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Toxicity to microorganisms

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For H2S toxicity, no reliable studies have been identified for this endpoint. Because of testing difficulties and also because of exposure-based considerations, it is not considered necessary to conduct additional tests with sodium sulfide compounds. For sulfate toxicity, the study of Tokuz and Eckenfelder (1979) was considered as a key study. In this study, the most sensitive endpoint was the presence of stalked ciliates, with a NOEC of ca. 8 g Na2SO4/L, yielding a NOEC of ca. 5.4 g/L for sulfate.

Key value for chemical safety assessment

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Some of the available studies reported toxicity data for less relevant species or endpoints (e.g., inhibition of bioluminescence of the marine bacteria Vibrio fischeri and Photobacterium phosphoreum or the inhibition of glucose metabolism by the intestinal bacterium Escherichia coli, Postma et al., 2002; Van Leeuwen et al., 1985; Bringmann and Kühn, 1959). The information considered relevant included growth inhibition studies with the bacterium Pseudomonas putida and flagellate or ciliate protozoans such as Entosiphon sulcatum, Uronema parduczi and Chilomonas paramaecium, and nitrification inhibition tests with species of the genera Nitrosomonas and Nitrobacter. However, all studies were considered unreliable because it was not clear whether sulfide concentrations were measured or not and/or because pH was not reported or only initial pH was reported. Therefore, the reported effect concentrations (mostly expressed as mg S2-/L) could not be recalculated to H2S concentrations in a reliable way. Because of the oxic nature of typical test systems for this endpoint (activated sludge respiration inhibition tests or single-species tests with ciliates), it is considered technically very difficult to ascertain continuous exposure to a constant H2S concentration in these specific tests. Therefore, waiving of any new tests is proposed. Instead, an estimate may be considered based on the lowest available effect concentration, which is the 16-h EC50 of 1.6 mg S2-/L for the bacterium Pseudomonas putida obtained in a cell multiplication inhibition test by Bringmann and Kühn (1977b; 1980b). Although pH is not reported, one may assume a worst case pH of 8.0, at which a maximum of 10% of the total dissolved sulfide could be present as molecular H2S. The key effect concentration would then be a 16-h EC50 of 0.16 mg H2S/L and can be used for calculation of a provisional PNECstp.

Because general conditions in sewage (i.e., organic load, redox conditions, microbial density) are often favourable for H2S formation, aeration is important to avoid possible H2S toxicity in sewage treatment plants. When active aeration is applied (during physicochemical pre-treatment or during the biological oxidation step), any dissolved sulfides present are expected to be oxidized to - eventually - sulfate in less than one hour. Next to oxidation, precipitation as metal sulfides will be an additional removal process (with metals present in the sewage or with added metals e.g. through addition of ferric chloride). Moreover, sulfide compounds as Na2S and/or NaHS are in their turn used in waste water treatment plants for the removal of metals through precipitation. During this use, care is taken not to add excess sulfides to avoid H2S formation. In sewage treatment plants where H2S formation is a realistic problem, the necessary measures are expected to be taken to avoid exposure of activated sludge to relatively high H2S concentrations (e.g., pre-oxidation step, sulfide precipitation). Further, whether discharged to surface water or to sewage treatment systems, industrial effluents have to comply with national standards. For sulfides, discharged waste water should generally not contain more than 1 mg S2-/L (total dissolved sulfides). When respecting these standards, generally no unacceptable complications directly related to the sulfide content of the effluent are to be expected. Moreover, if H2S formation poses a problem in a certain waste water treatment plant, it would be impossible to identify a single source as responsible for it, because any sewage source will have contributed to the problem, as well as the general conditions in the sewage. Because any dissolved sulfide will either be precipitated and removed from the system or oxidized to sulfate, again read across from Na2SO4 was suggested. One reliable (Klimisch 2) study from the OECD SIDS was included in this dossier (Tokuz and Eckenfelder, 1979). In this study, activated sludge units were fed with waste water with stepwise increasing Na2SO4 concentrations from 8 to 35 g/L over a time period of 37 days. Sludge performance was monitored by measuring BOD, COD and TSS in the effluent. Sludge composition was examined microscopically. The most sensitive endpoint was the presence of stalked ciliates, with a NOEC of ca. 8 g Na2SO4/L, or, recalculated to sulfate concentration, 5.4 g/L. It should however be kept in mind that this value may represent a worst case value because the observed toxicity of Na2SO4 is also affected by the presence of sodium ions and not only by the presence of sulfate ions.

No reliable data were found for Na2CO3 or NaOH, although two NaOH studies are included in the dossier, one reporting a 15 -min EC50 of 22 mg/L for Photobacterium phosphoreum.