11:30 AM - 11:45 AM
[AHW18-10] Role of reactive-transport through water film in rock pores in vadose zone chemical weathering
★Invited Papers
Keywords:Chemical weathering, Vadose zone, Water film, Reactive-transport model, Electric double layer
Chemical weathering of silicate and carbonate minerals affects the chemistry of the subsurface aqueous environment. Chemical weathering often occurs in the vadose zone, where air and water coexist in pores. Although numerous studies have been conducted on the weathering processes in the water-saturated condition, how the presence of air affects chemical weathering has yet to be fully investigated. In the pores where air is present, a water film, varying in thickness from submicron to nanometer scales, exists on the pore surfaces. To understand the role of the water film in chemical weathering, we investigated the physicochemical properties of the water film and how the mineral dissolution and mass transport occur in the water film.
To investigate the dissolution behavior of silicate and carbonate covered by the water film, flow-through dissolution experiments were conducted under both water-saturated and unsaturated conditions. Similar experiments were conducted with two types of sandstone: one composed of ~100 % quartz, and the other containing not only quartz but also Ca-Mg-Fe carbonate. Ultrapure water was infiltrated into sandstones adjusted to saturated and unsaturated conditions, and the flow rate and element concentration in the effluent were measured to calculate the bulk dissolution rate under each condition. The results showed that for both sandstones, the Si dissolution rate was almost the same in the saturated and unsaturated conditions. In contrast, experiments with Ca-Fe-Mg carbonates showed that the Ca dissolution rate in the unsaturated condition was slower than that in the saturated condition. These facts suggest that quartz covered by the film dissolves at the same rate as that under the saturated condition, while the dissolution of the carbonate becomes slower in the presence of the film.
What physicochemical properties control the water film thickness? A model was developed to predict the film thickness by considering the van der Waals and electric double layer forces between mineral, water film, and air. The model shows that the film thickness is influenced by pH, ionic strength, surface charge of mineral and air/water interface, and pore size. In particular, the sign of the surface charge is a crucial factor. At neutral pH, positively charged surfaces (e.g., quartz) are covered by a thick, thermodynamically stable film. In contrast, negatively charged surfaces (e.g., calcite) are wetted by a thin film due to the absence of the electric double layer force that thickens the film.
To quantitatively understand dissolution and mass transport in the water film in rock pores, we developed a reactive-transport model for the film. The model shows that due to a low dissolution rate of quartz, the diffusion of dissolved Si through the film is rapid enough to maintain the Si concentration far from equilibrium. As a result, the quartz dissolution in the film is equivalent to that in the saturated condition. For the carbonate having a high dissolution rate, less Ca is washed away through the film than the influx of Ca by dissolution. This leads to an increase in the Ca concentration close to the equilibrium, which significantly retards the carbonate dissolution in the presence of the film. These results are consistent with those obtained from the flow-through dissolution experiments. Reactive-transport modeling that accounts for the role of the water film is useful in quantitatively predicting the chemical weathering of silicate and carbonate in the vadose zone.
To investigate the dissolution behavior of silicate and carbonate covered by the water film, flow-through dissolution experiments were conducted under both water-saturated and unsaturated conditions. Similar experiments were conducted with two types of sandstone: one composed of ~100 % quartz, and the other containing not only quartz but also Ca-Mg-Fe carbonate. Ultrapure water was infiltrated into sandstones adjusted to saturated and unsaturated conditions, and the flow rate and element concentration in the effluent were measured to calculate the bulk dissolution rate under each condition. The results showed that for both sandstones, the Si dissolution rate was almost the same in the saturated and unsaturated conditions. In contrast, experiments with Ca-Fe-Mg carbonates showed that the Ca dissolution rate in the unsaturated condition was slower than that in the saturated condition. These facts suggest that quartz covered by the film dissolves at the same rate as that under the saturated condition, while the dissolution of the carbonate becomes slower in the presence of the film.
What physicochemical properties control the water film thickness? A model was developed to predict the film thickness by considering the van der Waals and electric double layer forces between mineral, water film, and air. The model shows that the film thickness is influenced by pH, ionic strength, surface charge of mineral and air/water interface, and pore size. In particular, the sign of the surface charge is a crucial factor. At neutral pH, positively charged surfaces (e.g., quartz) are covered by a thick, thermodynamically stable film. In contrast, negatively charged surfaces (e.g., calcite) are wetted by a thin film due to the absence of the electric double layer force that thickens the film.
To quantitatively understand dissolution and mass transport in the water film in rock pores, we developed a reactive-transport model for the film. The model shows that due to a low dissolution rate of quartz, the diffusion of dissolved Si through the film is rapid enough to maintain the Si concentration far from equilibrium. As a result, the quartz dissolution in the film is equivalent to that in the saturated condition. For the carbonate having a high dissolution rate, less Ca is washed away through the film than the influx of Ca by dissolution. This leads to an increase in the Ca concentration close to the equilibrium, which significantly retards the carbonate dissolution in the presence of the film. These results are consistent with those obtained from the flow-through dissolution experiments. Reactive-transport modeling that accounts for the role of the water film is useful in quantitatively predicting the chemical weathering of silicate and carbonate in the vadose zone.