Enhanced reduction of hexavalent chromium by hydrogen sulfide in frozen solution

Abstract

Significant enrichment of hexavalent chromium [Cr (VI)] has been detrimentally influenced both environment and human's health in the polar regions. We found that reduction of Cr (VI) by hydrogen sulfide (H2S) was significantly enhanced in ice (-20 degrees C) but relatively slow in water (25 degrees C) under the same experimental conditions. This enhancement is ascribed to the freeze concentrations of Cr (VI), H2S, and proton (H+) in ice grain boundaries during freezing. Sulfate (SO42-) was produced more appreciably in ice than in water. Cr (VI) reduction in ice was enhanced effectively under various H2S concentrations (5.0-100 mu M), pHi (2.0-7.7), and increasing the freezing temperature from -30 to -15 degrees C. Optical and chemical observations with a confocal Raman microscope under frozen state supported the freeze concentration of Cr (VI) in ice grain boundaries. The accumulation of H+ (pH decrease) in ice grain boundaries was verified with the UV-visible absorption spectra of cresol red (CR) in ice. Removal of Cr (VI) in electroplating wastewater was significant and comparable to that in distilled water by freezing. Therefore, Cr (VI) reduction by H2S in ice phase indicates the occurrence of Cr (VI) removal in cold regions.