21
Iodine [I, 53]
21.1 INTRODUCTION
Iodine (I) is an element of the halogen group 17 in the periodic table of elements, and
is the least reactive of the elements in this group. Its content in the continental Earth’s
crust is variable and is estimated between 0.15 and 1.4 mg/kg, in the upper layer, and
0.45–0.8 mg/kg, in the bulk crust. When I is heated, it sublimates, that is, it goes
from being a solid to a vapor without going through the liquid state.
The highest contents of I are in sedimentary rocks; up to 38 mg/kg in argillaceous, and in some carbonate sediments (especially hardened calcium carbonate, caliche deposits). In organic-rich shales, its content is up to 17 mg/kg. Igneous
acidic rocks contain I up to 0.5 mg/kg. During weathering, I is rapidly released and
transported to ocean and sea basins. Most of I, nearly 70%, exists in ocean and sea
sediments.
Increased I contents in some phosphates, bituminous shales, and coal are associated with organic matter (OM). Its mean content in coal is about 1  mg/kg. It is
concentrated in some nitrate deposits, especially in Chile saltpeter, up to 400 mg/kg.
The suggestion of its atmospheric origin in saltpeter seems to be the most reasonable,
as I is very volatile from seas as CH 3 I and I 2 .
Iodine, occurring mainly at –1 oxidation state, is a gray-black substance, very easily sublimate to a blue-violet vapor, without going through the liquid phase. It forms
only a few own minerals, such as lodargirite (AgI) and marshite (CuI). They are rare
in the oxidized zones of metamorphosed Pb–Zn–Ag deposits and of porphyry Cu
deposits. Other I minerals, also not very common, are lautarite, Ca(IO 3 ) 2 ; bellingerit,
Cu 3 (IO 3 ) 6 ·2H 2 O; dietzete, Ca(IO 3 ) 2 ·CrO 4 ; and salesite, Cu(IO 3 OH). Sodium iodate,
NaIO 3 , is present in saltpeter, which is its main source. Iodine may also be obtained
from some seaweeds (e.g., kelps).
In 2010, iodine world mine production, without the United States (rounded) was
29,000 t, of which (in t) Chile produced 18,000; Japan produced 9,000; and China
produced 590 (USGS 2011). As in previous years, Chile is the world’s leading producer of I, followed by Japan and the United States. Chile accounted for more than
50% of the world I production.
Seawater contains I at about 0.05 mg/L. Its total content in seawater is approximately 34 Mt. Seaweeds of the Laminariaceae family are able to extract and accumulate up to 0.45% of I on a dry weight basis, and has been its source (Szefer 2002).
The seaweed industry represents its major source. Although not economical, the production of I is also as a by-product of gas, nitrate, and oil. Now, the most economic I
production is from caliche deposits.
Among many I isotopes, 127 I is the only stable isotope in nature. Two radioactive
isotopes 129 I and 131 I are the most common products of atomic reactors, and have
been released in various proportions into the environment.
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