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Biodegradability testing of poorly soluble organic compounds

The aim of ready biodegradability testing is to obtain a basic statement about whether a substance is rapidly and finally biodegraded in the environment. Due to the strict test conditions, negative results are not considered as evidence of persistence. Instead, there exist the option to investigate inhibitory or poorly water-soluble substances or materials under modified test conditions. Compounds with a water solubility of <100 mg/L are usually referred to as poorly water-soluble. In this case, the surface / volume ratio can be so low that the hydrolysis or degradation rates are significantly reduced Further on, substances with a low specific weight can also float up and thereby decouple from the inoculum.

The updated standard EN ISO 10634 (August 2020) describes various methods for the "preparation and treatment of organic compounds that are sparingly soluble in water for the subsequent determination of their biodegradability in an aqueous medium":

  1. Direct addition and addition with inert support: The test substance is weighed directly into the test vessels or weighed on an inert carrier (e.g. polyethylene film, stainless steel foil, silica gel) and placed in the test vessel. Solid substances can be ground before weighing.
  2. Ultrasonic treatment: The test vessels with the required concentration of the test substance are dispersed in the ultrasonic bath at approx. 20 kHz to 35 kHz for approx. 5 min to 30 min.
  3. Adsorption on an inert support with a volatile solvent: The test substance is dissolved in a volatile solvent (e.g. trichloromethane) and the required amount is introduced into the test vessels via an inert carrier such as silica gel. The test is started after the solvent has completely evaporated.
  4. Addition via a non-degradable solvent or emulsifier: The test substance is added to the test vessels via a suitable non-bacterial solubilizer such as block copolymers of ethylene oxide and propylene oxide or silicone oil (polydimethylsiloxane and polyphenylmethylsiloxal) without removing the support.

Similar methods are used e.g. also for testing poorly water-soluble lubricants according to DIN EN 17181 (May 2019) "Determination of the aerobic biological degradation of formulated lubricants in aqueous solution - test method using CO2 production" (solution in hexane via transfer to glass fibre filter).

All described addition methods have their advantages and disadvantages, which depend on the properties of the test substances. Since the addition of auxiliaries can influence the test result, control vessels with the auxiliaries must always be run in parallel. Hydrotox GmbH has many years of experience with the use of the various forms of addition of substances that are difficult to dissolve in water.