EFFECT OF NANOPORE SIZE ON SILICA SOLUBILITY AND PRECIPITATION

Nanopore surfaces are dominant reactive surfaces in the critical zone where the fluids meet the solid earth. It is proposed that the behavior of solvent (water) and certain geochemical reactions (such as sorption, desorption, precipitation and dissolution) in nanoporous environment are greatly different from those in their normal / bulk solution systems. In here, we report silica precipitation in nanoporous environment. Both non-porous and nanoporous silica were used for the experiments, in order to investigate the effect of pore size on silica precipitation. Nanoporous silica samples with pore diameters ranging from 2 nm to 6 nm were prepared by using a self-assembling template method. The samples were characterized by X-ray Diffraction (XRD), surface area and pore size analysis before the dissolution kinetics and precipitation experiments. Non-local density functional theory (NLDFT) model was chosen to fit the N2 sorption and desorption isotherm data. The results indicate very uniform pore size distributions for the nanoporous samples. The samples were then immersed in a batch of DI water and NaCl-bearing solutions, in order to study kinetics of silica precipitation in nanopores and the reaction products monitored by pore size and pore surface area measurement, XRD analyses and ICP-OES. The results show silica can precipitate very fast in nanopores from a solution that is under-saturated with respect to normal amorphous silica. Precipitation rate is inversely proportional to the pore diameter. The results also support that solubility of silica in nanoporous environments is much lower than that in bulk solution. We propose that behavior of solvent water in nanopores is different from that of bulk water. Our FTIR study shows that vibration modes at the range of ~1620 to ~1640 cm-1 from nanopore water (1625 cm-1 for water in 2 nm pores, and 1635 cm-1 for water in 3 nm pores) are different from that from bulk water (1643 cm-1). It is expected that the change in water properties in nanopores will affect silical precipitation, and enrichment of other trace elements. This study will also provide a missing link between weathering rates observed in the field and measured in laboratory.