Reactive oxygen species dependent degradation pathway of 4-chlorophenol with Fe@Fe2O3 core–shell nanowires
Introduction
Chlorophenols are classified as the hazardous and top priority pollutants by the United States Environmental Protection Agency [1], [2]. They are chemical intermediates or byproducts in various synthetic industries, and can also be generated during drinking water disinfection process [3], [4]. For example, 4-chlorophenol (4-CP) has been widely used in the synthesis of pesticides, herbicides, disinfectants, and wood preservatives. They are irritant to the respiratory and central nervous systems at low levels, and can induce cancer at higher doses. Because of their high toxicity, poor biodegradability, and carcinogenic potential, the removal of chlorophenols is of great significance for environmental protection [5], [6].
Advanced oxidation processes (AOPs) could effectively degrade and mineralize chlorophenols via generating highly reactive hydroxyl radicals [7]. However, most of AOPs require the utilization of costly oxidants (ozone and hydrogen peroxide, etc) and/or intense energy (e.g. ultraviolet light and electricity) [7]. Comparing with ozone and hydrogen peroxide, molecular oxygen is a greener and lower cost oxidant ubiquitously existed in the environment. Unfortunately, the reactions between molecular oxygen and chlorophenols are spin-forbidden at ambient conditions. Therefore, it is a challenge to degrade chlorophenols with activated molecular oxygen.
Iron is the fourth most abundant element in the Earth's crust and therefore involved in many important environmental and biochemical processes, including the generation and consumption of reactive oxygen species (ROS), the decomposition and conversion of organics, as well as the respiration and nutrition intake of some microorganism, and so on. Although the generation of ROS via the reaction of zero-valent iron (ZVI) and molecular oxygen has been reported for many years, the application of iron/oxygen chemistry for environmental pollutant control and remediation is far from satisfactory because of its low ROS generation efficiency. The development of nanotechnology provides a solution to solve this low efficiency problem as scientists have found that the utilization of nanoscale zero-valent iron (nZVI) could enhance the ROS generation to some degree [8]. Fe@Fe2O3 core–shell nanowires are a special kind of air stable and highly active nZVI developed by our group [9]. Very recently, we found Fe@Fe2O3 nanowires showed interesting core–shell structure dependent reactivity on the aerobic 4-CP degradation. However, only 77.8% of 4-CP (1.1 mmol/L) could be aerobically degraded with the most reactive Fe@Fe2O3 nanowires in 7 h, accompanying with 30% of mineralization in 10 h. Obviously, further efforts must be made to improve the ROS generation by iron/oxygen chemistry for its practical applications.
It is an important breakthrough for scientists to find that ethylenediamine tetraacetate (EDTA), one of polyaminocarboxylic ligands, could significantly improve the generation of ROS via the reaction of ZVI and molecular oxygen for the degradation of various organic pollutants [10]. Nevertheless, EDTA may cause some unfavorable environmental consequences because of its poor biodegradability and superior heavy metal chelating ability. For example, EDTA may form stable water-soluble complexes with many radionuclides to increase the risk of radionuclide transport in the subsurface environment [11]. It is indispensable to find some safe and environmentally benign replacement for EDTA for the environmental control and remediation purposes. In comparison with EDTA, diethylenetriamine pentaacetate (DTPA) ligand is more environmentally friendly [12], [13], [14], but much less been paid attention. In this study, we compare the performances of DTPA and EDTA on the aerobic 4-CP degradation with Fe@Fe2O3 nanowires by investigating the generation of ROS as well as the degradation and mineralization of 4-CP systematically. The purposes of this study are to clarify the roles of polyaminocarboxylic ligands on the molecular oxygen activation over nZVI and develop a green method to remove chlorophenols.
Section snippets
Chemicals and materials
DTPA, FeCl3·6H2O, NaBH4, TA, and 1,10-phenanthroline monohydrate were all of commercially available analytical grade and purchased from National Medicines Corporation Ltd. of China. 4-CP was purchased from Acros. Superoxide dismutase (SOD), horseradish peroxidase, and p-hydroxyphenylacetic acid (POHPAA) were purchased from Aladdin. CH3OH, C2H5OH, C3H7OH, and C4H9OH were purchased from National Medicines Corporation Ltd. of China. High performance liquid chromatography (HPLC) grade acetonitrile
The removal of 4-CP and TOC
4-CP could not be degraded with air in the absence of Fe@Fe2O3, while DTPA or EDTA did not induce the aerobic 4-CP degradation alone. Only 22.3% of 4-CP was aerobically degraded with Fe@Fe2O3 in 2 h. The 4-CP removal efficiencies in 2 h reached 100% and 97.3% for the Fe@Fe2O3/EDTA/Air and Fe@Fe2O3/DTPA/Air systems, respectively (Fig. 1a). The aerobic 4-CP degradation processes were found to follow pseudo-first order kinetics within 90 min (Fig. 1b), while the 4-CP degradation rate constants were
Conclusions
In summary, we have demonstrated that the environmentally friendly diethylenetriamine pentacetate ligand is more effective to promote molecular oxygen activation with Fe@Fe2O3 core–shell nanowires than the most used ethylenediamine tetraacetate of poor biodegradable property. Both diethylenetriamine pentacetate ligand and 4-chlorophenol could be rapidly mineralized by more hydroxyl radicals generated through Fe@Fe2O3 nanowires induced molecular oxygen activation in the presence of DTPA. We
Acknowledgements
This work was supported by National Natural Science Foundation of China (Grants 21173093 and 21177048), Key Project of Natural Science Foundation of Hubei Province (Grant 2013CFA114), and self-determined research funds of CCNU from the colleges’ basic research and operation of MOE.
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−. The mechanistic analysis showed that in the first stage, the lower degradation efficiency could due to the self-quenching of the free radicals, with the uncomplexed Fe(II) and Fe(III) on the surface, the free NTA in the solution, or with the PMS, in addition to reacting with the pollutants. In the second stage, the ATZ achieved rapid degradation as the concentration of substances competing for free radicals decreased. Our work elucidates the key role of NTA functional ligand in the Fe3O4/PMS advanced oxidation processes, which may provide new approach for the efficient degradation of EOPs.