Fe Crust In Spodosols. A Chemical Barrier towards Many Groundwater Contaminants

Sandy spodosols with an underlying iron (II)- rich groundwater may develop, below the albic horizon, an Fe oxyhydroxide rich horizon at depths where the watertable level fluctuates. The autocatalytic oxygenation of Fe²⁺, first described by Tamura et al., may further lead to the formation of an iron crust which acts as a physical barrier.We will first show how new “Pourbaix Diagrams”developped on sound thermodynamic basis allow to represent the redox potential of sorbed ²⁺ species as a function of pH.

The interactions of Fe oxides and clays with a number of chemicals such as explosives (nitro-compounds), organic contaminants (chlorinated solvants), heavy metals (U, Cr, Hg, Se) and plant nutrients (nitrates) are also strongly correlated with the surface redox chemistry of these minerals in presence of Fe²⁺. A combination of XAFS, Môssbauer spectroscopy and AFM microscopy investigations lead to the identification of reaction pathways, reaction intermediate species, such as sorbed hydrogen, and reaction products. In this presentation we will analyse a few examples, which mechanisms have been identified.

The “chemical barrier” function of the iron crust in the field is finally discussed at the light of a detailed field study, performed on a uranium contaminated military field located in the South West of France.