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Environmental fate & pathways

Biodegradation in water: screening tests

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Description of key information

Not relevant, but due rapid removal from the water column regarded as “rapidly biodegradable” for classification purposes

Key value for chemical safety assessment

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 in accordance with column 2 restrictions. Biodegradation as such is not a relevant endpoint for these inorganic salts. Environmental fate is dominated by abiotic and physico-chemical transformation processes (speciation). The standard guideline tests are not appropriate and thus a waiving argumentation is provided. However, since iron is an essential element, it is also subject to biological activity and control. Bacterial transformations of iron are extensively studied and basically well-known processes. Iron transformations and the whole iron cycle in the environment is a combination of abiotic and biological processes. These processes were discussed in the section summing up on “Biodegradation” above.

Treatement with regard to classification criteria

It was considered in the recent EU classification (DSD) that the reduction of soluble iron concentrations below measured chronic NOECs could be considered as equivalent to “rapid degradability” leading to a “no environmental hazard” classification decision (Wildey et al. 2009, p 16).

As first defined in the Glossary of OECD (2001), the term “degradation” refers to the decomposition of organic molecules. “For inorganic compounds and metals, clearly the concept of degradability, as it has been considered and used for organic substances, has limited or no meaning. Rather, the substance may be transformed by normal environmental processes to either increase or decrease the bioavailability of the toxic species. Equally, the log Kow cannot be considered as a measure of the potential to accumulate. Nevertheless, the concepts that a substance or a toxic metabolite/reaction product may not be rapidly lost from the environment and/or may bioaccumulate are as applicable to metals and metal compounds as they are to organic substances” (OECD 2001 STA 27 paragraph 293 p 97, IHCP 2009 p 485, ECHA 2011 p 471, ECHA 2012a p 497, ECHA 2012b p 514).

“Speciation of the soluble form can be affected by pH, water hardness and other variables, and may yield particular forms of the metal ion which are more or less toxic. In addition, metal ions could be made non-available from the water column by a number of processes (e.g. mineralisation and partitioning). Sometimes these processes can be sufficiently rapid to be analogous to degradation in assessing chronic classification. However, partitioning of the metal ion from the water column to other environmental media does not necessarily mean that it is no longer bioavailable, nor does it mean that the metal has been made permanently unavailable” (OECD 2001 STA 27 paragraph 294 p 98, IHCP 2009 p 485, ECHA 2011 Annex IV.1 p 471-472, ECHA 2012a Annex IV.1 p 498).

Annex IV.3 (IHCP 2009 p 510, ECHA 2011, p 475, ECHA 2012 p 503, ECHA 2012b p 514), Assessment of environmental transformation, lays down criteria for replacing degradation by bioavailability (wording almost taken from OECD 2001 STA 27 paragraph 308 p 100): “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 as a result 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.”

In the same section these sources (IHCP 2009 p 489, ECHA 2011 p 475) state more precisely how acceptable evidence for rapid removal from the water column can be derived. “It can be based on (1) laboratory tests evaluating changes of metal species to less soluble metal species or (2) laboratory/mesocosm and/or field tests evaluating removal of soluble metal species through precipitation/partitioning processes over a range of environmentally relevant conditions.” This part is removed since Version 2.0 of the ECHA guidance (ECHA 2012a, ECHA 2012b) guidance due to ongoing discussions on its general applicability of the concept (ECHA 2012a p 503, ECHA 2012b p 519). Nonetheless it can be assumed that the reasoning regarding iron is still valid and applicable as the Example D (Hazard classification of a soluble metal salt: the case of rapid environmental transformation through speciation in the water column, p 535), which demonstrates obviously the case of iron (Me = Fe as they share the molecular weight of 55.84, ECHA 2012a p 536, ECHA 2012b p 553) sulphate, is kept in the guidance document.

In the explanation to (1) iron is used as an example by IHCP (2009 p 489) and the former ECHA guidance (2011 p 475): “Changes in metals species may result in initial solubilisation but rapid formation of less soluble metal species and subsequent rapid removal from the water column through precipitation processes as observed for some metals (i.e. Fe and Al). After an initial solubilisation (soluble metal compounds) or transformation/dissolution (sparingly soluble and metal compounds) of 1 mg metal/metal compound/L, test solutions are left unstirred and metal concentrations are monitored as a function of time during a 28 days period. The potential for rapid removal of the solubilised metal ions from the water column is assessed during the 28 days period at the relevant pHs. If it can be demonstrated that during the 28 day period, the dissolved metal species are removed to a level below the chronic toxicity (NOECs) of the soluble metal species, then this can be taken as fulfilling the criterion for rapid removal from the water column. This removal process should be supported by thermodynamic modelling of chemical speciation changes in support of the loss of metal from the water column to insure that the loss mechanism is not sorption to the test vessel. Aluminium, iron and tin all form metal hydroxides that are rapidly removed from the water column at various pH values. With time, these hydroxides either polymerise to form larger insoluble stable complexes or they are trapped and buried in sediments.”

In conclusion iron can be treated as “rapidly biodegradable” with regard to classification and labelling.

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  • ECHA European Chemicals Agency (2012a). Guidance on the Application of the CLP Criteria. Version 2.0, April 2012. Self-published, Helsinki, Finland. Reference: ECHA-12-G-06-EN, Date: 04/2012. 539 p.
  • ECHA European Chemicals Agency (2012b). Guidance on the Application of the CLP Criteria. Version 3.0, November 2012. Self-published, Helsinki, Finland. Reference: ECHA-12-G-14-EN, Date: 11/2012. 573 p.
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  • OECD Organisation for Economic Co-operation and Development (2001). Guidance Document on the Use of the Harmonised System for the Classification of Chemicals which are hazardous for the Aquatic Environment. Self-published OECD Environment Directorate, reference ENV/JM/MONO(2001)8. OECD Environment, Health and Safety Publications Series on Testing and Assessment Number 27. 115 p.
  • Wildey RJ, Girling AE, Fisk PR (2009). Position paper: Arguments against additional long-term ecotoxicity testing for the REACH compliance of the iron salts category with supplementary discussion of the approach to PNEC. Peter Fisk Associates Ltd., Kent, U.K. in October. 17 p.