Forward Osmosis Processes in the Limit of Osmotic Equilibrium

Forward osmosis processes are an emerging set of technologies that show promise in the treatment of complex and impaired water streams (e.g., those encountered in industrial wastewater treatment and the extraction of unconventional oil resources). The effective operation of these systems requires that the operating conditions be chosen wisely based on the membrane to be used and the streams to be treated. In this work, to aid in the design of these systems, an analytical model was developed that describes the module-level performance (i.e., water recovery rate and separation factors of the feed and draw solutes) for cocurrent and countercurrent forward osmosis systems in the thermodynamic limit of osmotic equilibrium. In the limit of osmotic equilibrium, the model expresses the recovery rate and separation factors in terms of the operating conditions (e.g., the flow rates and concentrations of the feed and draw solutions) and the characteristic membrane transport properties (e.g., the hydraulic and solute permeability coefficients). The model was validated by comparing its predictions with numerical simulations of the full system of governing equations: strong agreement between the model predictions and numerical simulations was observed. Analysis of the model demonstrates that the reverse flux selectivity, the ratio of the forward water flux to the reverse draw solute flux, is a key parameter in the design of forward osmosis systems that controls the maximum solute rejection that the systems can achieve at osmotic equilibrium. Further analysis shows that the flow ratio, the ratio of the inlet flow rate of the draw solution to the inlet flow rate of the feed solution, is an important design parameter. Specifically, in countercurrent operation, a critical value of the flow ratio that maximizes the recovery rate was identified

Polymeric Ion Pumps: Using an Oscillating Stimulus To Drive Solute Transport in Reactive Membranes

The development of membranes that separate molecules on the basis of chemical factors, rather than physical factors, is one promising approach to meeting the demand for membranes that are more selective. In this study, the design of multifunctional, pH-responsive membranes that selectively pump a target solute is detailed. The membranes consist of two functional components: a gate layer made from an amine-functionalized copolymer and a reactive matrix lined by iminodiacetic acid groups that bind divalent cations reversibly. These two chemistries exhibit concurrent changes in the cation binding affinity and gate permeability in response to the pH value of the surrounding solution such that when the membranes are exposed to an oscillating pH, the combination drives a facilitated transport mechanism that pumps ions. In mixed solute systems, calcium permeated through the membrane four times faster than sucrose in the presence of an oscillating pH even though the solutes possess similar hydrodynamic sizes and permeated through the membrane at the same rate when the pH value was constant. The development of polymeric ion pumps was guided by a model that provided several critical insights. First, the solute binding capacity and thickness of the membrane define the asymptotic limit for enhanced selectivity. Second, the maximum enhancement in selectivity is realized in the limit of infinitely rapid oscillations. The multifunctional membranes discussed here provide a platform for the development of processes that can fractionate molecules of similar size but varying chemistry

Ion Selective Permeation Through Cellulose Acetate Membranes in Forward Osmosis

Solute–solute interactions can have a dramatic impact on the permeation of solutes through dense polymeric membranes. In particular, understanding how solute–solute interactions can affect the design of osmotically driven membrane processes (ODMPs) is critical to the successful development of these emerging water treatment and energy generation processes. In this work, we investigate the influence that solute–solute interactions have on nitrate permeation through an asymmetric cellulose acetate forward osmosis membrane. A series of experiments that included systematic modifications to the cation paired with nitrate, the identity of the draw solute, and the solution pH were conducted. These experiments reveal that in the unique operating geometry of ODMPs, where solute containing solutions are present on both sides of the membrane, nitrate fluxes are significantly higher (>15 times in some cases) than predicted by existing models for solute permeation in ODMPs. The identity of the cation paired with nitrate influences the flux of nitrate; the identity of the cation in the draw solution does not affect the flux of nitrate; however, the identity of the anion in the draw solution has the most significant impact on the flux of nitrate. These results suggest that an ion exchange mechanism, which allows nitrate to switch rapidly with anions from the draw solution, is present when cellulose acetate based membranes are used in ODMPs

Nanoporous Block Polymer Thin Films Functionalized with Bio-Inspired Ligands for the Efficient Capture of Heavy Metal Ions from Water

Heavy metal contamination of water supplies poses a serious threat to public health, prompting the development of novel and sustainable treatment technologies. One promising approach is to molecularly engineer the chemical affinity of a material for the targeted removal of specific molecules from solution. In this work, nanoporous polymer thin films generated from tailor-made block polymers were functionalized with the bio-inspired moieties glutathione and cysteamine for the removal of heavy metal ions, including lead and cadmium, from aqueous solutions. In a single equilibrium stage, the films achieved removal rates of the ions in excess of 95%, which was consistent with predictions based on the engineered material properties. In a flow-through configuration, the thin films achieved an even greater removal rate of the metal ions. Furthermore, in mixed ion solutions the capacity of the thin films, and corresponding removal rates, did not demonstrate any reduction due to competitive adsorption effects. After such experiments the material was repeatedly regenerated quickly with no observed loss in capacity. Thus, these membranes provide a sustainable platform for the efficient purification of lead- and cadmium-contaminated water sources to safe levels. Moreover, their straightforward chemical modifications suggest that they could be engineered to treat sources containing other recalcitrant environmental contaminants as well

