Spectroscopic Characterization of TiO2 Polymorphs in Wastewater Treatment and Sediment Samples

The reactivity and toxicity of nano-TiO2 are controlled to a large extent by its phase composition: Although numerous studies have focused on the environmental impacts of nano-TiO2, the phase composition of TiO2 in environmental samples remains unknown. Using Ti K-edge X-ray absorption near edge structure (XANES) spectroscopy, we reveal the average phase composition of TiO2 present at various stages of a full-scale wastewater treatment plant (WWTP) and in sediments collected both upstream and downstream of the WWTP discharge point The XANES spectra provide direct evidence of the presence of TiO2 in these samples, in particular the coexistence of both anatase and rutile phases Spectral decomposition using least squares linear combination fitting shows that Ti in the WWTP samples contained similar to 30% anatase, 60% rutile, and 10% ilmenite. This composition was stable over a two month sampling period (nine sampling events) and did not change among primary sludge, activated sludge, and the final effluent In contrast, sediments located upstream and downstream of the WWTP discharge exhibited distinct XANES spectra reflecting a different TiO2 phase composition with a reversed relative abundance of anatase to rutile. Electron microscopy imaging of Ti-rich clusters in these various samples shows the presence of aggregates composed of rianoparticles (having a dimension of <100 nm) and larger particles with sizes of up to similar to 300 nrn. This is the first report of the average TiO2 phase composition in environmental samples based on a methodology that improves the assessment of the environmental risks of TiO2 under ambient conditions