Strontium

Adsorption / desorption

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

adsorption / desorption: screening

Remarks:

other: monitoring data in water and sediment

Type of information:

read-across based on grouping of substances (category approach)

Adequacy of study:

weight of evidence

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: see 'Remark'

Remarks:

Kd-values are based on reliable field data of concentration levels in water and sediment; no standard guideline test was conducted to generate Kd-values. Kd-values are calculated based on the hypothesis that equilibrium exists between the baseline levels in water and in sediment.

Justification for type of information:

Strontium metal is highly reactive and instantly oxidizes upon contact with water. It decomposes completely. During the redox-reaction with water, a strong evolution of hydrogen gas and an immediate precipitation of a white, crystalline solid (i.e. Sr(OH)2) is observed (Sr2+ + 2OH- + H2 (g). The amount of dissolved Sr cations is determined by the solubility of the Sr(OH)2 precipitate. According to OECD guideline 105 (1995) and EU method A.6 (2006), the water solubility of strontium was determined to be 6.74 ± 0.14 g/L under the conditions of the test (flask method under protective gas atmosphere; loading of 41 g Sr/L, at 20.0 ± 1.0 °C, pH >13).
Due to the buffering capacity of most environmental systems, it may reasonable be assumed that the formed hydroxide ions are neutralised in the environment by different processes including precipitation.

The solubility of strontium is not greatly affected by the presence of most inorganic anions as there is little tendency for strontium to form complexes with inorganic ligands (Krupka et al. 1999. EPA 402-R-99-004B and references therein). Free Sr2+ cations are mobile under most environmental conditions, despite the relatively low solubility of strontium carbonate and strontium sulfate at neutral to high pHs. In solutions with a pH below 4.5, the Sr2+ ion is dominant. Under more neutral conditions (pH 5 to 7.5), SrSO4 forms. Strontium carbonate controls strontium concentrations in solutions only under highly alkaline conditions. Further, dissolved strontium forms only weak aqueous complexes with chloride and nitrate (Salminen et al. 2015 and references therein, Krupka et al. 1999. EPA 402-R-99-004B). Regarding monodentate and bidentate binding to negatively-charged oxygen donor atoms, including natural organic matter, alkaline earth metals, such as strontium, tend to form complexes with ionic character as a result of their low electronegativity. Ionic bonding is usually described as resulting from electrostatic attractive forces between opposite charges, which increase with decreasing separation distance between ions (Carbonaro and Di Toro. 2007. Geochim Cosmochim Acta 71 3958–3968; Carbonaro et al. 2011. Geochim Cosmochim Acta 75: 2499-2511 and references therein). Thus, strontium does not form strong complexes with fulvic or humic acids based on the assumption that strontium would exhibit a similar (low) stability with organic ligands as calcium and that strontium could not effectively compete with calcium for exchange sites because calcium would be present at much greater concentrations (Krupka et al. 1999. EPA 402-R-99-004B). In sum, strontium ions are highly mobile, occur only in one valence state (2+), i.e. are not oxidized or reduced, and do not form strong complexes with most inorganic and organic ligands (Krupka et al. 1999. EPA 402-R-99-004B; Salminen et al. 2015). Thus, it may further be assumed that the behaviour of the dissociated strontium ions in the environment determine the fate of strontium upon dissolution with regard to (bio)degradation, bioaccumulation, partitioning as well as the distribution in environmental compartments (water, air, sediment and soil) and subsequently the ecotoxicological potential.Thus, the chemical safety assessment is based on elemental strontium concentrations and read-across of environmental fate and toxicity data available for soluble strontium substances as well as monitoring data of elemental strontium concentrations in the environment. The reliable data selected for the assessment of environmental fate of strontium, including adsorption/desorption and bioconcentration/bioaccumulation, are based on elemental strontium concentrations of water, soil, sediment, and biota.

