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Toxicity studies investigating the toxicity of Na2S or NaHS to aquatic organisms have focused in most cases on the toxicity of H2S, which is the most toxic sulfur species that can be formed upon dissolution of Na2S and NaHS. The data set for acute toxicity to aquatic organisms therefore not only contains toxicity tests in which Na2S, NaHS or its respective hydrates have been used as test substance, but also contains toxicity tests in which H2S was immediately dosed to the test medium. The toxicity of Na2S and NaHS depends on the relative presence of H2S in the test medium. In standard test systems - with a typical oxidizing nature - H2S formation will be limited through partial oxidation of the sulfides to (eventually) sulfate, and high effect concentrations will be obtained when expressed as mg Na2S/L, mg NaHS/L or mg S2-/L (total dissolved sulfide, i.e. [H2S]+[HS-]+[S2-]). In test systems especially adapted to sustain stabile H2S exposure levels (e.g., high flow-through rates, hypoxic conditions, etc.), the relative presence of H2S will be higher and much lower effect concentrations will be obtained when expressed as mg Na2S/L, mg NaHS/L or mg S2-/L. Consequently, Na2S or NaHS concentrations are poor predictors of toxicity, and it is much more useful to consider effect concentrations expressed as H2S concentrations. On the other hand, because in most natural aquatic systems the oxidizing conditions will counter the presence of H2S through oxidation of sulfides to sulfate, the data set was extended with the minimal critical information for derivation of a PNECaquatic for sulfate. This information was extracted from the OECD SIDS for Na2SO4, which was considered the most relevant compound for read-across. However, it should be kept in mind that the observed toxicity of Na2SO4 is affected by the presence of both sodium ions and sulfate ions, and that recalculation of the observed effect concentrations from Na2SO4 concentration to sulfate concentration represents a worst case approach.

In oxidizing aquatic conditions, the alkalinity of the reaction mass (pH >12) and its NaOH content are likely to have the most significant ecotoxicological effects. Ecotoxicological data also are presented for both NaOH and Na2CO3. While the data is not extensive, the difference between the reported toxic concentration levels of NaOH and, on the other hand, those of Na2CO3 and Na2SO4 is clear.