An alternative approach to calculating chemical equilibrium in soil reactions

Abstract

In most simulation models dealing with complex chemical reactions in soils, the hypothesis of instantaneous equilibrium (IE) is adopted. The use of this hypothesis (a form of the adiabatic elimination hypothesis) is based partially on the fact that the relaxation times of these reactions are much shorter than the corresponding ones of the external forces, and partially on the current ignorance about the kinetics of most of these reactions, which results in assuming that the system moves forward in time through steps of chemical equilibrium. The alternative method proposed here, on the contrary, does not move stepwise through equilibrium values, thus avoiding the intermediate numerical complications. It makes use of a single scale factor which can be adjusted in order to get “as close as desired” to the concentration values generated with the conventional IE method. Furthermore, a practical criterium is also deduced in order to define a convenient order of magnitude for the scale factor.