Long-term in vitro transformation of 2-line ferrihydrite to goethite/hematite at 4, 10, 15 and 25°C

2-line ferrihydrite stored in water at ambient temperatures from 4 to 25degreesC and at ten different pH values between 2.5 and 12 for up to 10-12 y transformed to both goethite and hematite at all temperatures and pH values except at pH 12 where only goethite was formed. The rate and degree of transformation (20-100%) increased with increasing pH and temperature. The hematite/(hematite+goethite) ratio varied between 0 and similar to0.8, increased with increasing temperature and showed a strong maximum at pH 7-8 which increased from 0.1-0.2 at 4degreesC to 0.7-0.8 at 25degreesC. The maximum coincides with the zero point of charge of ferrihydrite where its solubility and, thus, its via-solution transformation rate to goethite are minimal. We assume, therefore, that in this pH-range the (slower) via-solution transformation to hematite can more efficiently compete with that to goethite.