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Environmental fate & pathways

Biodegradation in water and sediment: simulation tests

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Endpoint:
biodegradation in water: sediment simulation testing
Data waiving:
study technically not feasible
Justification for data waiving:
other:
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JUSTIFICATION FOR DATA WAIVING
The fate and behaviour of substances in the environment is an important consideration in terms of understanding potential for bioaccumulation, biodegradation and possible ecotoxicological effects. PAS fibers possess a stable, macroscale structure and low solubility and as such have a low potential for bioaccumulation and biodegradation (inorganic substance) in soils and sediments. Furthermore, when considering PAS access to soils and sediments, there are two key considerations:

1. PAS fibers are used as a high temperature insulation material as an incorporated component of commercial products. As such, potential for environmental release is low;

2. PAS fibers are composed of SiO2, Al2O3 and K2O which are all common components of soils and sediments. If released into the environment, PAS fibers are expected to combine indistinguishably with the soil/ sediments with negligible alternation of soil/sediment composition even with high (e.g. equal volume) exposure. Within the soil/sediment, the elements comprising PAS fibres will be subjected to natural processes under environmental conditions (cation exchange, dissolution, sedimentation).

Environmental Release

When considering access to the soil/sediment phase, the use profile of PAS fibers is important. They are used as high temperature insulation materials, typically in a blanket or bonded form to prevent release, and integrated into an article (e.g. automotive part). They are not used in any environmental applications such as waste remediation and at the end of useful life, PAS fibers are recycled by the manufacturer or buried in deep landfill. In addition, the solid form of PAS fibers means that should, in the rare instances that release occurs the spread of any material (e.g. via surface water run-off etc.) would be isolated. Overall, environmental exposure to PAS fibers under normal use and accidental conditions is severely restricted.

Common Elemental Composition – PAS Fibers and Soils/ Sediments

PAS fibers share common elemental composition with soils meaning that where access to soils occurs, they would combine indistinguishably.
Whilst PAS fibers are stable and poorly soluble, the relative contribution and impact of each individual element comprising PAS fibers is considered with the conservative assumption of release from the fiber:

• Silicon dioxide – PAS fibers are comprised of ~35% SiO2 and silicon at 28% is only the second most abundant element in the earth’s crust after oxygen. It is a major constituent of nearly all rocks and as such, is a major constituent of soils with a median content of 68.0% in subsoil and 67.7% in topsoil [1]. In soil, SiO2 is the most resistant mineral and because of its very low aqueous solubility is one of the residual minerals remaining in the soil after others have altered or dissolved [2]. Indeed, the low solubility of SiO2 renders it biogeochemically immobile. Where Si is found in a soluble form, this is as poly- and monosilicic acids although the monosilicic acid form is poorly absorbed and has a low capability to migrate within soils [22, 3] while polysilicic acids are essential soil components [2]. Due to the highly stable nature of Si-O bind in SiO2, no photo- or chemical degradation is expected and so biodegradation is not applicable to this substance [4]. The presence of Si in soils can improve their condition, minimising the toxicity of heavy metals and increasing the bioavailability of phosphorus as well as promoting plant resistance to environmental stresses such as drought [5].

The ubiquitous nature of SiO2 in soils combined with low potential for environmental release of PAS fibres means that anthropogenic contribution of SiO2 in soils is minimal in terms of amount and irrelevant in terms of toxicity. The contribution of SiO2 to soil (density 1800 kg/ m3) from an equal volume of PAS fibre product (density 128 – 250 kg/m3) would be 3.7 – 7.2%.

• Aluminium oxide –Aluminium is the most common metallic element in the earth’s crust accounting for 8% of its composition and as such, is a common constituent of soils and sediments. Overall, Al2O3 content in topsoil’s is 11.0% (0.37 to 26.7%) and in subsoils it is 11.7% (0.21 to 27.1%) [1]. Together with iron oxide minerals, Al2O3 play a major role in stabilising soil structure thereby having a favourable effect on soil physical properties [6]. This includes increasing aggregate stability, permeability, friability, porosity, and hydraulic conductivity, and reducing swelling, clay dispersion, bulk density, and modulus of rupture.

Owing to the low potential for environmental release of PAS fibres, the contribution of PAS derived Al2O3 to existing natural pools of aluminium in soils and sediments is minimal. As Al2O3 is a common and indeed beneficial component of soils, the very low contribution of anthropogenic (PAS derived) aluminium is not relevant either in terms of added amounts or toxicity. As such, additional sediment and/or soil testing is not warranted and due to their similarity to compounds naturally present in sediments and soils, testing is not technically feasible.

• Potassium Oxide - PAS fibers are comprised of ~25% K2O and in soils, the principal sources of potassium are minerals such as feldspar and micas [32]. Potassium is released from these minerals in the course of their weathering which makes it available to plants in the form of a cation. The concentration in topsoils is 2.022% (0.026 – 6.13%) [26] and it is an essential element for all living organisms and furthermore, is the most important nutrient cation for plants with respect to its physiological and biochemical functions [33]. As such, it is found at high concentrations in plant tissues where it plays a prominent role in metabolism and biosynthesis. It also plays an important role in water status by lowering water potential [33].

Due to the low potential for environmental release of PAS fibers, the potential contribution to soil K2O is minimal (see Table 2). As potassium is an essential and ubiquitous in soils and sediments, the very low contribution of anthropogenic (PAS derived) potassium is not relevant either in terms of added amounts or toxicity. Therefore, additional sediment and/or soil testing is not scientifically justified.
Summary

The potential for environmental dispersion of PAS fibers based on the production, use and disposal characteristics is low. Where release does occur, the inorganic particulate nature means that it is expected to distribute mainly to soils and sediments. Upon deposition into soils, PAS fibers would be expected to combine indistinguishably with the soil or sediment owing to ubiquitous of the elements comprising PAS fibres (i.e. SiO2, Al2O3 and K2O) in soils. PAS fibers are not expected to contribute significant amounts of these essential elements to soil, nor are they expected to cause toxicity even with significant release. As such, additional sediment and/or soil does not appear to scientifically warranted and due to their similarity to compounds naturally present in sediments and soils, testing is not technically feasible.

1. De Vos, W. and T. Tarvainen, Geochemical Atlas of Europe. 2006, Finland: Geological Survey of Finland.
2. Hinman, N.W., Silicon, Silica, in Encylopedia of Geochemistry, C.P. Marshall and R.W. Fairbridge, Editors. 1999, Kluwer Academic Publishers: Dordrecht, Germany. p. 572-575.
3. Khalid, R.A. and J.A. Silva, Residual effect of calcium silicate on Ph, phosphorus, and aluminum in a tropical soil profile. Soil Science and Plant Nutrition, 1980. 26(1): p. 87-98.
4. ECHA. Silicon Dioxide: Environmental fate & pathways: Endpoint summary. [cited 2019 9/09/19]; Available from: https://echa.europa.eu/sk/registration-dossier/-/registered-dossier/15556/5/1.
5. Sahebi, M., et al., Importance of silicon and mechanisms of biosilica formation in plants. BioMed research international, 2015. 2015: p. 396010-396010.
6. Goldberg, S., Interaction of aluminum and iron oxides and clay minerals and their effect on soil physical properties: A review. Communications in Soil Science and Plant Analysis, 1989. 20(11-12): p. 1181-1207.

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Materials and methods

Results and discussion

Applicant's summary and conclusion