Impact of ageing on the leaching behavior of a calcareous sample excavated from Grand Paris Express construction sites and naturally contaminated in Molybdenum and Selenium

One of the main consequences of digging works for the Grand Paris Express (GPE) project will be the excavation of a huge amount of earths, representing a 20% increase of the total wastes produced in the area over ten years. These earths are known to be naturally contaminated in Trace Elements (TE) such as Molybdenum (Mo) or Selenium (Se). Even though only small levels of TE are involved, earths leaching behavior often prevent public works companies from storing them in Inert Wastes Landfill. Indeed, results obtained according to the standardized leaching test NF EN 12457-2 with a liquid/solid ratio of 10 L.kg^-1 are likely to overpass acceptation criteria fixed by European regulations (Council Decision 2003/33/EC).

To avoid extra charges linked with earth’s evacuation, a chemical stabilization process may be efficient and industrially applicable (Geng et al., 2013 ; Manning and Burau, 1995), even for TE like Mo and Se known to be highly mobile in alkaline matrices such as GPE excavated earths (Rashid et al., 2002). However, the current French regulation does not allow stabilized materials to be considered as inert wastes, mainly because of the lack of guarantee on the long-term efficiency of such treatments (Coussy et al, 2014).

We present here the long-term impact of minerals amendments (3%wt maximum) on the leaching behavior of a calcareous sample coming from a GPE construction site. Standardized leaching tests (NF EN 12457-2) have been carried out before, after stabilization, and after specific ageing experiments. Leachate concentrations have also been followed during ageing experiments carried out on raw and stabilized samples. The ageing protocol was based on soxhlet extraction principle (Benzaazoua et al., 2004) consisting in multiplying humectation/desiccation cycles at approximatively 70°C for 10 days in a closed environment. Standardized leaching tests have shown that Mo and Se concentrations sharply decrease after stabilization in every case, showing concentrations below Inert Wastes landfill criteria (0.1 ppm and 0.5 ppm for Se and Mo respectively). On the other hand, variable impacts of ageing have been observed depending on the TE considered. For Mo, the best results have been obtained with a zero-valent iron amendment, leading to the retention of almost 90% of the total Mo in a calcareous sample after ageing. For Se, all scenarii studied have permitted a retention of at least 90% of total Se or more. The promising results presented here will have to be confirm by attesting the nature of bonding between stabilizing agent and TE.