Molecular transformation of natural and anthropogenic dissolved organic matter under photo-irradiation in the presence of nano TiO2

Abstract

Photochemical transformation of dissolved organic matter (DOM) plays a very important role in the cycling of organic carbon in aquatic systems. Increasing release of photoactive nanoparticles such as titanium dioxide nanoparticles (nano TiO2) into surface water may impact this process. The present study employed Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) to examine the molecular transformation of natural DOM (peat DOM, DOMp) and anthropogenic DOM (sludge-derived DOM, DOMs) under photo-irradiation as affected by nano TiO2. Differences in molecular components between DOMp and DOMs were observed. DOMs contained more heteroatom formulas (76%) with low aromaticity and low carbon oxidation state than did DOMp (22%). The presence of nano TiO2 resulted in significant decreases in both DOM content and molecular diversity under photo-irradiation. Consistent alterations were observed between DOMp and DOMs such that high molecular weight compounds, high aromaticity and/or heteroatom S-containing compounds were more easily photodegraded in the presence of nano TiO2; whereas the average carbon oxidation state decreased in DOMp but increased in DOMs, likely due to the significant differences in O abundance, especially in the contents of carboxyl moieties, between DOMp and DOMs. The findings of the present study suggest that the release of nano TiO2 into aquatic environment will accelerate the consumption of dissolved organic carbon and the attenuation of molecular diversity for both DOM in waters.