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Endpoint:
adsorption / desorption
Remarks:
other: Sediment toxicity study from which a Kp could be calculated
Type of information:
experimental study
Adequacy of study:
weight of evidence
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: GLP sediment toxicity study reporting measured concentrations in sediment and overlaying water after 28d
Principles of method if other than guideline:
GLP sediment toxicity study reporting measured concentrations in sediment and overlaying water after 28d
GLP compliance:
yes
Type of method:
batch equilibrium method
Media:
sediment
Test temperature:
18.8 - 20.9 °C
Analytical monitoring:
yes
Details on sampling:
Analytical verification of the control and all test substance concentrations within the overlying water and pore water (interstitial) were performed at test initiation (day 0) and termination (day 28). Replicate E was used on day 0 and replicate D was used on day 28. At each time point, a 10-mL sample of the overlying test solution was collected from each replicate into appropriately labeled culture tubes. The pore water was collected by centrifuging the sediment from the control and all test substance treatments at approximately 4,000 rpm for 20 minutes at 20oC. The resulting supernatant was transferred to another Nalgene® bottle and centrifuged at approximately 9,000 rpm for 20 minutes at 20oC.
A 0.400-mL volume of the overlying water and pore water samples were removed to another culture tube and diluted to 10 mL using reagent water. Samples were further diluted as necessary with reagent water to produce analyte concentrations that fell within the calibration curve (i.e., 0.00 to 0.140 mg B/L). The samples were analyzed using ICP-MS. Two QC fortification spikes were prepared in a similar manner at concentrations of 0.288 and 71.9 mg B/L.

