Nanofiltration Of Solutions Research Articles

Nanofiltration of solutions containing organic and inorganic salts: Relationship between feed and permeate proportions

Nanofiltration can be considered for the purification of valuable products obtained by fermentation, like organic acids. In this work, nanofiltration experiments are carried out with synthetic solutions of different compositions containing acetate (Ac), propionate (Pr), and butyrate (Bu), as well as inorganic salts (NaCl). Although the retention of charged solutes can be significantly modified according to the composition of the solution, it is observed that there is a filtration flux above which the solutes proportions in the permeate remain constant. These proportions do not depend on the total concentration but only on the solutes proportions in the feed. Finally, individual solute transfer is found to vary linearly with the total solute flux. Similar trends are observed with two NF membranes despite their different properties.

Thermodynamic perspective on negative retention effects in nanofiltration of concentrated sodium chloride solutions

Transport mechanisms for nanofiltration membranes are usually studied for solutions containing low solute concentrations, whereas practical applications of nanofiltration often use highly concentrated or even saturated solutions. These solutions are amongst others obtained from anti-solvent crystallization processes. This study shows that the NaCl retention for nanofiltration of a solution saturated in NaCl and containing either ionic or neutral anti-solvents can be predicted from thermodynamic considerations. The membrane resistance of nanofiltration membranes during NaCl transport through the membrane for solutions with a high NaCl concentration in the concentrate is negligible. Furthermore, the NaCl retention can be predicted based on simple anti-solvent crystallization experiments, provided that the concentrations of the anti-solvent in the permeate and concentrate are known. The concept has been proven for a range of ionic and neutral anti-solvents and a broad range of nanofiltration membranes.

Behavior of Alkali Metal and Ammonium Salts in Nanofiltration of Their Aqueous Solutions

The behavior of salts (chlorides, nitrates, sulfates, phosphates, borates, ferri- and ferrocyanides) of various alkali metals (Li, Na, K, Cs) and ammonium in the course of nanofiltration (NF) of their aqueous solutions was studied. The NF polymer membrane at high salt concentrations (>30 g dm−3) retains alkali metal and ammonium ions to a low extent (1–5%). At lower salt concentrations, the retaining ability of the NF membrane increases. In nanofiltration of solutions of Na and K salts with anions of different charge, the retaining ability of the NF membrane increases with an increase in the anion charge. The pH dependence of the ability of the NF membrane to retain sodium nitrate was studied. This parameter was constant at pH in the ranges 2.0–5.5 and 7.5–13.0, but at pH in the range 5.5–7.5 the retaining ability decreased approximately by half. Experiments on retention of sodium borates of various compositions showed that the solutions containing bulky polynuclear anions consisting of 4–5 boron atoms were retained by the NF membrane with 45–50% efficiency, whereas neutral boric acid molecules were not noticeably retained by the membrane. Thus, nanofiltration can be used for selective separation of inactive alkali metal salts from radioactive components of liquid radioactive waste.

The effect of NaCl and glucose concentration on retentions for nanofiltration membranes processing concentrated solutions

Most nanofiltration studies deal with solutions containing less than 60gL−1 salts and low sugar concentrations. For concentrated solutions limited information is available about interactions between neutral and charged solutes during nanofiltration, and their effect on flux and retention. This study reports results for nanofiltration of solutions containing NaCl in concentrations between 1gL−1 and 300gL−1 and glucose concentrations as high as 80gL−1. Five membranes were characterized and evaluated. For mixed glucose/NaCl solutions with NaCl concentrations higher than 90gL−1, a glucose concentration increase from 1gL−1 to 80gL−1 only affected glucose retention at the same flux for NTR 7450. For the tightest membranes slightly negative NaCl retentions were obtained for high glucose concentrations in the feed. Glucose retention for solutions containing 1gL−1 glucose decreased strongly when the NaCl concentration was increased from 0gL−1 to 100gL−1. The observed glucose retention reductions were not caused by flux changes. A change in the ratio of glucose radius over membrane effective pore radius could explain the obtained results. Determined pore radii and effective thickness can be used for development of nanofiltration applications for desalination of concentrated glucose solutions.

