Formation of environmentally persistent free radicals from thermochemical reactions of catechol

Highlights

• The underlying links between persistent free radicals and catechol were clarified.

• Significant amounts of EPFRs were formed via thermochemical reactions of catechol.

• The effects of metal oxides were clarified by X-ray photoelectron spectroscopy.

• The knowledge will enable better control of the EPFR formation from catechol.

Abstract

In many anthropogenic activities, catechol as a widespread organic chemical could be released and also environmentally persistent free radicals (EPFRs) can be unintentionally formed. However, the underlying links between EPFRs and the role of catechol as an important precursor are not well understood. In this study, EPFR formation from catechol during heating was monitored online by electron paramagnetic resonance spectroscopy. It was found that catechol can produce significant amounts of EPFRs via thermochemical reactions. The EPFR species formed from catechol on metal oxides were oxygen-centered phenoxy and semiquinone radicals. Their half-lives were evaluated to be in the range of 113–909 h. The promotional effects of CaO and CuO on EPFR formation from catechol were stronger than that of Fe₂O₃. The promotional abilities and underlying mechanisms of various metal oxides in EPFR formation were clarified by X-ray photoelectron spectroscopy. Significant EPFR formation was observed during the cooling stage of a heating reaction system when CaO was used as the reaction medium. The obtained knowledge on the formation of EPFRs from catechol and the key factors involved will enable better control of the formation of EPFRs from anthropogenic activities.

Introduction

Environmentally persistent free radicals (EPFRs) are emerging environmental pollutants and can induce DNA damage (Gehling et al., 2014; Kelley et al., 2013; Khachatryan et al., 2011; Lubick, 2008; Tohidi and Cai, 2015; Valavanidis et al., 2013; Yang et al., 2017). The half-lives of EPFRs are many orders of magnitude higher than those of normal reactive oxygen species (ROS) (Gehling and Dellinger, 2013; Niu et al., 2007). It has been suggested that EPFRs can lead to ROS formation in simulated lung (Tong et al., 2017). The health risks associated with EPFRs in PM_2.5 have been reported to be similar to those of cigarette smoking (Dellinger, 2008; Pryor et al., 1983). The health risks arising from EPFR inhalation along with PM_2.5 are emerging concerns.

Phenols are important precursors for the formation of EPFRs and other highly toxic organic pollutants such as carcinogenic dioxins.(Burcat et al., 2003; Choi et al., 2008; Cook et al., 1956; Evans and Dellinger, 2005; Nishinaga et al., 1977; Steelink, 1965) Catechol is an important organic chemical and is widely used as a chemical intermediate, antibacterial agent, preservative, and as an additive in industrial processes such as rubber production and galvanizing.(Sedo et al., 2013) Catechol can be produced and released from anthropogenic activities such as cooking, coal and biomass combustion, chemical manufacturing, waste incineration, metallurgical industries, and other industrial sources.(Dorrestijn et al., 2000; Kibet et al., 2015; Sedo et al., 2013) Catechol is therefore an important and widespread organic precursor in anthropogenic activities.

Although both EPFRs and catechol can be formed and released from waste incineration, metallurgical processes, combustion of coal and biomass, and other anthropogenic activities (Dorrestijn et al., 2000; Kibet et al., 2015; Sedo et al., 2013), the underlying links between EPFRs and catechol during such activities are not fully understood. Therefore it is important to clarify the potential for formation of EPFRs from catechol and the factors that affect their formation during anthropogenic activities. This will enable improved source control and risk reduction of EPFRs.

Organic precursors, metal compounds, and suitable reaction conditions are considered to be the basis for EPFR formation.(Lomnicki, 2008; Mas-Torrent et al., 2012; Vejerano et al., 2011) Reactions that are caused by heating, e.g., during waste incineration, metallurgical processes, and coal and biomass combustion, are the most important characteristics for primary sources of EPFRs. It is therefore essential to consider the effects of heating on EPFR formation from catechol. This could well reflect the conditions for EPFR formation from primary sources. The pivotal effects on EPFR formation from catechol of various metal compounds that are generally involved in these anthropogenic activities have not yet been clarified. These need to be clarified to identify the key influencing factors and to guide development of control techniques.

In this study, catechol was selected as a typical organic precursor because it is found in the majority of anthropogenic emissions. The potential for EPFR formation from catechol was evaluated under heating and in the presence of metal oxides, which are common features of systems that are primary sources of EPFRs. The results of this study will improve our understanding of EPFR formation from catechol during anthropogenic activities. They will provide practical guidance for source control and risk reduction of emerging EPFRs.