Application of nanoscale probes for the evaluation of the integrity of ultrafiltration membranes (original) (raw)
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Nanoscale probes for the evaluation of the integrity of ultrafiltration membranes
Journal of Membrane Science - J MEMBRANE SCI, 2006
Current integrity tests are not sufficiently sensitive to detect nanometric scale breaches in the active layer of ultrafiltration membranes. The current paper describes a new approach for the detection of such breaches. We introduce two representative types of nanoprobes emulating virus transport over the membrane. Gold nanoparticles and fluorescent-dye-labeled MS2 bacteriophages were introduced for seeding tests. The affordability of the former probe has become more realistic with the development of electrochemical detection of gold nanoparticles by anodic stripping voltammetry (ASV). Our ASV system showed high sensitivity, of the order of single parts per billion, indicating the feasibility of developing the experimental protocol for on-site analysis. However, the physical characteristics of gold nanoparticles and particularly their specific density differ from those of viruses. The fluorescent bacteriophage probe emulates viral transport much better, though this technique is less sensitive, and further lowering of the limit of detection is required. Application of the probes for testing membrane integrity will provide a basis for developing on-line testing for removal of virus-sized particles and in addition to being a valuable research tool may provide a means to confirm compliance of membrane systems with the stringent regulatory requirements of the drinking water industry.
Desalination, 2004
Nanofiltration membranes (NF) have applications in several areas. One of the main applications has been in water treatment for drinking water production as well as wastewater treatment. NF can either be used to treat all kinds of water including ground, surface, and wastewater or used as a pretreatment for desalination. The introduction of NF as a pretreatment is considered a breakthrough for the desalination process. NF membranes have been shown to be able to remove turbidity, microorganisms and hardness, as well as a fraction of the dissolved salts. This results in a significantly lower operating pressure and thus provides a much more energy-efficient process. Similar to other membrane processes, a major problem in NF membrane applications is fouling. Several studies have investigated the mechanisms of fouling in NF membranes and suggested methods to minimize and control the fouling of NF membranes. For NF membrane characterizations and process prediction, modeling of NF processes and the use of atomic force microscopy (AFM) are very important. The ability to predict the performance of NF processes will lead to a lower number of experiments, saving of time and money, and help to understand the separation mechanisms during NF. A comprehensive review of NF in water treatments is presented including a review of the applications of NF in treating water as well as in the pretreatment process for desalination; the mechanism as well as minimization of NF membrane fouling problems; and theories for modelling and transport of salt, charged and noncharged organic compounds in NF membranes. The review will also address the application of AFM in studying the morphology of membrane surfaces as part of the NF membrane characterization.
Atomic force microscopy (AFM) analysis of cleaned and fouled nanofiltration membranes
International Journal of Nano and Biomaterials, 2009
In membrane process industries, membrane cleaning is one of the most important concerns from both economical and scientific points of view. Surfaces roughness and phase angle of virgin and fouled membranes were measured and compared before and after chemical cleaning by using atomic force microscopy (AFM). Correlations between the data obtained by this technique (AFM) with the flux and rejection have also been discussed.
Organic Fouling and Chemical Cleaning of Nanofiltration Membranes: Measurements and Mechanisms
Environmental Science & Technology, 2004
Fouling and subsequent chemical cleaning of nanofiltration (NF) membranes used in water quality control applications are often inevitable. To unravel the mechanisms of organic fouling and chemical cleaning, it is critical to understand the foulant-membrane, foulant-foulant, and foulantcleaning agent interactions at the molecular level. In this study, the adhesion forces between the foulant and the membrane surface and between the bulk foulant and the fouling layer were determined by atomic force microscopy (AFM). A carboxylate modified AFM colloid probe was used as a surrogate for humic acid, the major organic foulant in natural waters. The interfacial force data were combined with the NF membrane water flux measurements to elucidate the mechanisms of organic fouling and chemical cleaning. A remarkable correlation was obtained between the measured adhesion forces and the fouling and cleaning behavior of the membrane under various solution chemistries. The AFM measurements further confirmed that divalent calcium ions greatly enhance natural organic matter fouling by complexation and subsequent formation of intermolecular bridges among organic foulant molecules. Efficient chemical cleaning was achieved only when the calcium ion bridging was eliminated as a result of the interaction between the chemical cleaning agent and the fouling layer. The cleaning efficiency was highly dependent on solution pH and the concentration of the chemical cleaning agent. FIGURE 14. Mechanism of humic acid solubilization by SDS: (a) low SDS concentration, (b) moderate SDS concentration, and (c) SDS concentration exceeding the CMC. The binding sites shown are solely for illustration purposes.
