Chlorine tolerant, multilayer reverse-somosis membranes with high permeate flux and high salt rejection

A new class of high molecular weight polyethersulfone ionomers is described in which the ionic content can be varied, at will, over a very wide and fully-controllable range. A novel type of coating process enables these materials to be deposited from alcohol-type solvents as cohesive but very thin (50 – 250 nm) films on porous support-membranes, giving high-flux membranes (3.3 – 5.0 L m-2 h-1 bar-1) with very good, though not outstanding salt rejection (typically 92 - 96%). A secondary layer, of formaldehyde-cross-linked polyvinyl alcohol, can be deposited from aqueous solution on the surface of the ionomer membrane, and this layer increases salt rejection to greater than 99% without serious loss of water permeability. The final multi-layer membrane shows excellent chlorine tolerance in reverse-osmosis operation.

Reclaiming spent liquor from cotton reactive dyebaths using nanofiltration membrane for possible reuse of water and salt

During dyeing, salts are placed in a dyebath to aid the fixation of various dyes on to the fabric while bases are added to raise the pH from around neutral to pH 11. Afterwards, the used dyebath solution, called dyebath spent liquor, is discharged with almost all the salts and bases added as well as unfixed dyes. Consequently, a lot of raw materials are lost in the waste stream ending up in the environment as pollutants. In this study, possibilities of reusing water and salts of dyebathes were investigated, using a nanofiltration membrane. When the salt concentration in the spent liquor was increased from 10 to 80 g/L, the salt rejection by membrane was found to decrease initially; however, the salt rejection increased over the time, which was not expected. The aggregation of dye was also studied and found to decrease in the concentrate when the salt concentration was increased. This may be due to the aggregation of salt in the concentrate, which explains the increase in salt rejection. This information is useful for the textile industry in evaluating the treated water quality for the purpose of reuse.

Nanofiltration for the possible reuse of water and recovery of sodium chloride salt from textile effluent

During the reactive dyeing of cotton, salts such as sodium chloride (NaCI) are placed in a dyebath to aid the exhaustion of various dyes onto the fabric while bases are added to raise the pH from around neutral to pH 11 to achieve fixation. Afterwards, the used dyebath solution, called dyebath spent liquor, is discharged with almost all the salts and bases added as well as unfixed dyes. Consequently, many raw materials are lost in the waste stream ending up in the environment as pollutants. In this study possibilities of reusing the water and salts of dyebaths were investigated using a nanofiltration membrane. When the NaCI concentration in the spent liquor was increased from 10 to 80 g/L, the NaC1 rejection by the membrane was found to decrease initially; however, the NaC1 rejection increased over time, which was not expected. The aggregation of dye was also studied and found to decrease in the concentrate when the salt concentration was increased. This information is useful for the textile industry in evaluating the treated water quality for the purpose of reuse.

Polyamide thin film composite membranes prepared from 3,4',5-biphenyl triacyl chloride, 3,3',5,5'-biphenyl tetraacyl chloride and m-phenylenediamine

Two novel of tri- and tetra-functional biphenyl acid chloride: 3,4',5-biphenyl triacyl chloride (BTRC) and 3,3',5,5'-biphenyl tetraacyl chloride (BTEC), were synthesized, and used as new monomers for the preparations of the thin film composite (TFC) reverse osmosis (RO) membranes. The TFC RO membranes were prepared on a polysulfone supporting film through interfacial polymerization with the two new monomers and m-phenylenediamine (MPD). The membranes were characterized for the permeation properties, chemical composition, d-space between polymer chains, hydrophilicity, membrane morphology including top surface and cross-section. Permeation experiment was employed to evaluate the membranes performance including salt rejection and water flux. The surface structure and chemical composition of membranes were analyzed by attenuated total reflectance infrared (ATR-IR) and X-ray photoelectronic spectroscopy (XPS). The results revealed that the active layer of membranes was composed of highly cross-linked aromatic polyamide with the functional acylamide (-CONH-) bonds. The TFC membranes prepared from biphenyl acid chloride exhibit higher salt rejection compared with that prepared from trimesoyl chloride (TMC) at the expanse of some flux.

