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Roberts, D., A.C. Scheinost and D.L. Sparks. 2003. Zinc speciation in contaminated soils combing direct and indirect characterization methods. p. 187-227. In Geochemical and Hydrological Reactivity of Heavy Metals in Soils, H. M. Selim and W. L. Kingery eds. Lewis Publishers, Boca Raton.
The contamination of surface and subsurface environments via the anthropogenic and natural input of heavy metals has established the need to investigate and comprehend metal-soil interactions. The pathways for heavy metal introduction into soil and aquatic environments are numerous, and include the land application of sewage sludge and municipal composts, mine wastes, dredged materials, fly ash, and atmospheric deposits. In addition to these anthropogenic sources, heavy metals can be introduced to soils naturally as reaction products via the dissolution of metal-bearing minerals that are found in concentrated deposits. Of a thousand Superfund sites named in the U.S. Environmental Protection Agency’s National Priority List of 1986, 40% were reported to have elevated levels of heavy metals relative to background levels. The fate and mobility of these metals in soils and sediments are of concern because of potential bioaccumulation, food chain magnification, degradation of vegetation, and human exposure.
The effective toxicity of heavy metals to soil ecosystems depends not only on total metal concentrations, but also, and perhaps more importantly, on the chemical nature of the most mobile species. The long-term bioavailability to humans and other organisms is determined by the resupply of the metal to the mobile pool from more stable phases. Thus, quantitative speciation of metal species as well as their variation with time is a prerequisite for long-term risk assessments. The complex and heterogeneous array of mineral sorption sites, organic materials, metal oxides, macro- and micro-pores, and microorganisms in soils provide a matrix that may strongly sequester metal ions. Noncrystalline aluminosilicates (allophanes), oxides, and hydroxides of Fe, Al, and Mn, and even the edges of layer silicate clays, to a lesser extent, provide surface sites for the specific adsorption and interaction of transition and heavy metals. Before any remediation strategy is attempted, it is wise to determine and understand the nature of the interactions of metal ions with these reactive sites. These interactions can be considered one portion of the overall concept of metal speciation in soils. However, the determination of metal speciation in complex and heterogeneous systems such as soils and sediments is far from a trivial task.
Speciation encompasses both the chemical and physical form an element takes in a geochemical setting. A detailed definition of speciation includes the following components: (1) the identity of the contaminant of concern or interest; (2) the oxidation state of the contaminant; (3) associations and complexes, metal-ligand bonds, surface precipitates); and (4) the molecular geometry and coordination environment of the metal. The more of these parameters that can be identified the better one can predict the potential risk of toxicity to organisms by heavy metal contaminants. Prior to the application of sequential extraction techniques and analytical tools, researchers often relied on total metal concentration as an indication of the degree of bioavailability of a heavy metal. However, several studies have shown that the form the metal takes in soils is of much greater importance than the total concentration of the metal with regards to the bioavailability to the organism. Metal speciation in soil and aquatic systems continues to be a dynamic topic and of interest to soil scientists, engineers, toxicologists, and geochemists alike, as there remains no sufficient method to characterize metal contaminants in all natural settings.
The lack of a universal method of determining heavy metal speciation in natural settings comes as a result of the complexity of soil, sediment, and aquatic environments. The multiple solid phases in soils include primary minerals, phyllosilicates, hydrous metal oxides, and organic debris. Metals can potentially bind to these sorbents by a number of sorption processes, including both chemical and physical mechanisms. The mechanism(s) of metal binding strongly influences the fate and bioavailability of metals in the environment. In addition to solid phases, the soil solution is also heterogeneous in nature, containing dissolved organic matter and other metal-binding ligands over a range of concentrations. This leads to metal-ligand complexes in the soil solution and ternary complexes at the solid-solution interface. The presence of ligands in an ion-sorbent complex has been shown to influence the atomic coordination environment of the ion and, therefore, may lead to differences in the stability of metal sorption complexes. The partitioning of metal contaminants between solid and solution phases is a dynamic process and an accurate description of this process is important in constructing models capable of predicting heavy metal behavior in surface and subsurface environments.
