Humic Acid Filtration Research Articles

Reorganization of the surface geometry of hollow-fiber membranes using dip-coating and vapor-induced phase separation

Phase separation is one of the major methods to prepare a hollow-fiber membrane in industry. Despite the strong demands for the control of the inner membrane structure and surface geometry, it is still difficult to obtain the desired membrane structure and surface geometry simply by phase separation. In this work, we employed thermally induced phase separation, dip-coating and vapor-induced phase separation to prepare a hollow-fiber membrane with the controlled surface geometry and homogeneous inner structure, which led to high water permeability and high mechanical strength. First a poly(vinylidene fluoride) (PVDF) hollow-fiber membrane was prepared via thermally induced phase separation (TIPS) and then the outer surface of a membrane was dip-coated with another PVDF solution, followed by vapor induced phase separation (VIPS) to reorganize the outer surface of the membrane. The dip-coating and the VIPS treatment produced a highly porous mesh-like layer on the membrane surface. Filtration experiments using nanospheres and protein solutions revealed that the newly-formed layer served as a “separation layer” to determine the separation properties. The membrane pore-size was controlled to some extent by the conditions for the dip-coating and the VIPS treatment. The surface-reorganized membrane kept its intrinsic high water permeability and mechanical strength. In addition, the membrane exhibited improved low-fouling properties in the filtration of humic acid and protein solutions.

Fouling mitigation in humic acid ultrafiltration using polysulfone/SAPO-34 mixed matrix membrane

Although ultrafiltration (UF) membranes are applicable in wastewater and water treatment, most UF membranes are hydrophobic and susceptible to severe fouling by natural organic matter. In this work, polysulfone (PSf) membrane was blended with silicaluminophosphate (SAPO) nanoparticles, SAPO-34, to study the effect of SAPO-34 incorporation in humic acid (HA) fouling mitigation. The casting solution was prepared by blending 5-20 wt% of SAPO-34 nanoparticles into the mixture of PSf, 1-methyl-2-pyrrolidinone and polyvinyl alcohol at 75 °C. All membrane samples were then prepared using the phase inversion method. Blending SAPO-34 zeolite into PSf membranes caused augmentation in surface hydrophilicity and pore size, leading to higher water permeation. In the HA filtration test, mixed matrix membranes (MMMs) with SAPO-34 zeolite showed reduced HA fouling initiated from pore blocking. The MMM with 20 wt% SAPO-34 loading exhibited the highest increment of water permeation (83%) and maintained about 75% of permeate flux after 2.5 h. However, the SAPO-34 fillers agglomerated in the PSf matrix and induced macrovoid formation on the membrane surface when excessive zeolite was added.

Humic acid removal and easy-cleanability using temperature-responsive ZrO2 tubular membranes grafted with poly(N-isopropylacrylamide) brush chains

New poly(N-isopropylacrylamide) brushes grafted with ZrO2 (PNIPAAm-g-ZrO2) composite membranes, which had been prepared in our laboratory, were used for humic acid (HA) removal. We found that the fluxes associated with such membranes, when compared to those obtained from unmodified ZrO2 membranes, declined slightly at both 25 °C and 35 °C. The PNIPAAm-g-ZrO2 membrane achieved a high rejection, of 98.0%, at a suitable steady flux of 111.9 L m−2 h−1 at 25 °C. This membrane exhibited good anti-fouling properties as well as improved membrane performance during filtration of HA. The important role of pH and Ca2+ concentration in HA removal was also investigated. Lower adsorption fouling and a higher rejection were obtained at higher pH levels. The Ca2+ ions reduced the electrostatic exclusion and played a cross-linking role between HA and the PNIPAAm-g-ZrO2 membrane surface. Fouling was severe in the presence of Ca2+. These tests led to the development of an environment-friendly membrane cleaning method, by means of temperature-change water elution around LCST of PNIPAAm-brushes. After the alternate temperature-change (25 °C/35 °C) cleaning, a flux recovery of 98.2% was obtained for the PNIPAAm-g-ZrO2 membrane. Moreover, after four repeated experiments, the anti-fouling and easy-cleaning properties were still maintained. It is implied that PNIPAAm-brushes were firmly “stuck” to the membrane surface, and could not easily be removed by water cleaning or HA filtration. The PNIPAAm-g-ZrO2 membranes exhibited good stability and great potential for HA removal

Studies on the surface properties of mixed‐matrix membrane and its antifouling properties for humic acid removal

