Chemistry of Freshwaters - Principles of Environmental Chemistry - page 108

the formation of such complexes increases mineral solubility by several
orders of magnitude over that predicted by the solubility product, K SP .
The presence of even trace concentrations of organic ligands can have an
even greater effect over a wide pH range on Al III speciation (Example
3.9). 12 More generally, complexation clearly has an important role to
play, not only in reducing metal toxicity (see Section 3.3.1.2) but also in
markedly increasing metal ion mobility.

Example 3.9: Determination of Al III speciation at pH 2, 6, and 10 in
an aqueous solution where { Al III T } ¼ 1 10 7 mol L 1 . Use log *b 1 ¼
4.95, log *b 2 ¼ 10.55, log *b 3 ¼ 17.25, log *b 4 ¼ 22.85,
log b 1 fluoride ¼ 7.00, log b 2 fluoride ¼ 12.70, log b 1sulfate ¼ 3.50,
log b 2 sulfate ¼ 5.00, and log b 1 org ¼ 5.9. Assume SF ¼ 5 10 8
mol L 1 , SSO 4 2 ¼ 1 10 4 mol L 1 , SOrg ¼ 1 10 7 mol L 1 and
that no precipitation of Al(OH) 3 occurs.

Hydrolysis only:

f Al II T g¼f Al 3 þ gþf Al ð OH Þ 2 þ gþf Al ð OH Þ 2 g
þf Al ð OH Þ 3 gþf Al ð OH Þ 4 g
¼ 10 7 mol L 1
¼f Al 3 þ g 1 þ b 1 =f H þ gþ b 2 =f H þ g 2

þ b 3 =f H þ g 3 þ b 4 =f H þ g 4

Main species: pH 2: Al 31 ; pH 6: Al(OH) 21 , Al(OH) 2 1 ; pH 10:
Al(OH) 4 .

Hydrolysis and one inorganic ligand, F :

f Al II T g¼f Al 3 þ gþf Al ð OH Þ 2 þ gþf Al ð OH Þ 2 gþf Al ð OH Þ 3 g
þf Al ð OH Þ 4 gþf AlF 2 þ gþf AlF 2 g¼ 10 7 mol L 1
¼f Al 3 þ gð 1 þ b 1 =f H þ gþ b 2 =f H þ g 2 þ b 3 =f H þ g 3
þ b 4 =f H þ g 4 þ b 1 fluoride f F gþ b 2 fluoride f F g 2 Þ