Influence of polypropylene membrane surface porosity on the performance of membrane distillation process

Two kinds of polypropylene capillary membranes were used in the membrane distillation (MD). These membranes exhibited a similar morphology, but one of them has an additional low porosity layer on the internal surface of capillaries. The changes of membrane performance during MD process of tap water were investigated. The presence of low porosity layer (thickness below 1 μm) caused that the air permeability was reduced from 1.365 to 0.863 dm³/m² s kPa, whereas the MD permeate flux was decreased only by 15%. A significantly larger decline of the flux was caused by CaCO₃ deposit formed during distillation of tap water. This deposit was removed every 30–70 h by rinsing the modules with a 2–5 wt.% HCl. Unfortunately, a repetition of this operation several times resulted in a gradual decline of the maximum permeate flux (distilled water as a feed). However, the module efficiency with the membranes covered by a surface layer of low porosity was found to decreases twice as slowly. The investigations revealed that a low surface porosity does not limit the possibility of surface wetting of polypropylene membranes, but hindered the scale formation inside the pores.     

Surface and electrochemical properties of polypropylene track membrane modified by plasma of non-polymerizing gases

The surface and electrochemical properties of polypropylene track membrane treated by plasma of nitrogen, air, and oxygen are studied. The effect of the plasma-forming gas composition on the surface morphology is considered. The membrane surface microrelief formed during the gas-discharge etching is found to change. Moreover, the non-polymerizing gas plasma treatment induces oxidation of the membrane surface layer and generates oxygen-containing functional groups, mostly carbonyl and carboxyl. The higher membrane roughness and its hydrophilization is shown to lead to its better wettability. In addition, the presence of polar groups in the membrane surface layer modifies its hydrodynamic and electrochemical properties so that water permeability and conductivity of modified membranes increase.     

Non-invasive monitoring of fouling in hollow fiber membrane via UTDR

Fouling is the most critical problem associated with membrane separations in liquid media. But it is difficult to control the inevitable membrane fouling because of its invisibility, especially on the inside surface of hollow fiber membranes. This study describes the extension of ultrasonic time-domain reflectometry (UTDR) for the real-time measurement of particle deposition in a single hollow fiber membrane. A transducer with a frequency of 10 MHz and polyethersulfone hollow fiber membranes with 0.8 mm inside diameter (ID) and 1.2 mm outside diameter (OD) were used in this study. The fouling experiments were carried out with 1.8 g/L kaolin suspension at flow rates 16.7 and 10.0 cm/s. The results show that UTDR technique is able to distinguish and recognize the acoustic response signals generated from the interfaces water/upper outside surface of the hollow fiber, lumen upside surface/water, water/lumen underside surface and lower outside surface/water in the single hollow fiber membrane module in pure water phase. The systemic changes of acoustic responses from the inside surfaces of the hollow fiber in the time- and amplitude-domain with operation time during the fouling experiments were detected by UTDR. It is associated with the deposition and formation of the kaolin layer on the inside surfaces. Further, the acoustic measurement indicates that the deposited fouling layer is denser on the lumen underside surface of the hollow fiber than that on the lumen upside surface as a result of weight. Moreover, it is found that the fouling layer grows faster on the inside surface of the hollow fiber at a flow rate of 10.0 cm/s than that at 16.7 cm/s due to the lower shear stress. The fouling layer formed is thicker at a flow rate of 10.0 cm/s than that at 16.7 cm/s. The flux decline data and SEM analysis corroborate the ultrasonic measurement. Overall, this study confirms that UTDR measurement will provide not only a new protocol for the observation of hollow fiber membrane fouling and cleaning, but also a quantitative approach to the optimization of the membrane bioreactor system.     

