Experimental study and modeling of the transfer of zinc in a low reactive sand column in the presence of acetate
Introduction
Urban stormwater contains high concentrations of organic compounds and heavy metals such as lead, zinc (heavy metal in highest concentrations) and cadmium Mikkelsen et al., 1994, Makepeace et al., 1995. Reported zinc concentrations ranged from 0.0007 to 25.0 ppm and pH ranged from 4.2 to 8.7. The organic content of this type of water is barely known and only the hydrocarbon content is regularly monitored. This water is collected and transported by a compartmentalised or unitary urban drainage system, generally towards the hydrological network. Due to the potential impact of this water on the environment to which it is directed, alternative techniques, involving soil infiltration of stormwater, are used if the hydrological network is too far away or non-existent. So rainwater and the associated pollutants can be infiltrated through soil to refill aquifers. The surface of these basins is very often constituted of a coarse sandy porous medium in order to facilitate infiltration. Therefore, it is necessary to study and model the interactions between the trace metal contaminants and soil components in order to determine their migration and consequently their ability to affect soil quality and groundwater resources.
Many papers have dealt with the interactions between heavy metals and soil components Bourg, 1988, Evans, 1989, Yong et al., 1992, Davis and Kent, 1990, Stumm, 1992, Reddy et al., 1995, Taylor et al., 1995, Wilkens and Loch, 1997. pH level is one of the most important factors controlling speciation and, in particular, it controls the sorption, hydrolysis and solubility of metallic cations such as zinc Anderson and Christensen, 1988, Bourg, 1988, Evans, 1989, Davis and Kent, 1990, Stahl and James, 1991, Domergue and Vedy, 1992, Stumm, 1992, Dove and Rimstidt, 1994, Fuller et al., 1996, Pardo and Guadalix, 1996.
Fewer studies concern the fate of metals in earth materials in the presence of organic acids. Dissolved organic acid anions can either increase the mobility of metals through aqueous metal organic complexation Davis and Kent, 1990, Fein and Delea, 1999, or they can enhance adsorption through the formation of ternary metal–organic surface complexes Schindler, 1990, Boily and Fein, 1996. Many experiments have studied the co-adsorption of metal and organic anions on reactive minerals such as oxides and clays Ludwig et al., 1975, Schindler, 1990, Boily and Fein, 1996. However, little is known about the occurrence of these types of interaction on silica (Schindler, 1990).
Interactions of soluted ions with solids can be modeled by a basic empirical partitioning relationship of the solute between the mineral and the water phases Davis and Kent, 1990, Taylor et al., 1995, Karimian and Moafpouryan, 1999. The empirical adsorption models have been widely used in natural systems, but this approach to data processing is very limited in terms of predicting and understanding the reactions and mechanisms involved. A more interesting method of modeling is based on analysis of the chemical equilibrium conditions involved. In such systems, surface reactions adopt the formalism of ion association reactions in solutions as a representation of adsorption reactions at the mineral–water interface Stumm, 1992, Davis et al., 1998, Wen et al., 1998, Martin-Garin et al., 2003. Davis and Kent (1990) reviewed the different approaches and models used to describe the adsorption of ions onto surfaces. Both outer and inner-sphere surface complex models have been used to describe the interactions between Zn and mineral surfaces Davis and Kent, 1990, Dove and Rimstidt, 1994.
Parallel to obtaining knowledge on geochemical interactions and modeling under static batch conditions, several reactive solute transport models have been developed that consider the effect of diffusion, dispersion, convection, sorption, production and decay simultaneously (Jury and Roth, 1990). The convection dispersion model is the most commonly used to model solute movement through soil. Van Genuchten and Wieranga (1976) presented an extension of this model to describe regionalized flow through porous media. This two-region model partitions the medium into mobile and immobile (or stagnant) regions (Gaudet et al., 1977). It allows modeling flows in porous media with heterogeneous grain size or in aggregated soils Zurmühl and Durner, 1996, Fesch et al., 1998. PHREEQC (Parkurst and Appelo, 1999) is one of the computer models that simulates both geochemical reactions (based on equilibrium chemistry) and 1D transport processes (with stagnant zones).
The specific aim of this study was to test a methodology (laboratory experiment and modeling) designed to obtain understanding of the influence of chemical parameters (pH, ionic strength) on the retention processes of heavy metals (Zn taken as a representative contaminant of urban areas) in a natural sand (coarse sands are used in infiltration devices) in the presence of an organic ligand (acetate taken as a low complexing compound) and to formulate a hypothesis on the mechanisms involved.
