Effect of ionic strength of the equilibrium solution upon lead adsorption/desorption in Brazilian Oxisols

The ionic strength of soil solution influences metal availability and mobil ity in soils. Laboratory experiments were conducted in order to evaluate th e effect of solution ionic strength (I) upon Pb adsorption/desorption in A- horizon samples of 17 Brazilian Oxisols. The effect of I upon Pb adsorption was evaluated after a 72-hour reaction of the soil samples with 0.15 mmol L-1 Pb(NO,), at pH 5.5, using 5 and 50 mmol L-1 Ca(NO3)(2) as background so lutions (I = 15 and 150 mmol L-1). Lead desorption was measured after a 72- hour reaction of the soil samples with 5 mmol L-1 Ca(NO3)(2) at pH 5.5. All experiments were performed with a 1:100 soil:solution ratio. Increasing I caused Pb adsorption to decrease in six soils, while no significant change in Pb adsorption was observed for the remaining 11 Oxisols. Soil properties such as specific surface area, CEC, clay and hematite content, oxalate- an d DCB-Fe, and sulfuric acid digestion-Si and -Fe correlated with Pb adsorpt ion equally for both values of solution ionic strength. Changing I had a sm all effect on Pb desorption. The little change in the amount of Pb adsorbed upon variation of I indicates that Pb is adsorbed as an inner-sphere compl ex in most Oxisols. Therefore, this experiment suggests that Pb may not be part of the exchange complex of these soils.