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) 13C-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 shear plane, yielding more positive SP values. For the hydrophilic TPIA foulant no displacement of the IEP was observed. Then the results of SP experiments conducted through the membrane with a homemade apparatus has indicated that HPOA is more retained inside the pores as compared to the TPIA that was mainly sorbed at the surface of the membrane. Furthermore the membranes acidic-basis properties were amplified after foulants deposit in comparison to the cleaned membrane where a dominant specific sorption of monovalents and divalents ions occurred.