Abstract
The effect of ionic interactions of the major components of natural waters on the oxidation of Cu(I) and Fe(II) has been examined. The various ion pairs of these metals have been shown to have different rates of oxidation. For Fe(II), the chloride and sulfate ion pairs are not easily oxidized. The measured decrease in the rate constant at a fixed pH in chloride and sulfate solutions agrees very well with the values predicted. The effect of pH (6 to 8) on the oxidation of Fe(II) in water and seawater have been shown to follow the rate equation
-d in [Fe(II)]/dt = k
1β
1α
Fe/[H
+] + k
2β
2α
Fe/[H
+]
2
where
k
1 and
k
2 are the pseudo first order rate constants,
β
1 and
β
2 are the hydrolysis constants for Fe(OH)
+ and Fe(OH)
0. The value of
α
FE
is the fraction of free Fe
2+. The value of
k
1 (2.0 ±0.5
min
−1) in water and seawater are similar within experimental error. The value of
k
2 (1.2 × 10
5 min
−1) in seawater is 28% of its value in water in reasonable agreement with predictions using an ion pairing model.
For the oxidation of Cu(I) a rate equation of the form
−d ln [Cu(I)]/dt = k
0α
Cu+ k
1β
1α
Cu[Cl]
was found where
k
0 (14.1 sec
−1) and
k
1 (3.9 sec
−1) are the pseudo first order rate constants for the oxidation of Cu
+ and CuCl
0,
β
1 is the formation constant for CuCl
0 and
α
Cu
is the fraction of free Cu
+. Thus, unlike the results for Fe(II), Cu(I) chloride complexes have measurable rates of oxidation.
