Barium and barium compounds
7
barium ores are barite (barium sulfate) and witherite
(barium carbonate). Barite occurs largely in sedimentary
formations, as residual nodules resulting from weather-
ing of barite-containing sediments, and in beds along
with fluorspar, metallic sulfides, and other minerals.
Witherite is found in veins and is often associated with
lead sulfide. Barium is found in coal at concentrations up
to 3000 mg/kg, as well as in fuel oils (IPCS, 1990;
ATSDR, 1992). Estimates of terrestrial and marine
concentrations of barium are 250 and 0.006 g/tonne,
respectively (Considine, 1976).
Barite ore is the raw material from which nearly all
other barium compounds are derived. Barite is mined in
Morocco, China, India, and the United Kingdom. Crude
barite ore is washed free of clay and other impurities,
dried, and then ground before use. Barite is usually
imported as crude ore or crushed ore for milling or as
ready-milled ore. Barite can be 90–98% barium sulfate.
World production of barite in 1985 was estimated to be
5.7 million tonnes.
Because of its high specific gravity, low abrasive-
ness, chemical stability, and lack of magnetic effects,
barite is used as a weighting agent for oil and gas well
drilling muds, which counteracts high pressures encoun-
tered in the substrata (IPCS, 1990). It is also used as a
filler in a range of industrial coatings, as a dense filler in
some plastics and rubber products, in brake linings, and
in some sealants and adhesives. The use dictates the
source of barite used. Some sources produce very pure
white barite, which is used in coatings, while barite from
other sources is off-white and is used in applications
where the colour is unimportant. The use will also dictate
the particle size to which barite is milled. For example,
drilling muds are ground to an average particle diameter
of 44 µm, with a maximum of 30% of particles less than 6
µm in diameter. Barium and its compounds are used in
diverse industrial products ranging from ceramics to
lubricants. Barium is used in the manufacture of alloys,
soap, rubber, and linoleum; in the manufacture of valves;
as a loader for paper; and as an extinguisher for radium,
uranium, and plutonium fires. Barium compounds are
used in cement, specialty arc welding, glass industries,
electronics, roentgenography, cosmetics,
pharmaceuticals, inks, and paints. They have also been
used as insecticides and rodenticides (e.g., barium meta-
borate, barium polysulfide, and barium fluorosilicate).
Anthropogenic sources of barium are primarily
industrial. Emissions may result from mining, refining, or
processing of barium minerals and manufacture of
barium products. Barium is released to the atmosphere
during the burning of coal, fossil fuels, and waste.
Barium is also discharged in wastewater from metallur-
gical and industrial processes. Deposition on soil may
result from human activities, including the disposal of fly
ash and primary and secondary sludge in landfills (IPCS,
1990). Estimated releases of barium and barium
compounds to the air, water, and soil from manufacturing
and processing facilities in the USA during 1998 were
900, 45, and 9300 tonnes, respectively.
1
5. ENVIRONMENTAL TRANSPORT,
DISTRIBUTION, AND TRANSFORMATION
Both specific and non-specific adsorption of bar-
ium onto oxides and soils have been observed. Specific
sorption occurs onto metal oxides and hydroxides.
Adsorption onto metal oxides probably acts as a control
over the concentration of barium in natural waters.
Electrostatic forces account for a large fraction of the
non-specific sorption of barium on soil and subsoil. The
retention of barium, like that of other alkaline earth
cations, is largely controlled by the cation-exchange
capacity of the sorbent. Complexation by soil organic
material occurs to a limited extent. The K
d
(soil sorption)
value, the dissociation constant between sediment and
barium in sediments, is 5.3 × 10
5
ml/g (McComish & Ong,
1988).
Examination of dust falls and suspended particu-
lates indicates that most contain barium. The presence of
barium is mainly attributable to industrial emissions,
especially the combustion of coal and diesel oil and
waste incineration, and may also result from dusts blown
from soils and mining processes. Barium sulfate and
carbonate are the forms of barium most likely to occur in
particulate matter in the air, although the presence of
other insoluble compounds cannot be excluded. The
residence time of barium in the atmosphere may be
several days, depending on the particle size. Most of
these particles, however, are much larger than 10 µm in
size and rapidly settle back to earth. Particles can be
removed from the atmosphere by rainout or washout wet
deposition.
Soluble barium and suspended particulates can be
transported great distances in rivers, depending on the
rates of flow and sedimentation. Cartwright et al. (1978)
studied the chemical control of barium solubility and
1
Toxic Chemical Release Inventory (TRI) database,
Office of Toxic Substances, US Environmental Protection
Agency, Washington, DC, 1998.
Concise International Chemical Assessment Document 33
8
showed that, for most water samples, barium ion con-
centration is controlled by the amount of sulfate ion in
the water.
While some barium in water is removed by preci-
pitation, exchange with soil, or other processes, most
barium in surface waters ultimately reaches the ocean.
