Registration Dossier

Environmental fate & pathways

Endpoint summary

Administrative data

Description of key information

Additional information

Negligibility of Chloride and Sulphate

The submission item category comprises several soluble iron salts marketed solid and/or in watery solution. When these substances get in contact with water in environmental media (washout from the atmosphere by rain, release to surface waters, soil moisture) they will deliver chloride (CAS 16887-00-6) and sulphate (CAS 14808-79-8) anions into the water column, while the iron kations will speciate in several steps. Thus the anions and kations will follow their own, independent pathways is the environment. Chlorides and sulphates are omnipresent in the environmental media and in biota, where they are involved in physiological processes or regulated and used as a source of supply.

Thus the environmental fate assessment of the submission item category members must be based on the iron kations considering their speciation, while the anions can be considered nontoxic and naturally ubiquitary present in the environment.

Environmental iron levels (natural background)

Values for the Chemical Safety Assessment

Iron is a frequent naturally occurring element that is present at considerable levels in all environmental media. Natural sources of iron tend to contribute more to the environmental occurrence of this element than anthropogenic sources. The following table summarizes the levels considered as background concentrations in the present assessment.

Table: Environmental medium concentrations of iron (upper level) used for Chemical Safety Assessment (CSA)

Urban air

Marine water

Surface freshwater, dissolved

Freshwater sediment

Soil

[µg Fe/m³]

Reference

[µg Fe/L]

Reference

[µg Fe/L]

Reference

[g Fe/kg dw]

Reference

[g Fe/kg dw]

Reference

6

WHO 2003

5

OECD 2007

66

Gaillardet et al. 2005

90

OECD 2007

38

OECD 2007

It should be noted that some of the figures may deviate from those in Table A3.1 of the SIAR document (OECD 2007). Nonetheless the above listed environmental levels are in comparable ranges.

The background concentrations have to be compared with the respective PNEC according to ECHA (2008, Appendix R.7.13-2), which means that depending on the calculation the Added Risk Approach or the Total Risk Approach applies accordingly. In case the background is found significant compared to the PNEC, the Added Risk Approach will be to be applied, while in case PNECs are in the order of the background the Total Risk Approach will be used. Whenever no PNEC can be derived the background levels will be compared with PECadd (increase of the total concentration caused by the uses of the submission item). Insignificant PECadd compared to the background concentrations, i.e. < 1 % increase, will be evaluated as not hazardous to the environment.

Natural occurrence of iron

Iron is the fourth most abundant element in the Earth's crust accounting for approximately 5 % by weight (Horne 1978, p 74, table 3.6; Lindsay 1979, p 7, table 1.1). Iron is found in various minerals (as ferric chloride, ferrous chloride, ferric sulphate, ferrous sulphate and their mixtures, oxides, hydroxides, oxyhydroxides and sulphides) and in nearly all soils, sediments and mineral waters (OECD 2007, p 97). Total (dissolved and suspended) iron levels range from 0.002 (Drever 1982, p 234, table 10-1; Stumm & Morgan 1981, p 568, table 9.7) or 0.01 mg/L in seawater (Horne 1978, p 322, table 8.10; Horne 1978, p 349, table 8.20) up to 0.1 - 10 mg/L in fresh water (OECD 2007, p 97). This range is basically in line with the overview of Hem (1985, p 84-5, table 14). A common range in surface soils is 0.5 - 5 % w/w with typical levels of 2.5 % w/w (Khalid et al. 1977, p 106) or 3.8 % w/w (Lindsay 1979, p 7, table 1.12). Nonetheless in iron-rich points of surface soils 7.97 to 29.65 % w/w can be found (Kabata-Pendias & Pendias 1984, p 37, table 16), but even 55 % w/w (Lindsay 1979, p 7, table 1.1) is not abnormal. A range of 1 - 9 % w/w in sediments resulted from literature surveys (OECD 2007, p 97; Khalid et al. 1977, p 106; Gambrell et al. 1997, p 222). Overall, the data show that iron is present naturally in abundance in all environmental compartments. In water the solubility of the hydroxide and oxides is a limiting factor for the ones of dissolved iron forms keeping them low (Morel 1983, p 113-6, 202-11; Stumm & Morgan 1981, p 238-49 and p 265-8). The concentration of iron in suspended forms including nanoscaled materials can be significantly higher. Generally literature stating iron concentrations and/or effects refer to dissolved iron (which is concluded to be measurable after filtration) or total iron (which is determined without any filtration step), but does not differentiate on the basis of particle size data. The dissolved concentrations of iron tend to be low whereas soil and sediment concentrations can be high.

