Elsevier

Environmental Pollution

Volume 222, March 2017, Pages 331-337
Environmental Pollution

Relative roles of H-atom transfer and electron transfer in the debromination of polybrominated diphenyl ethers by palladized nanoscale zerovalent iron

https://doi.org/10.1016/j.envpol.2016.12.030Get rights and content

Highlights

  • The overall debromination pathways of BDE-47 in n-ZVI and n-ZVI/Pd were elucidated.

  • The H-atom transfer was proved to be the dominant mechanism in n-ZVI/Pd system.

  • Mulliken charge is a good descriptor to predict the debromination of PBDEs.

Abstract

The relative significance of H-atom transfer versus electron transfer in the dehalogenation of halogenated organic compounds (HOCs) in bimetallic systems has long been debated. In this study, we have investigated this question through the case study of the debromination of 2, 2′, 4, 4’-tetrabromodiphenyl ether (BDE-47). The debromination rates of isomer products of BDE-47 by palladized nano zero-valent iron (n-ZVI/Pd) in the same reactor were compared. The results confirmed a shift in the debromination pathway of BDE-47 when treated with unpalladized nano zero-valent iron (n-ZVI) vs. treatment with n-ZVI/Pd. Study showed that BDEs could be rapidly debrominated in a palladium-H2 system, and the debromination pathway in this system is the same as that in the n-ZVI/Pd system. These results suggest that the H-atom species adsorbed on the surface of palladium are responsible for the enhanced reaction rates and the shift of the debromination pathway in the n-ZVI/Pd system. The Mulliken charges, calculated with density functional theory, on bromine atoms of PBDEs were directly correlated with the susceptibility to the e-transfer pathway in the n-ZVI system and inversely correlated with the susceptibility to the H-transfer pathway in n-ZVI/Pd system. These experimentally verified correlations in BDE-47 permit the prediction of the dominant debromination pathway in other BDEs.

Introduction

Polybrominated diphenyl ethers (PBDEs) have been among the most frequently used flame retardants in the past decades. As a result of their persistence and toxicity, various studies have focused on the technologies for removing PBDEs from the environment. One promising method is to use nano zero valent iron (n-ZVI) to completely debrominate PBDEs to diphenyl ether (Keum and Li, 2005, Zhuang et al., 2010). However, this process usually takes a long time, and complete debromination is sometimes unachievable, especially for lower-brominated congeners (Keum and Li, 2005). Therefore, noble metal catalysts have been widely used to amend n-ZVI to enhance its efficiency in dehalogenating a range of pollutants, such as polychlorinated biphenyls (PCBs) (Lowry and Johnson, 2004), polychlorinated dibenzo-p-dioxins (PCDDs) (Kim et al., 2008, Wang et al., 2015) and chlorinated benzenes (CBs) (Zhu and Lim, 2007). Although some studies have reported that palladium and nickel can greatly enhance the debromination of 2, 3, 4-tribromodiphenyl ether (BDE-21) (Zhuang et al., 2011) and 2,2′,4,4’-tetrabromodiphenyl ether (BDE-47) (Liu et al., 2015, Zhuang et al., 2012), the exact mechanisms involved are still unclear.

The role of the metal catalyst in the increasing dehalogenation rates and generating different product yields in a bimetallic system (FeM) has long been debated (Cwiertny et al., 2007, Kim et al., 2008, Zhuang et al., 2011). Some researchers attribute the enhanced contaminant reduction to a galvanic couple generated between iron and metal catalyst, which results in the acceleration of electron transfer from iron to pollutants; i.e., it is an electron transfer process (e-transfer) (Wang and Zhang, 1997, Xu and Zhang, 2000). Other researchers believe that the activated H-atom species (H*) formed in the process of water reduction (eq (1)) or the dissociative chemisorption of H2 (eq (2)) by the metal catalyst that is responsible for the reduction; i.e., it is a H-atom transfer (H-transfer) process (Cwiertny et al., 2006, Cwiertny et al., 2007, Zhuang et al., 2012, He and Zhao, 2008, Lin et al., 2004, Zhang et al., 1998).FeM + H2O + e → FeM-H+ OH2FeM + H2 → 2FeM-H

Some studies have shown that the addition of palladium greatly increases the H2 production from water reduction by n-ZVI and leads to different kinetics and debromination pathways of PBDEs from that occurring with n-ZVI alone, which has led some researchers to assume that H* might play some role in the enhanced debromination (Zhuang et al., 2011, Zhuang et al., 2012). However, such a conclusion lacks direct evidence.

One challenge is that the debromination intermediates in bimetallic system are usually short-lived, which means that the stepwise debromination is difficult to observe since the intermediates do not accumulate in relatively great quantities. Even if they could be detected, the accumulation of the degradation products in n-ZVI/Pd system cannot be used reliably to deduce the actual debromination pathway, because the debromination rates of isomer products might be different, which would affect the accumulation of intermediates.

Another challenge comes from the fact that the e-transfer and H-transfer processes are occurring simultaneously in the same reactor and both of them might be accelerated, which would make it so difficult to evaluate the relative significance of these two processes. Therefore, more information is needed to gain a better understanding about the mechanisms of PBDEs debromination by bimetal particles.

