Reaction mass of disodium sulphide and sodium carbonate and sodium hydrogensulphide

Endpoint summary

Additional information

For a multi-constituent substance there is no need to test the substance as such, if the hazard profile of the substance can be sufficiently described by the information of the individual constituents. The hazard profile of the multi-constituent Scrubber Liquid can be adequately described from the information on each of the REACH registered constituents: disodium sulphide [EC No 215-211-5; CAS No 1313-82-2]; sodium carbonate [EC No 207-838-8; CAS No 497-19-8] and sodium hydrogensulphide [EC No 240-778-0; CAS No 16721-80-5]. The toxicological hazards of sodium hydrogen sulphide and disodium sulphide were determined to be greater than that for sodium carbonate. The majority of the assessments of the toxicological hazards for both disodium sulphide and sodium hydrogensulphide were obtained by read-across to hydrogen sulphide (H₂S) data based upon their respective sulphide (S^2-) content. The sulphide content of the Scrubber Liquid was determined as 15.6%. As such, this figure was then used to calculate the hazards for Scrubber Liquid by read-across to hydrogen sulphide in same manner as for disodium sulphide and sodium hydrogensulphide.

General read-across concept between sodium sulphide, sodium hydrogensulphide and hydrogen sulphide:

Given that sodium sulphide and sodium hydrogensulphide dissociate in aqueous media (please refer to the Hägg-graph give below), it can safely be assumed that under most physiologically relevant conditions (i.e., neutral pH) sulphide and hydrogen sulphide anions are present at almost equimolar concentrations, thus facilitating unrestricted read-across between both species. Only under extreme conditions such as gastric juice (pH << 2), sulphides will be present predominantly in the form of the non-dissociated hydrogen sulphide. In turn, hydrogen sulphide (H₂S) may be formed from both soluble sulphides, according to the following equilibria:

                           Na₂S + H₂O   →  NaOH + NaHS (2Na⁺/ OH^-/ HS^-)

                           NaHS + H₂O  →  NaOH + H₂S (Na⁺/ OH^-/ H₂S)

Similarly, hydrogen sulphide dissociates in aqueous solution to form two dissociation states involving the hydrogen sulphide anion and the sulphide anion, according to the following equilibrium:

                           H₂S  ↔  H⁺  +  HS^-  ↔  2 H⁺  +  S^2-

In conclusion, under physiological conditions, inorganic sulphides or hydrogensulphides as well as H₂S will dissociate to the respective species relevant to the pH of the physiological medium, irrespective the nature of the “sulphide”, which is why read-across between these substances and H₂S is considered to be feasible without any restrictions.

Inhalation absorption

No data on inhalation absorption are available for sodium sulphide and sodium hydrogensulphide, and little reliable information (if at all) is available on hydrogen sulphide absorption and distribution after inhalation. No toxicokinetic information is available for sodium carbonate.

Nevertheless, after completion of a testing programme on dustiness testing and particle size analysis of the airborne fraction on commercially available forms of sodium sulphide and sodium hydrogensulphide, the collected information can be used to estimate inhalation absorption factors based on a prediction of deposition patterns in the respiratory tract (MPPD model), in accordance with guidance developed under HERAG

The fate and uptake of deposited particles depends on the clearance mechanisms present in the different parts of the airway. In the head region, most material will be cleared rapidly, either by expulsion or by translocation to the gastrointestinal tract. A small fraction will be subjected to more prolonged retention, which can result in direct local absorption. More or less the same is true for the tracheobronchial region, where the largest part of the deposited material will be cleared to the pharynx (mainly by mucociliary clearance) followed by clearance to the gastrointestinal tract, and only a small fraction will be retained (ICRP, 1994). Once translocated to the gastrointestinal tract, the uptake will be in accordance with oral uptake kinetics.

