Reductive transformation of hexavalent chromium by ferrous ions in a frozen environment: Mechanism, kinetics, and environmental implications.

The transformation between hexavalent chromium (Cr) and trivalent chromium (Cr) has a significant impact on ecosystems, as Cr has higher levels of toxicity than Cr. In this regard, a variety of Cr reduction processes occurring in natural environments have been studied extensively. In this work, we investigate the reductive transformation of Cr by ferrous ions (Fe) in ice at -20 °C, and compare the same process in water at 25 °C. The Fe-mediated reduction of Cr occurred much faster in ice than it did in water. The accelerated reduction of Cr in ice is primarily ascribed to the accumulation of Cr, Fe, and protons in the grain boundaries formed during freezing, which constitutes favorable conditions for redox reactions between Cr and Fe. This freeze concentration phenomenon was verified using UV-visible spectroscopy with o-cresolsulfonephthalein (as a pH indicator) and confocal Raman spectroscopy. The reductive transformation of Cr (20 µM) by Fe in ice proceeded rapidly under various Fe concentrations (20-140 µM), pH values (2.0-5.0), and freezing temperatures (-10 to -30 °C) with a constant molar ratio of oxidized Fe to reduced Cr (3:1). This result implies that the proposed mechanism (i.e., the redox reaction between Cr and Fe in ice) can significantly contribute to the natural conversion of Cr in cold regions. The Fe-mediated Cr reduction kinetics in frozen Cr-contaminated wastewater was similar to that in frozen Cr solution. This indicates that the variety of substrates typically present in electroplating wastewater have a negligible effect on the redox reaction between Cr and Fe in ice; it also proposes that the Fe/freezing process can be used for the treatment of Cr-contaminated wastewater.