Plant-available speciation methods

Functionally defined speciation. Functionally defined species are exemplified by the plant-available species or chemical pools in which the function is plant availability. Available forms of trace metal cations are not necessarily associated with one particular chemical species or a specific soil component. Hence, to predict the availability of trace metals, we either have to establish the species involved and develop methods that specifically determine those forms only, or we have to establish an empirical relationship between an accepted diagnostic measure of the metal and plant growth. Both speciation in solution and fractionation of the solid phase to identify the chemical pools can affect plant uptake (phytoavailability) of trace metals and water pollution. 

In soils and sediments, chemical fractionation or sequential extraction methods offer another approach to speciation of the forms of phosphorus. Traditionally, there has been more emphasis on bioavailable or plant available inorganic forms, e.g., Olsen s extraction used for soils. 

A detailed review of the methods for deterrnination of low manganese concentration in water and waste is available (179). A review on the speciation of Mn in fresh waters has been reported (180). Reviews for the chemical analysis of Mn in seawater, soil and plants, and air are presented in References 181, 182, and 183, respectively. 

The availability of the analytes for uptake by plants, for transport through the soil, and for dissolution into water can be estimated from a well-studied speciation scheme. Risk assessment for disposal of wastes in landfills or for land disposal of dredge spoils or sewage sludges requires knowledge not only of the total metal content but also of the content in each separate fraction to begin to understand how the metals will act in the environment. Table 5.7 summarizes the methods available for speciation of metals in samples. 

More widely applied to determine the potential, plant and human bioavailability are the methods of PTMs speciation which involve selective chemical extraction techniques. Estimation of the plant- or human-available element content of soil using single chemical extractants is an example of functionally defined speciation, in which the function is plant or human availability. In operationally defined speciation, single extractants are classified according to their ability to release elements from specific soil phases. Selective sequential extraction procedures are examples of operational speciation (Ure and Davidson, 2002). 

Recently, attempts have been made to develop biomimetic methods, simulating plant uptake of metals. An example of such a method is DGT (diffusive gradients in thin films), developed by Zhang et al. (2001), for measuring metal availability to plants. In this case, metal accumulation in a chelex layer is measured. By taking into account thickness of the diffusive layer covering the chelex layer and contact time with the soil sample, it is possible to estimate the available metal concentration in the soil solution. The DGT method may also be used to estimate metal speciation in surface water (Zhang 2004). 

In this article, flowing-stream systems assembled for environmental and agricultural analysis are classified according to their application area, i.e., water, air, soil, and plant analysis. Within each area relevant flow methodologies for the determination of individual analytes are briefly reviewed and specific features of particular methods are outlined. Additional information about the analytical performance of several flow assemblies is listed comprehensibly in the tables. The likelihood of direct introduction and treatment of solid samples in an automated fashion is also highlighted in the bulk of the text. Finally, attention is also paid to the different schemes available for online speciation studies, which are of increasing significance in environmental assays. 

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