Fouling studies of a polyamide nanofiltration membrane by selected natural organic matter: an analytical approach

The present study compares the fouling properties of two selected and well-characterized hydrophobic (denoted HPOA) and hydrophilic (denoted TPIA) natural organic matters (NOM) fractions with a commercial polyamide (PA) nanofiltration (NF) membrane (denoted NF-55). Analytical tools such as elemental analyses using microanalyses, specific UV absorbance, solid-state cross-polarization magic angle spinning (CPMAS) 13 C-NMR, high pressure size exclusion (HPSEC) chromatography, HPLC system and acid/base titration for characterizing NOM fractions and contact angle, hydraulic permeability, streaming potential (SP) and observed rejection of a NaCl aqueous solution to evaluate the affinity of the PA material with both selected NOM fractions isolated from the Blavet river (Britany region, France). The PA material was found to be more sensitive to hydrophobic NOM adsorption leading to irreversible fouling with drastic modifications of the initial physico-chemical properties of the membrane: (a) increase of its hydrophobicity; (b) decrease of its hydraulic permeability associated with a decrease in its pore size and consequently (c) increase the observed rejection of salty solutions. The higher decrease in the performances of this PA NF membrane is observed for the more hydrophobic foulant, HPOA. At the same time a displacement of the isolectric point (IEP) of the membrane material was observed from 4.5 for the clean membrane (KCl 10−4 mol.L−1) to 3.4 after HPOA sorption. At a lower pH range than the IEP, the effects of cations and H+ on the charge properties of the membranes increases near the * Corresponding authors 0011-9164/05/$– See front matter © 2005 Elsevier B.V. All rights reserved

Natural organic matters play an important role in membrane fouling. Today’s NF industrial plants, just starting on a large scale, need longterm studies that they do not have. The present study is presented as an alternative for the limitation of NOM fouling during NF operation. In the present paper we have compared the fouling properties of two selected hydrophilic and hydrophobic NOM fractions extracted from the Blavet river (France) in contact with a hydrophilic PA membrane. The analytical tools accounted microanalyses, specific UV absorbance, solid-state cross-polarization magic angle spinning (CPMAS) 13C-NMR, high pressure size exclusion (HPSEC) chromatography, HPLC system, acid/base titration, contact angle, hydraulic permeability, SP and observed rejection of a NaCl aqueous solution) have shown that HPOA NOM is higher limiting PA membrane performances than the TPIA. But for each foulant the membrane initial properties has been dramatically changed due to NOM deposit. The modifications observed are dependant on which compounds are mainly adsorbed in, or on the membrane. We tried to identify the compounds which are the more important on the fouling phenomena observed. We have obtained that the NF-55 membrane modified with the selected foulants became two novel membranes with smaller pores size and with novel acidicbasis properties that they acquired after NOM sorption. All the analytical tools developed during this work to qualify both NOM and membrane material physico-chemical properties are the best approach in order to determine the optimum membranes material to be selected for a done water feed. Further investigations are being conducted now in order to elaborate synthetic solution by combining HPOA, TPIA, proteins and polysac- charides, in order to simulate the contents of a real water feed. The interest in the good knowledge of the water resource contents can facilitate an antifouling strategy in order to optimise the chemical washing procedures and/or the pretreatments of the water feeds.