Once freshwater sources discharge into seawater, barium
and the sulfate ions present in salt water form barium
sulfate. Due to the relatively higher concentration of
sulfate present in the oceans, only an estimated 0.006%
of the total barium brought by freshwater sources
remains in solution (Chow et al., 1978). This estimate is
supported by evidence that outer-shelf sediments have
lower barium concentrations than those closer to the
mainland.
Marine concentrations of barium generally
increase with depth, suggesting that barium may be
incorporated into organisms in the euphotic zone and
subsequently sedimented and released in deeper waters
(IPCS, 1990). In laboratory testing, the uptake of barium
by algae in culture media was 30–60% after 15 days of
exposure to barium concentrations of 0.04, 0.46, and
4.0 mg/litre of medium, the relative accumulation being
inversely related to the barium concentration in the
medium and directly related to the exposure duration
(Havlik et al., 1980). Barium was not incorporated into
organic components but was bound primarily to the cell
membrane or other non-extractable components.
Accumulation of barium ions (
133
Ba) in the cells of the
alga
Scenedesmus obliquus has been shown to increase
with increasing pH between pH 4 and 7, then remain
constant over the pH range 7–9 at a barium concentra-
tion of 10
–6
mol/litre, with a calculated affinity constant
(
K
m
) of 4.8 (Stary et al., 1984). In a marine environment
contaminated with heavy metals (including barium),
Guthrie et al. (1979) measured barium concentrations of
7.7 mg/litre in water and 131.0 mg/kg wet weight in
sediment. Among barnacles, crabs, oysters, clams, and
polychaete worms tested for barium content in this
marine environment, only barnacles showed higher
concentrations of barium (40.5 mg/kg wet weight) than
that of the water.
Barium sulfate is present in soil through the natural
process of soil formation; barium concentrations are
high in soils formed from limestone, feldspar, and biotite
micas of the schists and shales (Clark & Washington,
1924). When soluble barium-containing minerals weather
and come into contact with solutions containing
sulfates, barium sulfate is deposited in available geologi-
cal faults. If there is insufficient sulfate to combine with
barium, the soil material formed is partially saturated with
barium. In soil, barium replaces other sorbed alkaline
earth metals from manganese dioxide, silicon dioxide, and
titanium dioxide under typical environmental conditions,
by ion exchange (Bradfield, 1932; McComish & Ong,
1988). However, other alkaline earth metals displace
barium from aluminium oxide (McComish & Ong, 1988).
Barium sulfate in soils is not expected to be very
mobile because of the formation of water-insoluble salts
and its inability to form soluble complexes with humic
and fulvic materials. Under acid conditions, however,
some of the water-insoluble barium compounds (e.g.,
barium sulfate) may become soluble and move into
groundwater (US EPA, 1984).
Despite relatively high concentrations in soils,
only a limited amount of barium accumulates in plants.
Barium is actively taken up by legumes, grain stalks,
forage plants, red ash (Fraxinus pennsylvanica) leaves,
and black walnut (Juglans nigra), hickory (Carya sp.),
and brazil nut (Bertholletia excelsa) trees; Douglas-fir
(Pseudotsuga menziesii) trees and plants of the genus
Astragallu also accumulate barium (IPCS, 1990). Barium
has also been shown to accumulate in mushrooms
(Aruguete et al., 1998). No studies of barium particle
uptake from the air have been reported, although
vegetation is capable of removing significant amounts of
contaminants from the atmosphere. Plant leaves act only
as deposition sites for particulate matter. Although
levels of barium in wildlife have not been documented,
barium has been found in dairy products and eggs
(Gormican, 1970; IPCS, 1990), indicating that barium
uptake occurs in animals.
A bioconcentration factor (BCF) for soil to plants
was estimated as 0.4 (0.02 standard error of the mean
[SEM]), based on samples of a variety of plant species
(mean barium concentration of 29.8 mg/kg [13.7 SEM])
that were taken from a site in which the mean concentra-
tion of barium in the soil was 104.2 mg/kg (9.5 SEM)
(Hope et al., 1996). Based on the ratio of barium concen-
tration in the soil to whole-body barium concentration,
the same authors computed bioaccumulation factors of
0.2 (0.002 SEM) for terrestrial insects, 0.02 (0.0004 SEM)
for white-footed mice (Peromyscus leucopus), and 0.02
(0.0005 SEM) for hispid cotton rats (Sigmodon hispidus).
Based on dissolved barium concentrations in surface
water of 0.07 mg/litre (0.02 SEM) and whole-body barium
concentrations of 2.1 mg/kg (0.5 SEM) in fish, measured
at the same study site, a BCF of 129.0 litres/kg (13.5
SEM) was estimated. The authors also estimated mean
depuration rates in white-footed mice and hispid cotton
rats to be 0.4/day (0.01 SEM) and 0.2/day (0.01 SEM),
respectively, indicating that barium is “lost from these
receptors at a fairly rapid rate.” Field data were collected