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Environmental fate & pathways

Biodegradation in water: screening tests

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Description of key information

Articles published in peer reviewed journals show  the complete oxidation, the widespread occurrence, and the high growth rates of organisms

capable of growing on reduced sulfur.

Key value for chemical safety assessment

Biodegradation in water:
readily biodegradable

Additional information

Ready biodegradability

A ready biodegradability test result is not available for polysulfides because inorganic substances should according to the guidelines not be assessed in standard OECD TG 301 tests for ready biodegradability. All aspects important for achieving a ready biodegradability test result with polyslfides i.e. ultimate (complete) biodegradation, rate of biodegradation and number and occurrence of competent micro-organisms in “unacclimated” ecosystems and biological treatment plants have been investigated.

Ready biodegradability tests only detect growth-linked biodegradation. Microorganisms are capable of growth on polysulfides in the presence (aerobic) and absence (anaerobic conditions) of oxygen. In the presence of oxygen, phylogenetically highly diverse prokaryotes use energy derived from the oxidation of reduced sulfur including polysulfides for growth (Dahl and Prange, 2006). Some of these organisms use carbon dioxide (chemolithoautotrophs) others use organic compounds (chemolithoheterotrophs) as carbon source. Polysulfides are under aerobic conditions oxidized completely to sulfate. The microorganisms capable of oxidizing reduced sulfur compounds are found in wastewater treatment plants, freshwater systems, soils, and marine habitats (Germida and Sisiliano, 2002; Ito et al, 2004, Sorokin et al 2006; and many other).It is expected that the growth kinetics microorganisms utilizing elemental sulfur, sulfide and polysulfides as energy source are comparable. The maximum growth rates of sulfide-oxidizing microorganisms on sulfide and elemental sulfur are > = 0.8h-1(Jannasch et al, 1985;Ito et al, 2004).These growth rates are higher than maximum growth rates of nitrifying bacteria. Ammonium is oxidized readily in OECD TG 301 tests due to these nitrifying bacteria. Painter and King (1983) used a model based on the Monod equation to interpret the biodegradation curves in ready biodegradability tests. According to this model, growth rates of competent micro-organisms of 0.03 h-1or higher do result in a ready biodegradation of the test substance. The growth rates of reduced sulfur-utilizing bacteria therefore allow classification as readily biodegradable.

The complete oxidation, the widespread occurrence, and the high growth rates of organisms capable of growing on reduced sulfur led to the conclusion that polysulfides should be classified as readily biodegradable.

Polysulfides, naturally occurring compounds

 

Geochemically, many inorganic sulfur compounds are ubiquitous and several natural sources of production, emission and storage have been identified. The sulfur chemistry is complicated by the many oxidation states sulfur can assume. The most important oxidation states of are -2 (sulfide), -1 (polysulfides) , 0 (elemental sulfur), +2 (thiosulfate), +4 (sulfite) and +6 (sulfate). In the natural environment the sulfur is part of a cycle with alternating oxidized and reduced sulfur species, in both organic and inorganic forms. The sulfur cycle strongly interacts with biological activity resulting in utilization, transformation and storage of sulfur compounds (Figure).

Polysulfides (-S(S)nS-)are an integral part of the sulfur cycle (Figure).The chemical (Steudel, 2000), geochemical (Middelburg, 2000) and biochemical (Brueseret al., 2000) aspects of the sulfur cycle have been reviewed.

Polysulfide ions may be formed chemically in natural waters by interaction of aqueous sulfide with sulfur, the later being formed microbiologically or by partial oxidation with molecular oxygen (van der Gun et al, 2000; Bura-Nakic et al, 2009). The oxidation of hydrogen sulfide ions (by suitable oxidants in single electron steps) results in disulfide ions which may be oxidized further to higher polysulfides. The equilibrium may eventually be reached among the various polysulfides.The chain length at equilibrium varies from 2 to 9 but at moderately alkaline conditions, polysulfides with 4 to 6 sulfur atoms dominate.

Naturally occurring organic polysulfides are probably most abundant (Tolstikov et al 1997; Dahl et al, 2002). Polysulfides and organic polysulfides are stored in microorganisms intracellulary and extracellulary in globules (Dahl and Prange, 2006).

Sulfides and polysulfides are oxidized by a large and diverse group of prokaryotes, such as phototrophic sulfur bacteria, chemolithoheterotrophs and chemolithoautotrophs. Chemolithoheterotrophic and chemolithoautotrophs growth on polysulfides in the presence of oxygen has been discussed above. Oxidation of polysulfides in the absence of oxygen but in the presence of light has been found with a green sulfur bacteriumChlorobium limicolaand a purple sulfur bacteriumChromatium vinosum.Thiocapsa roseopersicina, being the dominant anoxygenic phototroph in microbial mats, was shown to be capable of growth on polysulfide (Visscher et al, 1990).The growth rates ofThiocapsa roseopersicinaon polysulfides and sulfide of 0.065, and 0.091h-1, respectively, are comparable.

Sulfate-reducing bacteria are important micro-organisms in the environment sulfur cycle. These bacteria have the ability to use sulfate as a terminal electron acceptor under anaerobic conditions. Some sulfate reducing bacteria can use other sulfur compounds including sulfite, sulfur and thiosulfate. Polysulfides are also readily reduced by sulfate-reducing bacteria (Schauber and Muller 1993; Blumenthals et al 1990; Klimmek et al 1991, Takahashi et al 2008). 

Polysulfides being part of the sulfur cycle explains the enormous potential for both the oxidation and reduction of polysulfides.