Release of TiO2 nanoparticles from cement during their life cycle : step of use

The industrial scale production and wide variety of applications of manufactured nanoparticles (NPs) and their possible release during use step into the natural aquatic environment have produced an increasing concern among the nanotechnology and environmental science community. A part of this production concerns a new type of cement, called self-cleaning cement which maintain clean and white wall fronts. This property come from the incorporation of photocatalytic titanium dioxide nanoparticles (TiO2 NPs anatase) in the cement matrix. During continuous UV radiation exposure, the TiO2 NPs contribute to the oxidation (i.e. degradation) of adsorbed compounds at the cement surface. This recent nanomaterial (nanoparticles-based material) is promising as it exhibits improved properties but its environmental validation (in terms of impacts and risks associated with the incorporation of TiO2 NPs) is also required. Cement matrix is altered when exposed to water (e.g. rain draining on cement wall). An altered porous layer is then formed at its surface where numerous and complex reactions (primary cement phases congruent or incongruent dissolution, secondary phase formation, etc.) occur. This layer shows an increase of porosity. Cement leaching behavior, and associated elements released into the environment, are well described in the literature but the behavior of the incorporated TiO2 NPs is currently unknown. Release of TiO2 NPs is suspected, more precisely, the emission of nanomaterial degradation residues (NDR) into the environment (waters, soils ...). In this study we hypothesize the release of TiO2 NPs is controlled by the surface layer porosity, as TiO2 anatase NPS are very few soluble. This study focuses on the influence of the cement porosity on leaching behavior of TiO2 NPs. Samples are industrial cements incorporating with TiO2 NPs that were hydrated at the lab-scale in various conditions (addition of 30, 40 or 50 % of water) in order to evaluate the role of the porosity of the cement paste. Initial cement matric porosity was assessed by helium porosimetry and X-ray computed micro and nano-tomograghraphy analysis. To simulate the alteration phase, static leaching tests (liquid/solid ratio (L/S) of 100) were performed during 7 days. Each sample was placed within a dialysis membrane (10 kDa) filled with ultrapure water and submerged in a leachate solution (ultrapure water) to isolate the released particulate fraction from the subreleased soluble fraction. The released elements (particulate and soluble fractions) and their kinetics were quantified by ICP-OES; the chemical evolution of the altered layer was characterized by SEM and micro-XRF. As assumed, no soluble Ti was observed in the leachates. But Ti was dosed in the particulate fractions. The release kinetic of Ti is increasing with time for all the three cements with various initial porosity. The release of Ti starts at 11 hours of leaching for cement 40 and 50% and after 2 days for the cement 30%. After 7 days the cumulated TiO2 NPs release is not significantly different for the cement 50 and 40% but is significantly different for cement 30% respectively at 6400; 5200 and 2700 ng of Ti/ g of cement. It appears that the initial porosity is not the only parameter to predict the release of TiO2 NPs. The hypothesis is that the release is controlled by the porosity in the altered layer and more precisely by their connectivity and pore size. Microtomography with a spatial resolution of 1 µm coupled with nanotomomaphy with resolution of 150 nm and 3D image analysis (Software AVISO) are performed for the analysis of void structure in altered layer and allow to identify more precisely parameters which controlled TiO2 NPs release