The effect of soil pH on heavy metal transport in the vadose zone.

摘要

This research study provides the experimental information necessary for numerical analyses, structured to account for vadose zone transport of heavy metals. Particular attention is payed to the effect of degree of saturation, the presence of carbonate, soil pH and heavy metals concentrations. In addition, attention is directed to an evaluation of the coupled solute-water transport mechanisms. A method that fully describes the coupling effects on the transport coefficient based on experimental evidence provides successful predictions of the rate of transport of the heavy metals through the unsaturated soil in a pH-controlled environment.; In the experimental part of this research, one dimensional solute and moisture flow (leaching) tests, using different heavy metal permeants, were conducted on an unsaturated illitic soil at varying pH values. Experimental results showed that the retention and migration of heavy metals are highly dependent on the soil pH, the presence of carbonates, the degree of saturation, the influent concentration and the time duration. At high soil pH and carbonate content, heavy metals were retained in the soils if the buffering capacity was high enough to resist the acidic input solution, and sorption processes will prevail in the carbonate phase. As the soil pH decreases, the dissolution of carbonates increases and cation exchange capacity becomes the more dominant process in heavy metals retention.; The numerical study developed a model to analyse and predict the transport of the contaminant in unsaturated clayey soils in which some of the species were adsorbed on clay particles surfaces. The proposed mathematical model was based on the postulates of irreversible thermodynamics and is also applicable in a one-dimensional case. In this model, various solute transport mechanisms such as diffusion and sorption were considered. Results indicated that the diffusion coefficient is necessary to provide a good agreement between the experimentally measured and the theoretically predicted values of contaminant transport through the soil. The numerical results of the coupled solute and moisture equations showed that the transport coefficients strongly and accurately depend on solute and volumetric content. (Abstract shortened by UMI.)