Persistent Free Radicals from Low-Molecular-Weight Organic Compounds Enhance Cross-Coupling Reactions and Toxicity of Anthracene on Amorphous Silica Surfaces under Light.

Polycyclic aromatic hydrocarbon (PAH) contamination has raised great environmental concerns, while the effects of low-molecular-weight organic compounds (LMWOCs) on PAH photodegradation at amorphous silica (AS)/air interfaces have been largely ignored. In this study, the phototransformation of anthracene (ANT) at amorphous silica (AS)/air interfaces was investigated with the addition of LMWOCs. ANT removal was attributed to •OH attacking and the energy transfer process via 3ANT*. Light irradiation induced the fractured ≡SiO• or ≡Si• generation on AS surfaces, which could react with absorbed H2O and O2 to generate •OH and further yield a series of hydroxylated products of ANT. The presence of citric acid and oxalic acid improved •OH generation and enhanced ANT removal by 1.0- and 2.2-fold, respectively. For comparison, the presence of catechol and hydroquinone significantly decreased ANT removal and produced coupling products. The results of density functional theory calculations suggest that persistent free radicals (PFRs) on AS surfaces from catechol or hydroquinone after •OH attacking prefer to cross-couple with ANT via C-C bonding rather than self-couple. Dianthrone and cross-coupling products might possess higher ecotoxicity, while hydroxylated products were less ecotoxic than their parent compounds based on Ecological Structure Activity Relationships (ECOSAR) estimation. The results of this study revealed the potential ecotoxicity of PAH-adsorbed particulates coexisting with LMWOCs and also provided a new insight into PAH transformation through PFR pathways.