Section snippets
Theory for solute transfer in a porous medium
The classical convective–dispersive equation generally used to describe the 1D transport of a non-reactive solute in soils under steady-state water flow iswhere t denotes time (T), z is distance (L), D is the dispersion coefficient (L2T−1), q is the water flux (L T−1), θ is the volumetric water content (L3L−3), and C is the water solute concentration (M L−3).
If we consider the regionalization of water in a porous medium, the equation for the transport of a non-reactive solute
Natural sand
Natural sand (0.3–2 mm diameter) was used in all the experiments (half of the grains had a mean size of 0.55 mm, while the other half had a mean size of 0.95 mm). The sand was washed with a mixture of concentrated nitric and hydrochloric acids (2:1 vol. acids/vol. sand), rinsed with distilled water and dried at 500 °C for 2 h to eliminate carbonates and organic matter. The cation exchange capacity (CEC) of the washed sand was 3 meq/kg (AFNOR NF X31-130). The sand was analyzed by X-ray
Speciation of zinc in the initial solutions
Zn Acetate–H+solution speciation (calculated with PHREEQC) at initial experimental conditions is given in Fig. 2. The equilibrium constants (Smith and Martell, 1976) used in the calculations are given in Table 2. Within the considered Zn concentration range 2–20 mg/l, the Zn Acetate+ species is dominant at the relevant pH values of 5, 6, and 7, with 52%, 68%, and 70%, respectively, of the total Zn (Fig. 2) whatever the concentration studied. The free Zn2+species is the second major species,
Silica surface
The composition of the sand was obtained by mixing equal masses of 0.55- and 0.95-mm-diameter sand grains. This corresponds to a 0.033 m2/g surface area when considering the sand grains as idealized spheres. Combining this and the estimated proton donor site density of 4.5–12 sites per nm2 given by James and Parks (1982) cited by Davis and Kent (1990), leads to an estimated total concentration of acid–base reactive surface SiOH sites from 2.2×10−7 to 6.5×10−7 mol sites/g. As the sand was
Discussion
The SiOH–Zn Acetate+ species may be questioned with respect to its effective existence. The complex would correspond to a monodentate ternary surface species with a surplus bound H+ ion. This makes this species hard to accept for itself. We rather consider this species to represent a moiety of surface complexes formed in the inner and outer sphere of the surface and not as a species that actually exists. For example, this species may represent parts of an outer-sphere bound Zn Acetate+
Conclusion
This study shows that the retention of zinc in a sandy medium is favored in the presence of high quantities of a low complexing organic ligand, through the formation of ternary surface complexes on a silica surface.
It also shows the synergy between batch and column experiments for studying cation transfer in a low reactive porous medium. It provides understanding of the adsorption processes occurring when the solid and liquid phases are in equilibrium. It also underlines the importance of
References (35)
- et al.
Experimental study of cadmium–citrate co-adsorption onto αAl2O3
Geochim. Cosmochim. Acta
(1996) - et al.
Experimental study of the effect of EDTA on Cd adsorption by Bacillus subtilis: a test of the chemical equilibrium approach
Chem. Geol.
(1999) - et al.
Effect of water content on solute transport in a porous medium containing reactive micro-aggregates
J. Contam. Hydrol.
(1998) - et al.
Characterization of metal adsorption variability in a sand and gravel aquifer
J. Contam. Hydrol.
(1996) - et al.
Aqueous cadmium uptake by calcite. A stirred flow-through reactor study
Geochim. Cosmochim. Acta
(2003) - et al.
Preferential flow and solute transport in a large lysimeter, under controlled boundary conditions
J. Hydrol.
(1999) - et al.
Adsorption, partition, ion exchange and chemical reactions in batch reactors or in columns—a review
J. Hydrol.
(1981) - et al.
Modeling adsorption of metal ions from aqueous solutions: reaction-controlled cases
J. Colloid Interface Sci.
(1995) - et al.
Distribution coefficient of Cd, Co, Ni and Zn in soils
J. Soil Sci.
(1988) Metals in aquatic and terrestrial systems: sorption, speciation and mobilization
Surface complexation modeling in aqueous geochemistry
Application of the surface complexation concept to complex mineral assemblage
Environ. Sci. Technol.
Mobility of heavy metals in soil profiles
Int. J. Environ. Anal. Chem.
Silica–water interactions
Chemistry of metal retention by soils
Environ. Sci. Technol.
Solute transfer with exchange between mobile and stagnant water, through unsaturated sand
Soil Sci. Soc. Am. J.
Multicomponent cation exchange including alkalinization/acidification following flow through sandy sediment
Water Resour. Res.
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