Fouling in direct contact membrane distillation process

The formation of deposit on the membrane surface (fouling) is one of the major operating problems of membrane distillation process. The influence of fouling on the performance of this process was investigated during the concentration of wastewater with proteins, bilge water, brines, and the production of demineralized water. The experiments were performed with polypropylene capillary membranes. The morphology and composition of the fouling layer were studied using Fourier transform infrared with diffuse reflectance spectroscopy and scanning electron microscopy coupled with the energy dispersing spectrometry. Fouling with various intensity was observed in most of the studied cases. Permeate flux decline was mainly caused by an increase in the heat resistance of the fouling layer. However in the case of non-porous deposit, a magnitude of the permeate flux was also determined by a resistance of water transport through the deposit layer. It was found the deposits were formed not only on the membrane surface, but also inside the pores. Salt crystallization in the membrane pores besides their wetting, also caused the mechanical damage of the membrane structure. The intensity of the fouling can be limited by the pretreatment of feed and a selection of the operating conditions of membrane distillation.     

Wettability of polypropylene capillary membranes during the membrane distillation process

Studies of membrane wettability in the membrane distillation process were performed with the application of hydrophobic capillary membranes made of polypropylene. Three kinds of Accurel PP membranes (Membrana GmbH, Germany) differing in the diameter of capillaries and pores as well as in the wall thickness were used. It was confirmed that membranes with lower wall thickness and larger pore size provide higher yields of the process. The studies demonstrated that the pores of used membranes located close to the external surface of capillaries are several times larger than those located inside the membrane wall. Based on air permeability measurements it was found that external surface of the membranes with such large pores was completely wetted by water after 50–80 h of membrane distillation. However, the pores located inside the wall with the diameter below 1 μm were not wetted and electrical conductivity of the obtained distillate was maintained at the level of 3–6 μS cm⁻¹.     

Mitigated membrane fouling in an SMBR by surface modification

Fouling is a major obstacle in membrane processes applied in membrane bioreactor. To improve the antifouling characteristics of polypropylene hollow fiber microporous membranes (PPHFMMs) in a submerged membrane bioreactor (SMBR), the PPHFMMs were surface modified by NH₃, CO₂ plasma treatment, photoinduced graft polymerization of acrylamide and acrylic acid. The structural and morphological changes on the membrane surface were characterized by X-ray photoelectron spectroscopy (XPS), attenuated total reflection-Fourier transform infrared spectroscopy (FT-IR/ATR) and field emission scanning electron microscopy (FE-SEM). The change of surface wettability was monitored by contact angle measurements. The results of XPS and FT-IR/ATR clearly indicated the successful modification on the membrane surface. The static water contact angle of the modified membrane reduced obviously. The antifouling characteristics of the modified membranes in an SMBR were evaluated. The modified membranes showed better filtration performances in the submerged membrane bioreactor than the unmodified one, and the acrylic acid-grafted membrane presented the best antifouling characteristics. The results demonstrated that (1) the surface carboxyl-containing membranes were better than the surface amido-containing membranes; (2) surface-grafted membranes were better than the plasma-treated membranes.     

The long-term studies of osmotic membrane distillation

The results of osmotic membrane distillation carried out for 2.5 years were presented in this work. The influence of the process conditions, such as temperature and brine concentration on the permeate flux, was investigated. The saturated NaCl solutions and distilled water were used as a stripping solution and feed, respectively. A continuous regeneration of stripping solution was conducted using a method of natural evaporation from the surface of Bia?ecki rings to the air surrounding the installation. The possibilities of application of Accurel PP S6/2 hydrophobic polypropylene membranes were tested. It was studied whether a saturation stripping solution does not cause scaling and wettability of membranes. It was found that most of the pores in the used membranes were non-wetted, and the salt retention over 99% was maintained during a study period. However, the obtained permeate flux was decreased by 10–20%. The SEM examinations revealed that it was caused by amorphous deposit, which was formed on the membrane surface on the brine side. The SEM–EDS analysis demonstrated that the deposit composition mainly included Si and O.     

