Environmental Engineering Reference
In-Depth Information
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Fig. 3
Some parameters in La Silva Stream as a function of distance
Secondly, in the case of Al, a similar process as with iron occurs. The Al con-
centration decreases from 234 mg/L at pH 3.2 to 4.54 mg/L at pH 5.2 by means of
hydrolysis and precipitation of Al over a stretch of nearly 8,000 m at the lower
reach of the stream, coinciding with the final hydrolysis range of this element (pH
from 3.5 to 5).
Thus, La Silva Stream has a slim layer of ochre precipitates all over its bed as a
consequence of the oxidation of Fe
2+
and the hydrolysis of Fe
3+
and Al
3+
. This layer
includes amorphous (in its initial phase) precipitate like hydroxides (ferrihydrite
and gibbsite) and oxyhydroxysulfates (schwertmannite, hydrobasaluminite and
alumina) with significantly quantities of silica and sulphate, and even little adsorbed
quantities of Cu, Zn, As, among others.
Acidity in La Silva Stream
The acidity produced by the ARD is due to the protons H
+
released to hydrolyzed
metals in solution:
