Predicting sulphate adsorption/desorption in forest soils: Evaluation of an extended Freundlich equation

• The proton coadsorption stoichiometry of SO4 in spodic B horizons is close to 2.
• A Freundlich model with two adjustable parameters described SO4 adsorption well.
• The model can be optimized using just two data points.
• It is easily integrated into dynamic soil chemical models, e.g. ForSAFE.

Sulphate adsorption and desorption can delay the response in soil acidity against changes in acid input. Here we evaluate the use of an extended Freundlich equation for predictions of pH-dependent SO4 adsorption and desorption in low-ionic strength soil systems. Five B horizons from Spodosols were subjected to batch equilibrations at low ionic strength at different pHs and dissolved SO4 concentrations. The proton coadsorption stoichiometry (η), i.e. the number of H+ ions co-adsorbed for every adsorbed SO42− ion, was close to 2 in four of five soils. This enabled the use of a Freundlich equation that involved only two adjustable parameters (the Freundlich coefficient KF and the non-ideality parameter m). With this model a satisfactory fit was obtained when only two data points were used for calibration. The root-mean square errors of log adsorbed SO4 ranged from 0.006 to 0.052. The model improves the possibility to consider SO4 adsorption/desorption processes correctly in dynamic soil chemistry models.