טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentNativ Paz
SubjectDevelopment and Investigation of Electrochemical Membrane
Processes for the Selective Separation of Ions
DepartmentDepartment of Civil and Environmental Engineering
Supervisor Dr. Youri Gendel
Full Thesis textFull thesis text - English Version


Abstract

One of the most important challenges facing modern science and technology is a development of new technologies for the effective, efficient and selective recovery and reuse of valuable chemicals from seawater, wastewater, process water and brines. Selective ion separation is essential in a variety of industrial and water treatment applications. This study focuses on selective ion separation via electrochemical membrane processes using either thin film composite membrane (TFCM), ion exchange membranes or a combination of both membrane types.

The research comprises four sections. In the first section, the mechanism of ions’ removal in a flow electrode capacitive deionization (FCDI) and Faradaic reactions that occur on the activated carbon electrodes were studied. These reactions underline an additional electro-dialytical desalination mechanism within the capacitive deionization (CDI), which proceeds in parallel to the well-known electro-sorption mechanism. In addition, a possibility of energy generation from mixing acidic and basic solutions in the FCDI reactor was investigated.

The second section was dedicated to the investigation of the electrical conductivity of nanofiltration (NF) thin film composite NF270 membrane which was successfully improved by a factor of 20 for the implementation in electrochemical membrane processes. Afterwards, the NF270 membrane was used instead of an ion exchange membrane (either anion or cation exchange membrane) in a FCDI reactor and its ion separation performance was studied.  This NF-FCDI reactors showed no separation capabilities towards cations but excellent separation capabilities towards anions. Within a third part of the study, the separation performance of the NF270 membrane was tested in an electrodialysis (ED) reactor and the specific ion permeabilities were obtained and compared to the pressure driven nanofiltration process operated with the same membrane. Our findings show that: (i) ionic transport in thin film composite membranes operated in electrochemical processes cannot be described by the conventional ion exclusion models such as Born or Donnan exclusion mechanisms; and (ii) a different, currently unknown, mechanism dominates the ion separation in NF-ED and NF-FCDI processes.

In the fourth and the last section, the use of monovalent selective ion exchange membranes in an electrodialysis reactor was tested for the removal of chloride from magnesium-rich NF brine. The influence of different operational parameters on the selectivity of the monovalent selective ion exchange membranes was studied. This novel process showed outstanding performance and Cl-to-Mg weight ratio of 4 was achieved in the produced Mg-rich brine which is significantly lower than Cl-to-Mg ratios of 10-70 previously reported for other available technologies.