Solid matter speciation

The present state of knowledge of solid matter speciation of trace elements is still somewhat unsatisfactory because the appropriate techniques are only operational tools with associated conceptual and practical problems. With respect to estimating bioavailable concentrations, one such conceptual problem is the effect of competition between binding sites on the solid substrate and selective mechanisms of metal translocation by the different organisms involved, a situation that... 

Estimation on the remobilization of metals under changing environmental conditions and on the potential uptake by biota are two major objectives of species differentiation on particle-bound trace metals. However, many authors have shown that with respect to bioavailability, as distinct from geochemical mobility, the present state of knowledge on solid matter speciation of metals is still somewhat unsatisfactory. The... 

Solid-phase speciation. While most speciation studies have been concerned with redox speciation in solution, speciation in the solid phase is also of interest. Both reduced and oxidized arsenic and selenium species can be adsorbed on minerals, sods, and sediments albeit with differing affinities (see Sections 9.02.5.3 and 9.02.7.2). Such adsorption has been demonstrated on metal oxides and clays and also probably takes place, to some extent, on carbonates, phosphates, sulhdes, and perhaps organic matter. Structural arsenic and selenium may also be characterized. 

Carbonates, organic matter, Fe and Mn oxides, and clay minerals play important roles in controlling overall reactivity of trace elements in soils and sediments. This chapter addresses the interaction of trace elements with carbonates, organic matter, Fe and Mn oxides and clay minerals. Analytical techniques for trace element speciation in solid-phase and their distribution among various solid-phase components in arid and semi-arid soils are reviewed. Solubilities of trace elements in solid phases and their mineralogical characteristics in arid and semi-arid soils also are discussed. 

In operationally defined speciation the physical or chemical fractionation procedure applied to the sample defines the fraction isolated for measurement. For example, selective sequential extraction procedures are used to isolate metals associated with the water/acid soluble , exchangeable , reducible , oxidisable and residual fractions in a sediment. The reducible, oxidisable and residual fractions, for example, are often equated with the metals associated, bound or adsorbed in the iron/manganese oxyhydroxide, organic matter/sulfide and silicate phases, respectively. While this is often a convenient concept it must be emphasised that these associations are nominal and can be misleading. It is, therefore, sounder to regard the isolated fractions as defined by the operational procedure. Physical procedures such as the division of a solid sample into particle-size fractions or the isolation of a soil solution by filtration, centrifugation or dialysis are also examples of operational speciation. Indeed even the distinction between soluble and insoluble species in aquatic systems can be considered as operational speciation as it is based on the somewhat arbitrary definition of soluble as the ability to pass a 0.45/Am filter. 

General Aspects Sample preparation for speciation analyses of solid foods is not as straightforward as previously shown for water and beverages. Extraction, clean-up, and preconcentration can be required and the preservation of the identity and original distribution of the species in the analyzed matrix is no simple matter. In order to obtain acceptable recoveries, extractions are usually repeated several times (each time the supernatant solution is separated from the... 

The interaction of radiation with matter can have profound effects. Whether in solid, solution, or gaseous states, radioactivity can impact the environment and therefore change the molecular speciation of the actinides. To put this into perspective, three examples are discussed below plutonium metal, americium crystals, and an aqueous solution of plutonium. 

The hioavailability of metal to plants and soil organisms is mainly through the solution phase. However, soil solutions are in intimate contact with the soil solid phase and are therefore influenced by mineral equilibria and exchange and sorption processes that involve soil organic matter and mineral phases, as well as complexation by organic matter in both the solid phase and that dissolved in the soil solution. Le Chatelier s principle of chemical equilibrium would suggest that metal hioavailability in soils is thus controlled by both solution speciation and the metal associated with the solid phase. 

Both speciation in solution and in sohd phase can affect plant uptake of metals. The amount of trace metal in soluble or exchangeable form can qualitatively indicate the easy uptake of metal cations by plants. At lower pH values, organic matter appears to be the only solid-phase component capable of retaining the trace metal cations and decreasing the soluble and exchangeable forms (Gerritse and Van Driel, 1984 Mann and Ritchie, 1993). The uptake of metals by plants is a function of their content and speciation in solution and the ability of the solid phase to replenish the solution metals. With respect to potentially toxic metal cations such as Cd, it would appear that the free ion and monomeric... 

In aquatic sediments or soils, there are also a range of trace elements species ranging from ions exchanged to particles, to those bound to organic matter or in various inorganic forms (e.g., oxides, carbonates, sulfides) or as more inert crystalline mineral phases. As in waters, speciation studies in soils and sediments are generally undertaken to better understand the bioavailability of toxic substances and to investigate transport pathways to and from other parts of the ecosystem. Sediment and soil pore waters (soil solutions) are of particular interest because they are in equilibrium with the solid phase and are the medium for contaminant uptake by plants and many other biota. The techniques used for speciation analysis in these aqueous samples differ little from those for waters. 

Dietz, C. Landaluze, J.S. Ximenez-Embun, P. Madrid-Albarrai, Y. Camara, C. Volatile organo-selenium speciation in biological matter by solid phase microextractiotMnoderate temperature multicapillaiy gas chromatography with microwave induced plasma atomic emission spectrometry detection. Anal. Chim. Acta 2004, 501,157-167. 

Martfnez-Villegas, N., and C. E. Martinez. 2008. Solid- and solution-phase organics dictate copper distribution and speciation in multicomponent systems containing ferrihydrite, organic matter, and montmorillonite. Environmental Science Technology 42, no. 8 2833-2838. doi 10.1021/es072012r. 

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