Interfacial formation of environmentally persistent free radicals-A theoretical investigation on pentachlorophenol activation on montmorillonite in PM 2.5 .

Environmentally persistent free radicals (EPFRs) in atmospheric fine particulate matters (PM2.5 ) possess high bioactivity and result in severe health problems. The facile transformation of aromatic pollutants into EPFRs on montmorillonite (MMT), an important solid component in PM2.5 , is an activation of air pollutants into more toxic chemical species and also attributes to the secondary source of EPFRs in PM2.5 . In this study, the interfacial reactions of pentachlorophenol (PCP), a typical EPFR precursor in air pollution, on the Fe(III)-, Ca- and Na-MMT surfaces have been explored by the density functional theory (DFT) calculations using the periodic slab models. The PCP molecule is found to be exothermically adsorbed on the three MMT surfaces. Moreover, significant charge transfer from PCP to Fe takes place and finally leads to the surface-bound phenoxyl radical formation on the Fe(III)-MMT surface since the half-filled 3d orbital of Fe3+ in Fe(III)-MMT could act as electron acceptor allowing the electron transferring from the 2p orbital of the phenolic O in PCP to Fe ion. However, similar charge transfer is not found in the Ca- and Na-MMTs, and the PCP transformation reaction is hindered on the Ca- and Na-MMT surfaces. Namely, the PCP activation to the corresponding EPFRs is impossible on the Ca-MMT and Na-MMT surfaces, while the catalytically active Fe(III)-MMT in PM2.5 can transform the chlorinated phenols into more toxic phenoxy-type EPFRs at ambient temperatures. Accordingly, more attention should be paid on the effect of MMT with catalytical capacity on the toxicity of PM2.5 .