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Environmental fate & pathways

Biodegradation in water: screening tests

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Description of key information

Based on all available information from aromatic sulphonic acids and the hydrotropes, the substance is considered to be readily biodegradable.

Key value for chemical safety assessment

Biodegradation in water:
readily biodegradable

Additional information

Although limited well documented biodegradation studies are available for the aromatic sulphonic acids, and with no clear conclusion on ready biodegradability, the conclusion of ready biodegradability is strengthened by the number of supporting literature tests and the fact that seven biodegradation studies are performed with the closely related hydrotropes (the salts) for which it was concluded that these are readily biodegradable. As the cation has a limited affect on the biodegradation potential, and in principle the salt gets dissociated when in contact with water thus forming the acid, it is considered justified to conclude that these substances are readily biodegradable, taking into account all the available information. That information includes the 14 studies with the acid forms and the seven studies with the salt forms, which are all summarized below.

Aromatic Sulphonic Acids-

Amongst all the biodegradability studies for the aromatic sulphonic acids, the most reliable (Klimish 2) study is a 1995 OECD 301D (Ready Biodegradation - Closed Bottle Test) with CAS 98-11-3 (Huntsman, 1995a). The source of the inoculum was a secondary treatment sewage plant. Sodium benzoate, the reference substance, attained 98% degradation after 28 days. Based on DO measurements, the test substance attained 54% degradation after 28 days which cannot be considered as ready biodegradable under the guideline's 60% criteria. The toxicity control confirmed at 6 mg/L the test substance was not toxic to the microorganisms.

A test of PTSA's biodegradability (100 mg/L) with non-adapted activated sludge (30 mg/L) as inoculum, at 20C +/- 1 and pH 7, biological oxygen demand was measured by respirometer over a period of 14 days (BOD14). Following a lag period of 50-60 hours and a degredation time of 125 to 135 hours, at a rate constant of 0.041 - 0.045 h-1, PTSA was found to be 79-80% biodegradable (Urano and Kato, 1986).

In the OECD test for inherent biodegradability, according to test guideline 302B (Zahn-Wellens Test) at a concentration of 720 mg/L PTSA monohydrate was eliminated to 95% after 15 days (Wellens, 1979), while PTSA from the 65% aqueous solution at a concentration of 307.5 mg/L was eliminated to almost 100% after 5 days (Wellens, 1983).

In another study (Zahn-Wellens Test) PTSA monohydrate was degraded to 90% after 10 days and 94% after 20 days while the 65% solution was degraded to 75% after 25 days. When the test with the 65% aqueous solution was repeated with an adapted activated sludge, over 90% degradation was achieved after 10 days (Wellens, 1981).

In a recently published paper (1990) Wellens reports on the course of biodegradation of PTSA in the Zahn-Wellens test according to OECD Fuideline 302B and DIN-Norm 38412 (L12). The results showed 97% degradation after a maximum of 12 days incubation, whereby following an adaptation phase of 1-3 days, 85% degradation took place within 5-9 days.

Two studies (Pitter, 1976) demonstrate ultimate biodegradability for aromatic sulphonic acids and were conducted with methods similar to OECD guidelines for aerobic activated sludge tests with 28 day durations, reference substances and controls. Neither study is fully documented and hence received a Klimish 4. The author reported benzenesulfonic acid and p-toluene sulfonic acid are biodegradable and achieving 98% and 94% in 28 days, respectively.

Another 2 biodegradation studies are considered "weight of evidence". They have relatively little documentation. A 1980 review of MITI tested chemicals reports CAS 98-11-3 as biodegradable under aerobic conditions. A 1988 study of industrial, adapted activated sludge reports 90% TOC removal for CAS 104-15-4.

While not a microbial toxicity study per se, a 1966 study conducted with 500 mg/L hydroxybenzene sulfonic acid, reported inhibition of the microorganisms in the aerobic activated sludge test. However, this study is labeled as "disregarded study" as the 500 mg/L concentration was inhibitory to the microorganisms and there was no biodegradation. OECD guidelines recommend 30 mg/L maximum concentration.

In addition, two anaerobic biodegradability tests were performed with aromatic sulphonic acids. A 1989 anaerobic test of aquifer microorganisms showed no biodegradation of CAS 98-11-3 in 13 months. A 1999 anaerobic test using a "lab-made sludge", showed no degradation of 100 mg/L of CAS 98 -11 -3, at 37C for 8 weeks.

Hydrotropes

 There are four Klimisch 1 studies on the biodegradability of the hydrotropes and all 4 indicate ready biodegradability. The studies are conducted with sodium toluene sulphonate, sodium cumene sulphonate, sodium xylene sulphonate and calcium xylene sulphonate. There are an additional three Klimisch 2 studies. The study with sodium xylene sulphonate indicates ready biodegradability. The studies with sodium cumene sulphonate and sodium xylene sulphonate indicate inherent biodegradability.Based on all available information from these seven studies, the hydrotropes, and by read-across the aromatic sulphonic acids, are considered to be readily biodegradable.

 

The key hydrotrope studies are:

A 1992 OECD Guideline study - 301B Modified Sturm Test - based on CO2 evolution (Ruetgers Nease, 1992). The study was conducted according to GLP requirements and is fully documented. The test substance (sodium xylene sulphonate - CAS No 1300-72-7) achieved the 60% biodegradation threshold by Day 6 and reached 83% of theoretical CO2 in 28 days.

A 1993 OECD Guideline study - 301B Modified Sturm Test - based on CO2 evolution with sodium cumene sulphonate (CAS No 28348-53-0) (Ruetgers-Nease, 1993). The study was conducted according to GLP requirements and is fully documented. The test substance achieved the 60% biodegradation threshold by Day 6 and reached 103-109% of theoretical CO2 in 28 days.

A 1994 OECD Guideline Study - 301B Modified Sturm Test - based on CO2 evolution study with calcium xylene sulphonate (CAS No 28088-63-3) (Ruetgers-Nease, 1994). The study was conducted according to GLP requirements and is fully documented. The test substance reached 82-87% CO2 evolution in 28 days but took 16 days to reach the 60% "ready biodegradability" threshold.

A 2004 OECD Guideline Study - 301B Modified Sturm Test - based on CO2 evolution study with sodium toluene sulphonate CAS No 12068-03-0) (Brunswik-Titze, 2004).. The study was conducted according to GLP requirements and is fully documented. The test substance reached 100 -115% CO2 evolution in 28 days and reached the 60% "ready biodegradability" threshold in 4 days.