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United States Department of Agriculture

Agricultural Research Service

Title: Modeling Polydimethylsiloxane Degradation Based on Soil Water Content

Authors
item Singh, U - UNIVERSITY OF MINNESOTA
item Gupta, S - UNIVERSITY OF MINNESOTA
item Flerchinger, Gerald

Submitted to: Environmental Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 1, 1999
Publication Date: N/A

Interpretive Summary: 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 environment as a result of sludge application to soils. PDMS breaks down when in contract with dry soils and the byproducts eventually biodegrade or evaporate. The objective of this study was to develop a computer model that can predict the degree of PDMS breakdown based soil drying under different climatic conditions. The Simultaneous Heat and Water (SHAW) model was used to predict soil water content throughout the year, which were used to estimate PDMS degradation rate from observed data. Field testing of the model at Columbus, OH showed that the model was able to predict the general trends in PDMS degradation. Model estimates showed that >95% PDMS degradation occurred in various soil in San Juan, PR, Columbus, OH, and Athens, GA 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 and Athens than at Columbus primarily because of better soil drying conditions in Puerto Rico and Georgia than in Ohio. This research demonstrates the utility of the SHAW model to address contaminant degradation and the limitation in soil application of PDMS in some areas.

Technical Abstract: 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 terrestrial environment as a result of sludge amendment to soil. Laboratory studies have shown that PDMS extensively breaks down into monomeric units when in contract with dry soils. The byproducts of hydrolysis eventually biodegrade or evaporate. Th 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 Simultaneous Heat and Water (SHAW) model that predicts daily soil water content profiles throughout the year. Predicted soil water contents 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. The sensitivity analysis of the model showed that >95% of PDMS degradation at the soil surface in Bayaman 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 and Athens than at Columbus. 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.

Last Modified: 12/18/2014
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