PNNL

Abstract

Floating, mixed-valent Fe films have been observed worldwide in wetlands, ferrous iron rich seeps, and in seasonally reduced soils, but are usually misidentified as oil or biofilms. Little characterization or explanation to their formation has taken place. Along the Oregon coast such films were found on ephemeral pools where Fe(II) rich groundwater (~ 100 µM Fe) discharged at the base of Pleistocene sand dunes. Fe(II) oxidized to Fe(III) at the air-water interface to form ~ 100 to 300nm thick films. Analyses indicated that the films contained both Fe(III) and Fe(II) in a ratio of 3:1. Si was the other main cation, OH was the main anion and some C was identified as well. The film morphology was flat, under optical and electron microscopy with some attached floccules having a stringlike morphology. Energy filtered electron diffraction patterns (EFED) showed three diffraction rings at 4.5, 2.6, and 1.4 Å in some places and 2 rings (2.6 and 1.4 Å) in others. Upon further oxidation the films became 2-line ferrihydrite. We are proposing the name „Schwimmeisen“ for the floating, mixed-valent Fe film.