Degradation of Silicone Polymer at Different Soil Moistures

Silicone polymers [poly(dimethylsiloxane)s, PDMS] are used in down-the-drain consumer products and enter soil through disposal of municipal sludge. We examined PDMS hydrolysis over a range of soil moistures in soils from different geographical regions. [ 14 C]PDMS 350 cSt was added (50 mg kg - 1 ) to a Michigan Alfisol (Londo sandy clay loam) at six water potentials. Hydrolysis (5-week incubation) was 16, 2.0, 0.14, and 0.09% wk - 1 at −76, −18, −5.4, and −1.2 MPa, respectively. Hydrolysis at −0.26 and −0.05 MPa was too slow to distinguish from experimental variability. When dry Londo soil was amended with PDMS and then remoistened to −0.05 MPa, hydrolysis was 0.19% wk - 1, meaning that remoistening of soil slows the reaction. Similarly, PDMS hydrolysis in remoistened soils (−0.04 to −0.1 MPa) from Ohio (Alfisol), Georgia (Ultisol), and Puerto Rico (Oxisol) was 0.41, 1.25, and 1.26% wk - 1, respectively. Best-fit regression equations ( R 2 > 0.99) showed a linear dependence of rate constant on water potential from −0.04 to about −100 MPa. Results show that PDMS hydrolysis increased with decreased soil moisture and was faster in Ultisols and Oxisols than in Alfisols at the same soil water potential.