M.Sc Thesis


M.Sc StudentHong Zihao
SubjectFacile Superhydrophobic Surface Modification of
Commercial Polyvinylidene Fluoride Membrane with
TiO2 Nanoparticles for Improvement of
Membrane Distillation...
DepartmentDepartment of Chemical Engineering
Supervisors DR. Bo Kong
PROFESSOR EMERITUS Raphael Semiat


Abstract

Membrane distillation (MD) is an emerging separation technology combining distillation with the membrane separation process. Different from the conventional membrane processes, the driving force of MD is the vapor pressure difference caused by the temperature gradient on the two sides of the membrane. The absence of expensive components, such as high-pressure pumps and pressure exchangers, reduces the capital cost to a certain extent. Furthermore, 100% theoretical rejection of non-volatile components and the capability of utilizing low-grade heat energy render MD the potential for desalination, wastewater treatment, removal of volatile components, etc. However, during the long-term operation, the inevitable wetting and fouling phenomenon could lead to the deteriorated permeate quality and efficiency of MD.

Herein, a modified PVDF membrane was fabricated to mitigate the fouling and wetting phenomenon by grafting fluorinated TiO2 nanoparticles onto the substrate. The modification enhanced the roughness and lowered the surface energy, endowing the PVDF membrane a superhydrophobic surface. When immersed into the solution, an interface of solid-vapor-liquid will be formed on the superhydrophobic surface, which minimizes the direct contact between membrane surface and foulants (dissolved organic matters, surfactants, salts, etc.).

The surface morphology, surface wettability and chemical composition of resultant membrane were characterized and evaluated through contact angle measurements, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR) spectroscopy, etc. The maximal water contact angle of the modified membrane was up to 157.4° while the sliding angle was smaller than 5°. The hierarchical micro/nano rough structures were observed on the membrane surface via the SEM images, which were constructed by the distributions of aggregated TiO2 nanoparticles. The results of FTIR-ATR and XPS spectra further confirmed that fluorinated TiO2 nanoparticles were successfully attached to membrane surface.

The anti-fouling property of the commercial and modified membrane was investigated by direct contact membrane distillation (DCMD) experiments with feed solution containing NaCl, CaCl2, and natural organic matters (NOM). The scaling resistance was evaluated by the scaling experiment of CaSO4. The anti-wetting property was tested with feed solutions containing NaCl, Sodium dodecyl sulfate (SDS) and Dodecyl Trimethyl Ammonium Chloride (DTAC). An elevated performance on fouling and wetting resisting was observed from the modified membrane through the relatively stable fluxes and permeate conductivities.