Effects of feed solution characteristics on the rejection of N-nitrosamines by reverse osmosis membranes
The rejection of eight N-nitrosamines was investigated in this laboratory-scale study, focusing on the influence of feed solution characteristics on their separation by low pressure reverse osmosis membranes. The rejection mechanisms of N-nitrosamines were first examined using one nanofiltration (NF90) and two reverse osmosis (TFC-HR and SWC5) membranes. The TFC-HR membrane was used to investigate the effects of feed solution characteristics. The rejection of a particular N-nitrosamine was generally membrane dependent and increased in the order of NF (NF90), low pressure RO (TFC-HR) and seawater RO (SWC5) membranes. In general, the rejection of N-nitrosamines by a given membrane also increased in the order of increasing molecular weight. These results suggested that steric hindrance was a dominating rejection mechanism of N-nitrosamines. Nevertheless, it was also observed from the result of N-nitrosomorpholine (NMOR) that the rejection of N-nitrosamines may also depend on other physicochemical properties such as hydrophobicity. A decrease in the feed solution pH (from 9 to 3) resulted in a decrease in the rejection of the two smallest molecular weight N-nitrosamines, namely N-nitrosodimethylamine (NDMA) and N-nitrosomethylethylamine (NMEA). Changes in the feed solution ionic strength (from 26 to 260 mM) caused a discernible decrease only in NDMA rejection, while no apparent impact on rejection was observed for an increase in the feed concentration. On the other hand, it is striking that an increase in the feed temperature led to a significant decrease in the rejection of all N-nitrosamines and the impact was more pronounced for the small molecular weight N-nitrosamines. For example, a significant drop in NDMA rejection (from 49 to 25%) was observed as the feed temperature increased from 20 to 30 degrees C. The results also indicate that pH, ionic strength, and temperature of the feed solution can exert some influence on the rejection of NDMA and in some cases other N-nitrosamines. The combined effects of these feed solution characteristics, particularly feed temperature, may account for some of the variation of NDMA rejection by RO membranes previously reported in the literature.