低温溶液中のケイ酸の重合に及ぼすpHおよび金属陽イオンの影響について

抄録

In this study, the experiments on the polymerization of silicic acid were carried out at a fixed low temperature (35°C) . The effects of a little differences in both pH and initial silicic acid concentration at near neutral pH (6-8) are experimentally examined on the polymerization of silicic acid. The effects of metal cations (Na⁺, K⁺, Mg²⁺, Ca²⁺) were also examined.
Even a little difference in both pH and initial silicic acid concentration affect clearly on the polymerization rate of silicic acid : the polymerization rate of silicic acid increased with increasing pH and initial silicic acid concentration. In all the metal cation solution, as the molarity of metal cation was higher, the polymerization rate was higher and the concentration of silicic acid after 2 hours was lower. These results are consistent with the tendency on decrease in equilibrium concentration of dissolved silica C_e due to salt out effect.Regardless of pH or initial silicic acid concentration C_o, the polymerization rate of silicic acid became maximum as the initial supersaturation concentration of silicic acid decreased by 25%. Then the maximum polymerization rate of silicic acid R_max is represented by the following equation.
log R_max=k₁·pH+k₂·log{(C_o-C_e)/C_e)}+K(T)At the range of the experimental condition (pH=6.4∼7.9 ; C_o-C_e=490∼650 mg/1 ; T=35±2°C), k₁=0.632±0.002, k₂=3.73±0.01, K(T=35°C)=-5.33±0.11.
The effects of adding each sodium, potassium and calcium ion on the polymerization rate of silicic acid are explained as the increase in supersaturation degree of silicic acid due to the salt out effect. On the other hand, the increase in polymerization rate of silicic acid in adding magnesium ions can not be explained as only the increase in supersaturation degree. From the fact that the concentration factor of magnesium between silica scale and geothermal brine is higher, magnesium ion is expected to be taken in the polysilicic acid produced.