Precipitates on granular iron in solutions containing calcium carbonate with trichloroethene and hexavalent chromium

Mineralogical examination, using scanning electron microscopy (SEM), X-ray diffractometry (XRD), and optical microscopy, was conducted on the Fe ⁰-bearing reactive materials derived from long-term column experiments undertaken to assess the treatment capacity of Fe⁰ under different geochemical conditions. The columns received either deionized water or solutions of differing dissolved calcium carbonate concentrations, together either with trichloroethene (TCE) or hexavalent chromium (Cr(VI)). The major reaction product in the columns receiving deionized water was magnetite-maghemite, and for the columns receiving dissolved calcium carbonate, the main products were iron hydroxy carbonate and aragonite. Replacement of Fe⁰ by reaction products occurred mainly at the edges of the Fe ⁰ particles, and penetrative replacement was focused along cracks and along and around graphitic inclusions. Fibrous or flake-shaped iron hydroxy carbonate mostly replaced the edges of the Fe⁰ particles. Aragonite had needle-shaped morphology, and some occurred as clusters of crystals. Aragonite was deposited on iron hydroxy carbonate, thus providing at least a partial armoring effect. The mineral was also observed to cement groups of Fe⁰ particles into compact aggregates. The Cr was present mostly as Cr(III) in Cr(III)-Fe(III) (oxy)hydroxides and in trace amounts in iron hydroxy carbonate.