Polyamide desalination membrane characterization and surface modification to enhance fouling resistance.

Abstract

The m arket f or pol yamide de salination m embranes i s e xpected t o c ontinue t o grow dur ing t he coming decades. Purification of alternative water sources will also be necessary to meet growing water d emands. Purification of pr oduced w ater, a b yproduct of oi l a nd g as pr oduction, i s of interest due t o i ts dua l pot ential t o pr ovide w ater f or be neficial us e as w ell a s t o reduce wastewater disposal costs. However, current polyamide membranes are prone to fouling, which decreases w ater f lux an d s hortens m embrane l ifetime. T his r esearch ex plored s urface modification us ing pol y(ethylene glycol) di glycidyl e ther ( PEGDE) t o i mprove t he fouling resistance of commercial polyamide membranes. 4Characterization o f co mmercial p olyamide m embrane p erformance w as a n ecessary first s tep before unde rtaking s urface m odification s tudies. M embrane pe rformance w as f ound t o be sensitive t o c rossflow t esting conditions. C oncentration pol arization a nd f eed pH s trongly influenced NaCl rejection, and the use of continuous feed filtration led to higher water flux and lower NaCl rejection than was observed for similar tests performed using unfiltered feed.Two c ommercial pol yamide m embranes, i ncluding one reverse os mosis a nd one na nofiltration membrane, w ere m odified b y grafting P EGDE t o t heir s urfaces. T wo di fferent P EG m olecular weights (200 and 1000) and treatment concentrations (1% (w/w) and 15% (w/w)) were studied. Water flux decreased and NaCl rejection increased with PEGDE graft density (μg/cm 2 ), although the largest changes were observed for low PEGDE graft densities. Surface properties including hydrophilicity, roughness and charge were minimally affected by surface modification.The fouling resistance of modified and unmodified membranes was compared in crossflow filtration studies using model foulant solutions consisting of either a charged surfactant or an oil in water emulsion containing n-decane and a charged surfactant. Several PEGDE-modified membranes demonstrated improved fouling resistance compared to unmodified membranes of similar initial water flux, possibly due to steric hindrance imparted by the PEG chains. Fouling resistance was higher for membranes modified with higher molecular weight PEG. Fouling was more extensive for feeds containing the cationic surfactant, potentially due to electrostatic attraction with the negatively charged membranes. However, fouling was also observed in the presence of the anionic surfactant, indicating hydrodynamic forces are also responsible for fouling. Finally, I would like to thank my family and friends. Old friends and new friends made in graduate school have made this experience more enjoyable. I am grateful to have met my friend and fiancé, Jason Dees, during graduate school, and I look forward to our future together. My family stands by me in everything I do, always ready to help in any way possible. I am very thankful for their constant love and support. 5 ACKNOWLEDGEMENTS