Erich Koenigsberger

An internally consistent thermodynamic network for arsenic minerals and aqueous species is being developed based on a high quality starting point: arsenolite and arsenolite solubility. Current research is refining the properties of scorodite solubility as a function of temperature along with improved aqueous species stability constants and activity coefficients for ions and ion pairs based on potentiometric measurements of redox, pH, and conductivity for 5-90°C.

Reliable and consistent thermodynamic data for arsenic minerals and aqueous species are required given the worldwide recognition of As poisoning in more than 70 countries [1] and the need to model As mobility more quantitatively in groundwater systems. Arsenic contamination can also constitute a serious problem in hydrometallurgical processing [2]. Unfortunately, few useful thermodynamic measurements have been made on reactions involving As minerals because of experimental difficulties, including long equilibration times with aqueous solutions, formation of metastable phases, and effects of crystallinity and particle-size on solubilities [3]. Solutions to these impediments are discussed in this review of solubility and thermodynamic data for Fe(III) [4] and Ca arsenates, and for solid solutions of arsenates with phosphates and vanadates [5]. We will present models for solubility calculations that utilize the most recent critical evaluation of the thermodynamic properties of arsenic species [6]. We will also make recommendations for the resolution of some discrepancies and suggest directions for further investigations.

A critical evaluation of selected arsenic minerals and aqueous species and a separate review of aqueous arsenic speciation were completed recently as part of a new volume on arsenic geochemistry, mineralogy, toxicity, and microbiology (Reviews in Mineralogy and Geochemistry #79). We developed a thermodynamic network based on the most reliable and fundamental data in the arsenic system, using the most direct pathways to build the network, and checking for internal consistency at each step. Arsenolite, As2O3-cubic, is the cornerstone or anchor of the network from which thermodynamic properties of the aqueous species of arsenate and arsenite and their hydrolysis products are derived. Thermodynamic data on aqueous arsenate ions are essential for the evaluation of metal arsenate minerals and their solubilities. Very few measurements of metal arsenate and metal arsenite complexes are reported from the literature but by using a consistent chemical model and the same equation of state, several metal arsenate and metal arsenite complexes have been estimated by Marini and Accornero (2007, Environ. Geol. 52, 1343). The solubilities and solubility-product constants of several metal arsenates have been reported but many are of variable quality and in need of better measurements and further evaluation. Scorodite solubility is well characterized, consistent with recent calorimetric data at ambient temperatures. The solubility of some calcium arsenates is well characterized and there is some agreement between investigations but inconsistencies still exist and some phases are poorly characterized or not measured at all. Application of estimated metal-arsenate and metal-arsenite stability constants to waters with a wide range of composition and temperature shows that arsenite speciation is affected very little (98-99% as free forms of hydrolyzed arsenite) but that arsenate speciation can be affected substantially (24-71% of the arsenate species in the free forms). Calcium and magnesium arsenate complexes contribute 0-24%, and sodium arsenate complexes contribute 1.5-75%. These complexes should be included in speciation calculations for any waters involving arsenate species, especially water compositions with moderate or higher concentrations of sodium.

At the present state-of-the-art, the modelling of equilibria occurring in concentrated reactive electrolyte mixtures remains problematic: the theoretical deficiencies related to calculating the activity of species in multicomponent solutions seem unlikely to be overcome soon. However, thermodynamic data for reactive systems, e.g. solubility values, are widespread in the chemical literature and need to be better utilised. A key challenge for storing this information is expressing the complete chemistry of a large number of potentially complicated solutions – such as phosphate and borate buffers – in a compact, and ultimately machine-processable, form. The approach taken to date (e.g. [1]) records only the analytical concentrations of well-defined components. We are seeking to build the data repository which will be required in future to parameterise theoretical solution chemistry frameworks estimated to involve at least five million experimental data values. A major issue is that existing databases do not store information relating to the chemical behaviour of the solution components that is needed for processing experimental data. We find that defining all solution species and the set of allowed reactions is necessary and sufficient for describing the chemistry of the solution. A database system implementing this design is in development building upon our existing chemical reaction [2] and physicochemical property [3] databases. Accordingly, the new database can store a vast range of properties including solubility, pH, density, mean activity coefficients and equilibrium constants. Methods to harmonise these data, which is a preliminary step prior to extracting reliable thermodynamic parameters for modelling, are described.

The IUPAC Subcommittee on Solubility and Equilibrium Data has been selecting, editing and reviewing projects and manuscripts for publication as part of the IUPAC-NIST Solubility Data Series (SDS). This series provides comprehensive reviews of data from the primary literature for solubilities of gases, liquids and solids in liquids or solids. Data are compiled in a uniform format, critically evaluated and, where high-quality data from independent sources agree sufficiently, recommended values are proposed [1]. SDS Volume 102 was published recently [2]. A current IUPAC project is dealing with solubilities of subs tances related to urolithiasis [3]. Kidney stones consist of a range of inorganic and organic substances that are sparingly soluble in urine, thereby producing ionic and uncharged aqueous species which can undergo a wide variety of protonation and complexation reactions. Using these data as an example, this work explores the features of the JESS solubility database [4] that assist with the presentation, manipulation and critical evaluation of solubility data. In particular, the unique combination of databases available in JESS for speciation equilibria [5], physicochemical properties of solutions [6] and solubilities provides improved evaluation facilities, including the assessment of thermodynamic consistency among solubility, calorimetric and speciation data as described in a recent SDS volume [7]. Systems of different chemical complexity will highlight the benefits of applying the JESS software package to the production of future SDS volumes.

The extraction of rare earth metals from fluoroapatite rich phosphate concentrates often involves a weak acid leach of fluoroapatite and a sulphuric acid bake of leach residue followed by water leach and precipitation to obtain an intermediate product. The main aim of this study was to determine and rationalize the solubility of rare earth metal ions in synthetic solutions representing various process liquors at three temperatures 40, 60 and 80oC containing different acid and metal ion concentrations with respect to sulphuric acid, phosphoric acid and sulphate salts of sodium, magnesium, aluminium, potassium, calcium and iron(III). Solid mixture of rare earth metal carbonates was used as the source of rare earth metal ions. The solubility tests and characterization of solids using XRD were conducted at Murdoch University laboratories. The composition of initial carbonate solids and the solids and solutions formed after saturation were analysed for rare earth and other elements at TSW Analytical Laboratories in Perth using ICP-MS and ICP-OES. The precipitated solid in sodium rich acidic sulphate solutions is the double salt NaRE(SO4)2 based on the solid assays. However, in acidic solutions free of sodium or of low concentrations of sodium the precipitated solid appears to be RE2(SO4)3.

This presentation in honour of Professor Heinz Gamsjäger on his 80th birthday will highlight his seminal contributions to our understanding of solubility phenomena in aqueous media. These include the development of apparatus for accurate solubility measurements, particularly of solid solutions, and theoretical methods to extract thermodynamic quantities from these experimental data. Recent applications to systems of industrial, geochemical and medical interest will be discussed.

Reliable solubility data for oxides and hydroxides of heavy metals in alkaline solutions are relevant to a number of industrial and geochemical processes, including the production of high-purity alumina in the Bayer process, the formation of ores from geothermal solutions at high pH, hydrometallurgical leaching processes for the extraction of metal values from ores and plant residues, and the storage and processing of certain types of radioactive waste. Recent results obtained for various systems will be reported, including experimental and modelling aspects. A new facility for the storage, handling and processing of solubility data will be outlined.