Life Cycle of Silicone Polymer, from Pilot-Scale Composting to Soil Amendment

Abstract

Silicone polymers (polydimethylsiloxanes, or PDMS) are used in numerous downthe-drain products which can eventually enter wastewater treatment plants, where they become incorporated into the sludge. Since sludge is sometimes composted before being applied to the land, this work investigates the fate of PDMS from its incorporation into sludge, through pilot-scale com posting processes, to its degradation in compost-amended soil.Either yard or food waste streams were com posted in 60 liter vessels with sludge which contained either no PDMS, or had been amended with 350 cs PDMS at 2000 mg/ kg wet weight sludge. The four treatments (yard waste and food waste, no PDMS; yard waste and food waste with PDMS) were composted for six weeks and showed expected profiles of temperature, 0 2 , and C0 2 changes as the composts matured. Mature composts (pH of 8.4, ash content of 46%, degree of humification of 90%, and C:N ratio of 12) were extracted and showed no degradation of the PDMS during compost generation. Cress seeds (Lepidium sativum) were germinated and grown for two days in the presence of water extracts of the finished composts; results indicated no trends in phytotoxicity between the composts with and without PDMS.PDMS degraded when samples were submitted to weekly or twice-weekly wetting/drying cycles in the laboratory. Degradation in compost or compost/soil samples (15 to 20% in one month) compared to >80% for neat PDMS spiked into soil indicates that PDMS may need to move off the compost and onto the soil minerals for more rapid degradation to occur. PDMS degradation was confirmed in compost/ soil mixes under field conditions: -60% degraded during the 1998 growing season. In general, results show that PDMS will proceed inertly through the active composting process, and will then degrade both during outside storage of compost, and after compost is mixed with soil.

Fate of Polydimethylsilicone in Biosolids‐Amended Field Plots

“…The greater rate of PDMS degradation at the soil surface is the likely reason for lower concentrations of PDMS in surface soil layers observed during the incremental depth sampling conducted in 1996 and 1997. Similar observations of rapid loss of PDMS from the soil surface have been reported in compost‐amended soil (Lehmann et al 1999), and in a study where PDMS was applied to a soil surface as a spill (Lehmann et al 2000).…”

Solar Photochemical Degradation of Aminosilicones Contained in Liquid Effluents. Process Studies and Neural Network Modeling

“…Only a slow, acid-catalyzed hydrolysis of some silicone polymers has been observed in dry soils in combination with clay minerals, , resulting in a series of water-soluble silanol compounds. In this case, the main breakdown product, monomeric dimethylsilanediol [(CH 3 ) 2 Si(OH) 2 ], can be removed from soil by biodegradation to CO 2 and inorganic compounds 5 and by volatilization to the atmosphere where it is photolytically degraded in the presence of sunlight . In contrast, silicone biodegradation is not observed in wastewater treatment facilities because the Si−O bond is resistant to enzymatic attack. , Biodegradation of Si−C bonds also seems to be difficult .…”

“…2,3 Silicones and silicone derivatives are found to be exceptionally resistant to hydrolytic and oxidative breakdown under conventional wastewater treatment. 4 Only a slow, acid-catalyzed hydrolysis of some silicone polymers has been observed in dry soils in combination with clay minerals, 4,5 resulting in a series of water-soluble silanol compounds. In this case, the main breakdown product, monomeric dimethylsilanediol [(CH 3 ) 2 Si(OH) 2 ], can be removed from soil by biodegradation to CO 2 and inorganic compounds 5 and by volatilization to the atmosphere where it is photolytically degraded in the presence of sunlight.…”

Sensitivity Analysis and Parameter Optimization of a Heat Loss Model for a Composting System

“…L and D In this study, the length of the reactor was varied between 0.40 m and 2.50 m and the diameter was varied from 0.20 m to 1.20 m. The proposed ranges for the length and diameter included values that have been chosen when designing and experimenting on similar compost bioreactors (Liao et al, 1994;Nakasaki et al, 1997;Darrell et al, 1998;Day et al, 1998;Freeman and Cawthon, 1999;Lehmann et al, 1999;Vining, 2002;Ekinci et al, 2004a,b;Hess et al, 2004;Hong and Park, 2004;Petiot and de Guardia, 2004;Mason and Milke, 2005;Yu et al, 2005).…”