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EC number: 215-125-8
CAS number: 1303-86-2
of terrestrial toxicity studies
toxicity data on terrestrial organisms are from ecotoxicity tests that
study relevant ecotoxicological parameters such as survival, growth,
reproduction, and emergence. Relevant endpoints for soil microorganisms
focused on functional parameters (such as respiration, nitrification,
mineralization) and microbial growth. Enzymatic processes are considered
not relevant for this risk assessment. The ecological relevance of
enzymatic assays is questionable for several reasons:
of the test media
data from observations in natural and (OECD) artificial standard soil
media have been used for the
derivation of the PNEC.
data used in the effect assessment should ideally be based on organisms
and exposure conditions from Europe. This would, however, considerably
reduce the amount of data that can be used.
data based on soils collected outside Europe have also been used. Two
options may be followed here:
1 is selected because:
of the test substance
on the ecotoxicity of boron have been performed with various compounds,
such as boric acid
anhydrous sodium tetraborate (Na2B4O7),
and hydrated sodium tetraborates (Na2B4O7.xH2O).
For the purpose of this evaluation, all endpoints are converted to
concentrations of elemental boron (B) using the relative molar mass.
comprises “chronic exposure” is a function of the life cycle of the test
organisms. A priori fixed exposure durations are therefore not
relevant. The duration should be related to the typical life cycle and
should ideally encompass the entire life cycle or, for longer-lived
species the most sensitive life stage. Retained exposure durations
should also be related to recommendations from standard ecotoxicity
(e.g. ISO, OECD, ASTM) protocols.
chronic test durations for the higher plants are within the range of 4
(e.g. the root elongationtest
based on ISO 11269-1 (1995)) and 21 days (e.g. the shoot yield test
based on ISO 11269-2 (1995)). OECD n° 208 (plant seedling emergence and
growth test, 2006) recommended a test duration of between 14 and 21 days
after emergence of the seedlings. Testing with soil invertebrates have a
typical acute exposure duration of 7 to 14 days for the oligochaetes Eisenia
fetida/Eisenia andrei. Assessment of the chronic effects of
substances on sub-lethal endpoints such as reproduction on oligochaetes
has a typical exposure duration of 3 to 6 weeks for the standard
organism Enchytraeus albidus (OECD, 2000; ISO 16387). For another
standard species Folsomia candida survival and reproduction is
typically assessed after 28 days of exposure (ISO 11267, 1999). Test
durations using soil micro-organisms for the OECD 216 (carbon
transformation test) and the OECD 217 (nitrogen transformation test)
last 28 days.
boron is a necessary plant micronutrient, it is intentionally added in
some instances where required by crop plants but is limited in the
natural soil. Typical applied doses are 1-2 kg B/ha/yr, but may range
from 0.5 to 7.6 mg B/ha depending on local conditions (Shorrocks, 1997;
Borax 2002). This may be in the form of formulated fertilizers broadcast
to agricultural soils or sprays applied directly to the plant or
vicinity of the plants. In these instances, it might be appropriate to
use a PNEC for agricultural soil that protects the agricultural uses of
the soil, rather than a PNEC derived to protect non-agricultural or
non-industrial soil. This is consistent with the REACH Guidance Document
(2008) distinctions in developing PECs for agricultural,
natural/grassland, and industrial soil.
potential approach would therefore be to derive a PNEC for agricultural
soil based on toxicity, butalso
with consideration of the risk of deficiency. For natural soils, the
presumption is that locally adapted species will not be adversely
affected by boron deficiency, so only boron toxicity is relevant for
deriving a PNEC.
is a naturally occurring element that is essential to a variety of
organisms. In plants, it is necessary for a variety of metabolic
processes (e.g. nitrogen metabolism, nucleic acid metabolism and
membrane integrity and stability) and has been known to be an essential
micronutrient for terrestrial plants for several decades (Butterwick et
al., 1989; Eisler, 2000). Shorrocks (1997) documented the use of boron
applications for 132 crops in over 80 countries, demonstrating the
widespread nature of agricultural use of boron.
exists that it is essential for nitrogen fixation in some species of
algae (Smyth and Dugger, 1981), fungi and bacteria (Saiki et al., 1993;
Fernandez et al., 1984), some diatoms and algae and macrophytes (Eisler,
2000). Required levels may vary, especially among plants, such that
essential levels for one species may be toxic to another (Eisler, 1990).
