Iodine Chemistry Research Articles

ASTEC V2 severe accident integral code: Fission product modelling and validation

One main goal of the severe accident integral code ASTEC V2, jointly developed since almost more than 15 years by IRSN and GRS, is to simulate the overall behaviour of fission products (FP) in a damaged nuclear facility. ASTEC applications are source term determinations, level 2 Probabilistic Safety Assessment (PSA2) studies including the determination of uncertainties, accident management studies and physical analyses of FP experiments to improve the understanding of the phenomenology.ASTEC is a modular code and models of a part of the phenomenology are implemented in each module: the release of FPs and structural materials from degraded fuel in the ELSA module; the transport through the reactor coolant system approximated as a sequence of control volumes in the SOPHAEROS module; and the radiochemistry inside the containment nuclear building in the IODE module. Three other modules, CPA, ISODOP and DOSE, allow respectively computing the deposition rate of aerosols inside the containment, the activities of the isotopes as a function of time, and the gaseous dose rate which is needed to model radiochemistry in the gaseous phase.In ELSA, release models are semi-mechanistic and have been validated for a wide range of experimental data, and noticeably for VERCORS experiments. For SOPHAEROS, the models can be divided into two parts: vapour phase phenomena and aerosol phase phenomena. For IODE, iodine and ruthenium chemistry are modelled based on a semi-mechanistic approach, these FPs can form some volatile species and are particularly important in terms of potential radiological consequences. The models in these 3 modules are based on a wide experimental database, resulting for a large part from international programmes, and they are considered at the state of the art of the R&D knowledge.This paper illustrates some FPs modelling capabilities of ASTEC and computed values are compared to some experimental results, which are parts of the validation matrix.

Hypervalent Iodine‐Catalyzed Oxidative Functionalizations Including Stereoselective Reactions

Hypervalent iodine chemistry is now a well-established area of organic chemistry. Novel hypervalent iodine reagents have been introduced in many different transformations owing to their mild reaction conditions and environmentally friendly nature. Recently, these reagents have received particular attention because of their applications in catalysis. Numerous hypervalent iodine-catalyzed oxidative functionalizations such as oxidations of various alcohols and phenols, α-functionalizations of carbonyl compounds, cyclizations, and rearrangements have been developed successfully. In these catalytic reactions stoichiometric oxidants such as mCPBA or oxone play a crucial role to generate the iodine(III) or iodine(V) species in situ. In this Focus Review, recent developments of hypervalent iodine-catalyzed reactions are described including some asymmetric variants. Catalytic reactions using recyclable hypervalent iodine catalysts are also covered.

Fundamental chemistry of iodine. The reaction of di-iodine towards thiourea and its methyl-derivative: formation of aminothiazoles and aminothiadiazoles through dicationic disulfides

The reactivity of di-iodine towards thiourea (TU) and its derivative methylthiourea (MeTU) was studied. A diversity of products was obtained from these reactions. TU reacted with di-iodine in the absence or presence of hydroiodic or hydrochloric acids in a 1 : 1, 1 : 1 : 1 or 1 : 1 : 2 (TU : I2 : HX (X = I, Cl)) molar ratio to form the ionic compounds [(TU2)(2+)2(I(-))·H2O] (1), [2(TU2) (2+)·(Cl(-))·2(I(-))·(I3(-))] (2) and [(TUH)(+) (I3(-))] (3). The compounds [(TU2)(2+)(Br(-))(I3(-))] (4) and [(TU2)(2+)2(Br(-))·H2O] (5) were derived from the reactions of TU with di-iodine in the presence of hydrobromic acid in a 1 : 1 : 1 or 1 : 2 : 1 (TU : I2 : HBr) molar ratio. However, when the product of the reaction between TU and di-iodine in a 2 : 1 (TU : I2) molar ratio was crystallized in acetone-ethylether media the ionic salt of formula [(DAThdH(+))(I(-))] (6) (DAThd = 3,5-diamino-1,2,4-thiadiazole) was obtained. Methylthiourea (MeTU) reacted with di-iodine in the presence of hydrobromic acid (1 : 1 : 1, MeTU : I2 : HBr) in dichloromethane to form a solid product which gives [2(MeTU2) (2+)·(2Br(-))(I4(2-))] (7). Moreover, MeTU reacted with I2 in 2 : 1 (MeTU : I2) to form an intermediate powder product which was crystallized in acetone to give the 2-amino-3,4-dimethylthiazolium cation in [(DMeAThH(+))(I(-))(H2O)] (8). Upon changing the crystallization medium to ethanol, instead of acetone, the cationic 5-amino-3-methylamino-4-methyl-1,2,4-thiadiazolium (AMeAThdH)(+) in [(AMeAThdH(+))(I3(-))] (9) was formed. The compounds were characterized by m.p., FT-IR, UV-Vis, (1)H-NMR spectroscopy and mass spectrometry. The crystal structures of compounds 1-9 were determined by X-ray crystallography.