The application of polypropylene membranes for production of fresh water from brines by membrane distillation

Scaling and wettability of hydrophobic membranes were studied during the membrane distillation applied for the production of fresh water from the concentrated salt solutions. The studies were performed with the use of membrane modules in which the capillary membranes from polypropylene were assembled. A saline ground water containing several sparingly soluble salts was used as a feeding solution. The presence of such compounds caused an intensive surface and internal scaling. Due to the scaling, a partial wetting of the membrane walls and the permeation of salts into distillate were observed. These phenomena were eliminated for the membranes with thicker walls when the amount of deposit was limited by a periodic rinsing of the module with water. During this study, the feed was concentrated up to the supersaturation state, which caused a salt crystallization on the membrane surface, and as a consequence, the permeate flux was reduced to zero. In this case, the internal scaling can be limited using the capillary membranes with a net covering their surface.     

Application of hydroxy sodalite films as novel water selective membranes

Supported hydroxy sodalite (H-SOD) membranes were prepared on α-alumina disks using direct hydrothermal synthesis at 413 K for 3.5 h. The continuity of the membranes was verified using single gas permeation of He and N₂ at ambient conditions. The membranes were impermeable to N₂ and He, which validated absence of defects in the membrane structure. The membranes were used in dewatering several organic alcohol/water mixtures (organic alcohol being: methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, 1-pentanol, and 2-pentanol). The influence of feed temperature (303–473 K), feed concentration (0–100 mol% alcohol in the feed), and absolute feed pressure (1.6–2.4 MPa) on the water flux were analyzed. The absolute feed pressure had no effect on the water permeance. The membrane exhibited a water/alcohol separation factor larger than 10⁶ and showed excellent thermal, mechanical, and operation stability in continuously dehydrating a water/ethanol mixture (72 mol% water) by pervaporation at 473 K and 2.2 MPa for 125 h. The normalized water flux correlated well with the water feed concentration for the primary alcohols. Below 40 mol% water in mixtures with secondary alcohols the water flux was three orders of magnitude lower. The water mobility through the membrane had an activation energy of ∼15 kJ/mol.     

Enhanced desulfurization performance and swelling resistance of asymmetric hydrophilic pervaporation membrane prepared through surface segregation technique

The severe swelling behavior of most hydrophobic membranes has always been an obstinate problem when separating organic mixtures by pervaporation. In some cases, hydrophilic membranes may be an appropriate alternative. In this study, amphiphilic copolymer Pluronic F127 was employed as a surface modifier to fabricate polyethersulfone (PES) asymmetric pervaporation membranes via surface segregation. The scanning electron microscopy (SEM) photographs showed an asymmetric structure of PES/Pluronic F127 membranes. The Fourier transform-infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS) and static water contact angle measurements confirmed the hydrophilic modification of the membrane surface. Based on the distinct difference of solubility in water between thiophene and n-octane, the prepared membranes were utilized to remove thiophene from n-octane by pervaporation. The effect of Pluronic F127 content on the pervaporation performance was evaluated experimentally. It has been found that both the permeation flux and enrichment factor exhibited a peak value of approximately 60 wt% of the Pluronic F127 content. The highest enrichment factor was around 3.50 with a permeation flux of 3.10 kg/(m² h) for 500 mg/L sulfur in the feed at 30 °C. The influence of various operating parameters on the pervaporation performance was extensively investigated.     

