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Abstract "Heft 4"

Freiburger Bodenkundliche Abhandlungen

Schriftenreihe des
Institut für Bodenkunde und Waldernährungslehre
der Albert-Ludwigs-Universität Freiburg i.Br.
Schriftleitung: F. Hädrich

Heft 4

Ernst E. Hildebrand

Die Bindung von Immissionsblei in Böden

Freiburg im Breisgau 1974

ISSN 0344-2691


The aim of the present work was to investigate the interactions of lead with pedogenetic soil adsorption complexes, in order to get criteria for the valuation of lead accumulation in soils along highways and highly frequented roads.
To do this, isolated soil humic acids of different soil types, Oh -humus of a podzol, the clay minerals montmorillonite, illite and kaolinite and synthetisized Fe-oxides were treated by PbCl2-solutions at different pH levels. The quantitative lead fixation and the stability of the resulting lead compounds were investigated by chemical and physical methods.
The results, including the identification of the configuration of humic acid-lead,- clay mineral-lead- and iron oxide-lead -binding rendered it possible, to set up reaction schemes of lead adsorption and -desorption, depending on the adsorbing material and pH level. These results, regarded as hypothesis for the behaviour of lead in soils and ecosystems, were proved by applying different extraction methods on lead contaminated soils and soil -similar systems (sewage sludge, treated waste). Following characteristica of lead binding can be pointed out:

1) H u m u s: The soil organic matter contributes mostly to the immobilisation of lead in soils, due to its occurence in top soils and its high lead affinity. The influence of humus genesis on lead sorption capacity  can be explained to a high extend by its degree of polymerization (content of acid -precipitable humic acids and within this fraction content of grey humic acids)
Coordinative binding at groups with free electron pairs (e.g. carbonylgroups) by completing the coordination sphere of lead is regarded to be the most important reaction of lead adsorption by humus.
Besides of this , salt formation also takes place at polar groups (e.g. carboxylgroups).
According to these types of binding, the lead sorption capacity and the stability of humus-lead -binding decrease with decreasing pH.
Due to the prevailing of coordinative lead-humus -binding,lead can be extracted to a hihgh degree by solutions with high complexing energy (e.g. EDTA) from soil samples, rich in organic matter.

2) c l a y  m i n e r a l s: The low, but unequivocally comprehensible part of clay minerals in lead fixation by soils is based on two types of adsorption, depending on pH conditions. At pH 7 and>7  chemical  properties of clay minerals and lead are responsible for the interactions, which occur. Lead is adsorbed by surface SiOH,- A1OH - and A10H2 -groups, a reaction analogous to neutralisation.The lead, thus adsorbed, can not be liberated by exchange with cations but by acid hydrolysis. This kind of lead binding does not depend on the mineral type and causes a common high stability of the clay mineral/lead -binding at pH 7 and>7?
At pH-<6.5 physical  conditions determine the lead binding by clay minerals. Based on electrostatic interactions, lead is adsorbed in different proportions by outer and inner mineral surfaces, according to the mineral type. The cation exchangeable part' of the adsorbed lead decreases with increasing participation  of inner mineral surfaces  in lead adsorption.
Due to the K -similar ionic diameter of Pb, montmorillonite and illite  with low lead saturation  show a high lead selectivity, as it occurs with potassium fixation. Moreover, there is a specific of illite towards lead, based on a steric sorption preference  analogous to potassium.
In lead contaminated soils, free of carbonates and at a pH range 6-3, lead binding by clay minerals is physisorptive predominantly. This causes negative correlations of % Ct and pH with the percentage of extractable lead by exchange solutions.

3) F e-o x i d e s: Below the isoelectric range, lead is adsorbed by amorphous Fe -hydroxide, goethite and hematite and can not be liberated by cation exchange. The chemisorp-tive reaction is based on the high hydroxyl affinity of Pb -ions in aquous solution . The quantitative lead adsorption depends to a higher extend on surface characteristics than on crystal structure. Under soil conditions a genesis of crystallized Fe-Pb -compounds will not take place, as the energy , necessary for its formation would be to high. The stability of Fe-oxid-lead compounds corresponds to the clay inineral-lead-binding at pH 7 and 7.
The effect of soil sorption complexes on lead binding is highly influenced by pH conditions. With increasing pH an increasing percentage of acetic-acid extractable lead is found. This lead fraction can not be regarded as specific for organic or inorganic soil sorption complexes, as both show a tendency  to rising chemisorptive lead adsorption with increasing pH.
From these findings, the conclusion can be drawn, that by elevation of pH and increasing the stock of organic matter leao immobilisation in soils can be increased.
The tendency of lead to accumulate in surface
horizons by interacting with soil adsorption complexes is the reason for the high correlation of lead concentration in top soils with the age of the road.
With regard to the lead behaviour in soils, the conclution is drawn, that no lead contamination of ground water will occur,extreme conditions excluded . In contrast to this ,  an increased inclusion of lead in ecological cycles mainly by plant uptake must be taken into consideration.

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