Labile and Nonlabile Aqueous Silica in Acid Solutions: Relation to the Colloidal Fraction
- Shihe Xu and
- James B. Harsh
The significance of polymeric Al-Si complexes and Si associated with colloids on Si speciation in acidic solutions is unknown. To characterize Si solutions, the kinetics of the color reaction of Mo with Si in various synthetic Al-Si solutions was determined and a method to quantify the labile fraction of Si was developed. We determined labile Si, dialyzable Si, and nonpositively charged Si in both synthetic and natural solutions, and total Si in solutions passing through 0.45-, 0.2-, and 0.1-µm filter membranes. Silica in various solutions was divided into labile and nonlabile Si according to the rates of the reaction of Si species with Mo in 0.05 M H2SO4. Silica species in the labile fraction were <0.001 µm in diameter and were either nonpositively charged or were converted to nonpositively charged species during passage through a cation-exchange column. The major portion of nonlabile Si was associated with colloidal particles, some of which could pass through a 0.45-µm filter membrane. At pH 4 to 5.5, the quantity of polymeric Al-Si complexes with sizes between 0.001 to 0.1 µm was not significant in solutions in contact with soils or soil clays except for those from a Spodosol that contained allophane and imogolite. Therefore, ignoring soluble polymeric Al-Si complexes will not introduce a significant error in Al and Si speciation in this pH range in the absence of allophane and imogolite. Incomplete removal of colloidal particles may, however, cause significant overestimation of soluble Si and Al, if nonlabile fractions are determined.
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