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Trace Elements in Abiotic and Biotic Environments
r Ferrihydrite, Fe 2 O 3 ·nH 2 O, is not stable, but common mineral in soils.
r Ilmenite, FeTiO 3 , not common in soils, is inherited from parent rocks and
is resistant to weathering.
r Pyrite, FeS 2 , and other ferrous sulfide minerals are widely distributed in
submerged soils.
r Vivianite, Fe 3 (PO 4 ) 2 ·2H 2 O, and hydrous ferric phosphate, FePO 4 ·nH 2 O, are
formed mainly under the influence of bacterial processes.
r Siderite, FeCO 3 , occurs mainly in bog ores and sedimentary deposits.
Microbial oxidation of Fe occurs under both aerobic and anaerobic conditions,
and stimulates the precipitation of poorly crystalline Fe oxides, which have a great
affinity for the sorption of several cations, mainly Cu, Cd, Mn, Ni, Pb, and Zn.
About 18 Fe minerals are listed as possibly biologically induced. Siderophores are
Fe-chelated compounds secreted by microorganisms (also possibly by grasses), and
are the strongest soluble Fe 3+ binding agents.
Since about 3000 BC, iron has been used by humans and has played a dominant
role in human civilization. It is the most commonly used metal in all civilizations.
Estimated world mine Fe production (rounded) in 2010 was (pig iron and raw steel,
respectively) 1000 and 1400 Mt, of which China produced 600 and 630; Japan produced 82 and 10; Russia produced 47 and 66; the United States produced 29 and 90
(USGS 2011). A great proportion of Fe is obtained from Fe recycling and steel scrap.
Iron is the least expensive and the most widely used metal. In most applications,
Fe and steel compete with other nonmetallic and/or metallic materials. However, as
a very chemically reactive metal, Fe tarnishes rapidly in air and water. Alloys of Fe
with other metals increase its resistance to corrosion. Nevertheless, effects of corrosion, on a global scale, are a serious source of this metal in various environmental
compartments. Rusting (corrosion) is the reaction of Fe with oxygen present in air
and with water, and leads to the formation of rust, red-brown hydrated Fe 2 O 3 .
23.2 SOILS
Iron abundance in soils averages about 3.5%, but varies highly from below 1% in
light sandy soils, up to 5% in some organic soils (Table 23.1). Lateritic soils, formed
in hot and wet tropical areas, are especially enriched in Fe (up to 30%), and are
rusty-red because of Fe oxides. In soils, Fe occurs mainly in the forms of oxides and
hydroxides, as small nanoparticles or in amorphous forms, associated with other soil
particles and minerals. Iron minerals also are formed pedogenically and biologically. Free Fe minerals and compounds are used as a key characterization for soils
and soil horizons.
Goethite is the most frequently occurring mineral, and is involved in sorption
processes: (1) absorption of metals at external surface; (2) solid-state diffusion of
metals; and (3) metal binding inside mineral particles. A strong ability of goethite
to bind some metals suggests that it might be used, as well as other Fe hydroxides,
for amelioration of metal-contaminated soils (Kumpiene et  al. 2005). Especially
Fe–Mn concretions reveal a great sorption capacity to several trace elements. They
may accumulate Pb up to 12,000 mg/kg, and Zn up to 1,800 mg/kg. Relative metal