Influence of water organic and inorganic matter on the pesticide removal by nanofiltration
The intensive use of pesticides in agriculture has resulted in the contamination of groundwater and surface waters. The removal of these organic pollutants by the usual methods such as adsorption by activated carbon (in powdered or granular form) or oxidation by ozone have some disadvantages. Recently, the removal of organic pollutants by membrane retention (reverse osmosis, ultrafiltration and nanofiltration) has become increasingly popular and due to its low cost, nanofiltration has become an interesting option. This study examined the efficiency of two different nanofiltration membranes (Desal DK and NF200) in the removal of some pesticides (atrazine and its metabolite desethylatrazine (DEA), simazine, cyanazine, isoproturon and diuron) from water and, in addition, investigated the influence that organic and inorganic matter may have on the efficiency of this removal. Synthetic waters were made from distilled water and organic matter (humic acids) or inorganic matter (CaCl2 or CaSO4) was added, as well as 1 μg/l of each pesticide. The samples were then filtered by a nanofiltration pilot for 96 hours. Samples of the feed, permeate and the retentate were taken after 4, 24, 48, 72 and 96 h. The samples were replaced with equivalent volumes of the original solution in the feed tank. The different samples were analysed by an on-line SPE / HPLC system. The different concentrations obtained allowed the determination of the proportion of the pesticides that adsorbed to the membrane. The removal efficiency of pesticides from pure distilled water differed according to the membrane. For example, the Desal DK membrane eliminated more than 90% of all the pesticides (with the exception of diuron). In contrast, the NF200 membrane, eliminated more than 75% of all the pesticides (with the exception of diuron). The removal efficiency of Diuron was the lowest by both the membranes: 70 % by Desal DK and 45 % by NF200. The adsorption efficiency of the pesticides was similar for both membranes (between 30 and 40%). In pure water, pesticide removal is a function of both the specific properties of each pesticide (solubility, molecular mass, Stokes diameter, equivalent molar diameter, calculated equivalent diameter and polarity) and the physical characteristics of the membrane (molecular weight cut-off and current load). The influence of inorganic matter on pesticide removal efficiency changed according to the type of membrane. For example, we noted an improvement in removal efficiency with the NF200 membrane from low removal with CaCl2 to high removal with CaSO4 for all pesticides examined including diuron. In contrast, for the Desal DK membrane, very little change was noted (a slight decrease in the removal efficiency of DEA and simazine with CaCl2). Adsorption by the membranes remained stable for all the pesticides tested on the two types of membrane. The improvement in the removal of pesticides by the NF200 membrane was probably linked to pores being blocked by ions at high concentrations. It could be concluded from these results that elimination of pesticides is quantitatively linked to the physical characteristics of the membranes and that inorganic matter only has an effect with wide-pore membranes (NF200 membrane) and, furthermore, it has no influence on the adsorption of the pesticides on the membranes. For water containing organic matter, we have noted an improvement in the removal of certain pesticides with the NF200 membrane when compared to distilled water (except diuron). With the Desal DK membrane, we observed a slight decrease in the removal of DEA, simazine and isoproturon, and a substantial drop for diuron (20 %) with no change for cyanazine and atrazine. Adsorption of the pesticides on the membranes remained unchanged with the NF200 membrane but increased by about 10% on the Desal DK membrane for all molecules. Pesticides, notably triazines, adsorb easily on organic matter by physiosorption (weak links) and by chemisorption (ionic links) to form macromolecules. The steric congestion and the density of these pseudo-complexes is high, which facilitated the elimination of certain pesticides with the (wide-pore) NF200 membrane by accentuating the effects of steric exclusion and electrostsatic repulsion and decreasing adsorption. For the Desal DK membrane, the increased adsorption of the pesticides on the membrane generated a more significant transition of these molecules in the direction of the permeate. This had a negative influence on the removal of some pesticides, depending on their size; the largest molecules underwent the least change. Diuron behaved differently from the other pesticides examined. This molecule did not bind to humic acids and its removal rate did not change with a wide-pore (NF200) membrane. However a greater adsorption of organic matter by the narrower-pore (Desal DK) membrane favored diuron adsorption and, consequently, its diffusion into the permeate. The effect of organic matter and, more specifically, of humic acids on the elimination of pesticides depends not only on the structure of the molecules, but also on the cut-off threshold of the membrane. The two main mechanisms that govern the process of pesticide elimination by NF are repulsion (steric and electrostatic) by the membrane and adsorption on the membrane. This latter phenomenon must be minimized, to reduce the elimination of pesticides by fostering their transition in the direction of the permeate. In addition, removal of the pesticides by NF was favoured by the high-molecular weight organic matter fraction (i.e., humic acids). The influence of the inorganic matter (CaCl2 and CaSO4), for its part, is greater with the wide-pore membrane.