Studies on soluble silica in soils. I. The sorption of silicic acid by soils and minerals

Soils, representing a variety of great soil groups, were shaken with solutions containing monosilicic acid at concentrations not exceeding 135 p.p.m. The residual concentrations of monosilicic acid under near-equilibrium conditions depended on the nature of the soil, the level of added monosilicic acid, the solution : soil ratio and the pH of the soil suspension. The experimental results indicate that the residual concentration of monosilicic acid is controlled by an adsorption equilibrium which is dependent on pH; with acidification below pH 8-9 the residual concentration in soil suspensions steadily increases. Several samples of oxides and hydroxides of iron and aluminium were also shown to sorb monosilicic acid from solution. This sorption resembled that recorded for soils in its strong dependence on pH in the range 4-9. Sesquioxides appear to be responsible for much of the capacity of soils to sorb monosilicic acid. Wyoming bentonite and two kaolinite samples also sorbed monosilicic acid, at least from suspensions of pH above 7, and some natural and synthetic carbonates sorbed monosilicic acid at their natural pH values. Such minerals may contribute to the ability of some alkaline soils to sorb monosilicic acid. The avidity with which some soils sorb added monosilicic acid suggests that some native silica occurs in soils in the sorbed state. Some implications of this suggestion and of the other results are discussed in relation to some previous work on soils and natural waters, possible adsorption mechanisms, and some pedogenetic processes.