Nanoparticles (NPs) are present in a broad range of products and industrial processes. However, their intrinsic novel properties represent also a challenge on understanding their fate and behaviour in the environment. This is the case of titanium dioxide nanoparticles (TiO₂NPs), since they are increasingly used in many commercial products (e.g. sunscreens, cosmetics, paints and plastics). 50,400 tons of TiO₂-NPs were globally produced in 2010, making these NPs some of the most produced in the world. Thus, it is important to determine their potential harmful effects they may have on ecosystems and health once released into the environment. Previous studies have shown that TiO₂-NPs induce genotoxicity in cells, mainly through the generation of oxidative stress. In addition, the National Institute of Occupational Safety and Health (NIOSH) concluded that TiO₂-NPs are potentially carcinogenic (respiratory exposure). An important environmental concern is their release into wastewaters, ending up in the sludge produced in wastewater treatment plants (WWTPs). With the increasing use of WWTPs sludge as amendment in agricultural soils, a depth characterization of TiO₂-NPs in sludge samples is necessary for assessing potential environmental risks. This work aims to the physical and chemical characterization of TiO₂-NPs present in the sludge of WWTPs.

The sludge samples were collected from WWTPs in the State of Chihuahua, Mexico. This state is territorially located in semi-desert zone, a climatic condition that causes the habit of the population to use sunscreen or moisturizers daily (often containing TiO₂-NPs). The samples were analysed by synchrotron and laboratory X-ray based techniques, X-ray Fluorescence (XRF), X-ray Absorption Near-Edge Spectroscopy (XANES) and X-ray Diffraction (XRD), using different probe sizes, i.e. bulk, micro and nano. Clear evidence of nanoparticulate Ti material was found in the sludge, along with other Cu and Zn particles with size distributions in the range of 100 nm, 200 nm and 300 nm. XANES spectra from large particles (>3 μm) corresponds to amorphous TiO₂ whereas smaller particles shown rutile phase and, the most copious, anatase. XRD signal from these organized phases were detected by bulk XRD, and from small particles with micro-XRD (<1 μm).)

By combining several X-ray based techniques, the Ti concentration and its speciation in function of the particle size has been obtained. Sub-micron and nano particles with high ordinate phases were found in the sludge, which could correspond to engineered nanomaterials.