Rejection Of Uncharged Solutes Research Articles

Novel Positively Charged Metal-Coordinated Nanofiltration Membrane for Lithium Recovery

Nanofiltration (NF) with high water flux and precise separation performance with high Li+/Mg2+ selectivity is ideal for lithium brine recovery. However, conventional polyamide-based commercial NF membranes are ineffective in lithium recovery processes due to their undesired Li+/Mg2+ selectivity. In addition, they are constrained by the water permeance selectivity trade-off, which means that a highly permeable membrane often has lower selectivity. In this study, we developed a novel nonpolyamide NF membrane based on metal-coordinated structure, which exhibits simultaneously improved water permeance and Li+/Mg2+ selectivity. Specifically, the optimized Cu-m-phenylenediamine (MPD) membrane demonstrated a high water permeance of 16.2 ± 2.7 LMH/bar and a high Li+/Mg2+ selectivity of 8.0 ± 1.0, which surpassed the trade-off of permeance selectivity. Meanwhile, the existence of copper in the Cu-MPD membrane further enhanced anti-biofouling property and the metal-coordinated nanofiltration membrane possesses a pH-responsive property. Finally, a transport model based on the Nernst-Planck equations has been developed to fit the water flux and rejection of uncharged solutes to the experiments conducted. The model had a deviation below 2% for all experiments performed and suggested an average pore radius of 1.25 nm with a porosity of 21% for the Cu-MPD membrane. Overall, our study provides an exciting approach for fabricating a nonpolyamide high-performance nanofiltration membrane in the context of lithium recovery.

Relating nanofiltration membrane morphology to observed rejection of saccharides

Nanofiltration for the fractionation of uncharged solutes has become more interesting in recent times, due to the goal of valorizing side-streams and by-products of the food industry. Since the membranes suitable for this purpose are mainly utilized for demineralization processes, the specifications given by the manufacturer are insufficient for the prediction of the rejection of uncharged solutes. Nine membranes with a molecular weight cut-off between 150 and 1000 Da were investigated showing differences in their rejection of fructose at a transmembrane pressure difference ΔpTM of 1.0 MPa. These values were correlated with the root-mean-square roughness Sq and apparent pore size dap determined by atomic force microscopy, while the determined contact angle Θ did not influence the rejection. Thereby, a correlation between membrane morphology and rejection behavior was established.

Effect of post-polymerization anion-exchange on the rejection of uncharged aqueous solutes in nanoporous, ionic, lyotropic liquid crystal polymer membranes

The selectivity of a bicontinuous cubic, lyotropic liquid crystal (LLC) polymer nanofiltration membrane containing uniform cationic nanopores was manipulated via anion-exchange at the pore wall by soaking the fabricated membranes in aqueous salt solutions. Monovalent organosulfonate anions of various sizes were completely exchanged as the counterion at the pore walls to explore the impact of anion size on uncharged molecular solute rejection and water permeance. It was found that larger organosulfonate anions associated at the pore walls induced a higher rejection of uncharged solutes and a lower water permeance. The change in selectivity of the nanopores was described quantitatively through the calculation of the effective pore radius. The effective pore radius in these modified membranes was found to vary from 0.9±0.2 to 0.55±0.08nm depending on the resident organosulfonate counterion, demonstrating sub-1-nm resolution in pore size manipulation. An empirical model was used to demonstrate that physicochemical properties (molecular volume and hydrophobicity) of the resident counterion correlate with the observed solute rejection, suggesting that the counterion can be chosen to target a desired membrane performance. The ability to manipulate uniform, nm-sized pores with sub-1-nm resolution via simple anion-exchange offers a new avenue to tailor nanofiltration membrane performance.

Nanofiltration of glucose solution containing salts: Effects of membrane characteristics, organic component and salts on retention

The understanding on nanofiltration especially the rejection of uncharged solutes, charged solutes and mixture of solutes is crucial for its application in food industry. This is because process streams in food industry usually contain organic components without charge and salts. In this work, DK and CK membranes were utilized for the separation of multi-component feeds containing glucose, NaCl and multivalent salts (MgCl 2 or Na 2SO 4). Glucose rejection is slightly affected by salt concentration which may due to pore swelling of CK membranes at high concentration of NaCl. Meanwhile, rejection of NaCl is reduced by the increment of glucose and NaCl concentration which causes concentration polarization. The addition of multivalent salts even induces negative rejection of NaCl in DK membranes. However, both membranes show high rejection of multivalent salts with or without the presence of glucose. The rejection behavior follows the pattern expected from the structural and electrical properties of the membranes.

