Barium di(acetate)

Ecotoxicological Summary

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:

PNEC aqua (freshwater)

PNEC value:

539.4 µg/L

Assessment factor:

10

Extrapolation method:

assessment factor

Marine water

Hazard assessment conclusion:

no data: aquatic toxicity unlikely

STP

Hazard assessment conclusion:

no hazard identified

Sediment (freshwater)

Hazard assessment conclusion:

PNEC sediment (freshwater)

PNEC value:

2 814 mg/kg sediment dw

Assessment factor:

1

Extrapolation method:

equilibrium partitioning method

Sediment (marine water)

Hazard assessment conclusion:

no hazard identified

Hazard for air

Air

Hazard assessment conclusion:

no hazard identified

Hazard for terrestrial organisms

Soil

Hazard assessment conclusion:

PNEC soil

PNEC value:

386.3 mg/kg soil dw

Assessment factor:

2

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:

no potential for bioaccumulation

Additional information

Read across approach:

barium di(acetate) is considered a salt consisting of a barium cation and the acetate anion (conjugated base) of acetic acid. The ions of barium di(acetate) are considered as a hard acid and a hard base, respectively, without any known transformation processes that may result in different moieties relevant for the hazard assessment. Barium di(acetate) were analysed for the presence of covalent and ionic bonds between barium and the oxygen of the acetate group. It was confirmed that barium di(acetate) bonds are ionic (Mehrotra RC and Bohra R, 1983).

Upon dissolution in water, the salt with a water solubility of 729 g/L (25°C) dissociates completely and releases barium cations and acetate anions. The equilibrium equation does not indicate any pH dependency of the dissociation:

The dissociation ofbarium di(acetate) is reversible and the ratio of the salt /dissociated ions is dependent on the metal-ligand complexation constant of the salt, the composition of the solution and its pH.

A metal-ligand complexation constant of barium di(acetate) could not be identified. Data for alkaline earth metals appear to be generally limited. However, alkaline earth metals tend to form complexes with ionic character as a result of their low electronegativity. Further, the ionic bonding of alkaline earth metals is typically described as resulting from electrostatic attractive forces between opposite charges, which increase with decreasing separation distance between ions.

 

Based on an analysis by Carbonaro & Di Toro (2007) of monodentate binding of barium to negatively-charged oxygen donor atoms, including acetate functional groups, monodentate ligands such as acetate are not expected to bind strongly with barium. Accordingly, protons will always out-compete barium ions for complexation of monodentate ligands given equal activities of free barium and hydrogen ions. The metal-ligand formation constant (log KML) of barium with acetate was reported to be 0.39 (Furia, T. 2006) and points to a low strength of the monodentate bond between acetic acid and barium.

 

Further, the analysis by Carbonaro & Di Toro (2007) suggests that the following equation models monodentate binding to negatively-charged oxygen donor atoms of carboxylic functional groups:

 

log KML = αO×log KHL + βO; where

 

KML is the metal-ligand formation constant, KHL is the corresponding proton–ligand formation constant, and αO and βO are termed the Irving–Rossotti slope and intercept, respectively. Applying the equation and parameters derived by Carbonaro & Di Toro (2007) and the pKa of acetic acid of 4.75 results in:

 

log KML = 0.186×4.75 - 0.171

 

log KML = 0.713 (estimated barium- acetate formation constant).

 

Thus, it may reasonably be assumed that based on the barium-acetate formation constant, the respective behaviour of the dissociated barium cations and acetate anions under physiological conditions and in the environment determine the fate of barium di(acetate) upon dissolution with regard to (bio)degradation, bioaccumulation, partitioning resulting in a different relative distribution in environmental compartments (water, air, sediment and soil) and subsequently its (eco)toxicological potential.

 

Therefore, in the assessment of the (eco)toxicity of barium di(acetate), a read-across to data for acetic acid and soluble barium substances is applied since only the ions of barium and acetate are available in the environment and systemically and determine the (eco)toxicological potential.

