Diffusivity of dissolved silica in rock pore water at 25°C as characterized by through‐diffusion experiments

The diffusivity of dissolved Si in rock pore water at 25°C was evaluated by through‐diffusion experiments for three types of rock (Fontainebleau sandstone, Berea sandstone, and Tenjyosan rhyolite). Comparisons of the effective diffusion coefficients of Si, K + , and Cl − ( D e , Si  ,De , K +, andDe , Cl −) at neutral pH (unbuffered) showed that theD e , Si  values obtained for the three rocks were 2.3–3.0 times smaller than those ofDe , K +andDe , Cl −. The small value ofD e , Si  compared withDe , K +andDe , Cl −is primarily attributed to the difference in the diffusion coefficients of K + , Cl − , and Si in free water (a factor of 1.7) but might be also partly due to the precipitation of Si in the pores. The chemical form of the dissolved Si is altered depending on the solution pH. To examine how the diffusivity of Si is affected by its chemical form, theD e , Si  values were also determined at pH 6 and pH 11 (buffered solutions containing 0.5 M NaCl) using a Fontainebleau sandstone sample. The dominant chemical forms of Si predicted from thermodynamic calculations are an electrically neutral monomer (Si(OH) 4 ) at pH 6 and an anionic monomer (SiO(OH) 3 − ) with a certain amount of anionic multimers (e.g., dimer, trimer, tetramer) at pH 11 (total Si concentration ≤20.4 mM). The values ofD e , Si  obtained at pH 6 and pH 11 were quite similar (1.94 × 10 −7 to 1.97 × 10 −7 cm 2 s −1 ), andD e , Si  was hardly affected by the change in the charge state and the presence of multimeric species.