Natural Colloids and Nanoparticles in Aquatic and Terrestrial Environments - Environmental and Human Health Impacts of Nanotechnology - page 113

carbonates, natural organic matter (NOM) and iron coatings (Mukhopadhyay and
Walther, 2001 ).

4.3.1.2 Oxides and Hydrous Oxides

Iron exists in the environment in two redox states, Fe(II) and Fe(III). Fe(II) is stable
at low pH or in the absence of oxygen or other oxidants, is soluble and relatively
free from complexation, whereas Fe(III) is stable in the presence of oxygen and
insoluble at neutral pH. It is well documented that solid phase Fe(III) oxides and
oxyhydroxides are formed by oxidation and hydrolysis of Fe(II) by subsurface
aeration (Wolthoorn et al. , 2004) at oxic/anoxic boundaries in groundwater
(Christensen et al. , 2001), freshwater lakes (Balistrieri et al. , 1992 ) and coastal
marine water (Gunnars et al. , 2002). In freshwaters, the oxidation process typically
results in colloidal particles having a mean diameter in the range of 0.05- 0.5 m

(Lienemann et al. , 1999 ; Tipping et al. , 1981) and often lower in the nanoscale range.
The presence of dissolved species, such as silicate, phosphate and organic matter,
can affect the composition, structure, morphology and reactivity of these hydrolysis
products (He et al. , 1996 ; Kandori et al. , 1992; Mayer and Jarrell, 2000). The trans-
formation between dissolved Fe(II) species and solid Fe(III) oxyhydroxide phases
at oxic-anoxic interfaces is central in the cycling of iron in aquatic environments
(Davison, 1993), that is in the production and removal of particles in the different
environmental compartments. These processes are highly dependent on pH and can
be controlled by microorganisms (Fredrickson et al. , 1998 ; Lovley, 1997 ; Pronk and
Johnson, 1992). For more information, the reader is referred to the literature on
iron cycling and particles in freshwater (Davison, 1993; Davison and De Vitre, 1992;
Stumm and Sulzberger, 1992), the biogeochemistry of iron in seawater (Turner and
Hunter, 2001) and the formation and occurrence of biogenic iron-rich minerals
(Fortin and Langley, 2005).
Naturally occurring iron oxide particles are very complex in structure. They exist
under different crystalline and/or amorphous forms, such as haematite (

µ

α - Fe 2 O 3 ),

goethite ( - Fe 2 O 3 ), magnetite

(Fe 3 O 4 ) and ferrihydrite (amorphous Fe(III) phase) (Davison and De Vitre, 1992).
Iron oxides formed in natural aquatic systems are not pure oxides but contain a
signifi cant amount of other elements (Mavrocordatos et al. , 2000 ; Mavrocordatos
and Fortin, 2002; Perret et al. , 2000) and are usually complexed or coated by NOM
(Allard et al. , 2004). In aerated sediment porewaters, freshly formed iron oxides
co-precipitate heavy metals as lead, copper and zinc (v.d. Kammer et al. , 2003 ).
Iron oxides control different environmentally relevant processes in soil, fresh-
water, groundwater and oceans such as the attenuation of bacteria and viruses
(Scholl et al. , 1990), the fate and transport of trace contaminants (Tessier et al. ,
1996) and limited ocean primary productivity (Behrenfeld et al. , 1996 ; Martin and
Fitzwater, 1988). Iron particles are one of the main vectors of trace metals transport
in aquatic systems (Lyven et al. , 2003 ).
In aquatic and terrestrial environments, mangenese (hydr)oxides are strongly
affected by redox reactions. The oxidation of Mn(II) to Mn (III, IV) is thermody-
namically possible at neutral pH and atmospheric oxygen concentrations, though

α - FeOOH), lepidocrocite ( γ - FeOOH), maghaemite ( γ