title: Observations of iodine speciation and cycling in the hydrosphere
creator: Gilfedder, Benjamin Silas
subject: ddc-550
subject: 550 Earth sciences
description: Iodine is an important element in oceanic, atmospheric, and terrestrial systems. Firstly, radical reactions in the troposphere can lead to significant ozone depletion, and secondly, nucleation of gaseous iodine molecules can produce new aerosol formation events, presenting possible direct and indirect natural cooling effects on climate. In the terrestrial environment iodine is a vital micronutrient for all mammals, with a lack of iodine intake leading to several debilitating disorders such as goiter and cretinism. The aim of this study was to investigate iodine systematics, and particularly speciation, in the atmosphere (aerosols, rain, and snow) and terrestrial hydrosphere (lakes) in order to gain a better understanding of how iodine moves between and within each environmental compartment. A subsidiary aim was to develop an inexpensive, but sensitive and accurate method for iodine quantification in soils and sediments using conventional analytical equipment. Rain and snow samples were taken from both northern (Germany, Switzerland, Ireland, Greenland) and southern (Australia, New Zealand, Chile) hemispheres whereas aerosols were obtained from Mace Head, Ireland using cascade (5 stages) and PM 2.5 impactors. Iodine cycling in lakes was investigated in the Mummelsee, a small headwater lake in the Black Forest. Speciation measurements were conducted by coupling an ion chromatograph to an ICP-MS and the organic fraction calculated as total iodine minus the inorganic species iodide and iodate. Organically bound iodine was the most abundant fraction in the atmospheric aqueous phase, despite the fact that iodine oxides are currently thought to be the theoretical sink species. Aerosols from Mace Head, Ireland, contained a median of 50 pmol m-3 total iodine, with more than 90 % being associated with organic matter. Iodide was the next most abundant species (median 5 %) with iodate being the least abundant (median 0.8 %). Similar results were found in the precipitation samples from northern and southern hemispheres, with organic iodine composing over half of the total iodine, and in the snow from Greenland up to 88 %; although in general the organic fraction was lower in precipitation than in aerosols. Up to 5 unidentified peaks, representing iodine species in addition to iodide and iodate, were observed in aerosol and precipitation chromatograms, providing direct evidence for organic iodine compounds in aerosols and precipitation. While these species remain unidentified, they are thought to be anionic and relatively small (i.e. low molecular weight). It is suggested that these compounds and iodide form during (photolytic) decomposition of organo-I of high molecular weight, the organic material possibly stemming from the ocean surface microlayer. It was also found that orographically induced precipitation significantly effects iodine concentrations in snow, with iodine levels decreasing exponentially with altitude over a transect in the Black Forest; indeed, more than halving (38 to 13 nmol l-1) over an altitude change of 840 m and horizontal distance of only 5 km. It is suggested that orographic affects may be more important than lateral distance from the ocean in determining iodine levels in continental precipitation. Once precipitation enters terrestrial ecosystems it may interact with soils, rocks, and biota. Iodine levels in the Mummelsee were very similar to rain and snow, averaging 15.2 ± 2.4 nmol l-1, suggesting at very little iodine input from the catchment geology. Iodine in the lake and the spring inflow was dominantly associated with organic matter with, on average, 85 ± 7 % organically bound. However, inorganic iodine cycling in the lake was also important, and displayed pronounced redox chemistry, with both iodide release from the sediments and iodate reduction in the hypolimnion during anoxic stratified conditions. The iodide flux (up to 10.1 nmol m-2 d-1) back into the water column is probably due to the decomposition of detritus in the top few centimeters of the sediments. In contrast to the hypolimnion, iodide was removed from the epilimnion during the summer and autumn months, whereas iodate levels increased slightly over the same time period, suggesting at the importance of biological reactions. This was supported by a sediment core that contained high iodine concentrations, averaging 92 μmol kg-1 total iodine, and a significant correlation with organic carbon (p<0.001). The analytical method entailed combusting sediment or soil samples in the oven of an AOX apparatus at 1000 oC and trapping the vapours in Milli-Q water. The solution was then analysed for iodine by a kinetic UV/Vis photospectrometry whereby iodide quantitatively catalyses the oxidation of As3+ and reduction of chromophoric Ce4+. The method was shown to be sensitive (detection limit 49 ng at 95 % confidence) and precise with relative standard deviations less than 5%. In conclusion, while this work has shown that organic matter plays a very important role in the hydrosphere, particularly in regards to iodine cycling, considerably more work needs to be conducted on themes such as identifying the organic iodine species, how is the iodine bound to the organic material and what is the role of organisms in the formation of organic iodine. With the current interest in iodine chemistry it is hoped that these and many other pressing questions will be answered in the near future.
date: 2008
type: Dissertation
type: info:eu-repo/semantics/doctoralThesis
type: NonPeerReviewed
format: application/pdf
identifier: https://archiv.ub.uni-heidelberg.de/volltextserverhttps://archiv.ub.uni-heidelberg.de/volltextserver/8264/1/Observations_of_Iodine_Speciation_and_Cycling_in_the_Hydrosp.pdf
identifier: DOI:10.11588/heidok.00008264
identifier: urn:nbn:de:bsz:16-opus-82645
identifier:   Gilfedder, Benjamin Silas  (2008) Observations of iodine speciation and cycling in the hydrosphere.  [Dissertation]     
relation: https://archiv.ub.uni-heidelberg.de/volltextserver/8264/
rights: info:eu-repo/semantics/openAccess
rights: http://archiv.ub.uni-heidelberg.de/volltextserver/help/license_urhg.html
language: eng