Fjölrit RALA - 15.06.2004, Page 60

heavy metal sorption (at a heavy metal presence of 7.5 g per kg soil and ambient soil pH)
while the subsoil of the Andosol N3 showed the weakes. When one considers the horizons
with high allophane content, no unique influence can be observed. The subsoil with the
second highest allophane content (EUR03Bw2) is also the horizon with the lowest sorption
capacity.
Values of the adsorption studies were for Cd fitted into the Freundlich isotherm and from it
the adsorption constant Kf was calculated. In the case of Pb, Cu and Cr, sorption was
especially very strong at pH 7 such that no isotherm calculation could be done. Regarding Cd,
no correlation was observed between the Kf values and the allophane content of the soils. In
contrast, a high correlation was observed between Kf and pH value. In general, a
contradictory results behaviour of the horizons studied was noted. In EUR03 sorption capa-
city increased systematically with Corg content (Fig. 1 right) while in EUROl and EUR06 no
fundamental difference between the horizons existed (Fig. 1 left; for EUR06 not all horizons
are shown). The Freundlich desorption isotherms were always higher than for adsorption.
Thus, it can be said that sorption processes are not fully reversible.
Fig. 1 Cd-adsorption and desorption isotherms of the selected horizons (soil pH)
Fig. 2 shows a very strong effect of pH values and is most prominent for Cd and Ni. At pH
7, strong fixation occurred for the horizons of profile EUR06 such that Freundlich isotherm
for the AB2 could not be determined. The sorption of Cr and Pb are only less affected by pH
changes. At the pH of the soils, the horizons
varied in their sorption capacity. The highest
was the concentration of Cd in the
equilibrium solution at pH 4. At this point,
sorption capacity increased with the Corg
content of the horizons. The subsoil horizons
of the Andosol containing high amounts of
allophane had much lower sorption capacity
at pH 4 compared to the Ah horizon.
Fig. 2 Cd-adsorption isotherms at different pH
Conclusions
Volcanic soils exhibit a very high sorption capacity for heavy metals. In particular, they
exhibit strong sorption of copper, lead and chromium. The sorption of Cd and Ni is strongly
pH dependent. A relation of the sorption as well as of the desorption behaviour to the content
of allophane was not found. The high sorption capacity is mostly caused by organic matter.
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