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Adsorption / desorption

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

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 (Table xx). 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 10^th, 50^thand 90^thpercentiles of 0.53, 2.19 and 9.47 mg L/kg dw, respectively (see table xx).

 

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

	N	Min.	10^th percentile	Median	90^thpercentile	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 xx: Measured Kp values for European soils

	N	Min.	10^th percentile	Median	90^thpercentile	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 10: 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