Movement of dimethylsilanediol formed in-situ from octamethylcyclotetrasiloxane in soil – A combined experimental and modeling study

Polymeric and oligomeric methylsiloxanes serve a wide variety of functions in modern society. Siloxanes are known to degrade in the environment through various mechanisms, but recent years have seen increased interest in the properties, transport, fate, and impact on ecosystems of various degradation products. The purpose of this study is to better understand the fate of dimethylsilanediol (DMSD), the main monomeric hydrolysis product of methylsiloxanes in the environment. While the fate of DMSD itself was studied using pure DMSD in numerous laboratory and field studies, this study aims to study the fate of DMSD that is generated in-situ from the hydrolysis of radiolabeled octamethylcyclotetrasiloxane (¹⁴C-D₄) by a combination of laboratory measurements and mechanistic modeling. Under typical temperate (e.g. Michigan) weather conditions, ¹⁴C-D₄ spiked into 20-cm thick wet soil columns was effectively removed (82% removal) within weeks by volatilization and hydrolysis, with minimal recovery of ¹⁴C-labeled DMSD in soil runoffs or leachate (≤0.3 wt%). A new numerical model was trained with experimental data to predict the movement of both D₄ and its degradation products under different weather and soil conditions. The model allows the extrapolation of experimental data to understand environmental conditions under which D₄ and its degradation products are transported. Even after a long drought period of 133 days that would maximize DMSD concentration in the soil, a heavy rain period (15 h) does not lead to bottom drainage of DMSD (<400 ppb of the annual soil siloxane amendment flows out the bottom from cycle 2 onwards). In addition, the model shows that even in the case of yearly soil amendment, the 10% of the siloxane accumulated from the previous year does not lead to a net accumulation of siloxane, as even after 5 years, the spike layer concentration remains constant at the end of the season.