Work with rainbow trout and zebrafish has shown that embryo larval
development was adversely affected in waters deficient in boron (Rowe et
al., 1998, Eckhert, 1998). Fort et al. (1998) reported that abnormal
development in frog embryos (Xenopus laevis) was
larval stages were exposed to 0.003 mg/-B/L or less. Boron does not
appear to be essential for all species, however. Evaluation of
essentiality in some animals is limited by the innate boron content in
plant-based animal feeds.
concentration-response curve for boron is likely to be U-shaped for most
species, with adverse effects observed at high and low concentrations,
while no adverse effects are observed at the intermediate concentrations
(Lowengart, 2001). Figure 5 illustrates such a pattern for plants (Gupta
et al., 1985) although the response has been normalized to 100%, making
the curve an inverted U-shape.
and animal species vary in the concentrations associated with deficiency
(e.g. corn and grasses) require about one-quarter as much boron as
dicotyledones (e.g. tomatoes, carrots, clovers, beets) (Gupta 1985;
Butterwick et al., 1989). The mobility of boron within the plant may
help explain the observed deficiency and toxicity patterns. Boron is
more mobile in plants that produce the simple sugars known as polyols
(e.g. sorbitol and mannitol) than in species that do not produce
polyols. In polyol-producing
species, boron is translocated from one part of the plant to another
and so may reach the meristem
and affect growth. In the absence of polyols, boron is relatively immobile
within the plant (Brown et al., 2002). Apolyol-producing
plant may be both more tolerant of boron
deficiency and more sensitive to higher boron concentrations because of
of boron within the plant. This is important in agricultural
applications of boron, which may be applied as a soil treatment or as
application of boron depends on the plant and cultivar, as well as the
local soil. Recommended application rates range from 0.5 to 7.6 kg B/ha
(Borax, 2002), but typically are in the range of 1 to 2 kg B/ha
(Shorrocks, 1997). If one assumes typical soil densities of 1700 kg/m³
depth of 20 cm (default values used in the EUSES model), an application
rate of 1 to 2 kg B/ha results in an estimated added soil concentration
of 0.3 to 0.6 mg B/kg soil. Mortvedt et al. (1992) estimated soil
concentrations of 0.16 to 2.0 mg B/kg soil for several crops with
application rates of 0.45 to 5.7 kg/ha. The intentional application of
borates to achieve such soil concentrations for agricultural crops
should be acknowledged in the risk assessment process.
of boron in soils
may be considered a typical metalloid having properties intermediate
between the metals and the electronegative non-metals. Boron has a
tendency to form anionic rather than cationic complexes (Keren and
Bingham, 1985). Boron does not undergo oxidation reduction reactions or
volatilization reactions in soils (Goldberg, 1997). Boron chemistry is
of covalent boron compounds and not of B3+ions because of its
very high ionisation potentials.
oxide, B2O3 reacts with water to form boric acid, H3BO3.
Boric acid is moderately soluble (4.9g 100mL-1 water at
20°C). It acts as a weak Lewis acid by accepting a hydroxyl ion to form
the borate anion.
boron species other than B(OH)3and B(OH)4- can
be ignored for most practical purposes in soils (Keren and Bingham,
1985). In most soils with soil solutions in the pH range 4.0 to 9.0, the
uncharged B(OH)3 predominates. The borate ion is expected to
form a variety of complex salts with suitable metal acceptor ions.
However, there is relatively little evidence for the existence of metal
borate complexes in solutions. Among the organic borates, the tendency
is for boron to replace carbon or nitrogen in three-fold coordination
(Keren and Bingham, 1985). In regions of low rainfall, the boron content
of the soil is usually high. Boron in these soils probably exists
largely as sodium-calcium borates. However, there is no information on
the kinetics of dissolution of these minerals in water or on the
composition of their products (Keren and Bingham, 1985).
affecting the bioavailability of boron in soils
toxicity to plants and many soil microorganisms is a function of the
bioavailability of the dissolved boron species in the soil solution and
the ability of the soil to buffer boron concentrations in the soil
solution. The bioavailability of metals and other inorganic substances
in soil can be strongly affected by soil properties and slow
equilibration reactions (ageing) after application to the soil. Various
environmental factors can influence boron availability in soils,
including pH, soil texture, organic matter content, soil moisture, and
temperature. As boron is either neutral or negatively charged under
environmentally relevant conditions, cation exchange capacity is not
expected to play a relevant role. Investigations into properties,
including ageing, which modify boron availability to plants are
underway, but reports were not available in time to incorporate into
availability to invertebrates depends on the relative amounts taken up
by the organism by dermal adsorption and/or ingestion, although the
relative importance of each route has not been determined (Vijver et
amount of boron adsorbed by soil varies greatly with the contents of
various soil constituents. Boron is adsorbed onto soil particles, with
the degree of adsorption depending on the type of soil minerals present,
pH, salinity, organic matter content, iron and aluminium oxide
oxy/hydroxy content, and clay content (Hingston, 1964; Sims and Bingham,
1968; Bingham et al., 1970; Bingham, 1973). Boron adsorption can vary
from being fully reversible to irreversible, depending on the soil type
and environmental conditions (IPCS, 1998). As the pH is increased to
about 9, the B(OH)4-concentrationincreases
rapidly and the amount of adsorbed boron increases rapidly (Keren and
the critical range of extractable boron levels leading to deficiency is
generally higher in alkaline soils than in acid soils (Bell, 1999).
