- Research article
- Open Access
Determining the degradation efficiency and mechanisms of ethyl violet using HPLC-PDA-ESI-MS and GC-MS
- Wen-Hsin Chung†2,
- Chung-Shin Lu†3,
- Wan-Yu Lin†2,
- Jian-Xun Wang†1,
- Chia-Wei Wu†1 and
- Chiing-Chang Chen1Email author
© Wang et al.; licensee BioMed Central Ltd. 2012
Received: 21 February 2012
Accepted: 7 June 2012
Published: 30 June 2012
The Erratum to this article has been published in Chemistry Central Journal 2014 8:24
The discharge of wastewater that contains high concentrations of reactive dyes is a well-known problem associated with dyestuff activities. In recent years, semiconductor photocatalysis has become more and more attractive and important since it has a great potential to contribute to such environmental problems. One of the most important aspects of environmental photocatalysis is in the selection of semiconductor materials like ZnO and TiO2, which are close to being two of the ideal photocatalysts in several respects. For example, they are relatively inexpensive, and they provide photo-generated holes with high oxidizing power due to their wide band gap energy. In this work, nanostructural ZnO film on the Zn foil of the Alkaline-Manganese Dioxide-Zinc Cell was fabricated to degrade EV dye. The major innovation of this paper is to obtain the degradation mechanism of ethyl violet dyes resulting from the HPLC-PDA-ESI-MS analyses.
The fabrication of ZnO nanostructures on zinc foils with a simple solution-based corrosion strategy and the synthesis, characterization, application, and implication of Zn would be reported in this study. Other objectives of this research are to identify the reaction intermediates and to understand the detailed degradation mechanism of EV dye, as model compound of triphenylmethane dye, with active Zn metal, by HPLC-ESI-MS and GC-MS.
ZnO nanostructure/Zn-foils had an excellent potential for future applications on the photocatalytic degradation of the organic dye in the environmental remediation. The intermediates of the degradation process were separated and characterized by the HPLC-PDA-ESI-MS and GC-MS, and twenty-six intermediates were characterized in this study. Based on the variation of the amount of intermediates, possible degradation pathways for the decolorization of dyes are also proposed and discussed.
It is estimated that over 700,000 tons of dyes and pigments are produced annually worldwide, 20% of which are utilized for textile dyeing and finishing processes . Many of these synthetic dyestuffs cannot be removed using conventional treatments due to their complex polyaromatic structures, resulting in various environmental problems . The textile, paper, food, cosmetic, and leather goods industries are all major consumers of triphenylmethane dyes [1, 2]. Previous reports [3, 4] have demonstrated the photodegradation of triphenylmethane dyes containing N-alkylamine groups via consecutive N-de-alkylation reactions. Other studies have reported that thyroid peroxidase-catalyzed oxidation of triphenylmethane dyes could result in the formation of various N-de-alkylated primary and secondary aromatic amines, with structures similar to those of aromatic amine carcinogens . Previous studies [6, 7] on the photocatalytic degradation of nitrogen-containing aromatic compounds have demonstrated that both electrons and hydroxyl radicals transform amine functional groups.
Zinc oxide is an important solid state material possessing photocatalytic  and piezoelectric properties, as well as demonstrating field emission and lasing action with a wide range of potential technological applications [8, 9]. Recently, a variety of methods have been developed for the synthesis of nanostructural ZnO, including hydrothermal, vapor-liquid–solid, vapor solid, and other solution processes [10–21]. A low-temperature chemical-liquid deposition method has been employed to grow oriented ZnO nanorods by continuously supplying Zn ions from a Zn foil to form a ZnO thin film in aqueous formaldehyde solution . Similar reactions have been achieved using Zn2+ salt with ethanol in the presence of amine to produce one-dimensional nanostructures of ZnO . Hydrothermal reactions have also been used in the preparation of the ZnO nanorods, employing zinc acetate dissolved in ethanol with polyvinylpyrolidone and NaOH . Heating zinc nitrate and NaOH in a mixture of ethylenediamine and water at 180°C for 20 h produces ZnO nanorods . In the presence of ethylenediamine, the reaction of Zn foil with water under hydrothermal conditions (150-230°C) has reportedly yielded ZnO nanorods [26–31].
It has recently been discovered that cleaving a C-O single bond of the aliphatic alcohols on zinc metal surfaces produces ZnO nanoparticles . Unfortunately, these techniques often require high temperatures. In addition, the reaction of Zn metal with liquid water may also produce ZnO nanostructures in a reaction associated with the evolution of hydrogen in acidic conditions . The methods described in the literature generally use amines and other additives or zinc compounds at higher temperatures.
