Ionic separation of the IEX spent regenerant using Nanofiltration

Abstract

The need to treat waste streams produced during the water treatment processes is becoming a challenge nowadays due to the industries’ disposal expenditures, the limitations imposed by laws and the environmental impacts. The spent regenerant from ion exchange (IEX) processes is an example of such streams. This study investigated the use of Nano-filtration technique for separation as a part of the treatment step for IEX spent regenerant. This research is focused on the parameters such as membrane characteristics, ionic composition and operating conditions that affect the ion rejection of the IEX spent regenerant. This research aims to select a membrane that contributes to a maximum separation of monovalent and divalent ions of the IEX spent regenerant.
During this research six nanofiltration membranes and two reverse osmosis membranes were tested, of which the molecular weight cut-off (MWCO) was measured. Each membrane was placed at a flow Cell unit and fed with artificial water.
The MWCO of the nanofiltration membranes was varied from 115 to 508 Da. The purpose of testing different membranes was to assess how the pore size of the membrane influences the ion rejection of the IEX spent regenerant. In addition, the ion rejection was evaluated in different ionic strengths and different molar ratios. The aim was to investigate how the ionic composition influences the ion rejection. Finally, operating parameters such as permeate flux and temperature were tested to research their impact on the ion rejection of the IEX spent regenerant.
Results showed that the ion rejection of the IEX spent regenerant was strongly affected by the pore size of the membrane as the charge effect was insignificant due to the high ionic strength of the feed solution (1 M). The main exclusion mechanisms were the steric and the dielectric exclusion. Loose membranes with MWCO greater than 500 Da rejected less than 10% of both monovalent and divalent ions. Membranes with pore size from 200 Da to 300 Da rejected 30-60% of Ca2+ and contributed to negative rejections of Na+. NF membranes with MWCO of approximately 150 Da, rejected 90% of Ca2+ and 40% of Na+. Tighter NF membrane and RO membranes rejected more than 95% of divalent ions and more than 70% of monovalent ions.
It was also observed that the ionic composition did not influence the rejection of divalent cations as it remained almost constant in different ionic strengths and molar ratio of the solution. Contrary, the rejection of monovalent cations was greatly influenced by the molar ratio of the solution due to the presence of predominant amounts of ion of higher charge and the need of the ions to maintain electroneutrality. Higher molar ratio resulted in lower rejection of monovalent cations.
The permeate flux affected the ion rejections. Higher fluxes resulted in higher ion rejection and different ion separation. To keep high rejection of bivalent and low rejection of monovalent ions a compromise would be desired. Temperature had a great influence on the ion rejection of the IEX spent regenerant as a 5oC increase in the temperature cased 19% decrease in divalent cation rejection.
TS80, one of the nanofiltration membranes, performed better than other membranes as it rejected 90% of Ca2+ and 40% of Na+. Double Pass NF with TS80 membrane was proposed as the first treatment step of the IEX spent regenerant stream as it resulted in 97% Ca2+ and 45% Na+ rejection.