Identification of two iron-chromate precipitates in a Cr(VI)-contaminated soil

Two iron-chromate precipitates, KFe3(CrO4)(2)(OH)(6) (the chromate analog of the sulfate mineral jarosite) and KFe(CrO4)(2) . 2H(2)O, were discovered in a soil contaminated by chrome plating solutions. The precipitates were identified by electron microscopy and powder X-ray diffraction. KFe3(CrO4)(2)(OH)(6) was found as small crystals interspersed within the bulk soil. KFe(CrO4)(2) . 2H(2)O forms crusts in cracks and fractures of the soil. Powder X-ray diffraction of the whole soil indicates that most of the Cr(VI) in the soil is present as KFe3(CrO4)(2)(OH)(6) and that the overall amount of KFe(CrO4)(2) . 2H(2)O in the soil is relatively small. The reaction for the transformation between these two phases indicates that KFe(CrO4)(2) . 2H(2)O is likely to form in more acidic, K+- and HCrO4--rich environments than KFe3(CrO4)(2)(OH)(6). Although both of these chromate phases have been synthesized and described, to our knowledge, this study is the first report of their occurrence in the environment.