Cecil L. Frye

Fate and effects of polydimethylsiloxanes on pilot and bench-top activated sludge reactors and anaerobic/aerobic digesters

Abstract The effect of polydimethylsiloxanes (PDMS) on bench-scale activated sludge reactors and pilot-scale anaerobic and aerobic digesters was investigated, and a mass balance was performed using radiolabeled PDMS. Polydimethylsiloxane loadings of up to 10,000 mg kg −1 dry weight of MLVSS had no effect on the maximum specific substrate utilization rate, half-saturation constant, endogenous decay coefficient, and cell yield in model activated sludge units. In addition, the operating parameters pH, suspended solids, sludge volume index, and specific oxygen uptake rate showed no difference between PDMS-loaded and control systems. A mass balance, in which 14 C-labeled PDMS was added to bench-scale activated sludge systems, showed that essentially all of the PDMS partitioned onto the microbial biomass. Pilot-scale aerobic and anaerobic digesters loaded with 100 mg kg −1 PDMS showed no difference in pH and solids concentrations compared to control systems. In addition, PDMS-loaded aerobic digesters exhibited oxygen uptake rates equal to control systems; PDMS-loaded anaerobic digesters produced the same amount of gas as control digesters. These results show that PDMS behaves as an inert material with no significant effect on wastewater treatment processes (other than the expected benefit of foam control).

Tertiary alcoholysis of chlorosilanes via tetracoordinate silylated quaternary ammonium intermediates

Abstract The rate-enhancing effects of tertiary amine-HCl acceptors on the alcoholysis of Ph 2 SiCl 2 and Ph 2 Si(OR)Cl with ROH (R = ViMe 2 C) have been studied. Relative rates greater than 10 6 for Ph 2 SiCl 2 and 10 4 for Ph 2 Si(OR)Cl have been observed. These profoundly enhanced rates bear no relationship whatsoever to the amine basicity differences, but correlate with the nucleophilic propensities of the amines; however, this is true only if the additional assumption is made that the slow and rate-determining steps involve ionic quaternary ammonium chloride intermediates in which the nitrogen bears a tetracoordinate silicon substituent. The proposed tetracoordinate-silylated quaternary ammonium intermediates derive further credibility from a number of stable species reported previously and reviewed herein.

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 soill4C 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.

Ecological effects of PDMS-augmented sludge amended to agricultural microcosms

The potential ecological effects of polydimethylsiloxane (PDMS)-augmented sewage sludge resulting from use of the sludge as an agricultural amendment were evaluated in a preliminary pot test and a definitive test in intact, agricultural soil-core microcosms. Seed germination and early seedling survival were evaluated in the pot study. The parameters investigated in microcosms included soil microorganism populations, crop productivity, and nutrient loss in soil leachate. Intact, soil-core microcosms from two soil types (silty clay or sandy loam) were amended with PDMS-augmented sludge by single or multiple (4) applications. Micrososms were sequentially cropped with spring wheat (Triticum aestivum) followed by soybeans (Glycine max). Ecological effects comparisons were made between microcosms containing one of three PDMS treatment levels in the sludge ( ∼ 290, 1000, or 3500 p.p.m.) and control microcosms containing sludge without PDMS. The three treatments of PDMS in sludge tested had no effect on seed germination and seedling survival when incorporated into soil (< 13 p.p.m. PDMS) in pots. Furthermore, PDMS in sludge amended to microcosm topsoil (< 10 p.p.m. PDMS) appears to have no effect on: (1) cumulative loss of nitrate-nitrogen in leachate; (2) oven-dry biomass of spring wheat or soybeans; (3) Rhizobium bacteria, as indicated by the number of nodules on soybean roots in sandy loam soil; and (4) numbers of soil microorganisms, including bacteria, actinomycetes, and fungi. However, the quantity and timing of sludge amendment may affect (1) loss of nitrate-nitrogen in leachate and (2) soybean yields.

The environmental fate and ecological impact of organosilicon materials: A review

In order to assess reliably and safely the potential threats posed by the environmental presence of any class of materials, one must carefully consider a number of key parameters. In addition to chemical reactivity, one must consider a materials physical properties such as vapor pressure, density, water solubility, lipid solubility, and often most importantly, the molecular weight. These factors define how and where the substance is likely to be distributed in the environment and what, if any, undesirable ecological consequences are to be anticipated. It is particularly important to realize that a material does not represent an ecological threat simply because it is toxic or persistent, or lipid soluble, or bioconcentratable. Hazard is proportional to the exposure intensity which is defined by the parameters of time and concentration. Therefore, only those materials which are toxic and persistent and bioconcentratable are likely to manifest undesirable ecological consequences. Based on extensive toxicological and environmental fate studies, commercially important organosilicon materials do not appear to present any ecologically significant threat. Recurrent suggestions that methylsiloxanes might produce methylmercury species under aquatic conditions are also discussed and shown to be highly improbable.