Nanostructured Membranes from Triblock Polymer Precursors as High Capacity Copper Adsorbents

Membrane adsorbers are a proposed alternative to packed beds for chromatographic separations. To date, membrane adsorbers have suffered from low binding capacities and/or complex processing methodologies. In this work, a polyisoprene-<i>b</i>-polystyrene-<i>b</i>-poly­(<i>N</i>,<i>N</i>-dimethyl­acrylamide) (PI–PS–PDMA) triblock polymer is cast into an asymmetric membrane that possesses a high density of nanopores (<i>d</i> ∼ 38 nm) at the upper surface of the membrane. Exposing the membrane to a 6 M aqueous hydrochloric acid solution converts the PDMA brushes that line the pore walls to poly­(acrylic acid) (PAA) brushes, which are capable of binding metal ions (e.g., copper ions). Using mass transport tests and static binding experiments, the saturation capacity of the PI–PS–PAA membrane was determined to be 4.1 ± 0.3 mmol Cu²⁺ g^–1. This experimental value is consistent with the theoretical binding capacity of the membranes, which is based on the initial PDMA content of the triblock polymer precursor and assumes a 1:1 stoichiometry for the binding interaction. The uniformly sized nanoscale pores provide a short diffusion length to the binding sites, resulting in a sharp breakthrough curve. Furthermore, the membrane is selective for copper ions over nickel ions, which permeate through the membrane over 10 times more rapidly than copper during the loading stage. This selectivity is present despite the fact that the sizes of these two ions are nearly identical and speaks to the chemical selectivity of the triblock polymer-based membrane. Furthermore, addition of a pH 1 solution releases the bound copper rapidly, allowing the membrane to be regenerated and reused with a negligible loss in binding capacity. Because of the high binding capacities, facile processing method implemented, and ability to tailor further the polymer brushes lining the pore walls using straightforward coupling reactions, these membrane adsorbers based on block polymer precursors have potential as a separation media that can be designed to a variety of specific applications

A Method for the Efficient Fabrication of Multifunctional Mosaic Membranes by Inkjet Printing

Most conventional membrane systems are based on size-selective materials that permeate smaller molecules and retain larger ones. However, membranes that can permeate larger molecules more rapidly than smaller ones could find widespread utilization in multiple arenas of technology. Charge mosaic membranes are one example of such a system. Due to their unique nanostructure, which consists of discrete oppositely charged domains, charge mosaics are capable of permeating large dissolved salts more rapidly than smaller water molecules. Here, we present a combined inkjet printing and template synthesis technique to prepare charge mosaic membranes in a rapid and straightforward manner and demonstrate the unique transport properties that result from the mosaic membrane design. Poly­(vinyl alcohol)-based composite inks containing poly­(diallyldimethylammonium chloride) or poly­(sodium 4-styrenesulfonate) were used to pattern positively charged or negatively charged domains, respectively, on the surface of a polycarbonate track-etched membrane with 30 nm pores. The ability to control the net surface charge of the mosaic membranes through the rational deposition of oppositely charged materials was demonstrated and confirmed through nanostructural characterization, electrokinetic measurements, and piezodialysis experiments. Namely, mosaic membranes that possessed an overall neutral charge (i.e., membranes that had equal coverage of positively and negatively charged domains) were capable of enriching the concentration of potassium chloride in the solution that permeated through the membrane. These membranes can be deployed in the many established and emerging nanoscale technologies that rely on the selective transport and separation of ionic solutes from solution. Furthermore, because of the flexibility provided by the membrane fabrication platform, the efforts reported in this work can be extended to other mosaic designs with myriad other functional components

Unusually Stable Hysteresis in the pH-Response of Poly(Acrylic Acid) Brushes Confined within Nanoporous Block Polymer Thin Films