After the pore water was removed from the control and all test substance sediments, the moisture of the sediments was analyzed with a Mettler Toledo HR73 Halogen Moisture Analyzer. A sample of the control and each treatment level sediment were weighed (e.g., 5 ± 0.05 g dry weight) into appropriately labeled 50-mL Falcon tubes and were fortified, if necessary, with boric acid spiking solution. The tubes were brought to a volume of 30 mL with reagent water, capped, and shaken to mix at ambient temperature. The samples were shaken in a tube rotator at 100% speed for 15 minutes, followed by centrifugation at 4,000 rpm for 20 minutes. After centrifugation, aliquots of the supernatants were transferred into appropriately labeled 15-mL culture tubes and diluted as necessary with reagent water to produce analyte concentrations that fell within the calibration curve (i.e., 0.00 to 0.140 mg B/L). The tubes were capped, shaken to mix, and analyzed using ICP-MS.
Details on matrix:
The formulated sediment used for the definitive test was prepared on June 1, 2010 by mixing the following ratio of constituents: 75% fine industrial sand (T & S Materials, Inc., Gainesville, Texas), 20% kaolin clay (Brickyard Ceramics and Crafts, Indianapolis, Indiana), and 5% sphagnum peat (Westlakes Hardware, Columbia, Missouri) (1). The sediment constituents were mixed based on dry weight equivalents. The peat moss was sieved to a finely ground consistency and did not contain any visible plant remains. Calcium carbonate, CaCO3, was added to the artificial sediment to adjust the pH to 7.0 ± 0.5 units.
Sample no.:
#1
Duration:
28 d
Conc. of adsorbed test mat.:
1.56 mg/kg sediment d.w.
Sample no.:
#2
Duration:
28 d
Conc. of adsorbed test mat.:
4.27 mg/kg sediment d.w.
Sample no.:
#3
Duration:
28 d
Conc. of adsorbed test mat.:
10 mg/kg sediment d.w.
Sample no.:
#4
Duration:
28 d
Conc. of adsorbed test mat.:
18.1 mg/kg sediment d.w.
Sample no.:
#5
Duration:
28 d
Conc. of adsorbed test mat.:
32.6 mg/kg sediment d.w.
Sample no.:
#6
Duration:
28 d
Conc. of adsorbed test mat.:
81 mg/kg sediment d.w.
Phase system:
solids-water in sediment
Type:
log Kp
Value:
0.29 L/kg
Remarks on result:
other: geomean of 6 samples
Endpoint:
adsorption / desorption
Remarks:
adsorption
Type of information:
experimental study
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: No guideline followed, methods and results well described
Qualifier:
no guideline followed
Principles of method if other than guideline:
Analysis of solution phase B-concentration after 72-h equilibration of soil with 5 mg B/kg in a 1:10 suspension.
GLP compliance:
no
Type of method:
batch equilibrium method
Media:
soil
Radiolabelling:
no
Test temperature:
not reported
Analytical monitoring:
yes
Details on sampling:
Suspensions were centrifuged at 1200 g for 20 min and supernatants filtered to <0.22 µm (Minisart, Sartorius)
Details on matrix:
A total of 4813 natural topsoils from arable land (0-20 cm top layer) and grassland (0-10 cm top layer) across Europe were sent to CSIRO by Eurometaux for this study. A total of 516 soils (500 samples + 16 replicates) were selected for IR analysis to be used in the development of calibration and validation models. The soils were air dried and sieved to < 2 mm prior to shipment to CSIRO. Soils were oven dried at 40°C for 12 hrs and cooled in a desiccator prior to MIR analysis or experimental Log-Kd value determinations.
Soils selected for experimental Kd determinations cover a wide range in pH (3.3 - 8.0, in 0.01 M CaCl2), organic carbon content (0.5 - 49 %), effective CEC (2.2 - 48.4 cmolc/kg, measured at soil pH) and aqua regia extractable background B concentrations (0.3 - 48.7 mg B/kg).
Details on test conditions:
Approximately 2.0 ± 0.05 g of < 2 mm sieved soils was weighed into 50 ml centrifuge tubes (Cellstar, Greiner Bio-one) and mixed end over end for 48 h with 20 mL of 0.01 M CaCl2 (1:10 m/v). Due to the large number of samples the variability in soil Kd values for B was assessed every 10 samples through the analysis of duplicate soil samples. The variability in experimental Kd values for B was found to be < 6.5 %.
After this initial equilibration period, samples were spiked with 100 µl of 100 mg B/L solution as boric acid (5 mg B/kg) and mixed end over end for a further 72 h. After this spike equilibration period, samples were centrifuged at 1200 g for 20 min and supernatants filtered to <0.22 µm (Minisart, Sartorius).
The background B concentrations in soils were determine by weighing approximately 2.0 ± 0.05 g of < 2 mm sieved soils into 50 ml centrifuge tubes (Cellstar, Greiner Bio-one) and mixing end over end for 5 days with 20 mL of 0.01 M CaCl2 (1:10 m/v).
The measured Kd values were used to develop a MIR-based model for prediction of Kd in the remaining approx. 4000 samples. Only measured Kd values for 474 soils (all values discarding all analytical repliactes) are taken into account here in order to eliminate the uncertainty on the predicted Kd values.
Duration:
72 h
Initial conc. measured:
5 mg/kg soil d.w.
Computational methods:
The solid-solution partitioning (Kd values) values for B in soils were determined using the following equation:

Kp (L/kg) = {initial added nominal B concentration (mg B/kg) - final measured B concentration in solution (mg B/l)/soil:solution ratio (kg/l)} / final measured solution-phase B concentration (mg B/l)

where, final measured B concentrations in solution (mg B/l) were corrected for background B concentrations.
Phase system:
solids-water in soil
Type:
log Kp
Value:
-0.45 L/kg
Remarks on result:
other: minimum of 474 soils
Phase system:
solids-water in soil
Type:
log Kp
Value:
-0.28 L/kg
Remarks on result:
other: 10th percentile of 474 soils
Phase system:
solids-water in soil
Type:
log Kp
Value:
0.34 L/kg
Remarks on result:
other: median of 474 soils
Phase system:
solids-water in soil
Type:
log Kp
Value:
0.98 L/kg
Remarks on result:
other: 90th percentile of 474 soils
Phase system:
solids-water in soil
Type:
log Kp
Value:
1.71 L/kg
Remarks on result:
other: maximum of 474 soils

Kd (L/kg) values for European soils as predicted by MIR spectra and pH:

 

N

Min.

10th

percentile

Median

90thpercentile

Max.

