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Environmental fate & pathways

Adsorption / desorption

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Description of key information

Key value for chemical safety assessment

Other adsorption coefficients

Type:
log Kp (solids-water in soil)
Value in L/kg:
1.73

Other adsorption coefficients

Type:
log Kp (solids-water in sediment)
Value in L/kg:
2.5

Other adsorption coefficients

Type:
log Kp (solids-water in suspended matter)
Value in L/kg:
3.49

Additional information

There is substantial reliable (all Klimisch 2) information available for adsorption/desorption of selenium, reporting partition coefficients (Kp values, i.e. ratio of Se concentration in solid phase over Se concentration in solution phase) for soil, sediments and suspended matter. All the information available for sediment and suspended matter is based on paired monitoring data of elemental Se concentrations in sediment or suspended matter and water. The reliable information for Kp values in soil is based on batch adsorption or desorption experiments with added sodium selenite or sodium selenate.

In total, 26 reliable Kp values for sodium selenite are available for a range of soils with varying properties (pH: 4.3-8.3; organic carbon: 0.3-6.3% + peat of 91% organic matter; clay: 1-60%). Log Kp values range between 0.96 and 2.69 L/kg dry weight. When several Kp values were reported for the same soil, a geomean value is calculated and taken forward for further analysis. The 10th, 50th and 90th percentiles of the 14 selected log Kp values according to the best-fitting distribution (log uniform) are 1.02, 1.73 and 2.44 L/kg, respectively. Additionally, reliable Kp values are available for sodium selenate in 4 soils (pH: 4.3-7.3; Organic carbon: 0.5-6.6%; clay: 1-11%), with log Kp values ranging between 0.11 and 0.99 L/kg dry weight (median: 0.87 L/kg). Because of the significant difference in sorption between selenite and selenate, data were not combined and the 50th percentile of the Kp values for selenite was selected for the chemical safety assessment of zinc selenite.

For freshwater sediment, 8 studies report reliable Kp values for 5 river systems: Arkansas river, USA (1 study), Colorado river, USA (1 study), Blackfoot river, USA (1 study), A riparian wetland from South Africa (1 study) and San Joaquin/Kesterson reservoir system, USA (4 studies). For each study, a geomean Kp value was calculated for all data reported. When several studies reported Kp values for the same river or water system, an overall geomean value was calculated. Log Kp values for the 5 river systems vary between 1.79 and 2.86 L/kg dry weight. The 10th, 50th and 90th percentiles of the log Kp values according to the best-fitting distribution (log logistic) are 1.95, 2.50 and 3.04 L/kg, respectively. The 50th percentile of the Kp values was selected for the chemical safety assessment. No data are available for marine sediments.

Reliable data for Kp of suspended matter in 3 water systems from the USA: Colorado river, a salinity gradient in the San Francisco Bay-delta and rivers from New Mexico. No effect of salinity on Kp of Se was observed. For each study, a geomean Kp value was calculated for all data reported and geomean log Kp values range from 2.80 to 4.34 L/kg dry weight. The median of 3.49 L/kg dry weight was selected for the chemical safety assessment.