D. Wilms

Influence of molecular size, polarity and charge on the retention of organic molecules by nanofiltration

Because of the growing interest in nanofiltration for industrial use, a better insight in the retention mechanisms in nanofiltration is needed, which will make it possible to understand membrane performances for specific applications. In this paper, the retention of a series of organic molecules by four nanofiltration membranes was studied. The membranes that were used are NF70 (Dow/FilmTec), NTR 7450 (Nitto-Denko), UTC-20 (Toray Ind.) and Zirfon® (VITO). In order to correlate the retention with the size of the molecule, which is the main factor that determines the retention, use was made of different parameters for the molecular size: molecular weight, the Stokes diameter, the equivalent molar diameter, and a diameter obtained with energy minimisation calculations. For each size parameter, the correlation with retention in nanofiltration experiments was calculated. For the Zirfon® membrane, retentions were too low to obtain a good correlation. For the three other membranes, a good correlation with retention was found for each of the size parameters. Two other factors were found to have an influence on retention of organic molecules: the polarity of the molecule, and the charge of the molecule. The importance of these factors depends on the molecules as well as on the type of membrane.

Influence of ion size and charge in nanofiltration

Three commercial nanofiltration membranes were studied: two negatively charged (NF 40 and NTR 7450) and one positively charged membrane (UTC 20). The membrane pore radii were estimated using filtration experiments with uncharged solutes. For the analysis of the results the Spiegler–Kedem equation and the steric hindrance pore model were used. Salt retention measurements were carried out for NaCl, MgCl2, Na2SO4 and MgSO4 at different concentrations. The experimental results were evaluated with the Donnan exclusion theory. For both NF 40 and UTC 20 the results could not be ascribed to charge effects alone and the retention sequence of the salt solutions was found inversely proportional to the salt diffusion coefficients in water. For the NTR 7450 membrane, which had larger pores, the Donnan exclusion theory could be used and salt retention was mainly determined by charge effects. It was also shown that transport by diffusion could not be neglected.

Nanofiltration as a treatment method for the removal of pesticides from ground waters

Abstract A major problem in the drinking water production is the presence of pesticides in raw water, not only in surface waters, but also in ground waters. Concentrations of several μg/l have been found. Traditionally, pesticides are removed using granular or powdered activated carbon, which is an expensive process. The cost is enhanced by competition of pesticides and natural organic matter (NOM) for adsorption sites on the activated carbon. Nanofiltration (NF) can be a valuable alternative for the removal of pesticides. Moreover, pesticides and hardness can be removed in one step. In this paper, the retention of four pesticides (atrazine, simazine, diuron, isoproturon) was determined experimentally for four NF membranes: NF-70 (Dow/Film Tec), NF-45 (Dow/Film Tec), UTC-20 (Toray Inc.) and NTR-7450 (Nitto-Denko). The experimental retentions indicate that NF-70 is a suitable membrane for removal of pesticides. The experimental retentions were related to structural properties of the molecules. The size of the molecule is the most important parameter, and may be represented by molecular weight, or more accurately by a molecular diameter calculated from the molecular structure. Furthermore, a comparison with retentions of a series of saccharides confirms the earlier found effect of the dipole moment: a higher dipole moment results in a lower retention. No effect of the pesticide concentration was found. The matrix of the ground waters caused an increase of the pesticide retention, together with a decrease of the water flux through the membrane.

Mechanisms of retention and flux decline for the nanofiltration of dye baths from the textile industry

Removal of dye compounds from colour baths used in the textile industry is a possible application of nanofiltration. However, the mechanisms involved in this process are not clearly understood and the practical application of the process is facing many problems such as fouling and flux decline. The mechanisms of retention and flux decline were examined using two different approaches. Firstly, synthetic dye baths were prepared according to manufacterers recipes. Retention of two reactive dyes (reactive blue 2 and reactive orange 16) was studied in separate baths and with different concentrations of Na2SO4, Na2CO3, NaOH and a surfactant. Different nanofiltration membranes (UTC-60, NF70 and NTR 7450) were used. The water flux in each of the experiments was monitored. It was found that the retention of ions decreased with the ion concentration due to a decrease of the Donnan potential. The retention of the dyes was high and was not influenced by the dye concentration or the ionic strength. The water flux was dependent of the ion concentration in the feed solution: high ion concentrations caused a dramatic decrease of the water flux. The dye concentration in the bath was found to have only a minor influence, whereas surfactants did not change water flux or dye retention. A theoretical explanation for these effects is given. The phenomenon of flux decline limits strongly the applicability of nanofiltration for direct treatment of dye baths. In a second step, an industrial wastewater from a textile factory was treated biologically in an active sludge system. The effluent was used as feed for nanofiltration with the same membranes as in the first step. The overall results for these experiments were satisfactory. The ion concentration was much lower than in the earlier experiments due to mixing of different feed streams. Therefore, the water fluxes were not considerably lower compared with the clean water fluxes. The retentions were sufficiently high to make recirculation of the treated water possible, thereby providing a considerable saving of water.