 

 

 

L/kg dw

Grazing land

 

 

 

 

 

 

 

EU27 + Norway

1834

0.51

1.3

2.7

7.6

52.9

 

Total GEMAS database

2117

0.51

1.3

2.7

7.5

52.9

 

Arable land

 

 

 

 

 

 

 

EU27 + Norway

1930

0.26

1.2

2.4

6.0

44.0

 

Total GEMAS database

2212

0.26

1.2

2.4

6.0

44.0

 

Grazing + Arable land

 

 

 

 

 

 

 

EU27 + Norway

3764

0.26

1.3

2.5

6.8

52.9

 

Total GEMAS database

4329

0.26

1.3

2.5

6.6

52.9

 

Conclusions:
The median of 474 measured Kp values in a range of agricultural and grassland soils is selected as typical Kp for soil: 2.19 l/kg (log Kp=0.34 L/kg).
Endpoint:
adsorption / desorption
Remarks:
adsorption
Type of information:
experimental study
Adequacy of study:
weight of evidence
Reliability:
2 (reliable with restrictions)
Principles of method if other than guideline:
The adsorption properties of standard clay minerals have been examined under a variety of pH, temperature and concentration conditions.
GLP compliance:
not specified
Type of method:
batch equilibrium method
Media:
sediment
Test temperature:
5-40°C
Analytical monitoring:
yes
Details on matrix:
- Details on collection: Mississippi delta sediment was rinsed repeatedly with distilled and deionised water at room temperature until all adsorbed boron had been removed.
- Textural classification: The sediment consists predominantly of mixed layer illite-smectite, discrete illite and kaolinite, together with lesser amounts of quartz, feldspar, mica and organic matter.
Details on test conditions:
Constant size fractions of the sediment were pipetted as a slurry into 20 ml centrifuge tubes. A known weight of open ocean water was added, together with HCl or NaOH to adjust the pH to the final measured value. The sample was placed i a constant temperature water bath and agitated regularly.A 72 hour reaction time was employed here.
Phase system:
solids-water in sediment
Type:
log Kp
Value:
0.3 L/kg
Remarks on result:
other: pH= 7.4; 5°C
Phase system:
solids-water in sediment
Type:
log Kp
Value:
0.49 L/kg
Remarks on result:
other: pH = 8.1; 5°C

24 Kd values have been determined.

Temperatures of 5, 15, 25 and 40 °C were used.

The Kd ranged from 0.75 (40°C pH = 6.7) to 3.6 (5°C pH = 8.45)

Endpoint:
adsorption / desorption
Remarks:
adsorption
Type of information:
experimental study
Adequacy of study:
weight of evidence
Reliability:
2 (reliable with restrictions)
Principles of method if other than guideline:
Two separate experiments were conducted utilizing a rocking autoclave hydrothermal system, with a flexible reaction gold-cell, allowing reaction to take place under controlled pressure and temperature conditions.
GLP compliance:
not specified
Type of method:
other: Hydrothermal experiments
Media:
sediment
Test temperature:
Temperature range of 25 to 350 °C
Details on matrix:
Magnesium and sulfate-free solutions, and organic carbon-depleted sediments were chosen.
Details on test conditions:
TEST SYSTEM
- Type, size and further details on reaction vessel: Rocking autoclave hydrothermal system, with a flexible reaction gold-cell with a volume up to ca. 300mL volume.
- Amount of soil/sediment/sludge and water per treatment (if simulation test): 60g sediment and ca. 200 ml fluid.
Computational methods:
Kd = [B]adsorbed/[B]sol and can be calculated utilizing mass balance and measured solution concentrations, [B]sol :
Kd = {Bi,exch - [B]i,solVi}/(Bi,sol*M), where Vi is the experimental solution volume when sample i was collected and M is the mass of solids.
Phase system:
solids-water in sediment
Type:
log Kp
Value:
0.49 L/kg
Remarks on result:
other: pH 7.5; 25°C
Phase system:
solids-water in sediment
Type:
log Kp
Value:
0.46 L/kg
Remarks on result:
other: pH 6.1; 25°C

