Assessing interactions of hydrophilic nanoscale TiO2 with soil water

The implications of manufactured nanoscale materials (MNMs) in unsaturated soil are mostly unknown. Owing to its widespread use, nanoscale (n) TiO2 is expected to enter soils where its accumulation could impact soil processes. Yet fundamental information is lacking regarding nTiO2 in situ wettability, i.e., interactions with soil water that relate to nTiO2 exposure and bioavailability. To probe nTiO2 interactions with soil water, we amended a natural soil with 20 mg per g of P25 nTiO2, a high-production, hydrophilic MNM that, based on its small size (25 nm nominal), provides ample specific surface area (SSA) for water sorption. We then measured nTiO2-amended soil SSA, and conducted a dynamic water vapor conditioning experiment. Early time-course water sorption into soil, with and without nTiO2, was clearly diffusional. Over 9 months, soil water content asymptotically equilibrated. However, despite amending with nTiO2 levels that increased the soil SSA by 16 %, measured water sorption rates and endpoint soil water contents were mostly unchanged by P25 nTiO2. Our results indicate that as-manufactured hydrophilic P25 nTiO2 was hydrophobic in soil, a finding relevant to nTiO2 bioavailability and transport.