Experimental study and modeling of the transfer of zinc in a low reactive sand column in the presence of acetate

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Abstract

Nowadays, it is necessary to understand and identify the reactions governing the fate of heavy metals introduced into the environment with low complexing organic compounds, particularly when they are transferred through soils in urban areas. In this work the concomitant influence of pH and acetate on the fate of zinc on siliceous sand was studied in batch and non-saturated column experiments. Total zinc concentrations varied between 2 and 20 mg/l, and total acetate concentrations were fixed at 22, 72, 132, and 223 mM to obtain solution pHs of 4, 5, 6 and 7, respectively. Natural sand (diameter, 0.3–2 mm), mainly constituted of silica, was used. In batch adsorption experiments, zinc adsorption is insignificant at pH 4, low and linear at pH 5, and increasingly nonlinear, of the Langmuir type, at pH 6 and 7 indicating near-saturation conditions of surface sites at these high pH values. In column experiments, Zn retardation increases and the maximum outlet concentration of Zn decreases with rising pH and acetate concentrations. Previous column tracer experiments revealed the occurrence of regionalized water transport in the column. Modeling these data was based on a non-electrostatic approach. Batch and column data modeling was based on the PHREEQC code that allows concomitant resolution of chemical speciation and regionalized water transport. The speciation calculation indicates that the ZnAcetate+ species is the dominant Zn species in the solutions used. Batch experimental curves are correctly modeled assuming the formation of the three surface species SiOZn+, SiOH–Zn Acetate+ and SiO–Zn(Acetate)2. The column data could be adequately modeled assuming a two-region water transport and the formation of the same three species with the same thermodynamic constants determined in the batch experiments. The hypothesis of the modeling leads to a slight overestimation of the quantities of zinc eluted (10%) at pH 6 and 7, mostly in the desorption phase. These results show that the methodology used facilitates the correct modeling of both batch and transport experiments and formulation of the hypothesis on the interactions between the low reactive sand and a complex solution.

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 isCt=D2Cz2qθCzwhere 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

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