Robust Superhydrophobic Membrane for Membrane Distillation with Excellent Scaling Resistance

Abstract

We report in this study a scalable and controllable approach for fabricating robust and high-performance superhydrophobic membranes for membrane distillation (MD). This novel approach combines electro-co-spinning/spraying (ES2) with chemical vapor welding and enables the formation of robust superhydrophobic (r-SH) membranes that are mechanically strong, highly porous, and robustly superhydrophobic. Compared with superhydrophobic membranes obtained using surface deposition of fluorinated nanoparticles, the r-SH membranes have more robust wetting properties and higher vapor permeability in MD. MD scaling experiments with sodium chloride and gypsum show that the r-SH membrane is highly effective in mitigating mineral scaling. Finally, we also discuss the mechanism of scaling resistance enabled by superhydrophobic membranes with a highlight on the roles of the surface-bound air layer in reducing the crystal-membrane contact area, nucleation propensity, and ion-membrane contact time.

Understanding the fouling/scaling resistance of superhydrophobic/omniphobic membranes in membrane distillation

“…(2) Non-alkaline, as represented by CaSO4, which is pHindependent [26]; CaSO4 mainly forms rod-like structures, but needle-like and rosette-like ones have also been observed (Fig. 2c) [31,[44][45][46][47][48][49][50][51]. CaSO4 scaling has three forms as a function of temperature [44,52]: dihydrate (gypsum, CaSO4•2H2O), hemihydrate (bassanite, CaSO4•0.5H2O) and anhydrite (CaSO4) [34].…”

“…Inverse solubility to temperature [21]; Strong adhesion [22] Flux decline by pore blockage; negligible or detrimental effect on the distillate quality [28,[46][47][48][49]52]; Hard to remove by chemical and physical cleaning [22,52] Silica Produced water;…”

Enhancement of Flux Performance in PTFE Membranes for Direct Contact Membrane Distillation

“…These indicators are either affected by membrane or process parameters. In order to optimize MD performance, the membrane parameters that need to be considered are (i) the thickness, pore size, and porosity to increase flux, the chosen average pore sizes lie in the range of 100-1000 nm [12][13][14][15][16], (ii) the wetting resistance, i.e., hydrophobicity of the pores to maintain purity of the product [17,18], (iii) the module design [19], the heat conduction of the material, membrane thickness, and porosity to increase energy efficiency [12,20], and (iv) thickness and porosity to increase structural endurance [21]. Comparatively less important is the latter indicator since the differential pressure applied in an MD process is low (usually <500 mbar, including hydraulic pressure), and, therefore, structural integrity is often sacrificed for the optimization of the remaining performance indicators [17].…”

Mechanically improved superhydrophobic nanofibrous polystyrene/high‐impact polystyrene membranes for promising membrane distillation application

“…Neat nanofibrous membranes were suffered from lower mechanical strength and wider pore size distribution. Pressing processes (cold and hot‐press) 36,37 and other methods like dilute solvent welding 38 and vapor welding 14 were implemented by different scientific groups to improve mechanical strength and narrow pore size distribution simultaneously. In our previous work, 29 nanofibrous styrene–acrylonitrile (SAN) MD membranes experienced outstanding enhancement (up to 6‐folds) in tensile strength after using the hot‐pressing process.…”

“…However, applied methods somehow add difficulty to the fabrication process and act as a secondary pollution source. For example, using surface‐modifying macromolecules is dangerous for the environment and nanoparticle‐containing membranes suffer from lower mechanical robustness and suspect failure in long‐term MD applications 13,14 …”