Stimuli-responsive soft materials are a highly studied field due to their wide-ranging applications; however, only a small group of these materials display hysteretic responses to stimuli. Moreover, previous reports of this behavior have typically shown it to be short-lived. In this work, poly­(acrylic acid) (PAA) chains at extremely high grafting densities and confined in nanoscale pores displayed a unique long-lived hysteretic behavior caused by their ability to form a metastable hydrogen bond network. Hydraulic permeability measurements demonstrated that the conformation of the PAA chains exhibited a hysteretic dependence on pH, where different effective pore diameters arose in a pH range of 3 to 8, as determined by the pH of the previous environment. Further studies using Fourier transform infrared (FTIR) spectroscopy demonstrated that the fraction of ionized PAA moieties depended on the thin film history; this was corroborated by metal adsorption capacity, which demonstrated the same pH dependence. This hysteresis was shown to be persistent, enduring for days, in a manner unlike most other systems. The hypothesis that hydrogen bonding among PAA units contributed to the hysteretic behavior was supported by experiments with a urea solution, which disrupted the metastable hydrogen bonded state of PAA toward its ionized state. The ability of PAA to hydrogen bond within these confined pores results in a stable and tunable hysteresis not previously observed in homopolymer materials. An enhanced understanding of the polymer chemistry and physics governing this hysteresis gives insight into the design and manipulation of next-generation sensors and gating materials in nanoscale applications

Mixed Mosaic Membranes Prepared by Layer-by-Layer Assembly for Ionic Separations

Charge mosaic membranes, which possess distinct cationic and anionic domains that traverse the membrane thickness, are capable of selectively separating dissolved salts from similarly sized neutral solutes. Here, the generation of charge mosaic membranes using facile layer-by-layer assembly methodologies is reported. Polymeric nanotubes with pore walls lined by positively charged polyethylenimine moieties or negatively charged poly(styrenesulfonate) moieties were prepared <i>via</i> layer-by-layer assembly using track-etched membranes as sacrificial templates. Subsequently, both types of nanotubes were deposited on a porous support in order to produce mixed mosaic membranes. Scanning electron microscopy demonstrates that the facile deposition techniques implemented result in nanotubes that are vertically aligned without overlap between adjacent elements. Furthermore, the nanotubes span the thickness of the mixed mosaic membranes. The effects of this unique nanostructure are reflected in the transport characteristics of the mixed mosaic membranes. The hydraulic permeability of the mixed mosaic membranes in piezodialysis operations was 8 L m^–2 h^–1 bar^–1. Importantly, solute rejection experiments demonstrate that the mixed mosaic membranes are more permeable to ionic solutes than similarly sized neutral molecules. In particular, negative rejection of sodium chloride is observed (<i>i</i>.<i>e</i>., the concentration of NaCl in the solution that permeates through a mixed mosaic membrane is higher than in the initial feed solution). These properties illustrate the ability of mixed mosaic membranes to permeate dissolved ions selectively without violating electroneutrality and suggest their utility in ionic separations

Ultrafiltration of Uranyl Peroxide Nanoclusters for the Separation of Uranium from Aqueous Solution

Uranyl peroxide cluster species were produced in aqueous solution by the treatment of uranyl nitrate with hydrogen peroxide, lithium hydroxide, and potassium chloride. Ultrafiltration of these cluster species using commercial sheet membranes with molecular mass cutoffs of 3, 8, and 20 kDa (based on polyethylene glycol) resulted in U rejection values of 95, 85, and 67% by mass, respectively. Ultrafiltration of untreated uranyl nitrate solutions using these membranes resulted in virtually no rejection of U. These results demonstrate the ability to use the filtration of cluster species as a means for separating U from solutions on the basis of size. Small-angle X-ray scattering, Raman spectroscopy, and electrospray ionization mass spectrometry confirmed the presence of uranyl peroxide cluster species in solution and were used to characterize their size, shape, and dispersity

Facile Synthesis of a Pentiptycene-Based Highly Microporous Organic Polymer for Gas Storage and Water Treatment

Rigid H-shaped pentiptycene units, with an intrinsic hierarchical structure, were employed to fabricate a highly microporous organic polymer sorbent via Friedel–Crafts reaction/polymerization. The obtained microporous polymer exhibits good thermal stability, a high Brunauer–Emmett–Teller surface area of 1604 m² g^–1, outstanding CO₂, H₂, and CH₄ storage capacities, as well as good adsorption selectivities for the separation of CO₂/N₂ and CO₂/CH₄ gas pairs. The CO₂ uptake values reached as high as 5.00 mmol g^–1 (1.0 bar and 273 K), which, along with high adsorption selectivity values (e.g., 47.1 for CO₂/N₂), make the pentiptycene-based microporous organic polymer (PMOP) a promising sorbent material for carbon capture from flue gas and natural gas purification. Moreover, the PMOP material displayed superior absorption capacities for organic solvents and dyes. For example, the maximum adsorption capacities for methylene blue and Congo red were 394 and 932 mg g^–1, respectively, promoting the potential of the PMOP as an excellent sorbent for environmental remediation and water treatment