Equilibrium of trace metals partitioning between soils and solutions has been studied extensively but kinetics of the metals sorption and desorption on soil particles is still poorly understood. Because of the complexity of the soil system and mobility of soil solutions, the sorption/desorption of trace metals on soils is always time-dependent and does not always achieve equilibrium in natural conditions.
The kinetics of metal sorption/desorption on soils can be affected by both the soil compositions and solution chemistry. Some parameters, such as total metal loading, SOM, pH, and DOM have been recognized to be important to control the metal partition equilibrium between soils and solutions, but how quantitatively the sorption/desorption kinetics is affected by these parameters is still unknown. Both the equilibrium models and spectroscopic techniques have demonstrated the importance of SOM controlling the partition of metals in soil systems. How to consider the effect of SOM on metal sorption/desorption kinetics is thus very important for modeling.
A number of kinetics models have been used for metal reactions with soils. Most of the models are empirical and the reaction chemistry is usually not
considered in these models. The usefulness of models with many parameters is limited due to many adjustable parameters, which makes the specific chemistry property of each parameter ambiguous. The two-site model is a simple model but still gives good model performance which has been used by many researchers. Overall, present kinetics model for metal reactions with soils have limited application and predictive models which can be used at different solution and soil chemistry are highly desired.
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Chapter 3
MODELING METHODS