Cyclo-hydrosilylation: A novel route to siloxetanes and silanones

Abstract A novel and convenient method for the apparent liquid phase generation of dimethylsilanone, Me 2 SiO (D 1 ), based on readily obtainable reactants and employing conventional temperatures (50–150°) is reported herein. Platinum catalyzed hydrosilylation of vinyldimethylcaarbinoxydimethylsilane (I) appears to proceed by an exclusively intramolecular path to produce not only the expected 5-membered heterocyclic, 1,1,3,3-tetramethyl-2-oxa-1 silacyclopentane (V), but also the isomeric and highly unstable 4-membered siloxetane, 1,1,3,3,4-pentamethyl-2-oxa-1-silacyclobutane (IV). The intermediacy of IV is suggested by the products: i.e., 2-methyl-2-butene which is believed to arise along with Me 2 SiO from fragmentation of IV; D 3 and D 4 from D 1 self-coupling; a 6-membered cyclic derived from insertion of D 1 into the Si-O bond of IV, i.e., 1,1,3,3,5,5,6-heptamethyl-2,4-dioxa-1,3-disilacyclohexane (VI); a polymer which upon alkaline cracking produces more 6-ring (VI) but little or no 5-ring (V) suggesting that the polymer arose from copolymerization of D 1 , D 2 , and siloxetane (IV). Compound I is also an excellent thermolytic source of D 1 as evidenced by the formation of the expected derivatives upon heating in the presence of known silanone traps.

Cyclohydrosilylation dimer formation: evidence for Pt0 PtII PtIV catalysis☆

An eight-membered cyclic compound, 2,2,3,4,4,6,6,7,8,8-decamethyl-1,5-dioxa-2,4-disilacyclooctane (VIII), along with the expected five-membered 2,2,5,5-tetramethyl-1-oxa-2-silacyclopentane (V) were found to be the major products when vinyldimethylcarbinoxydimethylsilane (I) (i.e. 2-methyl-3-buten-2-oxydimethylsilane) was subjected to cyclohydrosilylation using platinum catalysis. The eight-membered product (VIII) readily undergoes β-elimination during gas chromatography (GC) analysis to extrude 2-methyl-2-butene giving the six-membered 2,2,4,4,5,6,6-heptamethyl-1,3-dioxa-2,4-disilacyclohexane (VI). Contrary to an earlier report (T.H. Lane and C.L. Frye, J. Organomet. Chem., 172 (1979) 213), no siloxetane or silanone intermediates are needed to rationalize the formation of the six-membered cyclic compound. When (Wilkinsons) rhodium catalyst was used for the hydrosilylation, only V was formed. The formation of VIII with platinum catalysis is rationalized by a mechanism utilizing successive oxidative additions to both Pt0 and PtII catalytic species; i.e. the absence of a second catalytically active level of rhodium is believed responsible for its inability to yield the eight-membered cyclic. Similarly, dimerization of either 1,1-dimethyl-1-silacyclobutane or 1,1,3,3-tetramethyl-1,3-disilacyclobutane to form the cyclooctanes can be readily achieved by platinum catalysis but not by rhodium catalysis. These results constitute the first examples of processes believed to require the presence of not only one but two catalytically active oxidation levels.

Selective interconversion of stereoisomeric cyclosiloxanes

Abstract Reaction conditions have been determined which permit the highly selective interconversion of unsymmetrically substituted cyclosiloxanes. The cis - and trans -2,6-diphenylhexamethylcyclotetrasiloxanes can be cleanly interconverted while generating little or none of the isomeric 2,4-diphenyl cyclic compounds. A strained ring system, cis - or trans -(PhMeSiO) 3 , can be interconverted to an equilibrium mixture of cis - and trans -(PhMeSiO) 3 without generating linear polymers or lower energy cyclic systems. These remarkably selective transformations are achieved with an electrophilic metal halide catalyst, such as ZnCl 2 or FeCl 3 , in conjuction with a polar solvent such as a nitroalkane. A mechanism is presented which involved metal halide cleavage of siloxane bonds to yield short-lived linear intermediates which undergo racemization of the asymmetric chlorophenylmethylsiloxy termini before regeneration of the cyclosiloxane by intramolecular condensation.