Generation of a buoyancy-driven coastal current by an Antarctic Polynya

Descent and spreading of high salinity water generated by salt rejection during sea ice formation in an Antarctic coastal polynya is studied using a hydrostatic, primitive equation three-dimensional ocean model called the Proudman Oceanographic Laboratory Coastal Ocean Modeling System (POLCOMS). The shape of the polynya is assumed to be a rectangle 100 km long and 30 km wide, and the salinity flux into the polynya at its surface is constant. The model has been run at high horizontal spatial resolution (500 m), and numerical simulations reveal a buoyancy-driven coastal current. The coastal current is a robust feature and appears in a range of simulations designed to investigate the influence of a sloping bottom, variable bottom drag, variable vertical turbulent diffusivities, higher salinity flux, and an offshore position of the polynya. It is shown that bottom drag is the main factor determining the current width. This coastal current has not been produced with other numerical models of polynyas, which may be because these models were run at coarser resolutions. The coastal current becomes unstable upstream of its front when the polynya is adjacent to the coast. When the polynya is situated offshore, an unstable current is produced from its outset owing to the capture of cyclonic eddies. The effect of a coastal protrusion and a canyon on the current motion is investigated. In particular, due to the convex shape of the coastal protrusion, the current sheds a dipolar eddy.

Sea ice production and water mass modification in the eastern Laptev Sea

A simple polynya flux model driven by standard atmospheric forcing is used to investigate the ice formation that took place during an exceptionally strong and consistent western New Siberian (WNS) polynya event in 2004 in the Laptev Sea. Whether formation rates are high enough to erode the stratification of the water column beneath is examined by adding the brine released during the 2004 polynya event to the average winter density stratification of the water body, preconditioned by summers with a cyclonic atmospheric forcing (comparatively weakly stratified water column). Beforehand, the model performance is tested through a simulation of a well‐documented event in April 2008. Neglecting the replenishment of water masses by advection into the polynya area, we find the probability for the occurrence of density‐driven convection down to the bottom to be low. Our findings can be explained by the distinct vertical density gradient that characterizes the area of the WNS polynya and the apparent lack of extreme events in the eastern Laptev Sea. The simple approach is expected to be sufficiently rigorous, since the simulated event is exceptionally strong and consistent, the ice production and salt rejection rates are likely to be overestimated, and the amount of salt rejected is distrusted over a comparatively weakly stratified water column. We conclude that the observed erosion of the halocline and formation of vertically mixed water layers during a WNS polynya event is therefore predominantly related to wind‐ and tidally driven turbulent mixing processes.

Carbon nanotube based composite membranes for water desalination by membrane distillation

New technologies are required to improve desalination efficiency and increase water treatment capacities. One promising low energy technique to produce potable water from either sea or sewage water is membrane distillation (MD). However, to be competitive with other desalination processes, membranes need to be designed specifically for the MD process requirements. Here we report on the design of carbon nanotube (CNT) based composite material membranes for direct contact membrane distillation (DCMD). The membranes were characterized and tested in a DCMD setup under different feed temperatures and test conditions. The composite CNT structures showed significantly improved performance compared to their pure self-supporting CNT counterparts. The best composite CNT membranes gave permeabilities as high as 3.3 x 10-12 kg/(m x s x Pa) with an average salt rejection of 95% and lifespan of up to 39 h of continuous testing, making them highly promising candidates for DCMD.

Comparison between nanofiltration and forward osmosis in the treatment of dye solutions

Colour removal and the flux behaviour of nanofiltration (NF-DOW FILMTEC-NF245) and forward osmosis (FO-a flat sheet cellulose triacetate membrane with a woven embedded backing support) membranes were investigated in this study. The NF membrane was employed to perform dye removal experiments with aqueous solutions containing 15 g/L of NaCl and different concentrations of Acid Green 25, Remazol Brilliant Orange FR and Remazol Blue BR dyes. The increase in dye concentration resulted in a decline in water permeability and an increase in colour removal. When the concentrations of dye solutions varied from 250 to 1000 mg/L, at 0.8 bar of trans-membrane pressure, the NF system exhibited a steady permeate flux of more 30 L/m2h and a colour removal of more than 99%; salt rejection was more than 20.0%. Furthermore, the FO system possessed high dye rejection efficiency (almost 100%), with low permeate flux of around 2.0 L/m2h, when using dye solutions as feed streams and seawater as draw stream. The mode of operation (either FO or pressure retarded osmosis (PRO) did not change the flux significantly but PRO mode always produced higher fluxes than FO mode under the operating conditions used in this study. While both NF and FO can be used to reduce the volume of effluent containing dyes from textile industries, the energy spent in NF on applied pressure can be substituted by the osmotic pressure of draw solution in FO when concentrated draw solutions such as sea water or reverse osmosis concentrate are readily available.

