Influences of molecular weight, molecular size, flux, and recovery for aromatic pesticide removal by nanofiltration membranes

Eleven aromatic pesticides were used for a removal study using a 4040 spiral-wound polyamide nanofiltration (NF) membrane. The influences of molecular weight, molecular size (length and width), flux, and recovery were studied. The molecular weights of these pesticides are from 198 Da to 286 Da. Molecular sizes were determined by theoretical calculation for their length and width by “Hyperchem” based on their structures and orientation. Furthermore, the study held constant for two operated recoveries and fluxes to determine their effects. The results showed that the NF membrane can remove pesticides from 46% to 100% based on their molecular weights, lengths, fluxes and recoveries. The rejections were increased as the molecular weight increased, and a sharp increase to complete rejection (100%) was observed around MW 200 Da. Therefore, a molecular weight cut-off (MWC) of 200 Da can be determined for this membrane from this result. In addition, the results showed the molecular length was more significant than molecular width for these pesticides. The rejections were not only dependent on molecular weight and length, but also on operational flux and recovery. For a particular pesticide in the two operational fluxes and recoveries, the highest percent rejections occurred on high flux and low recovery, and lowest percent rejection occurred on low flux and high recovery, which would indicate the basic diffusion control theory.

The results showed that the NF membrane can remove pesticides from 46% to 100%, and rejection increased with molecular weight. A sharp increase to complete rejection was observed around MW 200 Da except for diuron. Therefore, a MWC of 200 Da can be determined for this membrane from these results. Diuron was also the least rejected pesticide in two previous studies [9,10] for NF membranes. In addition, the results showed that molecular length was more significant than width for pesticide rejection. Rejection was not only dependent on molecular weight and length, but was also dependent on operational flux and recovery. For a particular pesticide in the two operational fluxes and recoveries, the highest percent rejection occurred at high flux and low recovery, and the lowest percent rejection occurred at low flux and high recovery, which would indicate the basic diffusion-control theory. From the above results, altering operation of flux and recovery can affect up to 30% of the rejection and can be viewed as one of the mechanisms for partially rejected pesticides.