Degradation of organic contaminants through the activation of oxygen using zero valent copper coupled with sodium tripolyphosphate under neutral conditions

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Abstract

In this study, sodium tripolyphosphate (STPP) was used to promote the removal of organic pollutants in a zero-valent copper (ZVC)/O2 system under neutral conditions for the first time. 20 mg/L p-nitrophenol (PNP) can be completely decomposed within 120 min in the ZVC/O2/STPP system. The PNP degradation process followed pseudo-first-order kinetics and the degradation rate of PNP gradually increased upon the decreasing ZVC particle size. The optimal pH of the reaction system was 5.0. Our mechanism investigation showed that Cu+ generated by ZVC corrosion was the main reducing agent for the activation of O2 to produce ROS. ·OH was identified as the only ROS formed during the degradation of PNP and its production pathway was the double-electron activation of O2 (O2→H2O2→·OH). In this process, STPP did not only promote the release of Cu+ through its complexation, but also promoted the production of ·OH by reducing the redox potential of Cu2+/Cu+. In addition, we could initiate and terminate the reaction by controlling the pH. At pH < 8.1, ZVC/O2/STPP could continuously degrade organic pollutants; at pH > 8.1, the reaction was terminated. STPP was recycled to continuously promote the corrosion of ZVC and O2 activation as long as the pH was <8.1. This study provided a new and efficient way for O2 activation and organic contaminants removal.

Introduction

Advanced oxidation processes (AOPs) involving the generation of reactive oxygen species (ROS), such as hydroxyl radicals (·OH), are regarded as one of the most promising techniques for the degradation of organic pollutants (Cinar et al., 2017, Lee et al., 2007, Matzek and Carter, 2016, Yan et al., 2015, Zhao et al., 2014). The traditional Fenton reaction generates ·OH by reacting Fe2+ with H2O2 under acidic conditions. However, the use of H2O2 in the system has the disadvantages of poor stability, low utilization efficiency, and substantial environmental impact (Joseph et al., 2009, Saritha et al., 2007). Oxygen is a green, economic and readily available oxidant. In recent years, researchers have confirmed that metal elements, including iron and aluminum, can activate oxygen to produce ·OH (Bokare and Wonyong, 2009, Lin et al., 2014, Liu et al., 2011, Michael et al., 2011, Noradoun and I Francis, 2005, Welch et al., 2002). For example, Sedlak and coworkers (Keenan and Sedlak, 2008) found that EDTA was effectively degraded by a ZVI/O2 system at pH 3.5 and reported the mechanism for ·OH production in the system in detail. Putschew and coworkers reached a similar conclusion. Similar to ZVI, zero-valent aluminum (ZVAl)/O2 was also able to remove organic pollutants such as 2,4-dichlorophenol under acidic conditions (Lin et al., 2014). In addition to the above two metals, the activation of oxygen by Cu+ has also been reported (Feng et al., 2017, Lee et al., 2016, Zhou et al., 2018). It can not only activate molecular oxygen to generate H2O2, but induce the decomposition of as-formed H2O2 to produce ·OH (Deng et al., 2019). It is well established that ·OH is produced upon the reaction of Cu+ and O2, but there is still a debate in regard the mechanism and pathway for the generation of ·OH in this system. Some researchers believe that the mechanism of ·OH production is the single-electron activation of O2 (O2→O2·-→H2O2→·OH) (Xiu et al., 2012, Zhou et al., 2017). Specifically, Cu+ activates O2 to generate superoxide radicals (O2·-) (Eq. (1)), and H2O2 and ·OH are then sequentially produced (Eqs. (2), (3)). However, the other researchers believe that the production of ·OH is through the double-electron activation of O2 (O2→H2O2→·OH) (Eqs. (3), (4)) (Dong et al., 2014, Zhang et al., 2017). Therefore, it is necessary to further study the mechanism of ROS production in the Cu+/O2 reaction system.Cu+ + O2 → Cu2+ + O2·-Cu+ + O2·- + 2H+ → Cu2+ + H2O2Cu+ + H2O2 → Cu2+ + ·OH + OH2Cu+ + O2 + 2H+ → 2Cu2+ + H2O2

