Formation of organobromine and organoiodine compounds by engineered TiO nanoparticle-induced photohalogenation of dissolved organic matter in environmental waters.

There are increasing concerns about the adverse effects of released engineered nanoparticles and photochemically formed organohalogen compounds (OHCs) on human health and the environment. Herein, we report that titanium dioxide nanoparticles (TiO NPs) can photocatalytically halogenate dissolved organic matter (DOM) to form a large number of organobromine compounds (OBCs) and organoiodine compounds (OICs), as characterized by negative ion electrospray ionization coupled with Fourier transform ion cyclotron resonance mass spectrometry. Compared with no OHCs produced in control samples in darkness and/or without TiO NPs under sunlight irradiation, various OBCs and OICs were detected in freshwater and seawater under sunlight irradiation for 12h and 24h even in the presence of 1mgL TiO NPs, indicating the photocatalytic roles TiO NPs played in DOM halogenation. Furthermore, TiO NPs could result in the photodegradation of newly formed OHCs, as evidenced by the intensity and the number of some OHCs decreased with reaction time. In addition, many TiO NP-induced OBCs contained two or three bromine atoms, and/or nitrogen and sulfur elements, belonging to lignin-like, tannin-like, unsaturated hydrocarbon and aliphatic compounds. While the OICs were primarily contained one iodine, and very few consisted of nitrogen and sulfur elements, most were lignin-like and tannin-like compounds. Finally, the OBCs in freshwater were found to be formed mainly via a substitution reaction or addition reaction and were accompanied by other reactions such as photooxidation, while the OBCs in seawater and OICs were formed primarily via substitution reactions. Given the abundance of produced OHCs and their toxicity, our findings call for further studies on the exact structure and toxicity of the formed OHCs, taking account the TiO NP-induced DOM photohalogenation in aquatic environments during the evaluation of the environmental effects of engineered TiO NPs.