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Description of key information

Partition coefficients for different environmental compartments (sediment, suspended particulate matter, soil) have been derived for strontium, based on literature data and on data generation in the FOREGS monitoring survey.
- The value for soil is the geometric mean of three data points, and is 157.03 L/kg
- For the sediment compartment the typical KD based on FOREGS data is put forward as a reliable value for Europe, i.e., 861.2 L/kg.
- A value of 1.5 has been proposed as a relevant ratio between the Kd for sediment and the KD for suspended particulate matter (Stortelder et al, 1989; Van de Meent et al, 1990), and this ratio was also used by RIVM for setting relevant Kd-values for various metals. Application of this factor on the KD, sediment of 861.2 L/kg, results in an estimated Kd, spm of 1291.8 L/kg

Key value for chemical safety assessment

Other adsorption coefficients

Type:
log Kp (solids-water in soil)
Value in L/kg:
2.2
at the temperature of:
20 °C

Other adsorption coefficients

Type:
log Kp (solids-water in sediment)
Value in L/kg:
2.94
at the temperature of:
20 °C

Other adsorption coefficients

Type:
log Kp (solids-water in suspended matter)
Value in L/kg:
3.11
at the temperature of:
20 °C

Additional information

Sediment compartment

Literature data:

Five literature values reporting KD-values for strontium in sediment were identified, and ranged between 27 and 117.4 L/kg (See Table below). These values were used for the determination of a typical Sr-KD-value for the sediment compartment. The geometric mean of the reported KD values was taken to derive this typical sediment KD since all the reported data are considered to be of equal quality. As such a KD value of 40.04 L/kg was obtained. The sediments used in the reviewed studies were mainly sandy sediments. The weaker strontium sorption by sand compared to clay, silt or organic matter yields in a relatively low KD value.

Table: Overview of partition coefficient between the water and sediment compartment

 Reference KD-value
Bunde et al (1997)  31.5 (log KD: 1.50) 
Bunde et al (1998)  58.36 (log KD: 1.77) 
Hemming et al (1997)  17.67 (log KD: 1.25) 
Kaplan et al (2000)  27 (log KD: 1.43) 
Liszewski et al (2000)  117.40 (log KD: 2.07) 
Geometric mean: 40.04 (log KD: 1.60)

One study which calculated the partition coefficient between the water and marine sediment compartment was reviewed (Caroll et al, 1999), and with the data presented in this study a marine KD, sediment of 75.76 L/kg (log KD: 1.88) was determined.

Data from FOREGS:

The FOREGS Geochemical Baseline Mapping Programs main aim was to provide high quality, multi-purpose environmental geochemical baseline data for Europe. The sampling sites selected for stream water analyses of dissolved metals were typical of locally unimpacted or slightly impacted areas. Consequently, the metal concentrations that are determined in these samples can be considered as relevant baseline concentrations. A total number of 808 water samples were analyzed for Sr by ICP-OES (detection limit 1 µg/L); dissolved strontium levels ranged between 1 and 13,600 µg/L. For the sediment compartment, the amount of analyzed samples was 852, with strontium levels ranging between 31 mg/kg and 1,3522 mg/kg dw. Sediment samples were analyzed by ICP-XRF (X-ray fluorescence; detection limit of 1 mg/kg dw). XRF analysis is a fast, non-destructive analysis method with very high accuracy and reproducibility. With XRF it is not necessary to bring solid samples into solution and then dispose of solution residues, as is the case with all wet-chemical methods. (e.g., aqua regia destruction, followed by ICP-OES).

Raw data were sub-categorized per country, and a typical baseline value (i.e., 50th percentile or median) of strontium in water and sediment were determined for each country. Assuming that the country-specific median values are relevant for both compartments and represent a state of chemical equilibrium, a typical Kd-value can be derived for each country. Typical country-specific log KD values are situated between 2.57 and 4.35, with an overall typical value of 2.94 for Europe (i.e., 861.2 L/kg).

Soil compartment

The performed literature review and data analysis on strontium partitioning coefficients for the sediment compartment also resulted in some KD-values that were more relevant for soil particles. The geometric mean of the reported KD values was taken to derive a typical soil KD since all the reported data are considered to be of equal quality. As such a KD value of 157.03 L/kg is obtained.

The table below gives an overview of the different relevant KD-values that were selected for the derivation of a typical soil-partition coefficient.

Table: Overview of partition coefficient between the water and soil compartment

Reference KD-value
Bunzl and Schimmack (1989) - E-horizon  44 (log KD: 1.64) 
Kami-Ishikawa and Tagami (2008) upland soil  220 (log KD: 2.34 ) 
Kami-Ishikawa and Tagami (2008) upland soil  400 (log KD: 2.60) 
Geometric mean 157.03 (log KD: 2.20)

Conclusion:

For the sediment compartment, two values were identified, i.e., a literature value of 40.04 L/kg based on a limited data set (n=5) and the value of 861.2 L/Kg which was derived for with the data generated in the FOREGS monitoring survey (Salminen et al, 2005).

The latter value was put forward as a typical value for the sediment compartment:

literature data were primarily relevant for sandy soils which have a low affinity for metals due to their low clay and organic matter content; therefore they are not representative for silty, loamy and clayey soils.

FOREGS data represents a large amount of samples (>800) representing the whole of Europe; and Sr-levels in water and sediment were determined in an uniform way

Based on the data provided in Crommentuyn et al (1997) can be concluded that the sediment KD of cationic metals (e.g., Ba, Be, Cd, Co, Cu, Pb, Ni, Zn) is always at least one order of magnitude higher than the soil KD, with differences up to 3 orders of magnitude. Taking into account that literature KD-values for soil were situated between 44 and 400 L/kg, a sediment KD of 40.04 L/kg (based on literature data) would be unlikely.

No data were identified for particulate suspended matter. A partition coefficient for this compartment , however, can be estimated, based on the partition coefficient for sediment which is increased by a factor of 1.5 (Stortelder et al, 1989; Van de Meent et al, 1990) to account for the weaker adsorption of sediments as compared to particulate matter (DBW/RIZA, 1989): the relatively strong adsorption of metals by particulate matter is probably related to the relatively high organic matter and clay content (size fraction < 2 µm). Bockting et al (1992) indicated that this factor of 1.5 remains an assumption and use of this value should be dome with caution. According to this methodology, a KD, spm of 1291.8 L/kg (i.e., log KD, spm of 3.11) is derived for this compartment.

For the soil compartment the geometric mean of three literature data points is put forward as a typical value for the Sr-KD for soil, i.e., 157.03 L/kg (log KD: 2.20)

A summary of the KD-values for the different environmental compartments is given below:

 Compartment Kd-value (L/kg)  Log Kd  Reference
Sediment  861.2  2.94  Salminen et al (2005; FOREGS data) 
Suspended particulate matter  1,291.8  3.11  Estimated data (ratio of 1.5 on KD, sediment) 
Soil  157.03  2.20  Crommentuyn et al (1997)