Iodine Chemistry Research Articles

Modeling of iodine radiation chemistry in the presence of organic compounds

A kinetic-based model was developed that simulates the radiation chemistry of iodine in the presence of organic compounds. The model's mechanistic description of iodine chemistry and generic semi-mechanistic reactions for various classes of organics, provided a reasonable representation of experimental results. The majority of the model and experimental results of iodine volatilization rates were in agreement within an order of magnitude.

Implications of iodine chemistry for daytime HO2 levels at Rishiri Island

The observed midday maximum in the mixing ratio of HO2 at Rishiri Island in June 2000 was ∼10 pptv, but photochemical box model simulations overpredicted HO2 at this location by an average of 70%. This overestimation was significant only when the mixing ratio of NO was lower than 300 pptv, and was coincident with overprediction of the NO/NO2 ratio. We detected several organoiodines, presumably emitted from seaweeds, and propose the presence of the IO radical. IO could reduce HO2 mixing ratios via the formation of HOI that may subsequently be scavenged by aerosols or lost by photolysis and may also convert NO to NO2 directly. Model calculations with known iodine chemistry could reproduce the observed HO2 with 12–25 pptv of IO. Although iodine chemistry is unlikely to explain the entire discrepancy in HO2, several pptv of IO could significantly reduce HO2 mixing ratios and NO/NO2 ratios.

Iodine Behavior Under Conditions Relating to Nuclear Reactor Accidents

The short-term radiological impact of some serious reactor accidents may be governed by the release of airborne radioiodine to the environment. The impacts of parameters affecting iodine volatility, including radiation, iodine concentration, and solution pH, were investigated under a range of postaccident chemical conditions expected in a reactor containment structure. A bench-scale apparatus, installed in the irradiation chamber of a Gammacell, was used to measure the rate of iodine volatilization from dilute, 10-6 to 10-4 M, CsI solutions with pH values from 5 to 9. Iodine volatilization dramatically increased in the presence of radiation. The volatilization rates were nearly proportional to iodine concentration over the range of concentrations and pH values examined. Volatilization rate increased significantly with a decrease in pH. A kinetic-based model containing a mechanistic description of iodine chemistry was developed to simulate the radiation chemistry of iodine. The majority of the model prediction and experimental results of iodine volatilization rates were in agreement, although some divergence was evident.

Novel applications of the hypervalent iodine chemistry. Synthesis of thiazolo-fused quinolinones

A synthesis of thiazolo(5,4-c)quinolinone derivatives featuring the use of two different hypervalent iodine reagents has been achieved starting from ethyl benzoylacetate in an efficient way. Hydroxy(tosyloxy)phenyliodine (HITB, Kosers reagent) has been employed to functionalize the starting β-ketoester 4, and phenyliodine(III)bis(trifluoroacetate) (PIFA) to perform a ring closure reaction on the corresponding N-methoxyamides 9 and 14.

Iodine chemistry reflects productivity and denitrification in the Arabian Sea: evidence for flux of dissolved species from sediments of western India into the OMZ

Abstract Dissolved iodine species and total iodine concentrations were measured in the Arabian Sea during the Spring Intermonsoon of 1995. Two separate regimes of iodine chemistry are highlighted in this study: (1) the well-oxygenated surface layer (WOSL) where iodide concentrations were in the range of 158–558 nM, and (2) the oxygen minimum zone (OMZ) where total iodine concentrations [primarily as iodide and in excess to the oceanic iodine/salinity ratio of ∼13] varied from ∼200 to 950 nM. Iodine data in the WOSL of the Arabian Sea are contrasted with data from the Bermuda Atlantic Time-series Station (BATS), the Hawaii Ocean Time-series Station ALOHA (HOT), VERTEX in the Pacific and the Black Sea. Total iodine concentrations in excess of 400 nM were observed in eastern portions of the OMZ. The eastern portion of the basin has a permanent denitrification zone as well as high concentrations of dissolved Mn2+ (d-Mn2+) and iodide. While there is precedent for high values of iodide and total iodine in several other isolated basins, this is the first report of such values in open-ocean waters. Potential sources of excess total iodine to the OMZ include advection along isopycnals, from hydrothermal vents or margin sediments; atmospheric deposition; and remineralization of sinking particulate organic iodine (POI) associated with elevated productivity in surface waters. We estimate that only 3.6% of the excess total iodine can result from remineralization of sinking POI from the WOSL to the OMZ. Advection from margin sediments off of India is the most plausible source of iodine to the OMZ and contributes ∼96% of the total excess iodine to the OMZ. I− is maintained as the dominant form of iodine via in situ reduction of iodate by bacteria.

