Nanofiltration membrane performance of layer-by-layer membranes with different polyelectrolyte concentrations

Nanofiltration membranes produced with polyelectrolytes via the layer-by-layer technique are frequently researched, but misunderstood parameter is the polyelectrolyte concentration. Higher polyelectrolyte (PE) concentrations are known to produce thicker PE layers, but its effect on the membrane performance has only been studied in a limited fashion, leading to premature conclusions. In this work, two well-known strong polyelectrolytes, PDADMAC and PSS were used to prepare membranes using coating solutions with polyelectrolyte concentrations of 0.01, 0.1, 1.0, 2.5 and 5.0 wt% and two different salt concentrations in the coating solution of 0.05 and 1 M, as higher salt concentrations lead to thicker PE layers. The membrane performance of the prepared membranes is researched in terms of pure water permeability (PWP), molecular weight cut-off (MWCO) and the retention of different salts. In the first bilayer, membranes coated with a 0.05 M salt solution showed lower PWPs and MWCOs and higher salt retentions by increasing the PE concentration. After a certain number of coated bilayers, the MWCO and salt retentions reach a plateau for all PE concentrations; but the plateau value was obtained earlier by coating with a higher PE concentration. The membranes coated with the 1 M salt concentration had lower or comparable retention rates, except for MgCl2, than those coated with 0.05 M salt. The higher salt concentration resulted in more abundant PDADMAC in the membrane, which promotes the MgCl2 retentions for all bilayers. In conclusion, we found that the polyelectrolyte concentration significantly alters the membrane performance, but after coating 7 bilayers, the same size exclusion plateaus are reached.</p

Nanofiltration membrane performance of layer-by-layer membranes with different polyelectrolyte concentrations

Nanofiltration membranes produced with polyelectrolytes via the layer-by-layer technique are frequently researched, but misunderstood parameter is the polyelectrolyte concentration. Higher polyelectrolyte (PE) concentrations are known to produce thicker PE layers, but its effect on the membrane performance has only been studied in a limited fashion, leading to premature conclusions. In this work, two well-known strong polyelectrolytes, PDADMAC and PSS were used to prepare membranes using coating solutions with polyelectrolyte concentrations of 0.01, 0.1, 1.0, 2.5 and 5.0 wt% and two different salt concentrations in the coating solution of 0.05 and 1 M, as higher salt concentrations lead to thicker PE layers. The membrane performance of the prepared membranes is researched in terms of pure water permeability (PWP), molecular weight cut-off (MWCO) and the retention of different salts. In the first bilayer, membranes coated with a 0.05 M salt solution showed lower PWPs and MWCOs and higher salt retentions by increasing the PE concentration. After a certain number of coated bilayers, the MWCO and salt retentions reach a plateau for all PE concentrations; but the plateau value was obtained earlier by coating with a higher PE concentration. The membranes coated with the 1 M salt concentration had lower or comparable retention rates, except for MgCl2, than those coated with 0.05 M salt. The higher salt concentration resulted in more abundant PDADMAC in the membrane, which promotes the MgCl2 retentions for all bilayers. In conclusion, we found that the polyelectrolyte concentration significantly alters the membrane performance, but after coating 7 bilayers, the same size exclusion plateaus are reached.</p

Doubled Power Density from Salinity Gradients at Reduced Intermembrane Distance

The mixing of sea and river water can be used as a renewable energy source. The Gibbs free energy that is released when salt and fresh water mix can be captured in a process called reverse electrodialysis (RED). This research investigates the effect of the intermembrane distance and the feedwater flow rate in RED as a route to double the power density output. Intermembrane distances of 60, 100, 200, and 485 μm were experimentally investigated, using spacers to impose the intermembrane distance. The generated (gross) power densities (i.e., generated power per membrane area) are larger for smaller intermembrane distances. A maximum value of 2.2 W/m2 is achieved, which is almost double the maximum power density reported in previous work. In addition, the energy efficiency is significantly higher for smaller intermembrane distances. New improvements need to focus on reducing the pressure drop required to pump the feedwater through the RED-device using a spacerless design. In that case power outputs of more than 4 W per m2 of membrane area at small intermembrane distances are envisage

Optimizing flocculation of digestate to increase circularity in manure treatment

The flocculation of (co-)digested cattle and pig manure has rarely been investigated, leading to a rather intuitive use of flocculants in manure treatment processes, resulting in overdosing and increasing costs. Here, we investigate the effect of molecular weight, charge density and branching of reference and commercially available flocculants by establishing the optimal flocculant dosage and the corresponding maximum organic matter removal. Higher molecular weight flocculants show increased turbidity removal as result of their long chains corresponding to a higher amount adsorption sites. Results presented show that polymers with an increased cationic charge density give moderate and unstable flocculation due to the low amounts of non-charged parts essential for the hydrophobic interactions and hydrogen bonding. Further, the results show that a linear high molecular weight flocculant with a nonionic or a low anionic charge density is the most effective as it reached the highest organic matter removal at a low dosage.</p

