Uptake mechanism of silicic acid by schwertmannite and its stabilization.

As part of a series of studies on the interaction between ferric ions and silicic acid in the hydrosphere, the exchange reaction between SO 4 ^2- in the schwertmannite tunnel structure and silicic acid in solution was investigated. Increasing pH of the solution from 6 to 11.5 resulted in an increase of adsorbed silicic acid. It was found that rapid substitution of SO 4 ^2- took place with silicic acid which inhibited the transformation of schwertmannite to goethite. This observation implies more effective stabilization of schwertmannite by silicate compared to that of SO 4 ^2-, which acts as a template ion in the formation of schwertmannite under acidic conditions. The substitution reaction was determined to be as follows: Schwertmannite-{SO 4 ^2-} + Si(OH) 3 O^- + OH^- → Schwertmannite-{Si(OH) 2 O 2 ^2-} + SO 4 ^2-+ H 2 O, where schwertmannite-{ SO 4 ^2-} and schwertmannite-{Si(OH) 2 O 2 ^2-} indicate that the SO 4 ^2- and Si(OH) 2 O 2 ^2- species are present in the schwertmannite tunnel structure. The uptake mechanism of silicic acid by schwertmannite is significantly different from its uptake mechanisms by ferrihydrite and goethite. [Display omitted] • The adsorption behavior of silicic acid onto schwertmannite was examined. • The amount of adsorbed silicic acid onto schwertmannite increased with increasing pH. • Schwertmannite transforms to goethite in the absence of silicic acid. • Schwertmannite is stabilized by the ion exchange of sulfate ion by silicate ion. [ABSTRACT FROM AUTHOR]