Pressure-driven Filtration Processes Research Articles

Chemically Converted Graphene Nanosheets for the Construction of Ion-Exclusion Nanochannel Membranes.

Water and ion transport in nanochannels is an intriguing topic that has been extensively investigated in several energy- and environment-related research fields. Recently developed two-dimensional (2D) materials are ideal building blocks for constructing confined nanochannels by self-stacking. Among these, graphene oxide (GO) is the most frequently employed as the starting material because of its excellent solution processability. Since solvation of the GO nanostructure usually impairs the function of nanochannels, in this study, chemically converted graphene was prepared using a one-step method, to simultaneously acquire the desired stability and functionality of the nanochannels. The confined channels with high charge densities are capable of excluding ∼90% NaCl solutes from water in a pressure-driven filtration process. This surpasses the performance of most GO desalination membranes reported in the literature. Thus, this study provides useful information for the feasible development of ion-exclusion nanochannel membranes based on the proposed nanochannel-confined charge repulsion mechanism.

Concentration polarization in membrane distillation: I. Development of a laser-based spectrophotometric method for in-situ characterization

Membrane-based processes have gained prominence for environmental and industrial separations, especially in water purification and desalination. In spite of these developments, a fundamental knowledge gap exists with regards to fully understanding the processes that occur at or near the membrane surface. Concentration polarization is a phenomenon that may have a significant impact on mass transfer and fouling of the membrane. While attempts have been made to experimentally characterize this phenomenon for pressure-driven filtration processes, most studies employ semi-empirical methods to estimate the extent of concentration polarization. This study presents a novel non-intrusive spectrophotometric method developed to measure the concentration profile of solute near the membrane surface in a direct contact membrane distillation system. A custom membrane cell was designed and fabricated for this study to allow accurate in-situ characterization of the concentration polarization layer with spatial resolution of 4.5 μm. The spectrophotometric method was developed and validated using transition metal salt (i.e., nickel chloride) in the feed solution to probe the concentration profile near the membrane surface. The method developed in this study can be extended to other membrane-based separation processes and other solutes with known absorption spectra.

Influence of module orientation and geometry in the membrane distillation of oily seawater

To improve the mechanistic understanding for advancing the design and engineering of the membrane distillation (MD) modules, the objective of the current study was to investigate via both experiments and simulations the impact of (i) module orientation, (ii) module geometry, and (iii) an oily feed on the permeate flux and pore wetting propensity of direct contact membrane distillation (DCMD). Three module orientations and four feed channel geometries were investigated via experiments and simulations for oily feeds. Two key highlights emanated from this study. Firstly, module orientation mattered for DCMD, particularly in view of the formation of natural thermal convective currents and when the particle density of the particulate foulants varied. Particulate foulants with density much lesser and greater than water only deposited when the membrane was oriented respectively atop and beneath the feed. Secondly, the lack of consideration of convection currents, oil coalescence and the corresponding cake-enhanced temperature polarization in the simulations caused disagreement with the experimental results, which underscores the importance of these factors. This highlights that the optimization of MD modules particularly for treating oily feeds requires more mechanistic studies, especially in view of the thermal gradients, rather than relying on analogy with pressure-driven filtration processes.

3D nanoporous crystals enabled 2D channels in graphene membrane with enhanced water purification performance

Two-dimensional (2D) materials have been received increasing attention in various fields such as physics, material science, chemistry and engineering. In particular, the graphene-based membrane is an emerging subject mainly due to the atomic thickness, simple processing and the compatibility with other materials. In the case of water treatment, stable graphene-based laminar structures as well as high separation performance are pursued. In this work, nanoporous crystals embedded graphene laminate membranes was reported for water purification. Reduced graphene oxide (rGO) nanosheets obtained from a solution chemical process act as building blocks to construct the 2D channels through a pressure-driven filtration process. By incorporating three-dimensional (3D) nanoporous crystals with sub-nano sized aperture size into the 2D graphene laminates, both the inter-layer spacing and numbers of nanofluidic channels are increased, leading to greatly-enhanced water separation performance. The optimized 3D/2D membranes exhibit 15 times higher water permeability than that of the rGO membrane with similar high dye retention rate. The significance of such 3D nanoporous structure and transport mechanism through the 3D/2D membranes is systematically studied. This general approach of enhancing the molecular transport through 2D nanofluidic channels proposed here may also find application in gas separation and battery membranes.

The Influence of Water Matrix On The Separation of Bisphenol A With Low And High Pressure Driven Membrane Techniques

Abstract The unrestricted access to clean and inexpensive water is perceived as one of the most basic humanitarian objectives; however, it still remains a challenge in 21st century due to many factors such as constantly increasing pollution of surface water. One of the remedies for this situation might be so called in-depth municipal waste water treatment. The objective of this paper was to examine the efficacy of the pressure-driven membrane filtration (ultra- and nanofiltration) to remove bisphenol A from a variety of environmental matrices including real effluent from municipal wastewater treatment plants. The process of pressure-driven membrane filtration was realized through dead-end flow filtration with the use of different commercial membranes. It has been proved that bisphenol A retention coefficient depended both on the process type and the physical and chemical properties of the membrane applied. The type of water matrix was also significant. It has been determined that efficient bisphenol A removal is possible only in the process of nanofiltration. However, in this case the selection of an appropriate membrane is of a great importance because of the physical and chemical parameters that determine both separation properties and the intensity of adverse reactions that accompany pressure-driven filtration processes.

A method to characterize the compressibility and stability of microfiltration membrane

In this study, the membrane compaction of polysulfone (PS), poly(ether sulfone) (PES) and poly(vinylidene fluoride) (PVDF) flat sheet membranes with finger-like porous structure during pressure-driven filtration process was investigated. The parameters of membrane compression index (sc), membrane rebound index (sr) and membrane base resistance (RT0) were proposed to explore the microfiltration membrane compressibility and its effects to performance degradation. The results revealed that the PS, PES and PVDF membranes all showed the maximum compression index and low rebound index in the initial pressure-driven filtration process. (sc−sr) decreased after three supercharging–decompression filtration cycles. sc and sr tended to be equal and stabilized at 0.41±0.06, 0.35±0.06 and 0.25±0.04 for the PVDF, PES and PS membranes respectively. It unveiled a reduction of the residual deformation and the stability of the membrane structure. The elastic deformation exerted a more important effect on the flux of PVDF membrane than that of PES and PS membrane. The study also indicated that the compression deformation resulted in smaller pore size and larger membrane base resistance after continuing pressure-driven filtration process, which lead to the decay of membrane flux.

Mitigating the hydraulic compression of nanofiltration hollow fiber membranes through a single-step direct spinning technique.

Most nanofiltration (NF) membranes have been made through complicated multistep or thin-film composite processes. They also suffer the compaction issue that reduces permeate flux in pressure-driven filtration processes. A single-step coextrusion hollow fiber fabrication technique via immiscibility induced phase separation (I(2)PS) process is presented in this study to fabricate NF hollow fiber membranes. A protective layer is concurrently formed on top of the selective layer during the phase inversion process. The fabricated hollow fiber membrane has a narrow pore size distribution with a molecular weight cutoff (MWCO) of 470 Da. The outer layer of the I(2)PS hollow fiber is found to serve as a buffering layer that mitigates hydraulic compression on the compaction of dense-selective layer and sublayer and helps to retain membrane performance during nanofiltration operations. The newly fabricated NF hollow fiber membrane exhibits an average pure water permeability of 3.2 L m(-2) h(-1) bar(-1) and shows good rejections toward the testing dyes. This study may offer a simple, direct, and cost-effective approach to fabricate NF hollow fiber membranes.