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Environmental fate & pathways

Biodegradation in water: screening tests

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Description of key information

Reading across results from sodium gluconate (CAS No. 527-07-1), the registered substance is considered to be readily biodegradable on the basis of the results of a well reported, GLP-compliant, guideline ready biodegradability study, without deviations, which is considered to be key, and which is supported by a anaerobic degradation study of equally high quality study that showed complete anaerobic degradation in 35 days, as well as an inherent biodegradability study that showed no lag phase and essentially complete degradation after only 2 days.

Key value for chemical safety assessment

Biodegradation in water:
readily biodegradable

Additional information

The key study is a Klimisch-1, GLP-compliant, guideline study without deviation, sponsored by Jungbunzlauer S.A. and carried out on the read-across substance sodium gluconate (CAS No. 527-07-1) in 2001 by Hydrotox GmbH (Stiene G) according to the Closed Bottle test (OECD 301 D, EC C.4-E). Nominal initial test substance concentration was 3 mg/L. The test was performed using 0.4 mL/L of activated sludge from the Breisgauer Bucht municipal treatment plant (500000 population equivalent), which treats approximately 3:1 municipal:industrial waste.  The test substance achieved a degradation of 61% by day 3, and a maximum of 89% by the end of the study. The validity criteria were met, and the day-3 result clearly shows that the 10-day window criterion was also met. The test substance is considered to be readily biodegradable under the conditions of the test, and this result is taken to indicate that the registered substance would be similarly readily biodegradable.

The first supporting study is another Klimisch-1, GLP-compliant, guideline study without deviation, sponsored by Jungbunzlauer S.A., also carried out on the read-across substance sodium gluconate (CAS No. 527-07-1) in 2001 by Hydrotox GmbH (Stiene G), but this time according to OECD Guideline 311 (Anaerobic Biodegradability of Organic Compounds in Digested Sludge: Measurement of Gas Production). Nominal initial test substance concentration was 100 mg C/L (≈303 mg sodium gluconate/L). The test was performed using 2.9 g total solids/L of activated sludge from the Breisgauer Bucht municipal treatment plant (500000 population equivalent), which treats approximately 3:1 municipal:industrial waste. The validity criteria were met, and the test substance achieved total (100%) anaerobic degradation in 35 days. This result is taken to indicate that the registered substance would be similarly completely biodegradable under anaerobic conditions.

Inherent biodegradability, again of the sodium gluconate read-across substance, was assessed using a test equivalent or similar to the Zahn-Wellens test (OECD 302B) (Magureanu G, 1981). The purpose of the study was to examine the degradation of Gluconate complexes with heavy metals. The publication states that the study followed standard international guidelines, but they were not specifically referenced, and limited methodological details are provided. The initial test substance concentration was 1000 mg DOC/L. The test was performed with 1 g/L dry weight of a non-adapted activated sludge, and the DOC removal was monitored over a 19-day period. The validity criteria were met, and the test substance showed degradation (DOC removal) of 16.9% after only 3 hours and 74% after 1 day, 97.8% after 2 days and 98.3% after 3 days. The reference substance, diethylene glycol exhibited a lag-phase of 3 days before achieving >90% DOC removal after 11 days. The test substance was considered to be inherently biodegradable under the conditions of the test, which is in keeping with the findings of the more stringent ready biodegradability study, and is again taken to indicate that the registered substance would have performed similarly in the test.

D-glucono-δ-lactone is a cyclic ester of gluconic acid which, in aqueous solution, forms an equilibrium mixture of the lactone and gluconic acid. Gluconic acid is a somewhat weak carboxylic acid with a dissociation constant of pKa = 3.6. The dissociation of an acid into a proton and an anion is an equilibrium, the reverse of which is the re-association of that same anion with a proton to reform the original acid. The pKa of 3.6 means that, when the ambient pH = 3.6, half the gluconic acid molecules will exist in the form of the uncharged acid, and half as the anion. At pH < 3.6, the undissociated form will predominate, and pH > 3.6 the anion will predominate. Sodium gluconate and potassium gluconate are both 1:1 salts of gluconic acid, which will each dissolve in water to generate separate sodium or potassium cations and gluconate anions. Sodium and potassium are both strong bases, and are therefore expected to remain ionized at essentially any pH, but the gluconate anions deriving from the salts will be subject to the same equilibrium as those deriving from the free acid. To be in equilibrium, both the forward and the backward reaction must possess the same pKa value, so the gluconate anion is predicted to posses the same pKa of 3.6 as the free acid. In this way, gluconic acid in aqueous solution is in equilibrium with its cyclic esters and its anion, according to the pH of the system, and in any system with sufficient buffering capacity, the effects of introducing equimolar amounts of gluconic acid, D-glucono-δ-lactone, sodium gluconate or potassium gluconate would be indistinguishable. Hence these four substances are considered to be appropriate surrogates for each other in sufficiently buffered aqueous systems, such as environmental waters, flora and fauna.