Assessment of cross-flow filtration for the size fractionation of freshwater colloids and particles

This research has evaluated the ability of cross-flow filtration (CFF) to perform correct size fractionation of natural aquatic colloids (materials from 1 nm to 1 μm in size) and particles (>1 μm) using scanning electron microscopy (SEM) combined with atomic force microscopy (AFM). SEM provided very clear images at high lateral resolution (ca. 2-5 nm), whereas AFM offered extremely low resolution limits (sub-nanometer) and was consequently most useful for studying very small material. Both SEM and AFM were consistent in demonstrating the presence of colloids smaller than 50 nm in all fractions including the retentates (i.e. the fractions retained by the CFF membrane), showing that CFF fractionation is not fully quantitative and not based on size alone. This finding suggests that previous studies that investigated trace element partitioning between dissolved, colloidal and particulate fractions using CFF may need to be re-visited as the importance of particles and large colloids may have been over-estimated. The observation that ultra-fine colloidal material strongly interacted with and completely coated a mica substrate to form a thin film has important potential implications for our understanding of the behaviour of trace elements in aquatic systems. The results suggest that clean, ‘pure’ surfaces are unlikely to exist in the natural environment. As surface binding of trace elements is of great importance, the nature of this sorbed layer may dominate trace element partitioning, rather than the nature of the bulk particle.     

Metal associations to microparticles, nanocolloids and macromolecules in compost leachates: Size characterization by asymmetrical flow field-flow fractionation coupled to ICP-MS

A methodological approach based on the size characterization of environmental microparticles (size larger than 1 μm), nanocolloids (1 μm to 15 nm) and macromolecules (lower than 1000 kDa) by asymmetrical flow field-flow fractionation (AsFlFFF), taking advantage of both normal and steric elution modes, is presented. The procedure was optimized to minimize the potential alteration of the size distribution and metal associations of the species characterized. Prior to separation by AsFlFFF, samples are subjected to gravitational settling of the solid suspension, followed by a centrifugation of the settled sample. The comparison between the fractograms of the settled and the centrifuged samples allows the characterization of the microparticles, which are eluted in steric mode in the AsFlFFF system. The characterization of nanocolloids and macromolecules is carried out on the centrifuged sample by applying different operational conditions under normal mode in the AsFlFFF system. A comparison with the conventional frontal filtration through 0.45 μm pore size membranes have shown that filtration removes particles below their nominal pore size, modifying the size distribution of the samples respect to the centrifugation. The methodology proposed has been applied to the size characterization of compost leachates. The contribution of these three differentiated fractions to the mobilization of metals has been determined by coupling the AsFlFFF system to an inductively coupled plasma mass spectrometer (ICP-MS).     

Preparation and application of zeolite/ceramic microfiltration membranes for treatment of oil contaminated water

NaA zeolite microfiltration (MF) membranes were prepared on α-Al₂O₃ tube by in situ hydrothermal synthesis method and investigated for water separation and recovery from oily water. NaA/α-Al₂O₃ MF membranes with average inter-particle pore sizes of 1.2 μm, 0.4 μm and 0.2 μm were prepared. The membranes were characterized by scanning electron microscope (SEM) and the inter-particle pore size distribution (PSD) was determined by gas bubble pressure method. Membranes with pore sizes of 1.2 μm (NaA1) and 0.4 μm (NaA2) were used to treat an oil-in-water emulsion containing 100 mg/L oil. Better than 99% oil rejection was obtained and water containing less than 1 mg/L oil was produced at 85 L m⁻² h⁻¹ by NaA1 at a membrane pressure of 50 kPa. Consistent membrane performance was maintained by a regeneration regime consisting of frequent backwash with hot water and alkali solution.     

Atomic Force Microscope Studies of Membranes: Surface Pore Structures of Diaflo Ultrafiltration Membranes

Noncontact atomic force microscopy (AFM) has been used to investigate the surface pore structure of eight Diaflo ultrafiltration membranes covering a range of nominal molecular weight cutoff (MWCO) from 3000 to 300,000 and manufactured from three different polymer types. Excellent high resolution images were obtained. Analysis of the pore images gave quantitative information on the surface pore structure, in particular the pore size distribution and surface roughness. Such data is compared to that obtained by other techniques. Noncontact AFM is a facile and informative means of studying the surface structure of porous materials such as synthetic membranes.     

