Effects of membrane fouling on the nanofiltration of trace organic contaminants

Organic fouling and its effects on trace organic retention by three nanofiltration (NF) membranes were investigated in this study. Accelerated fouling conditions were achieved with foulant cocktails containing humic acids in a background electrolyte solution. The effects of membrane fouling on the separation process were delineated by comparing the retention values of clean and fouled membranes and relate them to the membrane pore size as well as physicochemical characteristics of the trace organics. Fouling was more severe for the more open pore size TFC-SR2 and NF-270 membranes as compared to the tighter NF-90 membrane. Results reported here indicate that retention of trace organics by NF membranes was governed by steric hindrance, electrostatic, and hydrophobic interactions. However, steric hindrance (or size exclusion) appears to be the most prevalent mechanism controlling not only trace organic retention but also the membrane fouling process as well as the effects of fouling on retention. Pore blocking was a major reason for the more severe fouling by the loose nanofiltration TFC-SR2 membrane as compared to the NF-270 and NF-90 membranes which had smaller membrane pore size. Considerable increase in retention of trace organics by the TFC-SR2 membrane under fouled conditions was probably attributed to pore blocking, which was a dominating fouling mechanism for this loose NF membrane. While retention increase due to pore blocking was probably limited to only the TFC-SR2 membrane, retention of hydrophobic trace organics was enhanced under fouled conditions by all three membranes. This enhancement in retention of the hydrophobic trace organics could be explained by the formation of a fouling layer which reduced hydrophobic interaction between the hydrophobic trace organics and the membrane surface, hence reducing diffusional transport across the membrane.

Results reported here indicate that retention of trace organics by NF membranes can be governed by steric hindrance, electrostatic, and hydrophobic interactions. However, steric hindrance (or size exclusion) appears to be the most prevalent mechanism controlling not only trace organic retention but also the membrane fouling process as well as the effects of fouling on retention. A good correlation between trace organic retention and the membrane pore size was observed. Complete or near complete retentions of the trace organics were achieved with the tight nanofiltration NF-90 membrane. Pore blocking was a major reason for the more severe fouling by the loose nanofiltration TFC-SR2 membrane as compared to the NF-270 and NF-90 membranes which had smaller membrane pore size. This was consistent with the considerable increase in retention of trace organics by the TFC-SR2 membrane under fouled conditions. While retention increase due to pore blocking was probably limited to only the TFC-SR2 membrane, retention of the hydrophobic bisphenol A was enhanced under fouled conditions by all three membranes. This enhancement in retention of bisphenol A was attributed to the formation of a fouling layer which reduced hydrophobic interaction between bisphenol A and the membrane surface, hence eliminating or reducing diffusional transport of the compound across the membrane.