Ti conducting support macroelectrode. Adsorption activity of MWCNT-ME was proved to be weaker than powder adsorption media. The removal of refractory organic pollutants in coking wastewater mainly depends on electrochemical degradation occurred on MWCNT-ME. The CV curves for phenol and aniline have obvious oxidation and reduction peaks which reflect electrochemical reactivity of MWCNT-ME for some refractory organic pollutants. Such reactivity is furthermore reflected in the concentration decrease of the organic pollutants expressed in the UV-vis spectrum. The COD removal and change of organic pollutant species showed that MWCNT-ME has better performance than IrSnSb/Ti electrode on degradation efficiency. MWCNT-ME could be applied to process degradation of refractory organic pollutant in coking wastewater.
The financial support of Shanghai Science and Technology Cooperation Foundation (08230707200 and 0952nm06300) and Natural Science Foundation of China (91023029) is gratefully acknowledged.