Source term assessment with ASTEC and associated uncertainty analysis using SUNSET tool

Several accidental scenarios have been simulated using the ASTEC integral IRSN-GRS code for a French 1300MWe PWR, including several break sizes or locations, highlighting the effect of safety systems and of iodine chemistry in the reactor coolant system (RCS) and in the containment on iodine source term evaluations. Iodine chemistry in the RCS and in the containment is still subject to significant uncertainties and it is thus studied in on-going R&D programs. To assess the impact of uncertainties, ASTEC has been coupled to the IRSN uncertainty propagation and sensitivity analysis tool SUNSET. Focusing on a loss of feed-water of steam generator accident, ASTEC/SUNSET calculations have been performed to assess the effect of remaining uncertainties relative to iodine behaviour on the source term. Calculations show that the postulated lack of knowledge may impact the iodine source term in the environment by at least one decade, confirming the importance of the on-going R&D programs to improve the knowledge on iodine chemistry.

Theoretical study on the adsorption mechanism of iodine molecule on platinum surface in dye-sensitized solar cells

By means of density functional theory calculations, the adsorption process of I2 at Pt (111) surface in dye-sensitized solar cells (DSSCs) has been investigated. The obtained adsorption energies and stable structures depending on the adsorption sites of the Pt surface are in good agreement with experimental values. Our results show that the dissociative chemisorption and the non-dissociative chemisorption are competitive for the adsorption of I2 on the Pt surface, and the dissociative pathway is more favored in energy. This study is expected to enrich the understanding on the origin of the excellent heterogeneous catalytic performance of Pt for triiodide reduction and the complex iodine chemistry in DSSCs. Understanding of this adsorption mechanism is helpful for rational screening for redox couple and the Pt-free alternative counter electrode materials.

Chiral Hypervalent Iodine Reagents: Synthesis and Reactivity

Chiral hypervalent iodine chemistry has been steadily increasing in importance in recent years. This review catalogues enantioselective transformations triggered by chiral hypervalent iodine(III/V) reagents, in stoichiometric or catalytic quantities, highlighting the different reactivities in terms of yield and enantioselectivity. Moreover, the synthesis of the most remarkable and successful catalysts has been illustrated in detail.

ChemInform Abstract: Oxidative Rearrangements with Hypervalent Iodine Reagents

AbstractReview: 46 refs.

Thermal-hydraulic–iodine chemistry coupling: Insights gained from the SARNET benchmark on the THAI experiments Iod-11 and Iod-12

In the SARNET2 WP8.3 THAI Benchmark the capability of current accident codes to simulate the iodine transport and behavior in sub-divided containments has been assessed. In THAI test Iod-11 and Iod-12, made available for the benchmark, the distribution of molecular iodine (I2) in the five compartments of the 60m3 vessel under stratified and well mixed conditions was measured. The main processes addressed are the I2 transport with the atmospheric flows and the interaction of I2 with the steel surface. During test Iod-11 the surfaces in contact with the containment atmosphere were dry. In Iod-12, steam was released, which condensed on the walls.Nine post-test calculations were conducted for Iod-11 and eight for Iod-12 by seven organizations using four different codes: ASTEC-IODE (CIEMAT, GRS and TUS), COCOSYS-AIM (AREVA, FZ-Jülich and GRS), ECART (Pisa University) and MELCOR (Pisa University and VTT). Different nodalizations of the THAI vessel with 20–65 zones were applied.Generally, for both tests the analytical thermal-hydraulic results are in a fairly good agreement with the measurements. Only the calculated local relative humidity deviates significantly from the measured values in all calculations. The results in Iod-11 for the local I2 concentration in the gaseous phase are quite diverse. Three calculations show only minor deviations from the measurement, whereas the others are substantially different from the measured I2 concentrations. For Iod-12, no calculation delivers a satisfactory evolution of the I2 concentration in all five compartments of the vessel. There are three mediocre results standing out in the Iod-11 exercise which are from the same user–code combinations. The discrepancies derive from various reasons which are discussed in the paper.In the benchmark a significant user effect was detected, i.e. results achieved with the same code differed considerably.This work highlights the need of a detailed iodine adsorption/desorption model and precise thermal-hydraulic modeling for an accurate simulation of I2 transport in a sub-divided containment, as well as experienced users or straight forward user guidelines.