Rejection of trace organic pollutants with high pressure membranes (NF/RO)

AbstractThis article presents a modeling approach for the rejection of trace organic pollutants in high‐pressure membranes. The rejection values of uncharged organic solutes with a wide range of solute size and hydrophobicity are determined. Sigmoidal rejection curves are constructed and modeled for solutes of different hydrophobicity. The model is based on a log‐normal pore size distribution model. A model is also presented to model the rejection of charged organic solutes, based on the rejection of uncharged solutes and the concept of a “charge concentration polarization.” Moreover, scale‐up issues arising when translating results from bench‐scale units to full‐scale plants are addressed, by assessing the viability of a full‐scale rejection model. The full‐scale model is based on a convection‐diffusion model for the transport of organic solutes through the high‐pressure membranes and the use of mass balances in the system. © 2008 American Institute of Chemical Engineers Environ Prog, 2008

Effect of solute geometry and orientation on the rejection of uncharged compounds by nanofiltration

This work discusses a new approach to model the rejection of uncharged solutes by nanofiltration membranes. A mathematical description of the solute is developed – Geometric radius – in which the molecular geometry is represented by a prolate revolution ellipsoid and a possible preferential orientation (orientation angle) of the molecule during permeation through the membrane is considered in the calculations. The model simulations suggest that the target solute rejection depends strongly on its preferential orientation, especially for elongated molecules due to the higher difference in their Geometric radii. In order to evaluate the model, the rejection of defined solutes ( n-alcohols and di-alcohols) with different chain length, polarity, symmetry and functional group position was studied experimentally. It was found that the common approach to model the solute rejection, using the molecular Stokes radius, failed to predict the experimental results for the n-alcohols studied, but was applicable for some of the di-alcohols. On the other hand, the Geometric model proposed allowed in all cases for a better description of the experimental rejection data due to its extra degree of freedom – the molecular orientation angle during membrane permeation. Based on these calculated orientation angles, which were dependent of the transmembrane pressure applied, the model could be used as an interpretative tool, allowing for a better understanding of the dominant factors that determine the rejection of uncharged solutes with different molecular geometry.

Salt rejection in nanofiltration for single and binary salt mixtures in view of sulphate removal

Three commercial membranes (NF70, NF90 and TFC-SR) were firstly characterized in terms of pure water flux and the rejection of uncharged (alcohols and sugars) compounds. Subsequently, the rejection of monovalent (sodium and chloride) and divalent (calcium and sulphate) ions in single (NaCl, CaCl, and Na 2SO 4) and binary (NaCI/Na 2,SO 4 CaCl 2/CaSO 4, NaCI/CaCl 2, and Na 2SO 4/CaSO 4) salt mixtures was studied. According to the pure water permeability the TFC-SR membrane is a loosely packed NF membrane (12.3 L.m −2.h −1.bar −1), while both NF70 and NF90 are tightly packed (2.6 and 3.6 Lm −2.h −1.bar-). According to the uncharged solute rejection, the MWCO NF70 = 60, MWCO NF90= 200 and MWCO TFC-SR > 500. NF70 and NF90 were equally efficient in rejecting 1–2, 1–1 and 2–1 salts (>90%), while TFC-SR showed typical negatively charged surface behaviour, i.e., R (1–2) salt > R (11) salt > R (2-1). Sulphate rejection decreased in the presence of sodium chloride more significantly than in the presence of calcium chloride due to the more efficient retention of the bivalent calcium.

