Assessment of Molybdenum and Antimony speciation in excavated rocks and soils from the Parisian basin using mineralogical and chemical analyses coupled to geochemical modelling

Abstract

In the Parisian basin area, soils excavated from public works sites often reveal naturally occurring trace elements contaminations, especially in Molybdenum (Mo) and Antimony (Sb). More specifically, the relatively strong solubility of Mo and Sb in these excavated soils prevents them from being re-used or disposed of in inert waste type landfills according to the French regulation, which is based on leaching rather than total content. Furthermore, the observed total contents are in general relatively low making it difficult to determine correctly the speciation of the trace elements. Yet, this is a key information to estimate the potential environmental risks associated to Mo and/or Sb leaching.Various direct and indirect methods of mineralogical and chemical characterizations were used in this study and coupled to geochemical modelling in the objective to determine the speciation and potential mobility of Mo and Sb in 4 samples of excavated rocks and soils from the Parisian basin. Three of them were raw materials, each one excavated in a single (but different) geological formation during classic earthworks. The fourth was a tunnel muck (a slurry) dug at the border between two geological formations and having undergone various treatments (e.g. liming) during the process of excavation. Studied samples all showed low total content of Mo and Sb. Mobility was particularly concerning for Mo in the carbonated sample (CS) and the tunnel muck (TM). Mo was found systematically carried by strontium sulfates (celestite, SrSO4) in marly limestones samples. pH dependence leaching tests coupled to geochemical modelling confirmed celestite as a major sink of Mo, while also pointing out its influence on Sb speciation. In the calcareous sample, Mo and Sb mobility was mainly controlled by complexation onto iron oxides surfaces with high pH dependency. Furthermore, in this sample, correlations between Trace Elements (TE) and Sr concentrations were assumed to occur through incorporation in carbonates rather than through associations with celestite. Associations with iron sulfides were suggested to be responsible for a small fraction of total leachable Mo and only under basic conditions. Finally, geochemical modelling was used to describe accurately Mo and Sb speciations in the tunnel muck pointing out a more complex partitioning influenced by the treatment undergone during the excavation process. Results presented here suggested that Mo and Sb speciations show great similarities and vary significantly depending on the excavated material. It is also inferred that pH changes of materials especially expected for the alkaline tunnel muck could further increase TE mobility.