Modeling Elution Histories of Copper and Lead from Contaminated Soil Treated by Poly„amidoamine... Dendrimers

A dynamic “two-site” model was formulated and tested for simulating the elution histories of copper Cu2+ and lead Pb2+ from a contaminated soil treated by poly amidoamine dendrimers. In the model, the metal sorption sites of the soil were divided into two compartments: one with a fast desorption rate and the other with a slow desorption rate. The model was tested for simulating and predicting Cu2+ and Pb2+ elution histories obtained from column experiments. Compared to the classical “one-site” model and the modified “gamma distribution” model, the “two-site” model not only provides much improved power for simulating the observed metal elution data, but also can more accurately predict the metal elution histories under various experimental conditions including initial metal concentration in soil, dendrimer concentration, and pH. DOI: 10.1061/ ASCE 0733-9372 2008 134:4 238 CE Database subject headings: Abatement and removal; Models; Remedial action; Extraction procedures; Heavy metals; Soil pollution; Soil treatment. In recent years, extraction of heavy metals from contaminated soils by various chelating agents has been widely studied Allen and Chen 1993; Di Palma and Ferrantelli 2005; Furukawa and Tokunnaga 2004; Hong et al. 1999; Lim et al. 2004; Lo and Yang 1999; Mulligan et al. 2001; Neilson et al. 2003; Rampley and Ogden 1998; Samani et al. 1998 . In our previous work Xu and Zhao 2005, 2006 , we reported that poly amidoamine PAMAM dendrimers can effectively remove Cu2+ and Pb2+ from contaminated soils. However, to facilitate further process design and analysis, it is necessary to develop a mechanistic model that can simulate and predict the performance and efficiency of the remediation process. Furthermore, model simulation of the metal extraction process may aid in process optimization and assessing the fate and transport of metal ions in the soilgroundwater systems. Various mathematical models have been reported for describing transport of metals in porous media. The conventional approach to model contaminant transport considers the processes of convection, dispersion, and sorption Clement et al. 2004, 2000, 1998; Tran et al. 2002 , where equilibrium conditions are often invoked and classical isotherm models such as Langmuir, Freundlich, or the linear models are often used to Graduate Research Assistant, Dept. of Civil Engineering, 238 Harbert Engineering Center, Auburn Univ., Auburn, AL 36849. E-mail: [email protected] Huff Associate Professor, Dept. of Civil Engineering, 238 Harbert Engineering Center, Auburn Univ., Auburn, AL 36849 corresponding author . E-mail: [email protected] Professor, Dept. of Civil Engineering, 238 Harbert Engineering Center, Auburn Univ., Auburn, AL 36849. E-mail: clement@ eng.auburn.edu Note. Discussion open until September 1, 2008. Separate discussions must be submitted for individual papers. To extend the closing date by one month, a written request must be filed with the ASCE Managing Editor. The manuscript for this paper was submitted for review and possible publication on July 28, 2006; approved on July 19, 2007. This paper is part of the Journal of Environmental Engineering, Vol. 134, No. 4, April 1, 2008. ©ASCE, ISSN 0733-9372/2008/4-238–246/$25.00. 238 / JOURNAL OF ENVIRONMENTAL ENGINEERING © ASCE / APRIL 200 Downloaded 06 May 2009 to 131.204.15.244. Redistribution subject to interpret the local sorption or desorption isotherms Fetter 2001; Peters and Shem 1992 . However, the classical transport models do not simulate the dynamic leaching process of heavy metals desorbed by the water soluble chelators such as dendrimers because the models do not account for the metal-chelator interactions. A number of modified models have been explored to simulate the chelator-facilitated desorption and reactive transport of metals. Column studies have shown that the leaching of metals from soils by chelating agents is primarily controlled by the slow solubilization kinetics of metal ions rather than the diffusion-limited mass transport Kedziorek et al. 1998 . Shi et al. 2005 used a two-site model to simulate the kinetics of Cu2+ and Zn2+ release from soils with a stirred-flow method. Qafoku et al. 2005 investigated the sorption and desorption of U VI during reactive transport in a sediment with a one-dimension distributed rate coefficient model. Kedziorek et al. 1998 developed a model with a second-order kinetic metal solubilization reaction to simulate the leaching of Cd2+ and Pb2+ from soil during the percolation of ethylenediaminetetraacetic acid EDTA . Samani et al. 1998 simulated a lead removal process based on equilibrium and kinetic dissolution in a contaminated soil column using EDTA and showed that the model could adequately predict the observed removal process. Thayalakumaran et al. 2003 developed a more sophisticated model to simulate Cu2+ leaching following the application of EDTA while considering the competition between copper and iron for EDTA. Bryan et al. 2005 applied a mixed equilibrium-kinetic model to simulate the column experimental data involving transport of metal ions and humic substances. The overall goal of this work is to develop a numerical modeling framework to simulate the elution histories of Cu2+ and Pb2+ in soils following application of various dendrimers and under various operating conditions such as dendrimer concentration, solution pH, and metal concentration in soils. The specific objectives are to: 1 formulate a modified two-site model for simulating/predicting the dendrimer-facilitated transport of metal ions in the metal-dendrimer-soil-groundwater system;