Mulders, Josephina Jacoba Petronella Adriana; (2021) Experimental studies of mineral dissolution, nucleation and growth in aqueous solutions. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The dissolution and precipitation reactions of minerals play a critical role in controlling the fluid chemistry of the Earth’s surface and subsurface water. However, there are few constraints on how the rates and mechanisms of mineral growth, nucleation, or dissolution depend on aqueous solution compositions. This thesis quantifies the dissolution and growth of sepiolite and siderite as a function of solution composition and furthermore explores the nucleation pathways of siderite as a function of solution saturation state. The growth and dissolution rates of siderite (FeCO3) and sepiolite (Mg4Si6O15(OH)2) were quantified in seeded flow-through experiments, in which the pH and solution saturation index (SI) of the reactive inlet solution were varied. The nucleation pathway of siderite was also examined as a function of solution SI in batch experiments and turbidity experiments. Results show that sepiolite dissolution rates increase as the reactive solution pH decreases. Under acidic and circum-neutral pH conditions, sepiolite dissolves via proton exchange with Mg2+. Short-term sepiolite dissolution and growth can be described by transition state theory, whereas longer-term sepiolite growth rates decrease over time, implicating the poisoning of reactive surface sites (RSS). Siderite nucleation pathways are affected by the initial SI of the reactive solution. At a low SI, crystalline siderite nucleates directly from solution, forming rhombohedral particles with relatively few RSS. At a high SI, siderite nucleation is preceded by the precipitation of amorphous ferrous carbonate, forming particles with a high number of RSS. Experiments on siderite growth and dissolution indicate that rates fell below the analytical detection limit, with siderite reactivity likely hindered by a lack of RSS on the mineral surface. This thesis provides novel insights into the dependency of mineral reactivity on RSS availability and the SI of the reactive solution. Computed siderite and sepiolite rate laws furthermore provide evidence to improve reactive transport models.

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