High-performance robust 2D graphitic carbon nitride nanosheets embedded membranes for desalination through direct contact membrane distillation

Abstract

Two-dimensional (2D) graphitic carbon nitride (g-C3N4)-based membranes have received widespread attention in the last ten years due to their fantastic separation performance. In the present investigation, for the first time, we tried to assess the desalination performance of membranes incorporated with g-C3N4 nanosheets through the membrane distillation (MD) process. for this purpose, different concentrations (0-0.06 wt.%) of synthesized nanosheets were embedded as fillers within the hydrophobic polyvinylidene fluoride (PVDF) matrix by using simple phase inversion method. Various techniques such as FTIR, XRD, SEM, AFM, water contact angle and LEP, tensile strength measurements as well as direct contact membrane distillation (DCMD) experiments were utilized to study the effect of the g-C3N4 contents on the elemental properties, crystalline structure, morphology, topography, surface hydrophobicity, mechanical resistance, and desalination performance of the membranes. The surface roughness parameters suggest that the membrane surface became even rougher by integrating g-C3N4 nanosheets into the membrane matrix. Upon loading with a small amount of 0.03 wt.% g-C3N4 in the polymer network, the water contact angle and LEP augmented from circa 80.5º and 6 bar for the pure PVDF membrane to 95.2º and 8 bar, respectively. Adding the g-C3N4 nanosheets also improved the mechanical properties of the PVDF membrane; so that, both the tensile strength and the elongation at break increased to 21.32% and 36.46% in comparison to the pristine PVDF membrane, respectively. The prepared mixed matrix membranes favorably improved MD performance; so that, 0.03 wt.% g-C3N4 embedded membrane exhibited a flux of 27.63 kg/m2h with an enhancement of 70% relative to its pure counterpart, despite having a trace amount of the nanosheets. Continuous testing for up to 24 h filtration of a 3.5 wt.% NaCl solution still showed a stable flux and almost complete salt rejection (i.e., 100%) for the mentioned mixed matrix membrane. This work holds promise for developing next-generation MD membranes with superior desalination performance in terms of water vapor permeability and salt rejection.