Removal of hardness from groundwater by nanofiltration

In this paper the softening of groundwater with drinking-water quality using nanofiltration is studied. Experiments were carried out with three commercial nanofiltration membranes. The UTC 20 membrane was chosen for further experiments. The influence of water temperature was studied on both permeate flux and ion retention. Also, the effect of a higher recovery was studied: experimental and predicted values were compared for recoveries of 0 to 70%. Finally, experiments were carried out to study membrane fouling. The use of hydrochloric and sulphuric acid was compared and the effect of an ultrafiltration pretreatment on the water flux was examined.

A comparison of models to describe the maximal retention of organic molecules in nanofiltration

Nanofiltration is used in a growing number of applications for the treatment of drinking water, wastewater, and process water. Trial-and-error is generally used to test the applicability of nanofiltration and to select the membranes. In particular for organic molecules, a model that describes retention as a function of molecular parameters and membrane characteristics has not yet been established. In this paper four models for maximal retention, represented by the reflection coefficient, were compared: the steric hindrance pore model, the model of Zeman and Wales, the log-normal model, and an adapted version of the log-normal model. The calculated results were compared to reflection coefficients determined experimentally for a broad range of relatively small organic molecules. Each of the models yielded acceptable results, although the steric hindrance pore model and the model of Zeman and Wales are based on a somewhat idealized view of membrane structure. The log-normal model calculates reflection coefficients from a distribution of pore sizes. The adapted log-normal model also includes hydrodynamic lag, caused by sterical hindrance in the membrane pores. It was found that this effect is very small. The log-normal model appeared to be most useful to predict reflection coefficients in practical applications.

Recovery of silver by crystallization of silver carbonate in a fluidized-bed reactor

Heavy metals can be recovered from wastewaters by growing crystals of metal carbonate in a fluidized-bed reactor. Optimal conditions for crystallizing silver carbonate have been investigated on a laboratory scale pellet reactor initially seeded with quartz sand. With an effluent-pH of 10.2, a CT/Ag feeding ratio of 3 mol/mol, an Ag-load of less than 2 kg Ag per square meter reactor cross-section per hour and a hydraulic load of 45 m/h, the effluent silver concentration was below 8 mg/l. From chemical analysis it was found that the pellets, apart from the sand fraction, consisted (for more than 99%) of pure Ag2CO1. It can be concluded that the crystallization technique is a valuable alternative for the classical methods of silver recovery from wastewater.

Retention Mechanisms in Nanofiltration

Nanofiltration is a pressure-driven membrane separation process that falls in between reverse osmosis and ultrafiltration in its separation characteristics. In general, even low molecular weight organics (> 200 g/mol) and multivalent ions are highly retained, while monovalent ions are retained to a smaller extent. Nanofiltration can be used as well for the production of drinking water, as for the treatment of process and waste waters. The mechanism for separation can be explained in terms of charge effects and/or size effects. Experimental evidence is given for several salt solutions and saccharide solutions. Three commercial nanofiltration membranes are used. The results are interpreted qualitatively by the Donnan exclusion mechanism, which takes the charge effects into account. The steric hindrance pore model is used to determine the membrane pore radii. To predict the results for multicomponent mixtures quantitatively the extended Nernst-Planck model could be used.

Thermodynamics and composition of chlorine hydrate

Abstract The HWG and HIG equilibria for the system chlorine-water-chlorine hydrate have been measured for pressures below 2 atm (Eq. 1 and 2). The chlorine concentration and the density of the aqueous solution, in equilibrium with the hydrate along the HWG line, have also been determined (Eq. 3 and 4). The composition of chlorine hydrate has been calculated in three independent ways: ( a ) from the slopes of the HWG and HIG lines in the lower invariant point, ( b ) using the Miller-Strong method with NaCl in the temperature interval 4 to 9°C, ( c ) by a dilatometric method using the measured density of the hydrate. There is a good concordance among the three sets of results.

Nanofiltration for Removal of Organic Substances from Waste Water

The possibility of using nanofiltration as a treatment method for waste water in the textile industry was investigated. An effluent from a textile factory, after biological treatment and sand filtration, was taken as feed solution for a lab-scale nanoflltration unit. The retention of organic and inorganic compounds was tested for three nanoflltration membranes: NF70 (FilmTec), Nitto NTR 7450 (Nitto-Denko) and UTC-20 (Toray). As expected, considerable differences in retention were observed. The highest retentions were obtained for NF70. For UTC-20 and Nitto 7450, retentions were slightly lower. Colour removal was nearly complete for the three membranes.