The influence of solution chemistry and operating conditions on nanofiltration of charged and uncharged organic macromolecules.

摘要

For separations by nanofiltration (NF) membranes to be effective in removal of dissolved natural organic matter (NOM), an understanding of the nature of membrane fouling is necessary. This dissertation addresses the role of solution chemistry (pH, ionic strength, and electrolyte composition) on membrane fouling, through examination of NF performance during filtration of uncharged and charged macromolecules.; Polyethylene glycol (PEG) was used as an uncharged macromolecule to isolate the influence of ionic strength on size exclusion capabilities of the NF membrane, without the repulsion and size change effects associated with charged molecules. Suwannee River organic matter (SROM) was selected to examine the influence of NOM on NF membrane fouling and the influence of charge repulsion on macromolecular solute rejection. The apparent molecular size distribution (AMSD) of PEG and SROM were determined by ultrafiltration. No change was identified in the AMSD of PEG between low and high ionic strength. The AMSD of SROM, however, shifted in the direction of lower molecular size fractions with low pH and high ionic strength, as predicted by polyelectrolyte solution theory.; NF experiments were conducted in a recycle batch mode using cross-flow, hollow-fiber membranes constructed of sulfonated polysulfone (Zenon Environmental, Inc.). Feed solutions were varied in pH, ionic strength, and selection of organic macromolecule (PEG or SROM). With organic-free solutions NF membrane permeability decreased at low pH and high ionic strength owing to the compacted membrane structure produced by reduced electrostatic charge repulsion. The increase in PEG rejection at high ionic strength presented further evidence for the increase in membrane compaction, manifested as a shift to lower membrane molecular weight cut-off. Introduction of SROM into the feed showed that permeate flux and solute rejection were reduced at low pH and high ionic strength. These effects were caused by a combination of reduced electrostatic charge repulsion at the membrane-solution interface and a shift in AMSD of SROM to a smaller apparent size range. Examination of SROM attachment to the membrane surface also revealed greater adsorption at low pH and high ionic strength, in a manner consistent with electrostatic adsorption principles. Increase in cross-flow velocity generated some increase in flux but minimal improvement in solute rejection, indicating that SROM association with the membrane surface is primarily of a chemical nature.