Comparative study of zero-valent mercury immobilization by solidification/stabilization in a limestone backfill model

There is a lack of specific treatments to mitigate medium-range mercury concentrations in soil in the context of mercury-based industrial sites decommission. Treatability tests were carried out to assess the immobilization of Hg(0) using in situ solidification-stabilization (S/S). Various chemical treatments were investigated. They were based on solidifying agents and sulfur-based mercury stabilizers, used individually and in combination. These treatments were applied to a limestone backfill model material (BM), at concentrations around 1000 mg kg^-1 of Hg(0). Although its ultimate purpose was reagent injection, the treatment was performed through mixing to minimize result variability and enable a robust assessment of the applied chemical treatments. Chemical treatments were assessed considering the Hg-mobility from the treated BM, its mechanical strength, and its residual permeability. The apparently conflicting objectives of mercury sequestration, solidification of a macro-porous material, and maintenance of BM permeability were achieved using very low reagent concentrations ranging from 0.1 to 4% w/w. The study revealed the synergy operating between solidifying and stabilizing agents. After 5 contact days, reductions of up to 600-fold in GEM release and up to 80-fold of total mercury in leachates were observed relative to the untreated contaminated BM, whereas the permeability of the treated BM was only reduced by half.