In addition, because Cu+ cannot be present under ambient conditions (Moffett and Zika, 1987), how to continuously generate a sufficient amount of Cu+ is also an urgent problem to be solved. At present, there are two main methods, one is to reduce Cu2+ to Cu+ using a suitable reducing agent (Lee et al., 2016, Zhou et al., 2016). For example, Zhou et al. used ascorbic acid to reduce Cu2+ to produce Cu+, and subsequently produced ·OH through the reaction of Cu+ and O2. The other method is to produce Cu+ via the corrosive dissolution of zero valent copper (ZVC) (Dong et al., 2014). For instance, Wen et al. reported that diethyl phthalate (DEP) was completely oxidized in ZVC/O2 after 120 min at an initial pH of 2.5. Because the former method has the disadvantage of short duration time and great difficulty to control the reaction rate, the ZVC/O2 system is considered to have a better application prospect. However, since ZVC is difficult to corrode under neutral conditions (Lin et al., 2005, Huang et al., 2012), the system can only be used to degrade organic pollutants under acidic conditions. To the best of our knowledge, there is no report on the degradation of organic pollutants using the ZVC/O2 system under neutral conditions. In the field of corrosion protection engineering, researchers have found that ligands, such as EDTA and sodium oxalate, can effectively promote the corrosion of ZVC (Tamura et al., 2001). Therefore, we speculated whether the ZVC/O2 system can degrade organic pollutants under neutral conditions upon the addition of a suitable ligand.

In this study, p-nitrophenol (PNP) was used as the target contaminant. The effect of different ligands on the degradation efficiency of PNP in the ZVC/O2 system was evaluated. According to the experimental results, STPP, a commonly used polyphosphate ligand, was selected to improve the oxidation efficiency of the ZVC/O2 system. A series of experiments were carried out to reveal the mechanism for ROS generation and PNP degradation, and further explore the role of STPP in the reaction. Finally, we explored the influence of variables including the initial pH and STPP dosage on the reaction.

Section snippets

Materials and reagents

ZVC powder with different particle size (particle size: 100 nm, 1 μm, 5 μm and 44 μm, purity: >99.9%) and catalase from bovine liver (BR, 2094 μg/mg) were purchased from Aladdin Reagent Co., Ltd. (Shanghai, China). Coumarin-3-carboxylic acid and bathocuproine were purchased from Sigma-Aldrich (Shanghai, China). Sodium tripolyphosphate (STPP, Na5P3O10), ethylenediamine tetraacetic acid disodium salt (EDTA, C10H14N2Na2O8, 99%), sodium oxalate (C2Na2O4, 99%), 2,9-dimethyl-1,10-phenant-hroline

PNP degradation in the different reaction systems studied

In this study, STPP, EDTA and sodium oxalate were chosen as representative ligands to evaluate their contribution to the degradation of PNP in the ZVC/O2 system under neutral conditions. As shown in Fig. 1a, PNP was completely degraded in the ZVC/O2/STPP system within 120 min, while its degradation was <10% in the presence of EDTA or sodium oxalate. These results showed that STPP was a superior ligand to the other studied in the ZVC/O2 system. The control experiments displayed in Fig. 1b shows

Conclusions and environmental implications

In this study, 20 mg/L PNP was effectively degraded in the ZVC/O2/STPP system under neutral conditions. ·OH was identified as the main ROS during the degradation of PNP in this system and its main production pathway was the double-electron activation of O2 (O2→H2O2→·OH). STPP played a crucial role in the degradation process. On one hand, when the solution pH was <8.1, the complexation of STPP promoted the corrosion of ZVC by O2 to release Cu+. On the other hand, STPP promoted the production of

Acknowledgments

This work was financially supported by the Fundamental Research Funds for the Central Universities of China, and Key Project of National Natural Science Foundation of China (No. 41530636).

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