Biomineralisation of nano-Fe(III) phases in inland waterways

Discharge of Fe(II)-rich groundwaters to surface-waters results in biomineralisation and the formation of Fe(III)- mineral accumulations in inland waterways. This study describes their micromorphology, pore-water chemistry and environmental occurrence in a Tertiary floodplain setting. Fe(III) precipitates were pH 6.2-7.2, sub-oxic (Eh 59-453 mV), with dissolved Fe(II) ranging from 0.1 to 81.6 mg L-1. X-ray diffraction indicated a dominance of 2-line ferrihydrite, with lesser amounts of goethite and lepidocrocite. Electron microscopy revealed the nano-crystalline properties of the 2- line ferrihydrite (Fe5HO3·4H2O), goethite (á-FeOOH) and lepidocrocite (ã-FeOOH). Overall, the results demonstrated distinct variations in Fe(III) mineralogy and micromorphological characteristics. Preliminary groundwater oxidation data found the importance of Leptothrix ochracea in promoting the precipitation and stabilisation of these nanoparticulate Fe(III) minerals. This biomineralisation resulted in abundant sheath and aggregate microstructures of predominantly 2-line ferrihydrite (Fig 1a), and some lepidocrocite crystals (Fig 1b). The implications to water quality of this biomineralisation process are explored.