Environmental impact of nano-functionalized construction materials: leaching of titanium and nitrates from photocatalytic pavements under outdoor conditions

摘要

There is a growing use of nano-functionalized construction materials, which contain nanoparticles embedded in their bulk or deposited on their surfaces. In the case of photocatalytic materials, nano-TiO_2 is usually added to provide it's functionality. One concern about these materials, in addition to release of nanoparticles as airborne, is that they can be leached into the aquatic environment Moreover, water eutrophication could be caused due to the increase in NO~-_3 as a product of the photocatalytic oxidation of NOx in runoff. In this paper, a systematic long term campaign assessing these potential side effects in the real outdoor environment has been carried out. Rainwater leachates from 4 m~2 slabs of 7 different photocatalytic materials exposed outdoors in two different locations (platforms) were collected and analysed over more than 800 days. Ti, NO~-_3, pH and conductivity were analysed. Ti was found in the leachates of almost every material, without a clear relation with the type of application (percolated cementitious slurry, suspension/emulsion or TiO_2 built-in). The highest concentration found was of 60 μg/L, which seems to be rather small when comparing with some threshold values for drinking water. In all the cases, the detected TiO_2 nanoparticles from water leachates were embedded in large microparticle agglomerates coming from the construction material matrix, which are less dangerous than nanoparticles. Nitrates were leached in clear relation with the NOx oxidation photocatalytic performance, and the observed concentrations were not higher than those in the recycled water used by the Madrid City Council to clean the streets.