Difunctional MOF-wrapped graphene membranes for efficient photothermal membrane distillation and VOCs interception

Abstract

Photothermal membrane distillation (PMD) has the potential to address freshwater scarcity, but heat loss during heat transfer from the photothermal surface to the feed bulk inevitably lower the photothermal-vapor conversion efficiency of this process. The permeation of VOCs during PMD also threatens the recycling of distilled water. Herein, a zeolitic imidazolate framework-67 (ZIF-67) wrapped graphene membrane (ZGM) was developed and used for the PMD process. The results demonstrated that the hierarchical porous structure and low heat conductivity of ZIF-67 endowed the ZGM with improved light absorbance and heat localization, thus sustaining a high cross-membrane temperature gradient. Under simulated sunlight illumination, the ZGM attained an additional flux of 0.91 kg m−2 h−1, corresponding to a photothermal efficiency of 62.1%, which was 183.2% higher than that of the pristine membrane. The temperature change near the photothermal coating was simulated, which showed that ZGM had slower heat conduction into the feed, demonstrating a confined heat effect. Meanwhile, the ZGM exhibited a good interception for phenol when peroxymonosulfate (PMS) was added to the hot feed. The distilled phenol concentration was 88.0% lower than that of the PMD without PS. Electron paramagnetic resonance (EPR) and radical quenching experiments verified that singlet oxygen (1O2) was the dominant reactive oxygen species during in-situ phenol degradation. This work provides a new strategy to develop advanced photothermal membranes to simultaneously promote the photothermal performance and VOC interception.