Phosphates have been used for lead immobilization in soils but the influence of soil type is not fully understood. In this work, lead chemical behaviour in two Brazilian latosoils (LA and LV) was studied via treatment with phosphates. The Pb concentration in Toxicity Characteristic Leaching Procedure (TCLP) solutions was decreased in all treatments. After treatment with H3PO4 the Pb concentration in the LA remained within the regulatory limit established by EPA. The ecotoxicological results with Daphnia pulex showed that this treatment reduced the lead bioavailability. Sequential extraction analyses showed that the lead was transferred from the most available to the residual fraction. Relevant decrease of soluble lead was observed in all phosphate treatments.

Neste trabalho, rochas fosfatadas brasileiras (PRs) foram usadas na imobilização de Pb2+, Cu2+ e Cd2+ de soluções aquosas. Concluímos que estes metais são principalmente imobilizados por fluorapatita, mas outros minerais tais com carbonatos de cálcio contribuem para esta imobilização. No caso do chumbo, dois mecanismos são propostos: a dissolução da Ca10(PO4)6F2 seguida pela precipitação de (Ca,Pb)10(PO4,CO3) 6(OH,F,Cl)2.56.1.5H2O e a dissolução de CaCO3 com formação de PbCO3. Este segundo mecanismo aumenta a eficiência do processo de remoção, mas contribui para tornar o chumbo mais biodisponível porque o carbonato de chumbo é mais solúvel que a fluorpiromorfita. Este fato pode limitar a utilização das rochas fosfatadas na regeneração de solos contaminados. A imobilização de Cu2+ e do Cd2+ pelas PRs é controlada por mecanismos de adsorção mas mecanismos de dissolução/precipitação também contribuem. No caso do Cu2+ a dissolução da fluorapatita é seguida pela precipitação da libetenita, Cu2(PO4)(OH).

In this work Brazilian phosphate rocks, PRs, were used to reduce Pb2+, Cu2+ and Cd2+ concentrations from aqueous solutions. We concluded that these metals were mainly immobilized by fluorapatite but other minerals such as calcium carbonate also contribute to their immobilization. In the case of lead two mechanisms are proposed: the Ca10(PO4)6F2 dissolution followed by the precipitation of (Ca,Pb)10(PO4,CO3) 6(OH,F,Cl)2.56.1.5H2O and CaCO3 dissolution with the formation of a PbCO3. The occurrence of this second mechanism improves the uptake efficiency but contribute to the increases of lead bioavailability because lead carbonate is more soluble than fluorpyromorphite. This fact could limit the use of PRs for remediation of contaminated soils and wastes. The Cu2+ and Cd2+ removal by PRs is controlled by adsorption mechanisms but dissolution /precipitation mechanisms also occur. In the case of Cu2+, fluorapatite dissolution followed by Cu2(PO4)(OH) precipitation.