Atmospheric fate and pathways

Volatilisation can generally be ignored for metals, except for several organometallic compounds, which are neither present in the submission item nor formed in the environment. Entering the atmosphere from water is irrelevant for the submission item due to the ionic nature of the constituents and no relevant release to the atmosphere is expected. Iron as contained in the submission item may exist in air as suspended particulate matter originating from industrial emissions or erosion of soils. Most of the metal species present in the atmosphere will be bound to aerosols, i.e. the aerosol-bound fraction is almost one. Metal containing particles are assumed to be mainly removed from the atmosphere by gravitational settling, with large particles tending to fall out faster than small particles. The half-life of airborne particles is assumed to be in the order of days. Some removal by washout mechanisms such as rain may also occur, although it is of minor significance in comparison to dry deposition.

Indirect photolysis by hydroxyl radicals and direct phototransformation in the air are considered irrelevant, while speciation in airborne droplets may occur and include (photo-) reduction, oxidation and hydroxylation. Speciation of iron kations in airborne aerosol droplets can be assumed to be comparable to the processes in waters. These processes may take place also in dry airborne particles and are expected to gain importance with an increasing surface / volume ratio, i.e. with shrinking particles.

Aquatic fate and pathways

The substances in the submission item readily convert to the naturally occurring (metal) species. They will integrate into the equilibrium system between the large sediment reservoirs and the dissolved species under environmental conditions. Biodegradation is not relevant as the submission item's constituents and their environmental transformation products are inorganic and thus a priori mineralized. No abiotic formation of organometallic species is anticipated and the Haemoglobin formation and fate is a highly regulated process of no concern in biota. Bioaccumulation poses no concern as indicated by the inverse medium level - BCF relationships and biodilution with increasing trophic levels. Whereas significant BCF in the order of 10'000 L/kg occur in organisms of lower trophic levels, all reported BCF for freshwater fish are ≤ 930 L/kg.

Iron forms metal hydroxides that are rapidly removed from the water column at various pH values and tend to sorb on particles. It is thus to conclude that after release to the aquatic environment the submission item will be removed quickly from the water column due sorption and formation of insoluble compounds. The aquatic bioavailability is thus limited, but influenced by transformation/speciation processes. Under the influence of daylight iron is efficiently reduced to the ferrous (Fe2+) form. The kinetics of this process overcompensate the aerobic oxidation in light-flooded upper layer of the water column. Iron is subject to transformation form organically bound species to ionic forms under intense light conditions. Subsequent hydroxylation and precipitation are supposed mechanisms of the transport from the water bodies to the sediments. These hydroxides either polymerise to form larger insoluble stable complexes or they are trapped and buried in sediments (IHCP 2009, p 489-90).

Terrestrial and sediment fate and pathways

Measured partition coefficients are used to describe the sorption of metals to sediments, which base on determination of the sum of the respective metal species from environmental samples. These data are reflecting the speciation equilibrium occurring under environmental conditions, which is therefore regarded in the derived Kd values. Based on these environmental concentration ratios, iron is considered immobile or non-mobile and Log Kd sed 4.997 L/kg dw and Log Kd susp 2.34 L/kg dw are used. Comparable behaviour in soils is likely.

