Refractories, fibers, aluminosilicate

Adsorption / desorption

Administrative data

Endpoint:

adsorption / desorption: screening

Data waiving:

study scientifically not necessary / other information available

Justification for data waiving:

the study does not need to be conducted because the physicochemical properties of the substance indicate that it can be expected to have a low potential for adsorption

Justification for type of information:

In accordance with Annex VIII, Column 2 this study does not need to be conducted as based on the physicochemical properties the substance can be expected to have a low potential for adsorption.

Adsorption/ desorption is not a specific physiochemical property of substance but rather indicates the relative binding capacity of a substance to a solid surface. Its utility is understanding how a substance may behave in the environment such as adsorption of suspended matter in water columns or sewerage and subsequent concentration in surface water, sediments or sludge. Furthermore, substances that are highly sorptive may bind to biological surfaces such as gills which may lead to toxicity with through bioaccumulation. In addition, it is also considered that substances that are highly sorptive may display reduced degradation rates as they are less bioavailable to microorganisms and so may be more environmentally persistent.
Mineral fibres such as aluminosilicate wools (ASW/RCF) should be exempt from testing due to the following considerations:

1. Based on the physicochemical properties of the fibres, it can be expected to have a very low potential for adsorption due to large, macroscopic scale and low solubility (both aqueous and fat solubility <1 mg/l);

2. The fundamental principles of adsorption/ desorption, and partitioning more generally, are incompatible with large, insoluble particulates and fibres;

3. The test material is incompatible with current standardized guideline test methods and analytical principles used therein.

1. Physicochemical Properties
The principle behind adsorption is temporary (reversible) or permanent bonding between a surface (usually a solid) and a substance, typically in the form of a thin film of test liquid or gas. The forces dictating the bonding are, for example, van der Waals interactions, hydrogen bonding to hydroxyl groups, ionic interactions, covalent binding etc. forming between the solid surface and discreet molecules of the test substance. It is important to consider that the form of ASW/RCF fibers is a stable, macroscale solid and not as free ions or discreet molecules. Weak bonds such as van der Waal forces can dictate molecular interactions or facilitate binding of very small, nanoscale, particulates. However, these would be insufficient to facilitate biding of ASW/RCF fibers to solid matrices, not least as the macroscale means that contact area over which binding may occur is very small.

ASW/RCF fibres are formed predominantly of SiO2 and Al2O3 in a chemically stable form and as with other aluminatesilicates, phototransformation in air, water and soil is not expected. For the same reasons, biodegradation in water, sediment or soil such as via hydrolysis is not applicable as an environmental fate process.

2. The fundamental principles of adsorption/ desorption

As noted within OECD test guidelines 106 [1], 121 [2], preliminary estimations of substances can be made of the adsorption coefficient based on the octanol-water partition coefficient, and to some extent, water solubility. The octanol-water partition coefficient (Kow) is an important physicochemical parameter owing to its use in numerous estimation models including environmental partitioning, sorption, bioavailability, bioconcentration, bioaccumulation. However, a key requirement for the test substances is that it soluble in either the n-octanol or water phase and insoluble inorganic particulates, being insoluble in both, behave differently to organic molecules. Therefore, Guideline test methods for determining partition coefficient (i.e. OECD TG 107 [3], 117 [4] and 123 [5]) are not applicable to insoluble particulates [6] such as ASW/RCF fibres.

It is important to consider that adsorption-desorption distribution coefficient (Kd) measurements are based on the assumption that a thermodynamic equilibrium between liquid and solid phase can be achieved. In the case of inorganic, insoluble particles and fibres, equilibrium partitioning does not apply as they do not form solutions, only colloidal suspensions which are dynamic and thermodynamically unstable. The importance of these fundamental principles of adsorption/ desorption testing and subsequent incompatibility with inorganic particles and fibres is clearly evident within the standardised test guidelines (see next section).

3. Test Method Specific Considerations

ASW/RCF fibers are discrete particulates with a diameter in the micron scale which show very low solubility in water. These fundamental properties of the material mean that testing of adsorption/ desorption using currently used and standardized test methods would be confounded. Specific issues are:

• OECD TG 106: Adsorption -- Desorption Using a Batch Equilibrium Method [1] is not applicable to ASW/RCF fibres because as the guideline notes, difficulties may arise for low solubility test substances defined as <1mg/l as is the case for ASW/RCF fibers. This is because the concentration in aqueous phase cannot be measured with sufficient accuracy and is not representative of the whole fibre substance. Furthermore, it is not possible to differentiate between the adsorbed material or that which has settled (i.e. no bonding attachment) during the centrifugation step [6].

