The ubiquitous distribution of halogenated aromatic compounds (XAr) coupled with their carcinogenicity has raised public concs on their potential risks to both human health and the ecosystem.Recently,advanced oxidation processes (AOPs) have been considered as an environmentally-friendly technology for the remediation and destruction of such recaldtrant and highly toxic XAr.During our study on the mechanism of metal-independent production of hydroxyl radicals (.OH) by halogenated quinones and H2O2,we found,unexpectedly,that an unprecedented.OH-dependent two-step intrinsic chemiluminescene (CL) can be produced by H2O2 and tetrachloro-p-benzoquinone,the major carcinogenic metabolite of the widely used wood preservative pentachlorophenol.Further investigations showed that,in all.OH-generating systems,CL can also be produced not only by pentachlorophenol and all other halogenated phenols,but also by all XAr tested.A systematic structure-activity relationship study for all 19 chlorophenolic congeners showed that the CL increased with an increasing number of Cl-substitution in general.More importantly,a relatively good correlation was observed betveen the formation of quinoid/semiquinone radical intermediates and CL generation.Based on these results,we propose that.OH-dependent formation of quinoid intermediates and electronically excited carbonyl species is responsible for this unusual CL production;and a rapid,sensitive,simple,and effective CL method was developed not only to detect and quantify trace amount of XAr,but also to provide useful information for predicting the toxicity or monitoring real-time degradation kinetics of XAr.These findings may have broad chemical,environmental and biological implications for future studies on halogenated aromatic persistent organic pollutants.