Colloidal transport of uranium in soil: Size fractionation and characterization by field-flow fractionation–multi-detection

The aim of this study was to characterize colloids associated with uranium by using an on-line fractionation/multi-detection technique based on asymmetrical flow field-flow fractionation (As-Fl-FFF) hyphenated with UV detector, multi angle laser light scattering (MALLS) and inductively coupling plasma-mass spectrometry (ICP-MS). Moreover, thanks to the As-Fl-FFF, the different colloidal fractions were collected and characterized by a total organic carbon analyzer (TOC). Thus it is possible to determine the nature (organic or inorganic colloids), molar mass, size (gyration and hydrodynamic radii) and quantitative uranium distribution over the whole colloidal phase. In the case of the site studied, two populations are highlighted. The first population corresponds to humic-like substances with a molar mass of (1500 ± 300) g mol⁻¹ and a hydrodynamic diameter of (2.0 ± 0.2) nm. The second one has been identified as a mix of carbonated nanoparticles or clays with organic particles (aggregates and/or coating of the inorganic particles) with a size range hydrodynamic diameter between 30 and 450 nm. Each population is implied in the colloidal transport of uranium: maximum 1% of the uranium content in soil leachate is transported by the colloids in the site studied, according to the depth in the soil. Indeed, humic substances are the main responsible of this transport in sub-surface conditions whereas nanoparticles drive the phenomenon in depth conditions.     

Detailed analysis of membrane adsorber pore structure and protein binding by advanced microscopy

Commercial Sartobind^® porous cation exchanger membranes, based on stabilized regenerated cellulose and with sulfonic acid (S) or carboxylic acid groups (C), were analysed with respect to their pore structure in dry, slightly swollen and wet state by three microscopic methods, conventional scanning electron microscopy (SEM), environmental SEM (ESEM), and confocal laser scanning microscopy (CLSM). The dehydration behaviour of the membranes was in situ observed at varied vapour pressure in the chamber of the ESEM, indicating some deformations of the macropore structure (largest pore diameters up to 20 μm) and significant changes in dimension and mobility of smaller cellulose fibers within these macropores, both as function of water content of the membrane. The binding of mono-Cy5-labelled lysozyme inside fluoresceine-labelled and unlabelled Sartobind^® membranes was monitored by CLSM. The characteristic fluorescence intensity distributions in areas of (146 μm × 146 μm) indicated that protein binding takes place predominately in a layer which is anchored to a fine cellulose fiber network and, to a lower degree, directly to thick cellulose fibers. Due to the limited thickness of this binding layer, a significant fraction of the macropores remained free of protein. Protein binding as function of concentration and incubation times was also monitored by CLSM and discussed related to the binding isotherms for the membrane Sartobind^® S and C. Further, a flow-through cell for the in situ monitoring with CLSM of protein binding during the binding step was built, and the results obtained for binding of lysozyme in membranes Sartobind^® S indicate this experiment can give very important information on the dynamic behaviour of porous membrane adsorbers during separation: the lateral microscopic resolution in the x, y plane enables the identification of different breakthrough times as function of the location (pore structure), and this information can help to explain possible reasons for axial dispersion (in z-direction) observed in breakthrough analyses of the same separation in a chromatography system. The combination of advanced microscopy with detailed investigations of static and dynamic protein binding will provide a better understanding of the coupling between mass transfer and reversible binding in membrane adsorbers onto separation performance, and it will provide valuable guide-lines for the development of improved membrane adsorbers.     

Surface structure and phase separation mechanism of polysulfone membranes by atomic force microscopy

Atomic force microscopy (AFM) was used to investigate the surface of polysulfone (PSf) membranes. The AFM method provides information on both size and shape of pores or cavities on the surface as well as the roughness of the skin. The pore sizes obtained from AFM observation were found to be more accurate than those obtained from scanning electron microscopy (SEM) since the potential of altering the pore structure of the membrane during sample preparation was eliminated. It was observed that two different modes of phase separation existed during the formation of PSf membrane when the coagulation conditions were varied.     

