Evaluation of a recently created membrane incorporating titanium oxide nanotubes and polyethersulfone for water desalination through vacuum membrane distillation -

Evaluation of a recently created membrane incorporating titanium oxide nanotubes and polyethersulfone for water desalination through vacuum membrane distillation

Introduction

Membrane technology is widely acknowledged as a highly efficient separation method, especially in the realms of water/wastewater treatment and water desalination. The incorporation of nanoparticles or nanotubes into polymeric membranes has become a focal point of recent research efforts.

This paper presents a thorough investigation into the performance of a newly developed membrane comprising titanium oxide nanotubes (TNTs) incorporated into a Polyethersulfone (PES) blend for desalination through vacuum membrane distillation (VMD). The study explores the impact of various operational conditions. The findings reveal a peak salt rejection of 98% and a permeate flux of 15.2 kg/m2 h at a feed salt concentration of 7000 ppm for the TNTs–PES membrane, operating at 65 °C with a vacuum pressure of 300 mbar and a feed flow rate of 11 mL/s. Comparative analysis with a commercial Polytetrafluoroethylene (PTFE) membrane, under different feed salt concentrations, demonstrates a notable enhancement in the performance of the newly developed TNTs-PES membrane. At a feed concentration of 3000 ppm, the new membrane achieves a salt rejection of 99.3%, contrasting with PTFE membrane’s 90.6%, and at 35,000 ppm, the salt rejection is 96.7% for TNTs-PES and 62.5% for PTFE. The dense TNTs layer on the membrane’s surface acts as a selective barrier, impeding salt passage. To address a decline in permeate flux, periodic membrane washing is suggested. In the process of membrane fabrication, the incorporation of nanoparticles and nanotubes has emerged as a promising avenue for enhancing the performance of polymeric membranes. The noteworthy impact of integrating nanoparticles/nanotubes into membrane preparation can be ascribed to the interactions between the surfaces of nanoparticles and polymer chains and/or solvents. These interactions contribute to the formation of well-structured membranes with desirable properties. These structural modifications of the membrane give rise to improved selectivity, permeability, and overall satisfactory performance, particularly in the context of ultrafiltration and nanofiltration membranes.

Methodology

A self-organized synthesis approach utilizing the hydrothermal method was employed to prepare titanium oxide nanotubes. TiO2 nanopowder was dispersed in 150 mL of 10 M NaOH and stirred for 15 minutes. Subsequently, the dispersion was transferred to a Teflon-lined autoclave (KH-300) and subjected to heating at 150 °C for a duration of 16 hours. The resulting white precipitate was initially washed with 1 M HCl, followed by rinsing with distilled water, and ultimately dried at 80 °C for a period of 4 hours. To prepare the solution of titanium oxide nanotubes, an initial addition of 5 wt.% tetramethylsiloxane was made to the acetonitrile solvent. Subsequently, 5 wt.% of titanium oxide nanotube powder was introduced into the mixture with continuous stirring, maintained for a duration of 3 hours. These membranes were used in desalination process.

Results: The nanotubes that were fabricated underwent characterization through X-ray diffraction (XRD) analysis. This indicates that the nanotubes obtained are a combination of various sodium titanium oxides and titanium oxide nanotubes. The nanotubes exhibit a narrow size distribution, with an inner diameter of approximately 1.67 nm, an outer diameter of 8.35 nm, and a length measuring about 141 nm. The morphology of the membrane was characterized by a scanning electron microscope (SEM) showing top surface appears as a dense layer given the presence of the titanium nanotubes. It’s noteworthy to highlight that both permeate flux and salt rejection show an increase as the vacuum pressure decreases on the permeate side.

Incorporating titanium oxide nanotubes into the fabrication of polymeric membranes has led to substantial enhancements in membrane performance, particularly in terms of permeate flux and salt rejection. The study conducted vacuum membrane distillation (VMD) experiments for water desalination under various operating conditions. Results indicated a maximum salt rejection of 98% and a permeate flux of 15.2 kg/m2 h under optimal conditions: 65 °C feed temperature, 7000 ppm feed salt concentration, 300 mbar vacuum pressure, and 11 mL/s feed flow rate. Notably, the permeate flux was significantly influenced by the duration of the VMD experiment, with the formation of salt scales hindering fluid flow across the membrane after 40 minutes of operation.

The developed TNTs-PES blend membrane exhibited superior performance compared to the commercial PTFE hydrophobic membrane in terms of salt rejection. When employed in VMD with a feed concentration of 3000 ppm, the TNTs-PES membrane demonstrated a higher salt rejection of 99.3% and a greater permeate flux of 24.9 kg/m2 h, as opposed to 90.6% and 21.5 kg/m2 h for the PTFE membrane. Similarly, at a high salt feed concentration of 35,000 ppm, the new membrane achieved a 96.7% salt rejection compared to 62.5% for the PTFE membrane. However, the permeate flux of the new membrane declined with increasing feed concentration to 5.5 kg/m2 h, compared to 7.2 kg/m2 h using the PTFE membrane. This decline was attributed to the dense layer formed on the top surface of the TNTs-PES blend membrane, limiting permeate flux at feed salt concentrations exceeding 7000 ppm.

Conclusion

Based on these findings, the authors suggest the potential use of the developed TNTs-PES blend membrane in nanofiltration (NF) and reverse osmosis (RO) operational regimes in addition to VMD operations, owing to its dual hydrophobic/hydrophilic property.

Reference:

Abdallah, H., Moustafa, A.F., AlAnezi, A.A. and El-Sayed, H.E.M., 2014. Performance of a newly developed titanium oxide nanotubes/polyethersulfone blend membrane for water desalination using vacuum membrane distillation. Desalination, 346, pp.30-36.