Modeling polydimethylsiloxane degradation based on soil water content

Polydimethylsiloxane (PDMS) is a widely used silicone polymer that is introduced into wastewater treatment systems where it is removed with sludge. PDMS subsequently enters the terrestial environment as a result of sludge amendment to soil. Laboratory studies have shown that PDMS extensively breaks down into monomeric units when in contact with dry soils. The byproducts of hydrolysis eventually biodegrade or evaporate. The objective of this study was to develop a computer model that can predict the degree of PDMS breakdown based on level and duration of soil drying under different climatic conditions. The framework of the model was the SHAW (Simultaneous Heat and Water) model that predicts daily water content distribution in soil over the course of a year. The soil water contents predicted from the SHAW model were then linked to PDMS degradation rate data for various soils to predict soil and climate impacts on PDMS losses. Field testing of the model at Columbus, OH showed that the model was able to predict the general trends in PDMS degradation over 2 years. Predicted PDMS concentrations remaining in the 0-10 cm depth 2 years after sludge addition were 19.8 mg/kg of soil compared to the measured values of 23.0 mg/kg of soil. The sensitivity analysis of the model showed that >95% of PDMS degraded at the soil surface in Bayamon sandy clay loam (San Juan, PR), Miamian loam (Columbus, OH), and Wedowee sandy clay loam (Athens, GA) soils within 365 days after application. However in some years, >50% of applied PDMS was still remaining at 2.5-cm depth 365 days after its application. At any given day, there was less PDMS remaining in soil at San Juan, PR, and at Athens, GA, than at Columbus, OH. This is because of (1) higher rates of PDMS degradation in Bayamon and Wedowee soils than in Miamian soil and (2) better soil drying conditions in Puerto Rico and Georgia than in Ohio.