Qualifier:

no guideline followed

Principles of method if other than guideline:

Baseline levels of strontium in water and sediment were determined in >800 pristine locations. Assuming equilibrium between both environmental compartments, site-specific Kd's were derived

GLP compliance:

not specified

Type of method:

other: field data of baseline levels in water and sediment

Media:

sediment

Radiolabelling:

no

Test temperature:

environmental relevant temperatures

Analytical monitoring:

yes

Details on sampling:

Water compartment:- running stream water was collected form small, second order drainage basins (<100 km²);- whenever possible, sampling was performed during winter and early spring months, and was avoided during rainy periods and flood events;- a full description of sampling materials and sampling volumes is provided, and all materials were rinsed twice with unfiltered or filtered stream water (depending on the type of water sample);- all potential contaminating factors were reduced during the sampling period (wearing of gloves, no smoking in the area allowed, no hand jewelry was allowed , running vehicles during sampling was prohibited, etc..) Sediment compartment:- sediment samples were taken at the same locations that were selected for the determination of background concentrations in the surface water;- sampling was also performed in such a way that any kind of metal contamination was avoided (e.g., no hand jewelry, no medical dressing, etc…);- if it was not possible to use non-metal equipment (e.g., spades, sieves), unpainted steel equipment was used (no aluminium or brass);- a composite sample was made from subsamples taken from beds of similar nature (ISO-5667-12, 1995), and minimum amount of sediment sample was 0.5 kg dry wt.

Phase system:

sediment-water

Type:

log Kp

Value:

2.57 L/kg

Remarks on result:

other: lowest country-specifc typical value (Poland)

Phase system:

sediment-water

Type:

log Kp

Value:

4.35 L/kg

Remarks on result:

other: highest country-specific typical value (Norway)

Phase system:

sediment-water

Type:

log Kp

Value:

2.94 L/kg

Remarks on result:

other: typical EU-value

Adsorption and desorption constants:

Country-specific log KD-values are situated between 2.57 (Polanda) and 4.356 (Norway), with a median value of 2.94 (861.2 L/kg) which is derived from the best fitted distribution that was developed with all country-specific KD-values

Details on results (Batch equilibrium method):

A typical (i.e., median) KD-value was determined for each EU-country, and the median value of all these country-specifc KD's is considered as a reliable typical KD-sediment for Europe.

Statistics:

KD = country-specific median sediment concentration / country-specific median water column concentrationA distribution was fitted through all country-specific KD-values: From this distribution a median log Kd of 2.94 (861.2 L/kg) for strontium in European sediments is derived.

Conclusions:

Reliable baseline levels of strontium in pristine water/sediment samples were determined in >800 samples. Sampling and analytical procedures are considered adequate and resulted in reliable data. Assuming equilibrium between the typical concentration in water and sediment, relevant KD-values were generated for each country. Data are therefore considered useful for the determination of a relevant KD for the sediment compartment.

Reference 2

Endpoint:

adsorption / desorption: screening

Remarks:

adsorption

Type of information:

read-across based on grouping of substances (category approach)

Adequacy of study:

weight of evidence

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: Kd-values for Sr were dertermined in the O-horizon and E-Horizion soil samples of a podsol soil that were spiked and equilibrated with 85-Sr in batch experiments.

Justification for type of information:

Strontium metal is highly reactive and instantly oxidizes upon contact with water. It decomposes completely. During the redox-reaction with water, a strong evolution of hydrogen gas and an immediate precipitation of a white, crystalline solid (i.e. Sr(OH)2) is observed (Sr2+ + 2OH- + H2 (g). The amount of dissolved Sr cations is determined by the solubility of the Sr(OH)2 precipitate. According to OECD guideline 105 (1995) and EU method A.6 (2006), the water solubility of strontium was determined to be 6.74 ± 0.14 g/L under the conditions of the test (flask method under protective gas atmosphere; loading of 41 g Sr/L, at 20.0 ± 1.0 °C, pH >13).
Due to the buffering capacity of most environmental systems, it may reasonable be assumed that the formed hydroxide ions are neutralised in the environment by different processes including precipitation.