This study selected zinc foil obtained from waste Alkaline-manganese Dioxide-zinc cells as the substrate for the generation of ZnO nanostructures because the lattice matching between ZnO and Zn crystals facilitates the generation of ZnO nanostructures, and the zinc foil in these cells is waste material useful in the treatment of organic wastewater through photocatalysis. Zinc foil can serve as both reactant and substrate to support ZnO nanostructures without an additional substrate. The method is simple and practical, requiring only zinc foil, and may be performed at low temperatures. This simple method has not been previously reported in any studies. This makes it a suitable and economical approach to the treatment of organic wastewater. This study reports on the fabrication of ZnO nanostructures on zinc foil using a simple solution-based corrosion strategy, and provides detailed descriptions related to the synthesis, characterization, application, and implications of using Zn in this manner. Other objectives of this research include the identification of reaction intermediates to understand the underlying mechanisms in the degradation of EV dye as a model compound of triphenylmethane dye, with active Zn metal, using HPLC-ESI-MS and GC-MS. It is hoped that the results will provide a foundation for future environmental applications.
Materials and reagents
The Zn foils were ultrasonically washed in HPLC-grade acetone three times prior to use. A mixture solution was prepared by adding Zn foil (0.05 m × 0.05 m) to a 0.25 L aqueous solution containing EV at appropriate concentrations. The initial pH of the solution was adjusted by adding either NaOH or HNO3 solution to produce reactions of various pH values. At set intervals during the reaction, the solution was sampled. The residual dye and organic intermediates were analyzed using HPLC-PDA-ESI-MS and GC-MS. Dark experiments performed in a beaker with Zn foil also demonstrated the decolorization of the dye solution. Irradiation experiments were carried out for comparison using 15 W lamps to determine the stability of EV dye under UV or visible light irradiation. The 0.01 gL-1 EV solutions did not show significant de-coloration under UV irradiation without Zn foil. Following the reaction, the Zn foil was removed, washed with de-ionized water and ethanol several times, and then dried with nitrogen. These Zn foils were characterized using X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), and high resolution X-ray photoelectron spectrometry (HRXPS).
Instruments and analytical methods
XRD patterns were recorded on a MAC Science, MXP18 X-ray diffractometer with Cu α radiation, operating at 40 kV and 0.08 A. FE-SEM measurement was carried out using a field-emission microscope (JEOL JSM-7401 F) operating at an acceleration voltage of 1.5 × 104 V. HRXPS measurement was carried out with ULVAC-PHI XPS: PHI Quantera SXM to measure changes in the surface structure following reflux treatment. The binding energy values reported in the present work were corrected with a C1s peak at 284.8 eV to take into account charging effects.
A Waters ZQ LC/MS system equipped with Waters 1525 Binary HPLC pumps, a Waters 2998 Photodiode Array Detector, a Waters 717 plus auto sampler, and a Waters micromass-ZQ 2000 detector were used. The analysis of organic intermediates was accomplished using HPLC-PDA-ESI-MS following the readjustment of chromatographic conditions to make the mobile phase compatible with the working conditions of the mass spectrometer. Two types of eluent were employed in this study: solvent A, 0.025 M aqueous ammonium acetate buffer (pH 6.9); and solvent B, methanol. LC was carried out on an Atlantis TM dC18 column (0.25 m × 0.046 m i.d., 5 × 10-6 m film thickness). The flow rate of the mobile phase was set at 0.001 L.min-1. Column effluent was introduced into the ESI source of the mass spectrometer.
Solid-Phase extraction (SPE) was employed for the pre-concentration of irradiated samples prior to GC-MS analysis. GC/MS analysis was performed on a Perkin-Elmer AutoSystem-XL gas chromatograph interfaced with a TurboMass selective mass detector. Separation was carried out in a DB-5 capillary column (5% diphenyl/95% dimethyl-siloxane) with 60 m, 2.5 × 10-4 m i.d., and film thickness of 1.0 × 10-6 m. Electron impact (EI) mass spectra were monitored from 35 to 300 m/z. The ion source and inlet line temperatures were set at 220 and 280°C, respectively.
Results and discussion
The results obtained by reacting Zn metal with water are encouraging and have led to an examination of the reaction of ethyl violet dye in aqueous solutions. This study describes a very simple method to generate ZnO nanostructures in which ethyl violet dye is decomposed through a reaction of liquid water with metals. Similar reaction of Zn metal with liquid water may also produce ZnO nanostructures in a reaction associated with the evolution of hydrogen in acidic conditions. The methods described in the literature generally use amines and other additives or zinc compounds at higher temperatures .
