p18ctqouf5gk614re1k6k1lc31s0l4.pdf

Oxidation Rates of Cations by Mn( Ill/IV) Oxides 171
~
20.0
0 275K
0
E
::t
15.0
[(5 -Mn02]o = 250 mg 1- 1
"'C
CII
E ....
[Cr{III)]o = 24.0 Jlmol I - 1
0
u. 10.0
pH = 5.5
~
>
~
....
u 5.0
10 20 30 40
Time (min)
Figure 8.4 Effect of temperature on the kinetics of Cr(III) oxidation by 8-Mn02 at
pH 5.5. [From Amacher and Baker (1982) with permission.]
oxidation. Thus, the actual amounts able to react were different from the
theoretical maximum. Most of the oxidation occurred during the first hour.
At 274 K, the reaction followed zero-order kinetics. Amacher and Baker
(1982) theorized that at low temperature, the oxidation reaction was
slowed down considerably so that the Mn oxide surface was "saturated"
with Cr undergoing oxidation. At higher temperatures (Fig. 8.3), initially
there was a rapid increase in Cr(VI) formed, followed by a decrease in
oxidation. However, as Amacher and Baker (1982) correctly note, the
study of redox kinetics in soils is different due to sorption and redox
reactions occurring simultaneously.
Kinetics of Cr(IlI) oxidation on Mn(III/IV) oxides showed a trend
similar to that observed for soils (Amacher and Baker, 1982). The effect of
temperatures and the shape of the kinetic curves are similar (Fig. 8.3
versus Fig. 8.4). Even though a large excess of 8-Mn02 was used, all of the
Cr(IlI) was not used up (Fig. 8.4). The rapid drop-off in the reaction rate
and lack of complete oxidation of Cr(IlI) was ascribed to part of the
reduced Mn(lI) not being released to solution as Cr(IlI) was oxidized. This
Mn(lI) was probably retained on the oxide surface and prevented fresh
Mn(IV) from being available to oxidize Cr(IlI) (Amacher and Baker,
1982). Morgan and Stumm (1964) found that y-Mn02 has a high affinity for
its ·own divalent metal ion.