Modelling of membrane nanofiltration—pore size distribution effects

The effects of log-normal pore size distributions on the rejection of uncharged solutes and NaCl at hypothetical nanofiltration membranes have been assessed theoretically. The importance of pore radius-dependent properties such as solvent viscosity and dielectric constant is increased by the introduction of a pore size distribution in calculations. However, the effect of porewise variation in viscosity is less apparent when considered at a defined applied pressure rather than at a defined flux, showing a further advantage of basing theoretical analysis of nanofiltration in terms of applied pressure. Truncated pore size distributions gave better agreement than full distributions with experimental rejection data for a Desal-DK nanofiltration membrane. Such truncation is in agreement with the findings of atomic force microscopy (AFM). Analysis of uncharged solute rejection data alone could not give useful information about membrane pore size distribution. Neither could such a distribution be obtained quantitatively directly from AFM images. However, use of the shape of the distribution obtained by AFM in conjunction with experimental rejection data for an uncharged solute allows calculation of corrected distributions. Importantly, incorporation of such a corrected pore size distribution in calculations of NaCl rejection gave better agreement with experimental data, compared to calculations assuming uniform pores, at high pressure, the region of industrial interest.

Modelling the performance of membrane nanofiltration—critical assessment and model development

A critical assessment of previous theoretical descriptions of membrane nanofiltration shows that the good agreement with experimental data is due to the use of the ratio of effective membrane thickness to membrane porosity as an arbitrary fitting parameter. A more rigorous analysis shows that rejection is independent of membrane thickness. A model allowing calculation of uncharged solute rejection on the basis of a single membrane parameter (pore radius) is developed. The theoretical description is based on a hydrodynamic model of hindered solute transport in pores. The dependence of solute chemical potential on pressure is included, although its effect is shown to be small. A variation of solvent viscosity with pore radius is also included. For pores of a single size, such variation has no effect on rejection, though it will become important for overall membrane rejection if a pore size distribution is included. The good agreement between this model and experimental data confirms that uncharged solute rejection in nanofiltration membranes may be well-described by such a continuum model. A two-parameter model (pore radius and membrane charge) for electrolyte rejection has been developed that includes dielectric exclusion in the form of an energy barrier to ion partitioning into the pores. Reassessment of the pore dielectric constant using NaCl rejection at the isoelectric point of a Desal-DK membrane resulted in significantly better agreement with experimental rejection data than use of the high frequency solvent dielectric constant for an oriented layer of water molecules at the pore walls. The predicted values of effective membrane charge density were not only significantly reduced in magnitude, and hence more realistic, than those from previous models, but their variation with concentration for divalent salts was in better agreement with physical models of ion adsorption. The theoretical descriptions of the present paper are developed more rigorously than those previously published. Their overall agreement with experimental data is good and the resulting membrane parameters are likely to be more closely related to physical membrane properties than those obtained from previous models. The descriptions are useful for membrane process assessment and prediction. They also provide a sound basis for further developments.

Characterisation of nanofiltration membranes for predictive purposes — use of salts, uncharged solutes and atomic force microscopy

An asymmetric nanofiltration membrane (Hoechst, PES5) has been characterised by three different techniques: modelling of the rejection of simple salts, modelling of the rejection of uncharged solutes and atomic force microscopy. Interpretation of experimental data for the rejection of three salts having common co-ion (LiCl, NaCl, KCl) with model calculations allows a characterisation of the membrane in terms of three parameters: an effective pore radius ( r p), the ratio of effective thickness over porosity ( Δx A k ) and an effective charge density ( X). Interpretation of experimental data with model calculations for uncharged solutes (Vitamin B 12, raffinose, sucrose, glucose, glycerin) allows a characterisation in terms of r p and Δx A k . Atomic force microscopy (AFM) allows direct determination of surface pore radius r p s and surface porosity A k s. The AFM images provide direct confirmation of the presence of discrete surface pores in such membranes. Further comparison of the characterisation obtained with salts and that obtained with uncharged solutes shows that it is better to describe the transport through such membranes as occurring through discrete pores rather than using an homogenous description of the membrane structure. It is also shown that the complexity of a “space-charge” description of the electric field distribution in the nanometre dimension pores of such membranes is not warranted. Direct experimental evidence of the charging mechanism of the membranes is provided. Overall characterisation parameters suitable for predictive purposes are suggested.