Calculated absorption coefficients are highly temperature dependent. Kd values decrease abruptly from 3.5 at 25 °C, to 2 at 50 °C, and to essentially zero at temperatures above 100 °C. By definition, a calculated Kd of zero in our experiments indicates not only that no exchangeable B remains adsorbed on the sediments but also implies that Bincorporation into secondary phases is of a magnitude that cannot be resolved. The Kd value may be also pressure dependent. One measurement at high pressure (25°C, 800 bar) has significantly higher Kd than that of at room conditions (9.3 and ca. 3.5, respectively). We, however, present this data with caution due to the low concentrations and since it is a single sample pair.

Description of key information

For the risk characterization, mean partition coefficients for boron in soil and sediments need to be estimated. This is a simplification, as soil and sediments show a high heterogeneity, influenced by the properties of the parent material, the state of pedogenesis, the vegetation cover and human activities. In general, the boron sorption capacity of soil and sediments is low.  Because of the large dataset, the wide range of soil types covered and the consistent methodology, the Kp for soil is calculated as the median of all measured Kp values from the GEMAS project: 2.19 L/kg dry weight. The chemistry of boron in soils and aquatic systems is simplified by the absence of oxidation- reduction reactions or volatilization. Redox processes can mobilize Fe oxides and Mn oxides, which may lead to a release of boron in aquatic systems. Generally, sediments are characterised with higher pH values than the soil matrix, which increases the boron sorption capacity. A median value of 3.0 L/kg is proposed as a tentative sorption value for boron in the marine sediment phase and 1.94 L/kg for the freshwater sediment phase. The Kp value of 3.5 L/kg (You e al, 1996) is put forward as a sorption value for the suspended solids phase.

Key value for chemical safety assessment

Other adsorption coefficients

Type:
log Kp (solids-water in soil)
Value in L/kg:
0.34

Other adsorption coefficients

Type:
log Kp (solids-water in sediment)
Value in L/kg:
0.48

Other adsorption coefficients

Type:
log Kp (solids-water in sediment)
Value in L/kg:
0.29

Other adsorption coefficients

Type:
log Kp (solids-water in suspended matter)
Value in L/kg:
0.54

Additional information

The following plausible mechanisms are responsible for the chemical interactions of boron with soil constituents: anion exchange, precipitation of insoluble borates with sesquioxides, sorption of borate ions or molecular boric acid, formation of organic complexes, and fixation of boron in a clay lattice (e.g. Goldberg, 1997; Adriano, 2001). Major sorption sites for boron in soils are: (1) Fe-, Mn-, and Al-hydroxy compounds present as coatings on or associated with clay minerals, (2) Fe-, Mn-, and Al-oxides in soils, (3) clay minerals, especially the micaceous type, (4) the edges of aluminosilicate minerals and (5) organic matter (Goldberg, 1997; Adriano, 2001).

Keren and Bingham (1985) reported that the B(OH)4- concentration and the amount of adsorbed boron increased rapidly when the pH is increased to about 9. Maximum retention was reported at alkaline pH levels of up to 9.5 when boron is mainly present as the borate ion (WHO, 1998; Blume et al., 1980).

Boron was reported to react more strongly with clay than sandy soils (Keren and Bingham, 1985). The rate of boron adsorption on clay minerals is likely to consist of a continuum of fast adsorption reactions and slow fixation reactions. Short-term experiments have shown that boron adsorption reaches an apparent equilibrium in less than one day (Hingston, 1964; Keren et al., 1981). Long-term experiments showed that fixation of boron continued even after six months of reaction time (Jasmund and Lindner, 1973). The magnitude of boron adsorption onto clay minerals is affected by the exchangeable cation. Calcium-rich clays adsorb more boron than sodium and potassium clays (Keren and Gast, 1981; Keren and O'Connor, 1982; Mattigod et al., 1985). A higher organic matter content increases the B-retention capacity of soil (Yermiyahu et al., 2001). Sorbed boron amounts and boron retention maxima have been significantly correlated with organic carbon content (Gupta, 1968).

