Removal of Fe(II) from groundwater via aqueous portlandite carbonation and calcite-solution interactions

Abstract

Fresh groundwater is sometimes enriched with dissolved ferrous iron (Fe(II)) that restricts its consumption as potable water because it forms colloidal red matter (mainly ferric oxyhydroxides) under oxic conditions at near neutral pH (>6) conditions. As already demonstrated, natural or synthetic calcite material can be used to accelerate the iron oxidation process from Fe(II) to Fe(III), a process that then enhances its precipitation at the calcite-solution interface as confirmed by in situ atomic force microcopy (AFM) observations in this study. The present study mainly reports on a simplified water treatment method to remove ferrous iron (Fe(II)) from water via aqueous carbonation of calcium hydroxide (Ca(OH)2) at ambient temperature (≈20 °C) and moderate CO2 pressure (10 bar) conditions. In practice, high concentrations of dissolved iron (up to 100 mg/L) can be successfully removed using only 4 g of Ca(OH)2 per liter of Fe-rich solution (close to 100% of efficiency) and a short treatment time is required (<1 h). This method offers various advantages compared with other calcite-based water treatments. For example, other pre-existent dissolved toxic and eutrophic ions such as As, Cu, Cd, Se, P, S, N, etc. can be simultaneously removed from water during the precipitation of calcite and iron oxyhydroxide nanoparticles (<100 nm). Additionally, the dissolution of calcium hydroxide prior to the carbonation process increases the pH (12.4), a process that can act as a softening agent in the water being treated. Finally, the resultant red solid-residue containing mainly calcite and iron oxyhydroxide (FeOOH type) nanoparticles could be reused as pigment or mineral filler powder for industrial applications. This integrated method could be used successfully to remove toxic dissolved ions from water while generating solid residues with industrial uses.