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.

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.

The purpose of this work is to study the influence of oil and surfactant present in produced water on the membrane distillation and membrane crystallization performance. The latter is evaluated in terms of permeate flux, quality of permeate and sodium chloride crystallization kinetics. Polypropylene and Hyflon AD40H/PVDF composite membranes with 0.2 $$\mathrm{\mu m}$$μ m pore size were used in the investigation. The tests were carried out in direct contact configuration using two synthetic feed solutions: the first one without oil and surfactant, and the second one with oil and surfactant. The achieved results showed a permeate flux reduction of 20 and 40% for PP and AD40H, respectively, in membrane distillation and of 35% in membrane crystallization. These results may be attributed to the interaction between the salts and surfactant, which led to the deterioration of the membrane performance. Despite this, high salt rejection factors greater than 99.9% and total carbon rejections ranging between 80 and 90% indicated the good potential of membrane distillation technology for the treatment of produced water. Moreover, good quality crystals and high total water recovery factor (97%) were achieved using the membrane crystallization process. Nevertheless, the presence of oil and surfactant in the feed caused an increase in the induction time compared to the system without oil and surfactant. In addition, in the performed experiments, simple physical cleaning with distillate water was sufficient to recover the initial trans-membrane flux of the membranes.

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.