Chromium is a widely used, toxic industrial metal that has been released into the environment at many sites. Chromate-laden solutions released into soils by leakage from industrial facilities or by improper waste disposal can alter the chemical environment of soils resulting in the dissolution of native soil minerals and the precipitation of new phases that incorporate Cr(VI). These precipitates can limit the mobility of Cr(VI) in the subsurface and regulate the bioavailability of Cr(VI). The identification of such precipitates can improve our estimates of the potential risks to human health and the environment at contaminated sites and greatly contribute to the rational design of remediation systems.
We have identified two iron-chromate precipitates, KFe3(CrO4)2(OH)6 (the chromate analog of the sulfate mineral jarosite) and KFe(CrO4)2*2H2O, in a soil contaminated by chrome plating solutions. The precipitates were identified by powder x-ray diffraction and electron diffraction patterns obtained under a transmission electron microscope. KFe3(CrO4)2(OH)6 was found as small (2-5 mm) crystals interspersed within the bulk soil. KFe(CrO4)2*2H2O forms crusts of larger crystals (10-50 mm) in cracks and fractures of the soil. Although both of these iron-chromate phases have been synthesized and described, to our knowledge, they have not previously been identified in the environment.
Dissolution and precipitation experiments with synthetic KFe3(CrO4)2(OH)6 and KFe(CrO4)2*2H2O were conducted to measure the solubility of these phases. The log KSP for the reaction
KFe3(CrO4)2(OH)6 + 6H+ ¤ K+ + 3Fe3+ + 2CrO42- + 6H2O (1)
at 25°C is -18.7±0.5. Ion activity products calculated from dissolution experiments at 4, 15, 25, and 35°C do not show a statistically significant trend, indicating a weak temperature dependence of the solubility over this temperature range. The log KSP for the reaction
KFe(CrO4)2*2H2O ¤ K+ + Fe3+ + 2CrO42- + 2H2O (2)
at 25°C is -19.34±0.13. Ion activity products calculated from dissolution experiments at 4, 15, 25, 35, 50, and 75°C show a non-linear trend of increasing solubility with increasing temperature. Based on the temperature dependence of the solubility product, the enthalpy of reaction, DH0r,289, is 18.8±1.7 kJ mol-1, the entropy of reaction, DS0r,298, is -310±45 J mol-1 K-1, and the heat capacity of reaction, DCp,r, over the temperature range of the experiments is -460±130 J mol-1 K-1.
The calculated thermodynamic data indicates that KFe3(CrO4)2(OH)6 is stable over a wide range of conditions and could form in large parts of a Cr(VI)-contaminated aquifer. Thus, it may control Cr(VI) concentrations in the groundwater and interfere with the cleanup of such aquifers. KFe(CrO4)2*2H2O is stable at low pH and very high Cr(VI) concentrations, typical for the immediate vicinity of a release of acidic, chromate-rich solutions, or for preferential flowpaths of such solutions within a soil.
Fig. 1: Predominance regions of KFe3(CrO4)2(OH)6, KFe(CrO4)2*2H2O, and hydrous ferric oxide, represented here as Fe(OH)3 (log KSP(Fe(OH)3 = -39), in the Fe(III)-Cr(VI)-K-H2O system (activity of dissolved Fe(III) is 10-4).