The Solubility of Ferric Oxyhydroxides in Natural Waters

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Abstract
Iron in ground water is often present both in solution and as suspended ferric oxyhydroxides. In most instances amounts of dissolved iron in natural waters are limited by the solubility of the ferric oxyhydroxides, whether suspended or present as part of aquifer materials. Oxyhydroxide solubilities, which range widely in natural waters, may be described by pQ =‐log [Fe3+] [OH‐]3, where [Fe3+] [OH‐]3 is the product of ion activities in the water. pQ. values calculated from Fe(II), Eh, and pH measurements in several types of high iron ground and surface waters (Fe(II) = 0.02 to 1460 ppm) indicate that most of the waters are in equilibrium with ferric oxyhydroxides which range from amorphous material to crystalline goethite (pQ values at 25°C from about 37 to 44, respectively). Stabilities generally increase (higher pQ's) in a given water with time. In general, the lower the dissolved iron, the more soluble is the oxyhydroxide precipitated and the slower its increase in stability. This was observed both in coastal‐plain ground waters (pQ= 36.6 ‐ 42.7), and in laboratory aged solutions. The faster the oxidation and hydrolysis rate of dissolved Fe(II), the lower the pQ, as shown by the reaction of mineralized spring waters with varying amounts of surface waters (pQ= 37.2 ‐ 41.5). Where saturation of water with siderite occurs, the siderite is in equilibrium with amorphous ferric oxyhydroxides (pQ= 36.9 ‐ 37.4 for calcium, magnesium bicarbonate ground waters). pQ values > 44.2, in a flooded coal mines for example, indicate that another iron mineral such as pyrite may control dissolved iron level.