LabEx OSUG@2020 - Nucleation, Growth and Aggregation Behavior of Al and Fe Oxyhydroxide Mineral Colloids

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Duration : 2013 - 2015

PI : Alejandro Fernandez-Martinez

Co-PI : Rafael Perez-Lopez (University of Huelva, Spain)

Funding : LABEX OSUG@2020, BQR
Funding : 24 k€

ISTerre team involved :
– Geochemistry

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The weathering of iron sulfide minerals and the subsequent production of Acid Mine Drainage (AMD) is a widespread important environmental problem due to the mobilization of toxic metals adsorbed to or incorporated into colloidal mineral phases. This is the case of the AMD-related mining wastes found in many areas of the Iberian Pyrite Belt (IPB ; South-West Spain). Highly acidic waters with high concentration in metals and anions are drained by the fluvial courses of the IPB, i.e., Tinto and Odiel rivers, causing their total pollution. Both rivers not only contaminate the fluvial basin ; in addition, they flow into a coastal wetland known as the ‘Ría’ of Huelva Estuary, which is part of ‘Marismas del Odiel’, a very important Natural Reserve declared as Biosphere Reserve by UNESCO in 1983. The contamination levels are so extreme than both rivers and estuary could be considered as one of the most contaminated aquatic systems in the world (Olías et al., 2006).

Both hydrochemistry and mineralogy of acid mine drainage (AMD) are mainly controlled by SO42—Fe3+ and SO42---Al systems at pH ranges between 2-4 and 4.5-6, respectively (Bigham et al., 1996 ; Nordstrom and Alpers, 1999). The oxidation of leached Fe2+ and the subsequent hydrolysis of Fe3+, and the mobilization of Al3+ in the acid waters, lead to the hydrolysis of Fe and Al when the pH of AMD increases by mixing either with pristine water courses or with alkaline additives in treatment systems, producing schwertmannite and basaluminite, respectively. Both phases are poorly oxy-hydroxysulphates and their precipitation plays an important role in the removal of trace elements from AMD-affected streams. Schwertmannite has been reported to remove mainly As and Cr (Courtin-Nomade et al., 2003). Basaluminite is less studied than schwertmannite, though it is also known that this phase can remove trace elements such as Cu and Si (Bigham and Nordstrom, 2000).

It is generally believed that solvated Fe3+ undergoes hydrolysis leading to the formation of low-molecular-weight Fe3+ species (hydrated and hydrolyzed monomers, dimers, trimers, etc.), that lead to the nucleation of Fe oxyhydroxide particles. The resultant Fe oxyhydroxide phases depend on the binding strength of the anions present in the solution, and on the solution pH value ; these parameters make that the nucleated minerals under acidic conditions vary between ferrihydrite (anions with low binding ability) to akaganeite or schwertmannite (high binding affinity) (Zhu et al., 2012). A study by Johansson (1963) demonstrated that Al13O40 units form the building blocks for several, if not all, Al hydroxysulfates. However, a major gap in knowledge concerns the nucleation pathway whereby aqueous monomeric Fe3+ species or Al13O40 ions are assembled into larger molecular clusters and nanoparticles.

****References
– Olias et al., (2006) Appl. Geochem. 21, 1733
– Bigham et al., (1996) GCA 60, 2111
– Nordstrom and Alpers, (1999) Geochemistry of Acid Mine Waters. In Society and Economic Geologists.
– Courtin-Nomade et al., (2003) Appl. Geochem. 18, 395
– Bigham and Nordstrom, (2000) Revs. Min. & Geochem. 40, 351
– Johansson, (1963) Arkiv. Kemi. 20, 321
– Fernandez-Martinez et al., (2010) Am. Min. 95, 1312
– Zhu et al., (2012) ES&T 46, 8140

In this project, we propose to study the structure and kinetics of schwertmannite and basaluminite formation, with the aim of characterizing the intermediated steps (molecular clusters) leading to the nucleation of these metastable phases as well as their final structures. Previous studies by the main proposer (A. F.-M.) resulted in the first structural characterization of the atomic structure of schwertmannite (Fernandez-Martinez et al., 2010). Here, that study will be extended to characterize the steps previous to the precipitation of schwertmannite and of basaluminite, and the effect of the presence of different anions on their structure. In situ time resolved techniques such as high energy X-ray scattering with Pair Distribution Function (PDF) analyses, Small-Angle X-ray Scattering (SAXS), UV-Vis and dynamic light scattering will be used here to characterize the nucleation of these iron and aluminum oxyhydroxides. The effect of different anions and pollutants (AsO43-, PO43-, CrO42-, SeO42-) on the nucleation kinetics will be also investigated to characterize the potential of these phases as possible vectors for the transport of contaminants. Given that AMD can contain a wide variety of toxic elements that are attenuated by adsorption onto particle surfaces or through co-precipitation during neutralization, knowledge of early stage particle formation and transformation processes contribute to an improved understanding of attenuation mechanisms.