Amine enrichment of thin-film composite membranes via low pressure plasma polymerization for antimicrobial adhesion

Thin-film composite membranes, primarily based on poly(amide) (PA) semipermeable materials, are nowadays the dominant technology used in pressure driven water desalination systems. Despite offering superior water permeation and salt selectivity, their surface properties, such as their charge and roughness, cannot be extensively tuned due to the intrinsic fabrication process of the membranes by interfacial polymerization. The alteration of these properties would lead to a better control of the materials surface zeta potential, which is critical to finely tune selectivity and enhance the membrane materials stability when exposed to complex industrial waste streams. Low pressure plasma was employed to introduce amine functionalities onto the PA surface of commercially available thin-film composite (TFC) membranes. Morphological changes after plasma polymerization were analyzed by SEM and AFM, and average surface roughness decreased by 29%. Amine enrichment provided isoelectric point changes from pH 3.7 to 5.2 for 5 to 15 min of plasma polymerization time. Synchrotron FTIR mappings of the amine-modified surface indicated the addition of a discrete 60 nm film to the PA layer. Furthermore, metal affinity was confirmed by the enhanced binding of silver to the modified surface, supported by an increased antimicrobial functionality with demonstrable elimination of E. coli growth. Essential salt rejection was shown minimally compromised for faster polymerization processes. Plasma polymerization is therefore a viable route to producing functional amine enriched thin-film composite PA membrane surfaces.

Caratterizzazione di membrane da nanofiltrazione

Nanofiltration (NF) is a pressure-driven membrane process, intermediate between reverse osmosis and ultrafiltration. Commercially available polymeric membranes have been used in a wide range of applications, such as drinking, process industry and waste water treatment. For all the applications requiring high stability and harsh washing procedures inorganic membranes are preferred due to their high chemical inertia. Typically, γ – Al2O3 as well as TiO2 and ZrO2 selective layers are used; the latter show higher chemical stability in a wide range of pH and temperatures. In this work the experimental characterization of two different type of membrane has been performed in order to investigate permeation properties, separation performance and efficiency with aqueous solutions containing strong inorganic electrolytes. The influence of salt concentration and feed pH as well as the role of concentration polarization and electrolyte type on the membrane behavior are investigated. Experimentation was performed testing a multi–layer structured NF membrane in α-Al2O3, TiO2 and ZrO2, and a polymeric membrane, in polyamide supported on polysulfone, with binary aqueous solutions containing NaCl, Na2SO4 or CaCl2; the effect of salt composition and pH in the feed side was studied both on flux and salt rejection. All the NF experimental data available for the two membranes were used to evaluate the volumetric membrane charge (X) corresponding to each operative conditions investigated, through the Donnan Steric Pore Model and Dielectric Exclusion (DSPM&DE). The results obtained allow to understand which are the main phenomena at the basis of the different behaviors observed.

Performance evaluation of reverse osmosis (RO) pre-treatment technologies for in-land brackish water treatment

Integration of renewable energy with desalination technologies has emerged as an attractive solution to augment fresh water supply sustainably. Fouling and scaling are still considered as limiting factors in membrane desalination processes. For brackish water treatment, pre-treatment of reverse osmosis (RO) feed water is a key step in designing RO plants avoiding membrane fouling. This study aims to compare at pilot scale the rejection efficiency of RO membranes with multiple pre-treatment options at different water recoveries (30, 35, 40, 45 and 50%) and TDS concentrations (3500, 4000, and 4500mg/L). Synthetic brackish water was prepared and performance evaluation were carried out using brackish water reverse osmosis (BWRO) membranes (Filmtec LC-LE-4040 and Hydranautics CPA5-LD-4040) preceded by 5 and 1μm cartridge filters, 0.02μm ultra-filtration (UF) membrane, and forward osmosis (FO) membrane using 0.25M NaCl and MgCl2 as draw solutions (DS). It was revealed that FO membrane with 0.25M MgCl2 used as a draw solution (DS) and Ultra-filtration (UF) membrane followed by Filmtec membrane gave overall 98% rejection but UF facing high fouling potential due to high applied pressure. Use of 5 and 1μm cartridge filter prior to Filmtec membrane also showed effective results with 95% salt rejection.

