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D Hunkeler

Publications and source records attributed to D Hunkeler.

At least 19 recordsLinked to original sources
PubMed2003

Monitoring oxidation of chlorinated ethenes by permanganate in groundwater using stable isotopes: laboratory and field studies.

Permanganate injection is increasingly applied for in situ destruction of chlorinated ethenes in groundwater. This laboratory and field study demonstrates the roles that carbon isotope analysis can play in the assessment of oxidation of trichloroethene (TCE) by permanganate. In laboratory experiments a strong carbon isotope fractionation was observed during oxidation of TCE with similar isotopic enrichment factors (-25.1 to -26.8 per thousand) for initial KMnO4 concentrations between 67 and 1,250 mg/L. At the field site, a single permanganate injection episode was conducted in a sandy aquifer contaminated with TCE as dense nonaqueous liquid (DNAPL). After injection, enriched delta13C values of up to +204% and elevated Cl- concentrations were observed at distances of up to 4 m from the injection point. Farther away, the Cl- increased without any change in delta13C of TCE suggesting that Cl- was not produced locally but migrated to the sampling point Except for the closest sampling location to the injection point, the delta13C rebounded to the initial 613C again, likely due to dissolution of DNAPL Isotope mass balance calculations made it possible to identify zones where TCE oxidation continued to occur during the rebound phase. The study indicates that delta13C values can be used to assess the dynamics between TCE oxidation and dissolution and to locate zones of oxidation of chlorinated ethenes that cannot be identified from the Cl- distribution alone.

Chlorine Compounds↗
PubMed2002

Permeability and stability of chitosan-based capsules: effect of preparation.

Capsules were obtained by interpolymer complexation between chitosan (polycation) and sodium hexametaphosphate (oligoanion). The effect of preparation variables such as the pH, ionic strength as well as the reagent and porogen concentration on the capsule characteristics was evaluated. By decreasing the chitosan/SMP ratio, adding mannitol up to 1% or maintaining the salt concentrations below 0.15 w/v%, the diffusion characteristics can be modulated without disturbing the capsule mechanical stability. Higher concentrations of the cross-linking agent (2.25 w/v%) produced stable capsules only in the absence of electrolyte and low polyol amounts. Furthermore, by increasing the ionic strength, or the pH of the initial chitosan solution, the membrane exclusion limit shifted to higher values concomitant with a significant loss in the membrane compression resistance. The results obtained showed that the capsule characteristics could be independently controlled by manipulating the coacervation conditions.

Capsules↗
PubMed2002

Carbon isotopes as a tool to evaluate the origin and fate of vinyl chloride: laboratory experiments and modeling of isotope evolution.

Accumulation of vinyl chloride (VC) is often a main concern at sites contaminated with chlorinated ethenes and ethanes due to its high toxicity. Since there can be several possible sources of VC and ethene at such sites, assessing the origin and fate of VC can be complicated. Aim of this study was to evaluate carbon isotope fractionation associated with various anaerobic processes that lead to the production of VC and ethene in view of using isotopes to evaluate the origin and fate of these compounds in groundwater. Microcosms were constructed using sediments and groundwater from a contaminated site and amended with potential precursors for VC and ethene production. In the microcosms with dichloroethene isomers, sequential reductive dechlorination was observed, and isotopic enrichmentfactors of -19.9 +/- 1.5 per thousand for cis-1,2-dichloroethene, -30.3 +/- 1.9 per thousand for trans-1,2-dichloroethene, and -7.3 +/- 0.4 per thousand for 1,1-dichloroethene were obtained. In microcosms with chlorinated ethanes, 1,2-dichloroethane (1,2-DCA) and 1,1,2-trichloroethane (1,1,2-TCA) were predominantly transformed by dichloroelimination to ethene and VC, respectively, and enrichmentfactors of -32.1 +/- 1.1 per thousand for 1,2OCA and -2.0 +/- 0.2 per thousand for 1,1,2-TCA were observed. Except for 1,1,2-TCA, a strong 13C enrichment in each of the potential precursor of VC was observed, which opens the possibility to trace the origin of VC based on the isotope ratio of potential precursors. Furthermore, it was possible to model the isotope evolution of VC present as substrate or intermediate product as a function of time. The study demonstrates that carbon isotope ratios can potentially be used for qualitative and possibly quantitative evaluation of the origin and fate of VC at sites with complex contaminant mixtures.

