Natural attenuation of TiO₂ nanoparticles in a fractured hard-rock

Successive transport experiments of TiO₂ nanoparticles (NP) suspension through fractured hard-rock column were done in laboratory. A low ionic strength (IS) water (0.8-1.3 10^-3 M) at pH ∼4.5 was used, corresponding to the chemical composition of groundwater where the rock was collected (Naizin, France). The surface charge of TiO₂ NP was positive while that of rock was negative favoring NP deposition. SEM/EDX reveals that NP were retained on a broad distribution of mineral collectors along the preferential flow pathways (i.e., fractures). However, a non-negligible amount of NP (∼10%) was transferred through the rock. Divalent cation (Ca²⁺) was responsible for the reduction of the negative charge of the rock and thus contributed to limit the NP deposition as attested by DLVO model. Blocking of rock surfaces by NP favored NP transfer while the ripening process and the size exclusion of aggregates decreased NP mobility. Decrease of water flow favored the exchange of solutes from the immobile to the mobile water in the porous medium, which in turn favored the aggregation of the NP and led to their natural attenuation. The result evidences how slight modifications of the environmental conditions can strongly influence the fate of NP in groundwater.