Evaluation of Transport Properties of Nanofiltration Membranes Exposed to Radioactive Liquid Waste

The application of membrane separation processes (PSM) for treatment of radioactive waste requires the selection of a suitable membrane for the treatment of waste, as the membrane will be directly exposed to the radioactive liquid waste, and also exposed to ionizing radiation. The nanofiltration membrane is most suitable for treatment of radioactive waste, since it has high rejection of multivalent ions. Usually the membranes are made of polymers and depending on the composition of the waste, type and dose of radiation absorbed may be changes in the structure of the membrane, resulting in loss of its transport properties. We tested two commercial nanofiltration membranes: NF and SW Dow/Filmtec. The waste liquid used was obtained in the process of conversion of uranium hexafluoride gas to solid uranium dioxide, known as "carbonated water". The membranes were characterized as their transport properties (hydraulic permeability, permeate flux and salt rejection) before and after their immersion in the waste for 24 hours. The surface of the membranes was also evaluated by SEM and FTIR. It was observed that in both the porosity of the membrane selective layer was altered, but not the membrane surface charge, which is responsible for the selectivity of the membrane. The NF membranes and SW showed uranium ion rejection of 64% and 55% respectively.

Design of advanced reverse osmosis and nanofiltration membranes for water purification

Most commercially available reverse osmosis (RO) and nanofiltration (NF) membranes are based on the thin film composite (TFC) aromatic polyamide membranes. However, they have several disadvantages including low resistance to fouling, low chemical and thermal stabilities and limited chlorine tolerance. To address these problems, advanced RO/NF membranes are being developed from polyimides for water and wastewater treatments. The following three projects have resulted from my research. (1) Positively charged and solvent resistant NF membranes. The use of solvent resistant membranes to facilitate small molecule separations has been a long standing industry goal of the chemical and pharmaceutical industries. We developed a solvent resistant membrane by chemically cross-linking of polyimide membrane using polyethylenimine. This membrane showed excellent stability in almost all organic solvents. In addition, this membrane was positively charged due to the amine groups remaining on the surface. As a result, high efficiency (> 95%) and selectivity for multivalent heavy metal removal was achieved. (2) Fouling resistant NF membranes. Antifouling membranes are highly desired for “all” applications because fouling will lead to higher energy demand, increase of cleaning and corresponding down time and reduced life-time of the membrane elements. For fouling prevention, we designed a new membrane system using a coating technique to modify membrane surface properties to avoid adsorption of foulants like humic acid. A layer of water-soluble polymer such as polyvinyl alcohol (PVA), polyacrylic acid (PAA), polyvinyl sulfate (PVS) or sulfonated poly(ether ether ketone) (SPEEK), was adsorbed onto the surface of a positively charged membrane. The resultant membranes have a smooth and almost neutrally charged surface which showed better fouling resistance than both the positively charged NF membranes and commercially available negatively charged NTR-7450 membrane. In addition, these membranes showed high efficiency for removal of multivalent ions (> 95% for both cations and anions). Therefore, these antifouling surfaces can be potentially used for water softening, water desalination and wastewater treatment in a membrane bioreactor (MBR) process. (3) Thermally stable RO membranes. Commercial RO membranes cannot be used at temperature higher than 45°C due to the use of polysulfone substrate, which often limits their applications in industries. We successfully developed polyimides as the membrane substrate for thermally stable RO membranes due to their high thermal resistance. The polyimide-based composite polyamide membranes showed desalination performance comparable to the commercial TFC membrane. However, the key advantage of the polyimide-based membrane is its high thermal stability. As the feed temperature increased from 25oC to 95oC, the water flux increased 5 - 6 times while the salt rejection almost kept constant. This membrane appears to provide a unique solution for hot water desalination and also a feasible way to improve the water productivity by increasing the operating temperature without any drop in salt rejection.

Towards enhanced performance thin-film composite membranes via surface plasma modification

Advancing the design of thin-film composite membrane surfaces is one of the most promising pathways to deal with treating varying water qualities and increase their long-term stability and permeability. Although plasma technologies have been explored for surface modification of bulk micro and ultrafiltration membrane materials, the modification of thin film composite membranes is yet to be systematically investigated. Here, the performance of commercial thin-film composite desalination membranes has been significantly enhanced by rapid and facile, low pressure, argon plasma activation. Pressure driven water desalination tests showed that at low power density, flux was improved by 22% without compromising salt rejection. Various plasma durations and excitation powers have been systematically evaluated to assess the impact of plasma glow reactions on the physico-chemical properties of these materials associated with permeability. With increasing power density, plasma treatment enhanced the hydrophilicity of the surfaces, where water contact angles decreasing by 70% were strongly correlated with increased negative charge and smooth uniform surface morphology. These results highlight a versatile chemical modification technique for post-treatment of commercial membrane products that provides uniform morphology and chemically altered surface properties.