Nanoaggregates of silica with kaolinite and montmorillonite: Sedimentation and transport.

Due to a wide range of applications in industrial fields, engineered nanomaterials (ENMs) have a high potential to enter the soil. The soil's major component of clay likely dictates the fate and transport of ENMs in the subsurface. Currently, few studies are available on the fate and transport of nanoparticle silica (nSiO2) in the presence of clay particles. Therefore, the sedimentation and transport of nSiO2 with two representative clays (montmorillonite (M) and kaolin (K)) in porous media were investigated in monovalent (Na+) and divalent (Ca2+) ion solutions with multiple characterizations including SEM/TEM-EDX, zeta potentials, particle sizes and colloid transport modeling. It was shown that nSiO2-nSiO2 homoaggregates and nSiO2-K (or M) heteroaggregates dominated in the nSiO2-clay nanoaggregate suspension. A distinct decrease in the stability and transport of nSiO2-M (or K) in NaCl solution and an increase in CaCl2 occurred when M or K was added to the nSiO2 suspension at pH 6.0. This was attributed to the faster settlement of the individual M or K in NaCl vs. the better stability in CaCl2 (compared to nSiO2 alone). Particularly, more negative individual M platelets occurred in the high NaCl solution until extensive flocculated structures built up, which contributed to the faster deposition of nSiO2-M compared to nSiO2-K, even though the nSiO2-M was more negatively charged. Comparably, the effect of M and K on the fate and transport of nSiO2 almost disappeared at pH 9.0. The values of the first-order attachment/detachment rate coefficients (k1/k1d) and first-order straining coefficient (k2) obtained from two-site kinetic attachment model fitting are responsible for the deposition of nSiO2-clay nanoaggregates in sand. This study suggests potential groundwater contamination due to the clay-facilitated transport of ENMs in calcareous soil.