Carbon Isotopes↗
PubMed2002

Sulfur-free lignins from alkaline pulping tested in mortar for use as mortar additives.

Sulfur-free lignin, obtained through the acid precipitation of black liquor from the soda pulping process, has been tested as water reducer in mortar. It has also been compared to existing commercial additives such as naphthalene sulfonates and lignosulfonates. The ash content and sugar content of these lignins are low in comparison to lignosulfonates, conferring on them higher purity. A procedure for small scale testing derived from the industrial norms SN-EN196 and ASTM (Designation C230-90) is presented. Specifically, all the sulfur-free lignins tested improved the flow of the mortar. Selected flax lignins performed better than lignosulfonates though still less than naphthalene sulfonates. Furthermore, certain hemp lignins gave comparable results to the lignosulfonates. Overall, the straw lignin prepared herein is comparable in performance to commercially available lignins, such as Organocell, Alcell and Curan 100. The plant from which the lignin was isolated, and the process of the pulp mill are the primary influences on the performance of the lignin.

Adhesives↗
PubMed2001

Hydrogen and carbon isotope fractionation during aerobic biodegradation of benzene.

The main aim of the study was to evaluate hydrogen and carbon isotope fractionation during biodegradation of benzene as a possible tool to trace the process in contaminated environments. Aerobic biodegradation of benzene by two bacterial isolates, Acinetobacter sp. and Burkholderia sp., was accompanied by significant hydrogen and carbon isotope fractionation with hydrogen isotope enrichment factors of -12.8 +/- 0.7 per thousand and -11.2 +/- 1.8 per thousand, respectively, and average carbon isotope enrichment factors of -1.46 +/- 0.06 per thousand and -3.53 +/- 0.26 per thousand, respectively. Inorganic carbon produced by Acinetobacter sp. was depleted in 13C by 3.6-6.2 per thousand as compared to the initial delta13C of benzene, while the produced biomass was enriched in 13C by 3.8 per thousand. The secondary aim was to determine isotope ratios of benzenes from different manufacturers with regard to the use of isotopes for source differentiation. While two of the four analyzed benzenes had similar delta13C values, each of them had a distinct delta2H-delta13C pair and delta2H values spread over a range of 66.5 per thousand. Thus, combined analyses of hydrogen and carbon isotopes may be a more promising approach to trace sources and/or biodegradation of benzene than measuring carbon isotopes only.

Acinetobacter↗
PubMed2001

Monitoring biodegradation of methyl tert-butyl ether (MTBE) using compound-specific carbon isotope analysis.

Methyl tert-butyl ether (MTBE), the most common gasoline oxygenate, is frequently detected in surface water and groundwater. The aim of this study was to evaluate the potential of compound-specific isotope analysis to assess in situ biodegradation of MTBE in groundwater. For that purpose, the effect of relevant physical and biological processes on carbon isotope ratios of MTBE was evaluated in laboratory studies. Carbon isotope fractionation during organic phase/gas-phase partitioning (0.50 +/- 0.15@1000), aqueous phase/gas-phase partitioning (0.17 +/- 0.05@1000), and organic phase/aqueous-phase partitioning (0.18 +/- 0.24@1000) was small in comparison to carbon isotope fractionation measured during biodegradation of MTBE in microcosms based on aquifer sediments of the Borden site. In experiments with MTBE as the only substrate and a cometabolic experiment with 3-methypentane as primary substrate, MTBE became enriched in 13C by 5.1 to 6.9@1000 after 95 to 97% degradation. For both experiments, similar isotopic enrichment factors were obtained (-1.52 +/- 0.06 to -1.97 +/- 0.05@1000). Biodegradation of TBA, which accumulated transiently in the cometabolic microcosms, was also accompanied by carbon isotope fractionation, with an isotopic enrichment factor of -4.21 +/- 0.07@1000. This study suggests that carbon isotope analysis is a potential tool to trace in situ biodegradation of MTBE and TBA and thus to better understand the fate of these contaminants in the environment.

