Formation and Immobilization of Cr(VI) Species in Long-Term Tannery Waste Contaminated Soils

Chromium speciation in naturally contaminated soils appears more complex than spiked studies have shown. This study characterized Cr speciation (oxidation states; availability; molecular geometry) intended to highlight the genesis of immobile Cr(VI) species in long-term tannery waste-contaminated soils. In a series of samples obtained from Shuitou in China, chemical extraction methods showed that Cr(III) was dominant(>96.7% of total Cr), with Cr(VI) concentration up to 144 mg kg^-1. Of the total Cr(VI) present, immobile Cr(VI) represents >90%. Synchrotron-based X-ray near-edge structure spectroscopy (XANES) showed the occurrence of Cr(VI), which was not removed by phosphate buffer extraction, confirming a significant amount of immobile Cr(VI) fractions in soils. X-ray fluorescence maps exhibited the heterogeneous distribution of Cr in soils associated with both Mn and Fe. Such a distribution suggests Cr(III) oxidation to Cr(VI) by Mn oxides and a possible immobilization of both Cr(III) and Cr(VI) onto Fe (hydr)oxides. Linear combination fitting of XANES spectra revealed that fractional weights (%) in samples were CrFeO₃ (49.3-53.6), CrOOH (22.3-30.8), and CaCrO₄ (13.2-25.3). Our results demonstrate that (i) Cr(VI) is immobilized in soils and (ii) mechanisms of Cr(VI) immobilization are CaCrO₄ precipitation and recrystallization with Fe (hydr)oxides.