42
River and Stream Sediments
Total dissolution of the samples was also carried out to establish the fraction of
metal removed at each pH. Digestion of 0.400 g samples was performed in Teflon
beakers by using concentrated nitric acid, hydrofluoric acid and perchloric acid in a
three-step process. First, 1 ml of perchloric acid, 1 ml of nitric acid and 3 ml of
hydrofluoric acid were added, a Teflon watch cover was put in place, and the sample
was heated at 50°C until a moist paste was obtained. The mixture was heated for 3 h
at 80°C with an additional 2 ml of nitric acid and 3 ml of hydrofluoric acid and then
brought to dryness. Finally, 1 ml of nitric acid and 30 ml of deionized distilled water
were added to the sample and heated to dissolve perchlorate salts and reduce the
volume. The completely dissolved and clear samples were then diluted to 25 ml with
deionized distilled water. Analysis by flame or flameless atomic absorption spectrometry followed.
Plots of metal leached vs pH for Mississippi River suspended particulates provide a
good representation of the overall trends observed. Copper, iron, and lead concentrations follow similar patterns of sharp increases in concentration below pH 4. At pH
values> 4, < IllS of copper and lead g-l is leached. For both copper and lead, the
same percentage (20 %) of total metal is removed at pH 2.3, whereas only 2 % of the
total iron is removed at the lowest pH.
In contrast to the iron group, copper and manganese show a more continuous
increase in the amount leached with decreasing pH. At pH 2.3, > 80 % of the total
cadmium and > 50 % of the total manganese are removed from this sample. At
higher pH values (4-6), 10-40 % of the total cadmium and manganese is removed.
Zinc removal follows the gradual increase with decreasing pH found for the manganese group, but with a small percentage of total metal leached at each pH, a characteristic of the iron group.
These results show that a higher percentage of the total cadmium and manganese
relative to copper, iron, lead, and zinc will be released from suspended particulates
with decreasing pH. In general, metal leaching is related to the final pH of the leaching
solution.
A pH approach to leaching may be less practical in cases where samples have a large
percentage of carbonates, metal oxides, or other acid-consuming components. In
carbonate-rich (30- > 95 % carbonate) samples, Trefrey and Metz [39] approached
the acid consumption problem by slowly titrating samples with 0.01 N hydrochloric
acid (or 1 N hydrochloric acid for large samples or carbonates) until the carbonate
reaction is completed. At this point, the pH may be adjusted by adding acid or base.
Such increased handling is less desirable, but use of a greater buffer capacity solution
in such samples will only increase the matrix problems already introduced by high
calcium concentrations.
Sinex et al. [35] have also investigated the use of nitric-hydrochloric acid in the
extraction of total elements from NBS SMS1645 standard river sediments prior to
analysis by atomic absorption spectrometry. They claim a recovery of '95 % for
chromium, manganese, nickel, copper, zinc, and lead and 75 % recovery for iron,
cobalt, and cadmium.
Legret et al. [52] demonstrated severe matrix interference in the determination of
heavy metals in sediments by electrothermal atomic absorption spectrometry, particularly with regard to lead, cadmium, and nickel.
River and Stream Sediments
Total dissolution of the samples was also carried out to establish the fraction of
metal removed at each pH. Digestion of 0.400 g samples was performed in Teflon
beakers by using concentrated nitric acid, hydrofluoric acid and perchloric acid in a
three-step process. First, 1 ml of perchloric acid, 1 ml of nitric acid and 3 ml of
hydrofluoric acid were added, a Teflon watch cover was put in place, and the sample
was heated at 50°C until a moist paste was obtained. The mixture was heated for 3 h
at 80°C with an additional 2 ml of nitric acid and 3 ml of hydrofluoric acid and then
brought to dryness. Finally, 1 ml of nitric acid and 30 ml of deionized distilled water
were added to the sample and heated to dissolve perchlorate salts and reduce the
volume. The completely dissolved and clear samples were then diluted to 25 ml with
deionized distilled water. Analysis by flame or flameless atomic absorption spectrometry followed.
Plots of metal leached vs pH for Mississippi River suspended particulates provide a
good representation of the overall trends observed. Copper, iron, and lead concentrations follow similar patterns of sharp increases in concentration below pH 4. At pH
values> 4, < IllS of copper and lead g-l is leached. For both copper and lead, the
same percentage (20 %) of total metal is removed at pH 2.3, whereas only 2 % of the
total iron is removed at the lowest pH.
In contrast to the iron group, copper and manganese show a more continuous
increase in the amount leached with decreasing pH. At pH 2.3, > 80 % of the total
cadmium and > 50 % of the total manganese are removed from this sample. At
higher pH values (4-6), 10-40 % of the total cadmium and manganese is removed.
Zinc removal follows the gradual increase with decreasing pH found for the manganese group, but with a small percentage of total metal leached at each pH, a characteristic of the iron group.
These results show that a higher percentage of the total cadmium and manganese
relative to copper, iron, lead, and zinc will be released from suspended particulates
with decreasing pH. In general, metal leaching is related to the final pH of the leaching
solution.
A pH approach to leaching may be less practical in cases where samples have a large
percentage of carbonates, metal oxides, or other acid-consuming components. In
carbonate-rich (30- > 95 % carbonate) samples, Trefrey and Metz [39] approached
the acid consumption problem by slowly titrating samples with 0.01 N hydrochloric
acid (or 1 N hydrochloric acid for large samples or carbonates) until the carbonate
reaction is completed. At this point, the pH may be adjusted by adding acid or base.
Such increased handling is less desirable, but use of a greater buffer capacity solution
in such samples will only increase the matrix problems already introduced by high
calcium concentrations.
Sinex et al. [35] have also investigated the use of nitric-hydrochloric acid in the
extraction of total elements from NBS SMS1645 standard river sediments prior to
analysis by atomic absorption spectrometry. They claim a recovery of '95 % for
chromium, manganese, nickel, copper, zinc, and lead and 75 % recovery for iron,
cobalt, and cadmium.
Legret et al. [52] demonstrated severe matrix interference in the determination of
heavy metals in sediments by electrothermal atomic absorption spectrometry, particularly with regard to lead, cadmium, and nickel.