7
maintained for 8d. Additionally to pH, Fe and Ca were determined in samples from the
underlying water.
Results
Sediment has a neutralizing capacity for acids and alkalis. The application of decreasing
conditions of pH caused that the pH lowered and each time increased to values near to 7.5 even
when the pH was lowered to 6.5 confirming the capacity of the wetland sediment to neutralize
acids. Dissolved phosphate concentrations were not detectable and Ca concentration increased in
the water with the addition of acid possibly to Ca minerals dissolution as CaCO
3
(s). Fe in the
water column concentration did not vary significantly with respect to the controls. Based on the
above, the wetland has the ability to buffer small changes in pH, which could be produced by the
input of acid rain or polluted water (Otte y Jacob, 2006).
Figure 1 shows the results obtained from the experimental columns. Phosphate mobility between
the sediment and water has been associated to dissolution and precipitation processes with Ca and
Fe. In our work, phosphate concentrations decreased under reductive conditions and increased
under oxidizing conditions. This behavior seemed to be related to redox processes affecting S and
so Fe; the mobility of the last one was not clearly detected however under reducing conditions the
concentrations of Fe were below the detection limit of the analytical method. Iron concentration
in the solution appears to be regulated by precipitation with sulfides in reducing conditions and
by the Fe hydroxides formed under oxidizing conditions. In this process, PO
4
3-
is precipitated
with Fe in reducing conditions (as Fe
3
(PO
4
)
2
(s)) and dissolved when this element is oxidized
(Fe(OH)
3
(s)). However, even if we haven’t determined Ca in these experiments, PO
4
3-
could also
be mobilized when the pH decrease affecting the Ca minerals dissolution. This in congruent with
our results because the initial pH first increased until pH 9.9 ± 0.2 under reducing conditions and
decreased to 8.4 ± 0.4 under oxidizing conditions. The mobilization of the phosphorus was more
evident in oxidizing conditions (stage 3) and when Fe was added (stage 5), due to the interaction
with iron compounds and precipitating as phosphate of iron or calcium. The reactivity of Fe
bound P was assumed to depend on redox potential and pH (Gomez et al., 1999).
The formation of iron sulfides compounds controls the P mobilization, while Ca can have an
important role to control phosphates in the wetland. The reduction of sulfates would suggest the
formation of sulfides. In this work, the ratio (
Σ
[HS
-
]/SO
4
2-
) was 2.5x10
-5
, extremely high
considering the pH (>8) and pE (-2.08 to -1.45V) determined in the experimental columns. High
sulfide concentrations as those determined (1.5x10
-4
a 3x10
-5
M; stage 2) in this work required
high sulfate concentrations (1.6x10
29
, and pH < 6; according to Ec 1); neither the FeS
2
(s)
dissolution could explain sulfides determined. Based on the above, it is possible that the co-
existence of sulfides with sulfates is due to a slow redox kinetic for S in the physico-chemical
conditions of the sediment columns. This probably reflects the natural conditions of the wetland
because a similar situation has been observed in the sediments porewater in the lake (unpublished
data).
SO
4
2-
+ 9H
+
+ 8e
-
↔ HS
-
+ 4H
2
O; K = 10
34
(1)
Roden and Edmonds (1997). Hydrobiology, 139: 347–378.
Giordani et al. (1996). Hydrobiology, 329: 211–222
