Solubility of colloidal ferric hydroxide

Abstract

Considerable evidence supports the concept that phosphate1–4, trace-metal cations5–7, and rare-earth elements8 are strongly influenced by chemical interactions with suspended solids in turbid rivers and estuaries. Among the most active of suspended particu-lates are metal-hydroxide colloids, which exist as dispersions or as surface coatings on larger suspended solids. Elucidation of the chemical mechanisms governing colloidal and dissolved species interactions is hampered by ignorance concerning fundamental chemical parameters describing the colloidal phases. Colloidal ferric hydroxides have long been known to interact with a variety of dissolved species in natural waters, yet reported solubility products for ferric hydroxides range over three orders of magnitude9. Without such fundamental information, more refined issues, such as the extent to which solid solution formation governs dissolved species, are impossible to resolve. Here we re-examine the solubility properties for colloidal ferric hydroxide reported over the past 60 years, and combine them with results from experi-ments with dialysed ferric-hydroxide suspension performed in this laboratory. After correction of historical data for ferric complexation and variations in ionic strengths, the combined data are consistent with the mass action equation, αFe3+α2.35OH = 10−31.7 where α, is the activity of species i.