Geochemical thermodynamic modelling of ion partitioning
Published:January 01, 2011
The partitioning of an ionic species between two or more co-existing phases is closely related to chemical speciation, solubility, sorption and solid-solution formation in the geochemical system. The aim of chemical thermodynamic modelling is to find the stable or metastable chemical speciation in such a system; from that, any possible ion partition or distribution coefficient can be retrieved. This chapter aims to highlight some typical features of modelling the equilibrium partitioning of inorganic ionic species between aqueous electrolyte and solid phases, including adsorption and ion exchange on their surfaces. Some guidance is also given on methods and computer codes that can be used in helping us understand and predict ion partitioning in complex scenarios, where several competing solids are involved in a minor element uptake by different mechanisms.
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Ion Partitioning in Ambient-Temperature Aqueous Systems
On the surface of the Earth, the intermingling of water and minerals gives rise to a diverse suite of reactions that determine the purity of water we drink, the fate of contaminants we emit, and the composition of minerals and biominerals that we use to interpret past environmental conditions from the sediment archive. Human societies have ubiquitous exposure to the outcome of these mineral—water reactions. Understanding in detail the ion partitioning in mineral—water interactions is of fundamental importance to geochemical studies and ultimately to society. The solid-solution properties of minerals are a significant part of the complexity, and also the importance, of these ion-partitioning reactions. Natural minerals always contain a certain proportion of trace elements in solid solution. These trace elements, precisely because of their rarity, often have a disproportionately large impact on living organisms as is the case for familiar toxic metals such as As and Cd. A clear understanding of ion partitioning behaviour is therefore essential for environmental objectives such as scavenging heavy metals from solution, remediating contamination in soils, or ensuring safe, long-term storage of anthropogenic CO2 or radionuclides in geological reservoirs. Materials science has also taken a new look at the role of trace-element and ion partitioning in regulating biomineralization. Finally, the last several decades have seen a surge in interest in reconstructing past climate and environmental conditions from the sediment archive. An accurate interpretation of ion partitioning is essential to the correct interpretation of records from diverse systems like stalagmites, corals, or shells of marine foraminifera. Given this wide range of applications for ion partitioning, it is fortunate that theoretical and thermodynamic frameworks for modelling ion partitioning have advanced significantly in the last decade. We believe that it is an opportune time to convene experts on ion partitioning from a range of perspectives, from theoretical to applied, to exchange knowledge across these topics and through this exchange, maximize the advances that have been made in the discipline. We are pleased to be able to convene these experts in person, at the European Mineralogical School in Oviedo in June 2010 to share these advances with each other and with the next generation of geochemists. It is our hope that this book will serve most crucially as a bridge through which researchers in one aspect of ion partitioning will be able to productively venture into complementary systems and models to better solve their research goals and perhaps be inspired with new research questions.