Reactivity of biogenic silica: Surface versus bulk charge density

Acid–base titrations were carried out at three different ionic strengths (0.01, 0.1 and 0.7 M NaCl) on a range of marine and continental biosiliceous materials. The large variability in electrical charging behavior of the various materials is consistent with the existence of two pools of ionizable groups, one on the outer surface of and the other within the silica particles. The relative amounts of internal and external silanols were estimated by fitting a two-site complexation model to excess proton versus pH curves obtained at the different ionic strengths. For fresh diatom frustules and phytoliths, as well as recently deposited biosiliceous sediments, the abundance of internal silanols was of the same order of magnitude as, or exceeded, that of silanols on the external surface. Older biosiliceous materials exhibited lower proportions of internal groups, while a decrease in the relative amount of internal silanols was also observed for diatom frustules artificially aged in seawater. The existence of internal ionizable functional groups explains measured charge densities of biogenic silicas that largely exceed the theoretical site density of silica surfaces. Variations in the relative abundance of internal versus surface silanols further explain the non-uniform dependence of electrical charging on ionic strength, the lack of correlation between total charge density and dissolution kinetics, and the variable 950 cm −1 peak intensity in the infrared spectra of biogenic silicas. Dissolution rates correlate positively with the external charge, rather than the total charge build-up, as expected if dissolution only involves the removal of silicate units from the external surfaces of the particles. The progressive reduction with time of the internal to external silanol concentration ratio represents one of the mechanisms altering the material properties that affect the recycling and preservation of biogenic silica in earth surface environments.