LEAD DISTRIBUTION BETWEEN SOIL GEOCHEMICAL PHASES AND ITS FRACTIONATION IN PB-TREATED SOILS

摘要

Eight horizons from four natural soils (Umbric Ac-risol (UA), Dystric Fluvisol (DF), Mollic Umbrisol (MU) and Tionic Fluvisol (TF) were treated with an acidic dissolution of Pb~(2+) with the aim of determining its sorption capacity and the distribution of Pb in the different geo-chemical phases of soil. The amount of Pb sorbed was determined, and fractioning was carried out by sequential extraction. XDR and FE-SEM techniques were also used to determine the mineralogy of the soils and the changes that occurred after treatment. The Pb content that was sorbed was found to be greater in the surface horizons rather than in the subsurface horizons, except in the Mollic Umbrisol. The majority of the Pb was sorbed in an exchangeable form (between 56.24 % in DF.A and 85.17 % in TF.G). Organic matter and amorphous Mn and Fe oxides were the soil components with greatest influence in Pb sorption. The crystalline Fe oxides and the residual fraction sorbed the least Pb (＜1%). By means of XRD and FE-SEM we verified that the acid treatment produced alteration in the soil minerals, leading to an increase in the content of amorphous and the formation of new crystalline phases such as nitratine. We also observed the Pb sorption on hydroxy compounds of Fe, which interacted with albite, muscovite and gibbsite, and the fixing of this metal in associations between organic material and gibbsite. We identified the presence of spherical and curved particles of aluminous clays with a low level of crystallinity with the ability to sorb Pb.