Effect of pH and dye concentration
Reusability of Zn-foil
Separation of the intermediates
UV-visible spectra of the intermediates
The UV-PDA adsorption spectra of the intermediates are depicted in the Additional file 1: Figure S2, identified as A-J and a-f, corresponding to the peaks A-J and a-f, in Figure 9, respectively. The maximum absorption of the spectral band shifted from 591.8 nm (spectrum A) to 561.7 nm (spectrum I), and from 371.6 nm (spectrum a) to 340.6 nm (spectrum f). Presumably, these shifts are due to the formation of a series of N-de-ethylated and N-hydroxyethylated intermediates. From these results, several groups of intermediates were identified.
The first group is marked in the chromatogram and illustrated in Figure 9(a). The major absorption bands of the intermediates of N-de-ethylated EV dye are shifted toward the blue region, λmax, A (EV), 591.8 nm; B, 585.6 nm; C, 571.9 nm; D, 573.2 nm; E, 582.1 nm; F, 569.7 nm; G, 565.6 nm; H, 563.6 nm; I, 561.7 nm. The N-de-ethylation of the EV dye causes the shift in wavelength because of an attack on the N, N-diethyl or N-ethyl group by the active oxygen species, as depicted in Additional file 1: Figure S2. It has been reported  that EV dye is N-de-ethylated in a stepwise manner (i.e., Ethyl groups are removed stepwise as confirmed by the blue shifts in the maximum absorbance of the separated intermediates).
The second and third groups are marked in the chromatogram in Figure 9(b). Destruction of EV yields DAP, DDBP, and N-de-ethylated products, N-hydroxyethylated intermediates. The N-de-ethylation derivatives of the DDBP and the N-hydroxyethylated intermediates of the N-de-ethylated DDBP species, produced by the cleavage of the EV chromophore ring structure, have their λmax blue shifted: a, 371.6 nm; b, 366.7 nm; c, 365.5 nm; d, 367.9 nm; e, 352.6 nm; f, 340.6 nm. The proposed intermediate a compared well with standard material of 4-(N, N-diethylamino)-4′-(N′, N′-diethylamino) benzophenone.
The fourth and the fifth groups are marked in the chromatogram and illustrated in Figure 9(c). The N-de-ethylation derivatives of the DAP, produced by the cleavage of the EV chromophore ring structure, also have their λmax blue shifted: α, 290.4 nm; β, 282.1 nm; γ, 272.6 nm as previously reported .
Mass spectra of the intermediates
Intermediates of the degradation of EV identified by HPLC-ESI-MS or GC-EI-MS
MS peaks (m/z)
Absorption maximum (nm)
Degradation mechanisms of EV
N-de-ethylation of EV
Destruction of the conjugated structure of the EV
As described above, electrons flow to the EV molecule via the positive diethylamine group. Following the transfer of electrons, the conjugated structure yields a carbon-centered radical, which is subsequently attacked by molecular oxygen, leading ultimately to a and α. The destruction of the conjugated structure of the EV dye most likely occurs through the attack of O2 on the carbon-centered radical of the EV, as intermediates a~f are isolated from the HPLC chromatogram. This process also occurs in N-de-ethylated EV derivatives (B to F), which are adsorbed on the Zn surface, implicating electrons in other similar events (electron attack, hydrolysis, or deprotonation, and/or oxygen attack) to yield the mono-N-de-ethylated derivative b. A similar process occurred in α to produce β. The N-de-ethylation process for a and α continues until the formation of the complete N-de-ethylated derivative f and γ. All of the above N-de-ethylation processes also produced a parallel series of N-de-hydroxyethylated intermediates through the hydroxylation of the N-ethyl group. All intermediates were further degraded to N, N-diethylaminobenzene, N-ethylaminobenzene, aminobenzene, acetamide, 2-propenoic acid, and acetic acid, which were subsequently mineralized to CO32– and NO3–. The degradation intermediates clearly reached their maximum concentrations, although some might have been under the detection limit. Mechanisms similar to those proposed here were also observed in a previous study of the MEK/TiO2 system .
This paper used HPLC-PDA-ESI-MS analysis to identify the mechanism underlying the degradation of ethyl violet dyes. In this study, a nanostructural ZnO film was produced on the Zn foil from Alkaline-Manganese Dioxide-Zinc cells, providing outstanding potential for future applications in the photocatalytic degradation of organic dye for environmental remediation. This study used HPLC-PDA-ESI-MS and GC-MS to differentiate and characterize twenty-six intermediates of the degradation process. According to variations in the quantity of intermediates, various possible degradation pathways for the decolorization of dyes were also proposed and discussed.
This research was supported by the National Science Council of the Republic of China (NSC 99-2113-M-142-001-MY2; NSC 100-2622-M-142-001-CC1).
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