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Dive into the research topics where Leon Kapetas is active.

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Featured researches published by Leon Kapetas.


Journal of Colloid and Interface Science | 2011

Kinetics of bacterial potentiometric titrations: the effect of equilibration time on buffering capacity of Pantoea agglomerans suspensions.

Leon Kapetas; Bryne T. Ngwenya; A.M. MacDonald; Stephen C. Elphick

Several recent studies have made use of continuous acid-base titration data to describe the surface chemistry of bacterial cells as a basis for accurately modelling metal adsorption to bacteria and other biomaterials of potential industrial importance. These studies do not share a common protocol; rather they titrate in different pH ranges and they use different stability criteria to define equilibration time during titration. In the present study we investigate the kinetics of bacterial titrations and test the effect they have on the derivation of functional group concentrations and acidity constants. We titrated suspensions of Pantoea agglomerans by varying the equilibration time between successive titrant additions until stability of 0.1 or 0.001 mV s(-1) was attained. We show that under longer equilibration times, titration results are less reproducible and suspensions exhibit marginally higher buffering. Fluorescence images suggest that cell lysis is not responsible for these effects. Rather, high DOC values and titration reversibility hysterisis after long equilibration times suggest that variability in buffering is due to the presence of bacterial exudates, as demonstrated by titrating supernatants separated from suspensions of different equilibration times. It is recommended that an optimal equilibration time is always determined with variable stability control and preliminary reversibility titration experiments.


Journal of Contaminant Hydrology | 2014

Evidence of preferential path formation and path memory effect during successive infiltration and drainage cycles in uniform sand columns.

Leon Kapetas; Ishai Dror; Brian Berkowitz

The formation of preferential flow paths in the partially saturated zone, and in naturally structured media, is well known. This study examines non-uniform flow in uniform sand columns under different pressure and infiltration/drainage conditions. Experiments were carried out in a vacuum box, with applied suction set to three different heads, and with infiltration fixed at two different flow rates. Tailing observed in some conservative tracer breakthrough curves suggests the formation of immobile resident water pockets which slowly exchange mass with the flowing water fraction. The applied suction controlled the degree of water immobilization whereas flow rate had minimal effect on the dynamic behavior. Trapping and exchange of water occurred repeatedly during successive infiltration and drainage cycles, implying a (hysteretic) memory effect of the previously formed preferential flow paths. Flow and solute transport modeling suggests that these dynamics can be described by a mobile-immobile model that corroborates measurements suggesting preferential flow path formation. These findings have implications for the natural attenuation of contaminants in the partially saturated zone, but also for the persistence of a contamination source exposed to repeated conditions of infiltration and drainage.


Environmental Science & Technology | 2012

Thermodynamic and kinetic controls on cotransport of Pantoea agglomerans cells and Zn through clean and iron oxide coated sand columns.

Leon Kapetas; Bryne T. Ngwenya; A.M. MacDonald; Stephen C. Elphick

Recent observations that subsurface bacteria quickly adsorb metal contaminants raise concerns that they may enhance metal transport, given the high mobility of bacteria themselves. However, metal adsorption to bacteria is also reversible, suggesting that mobility within porous medium will depend on the interplay between adsorption-desorption kinetics and thermodynamic driving forces for adsorption. Till now there has been no systematic investigation of these important interactions. This study investigates the thermodynamic and kinetic controls of cotransport of Pantoea agglomerans cells and Zn in quartz and iron-oxide coated sand (IOCS) packed columns. Batch kinetic studies show that significant Zn sorption on IOCS takes place within two hours. Adsorption onto P. agglomerans surfaces reaches equilibrium within 30 min. Experiments in flow through quartz sand systems demonstrate that bacteria have negligible effect on zinc mobility, regardless of ionic strength and pH conditions. Zinc transport exhibits significant retardation in IOCS columns at high pH in the absence of cells. Yet, when mobile bacteria (non attached) are passed through simultaneously with zinc, no facilitated transport is observed. Adsorption onto cells becomes significant and plays a role in mobile metal speciation only once the IOCS is saturated with zinc. This suggests that IOCS exhibits stronger affinity for Zn than cell surfaces. However, when bacteria and Zn are preassociated on entering the column, zinc transport is initially facilitated. Subsequently, zinc partly desorbs from the cells and redistributes onto the IOCS as a result of the higher thermodynamic affinity for IOCS.


