Sodium sulfide (Na2(Sx))

Endpoint summary

Additional information

Stability

Polysulfides in aqueous solutions will practically always be deprotonated. At neutral and slightly alkaline conditions, S₆^2-, S₅^2-, and S₄^2- dominate. These polysulfides are unstable especially under acidic conditions because polysulfides dissolved in water disproportionate as for instance according to the following reaction; S_n^2- + H⁺--> S_n-1+ HS^-. The rates of polysulfide oxidation are high compared to the oxidation rates of sulfide. The half-life of sulfide under oxic condions is considered to be in the order of a few hours to days.

Biodegradation

A valid ready biodegradability test result for polysulfides is not available because this substance is inorganic. Nevertheless all aspects important for achieving a ready biodegradability test result i.e. 1) ultimate (complete) biodegradation, 2) high rates of degradation and 3) occurrence of competent microorganisms in unacclimated ecosystems are fulfilled. Ready biodegradability tests only detect growth-linked biodegradation. Microorganisms are capable of growth on polysulfides under aerobic conditions (formation of sulfate (completely oxidized)) and anaerobic conditions in the presence of a reducing agent (formation of hydrogen sulfide (completely reduced)). Maximum growth rates of > = 0.8h^-1are sufficiently high to enable a ready biodegradability test result. Transformation of reduced sulfur compounds has been detected in soils, fresh water, marine ecosystems and biological wastewater treatment plants. These findings demonstrate the wide distribution of polysulfide transforming microorganisms. Polysulfides should therefore be classified as readily biodegradable.

 

 

Table Summary of degradation rate constants in various (eco)systems based on the ready biodegradability of polysulfides using TGD default values.

 

(Eco)system	Method	Result	Comment
Surface water (fresh)	TGD default value	Half live 15 days	Classification: readily biodegradable
Surface water (fresh)	Growth rate of sulfide oxidixing miroorganisms	Half live 1 hour	Result of publication in peer reviewed journal
Aerobic sediment layer	TGD default value	Half live 30 days	Kpsed 1.7 l/kg default of non hydrophobic
Anaerobic bulk sediment	TGD default value	Half live 30 days	Kpsed 1.7 l/kg default of non hydrophobic
Marine water	TGD default value	Half live 50 days	Classification: readily biodegradable
Marine water sediment	TGD default value	Half live 100 days	Kpsed 1.7 l/kg default of non hydrophobic
Soils	TGD default value	Half live 30 days	Kpsoil 0.7 l/kg default of non hydrophobic
Degradation in sewage treatment plants	Determined with bioreactor	>99%	Results of publications in peer reviewed journal (Vaiopoulou et al, 2005)

Bioaccumulation

Na2Sx will disassociate and react almost instantly when entering the environment (see chapter 5) to SO4 and therefore is considered a harmless substance that does not have bioaccumulation potential.

Transport and distribution

Na2Sx will disassociate and react almost instantly when entering the environment (see chapter 5) to SO4 and therefore a harmless natural substance.