The solubility of strontium is not greatly affected by the presence of most inorganic anions as there is little tendency for strontium to form complexes with inorganic ligands (Krupka et al. 1999. EPA 402-R-99-004B and references therein). Free Sr2+ cations are mobile under most environmental conditions, despite the relatively low solubility of strontium carbonate and strontium sulfate at neutral to high pHs. In solutions with a pH below 4.5, the Sr2+ ion is dominant. Under more neutral conditions (pH 5 to 7.5), SrSO4 forms. Strontium carbonate controls strontium concentrations in solutions only under highly alkaline conditions. Further, dissolved strontium forms only weak aqueous complexes with chloride and nitrate (Salminen et al. 2015 and references therein, Krupka et al. 1999. EPA 402-R-99-004B). Regarding monodentate and bidentate binding to negatively-charged oxygen donor atoms, including natural organic matter, alkaline earth metals, such as strontium, tend to form complexes with ionic character as a result of their low electronegativity. Ionic bonding is usually described as resulting from electrostatic attractive forces between opposite charges, which increase with decreasing separation distance between ions (Carbonaro and Di Toro. 2007. Geochim Cosmochim Acta 71 3958–3968; Carbonaro et al. 2011. Geochim Cosmochim Acta 75: 2499-2511 and references therein). Thus, strontium does not form strong complexes with fulvic or humic acids based on the assumption that strontium would exhibit a similar (low) stability with organic ligands as calcium and that strontium could not effectively compete with calcium for exchange sites because calcium would be present at much greater concentrations (Krupka et al. 1999. EPA 402-R-99-004B). In sum, strontium ions are highly mobile, occur only in one valence state (2+), i.e. are not oxidized or reduced, and do not form strong complexes with most inorganic and organic ligands (Krupka et al. 1999. EPA 402-R-99-004B; Salminen et al. 2015). Thus, it may further be assumed that the behaviour of the dissociated strontium ions in the environment determine the fate of strontium upon dissolution with regard to (bio)degradation, bioaccumulation, partitioning as well as the distribution in environmental compartments (water, air, sediment and soil) and subsequently the ecotoxicological potential.Thus, the chemical safety assessment is based on elemental strontium concentrations and read-across of environmental fate and toxicity data available for soluble strontium substances as well as monitoring data of elemental strontium concentrations in the environment. The reliable data selected for the assessment of environmental fate of strontium, including adsorption/desorption and bioconcentration/bioaccumulation, are based on elemental strontium concentrations of water, soil, sediment, and biota.

Qualifier:

no guideline available

Principles of method if other than guideline:

Sieved (2 x 2 mm) and air dried soil samples (10g) are equilibrated at 293 K for 20 days with 24.5 mL soil solution and 0.5 of a solution containing different radioisotope(carrierfree, as far as available) including 85-Sr. The concentration of the ions was always less than 6 x 10(-8) mol/L. After equilibration the solution was centrifuged at 4500 rpm and the activity in the clear supernatant was determined using a Ge-detector and a multichannel analyser. From the initial and final activity of each radionuclide in the solution the amound sorbed was determined, and used for Kd-determination.

GLP compliance:

not specified

Type of method:

batch equilibrium method

Media:

soil

Radiolabelling:

yes

Test temperature:

20 degrees Celcius

Analytical monitoring:

yes

Details on sampling:

Activity of radionuclides in the test solution was determined before and after the equilibration periodSupernatans after equilibration was obtained after centrifugation at 4500 rpm.

Phase system:

other: soil

Type:

other: soil

Value:

140 L/kg

Remarks on result:

other: O-Horizon; 95%CL: 120-180 L/kg

Phase system:

other: soil

Type:

other: soil

Value:

44 L/kg

Remarks on result:

other: E-Horizon; 95%CL: 32-53 L/kg

Adsorption and desorption constants:

O-Horizon: median Kd = 140 L/kg (95% CL: 120 - 180 L/kg)median log Kd: 2.15 (95% CL: 2.08 - 2.26)E-Horizon: median Kd = 44 L/kg (95% CL: 32 - 53 L/kg)median log Kd: 1.64 (95% CL: 1.51 - 1.72)

Transformation products:

not specified

Details on results (Batch equilibrium method):

KD determination was repeated 4 times by equilibrating further soil samples from the same sampling point with the corresponding soil solution containing the radionuclides. The error (standard deviation, as obtained from the four replicate measurtements) was for the determination of the KD-value about 15%

Statistics:

Median value represents the 50th percentile of a log-normal (O-horizon) or normal (E-horizon) that was fitted through the KD-value data set.

