Flux decline in nanofiltration due to adsorption of dissolved organic compounds: model prediction of time dependency

Flux decline during nanofiltration of aqueous solutions containing dissolved organic compounds is mainly caused by adsorption of these compounds in the membrane pores and on the membrane surface. In this paper, flux decline is modeled by incorporating the loss of permeability due to adsorption in the Spiegler-Kedem equation. This results in a logarithmic relation between normalized flux decline and time until the adsorption equilibrium is reached and the flux reaches a constant value. In this way, the expected flux decline due to adsorption of organic compounds can be estimated. Two different parameters were used in the model: the time delay t(0), which corresponds to the time at which the adsorption process in the membrane pores and on the membrane surface sets in, and the corresponding reduction of free (pore) volume (b). Both parameters depend on the hydrophobicity of the compounds and on the feed concentration.     

Characteristics and performance of new nanoporous PEEKWC films

This rapid communication reports a summary of the key findings of preparation and characterization of new polymeric membranes for nanofiltration of organic compounds. A series of nanoporous asymmetric membranes of PEEKWC, a modified poly(etheretherketone) was prepared by means of the dry-wet phase inversion method. In particular, the type and concentration of internal non-solvent were varied in order to obtain membranes for nanofiltration. The optimization of these factors led to reproducible membranes, which were characterized for hydrophobicity, roughness, morphology, surface charge. The performance was studied by doing dead-end filtration experiments with aqueous solutions of uncharged and charged organic compounds. The new polymeric films exhibited interesting performance compared to commercial nanofiltration membranes in terms of retention and relative flux for positively charged organic compounds.     

Adsorption behavior of phenolic compounds onto polymeric adsorbents modified with 2-carboxybenzoyl groups

<正>Two hypercrosslinked polymeric adsorbents(ZH-01 and Amberlite XAD-4 resin) were employed to remove three kinds of phenolic compounds including phenol,4-nitrophenol and 2,4-dinitrophenol from aqueous solutions.The study was focused on the static equilibrium adsorption behavior,the column dynamic adsorption and desorption profiles.The Freundlich model gave a perfect fitting to the isotherm data.The adsorbing capacities for these three compounds on ZH-01 were higher than those on Amberlite XAD-4 within the temperature range 288-318 K,which was attributed to the large micropore area and 2-carboxybenzoyl functional groups on the network of ZH-01 resin.The adsorption for phenol and 4- nitrophenol on ZH-01 was a physical adsorption process,while for 2,4-dinitrophenol it was a coexistence process of physical adsorption and chemisorption's transitions.The column test showed the advantages of ZH-01 in the dynamic adsorption processes of phenolic compounds.Being used as the desorption reagent,sodium hydroxide solution showed an excellent performance.     

Adsorption of designed pyrimidine derivative ligands on an activated carbon for the removal of Cu(II) ions from aqueous solution

The adsorption of five Nalpha-substituted amino acids with a 5-nitroso-6-oxo pyrimidine as substituent on a commercial activated carbon (AC) has been studied in aqueous solution at several pH values. The adsorption processes of these organic compounds have been analyzed on the basis of the electrolytic behavior of the adsorbates. In all cases, the adsorption process is highly irreversible due to strong pi-pi interactions between the arene centers of the AC and the pyrimidine residue of the adsorbates. This interaction is consistent with XPS data and HOMO-LUMO theoretical calculations. The adsorption of these organic compounds provides a new route for the functionalization of the AC surface with carboxyl groups. In addition, the adsorption capacity of the AC/organic compound systems for Cu(II) ions in aqueous solution has been studied at different pH values. These systems show an increase of the adsorption capacity for Cu(II) compared to the AC, which is related to the AC functionalization with carboxyl groups due to the adsorbed organic compounds.     

Influence of soluble organic matter on cadmium mobility in model compounds and in soils.

The role of soluble organic matter on the mobility of cadmium in model compounds and soils is discussed. For model compounds, a simple predictive model is proposed to describe the competition between metal adsorption and the formation of complexes in the solution, using conditional constants to predict the adsorption behaviour of metallic cations in the presence of an organic ligand. Experimental results are presented using a clay mineral and ethylenediaminetetraacetic acid as organic ligand to assess this model for cadmium. Soil experiments are also presented to illustrate the influence of soluble complexing organic matter on the mobility of cadmium. In this instance, mobility is shown to depend on three interactive parameters: pH, oxidation-reduction reactions and formation of complexes.     