Evaluation of fouling mechanisms in the nanofiltration of solutions with high anionic and nonionic surfactant contents using a resistance-in-series model

The effects of feed concentration and transmembrane pressure (TMP) on membrane fouling in the treatment of cleaning-in-place (CIP) wastewater originated from the production of liquid dishwashing detergent with a nanofiltration (NF) membrane were investigated. The resistance-in-series model was used to evaluate the flux decline caused by a gel layer, a concentration polarisation layer, and internal pore blocking in the NF membrane for CIP solutions of varying concentration termed CIP 5, CIP 10 and CIP 20. With an increase in feed concentration and TMP, it was observed that resistance of the gel layer ( R g) played a more important role in the flux decline than that of the concentration polarisation layer or internal pore fouling ( R cp + in ). Considering the membrane resistance ( R m) values, the CIP solutions containing high concentrations of anionic and nonionic surfactants and low dye and salt concentrations did not cause serious fouling of the NF membrane. Characterisation of the membrane surface by atomic force microscopy (AFM) and contact angle measurements also showed that the deposition of surfactant aggregates on the NF membrane surface likely played an important role in the gel layer fouling. The NF membrane showed a rejection efficiency of over 98% for anionic surfactants, nonionic surfactant and dye in all CIP solutions. Salt rejection was not achieved in the CIP 5 solution because of the Donnan effect, whereas salt rejections were around 11–34% and 28–54% in the CIP 10 and CIP 20 solutions, respectively. The resistance-in-series model was successfully tested to evaluate the flux decline for the CIP solutions containing anionic and nonionic surfactants, dye and salt at various TMPs.

Prediction of the concentration polarization in the nanofiltration/reverse osmosis of dilute multi-ionic solutions

The aim of this work was to develop a simple and accurate model for predicting the concentration polarization index in the nanofiltration (NF)/reverse osmosis (RO) of dilute multi-ionic solutions. On the grounds of this model, the total flux of the ion i at the feed-solution/membrane interface consists of the sum of the diffusion, convection and migration fluxes, the former of which is determined by conventional mass-transfer correlations duly corrected to take into account the permeation through the membrane (suction effect). The coupling of the ionic fluxes is enforced by the electroneutrality requirement at the feed-solution/membrane interface. The model developed dispenses with the arbitrary assumption of the thickness of a film layer in the vicinity of the membrane surface. Assessing the accuracy/validity of this model with multi-ionic solutions would be rather harsh, thus the model accuracy and ranges of validity were ascertained for a benchmark case: NF/RO of single salt solutions. The model predicts approximate concentration polarization indexes of the salts A +B −, A + 2B 2− and A + 3B 3− (or A 2B − 2 and A 3+B − 3) with positive deviations lower than 10% with respect to the benchmark concentration polarization index, for ions diffusivities ratios, D 1/ D 2 (or D 2/ D 1), in the range 0.16–5.5 and ϕ ≡ J v / k c < 3 , where J v is the permeation flux and k c is the mass-transfer coefficient of the salt for vanishing mass-transfer rates at impermeable walls. The main assumption of the model – the individual mass-transfer coefficients of the ions are independent of each other – appears to hold in a broad range of conditions, for single salt solutions. The model developed was expeditely applied to predict the concentration polarization in the nanofiltration of solutions containing Na +, Cl − and a dye 3− (experimental data of Bowen and Mohammad [AIChE J. 44 (8) (1998) 1799–1812]), and its predictions are in fair agreement with the predictions of the extended Nernst–Planck equations in the film layer of the “slowest” ion.

Factors influencing flux decline during nanofiltration of solutions containing dyes and salts

In this paper, factors influencing the flux decline of nanofiltration membranes for the treatment of dye bath wastewaters were investigated. Experiments were performed with synthetic wastewaters. Synthetic solutions were prepared in different dye and salt concentrations at laboratory conditions. Operating conditions including cross-flow velocity and pH were changed to observe the effects of interactions between NaCl, dyes and the membrane. Cake formation rate of dye molecules on membrane surface with time was investigated by using linearized forms of cake filtration equations. The results suggest that cake layer formation of dye molecules on membrane surface, especially at low salt concentrations was the principal cause of flux decline. Operating conditions had a strong influence on permeate flux. Effects of cross-flow velocity on permeate flux were more pronounced at low NaCl concentrations. Furthermore, the lowest permeate flux values were obtained at the alkaline conditions due to increased dye hydrophobicity at high pH values.

Nanofiltration of solutions containing surfactants—prediction of flux decline and modelling of mass transfer

Nanofiltration (NF) membranes are capable of separating surfactants from aqueous solutions with very high retentions. For the anionic surfactant sodium dodecylether sulfate (SDES) retentions exceeding 99.9% referring to the feed concentration have been found. The decrease of the permeate flux compared to the pure water flux in membrane filtration of solutions with surfactants is difficult to predict. In order to simplify membrane selection for a separation problem, an attempt is made to correlate flux decline due to adsorption to the contact angle measured on UF/NF membranes. The vague dependence shown cannot replace membrane screening. In the second part of the paper, a mass transfer model for NF of surfactant solutions based on the extended Nernst—Planck equation is presented. Convective mass transfer of the solute is included in the model, as well as the influence of adsorption of surfactant and concentration polarisation.