The ubiquitous distribution of halogenated aromatic pollutants (XAr)coupled with their carcinogenicity has raised public concerns on their potential risks to both human health and the ecosystem. Recently, advanced oxidation processes (AOPs) have been employed as an "environmental-green" technology for treatment and degradation of such recalcitrant and highly toxic XAr. During our study on the molecular mechanism of metal-independent hydroxyl radicals (·OH) production by halogenated quinones and H₂O₂, we unexpectedly find that an unprecedented·OH-dependent two-step intrinsic chemiluminescene (CL) can be produced by H₂O₂ and tetrachloro-p-benzoquinone, which is the major carcinogenic metabolite of the widely-used wood preservative pentachlorophenol. We further find that·OH-producing AOPs-mediated degradation of pentachlorophenol and all other XAr could produce intrinsic CL that is directly dependent on the formation of the extremely reactive·OH.A systematic structure-activity relationship study for all 19 chlorinated phenols demonstrates that the CL increases with an increasing number of chlorine-substitution in general. More interestingly, a relatively good correlation is noticed that not only between CL intensity and chlorinated quinoid intermediates, but also between CL emission and semiquinone radicals. Taken together, we propose that·OH-dependent formation of quinoid intermediates, quinone-1,2-dioxetane and electronically excited carbonyl species is responsible for this unusual intrinsic CL production.A rapid, sensitive, simple, and effective CL method has been developed to not only detect and measure trace amount of XAr in real environment, but also to provide useful information for predicting the toxicity or monitoring the degradation kinetics of XAr. These findings may have broad chemical, environmental and biological implications for future studies on remediation of other halogenated persistent organic pollutants by AOPs.
Contents
1 Introduction
2 An·OH-dependent CL could be generated by carcinogenic polyhalogenated quinones and H₂O₂
2.1 The CL system of tetrachloro-1,4-benzoquinone (TCBQ) and H₂O₂
2.2 Possible light-emitting intermediates
2.3 Molecular mechanism for·OH-dependent CL generation
2.4 Other halogenated quinones could generate similar·OH-dependent CL
3 Intrinsic·OH-dependent CL could also be produced from degradation of pentachlorophenol (PCP) during advanced oxidation processes
3.1 An intrinsic CL emission from advanced oxidation of PCP by the classic Fenton system
3.2 The primary intermediates and ring-opening product
3.3 Possible molecular mechanism for CL emission during·OH-dependent PCP degradation
4 The structure-activity relationship study on the CL emission from the degradation of chlorinated phenols
4.1 Chlorinated phenols could generate CL during the·OH-generating AOPs
4.2 A good correlation between CL emission and the formation of chlorinated quinoid intermediates
4.3 Possible molecular basis for the correlation between CL emission and chlorinated phenols
4.4 A good correlation between CL emission and the formation of the chlorinated semiquinone radicals
4.5 Relatively good correlations between CL and the toxicity/degradation rate of chlorinated phenols
5 Other halogenated aromatic compounds could generate intrinsic CL during their advanced degradation by·OH-generating systems
6 Conclusion