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This article in JEQ

  1. Vol. 31 No. 1, p. 162-167
     
    Received: Sept 8, 2000
    Published: Jan, 2002


    * Corresponding author(s): sbadawy4@hotmail.com
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doi:10.2134/jeq2002.1620

Soil Solid-Phase Controls Lead Activity in Soil Solution

  1. S. H. Badawy *ab,
  2. M. I. D. Helala,
  3. A. M. Chaudrib,
  4. K. Lawlorb and
  5. S. P. McGrathb
  1. a Soil Science Dep., Faculty of Agriculture, Cairo Univ., Giza, Egypt
    b Agriculture and Environment Division, IACR-Rothamsted, Harpenden, Hertfordshire, AL5 2JQ, UK

Abstract

Lead pollution of the environment is synonymous with civilization. It has no known biological function, and is naturally present in soil, but its presence in food crops is deemed undesirable. The concern regarding Pb is mostly due to chronic human and animal health effects, rather then phytotoxicity. However, not much is known about the chemistry and speciation of Pb in soils. We determined the activity of Pb2+, in near neutral and alkaline soils, representative of alluvial, desertic and calcareous soils of Egypt, using the competitive chelation method. Lead activity ranged from 10−6.73 to 10−4.83 M, and was negatively correlated with soil and soil solution pH (R 2 = −0.92, P < 0.01 and R 2 = −0.89, P < 0.01, respectively). It could be predicted in soil solution from the equation: log (Pb 2+ ) = 9.92 pH A solubility diagram for the various Pb minerals found in soil was constructed using published thermodynamic data obtained from the literature, and our measured Pb2+ activities compared with this information. The measured Pb2+ activities were undersaturated with regard to the solubility of PbSiO3 in equilibrium with SiO2 (soil). However, they were supersaturated with regard to the solubilities of the Pb carbonate minerals PbCO3 (cerussite) and Pb3(CO3)2(OH)2 in equilibrium with atmospheric CO2 and hydroxide Pb(OH)2 They were also supersaturated with regard to the solubilities of the Pb phosphate minerals Pb3(PO4)2, Pb5(PO4)3OH, and Pb4O(PO4)2 in equilibrium with tri-calcium phosphate and CaCO3 The activity of Pb2+ was not regulated by any mineral of known solubility in our soils, but possibly by a mixture of Pb carbonate and phosphate minerals.

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Copyright © 2002. American Society of Agronomy, Crop Science Society of America, Soil Science SocietyPublished in J. Environ. Qual.31:162–167.