Kinetics of Heavy Metal Desorption from Three Soils Using Citric Acid, Tartaric Acid, and EDTA

Wasay, S. A., Barrington, S., Tokunagal, S., Prasher, S., Journal of Environmental Engineering and Science

Abstract: Organic acids are relatively new soil heavy metal leaching agents requiring the development of desorption kinetics before being applied. The objective was to determine the heavy metal desorption rates for three heavily contaminated soils (clay loam, loam, and sandy clay loam), treated under optimum conditions with either citric acid, tartaric acid, or EDTA for comparison. A two-rate reaction (fast and slow) model was defined to describe desorption kinetics, where both first order rates were assumed irreversible and reversible, respectively. Experimental data were collected by initially subjecting the three soils to batch experiments at optimum leaching agent level, for up to 36 h. The results indicated that citric acid was a more consistent leaching agent compared to tartaric acid and EDTA, which were efficient mainly in treating the clay loam and the sandy clay loam, respectively. The two-rate reaction model fitted the data but its empirical coefficients need to be defined for each individual soil to be treated. For Pb common to all three experimental soils, [k.sub.1], representing the fast desorption rate, increased with the ratio of metal equivalence fraction: soil CEC. The coefficients [k.sub.2] and [[alpha].sub.0], representing the slower rate of desorption and the proportion of Pb involved in this slow rate, respectively, increased with the mass of Pb held by the oxide and organic matter fractions. The coefficient [m.sub.1], related to the relative rate of the backward reaction from the slow to the fast, varied according to the mass of Pb held by the exchangeable site. Despite the empirical values associated with the kinetics coefficients, their correspondence with individual processes indicate that organic acid remediation can be modeled using the two rate kinetic equation proposed in this project.

Key words: heavy metals (Cr, Mn, Hg, Pb, Cd, Cu, Zn), citric acid, tartaric acid, EDTA, two reaction kinetics model.

Resume : Les acides organiques sont des agents relativement nouveaux utilises pour la lixiviation des metaux lourds dans les sols; ils doivent cependant developper une cinetique de desorption avant de pouvoir etre appliques. L'objectif etait de determiner les vitesses de desorption des metaux lourds dans trois sols fortement contamines (loam argileux, loam et loam sablo-argileux), traites sous des conditions optimales avec soit de l'acide citrique, de l'acide tartrique ou de l'EDTA a des fins de comparaison. Un modele de reaction a deux vitesses (rapide et lente) a ete defini pour decrire la cinetique de desorption, ou les deux vitesses du premier ordre ont ete presumees respectivement irreversibles et reversibles. Les donnees experimentales ont ete colligees en soumettant initialement les trois sols a des experiences par lot a un niveau optimal d'agent de lixiviation, pour une duree allant jusqu'a 36 heures. Les resultats indiquent que l'acide citrique etait un agent de lixiviation plus constant que l'acide tartrique et l'EDTA, lesquels etaient principalement efficaces pour le traitement du loam argileux et du loam sablo-argileux respectivement. Le modele de reaction a deux vitesses correspondait bien aux donnees mais les coefficients empiriques doivent etre definis pour chaque sol individuel a etre traite. Quant au Pb, present dans les trois echantillons de sol, [k.sub.1], representant la vitesse elevee de desorption, augmentait avec le rapport de la fraction equivalent metal : la CEC du sol. Les coefficients [k.sub.2] et [[alpha].sub.0], representant respectivement la vitesse de desorption plus lente et la proportion de Pb impliquee dans cette vitesse lente, ont augmente avec la masse de Pb retenue par les fractions d'oxydes et de matieres organiques. Le coefficient [m.sub.1], lie a la vitesse relative de la reaction inverse de lente a rapide, a varie selon la masse de Pb retenu par le site d'echange. Malgre des valeurs empiriques associees aux coefficients cinetiques, leur correspondance aux procedes individuels indique que le retrait de l'acide organique peut etre modelise en utilisant l'equation cinetique a deux vitesses proposee pour dans projet.

