FORMATION OF IRON(III) – SILICIC ACID COMPLEXES AS A POSSIBLE MECHANISM FOR THE TRANSPORT OF IRON IN OXYGENATED NATURAL WATER

Iron is one of the most insoluble elements in oxygenated water because ferric iron is easily hydrolyzed and precipitated as ferric hydroxides with extremely low solubilities. Therefore, anoxic environments have been considered essential for the transport of Fe as ferrous iron. However, we have found that up to ~5 ppm of ferric iron dissolves in near-neutral oxygenated water containing high concentrations (~800 ppm) of silicic acid. Using spectrophotometry and gel chromatography, we have investigated the dissolution of ferric iron in solutions containing various amounts (75 – 800 ppm) of silicic acid at ~20 to 90°C. The dominant form of silicic acid changes from monosilicic acid to monosilicic and polysilicic acids when the concentration of silicic acid exceeds the solubility value of amorphous silica. The amount of ferric iron dissolved in solutions increases with increasing concentration of silicic acid. We have also recognized ferric iron dissolves in silicic acid-bearing solutions as two forms of stable colloids that are composed of ferric iron and silicic acids. Based on the Si/Fe ratios and UV absorption spectra of the colloids, we suggest the first colloid form (termed ISAC-I) is composed of particles of ferric hydroxide with adsorbed silica and the second colloid form (termed ISAC-II) is composed of particles of silicic acids with adsorbed ferric hydroxides. ISAC-I and ISAC-II form when the silica content of solution is below or above the saturation of amorphous silica, respectively. Our study suggests significant amounts of Fe can be transported for a long distance in oxygenated water in some environments containing high silica, such as the surface discharge of geothermal fluids.