Abstract The iron oxyhydroxide “Schwimmeisen” (or floating iron) plays an important role in biogeochemical iron cycling in surface waters. In a dewatering shaft in the Harz area, Germany, Schwimmeisen was observed as a floating iron oxyhydroxide film on water and subsequently analyzed. The precipitate was dominated by ferrihydrite, consistent with previous studies. In contrast to previous reports, no ferrous iron was detected, suggesting that Schwimmeisen should not be considered as a discrete mineral phase with a defined Fe(II)/Fe(III) ratio. Rather, it represents a transient interfacial phenomenon that occurs when anaerobic ferrous-iron bearing water contacts atmospheric oxygen in the presence of sufficient (and suitable) organic material that i. preserves the low crystallinity of the oxidation product and ii. hydrophobizes its surface. Oxidation was likely facilitated by microorganisms of the genus Gallionella, which were also characterized in this study. Schwimmeisen occurred both as iridescent thin films and as brown, voluminous masses, a variation attributed to flowing water aggregating the films and to ascending bubbles adhering to the hydrophobic surfaces, thereby reducing aggregate density. The precipitate was commonly enriched with organic material, including potential contaminants, indicating that its formation and transport may influence the risk of contamination mobility in the surrounding biosphere.
Kaufhold et al. (Wed,) studied this question.
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