Removal of tritium labeled compounds from waste solutions by Sephadex, mineral coal and alumina

Organic waste solutions containing tritium present two problems: the high toxicity of the solvents and the radioactivity of tritium. In this paper the sorption behavior of the radioactivity of tritium from aqueous and organic waste solutions, containing tritium labeled compounds, was studied in Sephadex, mineral coal and alumina. It was found that the tritium retention depends on the composition of the waste solutions. Mineral coal was the best of the three materials studied to retain the organic compounds labeled with this radionuclide.     

A Study on the Adsorption Characteristics of Orthophosphates on Rutile-Type Titanium Dioxide in Aqueous Solutions

Rutile-type titanium dioxide is widely used as a pigment for paint, coating ink, paper, plastic products, and cards because of its very whiteness and outstanding hiding property. It has two weak properties to be improved, however, one being its coagulation in compounding and the other its decreasing whiteness owing to poor heat resistance. To solve these problems, a study on the treatment of titanium dioxide surfaces by adsorption technology has been performed. Experiments have been carried out to establish the adsorption isotherms of orthophosphates on titanium dioxide and to investigate the effect of organic solvents on adsorption. It is shown that the adsorption isotherms vary with pH. A Freundlich adsorption isotherm is suitable for the acidic and basic regions, while a Langmuir adsorption isotherm is suitable for the region of pH 5-8 where a maximum adsorption has been achieved. In aqueous solutions containing organic solvents, the adsorption was strongest with aqueous solutions containing 1 wt% toluene and weakest with those containing 1 wt% ethanol. Among the alcohols used, the adsorption was strongest with the aqueous solution containing 1 wt% butanol and weakest with that containing 1 wt% ethanol, thus showing a correlation with the molecular weight of the alcohol. Copyright 2001 Academic Press.     

Determination of pharmaceutically related compounds by suppressed ion chromatography: I. Effects of organic solvent on suppressor performance

This overall study aims to investigate gradient elution ion-exchange chromatography of pharmaceutically relevant compounds using universal nebulisation detectors, such as evaporative light scattering detection (ELSD). Addition of organic solvents to the eluent is necessary to minimise hydrophobic adsorption on the polymeric stationary phase and improve solubility of analytes. It is also necessary to de-salt the eluent prior to detection, and in this work, ion chromatography suppressors were used for this step. Such suppressors have been designed for aqueous eluents, so the purpose of the present study was to investigate the effects of methanol and acetonitrile on suppressor performance. Chemical and electrolytic suppressors were evaluated for baseline drift, noise and efficiency of suppression using aqueous/organic eluents containing up to 40% (v/v) methanol or acetonitrile. Chemical suppression of aqueous/organic eluents showed minimal noise levels, uniform low baseline and low gradient drift. Electrolytic suppression gave good performance, but with higher baseline conductivity levels and baseline drift than chemical suppression. The elevated baseline was found not to be caused by incomplete suppression of the eluent, but was attributed to chemical reactions involving the organic solvents and facilitated by high electric currents and heat generation. It was demonstrated that suppressed ion-exchange separation using a complex KOH elution profile could be coupled with ELSD, with the suppressor effectively de-salting the eluent, producing a stable baseline. Finally, complementary separation selectivity was demonstrated using a set of pharmaceutically related organic acids separated by reversed-phase and ion-exchange methods.     

Thermodynamics of the adsorption-accompanied formation of new-phase particles

A thermodynamic model was proposed to describe the process of the formation of inorganic nanoparticles in a solution containing well-adsorbable organic compounds. An equation for the critical nucleus size was derived and analyzed. It was shown that intense adsorption of organic compounds leads to a strong decrease in the critical nucleation size of the forming nanoparticles.     

