The presence of organic ligands in the rhizosphere plays an important role in heavy metal solubilization in soils treated with sewage sludge. Low molecular weight organic acids exuded by roots and/or released by microorganisms are considered mobilizing agents of these metals. The present study determined Cd, Cu, Cr, Ni, and Zn solubilization kinetics in sewage sludge-treated soils using a mixture of four organic acids (acetic, citric, lactic, and oxalic) in 0.05 and 0.10 mol L-1 solutions. Based on the batch extraction method the 0.10 mol L-1 solution extracted greater quantities of Cu, Cr, Ni and Zn. Two phases of solubilization kinetics were observed: an initial one of fast metal release, followed by a slower one tending to equilibrium. The highest velocity constants were observed for Ni and Zn, followed by Cu and Cr. The values calculated for the labile pool varied from 2 to 15% of the total content of each metal under study. The percentage of this metal pool in relation to the total content followed the order: Zn > Cu <FONT FACE=Symbol>@</FONT> Ni > Cr.
biosolids; Cr; Cu; Ni; Zn; phytoavailability
