Fuellenbach, Lisa C; (2022) A microanalytical study of Pb and Cu uptake coupled to siderite dissolution. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Heavy metal contamination of aqueous environments is a worldwide problem and remediation is particularly challenged by natural redox fluctuations, causing acidity and increased metal solubility. Siderite (FeCO₃) is a promising candidate material for heavy metal sequestration, as its carbonate component may regulate acidity, while the redox active Fe(II) component may act as reductant or as sorbent upon oxidative precipitation as Fe(III) (oxyhydr)oxides. To tap the full potential of siderite and its auspicious properties, this thesis investigates the efficacy of metal uptake coupled to siderite dissolution from oxic and anoxic acidic aqueous solutions. Lead (Pb) and copper (Cu) are chosen as case studies for weakly (Pb) and strongly (Cu) redox active metals. In laboratory experiments, metal uptake and pH development are monitored during siderite dissolution in Pb- and Cu-bearing oxic and anoxic acidic waters. Special focus is laid on characterising the solid reaction products using high-resolution analyses to determine the metal speciation and type of bonding decisive for metal retention. Siderite surface-controlled adsorption and dissolution–precipitation reactions buffer the reactive solution pH, while the properties of the dissolved metals determine the reaction pathways. Under oxic conditions, metals predominantly adsorb onto secondary Fe(III) (oxyhydr)oxide precipitates, while under anoxic conditions, metals are captured by sparingly soluble cerussite (PbCO3) and native copper precipitation. Key to these processes is the continuous dissolution of siderite, which is promoted by non-epitaxial growth and phase transformations of the precipitates on the reacting siderite surface. This study corroborates the great potential of siderite dissolution to remediate metal polluted water-saturated dynamic redox environments by counteracting acidity and capturing metals in secondary minerals. The thorough documentation of the reaction products and mechanisms advances our understanding of siderite–metal (redox) interactions to predict sustainable metal contaminant immobilisation and metal cycling in other modern and ancient Fe(II) and carbonate-rich natural aqueous environments.

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