Enantioselective total synthesis of (-)-xialenon A.

The first total synthesis of (-)-xialenon A (1) via conjugate allylation of a 1,5-cyclooctadiene-derived bicyclo[3.3.0]octenone 3 and an alpha'-hydroxylation on the more hinderd face of enone 9 using hypervalent iodine chemistry, is described.

Organic iodine chemistry

A shared-cost action on Organic Iodine Chemistry has been completed as part of the CEC 4th Framework programme on Nuclear Fission Safety. Organisations from four EC countries are involved in an integrated programme of experiments and analysis to help clarify the phenomenology, and to increase confidence in the modelling of iodine behaviour in containment. The project is focused on identifying the main routes for organic iodine formation, and providing new experimental kinetic data which will be used to improve existing models and to stimulate code development.

ChemInform Abstract: Organic Iodine(I, III, and V) Chemistry: 10 Years of Development at the Medical University of Warsaw, Poland

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

ChemInform Abstract: Organic Iodine Chemistry: 10 Years of Development at the Medical University of Warsaw, Poland

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Organic Iodine(I, III, and V) Chemistry: 10 Years of Development at the Medical University of Warsaw, Poland

This review reports some novel (or considerably improved) methods for the synthesis of aromatic iodides, (dichloroiodo)arenes, (diacetoxyiodo)arenes, iodylarenes and diaryliodonium salts, as well as some facile, oxidative anion metatheses in crude diaryliodonium halides and, for comparison, potassium halides. All these new results were obtained in our laboratory over the past decade (1990-2000). A full list of our papers dealing with the organic iodine(I, III and V) chemistry, covering exlusively the aromatic derivatives, is also provided.

Steady-state mixing of iodine in shelf seas off the British Isles

A study of dissolved iodate and total iodine on the Hebridean shelf in several months of 1995/6 confirms an earlier hypothesis that the distribution of iodine in these waters is almost in steady state throughout the year. No evidence was found for changes in iodine chemistry that might correlate with seasonal changes in primary productivity, and the inappropriateness of adopting a Redfield-type model to explain the main trend of the iodine distribution is discussed. The system is determined by the mixing of three water types. To the west, the iodine distribution is governed by water of the North Atlantic Current (typically with salinity of 35.3 and iodate and total iodine concentrations close to 0.40 and 0.45 μM, respectively, and its mixing with deeper water containing a slightly higher concentration of iodate, but similar salinity. To the east, the distribution is governed by coastal waters with low salinity (∼33) and low iodate concentration (0.25 μM) but only a marginally lower total iodine concentration. The approximate iodate concentration, rationalised to 35 salinity, is given by RIO 3=0.060e −22 (35.42−S)+0.37e −0.20 (35.42−S)+0.010e −0.010 (35.42−S). Rationalised total iodine varied by only about 5%. Reasons are given for believing that the conditions found in the Hebridean waters probably applies to the whole of the shelf-seas of Britain, including the North Sea.

A modeling study of iodine chemistry in the marine boundary layer

An observationally constrained photochemical box model has been developed to investigate the atmospheric chemistry of iodine in the marine boundary layer, motivated by recent measurements of the iodine monoxide (IO) radical (Allan et al., this issue). Good agreement with the time series of IO measured at a midlatitude coastal station was achieved by using a reaction scheme that included recycling of iodine through marine aerosol. The strong diurnal variation in IO observed in the subtropical Atlantic was satisfactorily modeled by assuming a constant concentration of iodocarbons that photolyzed to produce roughly 1×104 iodine atoms cm−3 s−1 at midday. The significance of the occurrence of IO at concentrations of up to 4 parts per trillion in the marine boundary layer was then considered from three angles. First, the iodine‐catalyzed destruction of ozone was shown to be of a magnitude similar to that caused by odd‐hydrogen photochemistry, with up to 13% of the available ozone destroyed per day in a marine air mass. Second, the enrichment factor of iodine in marine aerosol compared with surface seawater was predicted to increase to values of several thousand, in sensible accord with observations. Most of the enrichment should be due to the accumulation of iodate, although other iodine species may also be present, depending on the rate of aerosol recycling. Third, the denoxification of the marine boundary layer was found to be significantly enhanced as a result of aerosol uptake of IONO2, formed from the recombination of IO with NO2.