Detoxifying effects of iron in sediments

The following two sections are taken from the Metal Environmental Risk Assessment guidance document (ICMM 2007) as they show that iron is generally considered to contribute to metal detoxification in the environment rather than causing toxic effects.

Simultaneously Extracted Metal – Acid Volatile Sulphides (SEM-AVS) concept

In anoxic sediments, sulphide produced by sulphate reduction reacts with Fe 2+ ions to form insoluble iron sulphides such as amorphous iron sulphide, mackinawite, greigite, pyrrthotite, troilite, and pyrite (Wang & Chapman 1999).

Although pyritic sulphide phases are both abundant and reactive towards trace metals, iron monosulphides, quite often referred to as Acid Volatile Sulphides (AVS), are considered to be a more reactive sulphide pool (ICMM 2007).

Di Toro et al. (1990 & 1991) have proposed an SEM/AVS Model based on the recognition that AVS is a reactive pool of solid phase sulphide available to bind to metals, forming insoluble metal-sulphide complexes that are non-bioavailable while releasing Fe 2+ kations (ICMM 2007).

Solubility constants have been experimentally determined for a large number of compounds and tabulated data are readily available. For ionic compounds the constants are called solubility products. Concentration units are molar in the following table.

Table: Metal sulphide solubility products (Ksp)

Compound

Formula

CAS

EINECS

Temperature [°C]

Ksp [molar]

Data Source
(legend below)

Mercury(II) sulphide

HgS *

1344-48-5

215-696-3

18

4×10–53to
2×10–49

C, K, L

Copper sulphide

CuS *

1317-40-4

215-271-2

25

4×10–38

K

Lead sulphide

PbS *

1314-87-0

215-246-6

25

1.1×10–29

K

Cadmium sulphide

CdS *

1306-23-6

215-147-8

18

3.6×10–29

C, L

Zinc sulphide

ZnS *

215-251-3

1314-98-3

25

7×10–26

K

Nickel sulphide

NiS *

16812-54-7

240-841-2

18-25

10–27

P (less soluble form)

18

1.4×10–24

C, L

18-25

10–21

P (more soluble form)

Manganese sulphide

MnS

18820-29-6

242-599-3

25

10–22

P (green form)

18

5.6×10–16

K

1.4×10–15

C, L (pink form)

Iron sulphide

FeS

1317-37-9

215-268-6

18-25

3.7×10–19

C, K, L

Data source legend:

  • C = Hodgman (1962 CRC Handbook 44th edn)
  • L = Lange & Forker (1956, Lange's Handbook of chemistry, 10th edn)
  • K = Kaltofen (1986 Table 12.8, p 174)
  • P = Pauling (1970)

* Reference point in the sequence according to ECHA (2008) Guidance on information requirements and chemical safety assessment, Appendix R.7.13-2, p 44

Fe (oxy)hydroxides

Although sulphides have been identified as a main factor for buffering the bioavailability of metals in (anoxic) sediments, toxicity may still not be seen even if the sulphide pool becomes exhausted. This shows the importance of other binding phases, e.g. organic ligands and dissolved/colloidal iron oxides (Müller & Sigg 1990), which in addition contribute to the reduction of metal bioavailability.

Conclusion

In conclusion, the submission item is transformed into naturally occurring inorganic, inert compounds after release to the environment, which get depleted from the atmosphere and the water column by fallout, washout and precipitation. Iron precipitate as sulphide, (oxo)hydroxides and oxides, which contribute to the detoxification of other metals. Sorption and with time polymerisation of the hydroxides keep them biounavailable. Nonetheless some temporary remobilization may happen due to pH and redox changes as it is the case for natural background iron, whose quantity outreaches by far the anthropogenic one. Eventually these iron compounds will fill the natural sinks with no chronic influence on bioavailability due to incorporation into the soil and sediment matrices. That way they are trapped and buried without relevantly changing these media as they contain already significant amounts of iron.

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