Soil Solid‐Phase Controls Lead Activity in Soil Solution

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 (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(Pb2+) = 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 tricalcium 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.

Bioaccumulation Factor of Selected Heavy Metals in Zea mays

“…Clay minerals and other soil colloids may also influence the bioavailability of heavy metals (a study by Zhou and Li (1996) suggested that for a given soil pH, increasing the proportion of particles with a size smaller than 0.002 mm can increase the soil capacity for Zn adsorption, thereby limiting the transfer of Zn to plants). Microbes are another factor affecting heavy metals uptake by plants 79–87. The number of microbes around the roots is higher than that in other parts of the soil.…”

A model for evaluation of the phytoavailability of trace elements to vegetables under the field conditions

“…Generally, with decreasing pH, pronounced increase in release rate of Pb was observed. The lower soil pH, the higher Pb 2þ concentrations can be found in soil solution (Badawy et al, 2002;Sukreeyapongse et al, 2002). However, among these calcareous soils Pb in soils was probably from air pollution and the Pb 2þ was loosely bond with soil particles.…”

Lead toxicity thresholds in 17 Chinese soils based on substrate-induced nitrification assay

“…Thus soil pH or CEC affects the solubility and bioavailability of Pb (Adriano, 2001), making it available for uptake and bioaccumulation by an organism. Badawy et al (2002) determined the activity of Pb 2+ in near-neutral and alkaline soils and found that Pb activity ranged from 10 -6.73 to 10 -4.83 mol/L, 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). Taken together, soil pH and CEC were found to be the two most important soil factors for explaining the variance in Pb toxicity thresholds in Chinese soils.…”