The Chemistry of Iodine in Containment

Recent investigations of iodine behavior under radiolytic conditions have demonstrated that kinetics, not thermodynamics, will govern iodine speciation and partitioning under conditions typical of those expected in a reactor containment during an accident. In the presence of radiation, iodine volatility is orders of magnitude higher than that expected based on thermodynamic calculations. Kinetic studies have contributed extensively to the existing database of iodine chemistry and have several implications for modeling iodine behavior for safety analyses. For example, as a result of these investigations, many uncertainties in the iodine database, such as those regarding thermal oxidation of iodine, which were formerly regarded as reactor safety issues, are now considered to be relatively unimportant. In contrast, previously unconsidered factors, such as the effect on aqueous chemistry of impurities originating from surfaces, are now recognized as playing major roles in determining iodine volatility. An updated review of the existing literature regarding iodine behavior is provided, with a focus on recent developments. A critical evaluation of the data in the context of developing a model for iodine behavior under reactor accident conditions is also provided.

Development of the INSPECT Model for the Prediction of Iodine Volatility from Irradiated Solutions

The prediction of iodine behavior in the containment of a pressurized water reactor following a loss-of-coolant accident requires a reliable model of the chemistry of iodine in aqueous solution. The INSPECT model, which has been developed over several years, contains a large number of the relevant chemical reactions of iodine and water radiation chemistry. Since the reaction set was first assembled, new data on rate constants and mechanisms have become available. In addition, the application of the model to various small-scale experiments has revealed problems in the modeling of some reactions, leading to an underprediction of the iodine volatility at high pH, although the experiments have demonstrated that the high-pH volatility remains satisfactorily low.The INSPECT model is described along with the recent modifications that have been made to take account of new data and to improve the modeling where appropriate. The most important of these were (a) changes to the H2O2 - I2 reaction mechanism, (b) the inclusion of an impurity-catalyzed first-order O2– disproportionation reaction, and (c) the treatment of atomic I as a volatile species. These modifications have led to an increase in the predicted iodine volatility under neutral and alkaline conditions. At pH 4.6, where the original model had been found to be satisfactory, the modifications did not result in a significant change in the predicted volatility.The predictions of the revised model are compared with the results of a comprehensive series of experiments, which are described in a separate paper. The model predictions are in generally good agreement with the experiments for the range of conditions studied (pH 4.6 to 9, 10–5 to 10–4 mol/dm3 I–, 0.02 to 0.2 Mrad/h, 25 to 70°C). The results at neutral and high pH show a significant improvement over the previous version of the model, which underestimated the volatility at pH 9 by more than two orders of magnitude.

Measurements of the Radiolytic Oxidation of Aqueous CsI Using a Sparging Apparatus

Radiolytic oxidation is considered to be the main mechanism for the formation of I2 from aqueous CsI in the containment of a water-cooled reactor after a loss-of-coolant accident (LOCA) in a pressurized water reactor. Despite the amount of study over the last 60 yr on the radiation chemistry of iodine, there have, until recently, been few consistent sets of experiments spanning a wide enough range of conditions to allow models to be verified with confidence.The results from a set of experiments carried out to remedy this deficiency are described. In this work the rate of evolution of I2 from sparged irradiated borate solutions containing CsI labeled with 131I was measured on-line over a range of conditions. This work involved the measurement of the effects of pH, temperature, O2 concentration, I– concentration, phosphate concentration, dose-rate, and impurities on the rate of evolution of I2. The range of conditions was chosen to span as closely as possible the range of conditions expected in a LOCA, but also to help to elucidate some of the mechanisms especially at high pH.The pH was found to be a very important factor in determining iodine volatility. Over the temperature range studied, the extent of oxidation decreased with increasing temperature, but this was counteracted, to a greater or lesser extent, by the decrease in partition coefficient. The oxygen concentration was more important in solutions not containing phosphate. The fractional oxidation was not particularly dependent on iodide concentration, but GI2 was very dependent on [I–]. There was no effect of added impurities, Fe, Mn, Mo, or organics although, in separate work, silver was found to have a very important effect.During attempts to interpret the data, it was found that it was necessary to consider the iodine atom as a volatile species with a partition coefficient of 1.9 taken from thermodynamic data. The modeling work is described in a separate paper.

OIO and the atmospheric cycle of iodine

IO and BrO radicals are intermediates in the atmospheric photo‐oxidation of iodo‐ and bromocarbons and can act as catalysts for ozone loss. We have studied the kinetics and mechanisms of the reactions of IO with itself and with BrO to establish their role in the atmospheric chemistry of iodine. We have found that iodine dioxide, OIO, is produced in these reactions. The results of these and other experimental observations together with a recent computational study suggest an unexpectedly high photochemical stability for OIO. It is shown that OIO formation and its attachment to particles could account for the high enrichment of iodine in the small size fraction of marine aerosol, which is important for the transport of iodine from the sea to the continents. OIO may be a route to the formation of iodate, which is present in atmospheric precipitation. OIO formation also implies a reduced efficiency for iodine catalysed ozone loss.

