INTEGRATED SYNCHROTRON APPROACHES TO CHARACTERISE ORGANIC MATTER-IRON PHASES RELEVANT TO PASSIVE MINE WASTE REMEDIATION SYSTEMS
Date
2023-07-25
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Journal ISSN
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Publisher
ORCID
Type
Thesis
Degree Level
Masters
Abstract
The work presented in this thesis used synchrotron X-ray scattering and X-ray absorption spectroscopy (XAS) techniques to characterize samples relevant to mine waste systems. Passive remediation systems such as permeable reactive barriers and constructed wetlands aim for long-term contaminant immobilization. Contaminant removal is achieved through mechanisms such as precipitation in sulfide phases and/or sorption onto iron oxyhydroxides. These processes occur in the presence of various dissolved organic compounds such as tannins that are released in natural environments through organic matter degradation. Research on tannin influences on mineral precipitation is necessary to better understand contaminant immobilization processes. This thesis aims to examine the role tannins (using tannic acid as the model compound) play during iron oxyhydroxide and iron sulfide precipitation. Tannic acid was found to favour formation of crystalline phases when present during ferrihydrite and mackinawite precipitation. The presence of tannic acid during ferrihydrite synthesis favoured the formation of lepidocrocite and goethite as tannic acid concentration increased. The presence of tannic acid during mackinawite synthesis favoured formation of greigite, however increasing tannic acid concentration also led to ferrihydrite precipitation . Tannic acid is redox sensitive and in the presence of redox sensitive metals such as Fe, it can change the system’s chemistry. During ferrihydrite synthesis, tannic acid reduced Fe(III) to Fe(II) even under aerobic conditions. The tannic-acid generated Fe(II) might have catalyzed the transformation of early formed ferrihydrite to lepidocrocite and goethite or they might have formed through oxidation of the produced Fe(II). During mackinawite synthesis, tannic acid catalyzed oxidation of Fe(II) to Fe(III) and S(-II) and S0; changing the system’s chemical conditions to favour greigite and ferrihydrite precipitation. This project suggests that the presence of tannic acid as well other redox sensitive tannin compounds may be important in passive remediation systems since they influence redox conditions and promote the formation of a mixture of crystalline and poorly crystalline phases at initial stages of iron oxyhydroxide and iron sulfide precipitation.
Description
Keywords
tannins, tannic acid, iron, coprecipitation, passive remediation, X-ray diffraction, X-ray absorption spectroscopy (XAS)
Citation
Degree
Master of Science (M.Sc.)
Department
Geological Sciences
Program
Geology