Enhanced Mobility of Lead in Soil Rhizosphere: Model Development and Validation

A multi-component reactive transport model is developed for understanding the fate of a typical heavy metal (lead) in soil rhizosphere, the root zone of a vegetated soil. Water movement in the soil is described by Richards equation, which is modified to include an extraction term for root water uptake. The transport model is coupled to a geochemical model that incorporates the important chemical interactions between the heavy metal of interest and root exudates (in the form of simple organic acids) typically present in the rhizosphere. Local equilibrium assumption (LEA) is adopted. The transport and geochemical models are externally coupled while solving for heavy metal concentrations. The model was used to interpret the lab experiments on enhanced leaching of lead in the presence of citric acid. The geochemical reactions indicated reduction in sorption of Pb to ion exchange sites due to its speciation with citric acid in the aqueous phase, resulting in the formation of complexes that facilitated the movement of Pb.