Dimethyl ether-boron trifluoride

Toxicity to microorganisms

Description of key information

The inhibition of the degradation activity of activated sludge is not anticipated when introduced in appropriate low concentrations.
The substance may generate a shift in the pH.

Additional information

In water, dimethyl ether - boron trifluoride rapidly decomposes to form dimethyl ether and boron trifluoride dihydrate. The latter reacts further to boric acid and fluoroboric acid, which finally hydrolyses to yield hydrofluoric acid/ fluoride ions. The assessment of toxicity to microorganisms was therefore based on studies on the substance itself as well as on boric acid, one of the final products of hydrolysis.

The acute toxicity of dimethyl ether – boron trifluoride on microorganisms was investigated, using the bacterium Pseudomonas putida as test organism. The test was conducted according to the EC Directive 79/831/EEC (C) and GLP (BASF AG, 1991). However, the pH was not adjusted and decreased to below 5 at the higher test substance concentrations (pH 2.1 at 1000 mg/L). The observed effects were therefore probably due to the low pH. Toxicity towards Scenedesmus subspicatus was also tested for the similar substance diethyl ether, with no effects observed at 100 mg/L in the neutralised sample (BASF AG, 1992).

Effects of boric acid on micro-organisms are reviewed in the EU Risk Assessment Report for boric acid. Inhibition was estimated using the standardized OECD 209 method, Hanstvelt and Schoonmade (2001) reported 24% inhibition at 175 mg-B/L, and an EC10 can be calculated from their data as 58 mg-B/L. A NOEC was reported by the study authors of 17.5 mg-B/L, but was not determined via any statistical means. In actuality, there was 4% inhibition at this exposure, so it is an EC4, thus more stringent than intended by the OECD 209 protocol. An EC10 value was calculated by linear regression from the data which showed a good fit (R2= 0.863) with a value of 58.0 mg-B/L. This study complies with GLP practice, is rated as highly reliable in quality, and is consistent with earlier tests of activated sludge, however they were not published.

Gerike et al.(1976), using an earlier version of the OECD activated sludge method reported LOEC and NOEC of 120 and 20 mg-B/L, respectively. Guhl (1992, 2000) reported activated sludge LOEC and NOEC of 50 and 20 mg-B/L, respectively.

Other reports include tests of the microbe, Pseudomonas putida, with NOEC values ranging from 7.6 to 1040 mg-B/L However, these study reports are of varying reliability and it appears that the high value (1040) is incorrect. The geometric mean value of the Henkel studies is 50.8 mg-B/L (EC10 values of 340 and 7.6 mg-B/L).

Guhl (1992, 2000) reported on ciliate growth inhibition to Entosiphon sulcatum, Opercularia bimarginata, and Paramecium caudatum, with NOEC values of 10, 15, and 20 mg-B/L, respectively. Bringmann and Kuehn (1980b) reported a toxicity threshold (EC5) to the ciliate Uronema pardaczi of 23 mg-B/L.

The data on sewage treatment plants appears consistent, specifically that no effects are seen at boron exposures of 8 to 20 mg-B/L.