A metal that has received a fair amount of attention due to its ubiquitous nature in soils and sediments and role as a plant essential nutrient, is Zn. Zinc is mined in 50 countries and smelted in 21 countries. At background levels it poses no serious threat to biota and vegetation, while in areas that have elevated levels of Zn as a result of smelting, land application of biosolids, or other anthropogenic processes, it is often a detriment to the environment. At acidic pH values, Zn toxicity to plants is the third most common metal toxicity behind Al and Mn. Under acidic oxidizing conditions, Zn is one of the most soluble and mobile of the trace metal cations. It does not complex tightly with organic matter at low pH; therefore, acid-leached soils often have Zn deficiencies because of depletion of this element in the surface layer. The degree of Zn bioavailability and, therefore Zn toxicity, is by and large determined by the nature of its complexation to surfaces found in soils, such as phyllosilicates, metal oxides, and organic matter. Research investigating Zn sorption using laboratory-based macroscopic sorption experiments using oxide and clay minerals as sorbents suggests Zn has variable reactivity and speciation in soils. Sorption studies have shown that Zn can adsorb onto Mn oxides, Fe (hydr)oxides and Al (hydr)oxides, and aluminosilicates. At alkaline pH values and at high initial Zn concentrations, the precipitation of Zn(OH)2, Zn(CO)3, and ZnFe2O4 may control Zn solubility. In these studies, however, direct determination of Zn sorption mechanisms and speciation using spectroscopic and/or microscopic approaches was not employed, allowing room for further interpretation of the results.
With the advent of more sophisticated analytical techniques and their application of soils and sediments, further information on the nature of Zn sorption complexes in clay mineral and metal oxide systems has been gleaned. Waychunas et al. studies Zn sorption to ferrihydrite using x-ray absorption fine structure (XAFS) spectroscopy and found that Zn forms inner-sphere absorption complexes at low Zn sorption densities, changing to the formation of Zn hydroxide polymers with increasing Zn sorption densities, and finally transforming to a brucite-like solid phase at the highest sorption densities in the study. In a study of Zn sorption on goethite, inner-sphere surface complexes were observed using XAFS. In investigations using Al-bearing mineral phases as sorbents and at neutral to basic pH values, researchers have demonstrated that Zn can form both inner-sphere surface complexes and Zn hydrotalcite-like phases upon sorption to Al-bearing minerals. Zn sorption on manganite resulted in both innersphere and multinuclear hydroxo-complexes. Perhaps the most significant finding in many of these studies is the fact that Zn-bearing precipitate phases often formed under reaction conditions well below the solubility limit of known Zn solid phases, suggesting that their formation in soils and sediments may have been overlooked using conventional approaches. For example, the sorption kinetics of Zn on hydroxyapatite surfaces had an initial rapid sorption step followed by a much slower rate of Zn removal from solution. It was conceded that x-ray diffraction (XRD) and scanning electron microscopy (SEM) were not sensitive enough to determine if precipitation was a major mechanism at high pH values (>7.0).
With a substantial amount of Zn sorption studies performed using a combination of sophisticated analytical tools such as XAFS in mineral and metal oxide systems, there is a natural progression to investigate Zn speciation in actual soils and sediments. By applying XAFS and electron microscopy to Zn-contaminated soils and sediments, Zn has been demonstrated to occur as ZnS in reduced environments, often followed by repartitioning into Zn hydroxide and/or ZnFe hydroxide phases, adsorption to Fe(oxyhydr)oxides, or incorporation into phyllosilicates upon oxidation. Manceau et al. employed a variety of techniques, including XRD, XAFS, and micro-focused XAFS to demonstrate that upon weathering of Zn-mineral phases in soils, Zn was taken up by the formation of Zn-containing phyllosilicates and, to a lesser extent, by adsorption to Fe and Mn (oxyhydr)oxides. In addition to adsorption and precipitation as the primary mechanisms for Zn removal from solution, Zn may be effectively removed from solution via diffusion of Zn ions into the micropores of Fe oxides. These studies demonstrate that in any given system, Zn may be present in one of several forms making direct identification of each species difficult using traditional approaches. The majority of studies employed to characterize the reactivity in Zn has dealt with relatively simplistic systems, with one or two sorbent phases in question. Clearly, natural environments are much more complex and only after extensive studies in the above systems can one focus on natural samples. To better illustrate this point, we now turn our attention to the various approaches that have been used to identify metal species in soils and sediments, followed by a specific scenario of applying these techniques to Zn-contaminated soils.