Iodine oxide in the marine boundary layer

A striking example of the influence of halogen chemistry on tropospheric ozone levels is the episodic destruction of boundary-layer ozone during the Arctic sunrise by reactive halogen species1, 2. We detected iodine oxide in the boundary layer at Mace Head, Ireland (53°20′ N, 9°54′ W) during May 1997, which indicates that iodine chemistry is occurring in the troposphere.

Iodine chemistry and its role in halogen activation and ozone loss in the marine boundary layer: A model study

A detailed set of reactions treating the gas and aqueous phase chemistry of the most important iodine species in the marine boundary layer (MBL) has been added to a box model which describes Br and Cl chemistry in the MBL. While Br and Cl originate from seasalt, the I compounds are largely derived photochemically from several biogenic alkyl iodides, in particular CH2I2, CH2ClI, C2H5I, C3H7I, or CH3I which are released from the sea. Their photodissociation produces some inorganic iodine gases which can rapidly react in the gas and aqueous phase with other halogen compounds. Scavenging of the iodine species HI, HOI, INO2, and IONO2 by aerosol particles is not a permanent sink as assumed in previous modeling studies. Aqueous-phase chemical reactions can produce the compounds IBr, ICl, and I2, which will be released back into the gas phase due to their low solubility. Our study, although highly theoretical, suggests that almost all particulate iodine is in the chemical form of IO-3. Other aqueous-phase species are only temporary reservoirs and can be re-activated to yield gas phase iodine. Assuming release rates of the organic iodine compounds which yield atmospheric concentrations similar to some measurements, we calculate significant concentrations of reactive halogen gases. The addition of iodine chemistry to our reaction scheme has the effect of accelerating photochemical Br and Cl release from the seasalt. This causes an enhancement in ozone destruction rates in the MBL over that arising from the well established reactions O(1D) + H2O → 2OH, HO2 + O3 → OH + 2O2, and OH + O3 → HO2 + O2. The given reaction scheme accounts for the formation of particulate iodine which is preferably accumulated in the smaller sulfate aerosol particles.

Iodine stress corrosion cracking of Zircaloy reactor cladding: iodine chemistry (a review)

This review covers chemical aspects of the iodine stress corrosion cracking (ISCC) pellet-cladding interaction (PCI) failures which have occurred in some reactors with UO 2 fuel in unlined Zircaloy tubes. The cracking mechanism is discussed in relation to chemical factors. Cesium and iodine are released as fission products and form CsI but its dissociation iodine pressure p(I) is many orders of magnitude greater than chemical thermodynamic calculations predict, due to radiolysis. This enhanced p(I) can form ZrI 4 in regions where Zr is exposed (not protected by surface scale) and a van Arkel vapour transport reaction then causes rapid Zr transport from localised cladding areas, i.e. pitting, which later becomes a crack site. Requirements are a critical strain rate (which cracks protective scale), iodine pressure and time, i.e. a power ramp. Factors include that a stress caused by a power ramp may relax before the required gap chemistry is achieved and that a ramp also releases short-lived I and Cs fission products which add to the iodine pressure available.

Experimental and Computational Investigations of the Reaction of OH with CF3I and the Enthalpy of Formation of HOI

The kinetics of the reaction of hydroxyl radicals with trifluoroiodomethane were investigated by the flash photolysis−resonance fluorescence technique. A rate constant of k = 5.8 × 10-12 exp((−11.3 kJ mol-1)/RT) cm3 molecule-1 s-1 was measured over the temperature range 280−450 K with accuracy limits of 20% (450 K) to 30% (280 K). Different product channels were investigated by ab initio methods, and the dominant products are CF3 + HOI. The enthalpy of formation of hypoiodous acid was analyzed with Gaussian-2 theory, in conjunction with G2 energies for INO, ICN, ClCN, and other species. The transition state and reaction coordinate for OH + CF3I was characterized at the G2(MP2) level, and the results suggest a negligible barrier to the reverse reaction of CF3 + HOI, so that the measured forward activation energy can be used to derive ΔfH298(HOI) = −69.6 ± 5.4 kJ mol-1. The implications of the kinetics and thermochemistry for iodine chemistry in flames and the atmosphere are discussed, and for the range 280−2000 K a proposed rate expression is k = 2.9 × 10-16(T/K)1.5 exp(−(960 K)/T) cm3 molecule-1 s-1.