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

The transformation between hexavalent chromium (Cr⁶⁺) and trivalent chromium (Cr³⁺) has a significant impact on ecosystems, as Cr⁶⁺ has higher levels of toxicity than Cr³⁺. In this regard, a variety of Cr⁶⁺ reduction processes occurring in natural environments have been studied extensively. In this work, we investigate the reductive transformation of Cr⁶⁺ by ferrous ions (Fe²⁺) in ice at -20 °C, and compare the same process in water at 25 °C. The Fe²⁺-mediated reduction of Cr⁶⁺ occurred much faster in ice than it did in water. The accelerated reduction of Cr⁶⁺ in ice is primarily ascribed to the accumulation of Cr⁶⁺, Fe²⁺, and protons in the grain boundaries formed during freezing, which constitutes favorable conditions for redox reactions between Cr⁶⁺ and Fe²⁺. 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 Cr⁶⁺ (20 µM) by Fe²⁺ in ice proceeded rapidly under various Fe²⁺ concentrations (20-140 µM), pH values (2.0-5.0), and freezing temperatures (-10 to -30 °C) with a constant molar ratio of oxidized Fe²⁺ to reduced Cr⁶⁺ (3:1). This result implies that the proposed mechanism (i.e., the redox reaction between Cr⁶⁺ and Fe²⁺ in ice) can significantly contribute to the natural conversion of Cr⁶⁺ in cold regions. The Fe²⁺-mediated Cr⁶⁺ reduction kinetics in frozen Cr⁶⁺-contaminated wastewater was similar to that in frozen Cr⁶⁺ solution. This indicates that the variety of substrates typically present in electroplating wastewater have a negligible effect on the redox reaction between Cr⁶⁺ and Fe²⁺ in ice; it also proposes that the Fe²⁺/freezing process can be used for the treatment of Cr⁶⁺-contaminated wastewater.