Effect of Directionally Aligned Channels in Composite Hydrogels on Solar Steam Generation and Adsorption of Perfluorooctanoic Acid

Abstract

Solar-powered steam generation is acknowledged as a highly efficient method for seawater desalination and wastewater purification. However, simultaneous generation of fresh water and removal of pollutants is still challenging for the treatment of saline wastewater containing organic pollutants. Herein, composite hydrogels with vertically aligned channels were prepared by directional freezing technology. The formation of vertical channels could greatly reduce mass transfer resistance and improve both interfacial evaporation rate and adsorption rate of pollutants. Compared to the non-vertically aligned composite hydrogel, this uniquely structured hydrogel exhibited superior desalination performance, attaining an evaporation rate of 1.99 kg·m⁻²·h⁻¹ and an energy conversion efficiency of 94.36% when exposed to one sun's illumination. Meanwhile, the hydrogel evaporator showed outstanding salt resistance even in the evaporation of brine with high salinity and stable desalination performance over successive evaporation for 10 cycles. In addition, the composite hydrogel was used as an adsorbent for the removal of perfluorooctanoic acid (PFOA). Due to the presence of directional channels, the composite hydrogel achieved higher adsorption capacity and faster adsorption rate than the non-vertically aligned composite hydrogel. This work realized integration of solar desalination and PFOA removal in a single evaporator, offering a facile strategy for treating saline wastewater containing organic pollutants.