Disodium sulphide

Toxicity to aquatic algae and cyanobacteria

Description of key information

Two studies were considered to yield reliable information on sulfide/H2S toxicity. The first study (Admiraal and Peletier, 1979) was an atypical study in which several pennate benthic and centric planktonic diatom species were exposed for 48 h to a series of sulfide concentrations in the absence of oxygen and light (not affecting control populations). In this study, the lowest 48-h NOEC values were < 30 mg S2-/L. The second study (Breteler et al., 1991) investigated the effects of sulfide on photosynthesis of the planktonic diatom Skeletonema costatum by exposing the diatoms for 4 h to a series of sulfide concentrations in the presence of a 14C-labeled carbon source and measuring 14C photoincorporation. The NOEC was determined to be 0.77 mg S2-/L, which corresponds to a H2S concentration of 0.041 mg H2S/L. The study of Breteler et al. (1991) was considered as a key study. The available studies only reported reliable results for marine algae. No reliable information was identified for freshwater algae. For sulfate toxicity, the study of Patrick et al. (1968) was identified as a key study. The 120-h EC50 (based on reduction in number of cells produced compared to the control) of Na2SO4 for the freshwater diatom Nitzschia linearis was reported to be 1900 mg/L. Recalculation to SO4 2- yields an EC50 of 1285 mg/L.

Key value for chemical safety assessment

EC50 for marine water algae:

0.104 mg/L

EC10 or NOEC for marine water algae:

0.041 mg/L

Additional information

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. Among the available studies using either Na2S, NaHS (or its respective hydrates) or H2S as test substance, the studies of Breteler et al. (1991) and Admiraal and Peletier (1979) yielded reliable information on sulfide toxicity, but only for marine diatoms. The study of Breteler et al. (1991) delivered the key NOEC of 0.041 mg H2S/L for the marine diatom Skeletonema costatum, which will be used for derivation of a PNECaquatic for H2S for both the freshwater and marine environment. In oxic environments, released sulfides will be oxidized to - eventually - sulfate. In these cases, the risks entailed by the released sulfur should be evaluated using toxicity data for sulfate. Na2SO4 was identified as the most relevant substance for this purpose. From the OECD SIDS for Na2SO4, one reliable (Klimisch 2) study was selected for inclusion in this dossier. The study of Patrick et al. (1968) was considered as the key study. This study reports a 120-h EC50 of 1900 mg Na2SO4/L for the freshwater diatom Nitzschia linearis, yielding a 120-h EC50 of 1285 mg/L for sulfate. This EC50 most likely represents a worst case value since the overall observed toxicity of Na2SO4 is also affected by the presence of sodium ions and not only by the presence of sulfate ions.