Natural Attenuation of Trace Element Availability Assessed by Chemical Extraction
The potential mobility and availability of trace elements depend on their total concentration in the soil, in soil solution, and in exchangeable forms. Retention and release reactions of solute with different components of the soil matrix govern the chemical behavior of trace elements. For elements freshly added to soils, the partitioning between the soil solution and the solid phase gradually changes with time until it reaches a state of pseudoequilibrium. The rate at which this equilibrium is attained is not only a time-dependent process, but it is also governed by the nature of the trace element and soil properties. These issues are discussed in more detail in Chapter 4 to Chapter 6. Predictions of persistence, the potential mobility, and the bioavailability of trace elements in contaminated soils currently require both chemical and biological methods (Kennedy et al. 1997), especially to quantify the fraction of trace elements available for biological uptake. However, chemical surrogates for assessing bioavailability have long been sought due to their simplicity and rapidity, in comparison to biological methods. Chemical methods include single and sequential extraction methods, which have both been used to quantify the concentration of trace elements available for plant uptake (He and Singh 1993a; Singh et al. 1995; Narwal and Singh 1998; Almås et al. 1999; Kukier and Chaney 2000). Sequential extraction procedures can provide more detailed information on the association of a trace element with soil components in comparison to a single extraction (Pickering 1986). The initial stages of a sequential extraction procedure have similarity to a single extraction and have been used to assess the potential short-term biological uptake of trace elements. The objective of this chapter is to discuss the use of chemical extraction procedures to assess natural attenuation of trace element availability in soils. Examples from different experiments, conducted under different soil and environmental conditions, will be cited to illustrate the effectiveness of different chemical extraction procedures in assessing the attenuation of trace element mobility or availability in soils.