Breaking the Symmetry: Mitigating Scaling in Tertiary Treatment of Waste Effluents Using a Positively Charged Nanofiltration Membrane

When salinity of municipal wastewater increases and approaches the limits of toxicity for plants, moderate desalting of wastewater becomes vital for keeping it suitable for irrigation. Nanofiltration (NF) is an attractive solution, as it partially removes NaCl. Unfortunately, commercial NF membranes (e.g., NF270) strongly reject multivalent ions present in wastewater, especially, scale-forming calcium and phosphate. This results in undesired demineralization, severe membrane scaling, and unacceptably low water recovery. To address this problem, we report here that a positively charged NF (p-NF) performs significantly better than NF270, owing to overall lower rejection of scale-forming ions. Therefore, for a commensurate flux and NaCl rejection, p-NF shows much less scaling than NF270, even at recoveries as large as 80%–85%. This suggests that p-NF may have an advantage over standard NF for moderate desalting of wastewater and other water sources with high scaling potential

Facile Modification of Reverse Osmosis Membranes by Surfactant-Assisted Acrylate Grafting for Enhanced Selectivity

The top polyamide layer of composite reverse osmosis (RO) membranes has a fascinatingly complex structure, yet nanoscale nonuniformities inherently present in polyamide layer may reduce selectivity, e.g., for boron rejection. This study examines improving selectivity by in situ “caulking” such nonuniformities using concentration polarization-enhanced graft-polymerization with a surfactant added to the reactive solution. The surfactant appears to enhance both polarization (via monomer solubilization in surfactant micelles) and adherence of graft-polymer to the membrane surface, which facilitates grafting and reduces monomer consumption. The effect of surfactant was particularly notable for a hydrophobic monomer glycidyl methacrylate combined with a nonionic surfactant Triton X-100. With Triton added at an optimal level, close to critical micellization concentration (CMC), monomer gets solubilized and highly concentrated within micelles, which results in a significantly increased degree of grafting and uniformity of the coating compared to a procedure with no surfactant added. Notably, no improvement was obtained for an anionic surfactant SDS or the cationic surfactant DTAB, in which cases the high CMC of surfactant precludes high monomer concentration within micelles. The modification procedure was also up-scalable to membranes elements and resulted in elements with permeability comparable to commercial brackish water RO elements with superior boric acid rejection

Influence of Ion Diffusion on the Lithium–Oxygen Electrochemical Process and Battery Application Using Carbon Nanotubes–Graphene Substrate

Lithium–oxygen (Li–O2) batteries are nowadays among the most appealing next-generation energy storage systems in view of a high theoretical capacity and the use of transition-metal-free cathodes. Nevertheless, the practical application of these batteries is still hindered by limited understanding of the relationships between cell components and performances. In this work, we investigate a Li–O2 battery by originally screening different gas diffusion layers (GDLs) characterized by low specific surface area (2 g–1) with relatively large pores (absence of micropores), graphitic character, and the presence of a fraction of the hydrophobic PTFE polymer on their surface (<20 wt %). The electrochemical characterization of Li–O2 cells using bare GDLs as the support indicates that the oxygen reduction reaction (ORR) occurs at potentials below 2.8 V vs Li+/Li, while the oxygen evolution reaction (OER) takes place at potentials higher than 3.6 V vs Li+/Li. Furthermore, the relatively high impedance of the Li–O2 cells at the pristine state remarkably decreases upon electrochemical activation achieved by voltammetry. The Li–O2 cells deliver high reversible capacities, ranging from ∼6 to ∼8 mA h cm–2 (referred to the geometric area of the GDLs). The Li–O2 battery performances are rationalized by the investigation of a practical Li+ diffusion coefficient (D) within the cell configuration adopted herein. The study reveals that D is higher during ORR than during OER, with values depending on the characteristics of the GDL and on the cell state of charge. Overall, D values range from ∼10–10 to ∼10–8 cm2 s–1 during the ORR and ∼10–17 to ∼10–11 cm2 s–1 during the OER. The most performing GDL is used as the support for the deposition of a substrate formed by few-layer graphene and multiwalled carbon nanotubes to improve the reaction in a Li–O2 cell operating with a maximum specific capacity of 1250 mA h g–1 (1 mA h cm–2) at a current density of 0.33 mA cm–2. XPS on the electrode tested in our Li–O2 cell setup suggests the formation of a stable solid electrolyte interphase at the surface which extends the cycle life