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Fig. 8.17 Changes with time
in soluble Mn (increment)
and Eh (+). (Green et al.
2003
)
factors. Reduction and oxidation processes also may occur simultaneously in a
partially saturated soil matrix at the aggregate level: reduction into the solid-phase
aggregate and oxidation at the aggregate surface. The reduction-oxidation process
may be enhanced by biological activity or by chemical catalysis, which affects
heavy metal solubility.
Examples of the ability of Mn oxides to oxidize metals directly or to catalyze
metal oxidation have been reported widely (e.g., Lindsay
1979
; McBride
1989
,
1994
; Stumm and Morgan
1996
; Xiang and Banin
1996
; Charlatchka and Cambier
2000
). As an example, we consider a case study by Green et al. (
2003
) on solu-
bilization of manganese and other trace metals from soils irrigated previously with
water affected by acid mine runoff. The experiment was designed to simulate
waterlogged irrigated soils, saturated for short-term periods of up to 5 days.
Increases in soluble Mn were observed, which are associated with a decrease in
redox potential (Eh; see
Sect. 2.2.2
)
. Dissolved Mn concentrations in the soil water
solutions, studied after 84 h of saturation, exceeded US EPA drinking water
quality standards of 0.05 mg/L. Changes in soluble Mn and in Eh as obtained in
one replicate of the experiment are presented in Fig.
8.17
.
As seen in Fig.
8.17
, Mn was released into solution after the Eh decreased
below approximately 450 mV. Once the critical Eh (*450-500 mV) needed for
dissolution of Mn was reached, time became the limiting factor in determining
soluble Mn concentration. The soluble Mn concentration continued to increase
throughout the duration of the experiment, suggesting that equilibrium conditions
were not reached. Concentrations of Zn and Ni also increased following soil
reduction. Lack of significant correlation for Cu was explained by the formation of
complexes
with
dissolved
organic
matter
(DOM).
Increases
in
the
soluble
