Assessment of the potential hazard of nano-scale TiO2 in photocatalytic cement: application of a tiered assessment framework

Photoactive nano-scale TiO2 (n-TiO2) is added to cement to reduce SO2 and NO2, and degrade organic contaminants in the presence of sunlight. This application of n-TiO2 introduces nano-specific environmental sustainability questions, including the potential for release and environmental impacts of free n-TiO2, or cement-TiO2 complexes. We assessed these issues using a Tiered Framework to guide categorization, characterization, and testing decisions to determine potential for release, fate, and toxicity of a TiO2-cement composite (TCC). Release scenarios included runoff during construction and abrasion of cured concrete. Tested material included as-delivered cement powder (with and without n-TiO2 and particles released using standardized abrasion testing). The primary hazard concern for these materials is the potential for their photocatalytic activity to translate directly to phototoxicity to water-column organisms. For this reason toxicity, using standard Ceriodaphnia dubia assays, was compared between tests performed under standard laboratory lighting and under simulated solar radiation (SSR). TiO2 in the cement was determined to be nearly 100% anatase; material released during abrasion was comprised of nano to micron sized particles containing free and matrix-bound TiO2. The results of comparative toxicity testing indicated no increase in toxicity in the presence of SSR (relative to laboratory lighting). This case study provides evidence that environmental risk of TCC may not be significantly increased relative to standard cement, but more importantly demonstrates the successful application of a Tiered Framework to assessing impacts of nanomaterials and nano-enabled products.