Electrochemical impedance spectroscopy of a reverse electrodialysis stack:a new approach to monitoring fouling and cleaning

When harvesting salinity gradient energy via reverse electrodialysis (RED), stack performance is monitored using DC characterizations, which does not provide information about the nature and mechanisms underlying fouling inside the stack. In order to assess the potential of natural salinity gradients as renewable energy source, progress in the fields of fouling monitoring and controlling is vital. To improve fouling and cleaning monitoring, experiments with sodium dodecylbenzenesulfonate (SDBS) were carried out while at the same time the electrochemical impedance spectroscopy (EIS) was measured at the RED stack level. EIS showed how SDBS affected the ohmic resistance of the stack, the non-ohmic resistance of the AEM and the non-ohmic resistance of the CEM on different time scales. Such detailed investigation into the effect of SDBS on different stack elements offered by EIS is not possible with traditional DC characterization. The results presented in this work illustrate the potential of EIS at the stack level for fouling monitoring. The knowledge presented shows the possibility to include EIS in up-scaled natural salinity gradient RED applications for fouling monitoring purposes.</p

Polyacrylonitrile (PAN)/crown ether composite nanofibers for the selective adsorption of cations

In this study, we prepared electrospun polyacrylonitrile (PAN) nanofibers functionalized with dibenzo-18-crown-6 (DB18C6) crown ether and showed the potential of these fibers for the selective recovery of K+ from other both mono- and divalent ions in aqueous solutions. Nanofibers were characterized by SEM, FTIR and TGA. SEM results showed that the crown ether addition resulted in thicker nanofibers and higher mean fiber diameters, in a range of 138 to 270 nm. Batch adsorption experiments were conducted in order to evaluate the potential of the crown ether modified nanofibers as an adsorbent for ion removal. The maximum adsorption capacity of the crown ether modified nanofibers for K+ was 0.37 mmol g−1 and the nanofibers followed the selectivity sequence of K+ > Ba2+ > Na+ ∼ Li+ for single ion experiments. Adsorption of Ba2+ ions onto crown ether-modified nanofiber was examined by XPS and the results confirmed the adsorption of the ion. Mixed ion adsorption experiments revealed competitive adsorption between K+ and Ba2+ ions for the available binding sites. This effect was not observed for the other monovalent ions present in the solution and exceptionally high selectivities for K+ over Li+ and Na+ were obtained. Also the crown ether modified nanofibers exhibited good regeneration properties and a good reusability over multiple consecutive adsorption–desorption cycles. Electrospinning is thus shown to be a very versatile tool to prepare crown ether functional polymer adsorbents for the selective recovery of ions

Influence of Pyrolysis Parameters on the Performance of CMSM

Carbon hollow fiber membranes have been prepared by pyrolysis of a P84/S-PEEK blend. Proximate analysis of the precursor was performed using thermogravimetry (TGA), and a carbon yield of approximately 40% can be obtained. This study aimed at understanding the influence of pyrolysis parameters—end temperature, quenching effect, and soaking time—on the membrane properties. Permeation experiments were performed with N2, He, and CO2. Scanning electron microscopy (SEM) has been done for all carbon hollow fibers. The highest permeances were obtained for the membrane submitted to an end temperature of 750°C and the highest ideal selectivities for an end temperature of 700°C. In both cases, the membranes were quenched to room temperatur

On the Performance of a Ready-to-Use Electrospun Sulfonated Poly(Ether Ether Ketone) Membrane Adsorber

