Thesis for the Degree of Doctor of Philosophy - DTU Orbit

of the atomic era in the late nineteen forties (Englund et al., 2010; Aldahan et al., 2007). Iodine is an
element with multiple oxidation states -1,0,+1,+3, +5 and +7, forming a wide variety of chemical
compounds in natural environmental Eh-pH conditions (Hou et al., 2009; Liu and Gunten, 1988). In
aqueous systems iodine occur as ioide (-1) and iodate (+5) with a minor amount of organic iodine in
seawater but considerable amounts in freshwater systems (Wong, 1991; Wong and Zhang, 2003).
Attempts to explain the reduction of iodate to iodide in seawater have demonstrated (Tsunogai and
Sase, 1969) that certain organisms enzymatically (nitrate-reductase) are able to reduce iodate to iodide
while another study (Waite and Truesdale, 2003) has not found the same results. Campos et al., (1999)
indicated that there might be a linkage between the iodide production and nitrate concentration,
showing that the iodide levels were increased as nitrate concentrations decreased. Through
observations of the iodate-iodide redox behavior in North Sea surface water samples, Spokes and Liss
(1996) showed that iodide is photochemically produced by iodate reduction and that organic matter
plays an important role in the process.
The oceans and seas are considered the main reservoirs of iodine in the Earth’s surface (Fuge &
Johnson, 1986) and the sources of iodine in the terrestrial environment such as soils and lakes originate
mainly from the seas and oceans through transport of atmospheric iodine species and subsequent
redistribution onto the earth’s surface environment by precipitation. Whereas 92% of 129 I occur as
iodide, iodate-127 accounts for 43 – 93% of total iodine in archived precipitation samples collected
during 2001-2006 in Roskilde, Denmark (Hou et al., 2009b). Iodine speciation analysis in lakes
samples has not been so numerous. Jones et al., (1984) detected both, iodide-127 and iodate-127 in
some British lakes and reported a reduction of iodate during spring and summer and oxidation of iodide
during autumn and winter. The authors suggested that the algae with active nitrate reductase are
responsible for iodate reduction during the spring and summer while iodide is oxidized in the surface
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