Thermodynamics of ion partitioning in solid solution–aqueous solution systems
While the study of ion partitioning in solid solution-aqueous solution (SS–AS) systems has garnered scientific attention for decades, the terminologies used in ion-partitioning studies are often inconsistent. Moreover, the equilibrium thermodynamics underlying the partitioning process is often disregarded or misunderstood, although this background is indispensable in interpreting effective (non-equilibrium) distribution of substituting ions between minerals and their growth environments. Here, after an introduction to the different terminologies and definitions, the equilibrium thermodynamics of ion partitioning is reviewed for the simplest case of solid solutions with isostructural end-members and one mixing position per formula unit. Variations of the equilibrium distribution coefficients with changes in the background solution or in the composition of the solid as well as the methods to determine equilibrium values are discussed in depth. The simplifications when one of the substituting ions occurs in trace amounts (dilute solid solutions) and the effect of temperature are introduced in a later tion, followed by a discussion of the thermodynamic treatment of more complex SS–AS systems: ion partitioning involving solids with more than one mixing position per formula unit, solid solutions with non-isostructural end-members, heterovalent substitutions, etc. Finally, the concept of supersaturation in SS–AS systems is introduced as a fundamental parameter to interpret non-equilibrium partitioning.
Figures & Tables
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.