Cryoprotective Polyol-Induced Ice Microstructure Development and Enhanced Chromium(VI) Reduction in Polycrystalline Structures

Cryoprotective polyols play a crucial role in inhibiting ice crystal growth, reducing the size of bulk ice crystals in polycrystalline structures, and increasing the volume of ice grain boundaries relative to the total solution volume. This study suggests that these properties enable cryoprotective polyols to accelerate the reduction of hexavalent chromium (Cr(VI)) in ice despite the reducing agents being limited to aqueous environments. This reaction is mainly caused by the accumulation of cryoprotective polyols and Cr(VI) at the ice grain boundaries formed during freezing (i.e., the freeze concentration effect). Upon freezing a mixture of cryoprotective polyols and a 20 μM Cr(VI) solution, over 90% reduction in Cr(VI) concentration was achieved within 24 h. Higher concentrations of cryoprotective polyols in Cr(VI) enhanced the reduction kinetics of Cr(VI) over the same reaction time, implying that the cryoprotective polyols function as reducing agents under freezing conditions. Confocal Raman spectroscopy confirmed cryoprotective polyols bind to ice crystal surfaces and concentrate Cr(VI) at the ice grain boundaries. Notably, the reductive capability of cryoprotective polyols toward Cr(VI) was observed under freezing conditions, using an electroplating wastewater sample contaminated with Cr(VI) as a model system, whereas negligible reduction was observed in the aqueous phase. Enhanced Cr(VI) reduction by cryoprotective polyols in frozen solutions presents a viable approach for treating contaminated wastewater and contributes to understanding the self-purification mechanism in natural environments where these polyols are present.