Sequential extraction tests

Chemical speciation is the process of determining and identifying specific chemical species or their binding forms. Chemical speciation reveals the availability and mobility of metals in solid materials in order to understand their chemical behaviour and fate (Kalembkiewicz, et al., 2008). It is widely recognized that the distribution, mobility and biological availability of trace metals in the solid phase depend not only on their total concentration but also on the physicochemical forms in which they occur (Nurmesniemi et al., 2008; Filgueiras, et al., 2002). Metal ions in solid materials are partitioned between the different phases in the materials such as organic matter, oxyhydroxides of iron, aluminium and manganese, phyllosilicate minerals, carbonates and sulfides. Metal ions also are retained on these solid phases by different mechanisms such as ion exchange, outer and inner-sphere surface complexation (adsorption), precipitation or co- precipitation (Rate, et al., 2000; Filgueiras, et al., 2002). To evaluate the availability and mobility of metals from solid materials such as fly ash and soil, some extraction tests have often been applied (Nurmesniemi et al., 2008). Extraction tests are widely used as tools to estimate the release potential of constituents from waste materials over a range of possible waste management activities, including recycling or reuse, for assessing the efficacy of the waste treatment process,

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and after disposal (Kosson et al., 2002; Daniels and Das, 2006; Nurmesniemi et al., 2008). Among several extraction methods, the sequential extraction procedure has been widely used to evaluate the speciation of particulate metals, i.e., the partitioning among the various form in which the might exist (Tessier, et al., 1979). The use of sequential extractions, although generally time consuming, provides detailed information about the origin, mode of occurrence, biological and physicochemical availability, mobilization and transport of trace metals. Sequential extraction experiments have been shown to provide a convenient means to determine the metals associated with the principal accumulative phases in sedimentary deposits. Fractionation is usually performed by a sequence of ‘selective’ chemical extraction techniques, which include the successive removal of these phases and their associated metals (Filgueiras et

al., 2002).

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According to Wan et al. (2006), a sequential chemical extraction procedure proposed by Tessier,

et al. (1979) has been widely used to investigate the chemical phases of heavy metals in fly ash.

In Tessier’s extraction procedure, extraction of metals is divided into five different fractions based on the inorganic matrix in which the metal ions are likely to bind to. These fractions include; (1) Exchangeable fraction, which represents the most easily available metals; (2) Bound to carbonates, which represent acid-soluble fraction; (3) Bound to iron and manganese oxides, which represents the reducible fraction; (4) Bound to organic matter, which represents the oxidizable fraction; and (5) residual fraction, tightly bound to the silicate matrix of the sample (Tessier, et al., (1979).

According to the International Ash Working Group (IAWG) (IAWG, 1997), the results of sequential extraction might not necessarily reflect the associations with the claimed phases, but rather represent the different leaching conditions within a landfill over time. The exchangeable phase is immediately available under neutral conditions; the carbonates phase is potentially available under neutral conditions; the phases of Fe–Mn oxides and organic matter are potentially available under reducing conditions; the residual phase is unavailable for leaching (Wan et al., 2006).

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Sequential extraction methods have been widely used to determine the chemical compositions and the partitioning of metals in waste materials such as fly ash (Chang et al., 2009; Smeda and Zyrnicki, 2002; Bódog et al., 1996; Smichowski et al., 2008; Marrero et al., 2007). Smeda and Zyrnicki, (2002), applied sequential extraction methods to study the partitioning of metals in fly ashes. The method initiated by Tessier, et al. (1979) was slightly modified by the introduction of leaching with deionized water as the first step. The sequential extraction procedure revealed much more information about the elements investigated than data obtained from measurements of their total concentrations. It was observed that the concentrations of some species vary at different extraction steps. For instance the concentrations of Ca and Mg in the water soluble fraction were lower than what was observed in the acid-soluble fraction. More than 60 % of the Ca and Mg were contained in the residue. Similar result was observed for Al and Fe with more than 98 % of their total concentrations found in the residual fraction. Chromium, boron and strontium were relatively easily extracted by deionized water as their concentrations in the water soluble fractions were significantly high. They recommended the inclusion of extraction using deionized water to determine the water soluble species as this could give very important information necessary to evaluate the risk of environmental pollution by fly ashes.

Bódog et al. (1996) reported a five-stage leaching procedure applied to fly ash samples collected at several emission sources in Europe. The analysis of the extracts by atomic absorption spectrometry (AAS) and graphite furnace atomic absorption spectrometry (GFAAS) showed different leachabilities and distribution patterns of Cd, Cr, Cu, Pb, V and Zn. They concluded that the difference in the partitioning of metals in the fly ashes depends considerably on the properties of the raw material and the operation conditions such as combustion temperature. As part of modification to Tessier, et al., (1979) extraction method, Smichowski et al. (2008) applied a three-step metal fractionation scheme to study the distribution of species in fly ashes. This was done by combining the original five-step sequential extraction methods to three fractions i.e. (i) soluble and exchangeable fractions, (ii) carbonates, oxides and reducible fractions and (iii) residual fractions. The results of the fractionation procedure showed that majority of the elements in the fly ashes were found in the residual fraction, except Mo and S with percentages of 41 % and 18 %, respectively. Mo and S concentrations were higher in the soluble and exchangeable fraction. The high content of soluble S is consistent with the

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physicochemical characterization that reported that S was present as sulfate compounds such as MgSO4 and CaSO4-Gypsum (Smichowski et al., 2008).