Preparation and characterization of granular FeS from its nanoparticulate precursor using a freeze and thaw method

Mackinawite (FeS), an effective reagent for treating inorganic and organic contaminants, is generally produced as nanoparticles. Since these nanoparticles tend to aggregate to decrease permeability in aquifers and even cause pore clogging, it is often necessary to modify them into such forms to be adequate for preamble reactive barrier (PRB) applications. To this end, this study aimed to produce granular FeS (GFS) from its nanoparticulate precursor using a freeze and thaw method, by which the nanoparticle suspensions were subjected to freezing and then thawing. According to X-ray diffraction, the crystallinity of the resultant GFS was not changed noticeably. By an environmental scanning electron microscope (ESEM), the size of the resultant GFS particles was affected by the type and concentration of electrolytes (e.g., Na₂SO₄, NaCl, and CaCl₂). While the particle size generally increased in the order of Na₂SO₄ ≪ NaCl ≈ CaCl₂, it decreased with the electrolyte concentration. The ESEM images processed with the proposed approach here allowed us to determine the macroporosity of GFS. The GFS prepared using 100 mM Na₂SO₄ solution had the largest population of the pores with ~0.2 µm in diameter. By batch experiments, the As(III) sorption capacity of GFS at neutral to basic pH was comparable to that of the nanoparticulate precursor, suggesting the PRB applicability of GFS to remedy As(III)-contaminated groundwater under anoxic conditions.