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 bulk-bound 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.
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