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Reference
Endpoint:
adsorption / desorption: screening
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
Remarks:
No data is available on the GLP compliance, as no guideline was followed this value was set to no, respectively.
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).

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 et 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 the table 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 (please refer to table below). 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 (see respective table below).

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

  N Min 10thpercentile Median 90thpercentile Max
L/kg dw
Grazing land
EU27 + Norway 1834 0.51 1.3 44014 7.5 52.9
Total GEMAS database 2117 0.51 1.3 44014 7.5 52.9
Arable land
EU27 + Norway 1930 0.26 1.2 43923 6.0 44.0
Total GEMAS database 2212 0.26 1.2 43923 6.0 44.0
Grazing + Arable land
EU27 + Norway 3764 0.26 1.3 43953 6.8 52.9
Total GEMAS database 4329 0.26 1.3 43953 6.6 52.9 

 

Measured Kp values for European soils 

 

N

Min

10thpercentile

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 behaviours (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.

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

Please also refer to the read-across statement attached to section 13.