Metals
17
precipitated and adsorbed metals, and leach out a certain amount of the metals from
the silicate lattice.
Much weaker extracting agents have also been used to extract metals of a nonresidual origin only. Methods involving the use of 0.5 N hydrochloric acid and 0.05 N
ethylenediamintetraacetic acid dissolve complexed, adsorbed and precipitated metals
in sediments with minimum attack on the silicate. A mixture of 1 N hydroxylammonium chloride and 25 % acetic acid has been used to dissolve adsorbed trace elements
in sediments and is similar to the above two methods.
Agemian and Chau [34] studed the relative simultaneous extraction of a large
number of metals from aquatic sediments in order to obtain a rapid, simple technique
for measuring non-residual metal. The non-residual metal phase includes the exchangeable metal carbonate, organic and sulphide phases, as well as oxides and hydroxides
of manganese and iron.
Leaching Conditions. The size of sediment particles strongly influences the extractable metal content of the samples. The < 80-mesh portion of an air dried (at 20 °C)
sediment provides the greatest contrast between anomalous and background samples.
To facilitate the dissolution necessary for determining the total metal, a sub-sample
from the < 80-mesh (0.177 mm) portion was ground to about 200-mesh.
Cold-extractable Metal Content. A 5-g sample of sediment was shaken overnight at
room temperature with 100 ml solutions of 0.05 N ethylenediaminetetraacetic acid at
pH 4.8, 1 N hydroxylammonium chloride plus 25 % acetic acid; and 0.5 N hydrochloric acid.
Acid Extractable Metal Content. A 1-g amount of sediment was digested with 25 ml of
nitric acid, boiled to dryness twice, with 25 ml of aqua regia and boiled to dryness
twice and with 25 ml of nitric-perchloric (1 + 1) acids and boiled to dryness twice. The
residue was dissolved in dilute hydrochloric acid in each instance.
Total Metal Content. The < 80-mesh sample was crushed to about 200-mesh and
100 mg of this powder was digested with 6 ml of hydrofluoric acid and 1 ml of
perchloric acid in a PTFE bomb.
Table 2.10 shows the degree of extraction of several metals by use of the methods
under consideration. The methods used were three of the four types of extraction
techniques, namely, those which extract total (last column), acid-extractable (columns 5-7), and cold-extractable (columns 2-4) metal. The acid leaching techniques
(columns 5-7) show varying degrees of attack on the crystal lattice and thus give an
intermediate value between cold-extractable and total metal extractions. The results
(Table 2.10) reflect this postulation.
It is apparent from Table 2.10 that, for the sample studied, perchloric acid does not
liberate all of the metal from the silicate matrix. The amount of metal extracted by
perchloric acid depends on the type of sample (both type of mineral and organic matter
content). For many types of sample, this acid is suitable for total metal extraction.
The nitric acid used in this method (Table 2.10, column 7) serves only as a safety
measure if large amounts of organic matter are present. The use of perchloric acid
alone for the sample in Table 2.10 gave results identical with those obtained using
nitric-perchloric acids.
17
precipitated and adsorbed metals, and leach out a certain amount of the metals from
the silicate lattice.
Much weaker extracting agents have also been used to extract metals of a nonresidual origin only. Methods involving the use of 0.5 N hydrochloric acid and 0.05 N
ethylenediamintetraacetic acid dissolve complexed, adsorbed and precipitated metals
in sediments with minimum attack on the silicate. A mixture of 1 N hydroxylammonium chloride and 25 % acetic acid has been used to dissolve adsorbed trace elements
in sediments and is similar to the above two methods.
Agemian and Chau [34] studed the relative simultaneous extraction of a large
number of metals from aquatic sediments in order to obtain a rapid, simple technique
for measuring non-residual metal. The non-residual metal phase includes the exchangeable metal carbonate, organic and sulphide phases, as well as oxides and hydroxides
of manganese and iron.
Leaching Conditions. The size of sediment particles strongly influences the extractable metal content of the samples. The < 80-mesh portion of an air dried (at 20 °C)
sediment provides the greatest contrast between anomalous and background samples.
To facilitate the dissolution necessary for determining the total metal, a sub-sample
from the < 80-mesh (0.177 mm) portion was ground to about 200-mesh.
Cold-extractable Metal Content. A 5-g sample of sediment was shaken overnight at
room temperature with 100 ml solutions of 0.05 N ethylenediaminetetraacetic acid at
pH 4.8, 1 N hydroxylammonium chloride plus 25 % acetic acid; and 0.5 N hydrochloric acid.
Acid Extractable Metal Content. A 1-g amount of sediment was digested with 25 ml of
nitric acid, boiled to dryness twice, with 25 ml of aqua regia and boiled to dryness
twice and with 25 ml of nitric-perchloric (1 + 1) acids and boiled to dryness twice. The
residue was dissolved in dilute hydrochloric acid in each instance.
Total Metal Content. The < 80-mesh sample was crushed to about 200-mesh and
100 mg of this powder was digested with 6 ml of hydrofluoric acid and 1 ml of
perchloric acid in a PTFE bomb.
Table 2.10 shows the degree of extraction of several metals by use of the methods
under consideration. The methods used were three of the four types of extraction
techniques, namely, those which extract total (last column), acid-extractable (columns 5-7), and cold-extractable (columns 2-4) metal. The acid leaching techniques
(columns 5-7) show varying degrees of attack on the crystal lattice and thus give an
intermediate value between cold-extractable and total metal extractions. The results
(Table 2.10) reflect this postulation.
It is apparent from Table 2.10 that, for the sample studied, perchloric acid does not
liberate all of the metal from the silicate matrix. The amount of metal extracted by
perchloric acid depends on the type of sample (both type of mineral and organic matter
content). For many types of sample, this acid is suitable for total metal extraction.
The nitric acid used in this method (Table 2.10, column 7) serves only as a safety
measure if large amounts of organic matter are present. The use of perchloric acid
alone for the sample in Table 2.10 gave results identical with those obtained using
nitric-perchloric acids.