Mineralogy and specific surface area are major controls on the stabilities of ferric oxyhydroxide microparticles in natural waters. The thermodynamic stabilities of ferric oxyhydroxides, as described by the activity product in solution pK = -log [Fe3+] [OH J3 range from pK = 37.1 for freshly precipitated amorphou oxyhydroxide to pK = 44.2 for well crystallized goethite. The sizes of suspended oxyhydroxide particles in natural waters range from less than 0.01 Am to greater than 5 Mm. Oxyhydroxides precipitated in the laboratory from solutions simulating high-iron natural waters are needlelike or lathlike in shape and have mean thicknesses as small as 60 A. Large specific surface areas resulting from the smali sizes of ferric oxyhydroxide particles cause increased solubilities and thus decreased pK values. Specific surface areas of 40-170 m2/g determined for laboratory precipitates gave computed decreases in pK of 0.4 to 1.6 units.
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