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Environmental fate & pathways

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Toxic substances that persist in the environment are considered more hazardous than substances that are readily degradable. Therefore, more severe chronic hazard classes are assigned to non-degradable substances (GHS, 2011 Table 4.1.1.). While it has been recognized that “rapid degradability”, as defined for organic substances, does not apply to metals (GHS, Annex 9, A9.7.3.1, 2011), the concept is critical to the environmental classification of metals and metal compounds.

 

Through multi-metal co-operation, in accordance with the principles outlined in the GHS and EU CLP guidance (ECHA, 2011), the potential for ‘rapid loss from the environment’ of metal ions (e.g. Cu, Zn, Pb, Ni, As, Cd) has been evaluated by assessing the removal rates of metal ions through partitioning and their subsequent potential for sediment mineralization/remobilization. The assessments have been done using a weight of evidence approach based on laboratory/mesocosm studies, field data and/or metal fate model.

 

The model assessment is based on The Tableau Input Coupled Kinetics Equilibrium Transport Unit World Model for Metals in Lakes (hereafter referred to as TICKET-UWM and available from http://blog.unitworldmodel.net), developed to assess the complexities and fate of metal speciation and its influence on effects of metals in the environment.

 

The principles and methodologies were discussed at the European Chemicals Agency (ECHA) in a workshop carried out on February 8th, 2012. The following conclusions were drawn:

·      Metals that methylate, such as Hg, are not rapidly degraded;

·      Metals that quickly hydrolyze and form different species that precipitate in the water column (Fe, Al, Sb, Sn, Mo, Cr...) are considered rapidly degraded;

·      For the third group of metals (Cu, Zn, Ni, Pb), where the partitioning as well as the binding to Acid Volatile Sulfides are important factors for determining the rate of substance removal from the aquatic compartment, a consensus was not reached. Different views were expressed as to what extent the intrinsic properties of metals drive partitioning and binding to the sediment and what are the parameters (type of information) required to determine irreversibility under different environmental conditions. Further discussions are needed.

Waeterschoot et al (2012) summarized a removal from the water column assessment using the TICKET-UWM andfrom this assessment, Cd, Cu, Ni, Pb, Sb and Zn are considered as rapidly degraded. This is integrated in this assessment.

For copper, the large weight of evidence obtained from laboratory experiments, field data and fate modelling exercises demonstrate that under most environmentally relevant conditions, copper ions are rapidly removed from the water column through partitioning and settling to sediments. Further processes at the water-sediment interface ensure the continuous formation of insoluble, unavailable Cu-species (e.g. copper-sulphides). Experimental data further demonstrate that remobilisation of copper ions from lake and river sediments to aqueous phases is not expected, not even during re-suspension and long-term oxygenation of sediments.

The data, therefore, supports the view that that under “environmentally relevant“ conditions, copper-ions are rapidly removed from the water-column and the processes involved at the water – sediment interface result in “irreversible” change in speciation to insoluble/non-available forms. In conclusion, copper ions are rapidly removed from the water-column, equivalent to the “biodegradability” of organic substances, and consistent with the metal-specific guidance and the conclusions from the Feb 2012 ECHA workshop.

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