NaNO2/FeCl3 dioxygen recyclable activator: An efficient approach to active oxygen species for degradation of a broad range of organic dye pollutants in water
Introduction
The textile industries are becoming a major source of environmental contamination because an alarming amount of dye pollutants are generated during the dyeing processes [1]. In fact, about 700 000 tonnes of different dyes are produced annually in the world, among which about 50 000 tonnes of dyes are discharged into the environment [2]. In China, over 1.6 × 109 m3 of dye-containing wastewater per year is drained into environmental water system without having been properly treated. Furthermore, one of the most marked features of modern dyes is their high degree of chemical, photolytic, and microbiological stability. Thus dyes are not readily degradable under aerobic conditions prevailing in the conventional biological treatment processes [3], [4], [5]. And other conventional treatment technologies such as chemical coagulation, activated carbon adsorption, electrochemical treatment and reverse osmosis are usually inefficient in treating those dye pollutants [6]. Therefore, it is greatly needed to develop practical and highly efficient technologies for treatment of dye-containing wastewater.
In the last decades, oxidative degradation methods aiming at treating persistent organic pollutants in industrial wastewaters as versatile strategies have been actively explored [7], [8], [9]. From an economical and environmental viewpoint, the pursuit of using molecular oxygen (or air) as the terminal-oxidizing agent constitutes a highly attractive target for oxidative degradation of pollutants [10]. Wet oxidation (WO) is considered as one of the most promising and efficient techniques for degrading the pollutants to CO2/CO or more biologically amenable intermediates [11]. Thus a large number of wet oxidation methods for the degradation of environmental pollutants have been developed [7], [12], [13], [14], [15], [16]. The typical conditions of industrial operation of WO are high temperature (200–320 °C) and high pressure (2–20 MPa), which adversely affect the cost benefit ratio of the wet oxidation process [11], [17]. Attempting to reduce the harsh process conditions, catalytic wet oxidation (CWO) has been studied extensively and numerous catalysts have been developed. Several research groups have studied the CWO of dyeing and printing wastewater and obtained some encouraging results [18], [19], [20], [21], [22], [23], [24], [25], [26]. Among these catalyst systems, copper salts were the most active homogeneous catalysts, but their use required a separation step to remove the toxic copper ion from the final effluent [27], [28]. This drawback can be overcome by using heterogeneous catalysts which are easily retrievable and reused [18]. However, these heterogeneous catalysts, in many cases, are easily deactivated by sintering, poisoning or fouling [13], [27]. Also, in hot acidic environments, the active components may dissolve into the liquid phase which is potentially toxic to the environment [13]. On the other hand, the degradation methods mainly focused on the treatment of azo dye pollutants, little information about CWO of the anthraquinone dyes was offered. Based on the above discussion, we sought to envisage and develop an innovative and inexpensive treatment method for a wide range of dye pollutants including anthraquinone dyes that would inherently facilitate industrialization. Specifically, we centered our attention on finding a catalytic system that utilizes inexpensive and readily available metal which is greener than copper as the catalyst, and cheap auxiliary agents as the co-catalyst. As such, one key feature is the catalytic system being effective for a broad range of dye pollutants at a wide pH range. Here we disclose that a combined catalyst of sodium nitrite and ferric chloride can preliminarily achieve these goals.
Section snippets
Materials and reagents
Acid Blue 129 (AB129) was purified by the well-known N,N-dimethylformamide–acetone method [29]. All the other acid dyes (Hangzhou Xiasa Hengsheng Chemical Co. Ltd., China) and reactive dyes (Shanghai Matex Chemicals Co. Ltd., China) were used without further purification. HCl, NaOH, NaCl, NaNO2, Na2SO4, FeCl3·6H2O, methanol and diethyl ether were of analytical grade and used without further purification. 0.2 M Me3S+OH− methanol solution (Tokyo Kasei Kodyo Co. Ltd., Japan) was purchased from
CWO of AB129
We have recently developed a novel catalyst (sodium nitrite), which showed a high catalytic activity for the aerobic oxidative degradation of trichlorophenol, a kind of persistent environmental pollutant [30]. Attempting to extend the oxidative degradation methodology, we designed the NaNO2-based aerobic oxidative degradation system for anthraquinone dye pollutants. AB129 (the molecular structure is shown in Fig. 1) was used as a prototypical pollutant to examine its degradation in such a
Conclusions
The FeCl3/NaNO2/O2 system efficiently bleaches a broad range of organic dyes and oxidizes the substrates into CO2 and biodegradable organic products with a high-conversion rate. To the best of our knowledge, such a broad range of organic dyes being bleached and relatively highly mineralized using dioxygen as the oxidant has not been reported previously. As such, the oxidative degradation method remains effective at a relatively wide pH range. Furthermore, the use of dioxygen as the oxidizing
Acknowledgements
We gratefully acknowledge the financial support from the grant of Knowledge Innovation Program of the Chinese Academy of Sciences (KJCX2-YW-H04), the National Natural Science Foundation of China (No. 20572110, No. 20707026) and Key Project of Knowledge Innovation Program of Chinese Academy of Sciences (KGCX2-SW-213). We also thank Xiaoli Dong (Dalian Polytechnic University) for total organic carbon analysis and Prof. Lefeng Zhang for his assistance in revision of the manuscript.
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