Nanofiltration of Natural Organic Matter: pH and Ionic Strength Effects

Abstract

Studies were conducted to examine the effect of solution chemistry, defined here as pH and ionic strength, on the permeability of negatively charged polymeric nanofiltration membranes. Water permeation through the membrane was demonstrated to decrease at conditions of low pH and high ionic strength in the absence of organic macromolecules. The reduction in membrane permeability was attributed to a compaction of the membrane matrix resulting from charge neutralization at the membrane surface and electric double layer compression. An uncharged model organic macromolecule (polyethylene glycol) was used to quantify the effects of solution chemistry on membrane compaction and solute rejection capabilities of the charged membrane. Studies of membrane permeability and rejection were then repeated with solutions containing natural organic matter (NOM), enabling concurrent evaluation of the effects of electric double layer compression as well as changes in both membrane structure and the apparent macromolecular size of charged NOM macromolecules.