Soil, lead surface binding

Lead is considered to be a non-essential metal to plants, and only a small proportion of the lead in soils is biovailable to plants (Alloway, 1990). Visible symptoms of toxicity, though unspecific to Pb, are smaller leaves and a stunted growth. Leaves may become chlorotic and reddish with necrosis and the roots may turn black. Several plant species, ecotypes and bacterial strains have been known to develop Pb tolerance. The phytotoxicity of Pb is low as it has very limited availability and uptake from soil and soil solutions. However, plant roots are usually able to take up and accumulate large quantities of Pb2+ in soil and culture solutions but translocation to aerial shoots is generally limited due to binding at root surfaces and cell walls (Lagerwerff, 1971 Jones et al., 1973 Lane and Martin, 1977). 

Lead within soils is distributed between solid and liquid phases, with the latter of major importance to the issue of lead bioavailability, for example, to plant roots where uptake can occur. Studies of lead species in this liquid mobile phase indicate that they exist as both complexed and ionic forms although the latter as simple ions are present in very low concentrations. The extent to which lead can move through soils, in turn, is the extent to which lead binds to insoluble organic and mineralogical inorganic species. The former are typically humic and fulvic acid derivatives, and the latter are surfaces of clays and metal oxides (U.S. EPA, 1986). The factors most important for lead movement within soils are pH, cation exchange capacity of... 

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