Microbial action can remobilize organic-bound boron (Banerji 1969, Su and Suarez 1995, Evans and Sparks 1983, as reviewed by Robinson et al. 2007). Boron sorption can vary from being fully reversible to irreversible, depending on the soil type and environmental conditions (Elrashidi and O’Conner, 1982, IPCS, 1998).

Partition coefficient of boron for soils

Only studies on natural soils were taken into account for the derivation sorption/desorption values. Boron sorption/desorption studies on pure soil constituents (e.g. clay, organic matter, oxides) were judged less relevant.

The GEMAS-project (Geochemical Mapping of Agricultural and Grazing Land Soil project) provides good quality and comparable data on Kp values and soil properties known to influence the adsorption and fate of inorganic elements (pH, organic matter content, clay content and effective cation exchange capacity [CEC]) in agricultural and grazing land soil in Europe. The aim of this project was to produce a harmonized and directly comparable dataset on soil quality and metal concentrations in soils at the EU scale and included samples from almost 4500 European soils. Kp values for boron were measured in 474 different soil samples at a low B dose (5 mg B/kg soil) added as boric acid. The Kp values for the remaining 4000 samples were assessed using a MIR based model (Janik et al 2010). A statistical overview of the results found is given in Table A below. Only measured Kp values are taken into account for the selection of typical Kp values in order to eliminate the uncertainty on the predicted Kp values (Table B). No significant differences were observed between the two land uses covered. The measured Kp values for B in European soils range from 0.35 to 51.9 L/kg dw, with 10th, 50thand 90thpercentiles of 0.53, 2.19 and 9.47 mg L/kg dw, respectively (Table B).

 

Table A: Kp values for European soils (measured and predicted by MIR and pH)

 

N

Min.

10th

percentile

Median

90thpercentile

Max.

 

 

 

L/kg dw

Grazing land

 

 

 

 

 

 

 

EU27 + Norway

1834

0.51

1.3

2.7

7.6

52.9

 

Total GEMAS database

2117

0.51

1.3

2.7

7.5

52.9

 

Arable land

 

 

 

 

 

 

 

EU27 + Norway

1930

0.26

1.2

2.4

6.0

44.0

 

Total GEMAS database

2212

0.26

1.2

2.4

6.0

44.0

 

Grazing + Arable land

 

 

 

 

 

 

 

EU27 + Norway

3764

0.26

1.3

2.5

6.8

52.9

 

Total GEMAS database

4329

0.26

1.3

2.5

6.6

52.9

 

 

Table B: Measured Kp values for European soils

 

N

Min.

10th

percentile

Median

90thpercentile

Max.

 

 

 

L/kg dw

Grazing land

292

0.39

0.50

2.20

9.75

51.9

 

Arable land

182

0.35

0.62

2.10

8.68

31.3

 

All

474

0.35

0.53

2.19

9.47

51.9

 

Other studies report Kp values between 0.09 and 8.4 L/kg, when the boron concentration in the equilibrium was 1 mg/L. The reliability of these partitioning coefficient data values is however limited due to the limited analytical precision used in the studies, reflecting the small amount of boron sorbed. The variability in sorption behaviors (linear, non-linear) reveals different sorption capacities for soils.

Partition coefficient of boron for sediments and suspended solids

Two studies reported partition coefficients for boron in marine aquatic systems.

One value is available for the freshwater aquatic system. A sediment toxicity study where sediment concentrations and water concentration have been monitored allowed to calculate Kp values for freshwater sediment. 

The following table summarizes the different sediment and suspended solids Kp values that have been identified from the open literature. No partition coefficient distribution was developed as an insufficient amount of data points were available for either the sediment phase or the suspended solid phase.

Table C: Overview of sediment and suspended solids Kp values

Marine sediment compartment

 

Kp value (L/kg)

pH

Reference

2.9

6.1

You et al, 1995

3.1

7.1

You et al, 1995

2.0

7.4

Palmer et al, 1987

3.1

8.1

Palmer et al, 1987

Median value: 3.0 L/kg

 

Freshwater sediment compartment

 

1.94

8-8.3

Gerke, 2011

Suspended solids

 

3.5

--

You et al, 1996