Effects of organic and inorganic matter on pesticide rejection by nanofiltration

This work explores the performances of two types of nanofiltration (NF) membranes (Desal DK and NF200) in the elimination of some selected pesticides presents in water (triazines and urea herbicides). Experiments were carried out on synthetic waters composed from distilled water to which has been added organic (humic acids) or inorganic (CaCl2 or CaSO4) matter and on a real water matrix collected at the outlet of the grain activated carbon. These matrices are fortified at a level of 1 µg/L of each pesticide and were treated on a NF pilot unit. The pesticides removal percent and that of their adsorption on the membrane have been calculated. A difference in the behaviour of the two membranes was observed, together with an improvement in the removal percent of some pesticides in the presence of humic matter. On the contrary, inorganic matter exercises little influence. The results obtained with the real water matrix reveal an increase in the rate of pesticides adsorbed on the membranes in comparison with those of the synthetic waters. This phenomenon facilitated the transition of these molecules in the direction of the permeate and led to an identical reduction percent except for diuron, which was furthermore the most poorly eliminated by the two membranes.

The two main mechanisms which govern the process of elimination of the pesticides by NF are repulsion (steric and electrostatic) by the membrane and adsorption on the membrane. This latter phenomenon must be minimized as far as possible for its increase negatively influences the rate of elimination of the pesticides by fostering their transition in the direction of the permeate by diffusion through the membrane. In addition, elimination of the pesticides by NF is more greatly favoured by the fraction of the high-molecular-weight organic matter such as humic acids. The influence of the inorganic matter (CaCl2 and CaSO4), for its part, is greater with the wide-pore membrane which has been studied. Lastly, NF does not totally eliminate the pesticides studied, in particular diuron. For this reason, we recommend associating it with another treatment process such as filtration on GAC so as to guarantee a high-quality water.