Freeze–Thaw Cycle-Enhanced Transformation of Iodide to Organoiodine Compounds in the Presence of Natural Organic Matter and Fe(III)

Abstract

The formation of organoiodine compounds (OICs) is of great interest in the natural iodine cycle as well as water treatment processes. Herein, we report a pathway of OIC formation that reactive iodine (RI) and OICs are produced from iodide oxidation in the presence of Fe­(III) and natural organic matter (NOM) in frozen solution, whereas their production is insignificant in aqueous solution. Moreover, thawing the frozen solution induces the further production of OICs. A total of 352 OICs are detected by Fourier transform ion cyclotron resonance mass spectrometry in the freeze–thaw cycled reactions of Fe­(III)/I–/humic acid solution, which are five times as many as OICs in aqueous reactions. Using model organic compounds instead of NOM, aromatic compounds (e.g., phenol, aniline, o-cresol, and guaiacol) induce higher OIC formation yields (10.4–18.6%) in the freeze–thaw Fe­(III)/I– system than those in aqueous (1.1–2.1%) or frozen (2.7–7.6%) solutions. In the frozen solution, the formation of RI is enhanced, but its further reaction with NOM is hindered. Therefore, the freeze–thaw cycle in which RI is formed in the frozen media and the resulting RI is consumed by reaction with NOM in the subsequently thawed solution is more efficient in producing OICs than the continuous reaction in frozen solution.

MnO2-Induced Oxidation of Iodide in Frozen Solution

“…Various iodine compounds (e.g., IO, I 2 , HOI, ICl, OICs) are found in the sea ice of Antarctic and the atmosphere of polar boundary layers. ,, The enhanced concentrations of iodocarbons were often observed in sea ice samples relative to the sea water below. ,, The emission of RI and volatile OICs from sea ice to the atmosphere via the brine channels of ice may arouse iodine cycling in and over sea ice . Although biotic processes have been proven to produce iodocarbons or RI in sea ice, , abiotic oxidation of iodide in ice has also been proposed as an alternative source in frozen environments. ,, Our previous studies revealed abiotic transformation pathways of iodide by Fe­(III)/iron oxides in frozen or freeze–thaw cycled solutions. , However, the effective oxidation of iodide in frozen iron­(III) systems requires an acidic condition (pH < 5.0), which limits the formation potential of OICs in natural environments. In this study, three ubiquitous forms of manganese dioxides were selected to investigate the oxidation of iodide in frozen media.…”

“…For samples containing FA, dissolved OICs were calculated from the difference between total dissolved iodine and total dissolved inorganic iodine (the sum of iodide, RI, and iodate) in the filtered sample. Total dissolved iodine was analyzed by the heating-activated persulfate oxidation method according to the previous study. , …”

“…Thus, chemical reactions in ice have been investigated to understand the chemistry of the polar caps and the atmospheric boundary layer or to reveal novel phenomena different significantly from bulk phases. − We also have reported that some environmental redox reactions which are very slow or negligible in ambient aquatic solutions can be markedly accelerated in frozen solutions. − The freezing-accelerated reactions are generally ascribed to the elevated concentration of substrates in the unfrozen ice grain boundary region (freeze concentration effect). , For example, the freeze concentration effect has been proposed to facilitate iodide oxidation. Kim et al reported that the oxidation of iodide to triiodide by dioxygen or iron oxides could be significantly accelerated in ice. , A recent study also observed that the frozen or freeze–thaw cycled solution enhanced iodide oxidation to generate OICs and RI in the presence of NOM and Fe(III)/iron oxides under acidic conditions . On the other hand, the frozen solution can also enhance the reductive dissolution of colloidal MnO 2 in the presence of ligands .…”

Enhanced anaerobic digestion of waste activated sludge with periodate-based pretreatment

“…For example, the P i,n of region III (regarded as fulvic acid-like compounds) and region V (regarded as humic acid-like substances) declined from 7.96% to 17.31% in the control to 5.05% and 10.87%, respectively with the addition of 100 mg PI per g TS. The potential reason for this decrease might be due to the powerful oxidative activity of PI (E 0 = +1.60 V vs. standard hydrogen electrode (SHE)) [ 37 ], which could destroy carbon double bonds and benzene rings and thereby benefit the degradation of recalcitrant organics [ 5 ]. On the other hand, the production of strong oxidizing radicals in PI-pretreatment systems, i.e., •OH, •O 2 − , and 1 O 2 , might promote the conversion of refractory substances.…”

Production of Molecular Iodine via a Redox Reaction between Iodate and Organic Compounds in Ice

“…The production of I 2 in the presence of I – and O 2 was considerably enhanced when the aqueous solution was frozen under both dark and light-irradiation conditions . Freezing accelerated the production of I 2 via the chemical reaction of I – with iron oxides, chromate, or ferric ions. − Although the production of I 2 via NO 2 – -mediated oxidation of I – was negligible in water, it was clearly observed in ice. − The freeze concentration effect is considered to be the main reason for the noteworthy enhanced production of I 2 from I – in ice. At temperatures between the freezing and eutectic points, frozen solutions contain both solid and liquid phases, which are referred to as bulk ice crystals and ice grain boundaries, respectively .…”