Geology Reference
In-Depth Information
Figure 13 Speciation of metal ions in seawater and the main controlling mechanisms
(Adapted from O ¨ hman and Sjo¨ berg, 1988. 20 )
that the uptake of trace elements is limited to free ions and some types of
lipid-soluble organic complexes. This is important in that some elements
may be essential, but toxic at elevated concentrations. Via complexation
reactions, organisms may be able to modify the free ion concentration.
Thus, siderophores can sequester Fe, often in limited supply. Alterna-
tively, metallothionein detoxify contaminants, notably Cd and Hg,
through chelation.
An element can exist in natural waters in a range of forms that exhibit
a size distribution as indicated in Table 5. Entities in true solution
include ions, ion pairs, complexes and a wide range of organic molecules
that can span several size categories. At the smallest extreme are ions,
which exist in solution with a co-ordinated sphere of water molecules as
discussed previously. Na 1 ,K 1 and Cl exist predominantly as free
hydrated ions. Collisions of oppositely charged ions occur due to
electrostatic attraction and can produce an ion pair. An ion pair is the
transient coupling of a cation and anion during which each retains its
own co-ordinated water envelope. While impossible to measure directly,
concentrations can be calculated with knowledge of the ion activities
and the stability constant. For the formation of the ion pair NaSO 4 via
Na þ þ SO 4 Ð NaSO 4
Search WWH ::




Custom Search