Perovskite membrane reactor for continuous and isothermal redox hydrogen production from the dissociation of water

The redox water splitting is one of the most promising routes for sustainable hydrogen production. Towards this goal, serious technological obstacles are set: (i) by the non-isothermal operation of the redox process, that causes serious reactor construction problems, and (ii) by the need for efficient high temperature oxygen/hydrogen separation technology which is a very challenging development. In this paper, perovskite materials having the formula La_0.3Sr_0.7FeO₃ were synthesized and subsequently tested for their high temperature oxidation/reduction behavior. The redox activity of the materials in relation to the water splitting reaction has been also investigated. Dense, disc shaped membranes of the materials were synthesized and placed in a membrane reactor. Experiments at 1133 K revealed the possibility of performing the reduction and oxidation steps simultaneously and isothermally on each side of the membrane reactor. A steady-state situation was thereby achieved where hydrogen was continuously produced on one side while the material was simultaneously regenerated on the other side. The created oxygen vacancy gradient formed the driving force for a continuous flux of vacancies from the membrane reduction surface to the membrane oxidation surface. The hydrogen production rate under the particular experimental conditions estimated to be ∼47.5 cm³ H₂ (STP) m⁻² min⁻¹. It could be increased by a factor of approximately 3, up to ∼145 cm³ H₂ (STP) m⁻² min⁻¹, if the membrane reduction was enhanced with a reductant such as carbon monoxide. This approach resulted in an efficient execution of the water gas shift reaction towards high purity hydrogen production.     

Effect of membrane parameters on the size and uniformity in preparing agarose beads by premix membrane emulsification

Agarose microbeads were prepared by premix membrane emulsification with Shirasu-Porous Glass (SPG) membrane and Polyethylene (PE) membrane. The effects of membrane parameters, including pore size, pore size distribution, contact angle between membrane surface and the water phase, shape of pore opening and membrane thickness on size and uniformity of agarose beads were investigated in this study. The results showed that pore size distribution and shape of pore opening did not affect the emulsification results apparently within a wide range in premix membrane emulsification, not as the result in general emulsification. The contact angle between the water phase and the membrane surface must be large enough to obtain uniform-sized agarose beads in both direct membrane emulsification and premix membrane emulsification. The results also showed that the membrane pore size and thickness affected the size distribution of emulsion. Thicker membrane resulted in more uniform and smaller emulsion when the number of pass through membrane is controlled. There was a linear relationship between the number average diameter of agarose beads and membranes pores size in premix membrane emulsification. Agarose beads with diameters from 3.06 to 9.02 μm were prepared by controlling membranes pore size. The ratio of the number average diameter of agarose beads to membrane pore diameters was found to be 0.486.     

Characterization of pore structure of a strong anion-exchange membrane adsorbent under different buffer and salt concentration conditions

The quantitative characterization of pore structure of Sartobind Q, a strongly basic membrane anion exchanger that is formed by cross-linked cellulose support and a hydrogel layer on its pore surface, was made combining the results obtained by several experimental techniques: liquid impregnation, batch size-exclusion, inverse size-exclusion chromatography, and permeability. Mercury intrusion and nitrogen sorption porosimetry were carried out for a dry cellulose support membrane in order to get additional information for building a model of the bimodal pore structure. The model incorporated the distribution of the total pore volume between transport and gel-layer pores and the partitioning of solutes of different molecular weights was expressed through the cylindrical pore model for the transport pores and random plane model for the gel layer. The effect of composition of liquid phase on the pore structure was investigated in redistilled water, phosphate and Tris–HCl buffers containing up to 1 M NaCl. Evident differences in the bimodal pore structure were observed here when both the specific volume and size of the hydrogel layer pores significantly decreased with the ionic strength of liquid phase.     

Novel membrane-based sensor for online membrane integrity monitoring

A novel membrane-based sensor device for upstream membrane integrity monitoring has been developed and evaluated in this study. The sensor is based on relative trans-membrane pressures created by two membranes in series inside the sensor device that detects deposition from the sample stream onto the first of the sensor membranes. The sensor pressure signals can distinguish between intact or damaged membranes in the upstream membrane filtration process. Studies were conducted to evaluate both stabilities and sensitivities of the relative trans-membrane pressure monitoring technique. Sensitivity, based on the response times of the membrane sensor for particle detection, was determined for a range of operating conditions, membrane sandwich configurations, and particle concentrations in both simulated membrane failures and for actual pin-hole defects on a submerged MF membrane. The results showed that both sensitivities and stability strongly depended on membrane sandwich configurations (membrane characteristics) in the sensor, and mode of operation (pressurized or vacuum). The membrane sensor detected bentonite particles with a concentration of 0.3 mg/L (turbidity ∼0.3 NTU) in approximately 35 min in the vacuum mode. The sensor is reliable, sensitive and low cost. It has potential applications in decentralized systems or in multichannel monitoring of local conditions in a large plant. Possible applications of the membrane sensor for fouling monitoring are also discussed.     

