Metal speciation and bioavailability to monitor contaminated soils

It is generally accepted that the distribution, mobility, biological availability and toxicity of trace elements in soil depend not simply on their total concentrations but, critically, on their forms. These may be soluble, readily exchangeable, complexed with organic matter, or hydrous oxides, substituted in stoichiometric compounds, or occluded in mineral structures. Changes in environmental conditions, whether natural or anthropogenic, can alter the forms of trace metals, thereby affecting their behaviour in soil. The main controlling factors include pH, redox potential, ionic strength of the soil solution, the solid and solution components and their relative concentrations and affinities for an element, and time. Metal bioavailability is the fraction of the total metal occurring in the soil matrix, which can be utilized by biota. Available forms of trace metals are not necessarily associated with one particular chemical species or a specific soil component. Although plant roots absorb nutrients from the soil solution, rarely is availability equated with solubility. This is because metal solubility in soil is a function of interacting biological, chemical and physical factors. The amounts and nature of nutrients in the soil solution are associated with nutrients adsorbed by or contained within the solid phase of the soil. Release from the solid phase may result from processes such as exchange, decomposition, dissolution or desorption. Rate of release as well as the capacity to release nutrients, the buffer power, has a great effect on the bioavailability of the ion and may strongly differ among soils, reflecting differences in bonding strengths. Estimation of metal bioavailability in soil relies on understanding the rate at which bioavailable fractions are replenished from the soil matrix. Speciation science seeks to characterise the various forms in which trace metals occur in soil or, at least, the main metal pools present in soil. Understanding speciation is important for assessing the potential of soil to supply micronutrients for plant growth or to contain toxic quantities of trace metals, and for determining amelioration procedures for soils at risk of causing the trace metal contamination of waterways. The residence time of an element in a soil depends on the mobility of its predominant forms. Speciation science is relevant to scientists with many different backgrounds and should be taken into consideration by legislators in the field of environmental protection.Trace metal speciation in soil can be achieved using either direct or indirect analytical methods. Direct methods include X-ray diffraction (XRD) and microanalysis (EDS/WDS), infrared absorption (FTIR) and Mössbauer spectrometry, electron microscopy (SEM/TEM), magnetic resonance (NMR, EPR) and photoelectron (EXAFS, XANES) spectroscopy. With such techniques the combinational forms of major elements in soil components, such as clay minerals, iron, manganese and aluminium oxyhydroxides and humic materials, and the chemical structures of these soil components can be precisely elucidated. Nevertheless, these direct, mainly non-destructive, methods for speciation are usually qualitative and often not sensitive enough to detect forms present in small amounts. Furthermore, their correct application requires sophisticated instrumentations and specialised operators. More widely applied are the methods of trace metal speciation which involve selective chemical extraction techniques. Estimation of the plant-available element content of soil using single chemical extractants is an example of functionally defined speciation, in which the ‘function' is plant 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.