The effect of uncertainties in stability constants on speciation diagrams
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The effect of uncertainties in stability constants on speciation diagrams by Arvid Ödegaard-Jensen1, Christian Ekberg1 and Gunther Meinrath2,3 1) Dept. Nuclear Chemistry, Chalmers University of Technology, S-412 96 Göteborg, Sweden. 2) RER Consultants Passau, Schießstattweg 3a, D-94032 Passau, Germany 3) Technical University Mining Academy Freiberg, Institute of Geology, D-09596 Freiberg, Germany ABSTRACT Speciation calculations are one of the more common ways to describe a chemical system. Speciation diagrams is a convenient way to display such calculations and may also be the basis on which experiments in neighbouring fields of interest, such as sorption are based. In earlier days only very simple problems could be described, e.g. hydroxide species dominance diagrams versus pH. With the aid of computers, however, more complicated systems such as groundwater chemistry may be used. However, as the complexity increase, so does the effect of uncertainties. In this paper we focus on the effect uncertainties in stability constants may have on speciation calculations. The common belief is that the curves will be replaced by narrow bands, while the truth is far from that. In many cases one or several species cover the entire region between 0 and 100%.
INTRODUCTION Speciation calculations are powerful tools to predict how chemical systems behaves. With the development of faster computers more complex systems may be evaluated. Speciation calculations are often shown as speciation diagrams. These diagrams clearly show at which state the different species dominate. However, since speciation calculations generally require thermodynamic data such as stability constants and as these constants are encumbered by different uncertainties [1], the resulting speciations are not as conclusive as one would hope. Rather, in a speciation diagram it is common that several species cover the entire region from 0100% [2]. In this paper we have used the newly developed Ljungskile program [3] to study a case given in the literature. This case was picked arbitrarily and our goal is not to show that the authors of that paper are wrong but rather to point at the huge effect that fairly small uncertainties have on the predictive power of a speciation calculation. THE CHEMICAL SYSTEM The system used is an aluminium system picked arbitrarily from the literature [4]. It is a very simple system, consisting of only one solid phase at a time. The solid phase used is either alunite, jurbanite, basaluminite or gibbsite, of which alunite is the most commonly used in the article. The water contains the following elements K+, F-, SO42-, H+, OH-, and dissolved organic carbon. The number of reactions chosen is about 20. The logarithmic values of the formation constants, however, are reported without uncertainties. The authors claim that the speciation of aluminium is important when evaluating the environmental and biological significance of aluminium concentration in soil solution. The free
II3.35.2
Al3+, Al(OH)2+, and Al(OH)2+ are the most toxic
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