Sorption, Kinetics, and Transport of Selected Heavy Metals in Soil: A Laboratory Study and Modelling Approaches.

Abstract

The retention and transport behavior of heavy metals in soils is affected by soil chemical heterogeneity. The objectives of this study were to evaluate (i) the effects of heterogeneity on solute transport of heavy metals under steady water flow, (ii) the applicability of single solute and ion exchange isotherms to model Zn and Cd transport in soil columns, (iii) the modelling of the sorption kinetics of Zn under batch and flow conditions, and (iv) the effects of organic matter and iron oxide removal on the kinetics of Zn sorption in soils. A sensitivity analysis showed that site affinity distribution functions describing chemical heterogeneity in soils are directly related to breakthrough behavior of reactive solutes. The general Langmuir-Freundlich and the Rothmund-Kornfeld equations adequately described heavy metal sorption and transport in soil columns for conditions of variable ionic strength. The modelled and measured Zn breakthrough curves in an acidic sandy soil agreed quantitatively whereas Cd transport was qualitatively predicted. Zn and Cd transport in a weakly acidic silt loam soil was only qualitatively described by mass transfer kinetics. The kinetics of Zn retention as measured under batch conditions were described using a multireaction model and a two-site second-order models and were successful only for a narrow concentration range. Based on noncompetitive high affinity and competitive low affinity sorption sites describing Zn-Ca ion exchange a new model was proposed. This model successfully described Zn sorption kinetics in an acidic soil at initial concentrations varying from 0.01 to 100 mg L$\sp{-1}$ between 2 and 288 h. It also qualitatively predicted Zn kinetics as measured by a thin disk flow method with flow interruptions varying from 0.5 min to 30 d. Modelling and experimental results suggest that equilibrium heavy metal-Ca competition governs the retention at high heavy metal concentrations above 0.1 mM, whereas slow noncompetitive sorption processes dominate the low concentrations below 0.1 mM. The removal of organic matter and iron oxides doubled and quadrupled the Zn sorption at low concentrations, respectively and decreased the rate of reaction.