Uptake mechanism of silicic acid by schwertmannite and its stabilization

Abstract

As part of a series of studies on the interaction between ferric ions and silicic acid in the hydrosphere, the exchange reaction between SO42- 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 SO42- 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 SO42-, which acts as a template ion in the formation of schwertmannite under acidic conditions. The substitution reaction was determined to be as follows: Schwertmannite-{SO42-} + Si(OH)3O- + OH- → Schwertmannite-{Si(OH)2O22-} + SO42-+ H2O, where schwertmannite-{ SO42-} and schwertmannite-{Si(OH)2O22-} indicate that the SO42- and Si(OH)2O22- 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.