Partitioning of Pb(II) during goethite and hematite crystallization: Implications for Pb transport in natural systems

The interaction (e.g., adsorption and incorporation) of Pb with iron(III) (oxyhydr)oxide minerals has a significant influence on its partitioning and transport in many natural systems (e.g., rivers). The incorporation of Pb during ferrihydrite crystallization to hematite and goethite at neutral and alkaline pH, in the presence and absence of sulphate SO 4 2 - has been studied using X-ray Absorption Spectroscopy (XAS), X-ray Powder Diffraction (XRD), electron microscopic techniques and chemical extraction procedures. The XRD data showed that hematite and goethite were the end-products of crystallization at pH 5, whereas goethite was the sole product at pH 13. The Pb partitioning data revealed that upon crystallization at pH 5, ∼60% of the initially adsorbed Pb remained on the surface of the crystalline hematite/goethite, while ∼20% became incorporated with the remaining ∼20% released back into solution. Lead incorporation occurred primarily during the initial stage of ferrihydrite crystallization prior to hematite/goethite formation at pH 5. The presence of SO 4 2 - at pH 5 had little influence on the partitioning of Pb or mineral phases formed. At pH 13, 52% of the adsorbed Pb was incorporated during crystallization to goethite. Lead incorporation into this phase occurred over the entire crystallization process with adsorbed Pb incorporated during goethite crystal growth. X-ray Absorption Spectroscopy and unit cell size data demonstrated that Pb did not replace Fe within the structure of hematite or goethite, but was incorporated into defects or nanopores within the iron (oxyhydr)oxides.