Metals
23
Although boiling perchloric-nitric acid may give total extraction of some metals
from some non-resistant samples, complete destruction of the silica matrix by hydrofluoric acid is necessary for total extraction of all metals of interest.
Of the cold-extractable metal extraction techniques, 1 N hydroxylammonium chloride plus 25 % acetic acid is inadequate for the simultaneous .extraction of a large
number of metals from a variety of sample types because it does not extract copper
that has been complexed by organic matter. Both 0.5 N hydrochloric acid and 0.05 N
ethylenediaminetetraacetic acid are suitable for the simultaneous extraction of 10
elements from the adsorbed, organic and precipitated phases of aquatic sediments.
The 0.5 N hydrochloric acid method is preferred because, owing to some natural
processes, the adsorbed metals in some sediments are bonded more strongly, rendering the 0.05 N ethylenediaminetetraacetic acid extraction method incapable of extracting them.
Zink-Nielsen [77] compared direct digestion for 4 h with 1 : 1 nitric acid and
autoclaving at 120°C with concentrated nitric acid followed by atomic absorption
spectrometry as a means of determining six elements in river sediments. The results
of an intercalibration exercise involving nine laboratories show that, in most cases,
smaller coefficients of variation were obtained by autoclaving at 120°C with concentrated nitric acid compared to 4 h digestion with 1 : 1 nitric acid. The procedure was
applied to determinations of lead, iron, manganese, zinc, copper, and chromium in
the 0.07 to IS mg kg-l concentration range.
Agemian and Chau [37] also carried out more detailed studies of some of these
extraction techniques (aqua regia, 0.5 N hydrochloric acid, 1 N hydroxylamine hydrochloride plus 25 % acetic acid, and 0.05 N EDTA at pH 4.S - see Table 2.10) for the
determination of heavy metals in sediments from the River Rideau, Canada. Their aim
was to find the metal extraction procedure which gave the greatest contrast between
background sediment samples and anomalous sediment samples with enhanced metal
levels due to mineral deposits occurring in the area from which the samples were taken.
Sediment extracts were analysed by atomic absorption spectrometry.
In this study, the analysis was performed on the SO-mesh portion of the sediments.
As this portion of a sediment provides the greatest contrast between anomalous and
background samples [44], Oliver [45] showed that the particle size of the sediment
strongly influenced the metal content. The metal content increased rapidly for surface
areas up to about 10 m 2 / g and then levelled off. Thus when samples with different
surface areas are compared, the SO-mesh portion of the sediment with surface area
greater than 10 m 2 / g must be used to normalize all samples. The samples were
air-dried and sieved to SO-mesh. A representative subsample of the SO-mesh fraction
of each sediment was treated in each of the five following ways.
(a) 100 mg of sample was accurately weighed into a Parr 4745 acid digestion bomb
and digested with a mixture ofHF, HN0 3 and HCI0 4 as described by Agemian and
Chau [37]. This provides a measure of total metal in the sediment.
(b) 1 g of sample was accurately weighed and digested in 100 ml of 4.0 N nitric-0.7 N
hydrochloric acid solution for 2 h at 70-90 °C.
(c) 5 g of sample was digested with shaking in 100 ml of 0.5 N hydrochloric acid
solution overnight at room temperature.
23
Although boiling perchloric-nitric acid may give total extraction of some metals
from some non-resistant samples, complete destruction of the silica matrix by hydrofluoric acid is necessary for total extraction of all metals of interest.
Of the cold-extractable metal extraction techniques, 1 N hydroxylammonium chloride plus 25 % acetic acid is inadequate for the simultaneous .extraction of a large
number of metals from a variety of sample types because it does not extract copper
that has been complexed by organic matter. Both 0.5 N hydrochloric acid and 0.05 N
ethylenediaminetetraacetic acid are suitable for the simultaneous extraction of 10
elements from the adsorbed, organic and precipitated phases of aquatic sediments.
The 0.5 N hydrochloric acid method is preferred because, owing to some natural
processes, the adsorbed metals in some sediments are bonded more strongly, rendering the 0.05 N ethylenediaminetetraacetic acid extraction method incapable of extracting them.
Zink-Nielsen [77] compared direct digestion for 4 h with 1 : 1 nitric acid and
autoclaving at 120°C with concentrated nitric acid followed by atomic absorption
spectrometry as a means of determining six elements in river sediments. The results
of an intercalibration exercise involving nine laboratories show that, in most cases,
smaller coefficients of variation were obtained by autoclaving at 120°C with concentrated nitric acid compared to 4 h digestion with 1 : 1 nitric acid. The procedure was
applied to determinations of lead, iron, manganese, zinc, copper, and chromium in
the 0.07 to IS mg kg-l concentration range.
Agemian and Chau [37] also carried out more detailed studies of some of these
extraction techniques (aqua regia, 0.5 N hydrochloric acid, 1 N hydroxylamine hydrochloride plus 25 % acetic acid, and 0.05 N EDTA at pH 4.S - see Table 2.10) for the
determination of heavy metals in sediments from the River Rideau, Canada. Their aim
was to find the metal extraction procedure which gave the greatest contrast between
background sediment samples and anomalous sediment samples with enhanced metal
levels due to mineral deposits occurring in the area from which the samples were taken.
Sediment extracts were analysed by atomic absorption spectrometry.
In this study, the analysis was performed on the SO-mesh portion of the sediments.
As this portion of a sediment provides the greatest contrast between anomalous and
background samples [44], Oliver [45] showed that the particle size of the sediment
strongly influenced the metal content. The metal content increased rapidly for surface
areas up to about 10 m 2 / g and then levelled off. Thus when samples with different
surface areas are compared, the SO-mesh portion of the sediment with surface area
greater than 10 m 2 / g must be used to normalize all samples. The samples were
air-dried and sieved to SO-mesh. A representative subsample of the SO-mesh fraction
of each sediment was treated in each of the five following ways.
(a) 100 mg of sample was accurately weighed into a Parr 4745 acid digestion bomb
and digested with a mixture ofHF, HN0 3 and HCI0 4 as described by Agemian and
Chau [37]. This provides a measure of total metal in the sediment.
(b) 1 g of sample was accurately weighed and digested in 100 ml of 4.0 N nitric-0.7 N
hydrochloric acid solution for 2 h at 70-90 °C.
(c) 5 g of sample was digested with shaking in 100 ml of 0.5 N hydrochloric acid
solution overnight at room temperature.