A method for characterizing membranes during nanofiltration at extreme pH
Journal of Membrane Science, 2010
This work presents a method for molecular weight cut off (MWCO) characterization of nanofiltration membranes, in a broad range of acidic and alkaline environments. Polyethylene glycols (PEG) have been identified as suitable marker molecules with sufficient chemical stability under the harsh conditions of interest. PEG molecular weight distributions have been analyzed using gel permeation chromatography (GPC). To allow quantitative GPC analysis, a protocol is presented to overcome the problem of an overlapping salt peak in the GPC elugram. The method is applied to a well-known commercial nanofiltration membrane (NF-270, DOW FILMTEC TM ) in the pH range 2-12. This membrane has similar MWCO (∼270 g mol −1 ) and permeance (∼10 L m −2 h −1 bar −1 ) in acid environment and at neutral conditions. At pH = 12 a reversible increase was observed for the MWCO (∼380 g mol −1 ) and the permeance (∼12 L m −2 h −1 bar −1 ). This demonstrates the added value of our method to observe the change of MWCO as a function of pH during nanofiltration at the relevant conditions.
Effect of Time in Chemical Cleaning of Ultrafiltration Membranes
Chemical Engineering & Technology, 2012
Chemical cleaning of ultrafiltration membranes is often considered successful when the flux through a cleaned membrane is much higher than through a pristine one. Here, a novel definition of cleaning intensity is proposed as the product of the concentration of the cleaning agent and the cleaning time (Ct), and it is shown that Ct values between 0.5 and 1.0 g h L -1 are sufficient for effective cleaning. Experiments with PES-30 and PVDF-30 membranes fouled by bovine serum albumin and cleaned with surfactant, oxidant, and formulated cleaning agents demonstrate that a good cleaning should last for 10-20 min and restore the flux through a virgin membrane. More intensive cleaning increases the membrane hydrophilicity and the water flux, but soon causes more severe fouling and even membrane disintegration.
Water Science and Technology, 2007
The efficiency of cleaning procedures to remove the fouling deposit from the surface of NF membranes operating in the drinking water plant of Méry sur Oise (Val d'Oise, France) was assessed by a combination of chemical analysis and fluorescence microscopy. The ATR-FTIR spectra of the fouled membranes revealed the presence of biological matter at the membrane surface, mainly composed of polysaccharides, nucleic acids and proteins. IR bands corresponding to the membrane material were detected for stage 1 but not for stage 3. Confocal laser scanning microscopy (CLSM) observations confirmed the microbial origin of the fouling deposit. After chemical cleaning, the analysis of the inorganic foulants revealed a significant decrease of the inorganic content. Moreover, ATR-FTIR spectra of the fouled membranes were modified, mainly in a broad complex region corresponding to polysaccharides and nucleic acids. The amide bands were also altered for stage 1, and some peaks corresponding to th...
Atomic force microscopy of nanofiltration membranes: Effect of imaging mode and environment
Journal of Membrane Science, 2012
The atomic force microscope (AFM) has become a useful tool for studying the morphology of membrane surfaces as well as their fouling characteristics. One principle advantage of the AFM over other high resolution imaging techniques is the ability to make observations in both ambient air and liquid environments. Diverse imaging modes also exist, each with their own advantages and disadvantages. In this study two different imaging modes in both air and water are compared when examining two different nanofiltration membranes, to compare the strengths and weakness of different methods of obtaining surface topography when applied to nanofiltration membrane characterization. When imaging the more hydrophobic of the two membranes using tapping mode in a water environment features consistent with the existence of surface adhered nanobubbles were observed. Such features have implications for the fouling of membranes by hydrophobic materials, as well as effects on the ability to image hydrophobic membrane surfaces under such conditions.
Comprehensive experimental studies of early-stage membrane scaling during nanofiltration
Desalination, 2011
Nanofiltration (NF) membranes have found more frequent use in recent years for the desalination of seawater and other sources of brackish water because they can be used at lower pressures than more traditional reverseosmosis (RO) technologies, and thus provide overall energy savings. However, membrane fouling still presents a common and significant challenge in practical applications. Currently, the performance of membrane-based liquid separation processes is most often monitored by external, volumetric flow-based techniques that provide delayed information on fouling layer development. The delay between initial growth and the observation of fully established fouling reduces the efficacy of cleaning and remediation measures. The focus of this study is the use of ultrasonic time-domain reflectometry (UTDR) as a non-destructive method for real-time, in-situ monitoring of early-stage inorganic scaling layer formation on NF membranes. This work utilizes miniature-scale ultrasonic transducers that are internally integrated into a flat-sheet cross-flow filtration module and in contact with the membrane. Comparisons are made with results obtained from externally mounted UTDR transducers, a more commonly used arrangement. Results show that while the internal sensors can be somewhat more sensitive, the significance of this improvement can be negated by scaling deposition that is hindered by the presence of the sensor.
Solvent-induced swelling of membranes — Measurements and influence in nanofiltration
Journal of Membrane Science, 2006
This paper describes improvements to an apparatus for in-situ determinations of swelling where a linear inductive probe and electronic column gauge with an overall resolution of 0.1 µm was used for measurements of seven variants of polyacrylonitrile (PAN)/polydimethylsiloxane (PDMS) composite nanofiltration membranes in a range of alkane, aromatic and alcohol solvents. The unswollen membranes incorporated PDMS layers between 1 and 10 μm nominal thickness and were manufactured with a radiation and/or thermal crosslinking step.