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Observations and assessment of iron oxide and green rust nanoparticles in metal-polluted mine drainage within a steep redox gradient

Carol A. Johnson A B F , Gina Freyer B , Maria Fabisch B , Manuel A. Caraballo A C D , Kirsten Küsel B E and Michael F. Hochella Jr A
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- Author Affiliations

A Department of Geosciences, Virginia Tech, 4044 Derring Hall, 1405 Perry Street, Blacksburg, VA 24061, USA.

B Institute of Ecology, Friedrich Schiller University Jena, Dornburger Strasse 159, D-07743 Jena, Germany.

C Geology Department, University of Huelva, Campus ‘El Carmen’, Avenida 3 de Marzo s/n, E-21071 Huelva, Spain.

D Mining Engineering Department, University of Chile, Avenida Tupper 2069, 8370451 Santiago, Chile.

E German Centre for Integrative Biodiversity Research (iDiv) Halle-Jena-Leipzig, Deutscher Platz 5e, D-04103 Leipzig, Germany.

F Corresponding author. Email: cjohns49@vt.edu

Environmental Chemistry 11(4) 377-391 https://doi.org/10.1071/EN13184
Submitted: 11 October 2013  Accepted: 15 January 2014   Published: 19 May 2014

Environmental context. Legacy contamination from mining operations is a serious and complex environmental problem. We examine a former uranium mine where groundwater leaving the site enters a stream with chemically dramatic effects resulting in a fundamental change in the way contaminant metals are transported to the surface environment. The results are important for our understanding of how these contaminants are dispersed, and how they could interact with the biosphere.

Abstract. In this study of iron- and silica-bearing nanoparticle and colloid aggregates in slightly acidic mine drainage, we combined bulk scale geochemistry techniques with detailed nanoscale analyses using high-resolution transmission electron microscopy (HR-TEM) to demonstrate the complexity of iron oxide formation and transformation at a steep redox gradient (groundwater outflow into a stream), and the resulting role in metal(loid) uptake. We also identified pseudohexagonal nanosheets of Zn-bearing green rust in outflowing groundwater using HR-TEM. This is only the second study where green rust was identified in groundwater, and the second to examine naturally occurring green rust with analytical TEM. In aerated downstream waters, we found aggregates of poorly crystalline iron oxide particles (20–200 nm in diameter). Inductively coupled plasma–mass spectrometry (ICP-MS) analysis of water fractions shows that most elements such as Ni and Zn were found almost exclusively in the dissolved–nanoparticulate (<0.1 μm) fraction, whereas Cu and As were primarily associated with suspended particles. In the underlying sediments composed of deposited particles, goethite nanoneedles formed on the ferrihydrite surfaces of larger aggregated particles (100–1000 nm), resulting in more reactive surface area for metal(loid) uptake. Sequential extraction of sediments showed that many metal(loid)s, particularly As and Zn, were associated with iron oxides identified as ferrihydrite, goethite and possibly schwertmannite. Amorphous silica co-precipitation with iron oxides was prevalent at all sampling sites, but its effect on metal(loid) sorption is unknown. Fine-grained iron oxide sediments are easily remobilised during turbulent flow events, adding to the mobility of the associated metals.

Additional keywords: colloids, electron microscopy, ferrihydrite, goethite, iron oxidation, schwertmannite.


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