Evaluating the ready biodegradability of two poorly water-soluble substances: comparative approach of bioavailability improvement methods (BIMs)

Abstract

Difficulties encountered in estimating the biodegradation of poorly water-soluble substances are often linked to their limited bioavailability to microorganisms. Many original bioavailability improvement methods (BIMs) have been described, but no global approach was proposed for a standardized comparison of these. The latter would be a valuable tool as part of a wider strategy for evaluating poorly water-soluble substances. The purpose of this study was to define an evaluation strategy following the assessment of different BIMs adapted to poorly water-soluble substances with ready biodegradability tests. The study was performed with two poorly water-soluble chemicals-a solid, anthraquinone, and a liquid, isodecyl neopentanoate-and five BIMs were compared to the direct addition method (reference method), i.e., (i) ultrasonic dispersion, (ii) adsorption onto silica gel, (iii) dispersion using an emulsifier, (iv) dispersion with silicone oil, and (v) dispersion with emulsifier and silicone oil. A two-phase evaluation strategy of solid and liquid chemicals was developed involving the selection of the most relevant BIMs for enhancing the biodegradability of tested substances. A description is given of a BIM classification ratio (R BIM), which enables a comparison to be made between the different test chemical sample preparation methods used in the various tests. Thereby, using this comparison, the BIMs giving rise to the greatest biodegradability were ultrasonic dispersion and dispersion with silicone oil or with silicone oil and emulsifier for the tested solid chemical, adsorption onto silica gel, and ultrasonic dispersion for the liquid one.

Keywords: Bioavailability improvement method; Biodegradability; Evaluation strategy; Poorly water-soluble substances; Ready biodegradability test; Silica gel; Silicon oil.

Fig. 1

Example of ready biodegradation test curve with parameters T _B, Biod10d, and BiodF identified

Fig. 2

Anthraquinone biodegradation screening test results. Anthraquinone at 20 mg carbon L⁻¹ and inoculum at 30 mg dry matter L⁻¹. Curves represent the median value of five replicates per test condition. Error bars represent the median absolute deviation. Operating conditions with anthraquinone: ultrasonic dispersion for 10 min at 35 kHz, silica gel at 5 g L⁻¹, dispersed with 10 mL silicone oil L⁻¹, dispersed with 10 mg Pluronic 9400® L⁻¹, and dispersed with 10 mg Pluronic 9400® L⁻¹ and 10 mL silicone oil L⁻¹

Fig. 3

Isodecyl neopentanoate: first screening biodegradation test. Isodecyl neopentanoate at 20 mg carbon L⁻¹ and inoculum at 30 mg dry matter L⁻¹. Curves represent the median value of five replicates per test condition. Error bars represent the median absolute deviation. a Operating conditions with isodecyl neopentanoate: ultrasonic dispersion for 10 min at 35 kHz, adsorbed onto silica gel at 5 g L⁻¹, dispersed with 10 mL silicone oil L⁻¹, dispersed with 10 mg Pluronic 9400® L⁻¹, and dispersed with 10 mg Pluronic 9400® L⁻¹ and 10 mL silicone oil L⁻¹. b Operating conditions with isodecyl neopentanoate: adsorbed onto silica gel at 0.05 g L⁻¹, dispersed with 10 mL silicone oil L⁻¹, and dispersed with 10 mg Pluronic 9400® L⁻¹ and 10 mL silicone oil L⁻¹. CO₂ consumption was not recorded for days 14 to 19

Fig. 4

Standard biodegradation tests. Curves represent the median value of two replicates per test condition. Error bars represent the median absolute deviation. a Operating conditions with anthraquinone: direct addition, ultrasonic dispersion for 10 min at 35 kHz, dispersion with silicone oil at 10 mL L⁻¹, and dispersion with 10 mg Pluronic 9400® L⁻¹ and 10 mL silicone oil L⁻¹. b Operating conditions with isodecyl neopentanoate: direct addition, ultrasonic dispersion for 10 min at 35 kHz, and adsorption onto silica gel at 0.05 g L⁻¹

Fig. 5

Test methodology. R _{BIM S} screening BIM, R _{BIM C} confirmatory BIM