Implementing the Mean Spherical Approximation Model in the Speciation Code CHEAQS Next at High Salt Concentrations
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Implementing the Mean Spherical Approximation Model in the Speciation Code CHEAQS Next at High Salt Concentrations Wilko Verweij1 · Jean‑Pierre Simonin2 Received: 17 November 2019 / Accepted: 25 February 2020 © Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract Speciation programs all struggle with the challenge of converting equilibrium constants to non-zero ionic strength. Equations like the Davies equation are semi-empirical and do not give satisfactory results at high salt concentrations. The Mean Spherical Approximation (MSA) model is a method with a solid theoretical basis that gives good results at high salt concentrations. We implemented the MSA model in a new 64-bit version of the speciation program CHEAQS Next which is under development. It was shown that the results obtained with this research version are identical to results obtained earlier by Simonin (Ind Eng Chem Res 56:9721–9733, 2017). This is the first time the MSA model has been implemented in a production program. Keywords Speciation program · MSA model · Activity correction · CHEAQS
1 Introduction Chemical speciation is an important factor in many aqueous processes in the environment. For example, the speciation of an element determines its physico-chemical behavior by processes like complexation in the solution phase, both by inorganic ligands and by the mixture of naturally occurring organic molecules. Also adsorption onto mineral phases and precipitation are related to the speciation of an element. Redox conditions can have a profound effect on the speciation of elements e.g. in the case of highly soluble iron(II) versus poorly soluble iron(III). The bioavailability and hence the toxicity of elements for aquatic organisms is also related to their speciation [1]. Especially for metals a lot of effort has been made to understand why an element is toxic in a certain situation but not toxic in another one. Electronic Supplementary Material The online version of this article (https://doi.org/10.1007/s1095 3-020-01008-9) contains supplementary material, which is available to authorized users. * Wilko Verweij [email protected] 1
Deltares, P.O. Box 177, 2600 MH Delft, The Netherlands
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CNRS, Laboratoire PHENIX, Campus P.M. Curie, Sorbonne Université, 75005 Paris, France
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Vol.:(0123456789)
Journal of Solution Chemistry
Different experimental approaches have been used to assess the speciation of elements. Physico-chemical methods are very useful in unravelling the speciation, but they sometimes lack the required analytic sensitivity. Biological methods can be used to more or less directly assess the effects of an element on organisms, so bypassing the need to quantify the speciation in chemical terms. Modelling is a way to calculate the chemical speciation based on information about total concentrations, equilibrium constants, etc. Many programs have been developed over time, including GEOCHEM [2], MINEQL [3], TiTrator [4], (Visual) MINTEQ [5], ORCHESTRA [6], ECOSAT [7], CHESS [8], JESS [9], an
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