AbstractA major factor limiting the use of ultrafiltration (UF) membrane in water treatment process is the membrane fouling by natural organic matter such as humic acid (HA). In this work, neat PVDF and PVDF/TiO2 mixed‐matrix membranes were prepared and compared in terms of their antifouling properties. Two commercial types of TiO2 namely PC‐20 and P25 were embedded to prepare the mixed matrix membranes via in situ colloidal precipitation method. The contact angles for the mixed‐matrix membranes were slightly reduced while the zeta potential was increased (more negatively charged) compared with the neat membrane. Filtration of HA with the presence of Ca2+ demonstrated that mixed‐matrix membrane could significantly mitigate the fouling tendency compared with the neat membrane with flux ratio (J/J0) of 0.65, 0.70, and 0.82 for neat PVDF membrane, PVDF/TiO2 mixed‐matrix membrane embedded with P25 and PC‐20, respectively. PC‐20 with higher anatase polymorphs exhibited better antifouling properties due to its hydrophilicity nature. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Ultrafiltration of humic acid solutions through unmodified and surface functionalized low-fouling polyethersulfone membranes – Effects of feed properties, molecular weight cut-off and membrane chemistry on fouling behavior and cleanability

In the present work, polyethersulfone (PES) ultrafiltration (UF) membranes with molecular weight cut-off (MWCO) from 5 to 300 kDa were analyzed with respect to fouling during filtration of humic acid (HA) model solution. The impact of MWCO and prefiltration of the feed solution on the fouling behavior of these membranes is presented. Moreover, an UV-initiated graft copolymerization of poly(ethylene glycol) methacrylate (PEGMA) onto PES membranes was performed and the effect of the applied modification was examined in terms of MWCO and water permeability changes as well as performance during filtration of HA. Moreover, the fouling behavior and principles during the filtration were compared. Membranes with higher MWCO exhibited stronger flux decline during UF but better cleanability. Prefiltration through 0.45 μm filter improved the membrane performance during filtration of HA but the efficiency of physical cleaning was deteriorated. The applied modification improved the membrane performance during ultrafiltration and facilitated the physical cleaning. This work contributes to a better understanding of the relationships between membrane MWCO, solute size and size distribution, membrane surface chemistry and membrane fouling.

Humic acid removal by deep-bed filtration and by UF membranes

Currently, rational choice of either the conventional filtration method or of the more expensive membrane pretreatment technique is hampered by the lack of sufficient experience and the paucity of design data. The main objective of the present study was to compare water qualities achieved by filtration of humic acid feed solutions by the two alternate pretreatment techniques under well controlled conditions. Deep-bed filtration of humic acid in the presence of the coagulant ferric chloride was performed in a continuous flow column (98 mm diameter, 750 mm long) filled with silica sand grains of 0.65 mm. Two PVDF tubular UF membranes were tested — a large pore size membrane of 100 kDa and a small pore size membrane of 20 kDa. In both systems, commercial humic acid solutions were tested at feed concentrations covering the range of 5–50 ppm. Water quality was monitored by TOC and turbidity measurements. Meaningful interpretation of conflicting water quality data was obtained by interlinking TOC and turbidity removal measurements with the particle size distribution of the feed solution. One of the main results of this study is that in the case of humic acid removal, both pretreatment techniques appear to be capable of providing substantially identical product qualities.

Coupled effects of internal concentration polarization and fouling on flux behavior of forward osmosis membranes during humic acid filtration

Forward osmosis (FO) is attracting increasing interest for its potential applications in water and wastewater treatment and desalination. One of the major drawbacks of FO is internal concentration polarization (ICP), which significantly limits the FO flux efficiency. In addition, FO membrane flux can be adversely affected by membrane fouling. The effects of ICP and fouling on FO flux behavior were systematically investigated in the current study. Both theoretical model and experimental results demonstrated that the FO flux was highly non-linear with respect to the apparent driving force (the concentration difference between the draw solution and the feed water) as a result of ICP. ICP played a dominant role on FO flux behavior at greater draw solution concentrations and/or greater membrane fluxes due to the exponential dependence of ICP on flux level. Compared to the active layer facing draw solution (AL-facing-DS) configuration, more severe ICP was observed when the membrane active layer faced the feed water (AL-facing-FW) as a result of dilutive ICP in the FO support layer. Interestingly, the AL-facing-FW configuration showed remarkable flux stability against both dilution of the bulk draw solution and membrane fouling. In this configuration, any attempt to reduce membrane flux was compensated by a reduced level of ICP. The net result was only a marginal flux reduction. In addition, foulant deposition was insignificant in this configuration. Thus, the AL-facing-FW configuration enjoyed inherently stable flux, however, at the expense of severer initial ICP. In contrast, the AL-facing-DS configuration suffered severe flux reduction as porous membrane support faced the humic acid containing feed water. The flux loss in this configuration was likely due to the combined effects of (1) the internal clogging of the FO support structure as well as (2) the resulting enhanced ICP in the support layer. The latter was caused by reduced porosity and reduced mass transfer coefficient of the support. The pore clogging enhanced ICP mechanism probably played a dominant role in FO flux reduction at higher flux levels. To the authors’ best knowledge, this is the first study to systematically demonstrate the coupled effects of ICP and fouling on the FO flux behavior.

