Identification of Two Iron− Chromate Precipitates in a Cr(VI)-Contaminated Soil

Two iron−chromate precipitates, KFe 3 (CrO 4 ) 2 (OH) 6 (the chromate analog of the sulfate mineral jarosite) and KFe(CrO 4 ) 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. KFe 3 (CrO 4 ) 2 (OH) 6 was found as small crystals interspersed within the bulk soil. KFe(CrO 4 ) 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 KFe 3 (CrO 4 ) 2 (OH) 6 and that the overall amount of KFe(CrO 4 ) 2 ·2H 2 O in the soil is relatively small. The reaction for the transformation between these two phases indicates that KFe(CrO 4 ) 2 ·2H 2 O is likely to form in more acidic, K + - and HCrO 4 - -rich environments than KFe 3 (CrO 4 ) 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.