Fate of sludge-applied silicones in agricultural soil microcosms

Our previous publications showed that silicone (polydimethylsiloxane, or PDMS) polymers degrade to monomeric silanols and eventually to C02 in laboratory soil incubations. In this study, 200 cs¹⁴C-PDMS was added to soil microcosms (Tuscola sandy loam and Fargo silty clay) in anaerobically digested sludge. Soybeans followed by wheat were grown for 7 months during which microcosms were subjected to 3 leaching events. Recoveries of¹⁴C in the microcosms ranged from 47 to 90%. The recovered¹⁴C was almost completely in the soils, with trace amounts in leachate and ≤ 2% of the total in plant shoots. Extraction of soils coupled with HPLC-GPC showed that the majority of soil^l4C was still polymeric, but with lower molecular weight than the original PDMS. From 1 to 5% of the remaining¹⁴C was probably small silanols. Results thus confirm laboratory studies and show that PDMS degradation occurs under conditions similar to the field.