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Environmental fate & pathways

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Important note:

All the data and information in this section relate to Indium and its compounds in general. Concentrations are expressed as "indium", not as specific In-substance. So, the data are relevant for In-substances in general.

Natural occurrence of indium

Indium is a metallic element found in nature mainly in zinc sulphide ores (sphalerites) at concentration of up to 1.1%. Concentration of Indium in rock-forming minerals is in the low mg/kg range. In sedimentary rocks, concentrations are about 0.05mg/kg. During weathering, Indium oxidises to In3+ , and usually precipitates under conditions forming hydrous Fe oxide. In soil, In is associated with organic matter and therefore may be concentrated in the surface soil horizon (FOREGs 2005).

In neutral aqeuous solutions, indium forms several insoluble compounds, such as In(OH)3, In2S3, In(CO3)3 and InPO4 (FOREGs 2005). The In(OH)3 form will be predominant at neutral pH values in natural water bodies (White and Shine 2016). More precisely, Indium will be in the insoluble In(OH)3 form (and thus precipitate in water) between pH values 5.6 and 9.7 (Chrysikopoulos and Kruger, 1986). Compounds of trivalent In are most stable and only the trivalent ion is stable in aqueous solution. Compounds of lower oxidation state undergo oxidation in the presence of water to form In3+ and elemental In (FOREGs 2005).

Environmental forms of In and relevance to toxicity

Indium occurs in the metallic state, or as indium compound, with two valency states (In+and In3+). The trivalent form is dominant and most stable under environmental conditions. While all environmental concentration data are expressed as “Indium”, toxicity is predominantly related to the In3+ion. For this reason, the sections on human toxicity and ecotoxicity are applicable to all indium compounds, from which indium ions are released into the environment. Most indium compounds (Cl-, Br-, NO3-, SO42-) are soluble in water; however due to very rapid transformation to the insoluble indium hydroxide form (In[OH]3) indium ions will not be present in the water column. This implies that the (eco-)toxicity potential will strongly decrease in natural systems.

For checking the potential of metal substances to release ions in the environment, a specific test, the transformation/dissolution (T/D) test is used. For metallic indium and some of the indium compounds, this test has been performed.

The results of this T/D test for the respective substances are presented in the respective IUCLID sections 4.8.

All T/D data obtained on In-substances are summarised in table below. It follows from these results that all indium substances seem to be characterised by this rapid transformation/precipitation out of the water column, under conditions of environmentally relevant pH. 

 substance  loading in T/D test (mg/L)  test pH  duration test (days)  In measured (µg In/L)
 InCl3  1  6  7  <0.05
   1  28  <0.02
   1  7  <0.01
   1 28  <0.01 
 In2O3  100 7    <1.0
   1 28 <1.0 
 In2S3  100 <1.0 
   1 28  <1.0 
 Indium powder  100 <1.0 
   1 28  <1.0 

Like well documented processes for mono- (Ag), di- (Cd, Cu, Ni, Pb, Zn), and tri-valent (Al, Fe) metals, when indium ions are formed in the environment, they will further interact with the environmental matrix and biota. As such, the concentration of indium ions that is available to organisms, the bioavailable fraction, will depend on processes like dissolution, absorption, precipitation, complexation, inclusion into (soil) matrix, etc. These processes are defining the fate of indium in the environment and, ultimately, its ecotoxic potential. This has been recognised e.g. in the guidance to the CLP regulation 1272/2008 (metals annex): "Environmental transformation of one species of a metal to another species of the same does not constitute degradation as applied to organic compounds and may increase or decrease the availability and bioavailability of the toxic species. However, because of naturally occurring geochemical processes, metal ions can partition from the water column. Data on water column residence time, the processes involved at the water – sediment interface (i. e. deposition and re-mobilisation) are fairly extensive, but have not been integrated into a meaningful database. Nevertheless, using the principles and assumptions discussed above in Section IV.1, it may be possible to incorporate this approach into classification."

The issue of degradation (IUCLID section 5.2.) is not applicable to inorganic compounds.

Environmental concentrations

Indium has been reported to occur at average concentration of 52 µg/kg in the earth's crust (Wedepohl 1995). Median In levels were estimated to be 56µg/kg (Rudnick and Gao 2004). Topsoil and subsoil levels were evaluated to be 50µg/kg; the range varying from <0.01 - 0.25 mg/kg in subsoils and up to 0.41 mg/kg in top soils (FOREGs 2005). Higher values are observed in mineralised areas in several EU countries e.g. N-Portugal, NW-Spain, the Pyrenees, the Central Massif and Brittany in France, Cornwall, Wales, W-Austria, Slovenia and coastal Croatia, and scattered point anomalies in Greece (FOREGs 2005).

Natural water contains picomolar concentrations (~=0.1ng/kg; White and Hemond 2012); dissolved background in EU was measured as <2ng/l (FOREGs 2005). FOREGs (2015) mentions indium in stream values ranging between <0.002 - 0.015 µg/l, with > 90% of the data below the analytical limit of quantification. The concentration of In in the atmosphere at the south pole has been measured as 53 fg/m3 (Maenhout and Zoller 1977). Present-day environmental levels relevant for the EU environment are reported in IUCLID section 5.5.

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