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Environmental fate & pathways

Adsorption / desorption

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Description of key information

The substance adsorbs significantly to sediment, soil and sludge substrates based on the available study data. While the binding is not necessarily to organic carbon, Kd values appear consistent with a log Koc (equivalent) value of approximately 4.2.

Key value for chemical safety assessment

Koc at 20 °C:
40 000

Other adsorption coefficients

Type:
log Kp (solids-water in sediment)
Value in L/kg:
3.3
at the temperature of:
12 °C

Other adsorption coefficients

Type:
log Kp (solids-water in soil)
Value in L/kg:
2.9
at the temperature of:
12 °C

Other adsorption coefficients

Type:
log Kp (solids-water in activated sewage sludge)
Value in L/kg:
4.18
at the temperature of:
12 °C

Other adsorption coefficients

Type:
log Kp (solids-water in raw sewage sludge)
Value in L/kg:
4.08
at the temperature of:
12 °C

Other adsorption coefficients

Type:
log Kp (solids-water in settled sewage sludge)
Value in L/kg:
4.08
at the temperature of:
12 °C

Other adsorption coefficients

Type:
log Kp (solids-water in effluent sewage sludge)
Value in L/kg:
4.18
at the temperature of:
12 °C

Other adsorption coefficients

Type:
log Kp (solids-water in suspended matter)
Value in L/kg:
3.3
at the temperature of:
12 °C

Additional information

This substance is a mineral-binding and complexing agent, with unusual chemical properties. HEDP and its salts adsorb strongly to inorganic surfaces, soils and sediments, in model systems and mesocosms, despite the very low log Kow; this has implications for the approach to environmental fate modelling. High adsorption is consistent with similar behaviour seen for structural analogues, and other common complexing agents such as EDTA.

 

Studies on analogous phosphonate complexing agents have revealed that adsorption is correlated with concentration in the aqueous phase and also relates significantly to the type and nature of inorganic content in the substrate.

 

The normal approach to modelling binding behaviour in environmental exposure assessment assumes that the substance is binding only to the organic carbon present in soils, sediments, and WWTP sludges. This assumption does not apply to HEDP and its salts. The extent of binding to substrates is fundamental to understanding and modelling of environmental exposure, for substances like this. Therefore, adsorption / desorption data, required in Section 9.3.1 of Annex IX, is an extremely important part of the data set for HEDP and its salts.

 

The nature of the adsorption is believed to be primarily due to interaction with inorganic substrate or generalised surface interactions. While Koc is the conventional indicator for adsorption, the interaction with organic carbon present in the substrate may be exceeded by these other interactions in the case of HEDP and its salts, meaning that Kocas such is not a meaningful parameter. It is convenient for comparison purposes to determine the value of log Koc that is consistent/equivalent to the degree of sediment or soil binding exhibited by the substance.

 

Thus, a log Koc (equivalent) value of 4.22 was obtained by evaluating Kp(sediment-water) data in a reliable study conducted according to generally accepted scientific principles (Michael, 1979). River sediments were analysed by using liquid scintillation on day 0, 1, 2, 4, 8. Methods and sample data were represented clearly and the test substance was being described adequately. The result is considered as reliable and has been assigned as key study.

 

From other various sources, an adsorption coefficient value of 0.91-0.98 is reported in reviewed literature paper (Steber and Wierich, 1987). The Freundlich isotherm constants values of ca. 2600-13000 for HEDP were determined by following the EPA test guide method in an activated sludge. A paper (Jaworska, 2002) supported the measured data for water sediment and water active sludge studies. Another study (Nowack, 2002) has reported the same phenomena using active sludge as testing medium.

 

Adsorption data for soils were derived in an OECD 106 study using radiochemical analysis (Springborn Laboratories, undated) and further values are reported in literature (Steber and Wierich, 1987). The adsorption values determined (Kd ~10 - ~200 l/kg in both tests) are consistent with a log Koc (equivalent) value of 3.8-4.7 from the silty soils and 2.8 from the sands, consistent with the log Koc(equivalent) derived from the key sediment study. In further studies (Nowack and Stone1999), goethite (an iron-based mineral commonly found in soil) was used as a testing substrate. High levels of adsorption were reported. A study by Fischer (1991) resulted in Freundlich constants 50 – 2400 for relevant soil minerals.

 

A screening study using the conventional HPLC method (OECD 121) to estimate the value of Koc (organic carbon-water partition coefficient) is considered not appropriate. Adsorption behaviour onto the normal aminopropyl column used in OECD 121 would not necessarily follow the pattern of adsorption onto substrates that are of importance in the environment. Understanding of sludge binding is informative, but much less significant in the chemical safety assessment than binding to matrices with a higher inorganic content or high surface area. It is important to understand Kd directly, and preferably as a function of variables such as solid phase composition and characteristics, water hardness, dilutions, and phase ratios.

 

[LogKoc: 4.6]