Transformation of benzo[a]pyrene on Al(III)-montmorillonite: Mechanism of environmental persistent radicals formation

Abstract

Environmental persistent free radicals (EPFRs) have been evaluated as a new kind of environmental risk substance, which are mainly detected in atmospheric particles and superfund sites. Clay minerals, which are composed of layered silicates and are abundant in soil environment, provide an appropriate site for organic pollutants to form EPFRs. Our previous studies showed that combination of clay minerals and transition metal species such as Fe and Cu ions/oxides can effectively degrade polycyclic aromatic hydrocarbons (PAHs), and significant EPFRs signal can be detected by electron paramagnetic resonance (EPR). Aluminum (Al) is the most abundant metal element in the earth’s crust, however, to the best of our knowledge, few studies have reported the formation of EPFRs by organic pollutants contaminated Al(III)-clay minerals, and an unambiguous of the relevant interface process and mechanism is still lacking. In this paper, benzo[a]pyrene (B[a]P), a strong carcinogenic PAHs compound, was used to contaminate Al(III)-montmorillonite, and Na(I)-montmorillonite served as the control group. The degradation rate of B[a]P was determined by high performance liquid chromatography (HPLC). Fourier transform Infrared spectrometer (FTIR) and X-ray photoelectron spectroscopy (XPS) were employed to analyze the changes of functional groups and elements in the course of degradation of B[a]P. Since water content is an important factor in soil, three different relative humidity (RH) conditions (RH 97%) were selected to explore the stability of B[a]P-EPFRs. In order to confirm that the formation of EPFRs was accompanied by generation of hydroxyl radical (•OH) and superoxide radical (O2 · −), reactive oxygen species (ROS) capture experiment and radical quenching experiment were carried out. The results confirmed that Al(III)-montmorillonite was capable of accelerating the degradation of B[a]P, and the reaction rate was 37 times higher than that of Na(I)-montmorillonite. FTIR characterizations showed that the decay of old functional groups (C=C) was accompanied by forming new functional groups (C=O). XPS analysis revealed that upon sorption the ‒C=O band and C=O band occurred by analyzing the C 1s and O 1s, indicating the formation of oxygen-containing compounds. The RH experiment demonstrated that the conversion of B[a]P and the formation of EPFRs were not affected under relatively dry conditions (RH · − radical respectively, and the degradation rate of B[a]P was decreased significantly to 19.1% and 23.4% at 9 d respectively. In addition, the EPR intensity of EPFRs was obvious decreased, and this indicated that the formation of EPFRs was accompanied by the generation of ROS. The degradation of B[a]P and the formation of EPFRs were mainly attributed to electron transfer on the surface of Al(III)-montmorillonite. The formed •OH and O2 · − radical in the system can oxidize B[a]P radicals and eventually generate oxygen-containing compounds.