Integrated chemical–physical processes modelling—I. Development of a kinetic-based model for mixed weak acid/base systems

A kinetic model for mixed weak acid/base systems is described. In the model, the weak acid/base equilibria have been formulated in terms of the kinetics of the forward and reverse reactions for the dissociation of the weak acid/bases. The compound H+ is explicitly included and pH is calculated from the H+ concentration via pH=−log(H+)=−log fm[H+]. For the conditions considered, the weak acid/bases included are the water, carbonate, ammonium, phosphate and short-chain fatty acids systems. However, the modelling approach is completely general and can be applied to include any other weak acid/base of importance. Ion pairing, precipitation of calcium carbonate and carbon dioxide gas exchange also are included. The model was validated by comparing model predictions with those obtained from conventional equilibrium chemistry based algorithms. A very good correlation was obtained. Compared to the equilibrium approach, the kinetic approach offers several advantages in that it facilitates seamless integration of (i) precipitation, gas stripping and dissolution kinetics of minerals and gases within the mixed weak acid/base chemistry structure and (ii) the biological kinetic models for wastewater treatment systems to extend application of these models to situations where the pH is not constant