Transport of anatase and rutile titanium dioxide nanoparticles in soils in the presence of phosphate: mechanisms and numerical modeling

Purpose It is known for titanium dioxide nanoparticles (nTiO 2 ) during production and application to inevitably enter the soil and aquatic environments, posing potential toxic risks to human health and living environments. This triggers the necessity to clarify the underlying mechanisms governing the fate and transport of nTiO 2 with different crystal structures in different soils. Materials and methods The anatase and rutile nTiO 2 were examined in saturated columns packed with varying the mass fraction ( λ ) of sand/soil (i.e., red soil and paddy soil) mixtures in the presence of phosphate. Moreover, numerical modeling based upon the two-site kinetic retention model was employed to unravel the retention mechanisms. Results and discussion This study showed that the degree of λ was found to have an obvious influence on the transport of nTiO 2 , irrespective of anatase and rutile. The transportability of nTiO 2 was greatly decreased with the increase of λ . The rutile nTiO 2 transport was more sensitive to the λ than anatase nTiO 2 , which especially happened in red soil due to the increased favorable retention sites resulting from the high contents of Fe (47.0 g kg −1 ) and Al (16.7 g kg −1 ) and the great specific surface area (38.6 m 2 g −1 ). The second-order attachment coefficient (k 2 ) and the maximum solid-phase concentration ( S max2 ), obtained by fitting the transport of both nTiO 2 in red and paddy soils using two-point kinetic retention model, increased linearly with increasing the λ . Conclusions Overall, combined mechanisms (e.g., secondary minimum, particle aggregation, crystalline structure, and surface charge heterogeneity) are responsible for nTiO 2 deposition in soils. Our findings provided a convincing theory for evaluating environmental risk of anatase and rutile nTiO 2 in different soils.