Standard Practices for Extraction of Trace Elements from Sediments
Industrialized and urban areas have been found to deposit a number of toxic elements into environments where those elements were previously either not present or were found in trace amounts. Consequently, it is important to be able to measure the concentration of these pollution-deposited elements to properly study pollution effects.
This procedure is concerned with the pollution-related trace elements that are described in 4.1 rather than those elements incorporated in the silicate lattices of the minerals from which the sediments were derived. These pollution-related trace elements are released into the water and readsorbed by the sediments with changes in general water quality, pH in particular. These elements are a serious source of pollution. The elements locked in the silicate lattices are not readily available in the biosphere (1-8).
When comparing the trace element concentrations, it is important to consider the particle sizes to be analyzed (8, 9).
5.3.1 The finer the particle the greater the surface area. Consequently, a potentially greater amount of a given trace element can be adsorbed on the surface of fine, particulate samples (4). For particle sizes smaller than 80 mesh, metal content is no longer dependent on surface area. Therefore, if this portion of the sediment is used, the analysis with respect to sample type (that is, sand, salt, or clay) is normalized. It has also been observed that the greatest contrast between anomalous and background samples is obtained when less than 80-mesh portion of the sediment is used (4, 5).
5.3.2 After the samples have been dried, care must be taken not to grind the sample in such a way to alter the natural particle-size distribution (14.1). Fracturing a particle disrupts the silicate lattice and makes available those elements which otherwise are not easily digested (6). Normally, aggregates of dried, natural soils, sediments, and many clays dissociate once the reagents are added (14.3 and 15.2).
1.1 These practices describe the partial extraction of soils, bottom sediments, suspended sediments, and waterborne materials to determine the extractable concentrations of certain trace elements.
1.1.1 Practice Ais capable of extracting concentrations of aluminum, boron, barium, cadmium, calcium, chromium, cobalt, copper, iron, lead, magnesium, manganese, molybdenum, nickel, potassium, sodium, strontium, vanadium, and zinc from the preceding materials. Other metals may be determined using this practice. This extraction is the more vigorous and more complicated of the two.
1.1.2 Practice Bis capable of extracting concentrations of aluminum, cadmium, chromium, cobalt, copper, iron, lead, manganese, nickel, and zinc from the preceding materials. Other metals may be determined using this practice. This extraction is less vigorous and less complicated than Practice A.
1.2 These practices describe three means of preparing samples prior to digestion:
1.2.2 Air-drying at room temperature.
1.2.3 Accelerated air-drying, for example, 95C.
1.3 The detection limit and linear concentration range of each procedure for each element is dependent on the atomic absorption spectrophotometric or other technique employed and may be found in the manual accompanying the instrument used. Also see various ASTM test methods for determining specific metals using atomic absorption spectrophotometric techniques.
1.3.1 The sensitivity of the practice can be adjusted by varying the sample size (14.2) or the dilution of the sample (14.6), or both.
1.4 Extractable trace element analysis provides more information than total metal analysis for the detection of pollutants, since absorption, complexation, and precipitation are the methods by which metals from polluted waters are retained in sediments.
1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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