Interaction of FeII and Si under anoxic and reducing conditions: Structural characteristics of ferrous silicate co-precipitates

Abstract

The interaction of FeII and Si is at the heart of many critical geochemical processes in diverse natural and engineered environments. The resulting FeII-silicate phases play important roles in regulating the concentrations and bioavailability of FeII and Si, as well as serve as sinks for trace and hazardous elements. Therefore, a detailed understanding of their structural characteristics and the underlying formation mechanisms may provide insights useful to predicting their reactivity and stability under different conditions. In this work, co-precipitates with different Si/FeII ratios (0.5, 1.0 and 2.0) were synthesized under anoxic and reducing conditions at different solution pH (7, 9 and 11). Thermodynamic calculations from solution chemistry data were carried out and co-precipitates were studied using X-ray diffraction (XRD), infrared (IR) spectroscopy and Fe K-edge extended X-ray absorption fine structure spectroscopy (EXAFS). Thermodynamic calculations predict the formation of phyllosilicate phases such as greenalite in all the samples. Solid characterization data, however, reveal significant structural variabilities and phase heterogeneity. Incipient phyllosilicate structures tend to be more pronounced in samples from pH 9 and 11, while at neutral pH conditions, polymeric silicate phases like amorphous SiO2 become predominant. These variabilities are possibly linked to heterogeneous formation processes arising from relative solubility differences between SiO2 and Fe(OH)2. High pH (>8) conditions favor the polymerization of Fe(OH)2 layers which likely serve as templates for layered silicate formation, while neutral pH conditions favor the precipitation of polymeric silicate phases like amorphous SiO2 to which aqueous Fe species may adsorb. In samples from Si/FeII = 0.5, relatively well developed Fe(OH)2 layers were identified from the EXAFS data. Increasing Si/FeII ratios lead to amorphous SiO2 precipitation and inhibition of Fe(OH)2 polymerization, resulting in smaller phyllosilicate domains embedded in a polymeric SiO2 matrix. The results of this work may be useful in interpreting structural variabilities of FeII-Si phases observed both in nature and in engineered environments. Such variabilities may influence subsequent phase recrystallization processes as well as reactivity towards environmentally relevant elements such as radionuclides.