Membrane characterization by solute transport and atomic force microscopy

Various ultrafiltration and nanofiltration membranes were characterized by solute transport and also by atomic force microscope (AFM). The molecular weight cut-off (MWCO) of the membranes studied were found to be between 3500 and 98,000 Daltons. The mean pore size (μ_p) and the geometric standard deviation (σ_p) around mean ranged from 0.7 to 11.12 nm and 1.68 to 3.31, respectively, when calculated from the solute transport data. Mean pore sizes measured by AFM were about 3.5 times larger than calculated from the solute transport. Pore sizes measured by AFM were remarkably fitted to the log-normal probability distribution curve. Pore sizes of the membranes with low MWCO (20,000 Daltons and lower) could not be measured by AFM because of indistinct pores. In most cases, the pore density ranged from 38 to 1291 pores/μm². In general, the pore density was higher for the membrane having lower MWCO. Surface porosity was around 0.5–1.0% as measured from the solute transport and was 9.5–12.9% as obtained from AFM images. When membranes were coated with a thin layer of sulfonated polyphenylene oxide, mean pore sizes were reduced for all the membranes. Surface roughness was also reduced on coating.     

Creation of regenerated cellulose microspheres with diameter ranging from micron to millimeter for chromatography applications

Regenerated cellulose microspheres (RCM) with different diameters were prepared from cellulose solution using 7 wt% NaOH/12 wt% urea aqueous solvent pre-cooled to −12 °C by the sol–gel transition method via a “green” process. By varying the hydrophile–lipophile balance, the amount of the surfactants, the proportion of the water to the oil phase and the stirring speed, the mean diameter of the cellulose microsphere with nano-scale pore size could be controlled easily from 5 μm to 1 mm. The structure and physicochemical properties of the microspheres were characterized by FT-IR spectroscopy, scanning electron microscopy, X-ray diffraction, mercury intrusion-porosimetry and particle size analyzer. The RCM microspheres exhibited spherical shape with the cellulose II structure. A preparative size-exclusion chromatography (SEC) column packed with the cellulose microspheres was used for the fractionation of a polyethylene oxide (PEO) in water, which indicated high efficiency for the fractionations and a large daily throughput of 4 g. Moreover, they had good adsorption capacity to dye particles through physical interaction. The cellulose microspheres would have potential applications in the fields of purification, separation and fractionation of polymers as chromatography packing and adsorbent both at laboratory and industrial scale.     

Characterization of heavy-metal-containing seepage water colloids by flow FFF, ultrafiltration, ELISA and AAS

Heavy-metal-containing humic colloids from seepage water samples of three different municipal waste disposal plants were characterized in terms of molecular weight, hydrodynamic radius and heavy metal content. The size distribution of the colloids was determined with ultrafiltration (UF) and flow field-flow fractionation (flow FFF). The humic colloids in the seepage water samples were characterized using an off-line coupling of flow FFF with an enzyme-linked immunosorbent assay (ELISA) for humic substances. The heavy metals in the different size fractions obtained by UF and flow FFF were determined using atomic absorption spectroscopy (AAS). The colloid size distributions obtained with UF showed a maximum of the distribution in the range 1–10 nm. Seepage water samples with high colloid concentrations had a second maximum in the range 0.1–1 m. The determination of colloid size with flow FFF gave different colloid size distributions for the three waste disposal seepage waters, whereas water from the oldest disposal plant showed the smallest colloid size with a maximum at 0.9 nm and water from the most recent plant showed the largest colloid size with a maximum at 1.3 nm. The determination of particle classes with regard to the chemical composition using a scanning electron microscope with energy dispersive X-ray fluorescence detector (SEM/EDX) showed that the particles can be divided into five classes: silicates, insoluble salts, iron(hydr)oxides, carbonates and organic colloids (humic colloids). Flow FFF/ELISA off-line coupling showed that the most frequently occurring colloids of the seepage waters were humic colloids and investigation of the UF-size-fractions with AAS showed that up to 77% of the total mass of a heavy metal element can be bound to particles, especially to humic colloids. Additionally, the distributions of the heavy metals Fe, Cu and Zn were investigated with flow FFF/AAS off-line coupling. These results also showed that a substantial amount of these heavy metals (up to 46%) was bound to humic colloids.     

