| Abstract |
We studied the interaction of lead with seven Portuguese soils with different physical and chemical properties in order to elucidate more fully the behaviour of Pb in soil. We studied these adsorption phenomena by voltammetric titrations with differential pulse polarography (DPP) at different pH (6.0-7.2) and ionic strengths, I (0.010-0.50 M) in order to clarify some of the factors that might control soil sorption capacity for Pb. From the voltammetric data, average formation constants, empty set, and binding capacity, C-c, have been estimated according to a surface complexation model based on Scatchard and van den Berg-Ruzic methods. Linear Scatchard and van den Berg-Ruzic plots (r >= 0.99) indicated that the results can be interpreted according to the existence of just one predominant active site for Pb(II) adsorption. The empty set values from both procedures (log empty set approximate to 6) agreed in all cases (r = 0.938, n = 66, P < 0.001). The same happened with C-c values that were statistically equivalent (r = 0.9998; n = 66; P < 0.001). The C-c values were found to depend on the pH and I, as well as on the soil properties. Either Langmuir or Freundlich isotherms fitted the experimental data well (r > 0.90, P < 0.05). The lead binding capacities were strongly and significantly correlated (P < 0.05) with pH, cation exchange capacity, organic carbon, loss-on-ignition, total Al2O3 content, extractable forms of Al and pyrophosphate extractable Fe, [Fe-p]. From a forward, stepwise regression model we concluded that [Al2O3], [Pb ] (concentration of labile lead in solution), [Fe-p], pH and I are able to explain more than 99.7\% of the variation in lead sorption in our soils. The soils surface groups with special affinity to Pb(II) are in the inorganic fraction associated with aluminium. |