EFFECT OF pH ON THE ADSORPTION OF p-AMINOBENZOIC ACID ON POLYSTYRENE-BASED ADSORBENTS

In the present study the adsorptive properties of p-aminobenzoic acid with hypercrosslinked and multi-functional polymeric adsorbents at different solution pHs were systematically investigated in accordance with the particular physicochemical characteristics of the aromatic amphoteric compound involving both Lewis acid and Lewis base functional groups. It was found that the equilibrium adsorption data of the three polymeric adsorbents fitted well in the Langmuir and Freundlich isotherm equations. Studies at various pH levels indicate that the capacity of the adsorbents for adsorption of the ionic forms of adsorbate is less than that for the corresponding neutral species. At pH 3.78, the adsorption capacities of the three adsorbents are the highest. Whereas the adsorption property of multi-functional polymeric adsorbent NJ-99 is the largest, which may be attributed to the strong hydrogen-bonding interaction between the amino groups on the resin and the carboxyl group of p-aminobenzoic acid. The trend of the adsorption capacities of the three adsorbents towards p-aminobenzoic acid with the solution pH is in accord with the dissociation curve of the neutral molecular p-aminobenzoic acid. The adsorption forces include π-π interaction, hydrogen-bonding interaction and electrostatic attraction or repulsion when there exist the molecular and ionic adsorbing species at different pHs in aqueous solution.     

Ion-exchange preconcentration and group separation of ionic and neutral organic compounds

In many cases sample pretreatment continues to be the most time-consuming and costly step in the analytical process. In the present work it is shown that macroporous ion-exchange resins of low exchange capacity can be used both to preconcentrate organic solutes from aqueous samples and to separate these solutes into groups. Thus, neutral and basic organic compounds are both taken up from aqueous solution by a very short column packed with a special cation-exchange resin. The neutral group of compounds is subsequently eluted with an organic solvent. The bases are then eluted 2 M methylamine in methanol. In a similar manner organic acids are concentrated on a special anion-exchange column. Extensive data are shown to demonstrate the efficiency of the preconcentration and group separation of neutral and basic compounds.     

Adsorption of aromatic compounds on porous covalent triazine-based framework

Covalent triazine-based frameworks (CTFs) are an emerging class of polymers whose adsorption properties of organic chemicals are not well understood. The main objective of this work was to evaluate combined effects of the functional groups of aromatic solutes and the triazine structure of a synthesized CTF on adsorption in aqueous solutions. Adsorption of the hydroxyl-, amino-, nitro-, and sulfonate-substituted monocyclic and bicyclic aromatic compounds was generally stronger than their non-substituted, nonpolar counterparts (benzene and naphthalene). When compared with Amberlite XAD-4 resin, one of the most common and widely used polymeric adsorbents, the CTF showed much stronger adsorption toward the polar and/or ionic compounds. To explain the adsorption enhancement of CTF, several specific, non-hydrophobic mechanisms were proposed, including hydrogen bonding (hydroxyl- and amino-substituted compounds), electrostatic attraction (anionized compounds), and π-π electron-donor-acceptor (EDA) interaction (nitroaromatic compounds) with the triazine structure of CTF. The hypothesized mechanisms were further supported by the observed pH dependence of adsorption. Resulting from size exclusion, adsorption of large-size dissolved humic acids on the homogeneous, nanopored (1.2 nm in size) CTF was negligible and did not affect adsorption of aromatic solutes. Additional advantages of fast adsorption/desorption kinetics and complete adsorption reversibility made CTF a superior adsorbent for aromatic compounds.     

Polymeric Membrane Nanofiltration and Its Application to Separations in the Chemical Industries

The application of membrane technology, particularly water-based nanofiltration, as a separation process in the chemical industries has increased tremendously in recent years. However, the use of membranes capable of molecular separation in non-aqueous systems (e.g. nanofiltration) is a relatively new and growing application of membrane technology. The main challenge in applying polymeric nanofiltration membranes to non-aqueous systems is that the polymers developed for water-based applications are not suitable. Polyimide is a particularly interesting polymer as it has excellent chemical resistance, and membranes produced from it provide desirable separation properties – i.e. economically viable flux and good separation of nanoscale molecules. Various research works have shown that commercial polyimide organic solvent nanofiltration (OSN) membranes, trademark STARMEM™, 1 are robust and suitable for performing molecular separations. This work will discuss in detail the use of STARMEM™ in a pharmaceutical application. The EIC-OSN process was developed for separating the enantiomers of chiral compounds in pharmaceutical applications. High optical purity (94.9%) of (S)-phenylethanol from rac-phenylethanol was achieved through the use of STARMEM™122. Process simulation of the ideal eutomer-distomer system predicted that the highest theoretical resolvability from this process would be 99.2%. Other application areas of OSN are varied, including purification and fractionation in the natural products industry, homogeneous catalyst recovery, monomer separation from oligomers, etc. Currently, OSN is used in a small number of processes including a very large petrochemical application, but it has the potential to be applied to a wide range of separations across the full spectrum of the chemical industries.     

