Transport, Deposition and Release Kinetics of Nano-TiO2 in Saturated Porous Media

Nanotechnology represents a fast growing economic sector that is constantly evolving with changes in technology, markets as well as product quality and demand. Novel nano-sized materials and particles are revolutionizing science and engineering due to their enhanced physico-chemical properties compared with their bulk counterparts. Among the engineered nanoparticles (NPs) derived from this industry, titanium dioxide is one of the most popular. In municipal wastewater treatment plants, the presence of Ti-based particles has been reported in effluents at concentrations of 10 – 100 μg/L (Kiser et al., 2009). These effluents are eventually discharged into surface waters which is a concern because the International Agency for Research on Cancer has categorized TiO2 in Group 2B of possibly carcinogenic materials (IARC 2006, volume 93). In order to evaluate the environmental consequences of the release and accumulation of engineered titanium dioxide nanoparticles (nano-TiO2) in subsurface and estuarine environments it is critical to comprehend the mechanisms controlling the deposition, mobility and stability of these nanoparticles in porous media because their bioavailability, reactivity and ecotoxicology will certainly be governed by their transport behavior. Thus, the overall objective of this study is to contribute to the state of science of the transport, deposition and release kinetics of nano-TiO2 in saturated porous media. The specific goals of the study included: (A) to explore the effects of physico-chemical parameters such as pH, presence of non-ionic (Triton X-100) and anionic (SDBS) surfactants as well as flow velocity on the aggregation and transport mechanisms of nano-TiO2 in saturated porous media; (B) to investigate the effects of ionic strength in the aggregation and deposition of nano-TiO2 in saturated porous media; (C) to examine the impact that the abrupt and gradual reduction in ionic strength possesses in the release of nano-TiO2 previously deposited onto sand grains; and (D) to characterize and predict through implementation of experimental data nanoparticle deposition coefficients, maximum transport distance and total interaction energy between nano- TiO2 and nano-TiO2 and collectors. The experimental results demonstrated that as the pH of the suspensions came close to the pHpzc, the surface ionization of the nanoparticles was suppressed, limiting the repulsive forces among nanoparticles and allowing for aggregate formation which induced an increase in the nanoparticles deposition rate coefficients. As the pH the suspensions increases, nano-TiO2 attained better stability. The presence and concentration of surface active agents such as Triton X-100 (non-ionic) and SDBS (anionic) in suspensions under similar ionic strength and pH enhanced the overall mobility of nano-TiO2 through the porous medium. The electrostatic and steric repulsion forces generated by solution chemistry in combination with an increase in hydrodynamic flow also favored the transport of nano-TiO2 through the porous matrix. The increase in electrolyte concentration induced an increase in nanoparticle aggregation that resulted in filter ripening of nano-TiO2 in the porous medium. Under conditions in which the attachment efficiency between nanoaggregates prevailed (i.e. favorable) over the attachment efficiency between nanoaggregates and collectors, previously deposited nano-TiO2 served as preferential sites for subsequent deposition. This had the potential to induce nano-TiO2 to nano-TiO2 deposition through secondary energy minimum at low and medium ionic strengths as well as to create deposits of nanoaggregates at high ionic strength that were altered (i.e. part of the nanoaggregates detached and cleared out from the porous matrix) upon abrupt or gradual reductions in the electrolyte concentration of the solutions utilized to flush the saturated porous medium.