A column study of soil contamination by lead: influence of pH and carbonate content. I. Experimental results.
ABSTRACT The influence of soil carbonate content on the fate of lead in soil was studied in a lab-scale column under different pH values of the contaminant solution. Results indicated that retention of this toxic heavy metal (up to 38% weight at pH = 5) occurred which was proportional to the total carbonate content. A decrease in the pH of the aqueous solution entering the column resulted in a decrease of the retention of lead in the soil. Furthermore, the concentration of lead in the effluent was increased dramatically. Concentrations of lead about 2.5 times higher than in the contaminant solution were measured at pH = 3. Knowledge of these phenomena is important for risk assessment and remediation feasibility studies.
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ABSTRACT: Chelating agents such as EDTA, when present in water percolating through polluted soils, are capable of solubilizing heavy metals, increasing their downward mobility to groundwater as they form water-soluble and negatively charged complexes. The mobilization and movement of heavy metals in an EDTA flux is investigated with both laboratory experiments and mathematical modeling. The solubilization of Cd and Pb is monitored during the percolation of EDTA, in both pulse and step modes, through columns filled with a contaminated soil. The experimental breakthrough curves (BTCs) are used to validate a numerical model that links solute transport of EDTA and EDTA−metal chelates to the metal solubilization process. An implicit difference scheme is used to solve advection−dispersion equations for free and complexed EDTA with, for both, inclusion of a second-order kinetic law to express the extraction reaction. The hydrodispersive parameters of the column are calculated using the bromide BTCs. The ability of the model to simulate various EDTA injection modes (pulse or step) at different chelate concentrations is tested by fitting the kinetic rate constant K of solubilization with the observed Cd and Pb BTCs. Pulse and step experiments give the same K values, 2.4 × 10-6 s-1 for Cd and 2.1 × 10-5 s-1 for Pb. The model accounts for the diminishing metal extraction efficiency as the metal in the solid is depleted or as the available EDTA concentration decreases. Methodological suggestions for necessary investigations prior to remediation operations are described.Environmental Science & Technology - ENVIRON SCI TECHNOL. 04/1998; 32(11).
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ABSTRACT: Conventional batch adsorption-isotherm experiments are conducted under conditions that are generally not representative of field condi- tions. Our objective was to develop and illustrate a technique that uses continuous, unsaturated flow through intact soil cores and gives information analogous to standard batch absorption experiments. Soil cores were placed on filter funnels and continuous flow was induced by controlled vacuum. Solution, 10 times the weight of the soil, was applied and recycled until apparent equilibrium concentrations were observed. While the total solution-to-soil ratio was 10:1, the amount in contact with soil at any instant was small. The procedure was replicated on separate cores with different initial solute concentrations to give different final equilibrium concentrations as a function of sol- ute adsorbed per gram of soil. The technique was illustrated by ob- serving adsorption of forest floor dissolved organic C to cores of AB horizon soil, and was compared with conventional batch adsorption data. Isotherms using both methods were linear but the slope of the batch isotherm was somewhat less. This flowthrough technique pro- vides a way to obtain adsorption-isotherm data under more realistic conditions.Soil Science Society of America Journal 01/1992; 56(2). · 1.82 Impact Factor
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ABSTRACT: The retention of lead by a very fine sandy loam was investigated. Aqueous lead concentrations between 10 and 1000 mg/L and soil concentrations ranging from 10 to 167 g/L were used. Lead retention by the soil was a strong function of pH. The width of the pH—adsorption edge decreased with increasing lead concentration. Experimental results were modeled using the Langmuir, Freundlich, and BET isotherms. Only the Langmuir and Freundlich isotherms successfully represented the experimental results. The role of surface precipitation was assumed to be small because of the failure of the BET isotherm to adequately predict metal retention. The Freundlich isotherm provided the best fit because a maximum surface concentration was usually not observed. Langmuir and Freundlich isotherms parameters varied in a way that suggested that the average binding energy and the distribution of bond strengths increased with increasing pH. The isotherm expressions determined in this study can be used as source-sink terms in the generalized mass transport model.Separation Science and Technology 08/1994; 29(12):1529-1551. · 1.16 Impact Factor