Photochemical transformation of anthracene (ANT) in surface soil: Chlorination and hydroxylation

Highlights

• Co-existing chloride inhibited the photolysis of ANT on moist silicon dioxide.

• ANT was attacked by •OH to form hydroxylated products.

• In presence of Cl^-, excited ANT may react with Cl• to produce Cl-PAHs.

• The formation of •Cl was attributed to the reaction of •OH with Cl^-.

• DFT calculation provided evidence for ANT excitation and the radical reaction site.

Abstract

To reveal the fate of anthracene (ANT) in soil, the photodegradation behavior of ANT was systematically studied using SiO₂ to simulate a soil environment. Under xenon lamp irradiation, more than 90% of ANT loaded on SiO₂ could be removed after 240 min. Moreover, the effects of water content, chloride ions (Cl^-) and humic acid (HA) were examined. It was found that the presence of water and HA can significantly inhibit the photolysis of ANT on SiO₂, while the addition of chloride alone has no obvious effect. However, when water is present, the inhibition effect of chloride became more obvious. According to radical quenching experiments and electron paramagnetic resonance (EPR) spectra, hydroxyl radicals (•OH) and chlorine radicals (Cl•) were formed in the system. Possible reaction pathways were speculated based on products identified by mass spectrometry. ANT was attacked by •OH to form hydroxylated products, which can be further hydroxylated and oxidized with the final formation of ring-opening products. ANT directly excited by light may also react with Cl• to produce chlorinated polycyclic aromatic hydrocarbons (Cl-PAHs). Finally, the experimental results were verified on real soil. This study provides important information for understanding the photochemical transformation mechanism of ANT at the soil/air interface.

Introduction

Polycyclic aromatic hydrocarbons (PAHs) are common pollutants in contaminated sites and are highly resistant to degradation [24]. Incomplete combustion of fossil fuels is the main source of PAHs, which is widespread in industrial activities and other human activities or geothermal reactions with fuel and mineral production. Under natural conditions, volcanic eruptions, oil spills, forest fires and biological production are the main sources of PAHs [18], [3]. Although PAHs have been detected in various environmental media, soil is considered the most important sink of PAHs. In fact, soil bears 90% of the global environmental PAHs burden [51]. The types of PAHs in soil are largely affected by local industrial types, but also related to non-human factors such as climate and temperature belt, and the available data from 1990 to 2014 showed that the 16 common PAHs are ubiquitous in soils worldwide, with those containing less than 4 rings being the main pollutants [24]. [57] believes that the PAHs in rural soil are mainly fluoranthene, naphthalene and pyrene derived from biomass combustion; However, the PAHs in urban soil are mainly originated from the emission of heavy traffic areas, which consist mainly of fluoranthene, naphthalene and benzo (b + k) fluoranthene. The bioaccumulation potential and carcinogenic activity of PAHs cannot be ignored [19]. In general, the carcinogenicity of PAHs increases with increasing molecular weight [22].

Chlorinated polycyclic aromatic hydrocarbons (Cl-PAHs) can also be detected in a variety of environmental media. As derivatives of PAHs, the concentrations of Cl^-PAHs are markedly lower than those of the parent compounds, but generally higher than those of dioxins [33]. Cl-PAHs are widely believed to be more mutagenic than their parent compounds [46]. According to [33], the main sources of chlorinated PAHs in the air are traffic and combustion facilities. The result of [55] showed that benzo[a] pyrene (BaP) can be activated by photoirradiation and further reacted with Cl⁻ to produce 6-ClBaP in saline waters. [40], [41], [43], have also performed much experimental work on the photochemical generation of Cl-PAHs, and they found that the amount of Cl-PAHs formed exhibited a significant positive correlation with salinity in the sediments of some tidal flats [40]. Under both simulated and real tidal flat conditions, anthracene (ANT) can produce chlorinated PAHs under UV irradiation [41], [46]. In addition, pyrene exposed to UV irradiation in synthetic seawater for various irradiation times can produce various halogenated derivatives [43]. Soil is the most polluted medium of PAHs, and some studies have shown the presence of Cl-PAHs in soil [32], [50]. However, few studies have focused on the complete photochemical fate of PAHs in soil, especially the potential chlorination reactions.

In this work, the effects of chloride ions, soil moisture and humic acid (HA) on the photolysis of PAHs were investigated, and the generation mechanism of Cl-PAHs under these conditions was further explored. Chloride ions and soil moisture are considered as two important factors affecting the formation of Cl-PAHs, while humic acid (HA) is a common factor to be examined in the photochemical system. ANT, a ubiquitous pollutant [36] with excellent photoreactivity [23] and is often selected as a model pollutant to study the degradation of PAHs in soil [12], was used in this work. First, we chose SiO₂, an important component of soil, as a carrier to simulate the photolysis process in soil [37], [59], and then verified the experimental results in real PAH-contaminated soil. Moreover, the reaction products were identified by high-performance liquid chromatographymass spectrometry (HPLCMS) and gas chromatographymass spectrometry (GCMS), from which possible degradation pathways were speculated by combining the results of theoretical calculations. This study show that the chlorination of PAHs can be extended to real soil, and will provide some new insights into the environmental fate of PAHs and the natural formation of Cl-PAHs on soil surface.