THERMODYNAMIC MODELLING OF THE BR PARTITION BETWEEN AQUEOUS SOLUTIONS AND HALITE

Abstract

Experimental work in the ternary system NaCl–NaBr–H2O at 25°C and 1 atm was carried out to determine the distribution of Br in halite and to close gaps in former sets of data. In order to model the incorporation of Br, a new link between the distribution coefficient DBr = (% Br in solid/% Br in aqueous solution) and the activity coefficient of the trace end-member in a solid solution system was developed. Because conventional solid solution models are not able to describe the behaviour of trace components satisfactory, a new generalized thermodynamic model for the extreme ends of such systems (Xtrace < 0.01) was created. The basis of the model is the assumption that the activity coefficient of the trace component in the extreme ends can be sufficiently described by the following form: 2.303RT log λtrace = p1Xmajor2 + p2Xmajorp3. The new model was implemented into the computer code EQ3/6. The influence of different alkali and earth alkali cations in aqueous solutions on the distribution coefficient was modelled. At a concentration of about 1% in the aqueous solution the distribution coefficient of Br in halite is reduced about 10% by K, 9.9% by Ca, 7% by Li, 4.5% by Rb, 3.5% by Sr and 2.5% by Mg. The model was validated by a comparison of calculated values for the Br concentration in halite precipitated from evaporating seawater and field observations of different authors. The calculated data are in good agreement with the experimental data as well as with other data from salt works.