Oxidative Rearrangements with Hypervalent Iodine Reagents

Hypervalent iodine chemistry is used as an important tool in synthetic and natural product chemistry. A number of hypervalent iodine reagents can be used to generate the functionalizations on various scaffolds by several chemical approaches including oxidation reactions, oxidative cyclization reactions, and rearrangements. Recently, these reagents have been employed to develop a variety of oxidative rearrangements. In this review article, various aspects of recently developed oxidative rearrangements using iodine(III) and iodine(V) reagents are described.

Analysis of the iodine gas phase produced by interaction of CsI and MoO3 vapours in flowing steam

In case of severe accident in a water-cooled nuclear reactor, the amount of gaseous iodine released from the primary circuit is of particular concern because it can have a direct impact on the short-term iodine source term to the environment. As revealed by the Phebus FPT tests, assessment of the iodine forms at the break is a complex issue that requires to take into account not only the thermal and flow conditions in the circuit but also the behaviour of several elements such as caesium and molybdenum involved in iodine chemistry at high temperature. Moreover, reactions between these elements and with the carrier gas could be kinetically-limited due to the low concentrations and the short residence time of the fission products in the primary circuit coupled to the strong thermal gradients encountered in this part of the reactor. In order to reduce the uncertainties in the fraction of gaseous iodine, experimental and modelling studies have been launched in the framework of the international CHIP program. The work presented in this paper is part of this program and aims to investigate the effect of molybdenum using an open flow reactor where MoO3 and CsI vapours are injected under steam/argon atmosphere. In these conditions, the experiments clearly demonstrate that molybdenum increases the fraction of gaseous iodine released at low temperature and show that this effect is highly sensitive to the Mo/Cs ratio. Caesium polymolybdates identified in the solid state by Raman microspectroscopy are consistent with the species of the Cs2MoO4–MoO3 diagram suggesting that some part of caesium could be present in the gas phase as Cs2MoO4. Calculations based on thermodynamic equilibrium in the gaseous phase give the right trend but predict HI as the major species whereas iodine is recovered mainly under molecular form (I2). Iodine speciation is better accounted with the new kinetic scheme developed for the {I, O, H} system and recently implemented in the ASTEC code.

Speciation analysis of 129I and its applications in environmental research

Summary 129I, a long-lived radionuclide, is important in view of geological repository of nuclear waste, and environmental tracing applications related to diverse natural processes of iodine. The environmental behaviors and bioavailability of 129I are highly related to its species. A number of methods have been reported for speciation analysis of 129I in a variety of environmental samples. These methods have been applied in many researches, including conversion processes of iodine species in marine and terrestrial systems, migration and retention of iodine in soil and sediment, geochemical cycling of iodine, as well as studies on atmospheric chemistry of iodine. This article aims to review these methods and their applications in environmental research.

Halogen species record Antarctic sea ice extent over glacial–interglacial periods

Abstract. Sea ice is an integral part of the earth's climate system because it affects planetary albedo, sea-surface salinity, and the atmosphere–ocean exchange of reactive gases and aerosols. Bromine and iodine chemistry is active at polar sea ice margins with the occurrence of bromine explosions and the biological production of organoiodine from sea ice algae. Satellite measurements demonstrate that concentrations of bromine oxide (BrO) and iodine oxide (IO) decrease over sea ice toward the Antarctic interior. Here we present speciation measurements of bromine and iodine in the TALDICE (TALos Dome Ice CorE) ice core (159°11' E, 72°49' S; 2315 m a.s.l.) spanning the last 215 ky. The Talos Dome ice core is located 250 km inland and is sensitive to marine air masses intruding onto the Antarctic Plateau. Talos Dome bromide (Br−) is positively correlated with temperature and negatively correlated with sodium (Na). Based on the Br−/Na seawater ratio, bromide is depleted in the ice during glacial periods and enriched during interglacial periods. Total iodine, consisting of iodide (I−) and iodate (IO3−), peaks during glacials with lower values during interglacial periods. Although IO3− is considered the most stable iodine species in the atmosphere it was only observed in the TALDICE record during glacial maxima. Sea ice dynamics are arguably the primary driver of halogen fluxes over glacial–interglacial timescales, by altering the distance between the sea ice edge and the Antarctic plateau and by altering the surface area of sea ice available to algal colonization. Based on our results we propose the use of both halogens for examining Antarctic variability of past sea ice extent.

