Membrane scaling in Vacuum Membrane Distillation - Part 1: In-situ observation of crystal growth and membrane wetting

Porous evaporators with special wettability for low-grade heat-driven water desalination

“…If MD is applied for the high salinity wastewater treatment, membrane scaling may become a major and unavoidable technical challenge because the feed solution will eventually become oversaturated. [150][151][152] The formation of mineral scales deposited on the membrane surface or inlaid in the membrane pores can both induce scaling that reduced water flux and amplify wetting that reduced salts' rejection efficiency. A schematic illustration of the possible anti-scaling behavior over the superhydrophobic membrane with a slippery surface is shown in Fig.…”

A Critical Review of Membrane Wettability in Membrane Distillation from the Perspective of Interfacial Interactions

“…Wetting mechanisms in MD are not yet clearly understood and several mechanisms have been proposed. The related mechanisms in membrane wetting may involve capillary effects, diffusion, , fouling and scaling, , chemical oxidative degradation, , adsorption, hydrophobic–hydrophobic interaction, , electrostatic attraction, and microbial growth and secretion of hydrophilic extracellular polymeric substances. , The hydrophobic–hydrophobic interaction and adsorption are among the widely proposed and studied. In MD process, the driving force is determined by the balance between the absolute pressure on the feed side, the capillary pressure caused by surface tension and the vapor pressure, which has an opposite effect. ,, On one hand, deposition of mineral salts on membrane surface or in membrane pores compromises the hydrophobic membrane into more hydrophilic, resulting in membrane wetting. , Surfactants, organics, or oils usually include a hydrophobic tail moiety that allows them to adsorb on the surface and pore walls of MD membranes through hydrophobic–hydrophobic interactions, as well as diffusion or electrostatic attraction, making the membrane surface and pores less hydrophobic. ,,,, On the other hand, both γ l and θ* are reduced at the pore entrance due to concentration polarization and temperature polarization, thereby lowering the LEP (eq ), which could lead to membrane wetting.…”

“…The same solution caused higher partial wetting of the PVDF membrane with 0.45 μm pore size compared to the membrane with 0.22 μm pores . When solvents are present in the feed solutions, volatile organic compounds (VOCs), including ethanol, butanol, and ethylene glycol, could accumulate on membrane surface and pores, thereby decreasing the γ l of feed mixtures and causing wetting. ,,, In comparison, some VOCs penetrated the membrane pores easily without the occurrence of membrane wetting, but the high concentration of VOCs in the permeate could be another challenge . The effect of organics on wetting propensity in MD is summarized in Table S5.…”

“…It is possible to visualize wetting propagation and dewetting behavior in MD systems using an in situ visualization systems. ,, Using optical microscopy coupled with an image processing routine, in situ observation of scaling induced wetting for NaCl and CaSO 4 was feasible, but this method was not as effective for small CaCO 3 deposits . On the basis of the ex-situ detection of a tracer (salt) intrusion by scanning electron microscope-energy dispersive spectrometer (SEM-EDS), it is possible to obtain totally wetted surface ratio (ω s , the proportion of the length covered by salt in the permeate side to the total observed length of the membrane) and pore wetting ratio (ω p , wetted thickness divided by the membrane thickness). , Kim at al .…”

“…116 Wetting mechanisms in MD are not yet clearly understood and several mechanisms have been proposed. The related mechanisms in membrane wetting may involve capillary effects, 118 diffusion, 118,119 fouling and scaling, 7,120 chemical oxidative degradation, 121,122 adsorption, 119 hydrophobic− hydrophobic interaction, 7,54 electrostatic attraction, 123 and microbial growth and secretion of hydrophilic extracellular polymeric substances. 124,125 The hydrophobic−hydrophobic interaction and adsorption are among the widely proposed and studied.…”