Cellulose bearing covalently linked copper phthalocyanine trisulphonate as an adsorbent selective for polycyclic compounds and its use in studies of environmental mutagens and carcinogens.

A method useful as a preconcentration technique for isolating mutagens and carcinogens is described. Cotton bearing covalently linked copper phthalocyanine trisulphonate as ligand (blue cotton) can adsorb selectively compounds having three or more fused rings. The adsorption takes place in aqueous media, involving 1:1 complex formation between the ligand and the polycyclic compound. The desorption can be done by elution with organic solvents, most effectively with methanol containing ammonia. As many important environmental mutagens and carcinogens are polycyclics, this adsorption is useful as a means of extracting this class of materials from crude samples such as food, urine and river waters. The use of copper phthalocyanine as a ligand for chromatographic supports has recently been initiated, yielding promising results for the effective separation of polycyclic aromatic compounds from each other.     

Synthesis of cellulose/silica nanocomposite through electrostatic interaction to reinforce polysulfone membranes

Cellulose/nanosilica (CNS) nanocomposite fiber has been synthesized via a novel surface modification of cellulose and nanosilica, prepared from rice husk as a low cost natural source, by anionic and cationic surfactants through electrostatic interaction. The effect of the prepared nanocomposite on the structural, mechanical, thermal and morphological properties of polysulfone nanofiltration membranes was comprehensively studied. The scanning electron microscope image was used to investigate the relationship between solidity aspect and morphological properties qualitatively and quantitatively. From the results, the membrane with 0.25 wt% of CNS fiber shows the highest mechanical strength and thermal stability with a glass transition temperature of about 201 °C. It was found that an increase in the filler content increases the surface roughness of the membranes. The same behavior was observed for hydrophilicity based on contact angle measurements (from 78.7° to 61.5°). The adsorption of dye molecules during the filtration process was studied by batch adsorption experiments obeying Langmuir isotherm (R² > 0.91). For all samples of fabricated membranes, the rejection of Crystal Violet dye from aqueous solution was higher than 80%.     

Polanyi-based models for the adsorption of naphthalene from aqueous solutions onto nonpolar polymeric adsorbents

In this research, naphthalene was adopted as the representative model compound of PAHs, and static adsorption of naphthalene from aqueous solution onto three commercial polymeric adsorbents with different pore structure was investigated. Nonlinear isotherms models, i.e., Freundlich, Langmuir, and Polanyi-Dubinin-Manes (PDM) models were tested to fit experimental data, and the experimental data were found to fit well by the PDM model. Through both isotherm modeling and constructing "characteristic curve," Polanyi theory was useful to describe the adsorption process of naphthalene by polymeric adsorbents, providing evidence that a micropore filling phenomenon was involved during the adsorption process. In addition, a good linear correlation was obtained between the naphthalene adsorption capacities and the micropore volume of adsorbents (Vmicro), whereas no linear relationship was found between the naphthalene adsorption capacities and the specific surface area of adsorbents. Based on the PDM model, the micropore volumes of adsorbents was introduced to normalize the equilibrium adsorbed volume (qv), plots of qv/V(micro) vs adsorption potential density for naphthalene on three different polymeric adsorbents were collapsed to a single correlation curve, which would be of great benefit to predict the adsorption capacity of adsorbent for the purpose of adsorption engineering design.     