Motivated by the need for efficient purification methods for the recovery of valuable resources, we developed a wire-electrospun membrane adsorber without the need for post-modification. The relationship between the fiber structure, functional-group density, and performance of electrospun sulfonated poly(ether ether ketone) (sPEEK) membrane adsorbers was explored. The sulfonate groups enable selective binding of lysozyme at neutral pH through electrostatic interactions. Our results show a dynamic lysozyme adsorption capacity of 59.3 mg/g at 10% breakthrough, which is independent of the flow velocity confirming dominant convective mass transport. Membrane adsorbers with three different fiber diameters (measured by SEM) were fabricated by altering the concentration of the polymer solution. The specific surface area as measured with BET and the dynamic adsorption capacity were minimally affected by variations in fiber diameter, offering membrane adsorbers with consistent performance. To study the effect of functional-group density, membrane adsorbers from sPEEK with different sulfonation degrees (52%, 62%, and 72%) were fabricated. Despite the increased functional-group density, the dynamic adsorption capacity did not increase accordingly. However, in all presented cases, at least a monolayer coverage was obtained, demonstrating ample functional groups available within the area occupied by a lysozyme molecule. Our study showcases a ready-to-use membrane adsorber for the recovery of positively charged molecules, using lysozyme as a model protein, with potential applications in removing heavy metals, dyes, and pharmaceutical components from process streams. Furthermore, this study highlights factors, such as fiber diameter and functional-group density, for optimizing the membrane adsorber's performance.</p

Thermodynamic, Energy Efficiency, and Power Density Analysis of Reverse Electrodialysis Power Generation with Natural Salinity Gradients

Reverse electrodialysis (RED) can harness the Gibbs free energy of mixing when fresh river water flows into the sea for sustainable power generation. In this study, we carry out a thermodynamic and energy efficiency analysis of RED power generation, and assess the membrane power density. First, we present a reversible thermodynamic model for RED and verify that the theoretical maximum extractable work in a reversible RED process is identical to the Gibbs free energy of mixing. Work extraction in an irreversible process with maximized power density using a constant-resistance load is then examined to assess the energy conversion efficiency and power density. With equal volumes of seawater and river water, energy conversion efficiency of ∼33–44% can be obtained in RED, while the rest is lost through dissipation in the internal resistance of the ion-exchange membrane stack. We show that imperfections in the selectivity of typical ion exchange membranes (namely, co-ion transport, osmosis, and electro-osmosis) can detrimentally lower efficiency by up to 26%, with co-ion leakage being the dominant effect. Further inspection of the power density profile during RED revealed inherent ineffectiveness toward the end of the process. By judicious early discontinuation of the controlled mixing process, the overall power density performance can be considerably enhanced by up to 7-fold, without significant compromise to the energy efficiency. Additionally, membrane resistance was found to be an important factor in determining the power densities attainable. Lastly, the performance of an RED stack was examined for different membrane conductivities and intermembrane distances simulating high performance membranes and stack design. By thoughtful selection of the operating parameters, an efficiency of ∼37% and an overall gross power density of 3.5 W/m2 represent the maximum performance that can potentially be achieved in a seawater-river water RED system with low-resistance ion exchange membranes (0.5 Ω cm2) at very small spacing intervals (50 μm)

Salinity Gradients for Sustainable Energy: Primer, Progress, and Prospects

Combining two solutions of different composition releases the Gibbs free energy of mixing. By using engineered processes to control the mixing, chemical energy stored in salinity gradients can be harnessed for useful work. In this critical review, we present an overview of the current progress in salinity gradient power generation, discuss the prospects and challenges of the foremost technologies — pressure retarded osmosis (PRO), reverse electrodialysis (RED), and capacitive mixing (CapMix) and provide perspectives on the outlook of salinity gradient power generation. Momentous strides have been made in technical development of salinity gradient technologies and field demonstrations with natural and anthropogenic salinity gradients (for example, seawater–river water and desalination brine-wastewater, respectively), but fouling persists to be a pivotal operational challenge that can significantly ebb away cost-competitiveness. Natural hypersaline sources (e.g., hypersaline lakes and salt domes) can achieve greater concentration difference and, thus, offer opportunities to overcome some of the limitations inherent to seawater–river water. Technological advances needed to fully exploit the larger salinity gradients are identified. While seawater desalination brine is a seemingly attractive high salinity anthropogenic stream that is otherwise wasted, actual feasibility hinges on the appropriate pairing with a suitable low salinity stream. Engineered solutions are foulant-free and can be thermally regenerative for application in low-temperature heat utilization. Alternatively, PRO, RED, and CapMix can be coupled with their analog separation process (reverse osmosis, electrodialysis, and capacitive deionization, respectively) in salinity gradient flow batteries for energy storage in chemical potential of the engineered solutions. Rigorous techno-economic assessments can more clearly identify the prospects of low-grade heat conversion and large-scale energy storage. While research attention is squarely focused on efficiency and power improvements, efforts to mitigate fouling and lower membrane and electrode cost will be equally important to reduce levelized cost of salinity gradient energy production and, thus, boost PRO, RED, and CapMix power generation to be competitive with other renewable technologies. Cognizance of the recent key developments and technical progress on the different technological fronts can help steer the strategic advancement of salinity gradient as a sustainable energy source