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EC number: 234-522-7
CAS number: 12007-92-0
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.
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;
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
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)
EU27 + Norway
Total GEMAS database
Grazing + Arable land
Table B: Measured Kp values for
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)
You et al, 1995
Palmer et al, 1987
Median value: 3.0 L/kg
Freshwater sediment compartment
You et al, 1996
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