A study of selected phytoestrogens retention by reverse osmosis and nanofiltration membranes - the role of fouling and scaling

Fouling and scaling are common phenomena that accompany membrane filtration and are caused by the presence of organic and inorganic matter in water, which may affect the removal of low-molecular mass organic micropollutants. Comparative filtration of deionized water containing selected phytoestrogens (biochanin A, daidzein, genistein, and coumestrol) was carried out using one new membrane and one contaminated with organic or inorganic matter. Two commercial Osmonics DS membranes were selected for the research, reverse osmosis DS3SE and nanofiltration DS5DK. Filtration was carried out in the dead-end mode. Higher removal of phytoestrogens was caused by reverse osmosis and retention depended on the molar mass of the compound. The decrease in membrane efficiency associated with fouling or scaling brings about an increase in the retention coefficient of phytoestrogens during both reverse osmosis and nanofiltration. The highest increase in phytoestrogen retention was found for the nanofiltraton membrane which was more susceptible to fouling than the osmotic one. This confirms the effect of membrane porosity on the phenomenon studied. The increase in micropollutants removal observed after fouling or scaling was caused by the modification of the membrane surface, hindered diffusion of the compound, and intensified or limited adsorption of micropollutants on the membrane surface.     

Relation between membrane characteristics and performance in nanofiltration

The performance of commercial membranes during nanofiltration of aqueous solutions containing dissolved uncharged or charged organic components, was studied on the basis of membrane characteristics by means of multiple linear regression.

The membrane characteristics studied were surface hydrophobicity, surface roughness, surface charge, molecular weight cut-off (MWCO), permeability and porosity of the top layer (expressed as the volume fraction of small and large pores, determined by Positron Annihilation Lifetime Spectroscopy). Filtration and adsorption experiments were performed in the presence of various components, which differ in molecular mass, hydrophobicity and (in the case of charged organic components) in charge.

It was concluded that in order to minimize fouling, the membrane should have a low volume fraction of small pores in the top layer. When the organic components are charged, a membrane with a large surface charge and a high hydrophilicity is also favourable. Not only the membrane, but also the feed characteristics have an influence on fouling: the best results during nanofiltration of dissolved uncharged or charged components were obtained with hydrophilic or negatively charged components, respectively. Dissolved organic components were the best retained by membranes with a low MWCO. In addition, uncharged organic components should be hydrophilic and small to obtain a high retention and minimal flux decline, while the interplay between membrane and component charge is crucial during filtration of dissolved charged organic components.     

Adsorption of cationic surfactants and subsequent adsolubilization of organic compounds onto cellulose fibers

Cationic surfactants with different hydrophobic chain length were adsorbed onto cellulose fibers in an aqueous medium. The adsorption isotherms exhibited three characteristic regions which were interpreted in terms of the mode of aggregation of the surfactant molecules at the solid–liquid interface. The hydrophobic layers were used as a reservoir to trap various slightly water soluble organic molecules. A quantitative study of these phenomena suggested typical partition behavior of the organic solutes between the aqueous phase and the surfactant layer. The surfactant chain length (from C12 to C18) was shown to play an important role in terms of the capacity to retain the organic solute and the capacity increased with the number of carbon atoms.     

Nanofiltration and ultrafiltration of endocrine disrupting compounds, pharmaceuticals and personal care products

Reports of endocrine disrupting compounds (EDCs) and pharmaceuticals and personal care products (PPCPs) have raised substantial concern among important potable drinking water and reclaimed wastewater quality issues. Our study investigates the removal of EDC/PPCPs of 52 compounds having different physico-chemical properties (e.g., size, hydrophobicity, and polarity) by nanofiltration (NF) and ultrafiltration (UF) membranes using a dead-end stirred-cell filtration system. EDC/PPCPs were applied to the membrane in one model water and three natural waters. Experiments were performed at environmentally relevant initial EDC/PPCP concentrations ranging typically from 2 to <250 ng/L. EDC/PPCP retention was quantified by liquid and gas chromatography with mass spectroscopy–mass spectroscopy. A general separation trend due to hydrophobic adsorption as a function of octanol–water partition coefficient was observed between the hydrophobic compounds and porous hydrophobic membrane during the membrane filtration in unequilibrium conditions. The results showed that the NF membrane retained many EDC/PPCPs due to both hydrophobic adsorption and size exclusion, while the UF membrane retained typically hydrophobic EDC/PPCPs due mainly to hydrophobic adsorption. However, the transport phenomenon associated with adsorption may depend on water chemistry conditions and membrane material.