IOR 2015: 18th European Symposium on Improved Oil Recovery, Dresden, Germany, 14-16 April 2015 | 2015

Effect of Permeability on Foam-model parameters - An Integrated Approach from Coreflood Experiments through to Foam Diversion Calculations

Leon Kapetas; S. Vincent-Bonnieu; R. Farajzadeh; Ali Akbar Eftekhari; S.R. Mohd-Shafian; R. Z. Kamarul Bahrim; W.R. Rossen

Foam reservoir simulations commonly suffer from poor foam parameter definition which makes their predictions ambiguous. To reduce uncertainty in foam parameter estimation, extensive core-flood experiments under variable experimental and physical conditions are required. A principal physical property which can influence foam parameters is permeability. We present a set of steady-state foam-flood experimental data for four sandstones with different permeabilities, ranging between 6 and 1900 mD, and with similar porosity. We derive permeability-dependent foam parameters with two modelling approaches, those of Boeije and Rossen (2013) and a non-linear least-square minimization approach (Eftekhari et al., 2015). The two approaches can yield significantly different foam parameters. Thus we critically assess their ability in deriving reliable foam parameter estimates. In particular, the way the two approaches treat shear-thinning foam behavior and foam coalescence is discussed. The foam parameter set acquired from the latter approach is further used as input in foam diversion calculations: this serves to evaluate mobility predictions in non-communicating reservoir layers to the foam parameters. This study aims to provide a framework to integrate experimental work, modelling and simple qualitative diversion calculations to provide a background for the upscaling of foam studies, with particular focus to heterogeneous systems.


Journal of Contaminant Hydrology | 2015

Transport and viability of Escherichia coli cells in clean and iron oxide coated sand following coating with silver nanoparticles.

Bryne T. Ngwenya; Philip Curry; Leon Kapetas

A mechanistic understanding of processes controlling the transport and viability of bacteria in porous media is critical for designing in situ bioremediation and microbiological water decontamination programs. We investigated the combined influence of coating sand with iron oxide and silver nanoparticles on the transport and viability of Escherichia coli cells under saturated conditions. Results showed that iron oxide coatings increase cell deposition which was generally reversed by silver nanoparticle coatings in the early stages of injection. These observations are consistent with short-term, particle surface charge controls on bacteria transport, where a negatively charged surface induced by silver nanoparticles reverses the positive charge due to iron oxide coatings, but columns eventually recovered irreversible cell deposition. Silver nanoparticle coatings significantly increased cell inactivation during transit through the columns. However, when viability data is normalised to volume throughput, only a small improvement in cell inactivation is observed for silver nanoparticle coated sands relative to iron oxide coating alone. This counterintuitive result underscores the importance of net surface charge in controlling cell transport and inactivation and implies that the extra cost for implementing silver nanoparticle coatings on porous beds coated with iron oxides may not be justified in designing point of use water filters in low income countries.


Journal of Industrial and Engineering Chemistry | 2016

Effect of temperature on foam flow in porous media

Leon Kapetas; S. Vincent Bonnieu; S. Danelis; W.R. Rossen; R. Farajzadeh; A.A. Eftekhari; S. R. Mohd Shafian; R. Z. Kamarul Bahrim


Desalination | 2009

A surface complexation framework for predicting water purification through metal biosorption

Bryne T. Ngwenya; Janette Tourney; Marisa Magennis; Leon Kapetas; Valerie Olive


Colloids and Surfaces A: Physicochemical and Engineering Aspects | 2017

Effect of permeability on foam-model parameters: An integrated approach from core-flood experiments through to foam diversion calculations

Leon Kapetas; S. Vincent Bonnieu; Rouhi Farajzadeh; A.A. Eftekhari; S. R. Mohd Shafian; R. Z. Kamarul Bahrim; W.R. Rossen


SPE Middle East Oil & Gas Show and Conference | 2015

Effect of Temperature on Foam Flow in Porous Media

Leon Kapetas; S. Vincent Bonnieu; S. Danelis; W.R. Rossen; R. Farajzadeh; A.A. Eftekhari; S. R. Mohd Shafian; R. Z. Kamarul Bahrim


Geochimica et Cosmochimica Acta | 2008

Microbial controls on contaminant metal transport in porous media [abstract]

Leon Kapetas; Bryne T. Ngwenya; A.M. MacDonald; Stephen C. Elphick

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W.R. Rossen

Delft University of Technology

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A.A. Eftekhari

Delft University of Technology

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R. Farajzadeh

Delft University of Technology

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A.M. MacDonald

British Geological Survey

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S. Danelis

Delft University of Technology

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