Adsorption-desorption reactions of lead in soils are affected by surface chemistry and solution composition. This study evaluated the effect of pH upon lead adsorption-desorption by A-horizon samples of Brazilian Oxisols. Soil suspension was previously titrated with either 7 mmol HNO3 or saturated Ca(OH)2 to determine the amount of acid or base necessary for pH adjustment. The effect of pH on Pb adsorption was evaluated after a 72 h-reaction of the soil samples with 0.15 mmol L-1 Pb(NO3)2 at pH 4.5, 5.5, and 6.5, using Ca(NO3)2 as the background solution (soil:solution ratio 1:100; ionic strength 15 mmol L-1). Lead desorption was measured after a 72 h-reaction of the soil samples with 5 mmol L-1 Ca(NO3)2 at pH 5.5. Increasing pH from 4.5 to 6.5 caused adsorption to increase up to 16.7 times, with an average of 2.9 times increase from pH 4.5 to 5.5, 1.4 times increase from pH 5.5 to 6.5 and 4.2 times increase from pH 4.5 to 6.5. The ratio Pb adsorbed/Pb added averaged 0.33 at pH 4.5, 0.75 at pH 5.5, and 0.94 at pH 6.5. The fraction of Pb desorbed (Pb desorbed/Pb adsorbed) decreased as the previous adsorption-pH value increased, averaging 0.36 at pH 4.5, 0.22 at 5.5, and < 0.06 at pH 6.5. The effect of soil properties upon Pb adsorption-desorption decreased as pH increased. Lead adsorption was positively correlated (and generally fractional desorption was negatively correlated) with specific surface area, kaolinite, hematite, oxalate- and DCB-Fe, and sulfuric acid digestion-Si and -Fe contents. The fact that a considerable fraction of Pb still remained adsorbed at pH 4.5 is noteworthy, as this may reduce lead availability in Oxisols even at low pH.
heavy metal; tropical soils; pollution