Publication: Toxicity mitigation and bioaccessibility of the cationic surfactant cetyltrimethylammonium bromide in a sorbent-modified biodegradation study

The biodegradation potential of long chain cationic surfactants may be hampered by toxic or inhibitory effects on the inoculum at test concentrations required to accurately determine inorganic carbon formation. Cetyltrimethylammonium bromide (CTAB) was found to inhibit or delay degradation of the reference compound aniline above 1 µM (0.36 mg/L). In this study, we used silicon dioxide and illite clay as inorganic sorbents to mitigate the toxicity of CTAB to the inoculum, by lowering the bioavailable (freely dissolved) concentration. We tested whether a potentially inhibitory total dose of 46 µmol CTAB/L (16.8 mg/L) was readily biodegradable in the presence of two different doses of either silicon dioxide (SiO2) or illite in a standard 28 day OECD 310 Headspace Test. Initially SiO2 adsorbed CTAB fractions of 90%, rendering bioavailable CTAB concentrations of ~4.6 µM, were mineralized >60% to CO2 within 2 weeks, showing ready biodegradability of CTAB. In systems with a 90% sorbed fraction to illite, 60% mineralization was reached within 3 weeks. At higher adsorbed CTAB fractions of 99%, rendering bioavailable CTAB concentrations 0.46 µM and thus close to the non-inhibiting effect concentration found in the toxicity study, it took longer to reach >60% mineralization to CO2; 17 days for SiO2 and 4 weeks for illite. Thus, while the higher sorbed fractions resulted in the desired non-inhibitory bioavailable concentrations, desorption kinetics likely decreased bioaccessibility, and CTAB did not pass ready biodegradability requirements. Using an appropriate dose of mitigating adsorbent may thus render a 28 d Headspace Test suitable to test for ready biodegradability based on carbon analysis, thereby avoiding the necessity of applying expensive radiolabeled compounds. However, too much sorbent can reduce bioaccessibility and limit degradation kinetics, particularly for relatively toxic cationic surfactants that require strong mitigation.

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