Validity criteria fulfilled:

not specified

Conclusions:

Based on the information that is provided in the study, it is concluded that the reported median KD-values for Sr in a O-horizon (organic layer) and E-horizon (mineral layer) of a forest podzol soil can be used in a weight of evidence approach for the determination of a typical Kd-value for soils.

Reference 3

Endpoint:

adsorption / desorption: screening

Remarks:

adsorption

Type of information:

read-across based on grouping of substances (category approach)

Adequacy of study:

weight of evidence

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: see 'Remark'

Remarks:

Batch equilibrium tests were conducted in a proper way, taking a sufficient equilibration period into account, and radioactive Sr-levels were derermined in an adequate way. It should be taken into account, however, that reported Kd are function of soil properties.

Justification for type of information:

Strontium metal is highly reactive and instantly oxidizes upon contact with water. It decomposes completely. During the redox-reaction with water, a strong evolution of hydrogen gas and an immediate precipitation of a white, crystalline solid (i.e. Sr(OH)2) is observed (Sr2+ + 2OH- + H2 (g). The amount of dissolved Sr cations is determined by the solubility of the Sr(OH)2 precipitate. According to OECD guideline 105 (1995) and EU method A.6 (2006), the water solubility of strontium was determined to be 6.74 ± 0.14 g/L under the conditions of the test (flask method under protective gas atmosphere; loading of 41 g Sr/L, at 20.0 ± 1.0 °C, pH >13).
Due to the buffering capacity of most environmental systems, it may reasonable be assumed that the formed hydroxide ions are neutralised in the environment by different processes including precipitation.

The solubility of strontium is not greatly affected by the presence of most inorganic anions as there is little tendency for strontium to form complexes with inorganic ligands (Krupka et al. 1999. EPA 402-R-99-004B and references therein). Free Sr2+ cations are mobile under most environmental conditions, despite the relatively low solubility of strontium carbonate and strontium sulfate at neutral to high pHs. In solutions with a pH below 4.5, the Sr2+ ion is dominant. Under more neutral conditions (pH 5 to 7.5), SrSO4 forms. Strontium carbonate controls strontium concentrations in solutions only under highly alkaline conditions. Further, dissolved strontium forms only weak aqueous complexes with chloride and nitrate (Salminen et al. 2015 and references therein, Krupka et al. 1999. EPA 402-R-99-004B). Regarding monodentate and bidentate binding to negatively-charged oxygen donor atoms, including natural organic matter, alkaline earth metals, such as strontium, tend to form complexes with ionic character as a result of their low electronegativity. Ionic bonding is usually described as resulting from electrostatic attractive forces between opposite charges, which increase with decreasing separation distance between ions (Carbonaro and Di Toro. 2007. Geochim Cosmochim Acta 71 3958–3968; Carbonaro et al. 2011. Geochim Cosmochim Acta 75: 2499-2511 and references therein). Thus, strontium does not form strong complexes with fulvic or humic acids based on the assumption that strontium would exhibit a similar (low) stability with organic ligands as calcium and that strontium could not effectively compete with calcium for exchange sites because calcium would be present at much greater concentrations (Krupka et al. 1999. EPA 402-R-99-004B). In sum, strontium ions are highly mobile, occur only in one valence state (2+), i.e. are not oxidized or reduced, and do not form strong complexes with most inorganic and organic ligands (Krupka et al. 1999. EPA 402-R-99-004B; Salminen et al. 2015). Thus, it may further be assumed that the behaviour of the dissociated strontium ions in the environment determine the fate of strontium upon dissolution with regard to (bio)degradation, bioaccumulation, partitioning as well as the distribution in environmental compartments (water, air, sediment and soil) and subsequently the ecotoxicological potential.Thus, the chemical safety assessment is based on elemental strontium concentrations and read-across of environmental fate and toxicity data available for soluble strontium substances as well as monitoring data of elemental strontium concentrations in the environment. The reliable data selected for the assessment of environmental fate of strontium, including adsorption/desorption and bioconcentration/bioaccumulation, are based on elemental strontium concentrations of water, soil, sediment, and biota.