Biodegradation, Environmental↗
PubMed2001

Maintenance of primary murine hepatocyte functions in multicomponent polymer capsules--in vitro cryopreservation studies.

BACKGROUND/AIMS: The potential of a new encapsulation system has been evaluated as an artificial housing for liver cells. METHODS: Murine hepatocytes were encapsulated in specially designed multicomponent capsules formed by polyelectrolyte complexation of sodium alginate, cellulose sulphate and poly(methylene-co-guanidine) hydrochloride, the permeability of which has previously been characterised. RESULTS: We demonstrate here the absence of cytotoxicity and the excellent biocompatibility of these capsules towards primary culture of murine hepatocytes. Experimental results demonstrated that the encapsulated hepatocytes retained their specific functions--transaminase activity, urea synthesis and protein secretion--over the first 4 days of culture in minimum medium. The cryopreservation of encapsulated hepatocytes, for periods of up to 4 months, did not alter their functional capacities, as no major differences were observed between unfrozen and frozen encapsulated cells for the functions tested. CONCLUSIONS: Because of the absence of cytotoxicity, and the ease of handling and cryopreservation, while maintaining liver specific functions, the described system appears to be valuable for murine liver cell encapsulation. It is also a promising tool for fundamental research into drug metabolism, intercellular regulation, metabolic pathways, and the establishment of banks for the supply and storage of murine hepatocytes.

Animals↗
PubMed2001

Carrageenan-oligochitosan microcapsules: optimization of the formation process(1).

The formation of new microcapsules based on polyelectrolyte complexes between carrageenans and oligochitosan has been investigated. The optimization of the process, which includes the selection of the most suitable solvent and investigation of the influence of reaction conditions on capsule properties, is presented. Iota-carrageenan (1.2-2% wt.) prepared in HEPES buffer was found to be the most suitable for the formation of mechanically stable capsules. These new capsules combine extremely high deformability (>90%) and elasticity with permeability control and can be applied in various bioencapsulation technologies. It has been shown that the reaction time influences the mechanical properties, whereas carrageenan concentration and the temperature during the capsule formation effect both mechanical and porosity characteristic of the membrane. Moreover, the temperature influences the kinetics of the diffusion through the complex iota-carrageenan/oligochitosan membrane. In general egress is faster above the sol-gel transition point, indicating applicability in thermo-induced releasing systems.

Journal Article↗
PubMed2001

Development of a coculture model of encapsulated cells.

In the whole animal, metabolic regulations are set by reciprocal interactions between various organs, via the blood circulation. At present, analyses of such interactions require numerous and uneasily controlled in vivo experiments. In a search for an alternative to in vivo experiments, our work aims at developing a coculture system in which different cell types are isolated in polymer capsules and grown in a common environment. The signals exchanged between cells from various origins are, thus, reproducing the in vivo intertissular communications. With this perspective, we evaluated a new encapsulation system as an artificial housing for liver cells on the one hand and adipocytes on the other hand. Murine hepatocytes were encapsulated with specially designed multicomponent capsules formed by polyelectrolyte complexation between sodium alginate, cellulose sulphate and poly(methylene-coguanidine) hydrochloride, of which the permeability has been characterized. We demonstrated the absence of cytotoxicity and the excellent biocompatibility of these capsules towards primary culture of murine hepatocytes. Encapsulated hepatocytes retain their specific functions--transaminase activity, urea synthesis, and protein secretion--during the first four days of culture in minimum medium. Mature adipocytes, isolated from mouse epidydimal fat, were embedded in alginate beads. Measurement of protein secretion shows an identical profile between free and embedded adipocytes. We finally assessed the properties of encapsulated hepatocytes, cryopreserved over a periods of up to four months. The perspective of using encapsulated cells in coculture are discussed, since this system may represent a promising tool for fundamental research, such as analyses of drug metabolism, intercellular regulations, and metabolic pathways, as well as for the establishment of a tissue bank for storage and supply of murine hepatocytes.

