Identification of molybdenum immobilization mechanisms in naturally contaminated excavated rocks and soils stabilized with zero-valent iron

Abstract

Chemical stabilization of naturally contaminated excavated materials could be used to limit environmental risks. Nevertheless, due to the low levels of contamination in such materials a more precise characterization of immobilization mechanisms is required. To overcome this lack of data, zero valent iron (Fe⁰) was used as a stabilizing agent at rates of 1 or 3 wt% in four different sets of excavated rock and soil from the Paris basin that were contaminated with low levels of molybdenum (with a total content of from 2 to 11 mg Mo kg⁻¹) but that involved leachability issues. Molybdenum (Mo) release was reduced by at least one order of magnitude after adding Fe⁰ over a wide pH range (i.e. 2 < pH < 12). Geochemical modeling of pH-dependent leaching tests was done to identify potential immobilizing mechanisms. For Mo, experimental results were well represented at pH close to and below the natural pH of the samples by considering surface adsorption with iron (hydr-)oxides involving non-, mono- and bi-protonated Mo surface species. Zero valent iron stabilization remained relatively significant under alkaline conditions where complexation of anionic species was not favored. These results suggested that stronger binding mechanisms are likely to occur concomitantly with surface adsorption. Finally, thanks to their high sensitivity, μ-particle-induced X-ray emission (μ-PIXE) and μ-Rutherford backscattering spectroscopy (μ-RBS) were successfully used to directly observe Mo presence in iron (hydr-)oxides formed after addition of Fe⁰.