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Environmental fate & pathways

Adsorption / desorption

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Description of key information

Iron is considered immobile or non-mobile, Log Kd sed 4.997 L/kg dw and log Kd susp 2.34 L/kg dw under environmental conditions (12 °C) are used.
Geochemical conditions control the bioavailability as they outreach anthropogenic releases, which are thus unlikely to influence chronic bioavailability due to speciation burying iron eventually into naturally existing sinks, which represent large reservoirs.

Key value for chemical safety assessment

Other adsorption coefficients

Type:
log Kp (sediment-water)
Value in L/kg:
4.997
at the temperature of:
12 °C

Other adsorption coefficients

Type:
log Kp (suspended matter-water)
Value in L/kg:
2.34
at the temperature of:
12 °C

Additional information

This endpoint is covered by the category approach for soluble iron salts (please see the section on physical and chemical properties for the category justification/report format).

Testing for this endpoint has been waived as it is considered scientifically unjustified.

Appropriateness

It is considered inappropriate to use the Kow and Koc concept for inorganic compounds as outlined in the waiving argumentation. Adsorption/desorption as a partitioning process associated with organic carbon, is not a relevant endpoint for these inorganic salts and bioavailability is considered the more relevant criterion. Thus a waiving statement is made.

Nonetheless some information derived from monitoring of water and corresponding sediment or suspended matter is available and discussed below. The reported Kd values reflect the total metal concentration ratio in equilibrium for all species under environmental conditions, where 12 °C are assumed. It is likely that the data reflect the environmental speciation behaviour.

In water, there is a rapid formation and deposition of iron oxides and/or hydroxides, and other salts in sediments. Iron species are rapidly removed from solution as insoluble precipitate at oxic conditions and moderate pH. Thus any direct impact of dissolved iron kations on the aquatic environment will be reduced.

The bioavailability of metals depend more on the geochemical situation but is characterized by equilibration between the dissolved fraction a large soil and/or sediment reservoirs making them frequent metals in the biosphere. Iron is present in all environmental media with large reservoirs in soils and sediments. Comparison of the environmental levels with the additional release according to the exposure scenario shows clearly that the additional releases contribute insignificantly.

Iron partitioning and mobility in soils

Roychoudhury & Starke (2006) examined the partitioning of trace metals between surface water and sediments and their fate within the sediments of a river in South Africa in which mine water was drained. A median Log Kd sed (solids-water in sediment) value of 4.997 L/kg dw for 20 locations with 5th, 10th, and 95th percentile values of 4.687, 4.74, and 5.501 L/kg dw respectively, were reported. The mean Log Kd sed was 5.078 L/kg dw and the values ranged from 4.38 to 5.81. Li (2000) lists a world average reference value of 6.08 L/kg.

Veselý et al (2001) studied partitioning between solids (suspended matter) and water by filtration and dialysis in situ in Czech freshwaters. Field-based distribution (partition) coefficients, Kd, between suspended particulate matter and filtrate (“dissolved” fraction) were derived. The median Log Kd susp (solids-water in suspended matter) for samples from 54 rivers in 119 localities was 2.34 L/kg.

Environmental fate is dominated by abiotic and physico-chemical processes, including precipitation and settling. Iron reactions in soil comprise precipitation, hydrolysis, complexation, and redox processes. The iron released precipitates readily as oxides and hydroxides. Iron integrates into the mineral structure of soil where it gets buried as it substitutes for manganese and aluminium in other minerals. Iron often form complexes with inorganic ligands and occurs mainly in the forms of oxides and hydroxides either as small particles or in association with the surfaces of other minerals. Soil iron shows a great affinity to form mobile organic complexes and chelates which are important compounds responsible for the migration of iron between soil horizons and the leaching of iron from soil profiles and also supply of iron to root of plants (Kabata-Pendias & Pendias 1984, Kerndorff & Schnitzer 1980).

A number of processes influence the iron bioavailability of iron, which interacts with the bioavailability of other metal species. Significant adsorption of other metals to iron oxides occurs under environmental conditions. The process is dependent on pH and is greatest for various ions at pH 4-5 (Drever 1982, Kabata-Pendias & Pendias 1984, Khalid et al 1977, Windom 1973).:

  • Precipitation of iron oxides during dredging operations has been reported to decrease dissolved concentrations of Cd, Cu, Pb and Zn.
  • Iron (and manganese) are used as scavengers in wastewater treatment and may be very effective in retarding the migration of pollutants in the subsurface.
  • Deep-sea nodules, concretions of manganese and iron oxides growing on the ocean floor, are known to concentrate trace elements, such as Co, Ni, Zn and Pb from seawater.
  • In soils, various trace elements are concentrated by iron oxides, including Zn, Pb, Mn, Ni, Cu, Co, V, Mo and Cr.
  • The most widely observed sorption capacities of iron oxides are those for phosphates, molybdates and selenites.
  • Iron oxides also reacts with carbonates in soil systems.
  • Fe (III) can be incorporated in hydrated phosphates and Fe (II) reacts with sulphur to form stable minerals pyrite and jarosite.

Concerning data interpretation of the measured Kd in sediments, the available data clearly indicate that the iron species have to be considered as “immobile” (Kd or Koc > 5000) or “non-mobile” (Kd or Koc > 4000) in soils or sediments according to the scales of McCall et al (1981) or Hollis (1991), respectively. Nonetheless the sorption to suspended matter is less strong.

  • Drever JI (1982). The geochemistry of natural waters; Prentice-Hall, Englewood Cliffs, NJ, U.S.A.
  • Hollis JM (1991). Mapping the vulnerability of aquifers and surface waters to pesticide contamination at the national/regional scale. Pesticides in Soils and Water: Current Perspectives (Walker A, ed), BCPC Monograph 47:165-74.
  • Kabata-Pendias A, Pendias H (1984). Trace elements in soils and plants; CRC Press, Boca Raton, FL, U.S.A.
  • Khalid RA, Gambrell RP, Verloo MG, Patrick WH (1977). Transformations of heavy metals and plant nutrients in dredged sediments as affected by oxidation reduction potential and pH; U.S. Army contract N DACW39-74-C-0076.
  • Kerndoff H, Schnitzer M (1980). Sorption of metals on humic acid. DOI 10.1016/0016-7037(80)90221-5 Geochim Cosmochim Acta 44(11):1701-08.
  • McCall PJ, Laskowski DA, Swann RL, and Dishburger HJ (1981) Measurement of sorption coefficients of organic chemicals and their use, in environmental fate analysis IN Test Protocols for Environmental Fate and Movement of Toxicants. Proceedings of AOAC Symposium, AOAC, Washington DC, U.S.A:94-109.
  • Windom HL (1973). Investigations of changes in heavy metals concentrations resulting from maintenance dredging of Mobile Bay Ship Channel, Mobile Bay, Alabama, Mobile District. U.S. Army Corps of Engineers, Mobile, Alabama. Contract No. DACW01-73-C-0136.