Mineralogical transformations controlling acid mine drainage chemistry

Abstract

The role of Fe(III) minerals in controlling acid mine drainage (AMD) chemistry was studied using samples from two AMD sites [Gum Boot (GB) and Fridays-2 (FR)] located in northern Pennsylvania. Chemical extractions, X-ray diffraction (XRD), scanning electron microscopy (SEM), and Fourier transform infrared spectroscopy (FTIR) were used to identify and characterize Fe(III) phases. The mineralogical analysis revealed schwertmannite and goethite as the principal Fe(III) phases in the sediments. Schwertmannite particles occurred as sub-micron sized spheroids. Their transformation into goethite occurred at the GB site where poorly-crystallized goethite rich in surface-bound sulfate was initially formed. In contrast, no schwertmannite transformation occurred at the FR site. The resulting goethite in GB sediments was also of spherical morphology and resulted from an in situ phase transformation involving the conversion of bulkbound schwertmannite sulfate ions into goethite surface complexes. Chemical extractions moreover showed that the poorly-crystallized goethite particles were subject to further crystallization accompanied by sulfate desorption. Changes in sulfate speciation preceded its desorption, with a conversion of bidentate-to monodentate-bound sulfate surface complexes. Laboratory sediment incubation experiments were conducted to evaluate the effect of mineral transformation on water chemistry. Incubation experiments were carried out with schwertmannite-containing sediments and aerobic AMD waters with different pH and chemical composition. The pH decreased to 1.9-2.2 in all suspensions and the concentrations of dissolved Fe and S increased significantly. Regardless of differences in the initial water composition, pH, Fe and S were similar in suspensions of the same sediment. XRD measurements revealed that schwertmannite transformed into goethite in GB and FR sediments during laboratory incubation. The incubation experiments demonstrated that schwertmannite transformation controlled AMD water chemistry under no-flow, batch conditions.

Iron Transformations Induced by an Acid-Tolerant Desulfosporosinus Species

“…Under acidic conditions, the transformation of schwertmannite to goethite may occur over periods of several years (5,30,40), while at circumneutral pH, the reaction may occur over periods of 100 days (57). Recent work by Burton et al (10,11) has suggested that Fe(II) enhances the schwertmannite-to-goethite transformation, with the Fe(II)-enhanced reaction occurring over periods of hours.…”

The Influence of Phototrophic Biomass on Fe and S Redox Cycling in an Acid Mine Drainage-Impacted System

“…This release of sulfate may be due to the reductive solubilization of sulfatecontaining Fe(III) phases. Schwertmannite (Fe 8 O 8 (OH) 8-2x (SO 4 ) x nH 2 O, where 1 B x B 1.5) is sulfate-containing Fe(III) phase that is frequently encountered in AMDimpacted systems (DeSa et al 2010;Murad and Rojík 2005;Peretyazhko et al 2009;Regenspurg et al 2004;Schwertmann et al 1995). Such minerals form in acidic, sulfate-rich fluids when sulfate ions are incorporated into the mineral structure (Bigham et al 1990(Bigham et al , 1996Gagliano et al 2004;Murad and Rojík 2005).…”

“…The pH of formaldehyde-deactivated incubations decreased, likely due to acidity associated with Fe(III) phases. For instance, Peretyazhko et al (2009) have suggested that Fe(III) phases (e.g. schwertamannite) in AMD-impacted systems may exert considerable control on the pH of fluids, regardless of the initial chemical composition of those fluids.…”

Arsenic Geochemistry of Acid Mine Drainage

“…in these precipitates by up to 61 mg/g. Other potential As scavengers in mine drainage areas include ferrihydrite (Fe(OH) 3 ), which is often present as an intermediate product of schwertmannite transformation to goethite (Peretyazhko et al 2009), and has been demonstrated to have a higher As retention affinity due to its higher surface area (Carlson et al 2002). Romero et al (2010) found that presence of hematite (Fe 2 O 3 ), goethite (FeOOH), or magnetite (Fe 3 O 4 ) in mine water can substantially reduce As content in schwertmannite-absent localities.…”