In consequence, the material deposited in the head and tracheobronchial regions would be translocated to the gastrointestinal tract, where it would be subject to gastrointestinal uptake at a ratio of 100%. The material that is deposited in the pulmonary region may be assumed by default to be absorbed to 100%. This absorption value is chosen in the absence of relevant scientific data regarding alveolar absorption although knowing that this is a conservative choice. Thus, the following predicted inhalation absorption factors can be derived for sodium sulphide and sodium hydrogensulphide. For further information on particle size and dustiness, see IUCLID section 4.5.

 

Absorption factors, sodium sulphide and sodium hydrogensulphide (rounded values)

test item	absorption factors via inhalation [%]
Sodium sulphide	45
Sodium hydrogensulphide	36

 

Dermal absorption

In the absence of measured data on dermal absorption, previous guidance primarily directed at organic chemicals with a defined lipophilicity and corresponding percutaneous transfer potential, suggests the assignment of either 10% or 100% default dermal absorption rates. In contrast, the currently available scientific evidence on dermal absorption of metal cations (predominantly based on the experience from previous EU risk assessments) yields substantially lower figures, which can be summarised briefly as follows:

Measured dermal absorption values for metal cations or inorganic metal substances in studies corresponding to the most recent OECD test guidelines are typically 1 % or even less. Therefore, the use of a 10 % default absorption factor is not scientifically supported for such ionic species. This is corroborated by conclusions from previous EU risk assessments (Ni, Cd, Zn), which have derived dermal absorption rates of 2 % or far less (but with considerable methodical deviations from existing OECD methods) from liquid media.

However, considering that under industrial circumstances many applications involve handling of dry powders, substances and materials, and since dissolution is a key prerequisite for any percutaneous absorption, a factor 10 lower default absorption factor may be assigned to such “dry” scenarios where handling of the product does not entail use of aqueous or other liquid media. This approach was taken in the in the EU RA on zinc. A reasoning for this is described in detail elsewhere (Cherrie and Robertson, 1995), based on the argument that dermal uptake is dependent on the concentration of the material on the skin surface rather than it’s mass.

Consistent with the methodology proposed in HERAG guidance for metals (HERAG fact sheet - assessment of occupational dermal exposure and dermal absorption for metal cations and inorganic metal substances; EBRC Consulting GmbH / Hannover /Germany; August 2007), the following default dermal absorption factors for metal cations have therefore been proposed (reflective of full-shift exposure, i.e. 8 hours):

From exposure to liquid/wet media: 1.0 %

From dry (dust) exposure: 0.1 %

Given that the primary cause between the lack of percutaneous transfer is considered to be the ionic nature, it is proposed to assume similar behaviour for sulphides anions as for metal cations, and to adopt the above stated dermal absorption factors for sodium sulphide and sodium hydrogensulphide.

Oral absorption

There are only few publications that allow an assessment of the oral bioavailability of sodium sulphide in rats, which are also somewhat of age. Nevertheless, the following conclusions can be drawn: after oral administration of sulphide to rats (Curtis et al., 1972) almost 70% are excreted within 48 hrs, 63% via urine and the remainder via faeces. In another study involving i.p.-administration, 90% of the injected dose could be recovered in urine and faeces (Dziewiatkowski, 1945). In conclusion, the assumption appears justified that upon oral uptake the systemic uptake is essentially complete for sulphides and hydrogen sulphides. Therefore, a conservative oral absorption factor of 100% will be taken forward for risk characterisation purposes.

 

Distribution, metabolism and elimination

 Following oral administration, sulphides are absorbed rapidly and extensively, and distributed widely throughout all tissues without any particular target tissue (Curtis et al., 1972; Dziewiatkowski, 1945). Upon distribution, sulphides rapidly excreted as sulphate, with thiosulphate having been identified as an intermediate metabolite (Bartholomew et al.,1980). The resulting sulphate is excreted almost quantitatively via urine; experiments with bile duct cannulated rats have shown that biliary excretion is minimal by comparison (Curtis et al., 1972). Methylation with subsequent elimination via exhaled air has been excluded for sulphides (Susman et al., 1978).