Mirko Frappa

Solutions with NaCl and Na₂SO₄ mixture simulating three real industrial RO waste brines were treated by membrane crystallization (MCr) with the aim of recovering the ions as crystals. In particular, the three different solutions have NaCl and Na₂SO₄ molar ratios of 1:2, 2:1 and 1:1, respectively. Experiments at two different feed temperatures were performed and analyzed. Results showed that the separation of the NaCl and Na₂SO₄ mixture can be realized by the MCr process and the Na₂SO₄ salts were successfully recovered from the mixture. Different morphologies of Na₂SO₄ crystals were obtained by selecting an appropriate feed temperature. Crystal size distribution (CSD) and cumulative fraction of the crystals were calculated to determine the distribution of the crystals’ size and their variation around the average size. The Na₂SO₄ salts obtained from all experiments were of high purity, indicating promising salt separation, recovery, and reuse from RO brine from a sustainable perspective.

In this study, porous polyvinylidene fluoride (PVDF) hollow fiber (HF) membranes for membrane distillation applications were successfully prepared using TamiSolve^® NxG, an innovative lower-hazard solvent, through the non-solvent induced phase inversion technique. The influence of the bore fluid composition and bore fluid flow rate on membrane morphology and performance was systematically investigated while maintaining a fixed polymer dope formulation. The prepared hollow fibers were thoroughly characterized in terms of morphology, membrane thickness, porosity, contact angle, mechanical resistance, pore size, and pure water permeability. In addition, their performance was investigated through Direct Contact Membrane Distillation (DCMD) tests in order to evaluate their applicability in desalination processes. The developed PVDF HF membranes exhibited pore sizes comparable to those of commercial polypropylene (PP) membranes and achieved high permeate flux together with excellent salt rejection, demonstrating their promising potential for sustainable membrane distillation and desalination applications.

Sustainable water desalination and purification membrane processes require new practical pathways to improve their efficiency. To this end, the inclusion of two-dimensional materials in membrane structure has proven to have a significant impact in various applications. In particular, in processes such as membrane distillation and crystallization, these materials, thanks to their characteristics, help to increase the recovery of clean water and, at the same time, to improve the quality and the production of the recovered salts. Therefore, a fundamental aspect of obtaining 2D materials with certain characteristics is the technique used for the preparation. This review provides a broad discussion on the preparation and proprieties of 2D materials, including examples of organic structures (such as graphene and structures containing transition metals and organic metals). Finally, the critical challenges, future research directions, and the opportunities for developing advanced membranes based on 2D materials are outlined.

Membrane crystallization (MCr) is a promising and innovative process for the recovery of freshwater from seawater and for the production of salt crystals from the brine streams of desalination plants. In the present work, composite polymeric membranes for membrane crystallization were fabricated using graphene and bismuth telluride inks prepared according to the wet-jet milling (WJM) technology. A comparison between PVDF-based membranes containing a few layers of graphene or bismuth telluride and PVDF-pristine membranes was carried out. Among the 2D composite membranes, PVDF with bismuth telluride at higher concentration (7%) exhibited the highest flux (about 3.9 L∙m⁻²h⁻¹, in MCr experiments performed with 5 M NaCl solution as feed, and at a temperature of 34 ± 0.2 °C at the feed side and 11 ± 0.2 °C at the permeate side). The confinement of graphene and bismuth telluride in PVDF membranes produced more uniform NaCl crystals with respect to the pristine PVDF membrane, especially in the case of few-layer graphene. All the membranes showed rejection equal to or higher than 99.9% (up to 99.99% in the case of the membrane with graphene). The high rejection together with the good trans-membrane flux confirmed the interesting performance of the process, without any wetting phenomena, at least during the performed crystallization tests.

Membrane distillation is envisaged to be a promising best practice to recover freshwater from seawater with the prospect of building low energy-consuming devices powered by natural and renewable energy sources in remote and less accessible areas. Moreover, there is an additional benefit of integrating this green technology with other well-established operations dedicated to desalination. Today, the development of membrane distillation depends on the productivity–efficiency ratio on a large scale. Despite hydrophobic commercial membranes being widely used, no membrane with suitable morphological and chemical feature is readily available in the market. Thus, there is a real need to identify best practices for developing new efficient membranes for more productive and eco-sustainable membrane distillation devices. Here, we propose engineered few-layer graphene membranes, showing enhanced trans-membrane fluxes and total barrier action against NaCl ions. The obtained performances are linked with filling polymeric membranes with few-layer graphene of 490 nm in lateral size, produced by the wet-jet milling technology. The experimental evidence, together with comparative analyses, confirmed that the use of more largely sized few-layer graphene leads to superior productivity related efficiency trade-off for the membrane distillation process. Herein, it was demonstrated that the quality of exfoliation is a crucial factor for addressing the few-layer graphene supporting the separation capability of the host membranes designed for water desalination.