Hollow fiber dead-end ultrafiltration: Axial transport variations during humic acid filtration

This paper describes the dead-end ultrafiltration of humic substances with hollow fiber modules. We show that the resistance of a fouling layer decreases with increasing flux. The lower resistance of the fouling layer is caused by a loss in retention of the membrane for humic acid at these higher fluxes. The ideal cake filtration model does not fit the filtration resistance appropriately even considering a retention smaller than unity. The extended model falls short since it does not incorporate an axially dependent retention. Using five small modules in series, as well as a specially designed flat sheet cell, we prove that both the flux and retention in a fiber module is a strong function of the axial coordinate (fiber length). The experimental results with the small modules gave a complete picture of the axial features in dead-end filtration of humic substances with fibers, and the results can be used to explain the observed behavior in the large module. The main contributor of the axial phenomena is hypothesized to be charge interactions between the membrane and the solute combined with the small size of the foulants causing diffusive concentration gradients of the humic acids.

Ultrafiltration of protein and humic substances: effect of solution chemistry on fouling and flux decline

The rate and extent of adsorption of a protein and a humic acid onto membranes was measured at varying conditions of pH and ionic strength. The resistance-in-series approach was used to calculate reversible and irreversible fouling resistances, which were then compared for static (no flow) and dynamic runs in order to determine the effect of convective flow and electrostatic interactions on fouling behavior. Although convective forces tended to increase the amount of material accumulated near the membrane surface, electrostatic interactions played a stronger role, as evident in the irreversible adsorption results for the static and dynamic cases. Electrostatic interactions affected reversible and irreversible resistances. Both resistances were higher at the isoelectric point (iep) of the protein and decreased at higher pH values. Humic acid adsorption decreased as pH was increased from 4.7 to 10. Humic acid filtration resulted in a higher resistance per unit mass than protein filtration.

Fouling of ceramic capillary filters in vacuum filtration of humic acid

In this study the fouling phenomenon of ceramic capillary filters in the filtration of humic acid was investigated by experimental work and by modelling. The fouling of humic acid on ceramic capillary filters was evident, and in most of the cases, irreversible. In some filtrations the fouling did not seem to be permanent, it looked more like a gel layer on the filter. The effect of pH was clear: the filtrate fluxes decreased to almost zero at pH 4 the charge of the filter medium is positive and the humic acid molecules are negative. The presence of ferric ions decreased the filtrate flux of humic acid faster than in the absence of them. The behaviour of humic acid was compared to other chelating agents. At pH 4 the difference between other chelating agents and humic acid was remarkable: the fluxes when filtering the others remained good during the filtration, but for humic acid the flux decreased very fast to close to zero. Elementary analysis showed that the inorganic elements did not adsorb on the filter surface. The fouling of humic acid fitted best to the standard blocking model, which means that the molecules fouled the filters by direct adsorption on the surface and the walls of the pores.

Complex formation between humic acid and clays as revealed by gel filtration and infrared spectroscopy

Complex formation between pure silicate clay minerals and humic acid (HA) extracted from a Eustis loamy sand (Psammentic Paleudult) was investigated by i.r. spectroscopy and Sephadex G50 and G25 gel filtration. Gel filtration of humic acid after treatment with clay resulted in the separation of normal amounts of high m.w., but reduced concentrations of low m.w. HA. This observation suggested that only the low m.w. fraction of HA was adsorbed by silicate clays. As compared to i.r. spectrograms of original HA, humic acid remaining in solution after interaction with clay exhibited i.r. curves with new bands at 940 and 840 cm −1 for O-Al-OH vibrations and increased absorption in the 1050, 1400 and 1620 cm −1 regions for Si-O and COO − stretching vibrations, respectively. The absorption band for Si-O was retained by the i.r. spectrum of the high m.w. fraction of HA, and the O-Al-OH absorption by that of low m.w. HA. The latter showed in addition the strong band for COO − stretching vibrations.

Adsorption Effects in Gel Filtration of Humic Acid.

Adsorption Effects in Gel Filtration of Humic Acid.