The Effect of Freezing on Reactions with Environmental Impact

The knowledge that the freezing process can accelerate certain chemical reactions has been available since the 1960s, particularly in relation to the food industry. However, investigations into such effects on environmentally relevant reactions have only been carried out since the late 1980s. Some 20 years later, the field has matured and scientists have conducted research into various important processes such as the oxidation of nitrite ions to nitrates, sulfites to sulfates, and elemental mercury to inorganic mercury. Field observations mainly carried out in the polar regions have driven this work. For example, researchers have found that both ozone and mercury are removed from the troposphere completely (and almost instantaneously) at the time of Arctic polar sunrise. The monitoring activities suggested that both the phenomena were caused by involvement of bromine (and possibly iodine) chemistry. Scientists investigating the production of interhalide products (bromine and iodine producing interhalides) in frozen aqueous solutions have found that these reactions result in both rate accelerations and unexpected products. Furthermore, these scientists did this research with environmentally relevant concentrations of reagents, thereby suggesting that these reactions could occur in the polar regions. The conversion of elemental mercury to more oxidized forms has also shown that the acceleration of reactions can occur when environmentally relevant concentrations of Hg(0) and oxidants are frozen together in aqueous solutions. These observations, coupled with previous investigations into the effect of freezing on environmental reactions, lead us to conclude that this type of chemistry could potentially play a significant role in the chemical processing of a wide variety of inorganic components in polar regions. More recently, researchers have recognized the implications of these complementary field and laboratory findings toward human health and climate change. In this Account, we focus on the chemical and physical mechanisms that may promote novel chemistry and rate accelerations when water-ice is present. Future prospects will likely concentrate, once again, on the low-temperature chemistry of organic compounds, such as the humic acids, which are known cryospheric contaminants. Furthermore, data on the kinetics and thermodynamics of all types of reaction promoted by the freezing process would provide much assistance in determining their implications to environmental computer models.

Iodine behaviour in the containment in Phébus FP tests

Amongst the fission products that can be released in the environment during a severe accident, iodine causes great concern because of its volatility and of its potential radiological impact on population. The Phébus FP program provides an extensive database on iodine behaviour in the containment of prime importance for a more realistic evaluation of the iodine source term that can be released in the environment in case of a severe accident.The four main results regarding the iodine behaviour in the containment are: an early presence of gaseous iodine in the containment, the importance of painted surfaces exposed to the containment atmosphere in adsorption/desorption of volatile iodine, the key role of silver in preventing the iodine volatilisation from the sump when in excess over iodine (i.e. in FPT0/1 tests) and the low steady state levels of gaseous iodine concentrations reached on the long term, independently of the amount of gaseous iodine present early in the containment.During the fuel degradation, an early presence of gaseous iodine in the containment was observed in all tests. The interpretation of this result suggests that this gaseous iodine was likely formed in the primary circuit (RCS), probably linked to non equilibrium chemical effects. In FPT3, performed with a B4C control rod instead of Ag–In–Cd control rod for the other tests, the gaseous iodine fraction was much higher pointing to the essential role of control rod material on iodine chemistry in the RCS. In all tests, more than half of the iodine fuel inventory reached the containment under aerosol and gaseous forms.The Phébus FP tests give evidence that the sump did not contribute as much as expected to the production of gaseous iodine in the containment. FPT0 and FPT1 results highlight the key role played by silver produced mainly by the control rod degradation. Silver rapidly reacts with iodine to form non soluble AgI. Thus, despite acidic pH, in so far as silver is in excess compared to iodine, it greatly reduces any gaseous iodine re-volatilisation from the sump either by radiolytic oxidation of iodide or by organic iodide formation from the immersed painted surface. For FPT2, the very low volatilisation of gaseous iodine is attributed to the alkaline pH. For FPT3, where the silver amount was negligible (Ag only present as FP) and the sump acidic, no significant iodine volatilisation from the sump was observed, due to low amounts of iodide aerosols transferred in it.Phébus FP results underline the importance of painted surfaces in the containment atmosphere through their interaction with gaseous iodine. Adsorption of gaseous iodine leads to the large decrease of the gaseous iodine concentration and to the formation of volatile organic iodides. In some conditions, organic iodides gain in importance and become the predominant volatile iodine species notably in the long term for FPT0 and FPT1. This is of particular significance for radiological consequences when operating filtered containment venting as organic iodide are difficult to remove by filtration.The Phébus FP experiments show that whatever the early gaseous iodine fraction was in the containment, low long-term steady concentrations are established. This indicates that an equilibrium between iodine formation/destruction mechanisms and/or reversibility of iodine adsorption/desorption processes yield a similar steady-state concentration in the long term in all the tests.The paper details the main results obtained and their up-to-date understanding.