Preparation of magnetite-loaded silica microspheres for solid-phase extraction of genomic DNA from soy-based foodstuffs

Solid-phase extraction has been widely employed for the preparation of DNA templates for polymerase chain reaction (PCR)-based analytical methods. Among the variety of adsorbents studied, magnetically responsive silica particles are particularly attractive due to their potential to simplify, expedite, and automate the extraction process. Here we report a facile method for the preparation of such magnetic particles, which entails impregnation of porous silica microspheres with iron salts, followed by calcination and reduction treatments. The samples were characterized using powder X-ray diffractometry (XRD), scanning electron microscopy (SEM), nitrogen adsorption/desorption isotherms, and vibrating sample magnetometry (VSM). XRD data show that magnetite nanocrystals of about 27.2 nm are produced within the pore channels of the silica support after reduction. SEM images show that the as-synthesized particles exhibit spherical shape and uniform particle size of about 3 μm as determined by the silica support. Nitrogen sorption data confirm that the magnetite-loaded silica particles possess typical mesopore structure with BET surface area of about 183 m²/g. VSM data show that the particles display paramagnetic behavior with saturation magnetization of 11.37 emu/g. The magnetic silica microspheres coated with silica shells were tested as adsorbents for rapid extraction of genomic DNA from soybean-derived products. The purified DNA templates were amplified by PCR for screening of genetically modified organisms (GMOs). The preliminary results confirm that the DNA extraction protocols using magnetite-loaded silica microspheres are capable of producing DNA templates which are inhibitor-free and ready for downstream analysis.     

Assessment of the distribution of pesticides on soil particle fractions in simulated irrigation run-off using centrifugal SPLITT fractionation and ELISA

This study was designed to measure the distribution of pesticides within the mobile phase of simulated irrigation run-off water, using centrifugal split-flow thin-channel (SPLITT) fractionation, a novel technique providing a gentle separation of natural sediment and suspended particles. Particular attention is paid to the extraction of pesticide residues for enzyme-linked immunosorbent assay (ELISA) analysis; ELISA was used because of the limited sample size.Centrifugal SPLITT fractionation combined laminar flow hydrodynamics and centrifugal sedimentation to obtain a continuous binary separation of suspended particles. The non-destructive technique allowed an accurate separation of particles into fractions with divisions at 0.5, 2 and 10 μm, with those above 25 μm being performed by wet sieving. ELISA was used to analyse the concentration of endosulfan and diuron for each fraction generated by the SPLITT technique.This data can be used to determine the role that particulate fines and colloidal fractions play in the transport of bound organic pollutants within the environment and to examine prospects for remediation on farms.     

Microwave assisted synthesis of manganese mixed oxide nanostructures using plastic templates

The synthesis method for obtaining sub-micrometric structures of rare earth manganese-based mixed oxide compounds is described. Pore wetting of porous polycarbonate templates with the liquid precursor was followed by a two-stage thermal treatment to obtain single phase La_0.325Pr_0.300Ca_0.375MnO₃ hollow and solid structures, with external diameter determined by the sacrificial template pore size. The first thermal stage, a microwave assisted denitration process, determines the shape of the structures. The second treatment, performed at 1073 K, allows to obtain the crystallographic structure of the compound. A variety of techniques (scanning and transmission electron microscopy, scanning probe microscopy) allowed to fully characterize the microstructure and morphology of these self-standing manganite nanostructures. For 1 μm pore size templates we obtained tubes, with external diameter around 800 nm and wall thickness around 150 nm; densely packed nanoparticles sized 20-50 nm are the building blocks of the walls. For pore size below 0.1 μm, solid nanowires were obtained, the size of constituent crystallites being around 10 nm. Overall obtained material exhibits ferromagnetic ordering below 200 K.     

