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TECHNICAL REPORT
Heavy Metals in the Environment

Soil Solid-Phase Controls Lead Activity in Soil Solution

^a Soil Science Dep., Faculty of Agriculture, Cairo Univ., Giza, Egypt
^b Agriculture and Environment Division, IACR-Rothamsted, Harpenden, Hertfordshire, AL5 2JQ, UK

* Corresponding author (sbadawy4{at}hotmail.com)

Received for publication September 8, 2000. 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 (R² = -0.92, P < 0.01 and R² = -0.89, P < 0.01, respectively). It could be predicted in soil solution from the equation: log = 9.9 - 2pH. 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.

Abbreviations: GF–AAS, graphite furnace atomic absorption spectrometry • DTPA, diethylenetriaminepentaacetic acid

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