The mobility and speciation of lead and cadmium in Bahr El Baqar region, Egypt

• Components and clay minerals of the studied soils were identified using XRF and XRD analyses.
• Metal sorption capacity of soils was due to their smectite minerals, amorphous Fe oxides and OM.
• Residual, oxides and organic fractions of Pb were the dominant species in the studied soils.
• Cd binds to carbonates lead to its greater mobility and availability to the plants.
• Langmuir and Freundlich models isotherm were fitted well for Pb and Cd adsorption by the studied soils.

Under continuous irrigation with Bahr El Baqar drain water in Egypt, chemical speciation of Pb and Cd in soils is critical to understand, their mobility and potential toxic effects. In the present study, sorption isotherm experiments of these metals in nine representative soils were assessed at varying metal concentrations (20, 40, 60, 80 and 100 mg l−1), solid to liquid ratio 5 g/50 ml and contact time (24 h). Additionally, the kinetic experiments were performed using initial concentration of 100 mg l−1 Pb or Cd and contact time of 2, 4, 8, 12 or 24 h. The two experiments were carried out at 298 K. Low amounts of Pb were found in exchangeable and carbonate fractions comparing with the residual, oxides and organic fractions for the most studied soils. On the other hand, the residual fraction of Cd speciation was dominant for all the studied soils, followed by carbonate fraction. Both Langmuir and Freundlich models isotherm for Pb and Cd adsorption by these soils were fitted. The second-order kinetic model was the best fit model for the adsorption mechanism in these soils. These soils are still able to be retained and more loaded with Pb and Cd, since they did not reach their saturation capacities. Taken together, continuous accumulation of these toxic metals will be exposed to the environment to real disaster under bad management in this region.