Modeling of heavy metal movement in rhizosphere soils.

摘要

Recent years have seen an increased interest in modeling multicomponent reactive transport systems to investigate the mobility of toxic heavy metals in subsurface environments. In this study, a mathematical model was developed for understanding the fate of a typical heavy metal (lead) in unsaturated soils. Water movement in the soil was described by Richards equation which was modified to include a sink term for root water uptake. A convection-dispersion type model was utilized to determine the fate and transport of heavy metals, other accompanying ions, root exudates, and biomass. The transport model was coupled to a biogeochemical model that incorporated the important chemical and biological interactions between the heavy metal of interest and the carbonate and phosphate minerals, reactive sorption sites, organic acids, and microbial biomass in the soil. Local equilibrium assumption (LEA) was adopted, and the transport and biogeochemical models were externally coupled while solving for heavy metal transport. The model was used to simulate lead movement through unsaturated soils, with a typical plant root system, at the laboratory column scale. Model results were shown to compare satisfactorily with experimental data for lead, zinc, and calcium in saturated soil columns. Fate and transport of lead was strongly influenced by ion exchange process in acidic soils, while precipitation was the dominant process in neutral to alkaline soils. The model predicted the enhanced leaching of lead in the presence of citric acid by explicitly incorporating the geochemical reactions between the metals Pb, Ca, Fe and citric acid. In another application, immobilization of lead in the presence of biomass was found to be dependent on the growth of biomass which in turn, depends on the availability of root exudates in the rhizosphere.