IMMOBILIZATION OF MERCURY(II) BY ETTRINGITE-TYPE PHASES: MODELING AND EXPERIMENTS

Reactive amendments such as Portland and super-sulfate cements offer a promising technology for the immobilization of problematic inorganic soil and sediment contaminants through sequestration in recalcitrant neophases. Ettringite (Ca₆[Al(OH)₆]₂(SO₄)₃.26H₂O) is an important mineral formed during cement hydration that can accommodate both divalent cations and oxyanion species in its structure. The ability of ettringite-type phases to sequester mercury (as Hg(II)) was investigated theoretically and experimentally in co-precipitation and substrate amendment experiments. Batch experiments were designed and analyzed with a kinetic-equilibrium model for the evolution of solid phases and water chemistry. The Lawrence Livermore National Laboratory thermodynamic database was augmented with the cement hydration model of Lothenbach and Winnefeld (2006) and updated with data for aqueous- and solid-phase mercury species. Experimentally, Al-ettringite was synthesized and co-precipitated in the presence of Hg(II) (0.1 mM). Portland cement type V (PCV) and ferrous sulfate (FeSO₄) were added to different substrates (quartz, quartz and clay minerals, natural sediments), reacted with Hg(II) solutions (0.001-0.25 mM) and aged for 1 to 90 days.

Reaction products were characterized by bulk sequential extraction (ion-exchangeable and poorly crystalline fractions) and by spectroscopic methods. Geochemical modeling accurately predicted the formation of ettringite and calcium silicate hydrate (C-S-H) gels as a function of reaction time. X-ray diffraction showed ettringite formation after 1 d of substrate reaction with PCV and FeSO₄. Extraction results indicated that after 1 d of reaction, 80% of Hg was retained in the solid phase and 5% was associated with the exchangeable fraction. Similar results were found after 7, 30, and 90 d. In synthesized ettringite, exchangeable Hg was about 2% of the total Hg retained. Structural characterization using synchrotron microfocused XRF/XRD and bulk XAS indicated Hg substitution for Ca(II) in the ettringite structure. Mercury EXAFS of natural sediments after 7 d of treatment was similar to the Hg co-precipitated ettringite, suggesting a similar immobilization mechanism of structural exchange of Hg for Ca.