Solar-driven evaporation is promising in oily wastewater treatment, in particular for emulsions, but conventional evaporators suffer from pore blocking by residual oil or contamination by volatile oil compounds in the condensed water.In the current research, we develop a suspended membrane evaporator integrating solar evaporation with oil-in-water emulsion separation. The heating and evaporating interface is separated from the rejecting interface to avoid oil escape and improve heat management. A temperature gradient forms on the membrane surface that can promote evaporation performance by combining both solar and environmental evaporation. Such an evaporator achieves a maximum evaporation rate of 1.645 kg/(m 2 •h) as well as an apparent evaporation efficiency of 111.9%. Moreover, the superhydrophilic and superoleophobic membrane shows excellent oil repellence and emulsion rejection, which can achieve an oil removal efficiency above 98.8% in oil-in-water emulsion separation, and high evaporation rate recovery in cycling tests. A scaled-up membrane evaporator array produces ∼8 kg/(m 2 •d) of clean water from oily wastewater in outdoor experiments, further demonstrating the strong purification performance of this evaporator in oily wastewater treatment.
Oil/water separation by porous materials has received
growing interest
over the past years since the ever-increasing oily wastewater discharges
seriously threaten our living environment. Purification of nano-sized
and concentrated emulsions remains a big challenge because of the
sharp flux decline by blocking the pores and fouling the surfaces
of those porous materials. Herein, we propose a solar-driven evaporator
possessing thin-film-composite architecture to deal with these two
bottlenecks. Inspired by plant roots, our evaporator composes of a
large-pore sponge wrapped by a thin hydrogel film, which is constructed
by the contra-diffusion and cross-linking of alginate and calcium
ions at the sponge surface. The dense superoleophobic hydrogel layer
serves as a selective barrier that prevents oil emulsions but allows
water permeation, while the inner sponge with large pores facilitates
water transport within the evaporator, ensuring sufficient water supply
for evaporation. By splitting the single evaporator into an array,
the evaporator performs a high evaporation rate of ∼3.10 kg·m–2·h–1 and oil removal efficiency
above 99.9% for a variety of oil emulsions. Moreover, it displays
a negligible decline in the evaporation rate when treating concentrated
emulsions for 8 h.
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