Development of polyelectrolyte multilayer-coated electrospun cellulose acetate fiber mat as composite membranes

Electrostatic multilayers of chitosan (CHI)/sodium alginate (SA) and CHI/poly(styrene sulfonate) sodium salt (PSS) were alternatively coated on electrospun cellulose acetate (CA) fiber mat. Morphologies of the composite membranes were characterized by scanning electron microscopy. The morphology of the CHI/SA-coated membrane was denser than the CHI/PSS-coated one. The top layers consisted of carboxyl and sulfonic functional groups for SA and PSS layers, respectively. Amino groups of CHI were only presented in slight quantity. X-ray photoelectron spectroscopy (XPS) confirmed the deposition of the amino groups of CHI on the multilayer membrane surface. These composite membranes were characterized for its water permeability where the water flux decreased with an increase in the number of the bilayers. The water flux was in the range of 60 and 40 L m⁻² h⁻¹ for 15 and 25 bilayered membranes, respectively. The sodium chloride (NaCl) solution flux was lower than the pure water flux due to the effect of osmotic pressure, and it decreased with an increase in the NaCl concentration. The rejection of NaCl increased substantially with the number of the bilayers of the polyelectrolytes multilayers. The level of NaCl rejection from this work was in the range of 6% and 15% for 15 and 25 bilayered membranes, respectively.     

Formation of porous poly(vinylidene fluoride) membranes with symmetric or asymmetric morphology by immersion precipitation in the water/TEP/PVDF system

The phase equilibrium boundaries of the membrane forming system, water/triethyl phosphate (TEP)/PVDF, at 25 °C were determined experimentally using cloud-point and equilibrium absorption methods. Based on the phase diagram, appropriate dope and bath compositions were selected to prepare microporous membranes by means of the isothermal immersion-precipitation technique. As a metastable casting dope with respect to crystallization was adopted, the formed membranes exhibited a uniform cross-section composed of interlocked crystal elements coexisting with the network of continuous pores, as was revealed by high resolution FESEM imaging. Morphologies of the membranes’ top surfaces were found to depend heavily on the bath strength, which was controlled by the TEP content. By changing the bath gradually from pure water to 70% TEP, the top surface evolved from a dense skin (asymmetric membrane) to a totally porous morphology (symmetric membrane). Wide angle X-ray diffraction analysis indicated that PVDF crystallized into α-type structure for all of the synthesized membranes. The crystallinity as determined from diffraction peak deconvolution was ≈65%, which value was confirmed by Differential Scanning Calorimetry (DSC). The obtained thermograms also showed a similar melting peak temperature (T_m ≈ 169 °C) for all membranes. Furthermore, water fluxes and tensile strengths of the membranes were measured. The results were found to correlate with the morphologies of the membranes.     

Crosslinked poly(ethylene oxide) fouling resistant coating materials for oil/water separation

Potential fouling reducing coating materials were synthesized via free-radical photopolymerization of aqueous solutions of poly(ethylene glycol) diacrylate (PEGDA). Crosslinked PEGDA (XLPEGDA) exhibited high water permeability and good fouling resistance to oil/water mixtures. Water permeability increased strongly with increasing the water content in the prepolymerization water mixture, going from 10 to 150 L μm/(m² h bar) as prepolymerization water content increased from 60 to 80 wt.%. However, molecular weight cutoff decreased as water content increased. These materials were applied to polysulfone (PSF) UF membranes to form coatings on the surface of the PSF membranes. Oil/water crossflow filtration experiments showed that the coated PSF membranes had water flux values 400% higher than that of an uncoated PSF membrane after 24 h of operation, and the coated membranes had higher organic rejection than the uncoated membranes.     