The main outcomes of the OECD Behaviour of Iodine (BIP) Project

An Organisation for Economic Co-operation and Development (OECD) status report on iodine behaviour published in February 2007 concluded that although the understanding of iodine behaviour in containment had advanced considerably over the past several decades, there were still areas where further investigation was warranted. The OECD initiated the Behaviour of Iodine Project (BIP) to investigate two of these areas:•The interaction of iodine with painted surfaces: adsorption of iodine from the gas phase to several containment surfaces was investigated under a variety of conditions, with focus on the role of water on the adsorption of iodine onto epoxy paints. The results show that the relative humidity is very important to the deposition velocity of iodine on paint; higher humidity caused faster deposition. Adsorption parameters are a critical input for containment codes.•The formation of organic iodide from painted surfaces: gas chromatography was used to monitor the evolution of CH3I during the irradiation of epoxy coupons pre-loaded with iodine. The results showed that organic iodide formation is greater, and has a different temporal behaviour, when the iodine is absorbed by the paint from the aqueous phase as opposed to from the gas phase. The results also highlighted the importance of destruction of organic iodides by gas phase radiolysis to the concentration of CH3I.The BIP organic iodide production experiments were different, but complementary, to the experiments performed in the EPICUR (Experimental Program for Iodine Chemistry Under Radiation) facility. However, both projects share an objective of supporting the explanation of iodine behaviour in the Phébus Fission Product (FP) tests. In this paper, the main outcomes of the BIP project will be outlined, and their relevance to the Phébus FP project will be examined.In addition to the new experiments performed on adsorption of iodine onto paint, and production of organic iodides from iodine adsorbed on paint, Atomic Energy of Canada Limited (AECL) provided the results from five Radioiodine Test Facility (RTF) experiments. The RTF was an intermediate-scale test facility that operated at AECL’s Whiteshell laboratories in the 1990s. It is expected that BIP members will use this data for code development and verification.

Asymmetric Dearomatizing Spirolactonization of Naphthols Catalyzed by Spirobiindane-Based Chiral Hypervalent Iodine Species

This report details the development of a spirobiindane-based chiral hypervalent iodine reagent, especially focusing on its structural elucidation for effective asymmetric induction of the chiral spiro center during the oxidative dearomatizing spirolactonization of naphthols. In this study we synthesized a new series of ortho-functionalized spirobiindane catalysts and demonstrated that the enantioselectivity can be dramatically improved by the presence of the substituents ortho to the iodine atom. The structural elucidation of a spirobiindane-based hypervalent iodine catalyst has led to further improvement in the stereoselective construction of the spiro center during the oxidative dearomatizing spirolactonization of naphthols. Thus, catalytic oxidation with the highest reported level of enantioselectivity in hypervalent iodine chemistry has been achieved with also an excellent level of asymmetric induction (92% ee for substrate 3a). As a result, this study, dealing with a series of modified iodine catalysts, can provide important clues about the transition state and reaction intermediate to help scientists understand the origin of the stereoselectivity. A plausible transition-state model and intermediate in the reaction for the stereoselective formation of spirolactone products are postulated by considering the ortho-substituent effect and the results of X-ray analysis. In this reaction model, the high enantiomeric excess obtained by using the spirobiindane catalysts could be well explained by the occupation of the equatorial site and extension of the surroundings around the hypervalent iodine bonds by the introduced ortho-substituent. Thus, this study would contribute to estimation of the chiral hypervalent iodine compounds in asymmetric reactions.