• OECD TG 121: Estimation of the Adsorption Coefficient (Koc) on Soil and on Sewage Sludge using High Performance Liquid Chromatography (HPLC) is not suitable for inorganic compounds. The principle of the test method is that a test substance is applied to a HPLC column packed with cyanopropyl solid phase containing lipophilic and polar moieties (the solid phase). As the test substance moves through the column it can interact with the solid phase where it may be retarded in its passage owing to adsorption. Non-adsorptive substances would continue through the column in the mobile phase. It is clear that the fundamental principle of the test method is that the only hinderance to the passage of a test substance through the column is adsorption to the stationary phase. Insoluble particles and fibers owing to their solid, macro-scale form are unable to move unhindered through the column irrespective of any adsorptive properties and would be highly unlikely to even enter the column.

• OECD TG 312: Leaching in Soil Columns [7]. The principle of this test method is that a test substance is applied to the top of a constructed soil column. The substance should be either dissolved in water or, in the case of solid formulations (e.g. granules), applied in the solid form without water. It is worth noting that the guideline also states that solid materials may be mixed with quartz (SiO2) sand and applied. Artificial rain solution is then applied, allowed to drain and the leachates collected and the soil column is divided up. The penetration of the test substance into/ through the soil column is then established though analysis of the leachate and sections.

The soil used is ‘A mixture of mineral and organic chemical constituents’ [7] which in itself presents a considerable issue for the detection of any possible leachates in the water run-off or soil sections. That is because ASW/RCF fibres and soil share a common composition; indeed soil can contain >40% silicon dioxide and various aluminosilicates [8]. Should elements of ASW/RCF fibers leach, which owing to its insolubility in water (and results of solution/extraction and transformation/dissolution tests) is highly limited, the raising of the limit of detection (LOD) for its elemental constituents against the background soil content would make detection impossible. This is further reinforced by the fact that the guideline states that ‘The amount of test substance applied to the soil columns should be sufficient to allow for detection of at least 0.5 % of the applied dose in any single segment’. Given the low solubility and high background levels of ASW/RCF fiber constituents in soil, such a requirement is physically unfeasible.

• ISO 18749: Batch test of adsorption of substances on activated sludge. This test method is not suitable for substances that are insoluble or allow for stable suspensions/ dispersions. Furthermore, it does not differentiate between adsorption and other elimination methods such as sedimentation [9].

Summary

The insolubility, inorganic nature of ASW/RCF fibres along with their macroscale means that they are both unlikely to adsorb and are incompatible with adsorption/ desorption testing. Therefore, testing of ASW/RCF fibers for adsorption/desorption is neither technically feasible nor scientifically warranted.

1. OECD, Test No. 106: Adsorption -- Desorption Using a Batch Equilibrium Method. 2000.
2. OECD, Test No. 121: Estimation of the Adsorption Coefficient (Koc ) on Soil and on Sewage Sludge using High Performance Liquid Chromatography (HPLC). 2001.
3. OECD, Test No. 107: Partition Coefficient (n-octanol/water): Shake Flask Method. 1995.
4. OECD, Test No. 117: Partition Coefficient (n-octanol/water), HPLC Method. 2004.
5. OECD, Test No. 123: Partition Coefficient (1-Octanol/Water): Slow-Stirring Method. 2006.
6. ECHA, Appendix to Chapter R.7.a for nanomaterials. 2017, ECHA: Helsinki.
7. OECD, Test No. 312: Leaching in Soil Columns. 2004.
8. Matichenkov, V.V. and E.A. Bocharnikova, Chapter 13 The relationship between silicon and soil physical and chemical properties, in Studies in Plant Science, L.E. Datnoff, G.H. Snyder, and G.H. Korndörfer, Editors. 2001, Elsevier. p. 209-219.
9. ECHA, Chapter R.7a: Endpoint specific guidance. 2017, ECHA: Helsinki.