Doctor of Philosophy (PhD)
School of Civil, Mining and Environmental Engineering - Faculty of Engineering
Nghiem, Long Duc, Removal of emerging trace organic contaminants by nanofiltration and reverse osmosis, PhD thesis, School of Civil, Mining and Environmental Engineering, University of Wollongong, 2005. http://ro.uow.edu.au/theses/377
This study investigates the retention mechanisms of three prominent classes of emerging trace organic contaminants . natural steroid hormones, hormone mimicking compounds, and pharmaceutically active compounds . by several nanofiltration (NF) and reverse osmosis membranes. Laboratory-scale experiments were carried out using both cross flow and dead end stirred cell filtration equipment with the goal of relating trace organic retention behaviour to membrane characteristics, physicochemical properties of the trace organic molecules, and solution chemistry. The results reported here show that retention of neutral trace organics by a tight NF or RO membrane is dominated by steric (size) exclusion, whereas both electrostatic repulsion and steric exclusion govern the retention of negatively charged trace organics by a loose NF membrane. In the latter case, speciation of trace organics may lead to a dramatic change in retention as a function of pH, with much greater retention observed for ionized, negatively charged trace organics. Retention of the negatively charged trace organics decreases as the solution's ionic strength increases due to charge shielding and double layer compression. For uncharged trace organic species, intrinsic physicochemical properties of the trace organic molecules can substantially affect their retention. In their neutral form, natural steroid hormones, hormone mimicking compounds, and pharmaceuticals such as ibuprofen adsorb considerably to the membrane because of their relatively high hydrophobicity. Similarly, polarity (represented by the dipole moment) can influence the separation of molecules that are cylindrical in shape as they can be directed to approach the membrane pores head on due to attractive interaction between the molecule polar centers and fixed charged groups on the membrane surface. This phenomenon is probably inherent for high dipole moment organic compounds and the governing retention mechanism remains steric in nature. The adsorption of trace organics to the membrane polymer has several important implications. Firstly, because the adsorptive capacity of the membrane is limited, the final retention stabilizes when the adsorption of trace organics into the membrane polymer has reached equilibrium. At this later filtration stage, the overall hormone retention is lower than what expected based on solely size exclusion mechanism. This behaviour is attributed to partitioning and subsequent diffusion of hormone molecules in the membrane polymeric phase, which ultimately results in a lower retention. Trace organic diffusion in the membrane polymeric matrix most likely depends on the size of the molecule, hydrogen bonding of the compound to membrane functional groups, and hydrophobic interactions of the compound with the membrane polymeric matrix. Secondly, the membrane can serve as a large reservoir for trace organics and their release may be possible during membrane cleaning or erratic pH variation during operation. Treatment of membrane cleaning solution should be carefully considered when such trace organics are amongst the target contaminants in NF/RO membrane filtration. The study also critically demonstrates the possible complexity of a real membrane filtration system where trace organic contaminants are of concern. Several factors including operating conditions, the solution chemistry, and other constituents such as organic and particulate matter that may be present in the feed solution can influence the filtration of trace organics. Findings in this study are crucial in understanding the removal mechanisms and filtration processes of trace organic contaminants. However, the application of such findings to a practical situation requires a careful consideration of these factors.