Iodine chemistry in the eastern Pacific marine boundary layer

Observations of gas‐phase iodine species were made during a field campaign in the eastern Pacific marine boundary layer (MBL). The Climate and Halogen Reactivity Tropical Experiment (CHARLEX) in the Galápagos Islands, running from September 2010 to present, is the first long‐term ground‐based study of trace gases in this region. Observations of gas‐phase iodine species were made using long‐path differential optical absorption spectroscopy (LP‐DOAS), multi‐axis DOAS (MAX‐DOAS), and resonance and off‐resonance fluorescence by lamp excitation (ROFLEX). These measurements were supported by ancillary measurements of ozone, nitrogen oxides, and meteorological variables. Selective halocarbon and ultrafine aerosol concentration measurements were also made. MAX‐DOAS observations of iodine monoxide (IO) display a weak seasonal variation. The maximum differential slant column density was 3.8 × 1013 molecule cm−2 (detection limit ~7 × 1012 molecule cm−2). The seasonal variation of reactive iodine IOx (= I + IO) is stronger, peaking at 1.6 pptv during the warm season (February–April). This suggests a dependence of the iodine sources on the annual cycle in sea surface temperature, although perturbations by changes in ocean surface iodide concentration and solar radiation are also possible. An observed negative correlation of IOx with chlorophyll‐a indicates a predominance of abiotic sources. The low IO mixing ratios measured (below the LP‐DOAS detection limit of 0.9 pptv) are not consistent with satellite observations if IO is confined to the MBL. The IOx loading is consistent with the observed absence of strong ozone depletion and nucleation events, indicating a small impact of iodine chemistry on these climatically relevant factors in the eastern Pacific MBL.

Iodine emissions from the sea ice of the Weddell Sea

Abstract. Iodine compounds were measured above, below and within the sea ice of the Weddell Sea during a cruise in 2009, to make progress in elucidating the mechanism of local enhancement and volatilisation of iodine. I2 mixing ratios of up to 12.4 pptv were measured 10 m above the sea ice, and up to 31 pptv was observed above surface snow on the nearby Brunt Ice Shelf – large amounts. Atmospheric IO of up to 7 pptv was measured from the ship, and the average sum of HOI and ICl was 1.9 pptv. These measurements confirm the Weddell Sea as an iodine hotspot. Average atmospheric concentrations of CH3I, C2H5I, CH2ICl, 2-C3H7I, CH2IBr and 1-C3H7I were each 0.2 pptv or less. On the Brunt Ice Shelf, enhanced concentrations of CH3I and C2H5I (up to 0.5 and 1 pptv respectively) were observed in firn air, with a diurnal profile that suggests the snow may be a source. In the sea ice brine, iodocarbons concentrations were over 10 times those of the sea water below. The sum of iodide + iodate was depleted in sea ice samples, suggesting some missing iodine chemistry. Flux calculations suggest I2 dominates the iodine atom flux to the atmosphere, but models cannot reconcile the observations and suggest either a missing iodine source or other deficiencies in our understanding of iodine chemistry. The observation of new particle formation, consistent with the model predictions, strongly suggests an iodine source. This combined study of iodine compounds is the first of its kind in this unique region of sea ice rich in biology and rich in iodine chemistry.

Topical iodophor preparations: Chemistry, microbiology, and clinical utility

Iodophor preparations are commonly used in all medical specialties for antisepsis of the skin prior to injections, invasive procedures, and surgery. Povidone-iodine has some very intriguing properties that make it extremely effective as a broad spectrum bacteriocidal agent with no known bacterial resistance, potentially lending itself to broader applications than its current uses. In this article the background, formulations, chemistry, and microbiology of iodine will be reviewed and recent clinical investigations of utility beyond skin antisepsis will be discussed.

Insights into the Photochemical Transformation of Iodine in Aqueous Systems: Humic Acid Photosensitized Reduction of Iodate

Marine aerosol is highly enriched in iodine, mostly in the form of iodate (IO(3)(-)) ions, compared to its relative abundance in seawater. This paper describes a laboratory study of the photochemical reduction of IO(3)(-) in the presence of humic acid. Spectroscopic analysis showed that ~20% of IO(3)(-) was converted to "free" iodide (I(-)) ions and this fraction remained constant as a function of time. Direct detection of an organically fixed fraction (i.e., ∼ 80%) was not possible, but a number of test reactions with surrogate organic compounds containing functional groups identified in humic acid structures indicate that efficient substitution of iodine occurs at aromatic 1,2 diol sites. These iodinated humic acids are stable with respect to photolysis at near-UV/visible wavelengths and are likely to account for a significant proportion of the soluble iodine-containing organic material occurring within aerosols. In the lower atmosphere, oxidation of I(-) to I(2) in marine aerosol occurs mostly through the uptake of O(3), with H(2)O(2) playing a very minor role. A model of iodine chemistry in the open ocean tropical boundary layer, which incorporates these experimental results, is able to account for the observed enrichment of iodine in marine aerosol.