Nitrite-Induced Activation of Iodate into Molecular Iodine in Frozen Solution.

A new mechanism for the abiotic production of molecular iodine (I2) from iodate (IO3-), which is the most abundant iodine species, in dark conditions was identified and investigated. The production of I2 in aqueous solution containing IO3- and nitrite (NO2-) at 25 °C was negligible. However, the redox chemical reaction between IO3- and NO2- rapidly proceeded in frozen solution at -20 °C, which resulted in the production of I2, I-, and NO3-. The rapid redox chemical reaction between IO3- and NO2- in frozen solution is ascribed to the accumulation of IO3-, NO2-, and protons in the liquid regions between ice crystals during freezing (freeze concentration effect). This freeze concentration effect was verified by confocal Raman microscopy for the solute concentration and UV-visible absorption spectroscopy with cresol red (acid-base indicator) for the proton concentration. The freezing-induced production of I2 in the presence of IO3- and NO2- was observed under various conditions, which suggests this abiotic process for I2 production is not restricted to a specific region and occurs in many cold regions. NO2--induced activation of IO3- to I2 in frozen solution may help explain why the measured values of iodine are larger than the modeled values in some polar areas.

Activation of Periodate by Freezing for the Degradation of Aqueous Organic Pollutants.

A new strategy (i.e., freezing) for the activation of IO4- for the degradation of aqueous organic pollutants was developed and investigated. Although the degradation of furfuryl alcohol (FFA) by IO4- was negligible in water at 25 °C, it proceeded rapidly during freezing at -20 °C. The rapid degradation of FFA during freezing should be ascribed to the freeze concentration effect that provides a favorable site (i.e., liquid brine) for the proton-coupled degradation process by concentrating IO4-, FFA, and protons. The maximum absorption wavelength of cresol red (CR) was changed from 434 nm (monoprotonated CR) to 518 nm (diprotonated CR) after freezing, which confirms that the pH of the aqueous IO4- solution decreases by freezing. The degradation experiments with varying experimental parameters demonstrate that the degradation rate increases with increasing IO4- concentration and decreasing pH and freezing temperature. The application of the IO4-/freezing system is not restricted to FFA. The degradation of four other organic pollutants (i.e., tryptophan, phenol, 4-chlorophenol, and bisphenol A) by IO4-, which was negligible in water, proceeded during freezing. In addition, freezing significantly enhanced the IO4--mediated degradation of cimetidine. The outdoor experiments performed on a cold winter night show that the IO4-/freezing system for water treatment can be operated without external electrical energy.

Bambusuril as a One-Electron Donor for Photoinduced Electron Transfer to Methyl Viologen in Mixed Crystals.

Methyl viologen hexafluorophosphate (MV2+·2PF6-) and dodecamethylbambus[6]uril (BU6) form crystals in which the layers of viologen dications alternate with those of a 1:2 supramolecular complex of BU6 and PF6-. This arrangement allows for a one-electron reduction of MV2+ ions upon UV irradiation to form MV+• radical cations within the crystal structure with half-lives of several hours in air. The mechanism of this photoinduced electron transfer in the solid state and the origin of the long-lived charge-separated state were studied by steady-state and transient spectroscopies, cyclic voltammetry, and electron paramagnetic resonance spectroscopy. Our experiments are supported by quantum-chemical calculations showing that BU6 acts as a reductant. In addition, analogous photochemical behavior is also demonstrated on other MV2+/BU6 crystals containing either BF4- or Br- counterions.