Renewing approaches to understanding the minerals and waters at alkaline waste sites

Highly alkaline (pH 9-12) waters can arise from a range of natural and anthropogenic processes. The latter include drainage waters from a range of globally significant anthropogenic by-products such as lime, cement and steel wastes, bauxite processing residue and combustion ashes. Such waste storage sites are often characterised by extreme geochemical conditions that can be hazardous to aquatic life, but are equally an increasing focus for resource recovery and carbon capture initiatives. The very high rates of mineral precipitation at these sites can give rise to the formation of transient minerals that are not currently well understood. As such our estimates of carbon budgets and understanding of trace metal dynamics at highly alkaline sites is currently limited. This thesis aimed to improve the basis for characterising hyperalkaline carbonate systems by (1) understanding the dominant carbonate fabrics found in secondary deposits and their formation processes, (2) identifying transient minerals forming at hyperalkaline sites, and (3) improving methods for characterising dissolved inorganic carbon and secondary mineral phases at high pH sites.Petrographic analysis showed distinctive shrubby carbonates forming in hyperalkaline (pH 9–12) and moderate conductivity (conductivity 425–3200μS) solutions at ambient temperature (12.5–13°C) at two disposal sites in the UK. Microfabrics in anthropogenic sites are comparable to travertines but lack the sub-surface facies and at extreme pH exhibit sparry crusts without clear equivalents in travertines. Despite the highly alkaline conditions, significant diatomaceous and cyanobacterial biofilms were reported growing in the presence of these carbonates, suggesting a bio-influence on their formation. This sedimentology of anthropogenic carbonates shows that calcite mineral formation is complex and not homogeneous or purely driven by thermodynamic processes.Whilst most of the secondary deposits at the study sites appear to be dominated by calcite, this study provides the first account internationally of ikaite (CaCO3.6H2O) crystallization within steel-slag leachate through novel field (Fourier Transform Infra-Red) and laboratory (X-Ray Diffraction) validation. This study suggests that ikaite is a secondary mineral with a primary phase being amorphous calcium carbonate (ACC). The ikaite forming in steel-slag leachate affected waters is incorporating large inventories of potentially harmful metals (e.g. lead and cadmium) which could be of environmental concern given ikaite is not thermally stable and could release a pulse of contamination in short duration warming events. The final component of the study develops a new protocol for assessing dissolved inorganic carbon (DIC) in alkaline waters via strontium carbonate precipitation. This method is compared to established methods for DIC using field and laboratory titration. The strontium method appears to perform much better than the existing methods and is likely to provide more robust estimates of alkalinity and saturation index of carbonate minerals.The combined findings provide an improved understanding of carbonate precipitation processes at highly alkaline sites which in turn should assist future research endeavours around mineral carbonation, trace metal dynamics and environmental remediation at these sites