It is promising to see that the model is applicable to a number of heavy metals that have different reactivity with SOM. The model shows predictive ability. The modeling approach provides a general framework to develop predictive models for the kinetic reactions of heavy metals with soils in natural environments if SOM is the major adsorbent. Although the impact of mineral phases is not discussed in this study, the existing kinetics model can be easily expanded to include metal reactions with mineral phases when needed, such as sorption to clay minerals and precipitation, as shown in our previous
study. Currently, all model parameters were obtained based on the data collected in a well-controlled stirred- flow reactor. In the field conditions, metal reactions with soils are complicated by other kinetic processes such as diffusion in soil pore water,transport by water flow, and uptake by organisms. For the field contaminated soils with long aging time, the metal speciation could differ significantly from those in laboratory short-term adsorption experiments, and the metal release from the field contaminated soils may be slower than that from freshly spiked samples. The kinetics model would need to be modified under these conditions. Therefore, it is desired to examine this model in a more realistic soil system in the field, and the applicability of laboratory-derived model parameters to field situations must be established.