Visualisation of an ultrafiltration membrane by non-contact atomic force microscopy at single pore resolution

Non-contact atomic force microscopy (AFM) has been used to investigate the furface pore structure of a polyethersulfone ultrafitration membrane of specified molecular weight cut off (MWCO) 25 000 (ES625, PCI Membrane Systems). Excellent images at up to single pore resolution were obtained. This is the first time that AFM images of a membrane at such high resolution have been presented. Analysis of the images gave a mean pore size of 5.1 nm with a standard deviation of 1.1 nm. The results have been compared to previously published studies of membranes of comparable MWCO using contact AFM and electron microscopy. Non-contact AFM is a powerful means of studying the surface pore characteristics of ultrafiltration membranes.     

Study on polytetrafluoroethylene aqueous dispersion irradiated by gamma ray

Gamma-radiation induced degradation of polytetrafluoroethylene (PTFE) in 60 wt.% dispersion was studied in the dose range of 20-200 kGy and the change in property of PTFE was characterized by differential scanning calorimetry (DSC), photon cross correlation spectroscopy (PCCS), X-ray diffraction (XRD), scanning electron microscopy (SEM), FT-IR spectroscopy and X-ray photoelectron spectroscope (XPS). It was found that the mean particle size of PTFE reduces from 250 nm of the control to 170 nm at 100 kGy, as confirmed by dynamic laser scattering and SEM. The crystallinity degree of PTFE increased at 20 kGy but remained unvaried at higher dose level. G-value of scission, G(s), was determined to be 0.46 μmol/J.     

Bacterial passage through microfiltration membranes in wastewater applications

Two polyvinylidenefluoride microfiltration membranes (GVHP and GVWP: Millipore, Bedford, MA) with a nominal pore size of 0.22 μm were challenged with mixed microbial cultures present in Milli-Q™ water and in secondary effluent, and with a Gram-negative model bacterium, SW8, to investigate bacterial passage. Total bacterial counts measured microscopically using the DNA fluorochrome DAPI revealed that the small bacteria in Milli-Q™ water passed MF membranes totally. The model bacterium, SW8 and bacteria from secondary effluent were mostly retained with log reduction values (LRV) of 4 and 3.5, respectively. Transmembrane pressure did not influence the levels of bacterial passage significantly. Pore size effects were investigated with track-etched membranes (Poretics™: Osmonics, Minnetonka MN) with nominal pore sizes of 0.2, 0.1 and 0.05 μm. The LRV of 0.2 μm membranes for SW8 and secondary effluent cells was 3 and 1.5, respectively (total counts). Both membranes with pore sizes smaller than 0.2 μm acted similarly, they still transmitted secondary effluent cells with LRV 2 log higher than 0.2 μm membranes, but for SW8 only 50% higher. In contrast to total count results, removal of bacteria was 100% with all membranes when assessed by cultureable counts, i.e. numbers of bacterial colonies recovered on R2A agar plates. Transmitted bacteria failed to grow on standard basal microbiology media most probably because they were injured during passage through the membranes to the extent that recovery in laboratory media did not occur. However, tests with CTC, an indicator of cell viability, indicated that approximately half of the cells of SW8 which passed the membranes had what appeared to be functional electron transfer chains in their membranes. All membranes had a pore size distribution which included pores larger than the nominal value. Field emission scanning electron microscopy (FESEM) provided evidence for entrapment of bacteria within the membrane matrix.     