reacts more strongly with clay than sandy soils (Keren and Bingham,
1985). Clay soils buffer boron in the soil solution better than sandy
soils. The magnitude of the boron adsorption onto clay minerals
is affected by the exchangeable cation (Keren and Gast, 1981; Keren and
Mezuman, 1981; Keren and O'Connor, 1982; Mattigod et al., 1985) Calcium-rich
clays adsorb more boron than sodium and
potassium clays (Keren and Gast, 1981; Keren and O'Connor, 1982;
Mattigod et al., 1985). Higher organic matter content increases the B-sorption
capacity of soils (Yermiyahu et al., 1995).Adsorbed
boron and boron adsorption maxima have been highly significantly
correlated with organic carbon content (Elrashidi et al., 1982; Gupta,
1968). The uptake of boron
by plants can be markedly affected by
the presence of other plant nutrients in soils. The most well known of
these is the effect of Ca (Gupta et al., 1985).
are few studies that compare boron toxicity for the same endpoint in
different soils (Aitken & McCallum, 1988; Gestring & Soltanpour, 1987;
Liang & Tabatabai, 1977; Liang & Tabatabai, 1978). The available results
indicate a significant variation in boron toxicity thresholds among
soils and show a tendency of increased boron toxicity in soils with low
organic matter content, low clay content and pH < 7.5. The information
is, however, too limited to allow conclusions on soil properties
controlling boron toxicity in soils.
Van Laer et al. (2010) studied the effect of soil type and ageing on the
toxicity of boric acid to root elongation of barley seedlings in a set
of 17 soils covering a large range in pH (4.4-7.8), organic carbon (0.14
-30.7%), clay content (2-59%) and background boron concentration (1.2-32
mg B/kg).The EC10 values based on added boron concentration
ranged between 3 and 27 mg B/kg dw among these soils. The percentage
clay and organic carbon (log) were positively correlated with the log ED50
values of the root elongation test. Van Laer et al. also found positive
correlations between %OC and %clay and %moisture content (%MC) of the
soils. They suggest that increased moisture content of the soil dilutes
added boron because adsorption is almost absent .Indeed, sorption of
boron is very low: the soil-liquid distribution of added boron (Kd)
ranges from 0 to 1.9 l/kg in the freshly spiked soils and hardly
increases upon ageing. Further, %MC correlates well with ED50 (R²=0.67
on log scale and 0.76 on untransformed scale)
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 have shown that fixation of boron increased
even after six months of reaction time (Jasmund and Lindner, 1973).
on the residual effect of boron application after a single application
also indicated decreasing boron toxicity to plants with increasing time
since application (Gupta & Cutcliffe, 1984; Gestring & Soltanpour, 1987).
Laer et al. (2010) studied the effect of ageing on the toxicity of boric
acid to barley root elongation and microbial nitrification by measuring
toxicity immediately after spiking soils and after ageing the soil for 1
or 5 months in closed containers (no leaching). Ageing at 20°C for up to
5 months in a closed container had only a slight effect on the boron
toxicity: EC10 values increased by an average factor of 1.3
(range 0.4 – 3.5) after 1 month of ageing and 1.4 (range 0.5 – 3.3) in
the 5 months.
of this negligible effect of ageing on boron toxicity, it was decided to
take into account the data for all equilibration times as replicates and
calculate a geomean value for each soil for PNECderivation.
contrast to this negligible ageing effect of added B, Van Laer et al.
(2010) observed a large difference between soil partitioning between
added boron and naturally present boron in these soils. Measured
background soil solution boron concentrations in 3 soils were 0.2 to 0.9
mg B/l, equivalent to Kd values of 3 to 15 L/kg (Van Laer et al., 2010).
Measured boron concentrations exhibiting effects on barley (EC10)
in the added-boron soils ranged between 5 and 104 mg B/L across 17
natural soils (mean: 26 mg B/L). The median Kd value for added boron was
0.4 L/kg. Therefore, it was concluded that ageing after spiking for 5
months does not affect boron availability in the same way as natural
geogenic or field equilibrated boron. The difference in boron speciation
between added boron and native boron may be related to boron
incorporation into silicate structures or boron in biomass. Van Laer et
al. (2010) noted that the amount of boron naturally present in soils, as
measured by aqua regia soluble boron, did not correspond to EC10 values
based on added boron, even with ageing. They found the aqua
regia-soluble boron to range from 1 to 32 mg B/kg in several natural
soils, but these did not cause barley toxicity, even though EC10 values
from freshly spiked soils ranged from 3 to 38 mg B/kg. This supports the
used of added-boron, rather than total boron, as the basis for
derivation of a PNEC for soil.
of the large difference in bioavailability between boron naturally
present in soils and added soluble B, risks of added soluble boron will
be assessed by using the added risk concept.
all information into account, it was decided not to implement
normalization models for soil properties in the PNEC derivation for
boron in soils because:
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