Sequential Extraction Methods

While total concentrations of nutrients and contaminants in sediments are measured to quantify spatial and temporal gradients, concentrations of specific fractions on the surface and within sediments may be further characterized using selective extraction techniques. These extraction techniques provide more detailed information on how those constituents are bound in sediments, as well as their bioavailability and conditions for their remobili-zation from the sediments. As noted at the beginning of this article, bioavailability and mobility are especially important in studies of nutrients and contaminants in sediments because they are dynamic systems which are subject to chemical and physical changes on the time-scales of seconds, days, seasons, years, decades, and longer.

To address these concerns, various sequential extraction techniques have been developed to determine semiquantitatively how elements and compounds, including nutrients and contaminants, are associated with sediments. The extractions are designed to sequentially extract from the most weakly bound (e.g., ion exchangeable) to the most strongly bound (e.g., refractory crystal lattice) fractions. While the fractions are operationally defined, rather than definitive and truly specific, they have proved to be relatively useful in characterizing their relative bioavailability and potential for diagenic remobilization from sediments, as well as their biogeochemical cycling between sediments and water.

Table 1 provides an example of a sequential extraction for sediments. It shows the extractants and conditions used in five sequential steps and the operationally defined fraction solubilized in each of those steps. There are numerous variations of these techniques (i.e., number of steps, extractants and conditions, and operationally defined fractions) with similar applicabilities and

Table 1 Sequential extraction steps and operationally defined fractions used to characterize the phase distribution of constituents, including nutrients and contaminants, in sediments

Step no. Extractants and conditions Operationally defined fraction

1 1 M NaOAc, pH 8.2,25°C Exchangeable

2 1 M NaOAc, pH 5 (HOAc), 25 °C Bound to carbonates

3 0.04 M NH2OH-HCl in 25% (v/v) HOAc, 100°C Reducible, bound to Fe-Mn oxides

4 0.02 M HNO3 + 30% H2O2, pH 2, 85°C; then 3.2 M NH4OAc in 20% (v/v) Oxidizable, strongly bound to organics and

HNO3 sulfide

5 Concentrated HNO3 + HF, 110°C Residual, recalcitrant

Modified from Tessier A, Campbell P G C, and Bisson M (1979) Sequential extraction procedure for the speciation of particulate trace-metals. Analytical Chemistry 51: 844-851.

limitations. Those limitations, along with previously noted limitations in measuring the speciation and fluxes of nutrients and contaminants in sediments, attest to their truly complex biogeochemical cycling and bioavailability.

See also-. Adsorption; Microbial Ecological Processes: Aerobic/Anaerobic; Physical Transport Processes in Ecology. Advection, Diffusion, and Dispersion; Transport in Porous Media.

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