No abstract

The adsorption of L-cysteine (L-Cys) onto a polycrystalline silver electrode surface was investigated by in situ spectroelectrochemical methods. Surface-enhanced Raman spectroscopy (SERS) and surface-enhanced second-harmonic generation (SESHG) measurements were performed in a 0.2 M KCl solution in the presence and absence of L-Cys. The experimental results indicated that L-Cys strongly adsorbs onto silver and remains on the surface at potentials as negative as -900 mV (vs Ag/AgCl). A peak around -650 mV was observed in the SESHG intensity versus applied potential plots obtained in the presence of L-Cys. The peak was indicative of an abrupt change in the electronic properties of the interface at that potential. The SERS spectra at potentials more negative than ca. -650 mV showed an increase in the intensity of vibrational modes assigned to the carboxylate group of L-Cys. The combination of the SERS and the SESHG results suggests a potentialinduced reorientation of the adsorbed L-Cys molecules for potentials more negative than -650 mV. The data interpretation considered the different possible conformational forms of L-Cys adsorbed on the Ag surface. At potentials more positive than ca. -650 mV, L-Cys molecules adsorb with the protonated amino group pointing toward the surface. In this case, the positively charged amino group is stabilized by the coadsorbed chloride anions. The molecule changes its conformation at potentials more negative than -650 mV as the chloride ions leave the surface. The C R -C bond rotation brings the carboxylate group closer to the surface at these potentials.

No abstract

No abstract

Accelerated degradation tests were performed on polydimethylsiloxane (PDMS) fluids in aqueous solutions and in extreme chemical conditions (pH 2-4 and 9-12). Results confirmed that silicones can be degraded by hydrolysis. Higher degradation levels were achieved in very acidic and alkaline conditions. Degradation products are probably polar siloxanols. In alkaline conditions, the counter-ion was found to have a strong influence on degradation level. Degradation kinetic studies (46 days) were also performed at different pH values. Supposing zeroth-order kinetics, degradation rate constants at 24 °C were estimated to 0.28 mgSi L(-1) day(-1) in NaOH solution (pH 12), 0.07 mgSi L(-1) day(-1) in HCl solution (pH 2) and 0.002 mgSi L(-1) day(-1) in demineralised water (pH 6). From these results, the following hypothesis was drawn: PDMS hydrolysis could occur in wastewater treatment plants and in landfill cells. It may be a first step in the formation of volatile organic silicon compounds (VOSiCs, including siloxanes) in biogas: coupled to biodegradation and (self-) condensation of degradation products, it could finally lead to VOSiCs.

No abstract

No abstract

Recently a lot of attention has been focused on volatile organic silicon compounds (VOSiC) present in biogases. They induce costly problems due to silicate formation during biogas combustion in valorisation engine. The cost of converting landfill gas and digester gas into electricity is adversely affected by this undesirable presence. VOSiC in biogases spark off formation of silicate deposits in combustion chambers. They engender abrasion of the inner surfaces leading to serious damage, which causes frequent service interruptions, thus reducing the economic benefit of biogases. It is already known that these VOSiC originate from polydimethylsiloxanes (PDMS) hydrolysis. PDMS (silicones) are used in a wide range of consumer and industrial applications. PDMS are released into the environment through landfills and wastewater treatment plants. There is a lack of knowledge concerning PDMS biodegradation during waste storage. Consequently, understanding PDMS behaviour in landfill cells and in sludge digester is particularly important. In this article, we focused on microbial degradation of PDMS through laboratory experiments. Preliminary test concerning anaerobic biodegradation of various PDMS have been investigated. Results demonstrate that the biotic step has an obvious influence on PDMS biodegradation.