Adipocytes↗
PubMed2001

Objectively assessing bioartificial organs.

The metrics used, thus far, to assess bioartificial organ function are shown to be subjective and requiring validation. Therefore, four categories of correlations are proposed based on, respectively, device, in vitro and in vivo evaluations, and clinical function. Examples are presented whereby the correlations among individual indicators are used as a means to expedite the development of immunoisolated cells. Specifically, a case study illustrating the validation of in vitro indicators of in vivo graft function for the bioartificial pancreas (microencapsulated islets) is summarized. This has revealed thresholds with respect to given metrics relating to in vivo device function, the necessity to couple bioartificial organ design with transplant site selection, as well as the lack of objectivity involved in the evaluation and establishment of hypotheses. Specific quantitative indicators illustrate the need for quality-controlled measures, for example, relating to the tolerance of microcapsule diameter and membrane thickness distributions. Qualitative indices representing fibrosis and device properties (e.g., sphericity) are also used to describe the need for in vitro experiments in the development of bioartificial organs.

Bioartificial Organs↗
PubMed2001

The compressive deformation of multicomponent microcapsules: influence of size, membrane thickness, and compression speed.

The clinical application of microcapsules for the immunoisolation of living tissue requires knowledge about the mechanical stability of polymer membranes. Microcapsules of 400-1000 microm in diameter were formed through the gelation of sodium alginate/sodium cellulose sulfate droplets through calcium chloride, with the membrane produced via complex coacervation between polyanions and poly(methylene-co-guanidine) hydrochloride. The deformation behavior of these multicomponent microcapsules was investigated by uniaxial compression experiments. Specifically, the influence of the deformation speed, capsule diameter, and membrane thickness on the mechanical properties was evaluated. The bursting force was found to be dependent on the deformation speed. Therefore, the measurement of the bursting work, a speed-independent value of the resistance to high stresses and deformations, was recommended as the most valid for capsule mechanical resistance. Furthermore, the bursting force was positively correlated with membrane thickness only for membrane-radius ratios up to 20%. For thicker membranes, the bursting event occurred because the opposite membranes touched each other, and not, because of insufficient strength. Indeed, the resistance to smaller deformations was positively correlated to the membrane thickness over the whole range of membrane-radius ratios. Moreover, the forces for constant deformation were linearly correlated to the total membrane volume, independently of capsule size and membrane thickness.

Biocompatible Materials↗
PubMed2001

Stability assessment of chitosan-sodium hexametaphosphate capsules.

The assessment of the stability of capsules based on chitosan-sodium hexametaphosphate complex formation has been carried out using two independent methods--compression and osmotic swelling, and the influence of the preparation variables was evaluated. The formulation containing 1.5% core polymer (chitosan) and 1.5% oligophosphate, in the absence of salt or at low ionic strength (0.15% NaCl) was found to provide the best membrane resistance. A higher concentration of cross-linker (2.25%) produced stable capsules only in absence of electrolyte. Mannitol, a porogen added to the preparation solutions, did not affect the stability of the obtained membranes. At elevated polyol (1%) and cross-linker levels (2.25%), and at 0% salt, membranes with decreased elasticity were obtained, having lower compression and osmotic bursting values and lower deformation at the breaking points. A significant influence of salt amount on the capsule stability was also found. This was attributed to changes in the membrane formation process resulting in membranes with different thickness and structure. Membrane compression stability was found to be dependent on the pH of both oligophosphate and chitosan solutions, as well as on the reaction time. The bursting force values decreased for capsule diameters below 1.6 mm. The increased membrane/capsule volume ratio for the small capsules decreased the capsule deformation freedom and caused capsule rupture at low force values. The capsules made at low salt amounts showed very good storage stability over time and at elevated temperatures. The results demonstrated that the capsules could be formulated with controlled properties for various biomedical applications.

