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Environmental fate & pathways

Biodegradation in soil

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Description of key information

Although biodegradation in soil has not been demonstrated for HEDP-H and its salts, the role of abiotic removal processes is significant. The key data for soil adsorption are from the study by Michael (undated) (refer to Section 5.4.1 for further information about this test). There is no evidence for desorption occurring. Effectively irreversible binding is entirely consistent with the known behaviour of complexation and binding within crystal lattices. The high levels of adsorption which occur are therefore a form of removal from the environment. After approximately 40-50 days, the phosphonate is >95% bound to sediment with only 5% extractable by ultrasonication and use of 0.25N HCl-xylene solvent (based on radiolabelling) in river and lake water microcosms (Monsanto internal report, cited by Gledhill and Feijtel, 1992). 66-80% removal (binding) is seen after 11 days in the same test. In the context of the exposure assessment, largely irreversible binding is interpreted as a removal process; 5% remaining after 40 - 50 days is equivalent to a half-life of 10 days which is significant for the environmental exposure assessment in the regional and continental scales. This abiotic removal rate is used in the chemical safety assessment of HEDP-H and its salts.

Key value for chemical safety assessment

Half-life in soil:
10 d
at the temperature of:
12 °C

Additional information

Two reliable studies of biodegradation in soil are available. Soil biodegradation rates of between 6.7 and 28.2% over 119 days in four soils were determined (Saeger et al., 1977). A soil biodegradation rate of 47% over 148 days in silt-loam soil was determined (Saeger 1978).