The use of inverse gas chromatography to study surface thermal oxidation of polypropylene

Thermo-oxidative effects on the surface energy of polypropylene were measured by inverse gas chromatography as a function of exposure time and temperature. Unaltered polypropylene had a surface energy of 33 mJ/m². Oxidized polypropylene, after exposure to air at temperatures of 100 °C and 110 °C, had a range of maximum surface energies from 38 to 41 mJ/m². Comparisons between FTIR carbonyl peak growth and the surface energy showed that both methods detect oxidation, though the increase in surface energy is detected before the carbonyl peak growth is noticeable. The work of adhesion predicted by the surface free energies obtained in this work between a coated calcium carbonate and polypropylene changes by 10% due to the oxidation of the polymer at 110 °C.     

Chlorine-induced permeability recovery for low-pressure membrane filtration of natural waters

Chlorine treatment is widely used by membrane filtration plants to recover the loss of membrane permeability encountered in low-pressure membrane (LPM) filtration of natural waters. However, there are few methodical studies in the literature addressing the efficacy of chlorine in cleaning membranes. Thus, the purpose of this study was to assess chlorine-induced permeability recovery (CIPR) of LPMs using the Ct concept (product of chlorine dose concentration and treatment time) commonly employed in the disinfection literature. The experimental work was conducted by evaluating the efficacy of CIPR for a membrane and water combination under variable Ct exposures and determining the presence of minimum effective Ct exposure and proper empirical models for the CIPR. The results showed that the efficacy of CIPR depended on both C and t. A minimum Ct exposure of approximately 2 × 10⁵ (min mg)/L was required for effective CIPR, and the relationship between the residual fouling and chlorine exposures was best fitted using a revised Chick–Watson model. These results may be explained by a conceptual model that considers CIPR as a sequential process of oxidation of organic foulants and diffusional detachment of the reaction products from membrane surfaces. Additional work is needed to validate the applicability of the model to other waters and membranes.     

Extraction and preconcentration of manganese(II) from biological fluids (water, milk and blood serum) using supported liquid membrane and membrane probe methods

A supported liquid membrane (SLM) technique was investigated to extract and preconcentrate Mn(II) from water, milk and blood serum. Di-2-ethylhexyl phosphoric acid (DEHPA) with kerosene as diluent was used as a carrier in the membrane to transport Mn(II) from the donor side to acceptor side. The membrane was modified with tri-n-octylphosphine oxide (TOPO) to increase its polarity. Various parameters were investigated to optimise the extraction efficiency: pH of the donor and acceptor phase, dilution factor, donor flow rate. Scanning electron microscope images of the membranes revealed that some matrix compounds were deposited on the surface, thus limiting the extraction process. The optimum conditions found were: pH 3 in the donor phase, 0.2 M nitric acid in the acceptor phase, donor flow rate between 1.0 and 0.3 ml min⁻¹, 15% (w/v) DEPHA and 10% TOPO in kerosene as a carrier in membrane, and dilution factors of 20 times for blood serum and 30 times for milk. The extraction efficiencies were found to be low but constant and highly reproducible showing, strong dependence on sample matrix. The new SLM extraction probe was developed and optimised for Mn(II) extraction. Compared to traditional SLM configurations, this is the simplest configuration. The use of stirring allows the same sample to be extracted many times giving higher extraction efficiency and to minimise the sample size. Adsorptive stripping voltammetry (AdSV) was applied to measure Mn(II) concentration. The optimised method was used to determine the concentration of Mn(II) in water, milk and blood serum samples.