Degradation of silicone polymers in soil

Abstract

Silicone polymers (polydimethylsiloxanes, or PDMS) are used in numerous personal care and household products, eventually enter wastewater treatment plants, and are later applied to the land as a component of sludge. The fate of silicones in soil is largely unknown, but this study shows that in a moist (0.2 MPa = 12% moisture) Londo sandy clay loam, 200 centi‐stoke (cs)14C‐labeled PDMS degraded slowly over six months to yield about 3% of applied 14C as low‐molecular‐weight, water‐soluble products. When the soil was allowed to dry in one week from 12 to 3% moisture, the degradation rate was much more rapid, and after several days at 3 % moisture about half of the applied 14C was water desorbable. HPLC‐GPC of tetrahydrofuran (THF) soil extracts showed that PDMS had been degraded to low‐molecular‐weight molecules of the general formula HO‐[Si(CH3)2O]n‐H. The range of moistures in this experiment was measured in a field of Londo sandy clay loam during the summer of 1992, indicating that PDMS should be unstable in the soil environment. Further work on the identification and biological degradation of these small products is ongoing.

Temperature Dependence of the Organic Carbon/Water Partition Ratios (KOC) of Volatile Methylsiloxanes

“…This indicates that some of the low molecular weight breakdown products may be tightly bound to soil, consistent with the findings of Lehmann et al (1994Lehmann et al ( , 1995, Lehmann and Miller (1996), Xu (1998), andXu et al (1998). These show that as the soil dries binding of dimethylsilanediol to soil increases (i.e.…”

“…Lehmann et al (1994) conclude that the degradation of PDMS is probably not biological in origin, as it is more rapid at lower soil moisture contents, conditions that are less favourable to microbial populations.…”

“…Lehmann et al (1994) showed that 14 C-PDMS (200 cst viscosity, number average molecular weight 6642 g/mol) degraded slowly when incubated in a Londo sandy clay loam soil with a water content of 12 per cent. The radiolabel in the substance tested was randomly distributed on the methyl groups.…”

“…• The amount of cyclic siloxanes and other volatile product emitted during the degradation in soil is around 0.5 per cent of the PDMS added to the soil (based on Lehmann et al, 1994). …”

“…The actual identity of the volatile products formed in the Lehmann et al (1994) study was not determined. This is a significant source of uncertainty in the estimates.…”