Abiotic and biotic degradation of five aromatic organosilicon compounds in aqueous media: Structure degradability relationships

Silicones have many applications and are produced in large quantities. Despite their potential toxicity, information on their environmental mineralisation is scarce. Therefore, we investigated a group of five organosilicon compounds (o-MeOC ₆H ₄SiMe ₃ (1), p-MeOC ₆H ₄SiMe ₃ (2), (p-MeOC ₆H ₄) ₂SiMe ₂ (3), o-Me ₂NC ₆H ₄SiMe ₃ (4) and p-Me ₂NC ₆H ₄SiMe ₃ (5)), recently developed to be ‘benign by design’ based on their readily degradable core structure. Five different degradability tests were performed, one assessing hydrolytic and two analysing biological and photolytic stability, respectively. All substances, except (p-MeOC ₆H ₄) ₂SiMe ₂ (3), hydrolysed within 24 h to 50% indicating that this is one of the major pathways of their primary elimination. In agreement with previous research, none of the substances was readily biodegradable. In contrast, 100% of p-Me ₂NC ₆H ₄SiMe ₃ (5) was primarily eliminated by photolytic and hydrolytic processes. The elimination rates of the other substances ranged from 7% to 64%. Irradiation at shorter wavelengths increased both the extent and speed of photodegradation. Eleven transformation products of p-Me ₂NC ₆H ₄SiMe ₃ (5) were detected, all of which were completely eliminated within 64 min of irradiation with a Hg lamp (200–400 nm). The insertion of an electron-donating group on the benzene ring like in p-Me ₂NC ₆H ₄SiMe ₃ (5) clearly enhanced photolytic degradability but further research is necessary to achieve truly biodegradable silicones.