Fjölrit RALA - 15.06.2004, Page 59

Heavy metal ad- and desorption of andic and non-andic horizons
R. Jahn and H. Tanneberg
Institute of Soil Science & Plant Nutrition, Univ. of Halle, Germany
Introduction
Andosols have a high binding capacity for heavy metals which may be naturally accumulated
or anthropogenically derived. These metals are bound in mineral structures, on mineral
surfaces, and complexed by organic matter. Some trace elements may be so strongly bound
that they become unavailable to plants. But in this respect, information is very scarce, but
extremely important for environmental protection. The dominance of variable charge sites in
Andosols has driven us to study the pH-depended sorption and desorption of trace elements.
Materials and Methods
Horizons of COST- profiles of Italy (EUROl, Humi-Tephric Regosol (Eutric); EUR03, Fulvi-
Silandic Andosol (Dystric)) and the
Azores (EUR06, Hydri-Silandic Andosol
(Umbric and Acroxic)) were selected
based on distinct differences of contents
in organic matter and short range ordered
minerals (see Tab. 1). For comparison,
an Ap-horizon of a Siltic Chemozem
(Germany) without short range ordered
minerals was also included in the study.
Sorption kinetics as well as adsorption
and desorption studies was done in
accordance with the OECD Guideline
106. For the sorption and the desorption
studies, modifications in the methodo-
logy was done due to limited availability
of materials. For sorption kinetics 2g soil
(based on dry weight) was added with 50 ml 0.01 M Ca(NOs)2 solution containing 300 mg of
Cd, Pb, Cu, Ni, and Cr as chloride. At an interval of 10 to 4 days, 0.4 ml solution was taken
for the determination of sorbed metals. The sorption behaviour was evaluated by Batch
procedure (48 hours) at the pH value of the soil as well as in soil samples with pH adjusted to
pH 4 and pH 7. The concentration of the heavy metals in the M Ca(N03)2 solution occurred
between 1 and 2000 mg L'1. Desorption was evaluated on the samples with the highest metal
loading. The samples were shaken in 20 ml M Ca(N03)2 solution 10 times. Quantification of
the metal concentration was done by flame-atomic absorption spectrophotometry and
inductively coupled plasma.
Results
From the sorption kinetics it can be concluded that in all samples the sorption of trace
elements is govemed by ion association, ion exchange and rapid sorption within a few hours.
After only 30 minutes, 75 % Cd, 90 % Pb, 65 % Cu and 75 % Ni of the existing amount were
sorbed. It was only with Cr that sorption was found slower. The sorption capacity of the
individual horizons varied, was also dependent on the kind of metal and was generally high.
Without any exception no clear maximum of sorption was reached up to a maximum concen-
tration of 2000 mg L'1 in the solution. The Ap horizon of the Chemozem showed the strongest
Tab. 1 Selected data of the investigated soil
samples (COST-data)
Soil Depth cm Hor. pH KCl Corg Allophane % % (Si0 ■ 7.1)
EUR01 0-16 Ap 4.7 2.7 0.7
EUR01 54-95 Bw2 5.2 0.7 0.5
EUR03 0-22 A1 4.9 6.7 7.7
EUR03 48-70 AB 5.4 2.1 13.4
EUR03 98-125 Bw2 5.5 0.6 13.2
EUR06 0-20 Ah 4.6 19.1 6.4
EUR06 40-60 2AB 5.4 11.0 15.1
EUR06 100-120 2Bw3 5.7 4.5 25.6
Chemozem 0-24 Ap 7.5 2.0 0.0
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