PNEC marine water:

A relevant PNEC for the marine environment cannot be determined, for the following reasons:

(i) Barium levels in sea water range from 2 to 63 μg/L with a mean concentration of about 13 μg/L (Bowen 1979).

(ii) Applying ECHA-guidance, the derived marine PNEC of 11.5 μg/L for barium (PNEC freshwater = 0.115 mg Ba/L and an AF of 100) would thus be within the range of typical barium seawater levels.

(iii) Seawater contains about 2700 mg/L sulfate (Hitchcock, 1975 cited in WHO, 2004).

(iv) Barium transported into marine systems combines with sulfate ions present in salt water to form barium sulfate.

(v) Barium in marine environments is in a steady state; the amount entering is balanced by the amount falling to the bottom as barium sulfate (barite) particles to form a permanent part of the marine sediment (Wolgemuth & Brocker, 1970). Thus, dissolved barium concentrations are controlled by the solubility of barium sulfate. The solubility product (K_sp) of barium sulfate is 1.08E^-10(CRC Handbook, 2008), resulting in maximum dissolved Ba levels of approximately 1.4 mg/L.

(vi) In sum, due to high sulfate levels in the marine environment and a low solubility of barium sulfate, dissolved barium levels will remain constant in marine waters, regardless of the amount of barium introduced to the system.

 

References:

Bowen HMJ (1979) Environmental Chemistry of the Elements. Academic Press, London, 333 pp.

Lide, D.R. (2008) CRC Handbook of chemistry and physics. 88^thedition.

Hitchcock DR (1975) Biogenic contributions to atmospheric sulphate levels. In: Proceedings of the 2nd National Conference on Complete Water Re-use. Chicago, IL, American Institute of Chemical Engineers.

WHO (1990) Barium. Environmental Health Criteria 107. International Programme on Chemical Safety.

WHO (2004) Sulfate in Drinking-water. Background document for development of WHO Guidelines for Drinking-water Quality. WHO/SDE/WSH/03.04/114.

Wolgemuth K & Broecker WS (1970) Barium in sea water. Earth planet. Sci. Lett., 8: 372-378.

 

PNEC sediment:

The PNECsediment can be derived from the PNECaquatic using the equilibrium partitioning method (EPM).

A distribution/partition coefficient (KD) between the water and sediment compartment for barium has been determined (see chapter 4). This resulted in a typical KD, susp-water of 5,217 L/kg (logKD: 3.72). In a first step the units have to be converted from L/kg to m3/m3using the formula below.

KD, susp-water(m3/m3) = 0.9 + [0.1 x (KD, susp-water(L/kg) x 2,500) / 1,000]

This results in a KD, sediment of 1,305 m3/m3. This value can be entered in the equation below to calculate the PNECsediment:

PNECsediment= (KD, susp-water/ RHOsusp) x PNECaquatic x 1,000

with the PNECaquatic expressed as mg/L, RHOsusp representing the bulk density of wet suspended matter (freshly deposited sediment) (1,150 kg/m3), and a KD, susp-water of 1,305 m3/m3, a PNECsediment that is expressed as mg/kg wet weight can be derived. This value can be converted to a dry weight-based PNEC, using a conversion factor of 4.6 (CONVsusp = RHOsusp/Fsolid-susp * RHOsolid) kg wet weight/ kg dry weight.

This results in a PNECsediment of 1,513 mg Ba/kg dry sediment.

 

PNEC soil:

 

Derivation of a PNEC for the terrestrial compartment according to the assessment factor method resulted in a PNEC well below typical background levels for the majority of EU-countries. Therefore a more relevant PNEC was derived based on reported baseline levels of Ba in EU top soil samples. The outlier cut-off level for Ba baseline levels (i.e., 415.7 µg/L) was used as a starting point. A factor of two was applied to the cut-off level and a PNEC soil of 207.7 mg Ba/kg dry wt is derived and considered as a reliable, provisional PNEC for the terrestrial compartment. For more information please refer to the CSR.

