Recovery of CaSO4 and NaCl from sub-soil brine using CNT@MOF5 incorporated poly(vinylidene fluoride-hexafluoropropylene) membranes via vacuum-assisted distillation

Abstract

The recovery of salts from sub-soil brine can fulfil the demand for the salts and chemicals for the industries and mitigate the problem of fresh water at the same time. This study reported the performance of a vacuum-assisted membrane distillation crystallization (VMDCr) for CaSO4 and NaCl salts recovery from sub-soil brine. The facile one-step synthesis of porous MOF5 and CNT@MOF5 composites was also reported. Poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) mixed matrix flat-sheet membranes incorporated with 1 wt% of MOF5 and CNT@MOF5 were used for VMDCr. The membranes and materials were characterized by scanning electron microscope (SEM), thermogravimetric analysis (TGA), X-ray diffraction (XRD) and Fourier-transform infrared (FT-IR) analyses. The incorporation of CNTs to MOF5 did not affect the MOF5 crystallinity, confirmed by XRD and FT-IR. The MOF5 and CNT@MOF5 incorporated PVDF-HFP matrix membranes (designated as M-ZM and M-CZM, respectively) showed rough and porous morphology. Moreover, the M-CZM showed improved porosity and number of pores than the M-ZM membrane. As a result, M-CZM produced 2.1 times the flux than the pristine PVDF-HFP membrane facilitating faster crystallization. In the density span of 15.83°Be' to 24.5°Be', CaSO4 precipitated over the membrane surface, and complete precipitation of CaSO4 was achieved once the feed (sub-soil brine) reached 24.5°Be'. NaCl crystals started forming on the membrane surface after the density of the feed reached 25°Be'. The NaCl salt precipitated completely once the sub-soil brine density reached 28.5°Be'. The SEM and XRD analyses of the recovered monoclinic CaSO4 and cubic NaCl crystals revealed their good geometrical and high purity. This study concludes that the M-CZM membrane can improve the membrane properties for faster salt crystallization from sub-soil brine.