ABSTRACT

Lead contamination of soils is caused by the supply of various compounds whose properties essentially influence the mobility and availability of the element. Some mechanisms of lead binding in soils can be evaluated basing on the data on the content and forms of this trace metal in soils and chemical properties of its compounds. Lead ‘species’ in soils, estimated by the procedures presented in the literature, have no unique chemical interpretation. Each method gives only some ‘operational’ values that can be compared only with that obtained with the same procedure for the soils of similar properties. The ‘exchangeable’ fraction cannot be taken as the amount of Pb(II) that can be exchanged, ‘carbonate’ fraction – as PbCO3, and so on. Basing on the data obtained with the existing methods of fractionation up to 10-70% of the total amount of lead in acid soils can exist in exchangeable form, bound either with inorganic or organic compounds. In neutral soils, the greatest part of lead may be ‘bound to Fe-Mn oxides’ and ‘organic’ fraction. In slightly alkaline soils, lead is retained as ‘carbonate’, ‘organic’, and ‘residual’ fractions. At pH <6, ion exchange can be the main process of lead retention by soils. The equivalent exchange between Ca2+ and Pb2+ occurred in soils containing halloysite, montmorillonite, vermiculite, or Alhumus complexes. Some ‘extra’ sorption was observed in soils abandoned with allophane, imogolite, iron oxides, or vermiculite-chlorite intergrades. Lead can be sorbed in the amounts that exceed the cation exchange capacity. This phenomenon hardly could be interpreted either in terms of inclusion of a single charged hydroxy species such as PbOH+, or by taking into account the precipitation of lead(II) hydroxide.