Modeling of Dimethylsilanediol (DMSD) Fate and Transport in Soil.

Produced on a scale of over two billion tons per year, methylsiloxanes are ubiquitous in our daily lives. Dimethylsilanediol (DMSD), while not sold commercially, has been demonstrated to be a degradation product of methylsiloxane polymers and volatile methylsiloxanes (VMS) in the environment. Polydimethylsiloxanes (PDMS) and VMS enter the soil compartment when biosolids from wastewater treatment are applied for soil amendment. Understanding the fate and transport of DMSD in soil is important for understanding the ultimate fate of methylsiloxanes. Recently published experimental studies in bare soil and plant-soil systems have significantly advanced the understanding of the fate of DMSD. The results supported the upward transport of DMSD to top surface soil along with pore water during dry periods, its volatilization into the air phase at the soil surface, and its phytovolatilization via transpiration through plants. However, a more systematic understanding was needed to interpret the data using mechanistic models. Thus, the objectives of this study were to: i) develop mathematical models for DMSD movement in soil systems, ii) optimize the model parameters using recent experimental data, and iii) test the models with varying parameters under different environmental conditions to evaluate DMSD movement in soils. Our models predicted DMSD concentrations and movements that were in good agreement with the experimental results frompreviously reported column studies in bare soil and soil-plant systems. Sensitivity analyses reveal the key factors governing hydraulic conductivity, DMSD vs water transport and rate of DMSD transfer to air are soil texture, KOC and KOA, respectively. The impact of plants on DMSD removal to air is also discussed. In the future, we hope to use this model in combination with experimental data to better understand the fate of DMSD in the environment.