Current nanofiltration (NF) models are based on the “diluted solution” assumption and cannot successfully predict permeate fluxes at high salt concentrations. The reasons behind the strong differences between the predicted and observed fluxes are still not fully understood. In this work, it is proposed that these deviations are possibly caused by the electrical charges inside the membrane pores. At a nanoscale level, the complex electrostatic interactions between the highly confined charged solutes and the charges inside membrane pores contribute to flow retardation and this phenomena can be characterized using an additional resistance factor, which is defined as the electric resistance factor in this study. To this extent, experiments were carried out with aqueous sodium chloride (NaCl) solutions in a wide range of concentrations (0.05 – 1.96 M) using two commercial membranes (NF270 and Desal-5 DL). Salt retention was fitted and analysed by means of the classical Spiegler-Kedem model (SK). The model has been modified to include the proposed empirical electric resistance factor, Relec, to account for this additional hydrodynamic flow resistance. The modified Spiegler-Kedem model (MSK) was verified by fitting experimental data at relatively low salt concentration to obtain model parameters and then comparing the model prediction with experimental data at higher concentrations. A mathematical equation was developed to describe the dependence of an important model parameter, reflection coefficient (σ), on operational conditions such as pressure and bulk salt concentration. The thesis also discussed the mechanisms of NF separation, highlighting the electrostatic interaction between the co-ions and the membrane charges in the confined nano-environment inside the NF membrane pores.

Current nanofiltration (NF) models are based on the “diluted solution” assumption and cannot successfully predict permeate fluxes at high salt concentrations. The reasons behind the strong differences between the predicted and observed fluxes are still not fully understood. In this work, it is proposed that these deviations are possibly caused by the electrical charges inside the membrane pores. At a nanoscale level, the complex electrostatic interactions between the highly confined charged solutes and the charges inside membrane pores contribute to flow retardation and this phenomena can be characterized using an additional resistance factor, which is defined as the electric resistance factor in this study. To this extent, experiments were carried out with aqueous sodium chloride (NaCl) solutions in a wide range of concentrations (0.05 – 1.96 M) using two commercial membranes (NF270 and Desal-5 DL). Salt retention was fitted and analysed by means of the classical Spiegler-Kedem model (SK). The model has been modified to include the proposed empirical electric resistance factor, Relec, to account for this additional hydrodynamic flow resistance. The modified Spiegler-Kedem model (MSK) was verified by fitting experimental data at relatively low salt concentration to obtain model parameters and then comparing the model prediction with experimental data at higher concentrations. A mathematical equation was developed to describe the dependence of an important model parameter, reflection coefficient (σ), on operational conditions such as pressure and bulk salt concentration. The thesis also discussed the mechanisms of NF separation, highlighting the electrostatic interaction between the co-ions and the membrane charges in the confined nano-environment inside the NF membrane pores.

This dissertation investigates the importance of diffusional and convective fluxes for salts in reverse osmosis (RO) and nanofiltration (NF) membranes. Moreover, the physical and thermodynamic factors controlling the salt permeability are analyzed. The study utilizes direct osmosis (DO) experiments and RO experiments, the later using both flat sheet and spiral wound membrane configurations. The salts considered are chlorides and acetates of alkali metals and alkaline earth metals. The equation governing the salt transport in DO experiments is derived and a phenomenon inverse to concentration polarization in RO is observed. The salt permeability in DO is equal to the salt permeability calculated for the valid cases of the used RO models. DO is suggested as an alternative method in characterizing the salt transport in membranes. The method can be more advantageous than RO due to the lower costs and simplicity of the apparatus. The models used to calculate the salt transport parameters in RO experiments are Spiegler-Kedem model, which considers both diffusion and convection of salt, and Kimura-Sourirajan model, which considers only diffusion of salt. It is found that diffusion is the dominant mechanism of transport in both RO and NF membranes. The percentage of the salt diffusional flux of the total flux is highest for seawater membranes and it is approximately equal for brackish water and nanofiltration membranes. The salt diffusive flux contribute more to the total flux for the 1:2 salts than for 1:1 salts. The two RO models are found equivalent in determining the salt permeability for only the seawater membranes. The Kimura-Sourirajan model overestimates the salt permeability coefficient for salts with rejection coefficient lower than 86%. The permeation rates for studied salts follow the lyotropic series regardless the membrane type (RO or NF), the membrane configuration (flat sheet or spiral wound), the process (DO or RO), or the models used for the calculations. This order of salt permeability is explained by the hydration of the cations, which is quantified by the enthalpy and entropy of hydration. The relative free energy theory can also be used to predict the salt permeability in a membrane based on preliminary data.

Includes vita. / Title from PDF of title page. / Document formatted into pages; contains 187 pages. / Thesis (Ph.D.)--University of South Florida, 2003. / Includes bibliographical references. / Text (Electronic thesis) in PDF format. / ABSTRACT: This dissertation investigates the importance of diffusional and convective fluxes for salts in reverse osmosis (RO) and nanofiltration (NF) membranes. Moreover, the physical and thermodynamic factors controlling the salt permeability are analyzed. The study utilizes direct osmosis (DO) experiments and RO experiments, the later using both flat sheet and spiral wound membrane configurations. The salts considered are chlorides and acetates of alkali metals and alkaline earth metals. The equation governing the salt transport in DO experiments is derived and a phenomenon inverse to concentration polarization in RO is observed. The salt permeability in DO is equal to the salt permeability calculated for the valid cases of the used RO models. DO is suggested as an alternative method in characterizing the salt transport in membranes. The method can be more advantageous than RO due to the lower costs and simplicity of the apparatus. / ABSTRACT: The models used to calculate the salt transport parameters in RO experiments are Spiegler-Kedem model, which considers both diffusion and convection of salt, and Kimura-Sourirajan model, which considers only diffusion of salt. It is found that diffusion is the dominant mechanism of transport in both RO and NF membranes. The percentage of the salt diffusional flux of the total flux is highest for seawater membranes and it is approximately equal for brackish water and nanofiltration membranes. The salt diffusive flux contribute more to the total flux for the 1:2 salts than for 1:1 salts. The two RO models are found equivalent in determining the salt permeability for only the seawater membranes. The Kimura-Sourirajan model overestimates the salt permeability coefficient for salts with rejection coefficient lower than 86%. / ABSTRACT: The permeation rates for studied salts follow the lyotropic series regardless the membrane type (RO or NF), the membrane configuration (flat sheet or spiral wound), the process (DO or RO), or the models used for the calculations. This order of salt permeability is explained by the hydration of the cations, which is quantified by the enthalpy and entropy of hydration. The relative free energy theory can also be used to predict the salt permeability in a membrane based on preliminary data. / System requirements: World Wide Web browser and PDF reader. / Mode of access: World Wide Web.