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Environmental solubility and dispersibility

Transformation/dissolution data (OECD Series No. 29) indicate that in environmental media at pH 6 BaSO4 nanomaterials are soluble (86 - 94 %) at the 1 mg/L-loading but of limited solubility (0.9 - 1.4 %) at the 100 mg/L-loading. At pH 8, BaSO4 in nanoform is of limited solubility (~20 %). Transformation/dissolution testing of different nanomaterials further demonstrates that Ba concentrations of the fractions < 0.2 μm and < 2.1 nm are similar at pH 6 and pH 8, at the low (1 mg/L) and high loading (100 mg/L) after 1 (1 and 100 mg/L), 7 d (1 and 100 mg/L) and 28 d (1 mg/L) of shaking. Thus, the tested nanomaterials dissolve considerably, but only at the low loading, and only a very limited fraction of BaSO4 nanoparticles (< 100 nm) may be expected to remain in dispersion. Thus, dissolution is considered to be important for the fate of barium sulfate in the environment and barium sulfate is expected to behave similar to barite, the BaSO4 mineral naturally abundant.


Ubiquitousness and environmental chemistry of barium

Barium is a lithophile and common element in the earth’s crust. Barium is released from weathered rocks and is not very mobile as it precipitates as barium sulfate (BaSO4) and barium carbonate (BaCO3) and is strongly adsorbed by clays. Barium readily displaces other sorbed alkaline earth metals from some oxides, e.g., MnO2 and TiO2, but it is displaced by alkaline earth metals such as Be and Sr from Al2O3. In temperate humid climate soils, Ba is likely to be fixed by Fe oxides and becomes immobile.

Barium has one oxidation state (2+). In the environment, barium does not exist in the elemental form, but occurs as divalent cation Ba2+. Its mobility is not strongly controlled by pH and redox potential. Compared to the other alkaline earth metals, Ba carbonate and sulfate show limited solubility. BaCl2 and Ba(NO3)2 are more soluble, and the presence of a high chloride concentrations may cause sulfate-rich water to retain more Ba in solution (Salminen et al. 2005, US EPA 2005).

Barium is naturally abundant in (agricultural) soils with concentrations ranging from 2.6 to 818 mg/kg (aqua regia extraction) in the top layer and the 5th, 50th and 95th P amounting to 15, 62 and 180 mg/kg, respectively (Reimann et al. 2014). The median total barium background concentration of European soils expressed as Ba (XRF analysis) is 375 mg/kg in topsoil ranging from 30 to 1870 mg/kg.

Barium is naturally abundant in sediments. The median total barium content of European stream sediment expressed as Ba (XRF analysis) is 386 mg/kg ranging from 8 to 5000 mg/kg, whereas barium concentrations of the < 45 µm fraction of European stream waters are highly variable ranging from 0.2 to 436 μg/L with a median of 24.9 µg/L. The presence and concentration of barium in surface water is strongly controlled by the abundance of Ba in the bed rock, as hydrogeochemical and biogeochemical processes show little variability from one environment to another. The dispersal of Ba in surface water is also controlled by the presence or absence of hydrous Mn and Fe oxides, which adsorb Ba(2+) ions. Adsorption onto the surfaces of clay minerals and organic matter can also be significant at higher pH (Salminen et al. 2005).



Existing bioaccumulation data for barium suggest that barium bioconcentration and bioaccumulation is negligible. The bioaccumulation factor of fish (whole body) was situated between 37.6 and 98.8 (geomean of 4 values: 65.6) (Nakamoto and Hassler, 1992). Whole-body concentrations are significantly higher than reported soft tissue concentrations due to the fact that Ba (like Sr) can replace Ca in the bones and hard tissue parts; indeed, according to the WHO (1990), approximately 91% of Ba found in the body are located in the bones. Reported whole-body Ba-levels in fish were similar in different studies; the following ranges were reported; 5.7-17.2 μg/g (Nakamoto and Hassler, 1992), 4.37 μg/g (Saiki and Palawski, 1990); 5.1-16 μg/g (Schroeder et al, 1988); 4.4-12 (Radtke et al, 1988) and 9-33 μg/g (three fish species; Allen et al, 2001). The data indicate a certain degree of homeostatic control of internal Ba levels by fish. Limited information on transfer of Ba through the food chain indicates that barium does not biomagnify in aquatic food chains.



Reimann C et al. 2014. Chemistry of Europe’s agricultural soils–Part B: General background information and further analysis of the GEMAS data set. Geologisches Jahrbuch, Reihe B, 103, 352

Salminen R et al. 2005. Geochemical Atlas of Europe. Part 1: Background Information, Methodology and Maps.

US EPA 2005. Toxicological review of barium and compounds (CAS No. 7440-39-3), In Support of Summary Information on the Integrated Risk Information System (IRIS).

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