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Environmental fate & pathways

Biodegradation in water: screening tests

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The biodegradation potential of aromatic hydrocarbons, C10-13, reaction products with branched nonene, sulphonated, sodium salts has been assessed in an inherent test (SCAS test) in combination with Closed Bottle tests. The results reveal that methylnaphthalenesulphonate, and part of the isomers of dimethylnaphthalenesulphonate, degrade. The other constituents are not readily biodegradable.

Key value for chemical safety assessment

Biodegradation in water:
inherently biodegradable, not fulfilling specific criteria

Additional information

Aromatic hydrocarbons, C10-13, reaction products with branched nonene, sulphonated, sodium salts have been tested in four screening tests. The substance did not pass the criterium for readily biodegradable in the tests. As some shortcomings were identified in the older tests, a new test was performed recently (van Ginkel, 2012) and this study has been selected as the key study.

Aromatic hydrocarbons, C10-13, reaction products with branched nonene, sulphonated, sodium salts was characterized comprehensively . Inhibition by the test substance in ready biodegradability tests is best detected prior to the onset of the biodegradation of the test substance through suppression of the endogenous respiration (lower oxygen consumption in the presence of a test substance as compared to the control). No inhibition of the endogenous respiration by Aromatic hydrocarbons, C10-13, reaction products with branched nonene (low nonene substitution), sulphonated, sodium salts was detected in the Sturm test (Wildlife, 1995). The aromatic hydrocarbons, C10-13, reaction products with branched nonene (low nonene substitution) was degraded partially i.e. 49% in the Sturm test (Wildlife, 1995). Partial degradation of aromatic hydrocarbons, C10 -13, reaction products with branched nonene, sulphonated, sodium salts was also found in a semi continuously fed activated sludge (SCAS) test (key study). In this test a removal percentage of organic carbon of 57% was achieved. Using sludge adapted for 70 days in the SCAS unit did result in 30% biodegradation after 14 days in the Closed Bottle test. This percentage did not increase after this period. The biodegradation percentage of 30 achieved in the Closed Bottle test demonstrates that microorganisms capable of growing do not have the ability to completely biodegrade all compounds present in this test substance. The biodegradation percentage of 30 obtained in the Closed Bottle test would result in a 50% removal of the test substance by biodegradation at most in the SCAS test. It is therefore very likely that approximately 10% of the test substance administered in the SCAS test is removed by adsorption and/or volatilisation.

 

The effluent of the SCAS test unit was analysed with LC-MS/MS for four constituents. The biodegradation percentage of methylnaphthalenesulphonate is almost 100%. It is very likely that all isomers of methylnaphthalenesulphonate present in the test substance are completely biodegraded. Almost complete mineralization (75 to 90%) of naphthalene-2-sulphonate was demonstrated with pure cultures (Song et al, 2005). The introduction of a methyl group does probably not deteriorate the potential of microorganisms to degrade these substances. The introduction of a second methyl group results in a decrease of the biodegradation. Approximately 25% of the isomers are not degraded by microorganisms.Structural features that were found to decrease the biodegradation of alkylated naphthalenes include an increase in the number of methyl substituents and an increase in the size of the substituent. Dimethylnaphthalene isomers with adjacent methyl substituents are resistant to biodegradation (open literature). It is therefore likely that the 75% of the dimethylnaphthalenesulphonate isomers not detected in the effluent of the SCAS unit are completely biodegraded. The complete degradation of methylnaphthalene sulphonates and part of the dimethylnaphthalenesulphonate isomers could already account for approximately 25% degradation in the Closed Bottle test.

Introduction of a branched nonyl chain instead of a methyl group does not further reduce the biodegradability of naphthalene sulphonates. The nonyl chain itself will degrade slowly because it is branched. The presence of the nonyl chain may result in the attack of the sulphonate from the far-end of the alkyl chain resulting in partial oxidation. The introduction of a third substituent on naphthalene sulphonates does, as found with alkylated naphthalenes, decrease biodegradability of many isomers.

Based on the NPOC removal, Closed Bottle tests and the specific analysis it is concluded that biodegradation of aromatic hydrocarbons, C10-13, reaction products with branched nonene, sulphonated, sodium saltswill only result in complete oxidation of methylnaphthalenesulphonates and part of the dimethylnaphthalenesulphonates. Removal of the nonylmethylnaphthalenes measured with LC-MS/MS is explained by oxidation of the far-end of the alkyl chain and adsorption onto the sludge.

Nonylmethylnaphthalenesulphonates are expected to adsorb due to the hydrophobic alkyl chain. 

Effluent from the control unit spiked with the parent surfactant and the effluent of the SCAS unit containing the water-soluble biodegradation products of aromatic hydrocarbons, C10-13, reaction products with branched nonene, sulphonated, sodium salts, are both slightly toxic to algae.On the basis of the evaluation of the biodegradability tests it is proposed that primarily naphthalene sulphonates with nonyl chains will persist. The nonyl chain is responsible for the surfactancy of the test substance. Surfactancy and toxicity to aquatic organisms usually coincide. Decrease in toxicity due to the partial degradation of the test substanceis therefore not expected.                    

           

In conclusion only part of the homologues and isomers of aromatic hydrocarbons, C10-13, reaction products with branched nonene, sulphonated, sodium salts are readily degraded.  Partial degradation of the test substance does not result in a significant increase nor decrease of the toxicity to algae.