Capsules↗
PubMed2001

Effect of preparation conditions on properties and permeability of chitosan-sodium hexametaphosphate capsules.

Capsules were obtained by interpolymer complexation between chitosan (polycation) and sodium hexametaphosphate (SMP, oligoanion). The effect of the preparation conditions on the capsule characteristics was evaluated. Specifically, the influence of variables such as pH, ionic strength, reagent concentration, and additives on the capsule permeability properties was investigated using dextran as a model permeant. The capsule membrane permeability was found to increase by decreasing the chitosan/SMP ratio as well as adding mannitol to the oligoanion recipient bath. Increasing the ionic strength or the pH of the initial chitosan solution was also found to enhance the membrane permeability, moving the membrane exclusion limit to higher values. Generally, the capsules prepared tinder all tested conditions had a relatively low permeability which rarely exceeded a molecular cut-off of 40 kD based on dextran standards. Furthermore, the diffusion rate showed a strong temporal dependence, indicating that the capsules prepared under various conditions exhibit different apparent pore size densities on the surface. The results indicated that, in order to obtain the desired capsule mass-transfer properties, the preparation conditions should be carefully considered and adjusted. Adding a polyol as well as low salt amount (less than 0.15%) is preferable as a means of modulating the diffusion characteristics, without disturbing the capsule mechanical stability.

Capsules↗
PubMed2000

Evidence of substantial carbon isotope fractionation among substrate, inorganic carbon, and biomass during aerobic mineralization of 1, 2-dichloroethane by Xanthobacter autotrophicus.

Carbon isotope fractionation during aerobic mineralization of 1, 2-dichloroethane (1,2-DCA) by Xanthobacter autotrophicus GJ10 was investigated. A strong enrichment of (13)C in residual 1,2-DCA was observed, with a mean fractionation factor alpha +/- standard deviation of 0.968 +/- 0.0013 to 0.973 +/- 0.0015. In addition, a large carbon isotope fractionation between biomass and inorganic carbon occurred. A mechanistic model that links the fractionation factor alpha to the rate constants of the first catabolic enzyme was developed. Based on the model, it was concluded that the strong enrichment of (13)C in 1,2-DCA arises because the first irreversible step of the initial enzymatic transformation of 1,2-DCA consists of an S(N)2 nucleophilic substitution. S(N)2 reactions are accompanied by a large kinetic isotope effect. The substantial carbon isotope fractionation between biomass and inorganic carbon could be explained by the kinetic isotope effect associated with the initial 1,2-DCA transformation and by the metabolic pathway of 1,2-DCA degradation. Carbon isotope fractionation during 1,2-DCA mineralization leads to 1,2-DCA, inorganic carbon, and biomass with characteristic carbon isotope compositions, which may be used to trace the process in contaminated environments.

Aerobiosis↗
PubMed1999

New microcapsules based on oligoelectrolyte complexation.

A new one-step microencapsulation procedure has been developed. For the alginate/oligochitosan system the molar mass of the chitosan is a key parameter in the formation of stable, elastic capsules with high modulus. Furthermore, the selection of an optimum molar mass provides an additional degree of freedom, permitting the simultaneous regulation of mechanical properties and permeability without the need for multicomponent organic-inorganic chemistries as have been previously employed. The effects of molar mass of chitosan, its concentration, the alginate molar mass and its metal salt on the preparation, physical properties, and release characteristics of the capsules have been studied.

Alginates↗