Osmotically Driven Membrane Processes for Water Treatment: Membrane Synthesis and Transport Mechanism Exploration
Loading...
Author(s)
Liu, Su
Advisor(s)
Editor(s)
Collections
Supplementary to:
Permanent Link
Abstract
Water is a necessity in people’s life. In the 21st century, the water shortage has become one of the global challenges. The limited freshwater resource, the population growth, and climate change exaggerate the situation. Effective strategies and technologies for water treatment and reuse are needed to address this issue. Membrane technologies have the potential to be used in water treatment and water reuse areas. A semi-permeable membrane is the key component of a membrane separation system. The objective of this dissertation is to design membranes for osmotically driven membrane processes for water treatment.
Forward osmosis (FO) is an osmotically driven membrane process, which can be used for water treatment. The FO system extracts water from the feed solution to draw solution under the osmotic pressure difference. The performance of the FO membrane is very important for ensuring the system efficiency. Novel tannic acid - ferric (TA-Fe) complex layer was coated onto a commercially available cellulose triacetate (CTA) FO membrane. The coating layer modified the membrane with a more hydrophilic, less rough surface and a reduced pore size. The coated membrane has a better salt/water selectivity while water permeability loss was mild (about 13.6%). Multiple membrane performances were enhanced, including the reverse salt selectivity, the anti-fouling property, and the rejection of the micropollutants. Mathematical models including the resistance-in-series model and XDLVO model were used to analyze the underlying mechanisms regarding the performance improvement.
Graphene oxide membranes (GOMs) with nano-sized pores and highly tortuous structures are promising separation technologies because of their high water permeability. We found that the reverse salt flux from the draw solution to the feed solution is low in FO. However, in the hydraulic pressure driven technologies such as RO, the rejection is low (the salt flux is high) compared to that in the commercially available RO membranes. This counterintuitive phenomenon was examined to understand the transport mechanisms in GOMs. Based on the analysis, the water flux from the feed solution to the draw solution can prevent ions transporting from draw solution to the feed solution. The reverse salt concentration profile in the GOM can be shortened by the water flux from the feed solution to the draw solution. In addition, the EDL thickness was also decreased with the increasing NaCl concentration in the draw solution.
To further explore the potential applications for GOMs to be used in osmotically driven membrane processes for water treatment, the investigation of forward solute transport was initiated with the freestanding GOMs. Both uncharged solutes and charged solutes were utilized as the feed solutes in the feed solution in FO. The water flux and forward solute flux were tested. The Donnan steric pore model model (DSPM), which considers diffusion, convection and electromigration was also utilized to calculate forward solute flux of the freestanding GOM in the FO mode. According to the analysis, freestanding GOM has a better separation performance for multivalent ions than the monovalent ions. The forward solute transport of the charged solutes is mainly governed by the steric exclusion, interfacial Donnan exclusion and also the EDL screening along the nanochannels in the GOMs.
This dissertation presents investigations on the development of osmotically driven membranes for water treatment. The findings and implications from the fundamental studies can provide further guidance for future membrane design and application.
Sponsor
Date
2021-01-19
Extent
Resource Type
Text
Resource Subtype
Dissertation