Detection and Characterization of TiO2 Nanomaterials in Sludge from Wastewater Treatment Plants

“TiO2 nanoparticles (TiO2-NPs) have a wide range of industrial applications (paintings, sunscreens, food and cosmetics) and is one of the most intensively used nanomaterials worldwide. Leaching from commercial products TiO2-NPs are predicted to significantly accumulate in wastewater sludges, which are then often used as soil amendment. In this work, sludge samples from four wastewater treatment plants of the Chihuahua State in Mexico were obtained during spring and summer (2017). A comprehensive characterization study was performed by X-ray based (laboratory and synchrotron) techniques and electron microscopy. Ti was detected in all sludge samples (1810–2760 mg/kg) mainly as TiO2 particles ranging from 40 nm up to hundreds of nm. Micro-XANES data was analyzed by principal component analysis and linear combination fitting enabling the identification of three predominant Ti species: anatase, rutile and ilmenite. Micro-XANES from the smaller Ti particles was predominantly anatase (68% + 32% rutile), suggesting these TiO2-NPs originate from paintings and cosmetics. TEM imaging confirmed the presence of nanoscale Ti with smooth surface morphologies resembling engineered TiO2-NPs. The size and crystalline phase of TiO2-NPs in the sludge from this region suggest increased reactivity and potential toxicity to agro-systems. Further studies should be dedicated to evaluating this.”

“Samples from both seasons and all WWTPs were used for bulk XRD. Agatha mortar ground sludge was introduced in thin glass capillaries and mounted in the high energy resolution powder diffractometer at ID22 [59] of the European Synchrotron Radiation Facility (ESRF) in Grenoble. Using a 29 keV incident photon beam, the diffracted X-rays were collected by the multianalyzer stage.”

The crystalline forms of TiO2 present in the sludge samples were investigated by synchrotron based High Resolution XRD. Results shown in Figure 3 indicate the most intense diffraction peaks, belonging to anatase and rutile forms. The characteristic diffraction peaks of anatase and rutile isoforms of TiO2 are shown in Figure A1.
Figure 3. XRD diffraction spectra from all sludge samples. Zoomed regions are shown for the most important diffraction peaks of anatase (Ana) and rutile (Rut): (a) anatase 101; (b) rutile 110; (c) rutile 101 and (d) anatase 200. A small vertical shift has been applied for visualization purposes only.

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