 

 

PNEC for sewage treatment plant:

In general, an AF of 10 is to be applied to the NOEC/EC₁₀of a sludge respiration test, reflecting the lower sensitivity of this endpoint as compared to nitrification, as well as the short duration of the test. The corresponding AF is 100 when based on the EC₅₀. The PNEC_{micro-organism}is set equal to a NOEC (AF = 1) for a test performed with specific bacterial populations such as nitrifying bacteria,P. putida, ciliated protozoa, the Shk1 Assay. An EC₅₀from this test is divided by an AF of 10 to derive the_{PNECmicro-organism}. No AF is needed to derive a PNEC_{micro-organism}based on good quality field data.

The lowest reliable observed NOEC/EC₁₀-value for respiration (inhibition of respiration after a 3h incubation period) using activated sludge was ≥943.1 mg BaCl2/L. Based on the guidance given in the RIP3.2 (ECHA, 2008) and the TGD (2003), an assessment factor of 10 should be used on this value, as respiration is the endpoint.

 

PNEC_oral(secondary poisoning):

No avian toxicity data are available

-         Data from an NTP (1994) study resulted in NOAELs for rats ranging between 61 and 115 mg Ba/kg/d, depending on the exposure period (13 wk, 2 yr). Evaluated endpoints were renal and cardiovascular effects.

-         Data from an NTP (1994) study resulted in NOAELs for mice ranging between 61 and 115 mg Ba/kg/d, depending on the exposure period (13 wk, 2 yr). Evaluated endpoints were renal and cardiovascular effects.

-         A 13 wk reproduction study with rat and mice (Dietz et al, 1992), resulted in a NOAEL of 200 mg Ba/kg/d for both test species.

According to ECHA-Guidance (ECHA, 2008: Chapter R.10 – Dose (concentration)-response regarding environment) a NOEC_mammalcan be derived from a NOAEL_mammal,using the following formula:

NOEC_{mammal_food_chronic}= NOAEL_{mammal_food_chronic} x CONV_mammal

with CONV_mammala species-specific conversion factor. The conversion factor for rats is 10-20, depending on the age of the test organisms.

As endpoints like mortality, growth and reproduction are strongly preferred for the derivation PNEC_oral, the value of 200 mg/kg bw/d (NOAEL_reproductionfor rats and mice) was used as reference value for the derivation of a PNEC_oral.

The age of the test organisms was not specified; therefore a conversion factor of 10 kg bw.d/kg food

NOEC_{mammal_food_chronic}= 200 mg /kg bw/d * 10 kg bw.d/kg_food= 2000 mg Ba/kg_food

The PNEC_oralthat can be derived from this NOEC-value by applying an adequate assessment factor on the NOEC. An assessment factor of 90 is required when a 90d-NOEC for mammals is used as reference value. An estimated PNEC_oralfor barium would be 2000 mg Ba/kg food / 90 = 22.2 mg/kg food

It should be noted that, according to the ECHA technical guidance on environmental hazard assessment, ‘if a substance has a bioaccumulation potential and a low degradability, it is necessary to consider whether the substance also has the potential to cause toxic effects if accumulated in higher organisms.’ It further states that the assessment of secondary poisoning takes place as a tiered process, where the first step is to evaluate the bioaccumulative potential of a substance, following the criterion that if BCF ≥ 100 (together with considerations regarding biodegradability). When this criterion is met, the subsequent step to calculate a PNECoral,predator is needed.

As barium does not meet this requirement, no PNEC_{oral,predator}is required for this substance.

 

Conclusion on classification

For the assessment of the environmental hazard potential of barium di(acetate), the category approach is applied and data for acetic acid and its salts and soluble barium substances are read-across since only the ions of barium di(acetate), Ba2+ + 2*CH3COO- are available in an aqueous environment and determine the toxicity.

Based on aquatic toxicity data of its moieties, i.e. barium and acetate, barium di(acetate) appears to be not toxic to aquatic organisms. Short-term and long-term toxicity data of barium are available for freshwater organisms covering three trophic levels. The respective reliable IC/LC/EC50 and NOEC values, respectively are above 1 mg/L. Furthermore, the potential of acetate for acute and chronic aquatic toxicity appears to be low. Hence, no classification and labelling for aquatic acute/chronic toxicity is required.