Ultrahigh and Stable Water Recovery of Reverse Osmosis-Concentrated Seawater with Membrane Distillation by Synchronously Optimizing Membrane Interfaces and Seawater Ingredients

Abstract

Membrane distillation holds promise for further recovery of reverse osmosis (RO)-concentrated seawater to realize zero-liquid discharge; however, severe scaling induced by high-concentration salts and fouling triggered by a naturally occurring substance would result in severe wetting of the hydrophobic distillation membrane. To retard membrane wetting during the concentration process, the membrane’s interfacial structure and the seawater’s ingredients were systematically investigated to improve the membrane’s durability. The results demonstrated that the dual-layer membrane with hierarchically rough beads-on-a-string structured fibers could enlarge the water evaporation area on the membrane surface to improve membrane water flux, and superhydrophobicity can reduce the contact area between the salt and membrane surface to prevent membrane scaling. Significantly, the membrane’s durability can be further improved by adjusting the seawater ingredients. This finding revealed that the scaling resulting from Ca2+ and Mg2+ when they exist alone is more severe than when they exist together. Therefore, eliminating the dominant Ca2+, Mg2+, and naturally occurring substance in seawater and maintaining it in a neutral state can stably concentrate the seawater with a high water recovery of ∼85%. The regulation of the membrane interface integrated with optimal seawater ingredients is essential for the application of membrane distillation in the treatment of RO-concentrated seawater.

All-Polymer and Self-Roughened Superhydrophobic PVDF Fibrous Membranes for Stably Concentrating Seawater by Membrane Distillation

“…A small amount of saline water was infiltrated into the membrane in 65 min, while a large amount of saline water was filled into the membrane as the time increased to 220 min, along with the decline of membrane flux (Figure A). The saline water permeating into the membrane pores greatly reduced the membrane heat transfer efficiency because the vapor–liquid–vapor–liquid multiple-phase transition greatly reduced the membrane thermal driving force than the traditional liquid–vapor–liquid phase transition . Interestingly, the membrane flux was significantly increased to 42.73 L m –2 h –1 after F-POSS was added to the PVDF FM.…”

“…Interestingly, the membrane flux was significantly increased to 42.73 L m –2 h –1 after F-POSS was added to the PVDF FM. This reason was attributed to the fact that the increased membrane roughness led to a larger contact point for saline water evaporation, as shown in Figure S4. , Nevertheless, the membrane flux was decreased to 35.99 L m –2 h –1 after the F-POSS/PVDF FM was sandwiched between the two glass panels. This reason was attributed to the fact that the increased pressure applied to the membrane surface obviously improved the membrane density, while decreasing the membrane porosity (Figure F).…”

“…The temperature of hot-pressing on the membrane surface wettability and pore structure may also affect the membrane performance . Hence, the concentration performance of the HP F-POSS/PVDF FM that was thermally treated at different temperatures is shown in Figure A,B.…”

“…The samples for membrane morphology, pore size, thickness, stress, roughness, contact angle, thermal stability, surface functional groups, and near-surface chemical information used the apparatus as described in our previous studies. , The membrane porosity ( P ) was investigated by the tert -butanol uptake method according to the previous study . The total organic carbon (TOC) and the ion concentration of the natural seawater and distilled water were also prepared as described previously. , The excitation–emission–matrix spectrum (EEM) of natural seawater and the water after washing the used membrane were recorded by a fluorescence spectrophotometer (Hitachi, F-7000).…”

“…The distillation and concentration performances of relevant membranes were estimated using the direct contact membrane distillation (DCMD) apparatus with an effective surface area of 9 cm 2 (3 cm width and 3 cm long) described in our previous study. , The initial feed volume was fixed at 500 mL. The temperature of feed and distillate was always maintained at 60 and 20 °C, respectively, and the flow rate of feed and distillate was fixed at 240 mL min –1 .…”