Novel fabrication of on-column capillary inlet frits through flame induced sintering of stainless steel particles

A novel fritting technology was introduced for the fused-silica capillary. The technique involved sintering of stainless steel (SS) particles at the tip of capillary through flame heating. A simple butane gas based welding torch was used for sintering the SS particles. The new fritting technique, flame induced sintering of SS particles (FIS/SSP), was applied for making frits with different inlet diameters (75 μm, 100 μm, 250 μm and 530 μm). The changes in morphologies of SS particles during sintering process were identified by scanning electron microscopy (SEM). Frits with the length of 0.5-1 mm and capillaries with inner diameter about 50-100 μm were fabricated through suitable selection of experimental conditions (size of SS particles and heating mode). The frits prepared by FIS/SSP technique exhibited adequate separation properties and mechanical strength. Columns packed with C₁₈ particles were stable with these frits in a few important chromatographic operations. Frits prepared by FIS/SSP technique was used in three typical separation modes namely, capillary electrochromatography (CEC), p-assisted CEC (p-CEC) and low pressure liquid chromatography (LPLC). Importantly, no bubble formation was noticed with the frit over a period of one week. A good peak symmetry and high efficiency for separation were obtained using pressure-assisted CEC, p-CEC and low pressure-driven separation modes.     

Enzymatic synthesis of pH-responsive polyaniline colloids by using chitosan as steric stabilizer

Polyaniline colloidal particles were prepared by enzymatic polymerization of aniline using chitosan as steric stabilizer and toluenesulfonic or camphorsulfonic acids as doping agents. Fourier transform infrared and UV-vis spectroscopic studies indicate that enzymatic polymerization of aniline in dispersed media results in the emeraldine salt form of polyaniline. The morphology of the colloids was studied by transmission electron microscopy. Toluenesulfonic acid produced mainly oblong particles whereas rod-like shaped particles were obtained using camphorsulfonic acid. Polyaniline particles with good colloidal stability and size below 200 nm were obtained using 1.0 wt% of chitosan in the reaction media, indicating that this polymer was highly efficient as a steric stabilizer. The content of chitosan attached to the polyaniline colloids was approximately 20 wt% as indicated by elemental analysis. The colloids synthesized either with toluenesulfonic or camphorsulfonic acid showed a strong pH-dependent colloidal stability and underwent rapid flocculation in near neutral or alkaline media. This interesting behavior could be exploited in separation technology applications.     

Development of polyoxadiazole nanocomposites for high temperature polymer electrolyte membrane fuel cells

Novel nanocomposite membranes were prepared with sulfonated polyoxadiazole and different amounts of sulfonated dense and mesoporous (MCM-41) silica particles. It has been shown that particle size and functionality of sulfonated silica particles play an important role when they are used as fillers for the development of polymer electrolyte nanocomposite membrane for fuel cells. No significant particle agglomerates were observed in all nanocomposite membranes prepared with sulfonated dense silica particles, as analyzed by SEM, AFM, TGA, DMTA and tensile tests. The T_g values of the composite membranes increased with addition of sulfonated silica, indicating an interaction between the sulfonic acid groups of the silica and the polyoxadiazole. Constrained polymer chains in the vicinity of the inorganic particles were confirmed by the reduction of the relative peak height of tan δ. A proton conductivity of 0.034 S cm⁻¹ at 120 °C and 25% RH, which is around two-fold higher than the value of the pristine polymer membrane was obtained.     

Swift heavy ion induced modification in dielectric and microhardness properties of polymer composites

Polymer composites with different concentrations of organometallics (ferric oxalate) dispersed PMMA were prepared. PMMA was synthesized by solution polymerization technique. These films were irradiated with 120 MeV Ni¹⁰⁺ ions in the fluence range 10¹¹-5 × 10¹² ions/cm². The radiation induced modifications in dielectric properties, microhardness, structural changes and surface morphology of polymer composite films have been investigated at different concentrations of filler and ion-fluences. It was observed that electrical conductivity and hardness of the films increase with the concentration of the filler and also with the fluence. The dielectric constant (?) obeys the Universal law given by ?αfⁿ⁻¹. The dielectric constant/loss is observed to change significantly due to irradiation. This suggests that ion beam irradiation promotes the metal to polymer bonding and convert polymeric structure into hydrogen depleted carbon network. This makes the composites more conductive and harder. Surface morphology of the films has been studied using atomic force microscopy (AFM) and scanning electron microscopy (SEM). The average surface roughness is observed to increase after irradiation as revealed by AFM studies. The SEM images show the blisters type of phenomenon on the surface due to ion beam irradiation.