Anions in seawater - Determination of Anions in Natural and Treated Waters - page 361

be formed under the conditions prevailing in seawater as well as inorganic chromium(III)
and chromium(VI) forms. Inconsistencies in earlier research may therefore, be at least
partly due to the fact that the possibility of organic chromium species was ignored
[ 41,42].
Nakayama et al. [38] have described a method for the determination of chromium(III),
chromium(VI) and organically bound chromium in seawater. They found that seawater in
the sea of Japan contained about 9 ×10−9 M dissolved chromium. This is shown to be
divided as ca. 15% inorganic chromium(III), ca. 25% inorganic chromium(VI) and ca.
60% organically-bound chromium.
These workers studied the co-precipitation behaviours of chromium species with
hydrated iron(III) and bismuth oxides.
The collection behaviour of chromium species was examined as follows. Seawater
(400ml) spiked with 10 −8 M chromium(III), chromium (VI) and chromium(III) organic
complexes labelled with 51 Cr was adjusted to the desired pH by hydrochloric acid or
sodium hydroxide. An appropriate amount of hydrated iron(III) or bismuth oxide was
added; the oxide precipitates were prepared separately and washed thoroughly with
distilled water before use [43]. After about 24 h, the samples were filtered on 0.4µm
Nuclepore filters. The separated precipitates were dissolved with hydrochloric acid and
the solutions thus obtained were used for γ-activity measurements. In the examination of
solvent extraction, chromium was measured by using 51 Cr, while iron and bismuth were
measured by electrothermal atomic absorption spectrometry. The decomposition of
organic complexes and other procedures were also examined by electrothermal atomic
absorption spectrometry.
3.13.2.1 Collection of chromium(III) and chromium(VI) with hydrated iron
(III) or bismuth oxide
Only chromium(III) co-precipitates quantitatively with hydrated iron(III) oxide at the pH
of seawater, around 8. In order to collect chromium(VI) directly without pretreatment, eg
reduction to chromium(III), hydrated bismuth oxide, which forms an insoluble compound
with chromium(VI) was used. Chromium(III) is collected with hydrated bismuth oxide
(50mg per 400mL seawater) to collect chromium(VI) in seawater is collected at about pH
4 and chromium(VI) is collected below pH 10. Both chromium(III) and chromium(VI)
are thus collected quantitatively the pH of seawater around 8.
3.13.2.2 Collection of chromium(III) organic complexes with hydrated iron
(III) or bismuth oxide
The percentage collection of chromium(III) with hydrated iron(III) oxide may decrease
considerably in the neutral pH range when organic materials capable of combining with
chromium(III) such as citric acid and certain amino acids, are added to the seawater [41].
Moreover, synthesised organic chromium(III) complexes are scarcely collected with
hydrated iron(III) oxide over a wide pH range [41].
As it was not known what kind of organic matter acts as the major ligand for
chromium in seawater Nakayama et al. [38] used EDTA and 8-quinolinol-4-sulphuric