The debromination pathway by n-ZVI has been reported in numerous studies, and several quantum chemistry parameters have been proposed for use in predicting the kinetics and pathways of PBDEs (Cao et al., 2015, Harju et al., 2007, Hu et al., 2005, Jiwei et al., 2012, Kim et al., 2014, Zhao et al., 2008). For example, Keum et al. found that the debromination rate constants of PBDEs were positively correlated with their heats of formation (Hf) and negatively correlated with their ELUMO (Keum and Li, 2005, Zhuang et al., 2010). That is, the reactions proceeded to form progressively less brominated products that have lower heats of formation (Keum and Li, 2005). Li et al. have calculated the mean net charge on each of the halogen atoms of selected PCBs and PBDEs and compared them with the susceptibility of halogens to electron attack at the ortho-, meta- and para-positions (Li et al., 2007). Zhao et al. found that the Csingle bondBr bonds in anionic BDEs were lengthened from those in corresponding neutral congeners, and that the most lengthening Csingle bondBr bond was preferentially broken or cleaved (Zhao et al., 2008). These efforts were aimed at providing a general rule for predicting the debromination pathway for all the PBDEs when reacting with n-ZVI. However, as we demonstrated above, an alternative H-transfer pathway is also possible for which the previous indicators would not be adaptable. So a new descriptor to predict debromination pathway in both e-transfer and H-transfer process is desperately needed.

In this study, BDE-47 was selected as the target pollutant, because the accumulation of different products in the n-ZVI system vs. the n-ZVI/Pd system has been described in previous studies (Zhuang et al., 2012), and we possess the standards all of these products, which makes it relatively easy to follow the mass balance throughout the entire debromination process. The main objectives of this study are 1) to confirm the different debromination pathways of BDE-47 in the n-ZVI system vs. the n-ZVI/Pd system; 2) to elucidate the role of Pd on the debromination pathway of PBDEs in n-ZVI/Pd system; and 3) to use quantum mechanical insights (Mulliken charges) to predict both e-transfer and H-transfer debromination pathways.

Section snippets

Chemicals

Analytical grade NaBH4, FeSO4, HCl, Cl4Na2Pd and ethanol absolute were purchased from Sigma Aldrich Chemical Company. HPLC grade isooctane was obtained from CNW (Shanghai, China). All the PBDE and PCB standards (i.e., BDE-47, BDE-28, BDE-17, BDE-15, BDE-8, BDE-7, BDE-4, BDE-3, BDE-1, DE and PCB-61) were purchased from AccuStandard, Inc, USA (See Supplementary Information Table S1 for PBDE congener nomenclature and definitions).

Synthesis of particles

The n-ZVI and n-ZVI/Pd particles were synthesized by aqueous-phase

Debromination of PBDEs by n-ZVI and n-ZVI/Pd particles

BDE-47 was transformed to lower brominated diphenyl ethers more rapidly by n-ZVI/Pd treatment (Fig. 1a) than by n-ZVI treatment (Fig. 1b), and the products of debromination of BDE-47 by n-ZVI treatment (BDE-28, BDE-15, BDE-3) are much different from those by n-ZVI/Pd treatment (BDE-17, BDE-4, BDE-1, DE). The solution pH was found to increase slightly from initially 6.3 to about 7.9 in all n-ZVI systems and from 6.3 to about 8.8 in all n-ZVI/Pd systems. Apparently, the addition of Pd onto n-ZVI

Conclusions

This study has confirmed the shift in debromination pathway of BDE-47 from n-ZVI to n-ZVI/Pd system and found that the palladium particles could utilize H2 to debrominate PBDEs through a H-transfer process. These results have generated valuable insight on the chemical processes involved in the dehalogenation of HOCs in n-ZVI and n-ZVI/Pd systems. The Mulliken charges on bromine atoms of PBDEs were found to be directly correlated with the susceptibility to the e-transfer pathway in the n-ZVI

Conflict of interest

The authors declare no competing financial interest.

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (Nos. 41573091 and U1501234), the Guangdong Natural Science Funds for Distinguished Young Scholar (No. 2015A030306005), the Tip-top Scientific and Technical Innovative Youth Talents of Guangdong Special Support Program (No. 2015TQ01Z233), the Science and Technology Program of Guangdong Province (No. 2014A020216004). We thank our visiting professor Dr. Donald G. Barnes for providing valuable comments and

References (29)

  • S. Agarwal et al.

    Reactivity of substituted chlorines and ensuing dechlorination pathways of select PCB congeners with Pd/Mg bimetallics

    Environ. Sci. Technol.

    (2009)
  • H.J. Cao et al.

    Theoretical investigation on mechanistic and kinetic transformation of 2,2 ',4,4 ',5-Pentabromodiphenyl ether

    J. Phys. Chem. A

    (2015)
  • D.M. Cwiertny et al.

    Exploring the influence of granular iron additives on 1,1,1-trichloroethane reduction

    Environ. Sci. Technol.

    (2006)
  • D.M. Cwiertny et al.

    Influence of the oxidizing species on the reactivity of iron-based bimetallic reductants

    Environ. Sci. Technol.

    (2007)
  • Cited by (45)

    View all citing articles on Scopus

    This paper has been recommended for acceptance by Charles Wong.

    View full text