Solvant | EPFL Graph Search

vignette|Schéma moléculaire de la dissolution du chlorure de sodium dans l'eau : le sel est le soluté, l'eau le solvant. Un solvant est une substance, liquide ou supercritique à sa température d'utilisation, qui a la propriété de dissoudre, de diluer ou d'extraire d’autres substances sans les modifier chimiquement et sans lui-même se modifier. Les solvants sont utilisés dans des secteurs très diversifiés tels que le dégraissage, les peintures, les encres, la détergence, la synthèse organique, et représentent des quantités considérables en termes de tonnage et de chiffre d'affaires. En Suisse romande et en Belgique, on utilise le terme anglais thinner pour les solvants organiques destinés à la dilution des peintures. L'eau est le solvant le plus courant, la solution étant alors qualifiée de solution aqueuse. Généralités Le terme « solvant organique » se réfère aux solvants qui sont des composés organiques qui contiennent des atomes de carbone. Habituellement, les solvants

Influence of pH on dehalorespiring consortia & first steps in the development of a methodology for acid neutralization in enhanced in situ bioremediation

Nicole Munz

Enhanced in situ bioremediation of chlorinated solvents is, especially in source zone remediation, often limited through the accompanied high acidification of groundwater. This acid built-up originating from the dechlorination itself and fermentation products, such as organic acids, presents a draw-back in the efficiency of dehalorespiring bacteria. It is therefore indispensable to develop and implement appropriate buffering strategies to ensure a constant transformation of toxic chloroethenes to the final non-toxic product ethene. In this study pH tolerance of different dehalorespiring bacteria was tested. The limiting acidic pH varies significantly among the consortia. The consortium SL23b4 (PCE to ethene) grows at all the examined pH (pH 5 to pH 7.5) with a pH optimum between 6 and 7 (depending on the dechlorination step), whereas Maroc 6b (PCE to ethene) was only found to grow at incubation pH of 7, and very slowly at pH 6.5. The consortium 2SO (cis-DCE to ethene) seems to be inhibited at acidic pH (no growth at pH 5 and 5.5) having a pH optimum above neutral pH. In most cases incomplete dechlorination was observed in cultures growing at acidic pH or below the pH optima, accumulating toxic intermediate dechlorination products as a consequence. Additionally, high rates of dechlorination were always accompanied by pH drops up to 1.2 pH units, giving evidence of high acid production. pH had also influence on the community composition. In the consortium SL23b4 a shift from a Dehalococcoides- (pH 6-7.5) to a Sulfurospirillum spp.- (pH5 -5.5) dominated community was observed. Consortium Maroc 6b showed significantly different community compositions due to a pH change of only 0.5 pH units. Dehalococcoides spp. and Dehalobacter spp. were dominating at pH 7 and pH 6.5, respectively. Consortium 2SO is dominated by Dehalococcoides spp. at all pH, although at pH 7 a much higher diversity seems to prevail, with the threefold number of OTUs. Tests on the tolerance of the dehalorespiring consortia for elevated PCE concentration showed similar patterns for the different dechlorination steps, as for pH cultures, and confirmed a generally higher sensitivity towards PCE and pH changes for the last steps of dechlorination. Conclusively, Dehalococcoides spp., the only known genera able to perform complete dechlorination, seem to be highly sensitive to pH and PCE changes. Further, abiotic experiments on silicate mineral dissolution, using forsteritic olivine, were performed to simulate the buffer capacity of the minerals for the acid production during microbial reductive dechlorination. In these experiments, the buffer capacity was insufficient to counterbalance the constant acid titration rate, resulting in a final pH between 3 and 4 (initial pH of 7). The formation of precipitates, such as iron oxide, and the re-precipitation of magnesium were observed in some cases, showing a decrease in magnesium concentrations.

2010

Mechanical Activation of Chemical Bonds at Polymer Interfaces

Julian Nicolas Bleich

Polymer brushes are a specific class of polymer thin films in which each chain is tethered to the underlying substrate via one chain end. In sufficiently densely grafted assemblies, the individual polymer chains are forced into an extended chain conformation and form a brush-like structure, colloquially referred to as polymer brush. Upon incubation in a good solvent, polymer brushes experience swelling which leads to a tension at the polymer brush - substrate interface. This tension is believed to mechanically facilitate cleaving reactions at the tethering point resulting in the detachment of polymer chains, a process which is known as degrafting. Degrafting, and in the case of crosslinked polymer brushes delamination, has been reported for a large variety of polymer brushes and surface materials. Even though this phenomenon is already known for more than two decades, in depth understanding of the underlying mechanism is lacking and quantitative description of the process is rare. This thesis aims to gain further understanding of the phenomenon by quantitative description of the degrafting process. Further, swelling induced mechanical activation at the tethering point of the polymer to the underlying substrate is discussed in the context of polymer mechanochemistry and presented as a tool to selectively facilitate mechanophore cleavage at the interface. Chapter 1 provides a short historical review on the development of polymer mechanochemistry and gives an overview of commonly applied techniques in the field. The concept of polymer mechanochemistry is then extended to polymer brushes and the discussion results in a number of issues and research questions which are addressed in the subsequent chapters. The preparation and characterization of polymer brushes is not trivial. In Chapter 2 limitations and challenges with focus on experimental reproducibility and thin film characterization are discussed. Chapter 3 discusses the influence of a number of factors, i.e. grafting density and molecular weight of the polymer brush, solvent quality and salt concentration in the incubation medium, on the degrafting process of hydrophilic brushes in aqueous media. Further, the degrafting behavior of polymer brushes prepared from different monomers is compared. It is attempted to correlate degrafting behavior with polymer brush swelling. Contradicting results concerning the actual cleaving point of chains in polymer brushes are found in literature. In Chapter 4 presents structures incorporating only one single potential cleaving point. However, non-specific degrafting could not completely be suppressed. Further, a disulfide bond and oxy-norbornene moiety, commonly used mechanophores, were introduced at the anchoring point. In Chapter 5 the limitation of unwanted background hydrolysis was circumvented by investigating degrafting processes in dry organic media. Specific cleavage of the two mechanophores, disulfide and oxy-norbornene, could be demonstrated.

EPFL2021