Mots-cles : metaux lourds (Cr, Mn, Hg, Pb, Cd, Cu, Zn), acide citrique, acide tartrique, EDTA, modele a deux reactions cinetiques.

[Traduit par la Redaction]

Introduction

The rate at which heavy metals are leached from a soil governs the cost and size of its treatment process. Leaching rates can be computed from models using kinetic coefficients that, for heavy metals desorption from soils, have been investigated to a limited extent, by authors such as Griffin and Burau (1974) for Pb, Kuo and Mikkelsen (1980) for Zn, Jopony and Young (1987) for Cu, and Bowman et al. (1981) for Ni, while desorption kinetics of plant nutrients, such as nitrogen, phosphorus, and potassium, have been studied extensively. A rigorous mathematical treatment of the kinetics of chemical reactions and diffusion in soils has been presented by Sposito (1994) and Bradl (2004). Desorption of heavy metals from soils is said to respect a tworate reaction model (Tuin and Tels 1990), influenced by the forms of heavy metal retention and the type of leaching agent. A recent study was conducted using a two-step first-order reaction equation to evaluate the heavy metals desorption using a surfactant and ligand (Shin et al. 2006).

Because of their moderate strength, organic acids such as citric acid, oxalic acid, and tartaric acid have been shown to effectively leach heavy metals from contaminated soils of various textures (Wasay et al. 1998b, 1998c; Khodadoust et al. 2005). At molarities as low as 0.10 mol/L, these mild leaching agents were able to desorb more than 85% of heavy metals such as Pb, Cd, and Zn, while leaving a major portion of the beneficial macro nutrients (Ca, Na, Mg) bound to the soil (Wasay et al. 1998b, 1998c). Because organic acids have only been recently applied as leaching agents for soils contaminated with heavy metals, limited information pertains to their desorption kinetics. Leaching of heavy metals from contaminated soils were also studied using ethylenediaminetetraacetic acid (EDTA) (Sun et al. 2001; Lim et al. 2004; Yukselen and Gokyay 2006). USEPA prepared a report pertaining to the effectiveness of soil heavy metal decontamination processes using complexing agents such as EDTA (Bricka et al. 1994). Oxalic acid was not found to be an efficient soil extraction agent for Pb and Zn because of the formation of insoluble metal complex as opposed to EDTA (Elliott and Herzig 1999). The use of chelating agents such as EDTA to remediate soils contaminated with heavy metals is viable but depends highly on the extraction parameters (Hong et al. 1999).

The first objective of the present study was to assess the rate of heavy metal desorption from different soil types using various chelating agents such as citric acid, tartaric acid, and EDTA. The second objective of the study was to evaluate the effectiveness of a two-rate reaction model in simulating the leaching rate of heavy metals from contaminated soils treated with one of the three agents. The coefficients of the two-rate reaction model were determined using the experimental data collected from the treatment of soil batches.

Theory of the two-rate reaction kinetic model

Different kinetic models have been used to predict the leaching rate of various ions from soils (Sparks 1986; Bradl 2004) in the presence of a leaching agent. A first order rate equation used to describe desorption of phosphate and boron (Griffin and Burau 1974; Griffin and Jurinak 1974) was found to fit the desorption data when plotted using two or three different slopes. This indicated the presence of at least two discrete types of soil binding sites. The fast desorption sites for [K.sup.+] was attributed to external surface sites on the inorganic and organic phases of the soil while the slow desorption of [K.sup.+] was attributed to the less accessible sites of organic matter and those of …

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Publication information: Article title: Kinetics of Heavy Metal Desorption from Three Soils Using Citric Acid, Tartaric Acid, and EDTA. Contributors: Wasay, S. A. - Author, Barrington, S. - Author, Tokunagal, S. - Author, Prasher, S. - Author. Journal title: Journal of Environmental Engineering and Science. Volume: 6. Issue: 6 Publication date: November 2007. Page number: 611+. © 2008 NRC Research Press. COPYRIGHT 2007 Gale Group.

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