Freezing-Induced Simultaneous Reduction of Chromate and Production of Molecular Iodine: Mechanism, Kinetics, and Practical Implications

摘要

A new method for the concurrent treatment of Cr(Ⅵ)-contaminated wastewater and production of the useful I_2 chemical was developed. The method is based on the redox reaction between Cr(Ⅵ) and I~- that occurs when an aqueous wastewater solution containing Cr(Ⅵ) and I~- is frozen, producing I_2 and allowing for the effective removal of Cr. The redox reaction occurs primarily because of the accumulation of Cr(Ⅵ), I~-, and protons in the ice grain boundaries formed during freezing (i.e., the freeze concentration effect). This effect was verified by confocal Raman spectroscopy and the experiments varying I~- concentration and pH. The reduction of Cr(Ⅵ) (20 μM) was near complete after freezing at I~- concentrations ≥ 100 μM, pH ≤ 3.0, and temperatures ≤ -10 °C. The freezing method (liquid cooling vs air cooling) had little effect on the final Cr(Ⅵ) reduction efficiency but had a significant effect on the Cr(Ⅵ) reduction rate. The freezing method was also tested with Cr(Ⅵ)-contaminated electroplating wastewater samples, and simultaneous Cr(Ⅵ) reduction and I_2 production proceeded rapidly in a frozen solution but was not observed in an aqueous solution. Additionally, other substances in electroplating wastewater did not reduce the rate and final efficiency of Cr(Ⅵ) reduction and I_2 production. Therefore, the freezing/Cr(Ⅵ)/I~- system can be considered a feasible approach to water-energy nexus technology for simultaneous I_2 production and Cr(Ⅵ)-contaminated wastewater treatment.