Qualifier:

no guideline followed

Principles of method if other than guideline:

Agricultural soil samples (n=112) are collected, dried at room temperature and then passed through a 2-mm sieve. One gram of each soil is placed into a 30 mL bottle together with 10 mL of deionized water. After shaking for 24h at 23°C, +/- 10 kBq of 85-Sr (as 85-SrCl2) with about 12.7 ng of stable Sr is added to the suspension.After shaking for 7 more days, the suspension is centrifuged at 3000 rpm fro 10 minutes, and the supernatant is filtered through a 0.45 um membrane filter. Radioctivity is measured in the liquid phase before and after the equilibration period. The difference is assumed to be adsorbed to the soil particles.

GLP compliance:

not specified

Type of method:

batch equilibrium method

Media:

soil

Radiolabelling:

yes

Test temperature:

23 degrees Celcius

Analytical monitoring:

yes

Details on sampling:

filtered (0.45 um) supernatant after a 10 min centrifugation at 3000 rpm (Hitachi CT5L)

Details on test conditions:

Test conducted at 23 degrees CelciusEquilibration period of 7 days1g of dried soil in 10 mL of deionized watertest concentration: +/- 10 kBq (nominal)

Computational methods:

KD =( Ci - Cl) * Wl) : ( Cl * Ws)Ci and Cl: radionucleide concentration in the liquid phase before (i) and after (l) the 7 day equilibration periodWl =solution volume : Ws = spoil dry weight

Phase system:

other: soil

Type:

other: soil

Value:

400 L/kg

Remarks on result:

other: geometric mean in paddy soil sample (range: 100-1,820 L/kg)

Phase system:

other: soil

Type:

other: soil

Value:

220 L/kg

Remarks on result:

other: geometric mean in upland soil samples (range: 60-640 L/kg)

Adsorption and desorption constants:

Paddy soil samples: KD-range: 100 - 1820 L/k ; geometric mean: 400 L/kgUpland soil samples: KD-range: 60 - 640 L/k ; geometric mean: 220 L/kg

Transformation products:

not specified

Details on results (Batch equilibrium method):

Differences are attibuted to soil differences. pH did not affect Sr-KD values of either paddy or upland soil samples.As the soil chemistry of Ca and Sr is similar, a strong competition can be expected between both elements for binding to soil particles. Although there was no correlation between exchangeable Ca and Sr-Kd values, the lower measured value of exchangeable Ca could be causing the higher Sr-Kd values for paddy soil samples than for upland soil samples.

Statistics:

No specific statistics were used to calculate the indivudual Kd-values

Validity criteria fulfilled:

not specified

Conclusions:

A KD geometric mean of 400 and 200 L/kg was derived for paddy and upland agricultural soil samples, respectively. Batch equilibrium tests were conducted in a proper way, taking a sufficient equilibration period into account, and radioactive Sr-levels were derermined in an adequate way. Therefore the reported KD levels can be used in a weight of evidence approach for determining a typical soil KD.

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 the FOREGS monitoring survey.
- The value for soil is the geometric mean of three data points, and amounts to 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 sediment)

Value in L/kg:

2.94

Other adsorption coefficients

Type:

log Kp (solids-water in suspended matter)

Value in L/kg:

3.11

Other adsorption coefficients

Type:

log Kp (solids-water in soil)

Value in L/kg:

2.2

Additional information

Sediment

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 coefficients for water and sediment

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 Kd(marine 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., 50^th 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

The performed literature review and data analysis resulted in some Kd-values that were 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 coefficients for porewater and soil

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)

Suspended particulate matter

No data were identified for suspended particulate matter. A partition coefficient for this compartment, however, can be estimated based on the partition coefficient for sediment, which is incresaed 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 done 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 suspended particulate matter.

Conclusions:

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. FOREGS data represents a large number 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. The literature data were primarily relevant for sandy sediments with a low affinity for metals due to their low clay and organic matter content; therefore they are not representative for silty, loamy and clayey soiediments. 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.

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).

No data were identified for particulate suspended matter. A partition coefficient for this compartment was estimated based on the partition coefficient for sediment which is incresaed by a factor of 1.5 to account for the weaker adsorption of sediments as compared to particulate matter (DBW/RIZA, 1989) and amounts to 1291.8 L/kg (i.e., log Kd(SPM) of 3.11).