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Environmental fate & pathways

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For inorganic substances present in the natural environment, discussion is necessarily qualitative and modelling processes cannot be used easily. For an ubiquitous substance, the measured environmental concentrations (see discussion of Environmental fate and pathways) constitute sufficient information about ultimate environmental fate and behaviour. Under normal aerobic environmental conditions, anthropogenic iron salt emissions will primarily mineralise with water, precipitate as ferric hydroxide and be incorporated into soil and sediment, and participate in the natural geochemical processes of iron. Under anaerobic conditions or low pH other processes will dominate the environmental fate of iron.

Soil is the primary reservoir of naturally occurring iron. It has its own surface geochemical cycle. Iron can be mobilized from soil or sediment to surface waters as colloidal ferric hydroxide, fine suspended particulates and inbound to clay silt. Factors like pH, CO2 concentration, redox conditions, availability of organic and inorganic complexing agents and soil type contribute to reactions of iron in soil. In soil iron can be bound to organic humic substances (see below), which can be soluble, colloidal or precipitates depending on the environmental factors (Lahermo et al 1996).

Mass balances and precipitation of iron were studied in two acidified (pH < 5) Czech lakes in 2000 - 2003, where influent iron was mainly in the organically bound form. Photochemical reactions were observed to liberate iron to the inorganic form, subject to precipitation of iron hydroxides. These hydroxides were concluded to decrease the availability of orthophosphate to phytoplankton, and to increase the amount of iron hydroxides in lake sediment, changing its phosphate sorption characteristics (Kopácek et al. 2005).

Sediments contain Fe(III) as insoluble oxides, but also in an organically complexed and colloidal state. Even though 60 – 80 % of iron is bound to silicates in marine sediments, Fe(III) hydroxides are considered to be the main species linked to binding and cycling of phosphorus in surface waters. They can act as chemical coagulants of external phosphorus loading but also as internal source of phosphorus under strong reducing conditions, caused e.g. by sedimented organic matter. Basic environmental factors are as availability of oxygen, pH, sulphate, microbiological reduction and sedimented organic matter. Dynamics of phosphorus has been studied in brackish marine sediments (Gulf of Finland) in detail (Lehtoranta 2003).

  • Lehtoranta J (2003). Dynamics of sediment phosphorus in the brackish Gulf of Finland. Monographs of the Boreal Environmental Research 24. Finnish Environmental Institute, Helsinki, Finland. 58:6-15.
  • Lahermo P et al (1996). Geochemical Atlas of Finland. Part 3: Environmental Geochemistry – stream waters and sediments. 5.12. Rauta (Fe). Pp. 113:79–81.
  • Kopácek J, Klementova S, Norton SA (2005). Photochemical Production of Ionic and Particulate Aluminum and Iron in Lakes. Environ Sci Technol 39(10):3656 -62.