Characterization of humic acid fouled reverse osmosis and nanofiltration membranes by transmission electron microscopy and streaming potential measurements

Reverse osmosis and nanofiltration membranes fouled by humic acid were systematically characterized by transmission electron microscopy. All fouled membranes, except those with very low initial flux, were completely covered by a layer of humic acid whose thickness and density were greatly affected by the feedwater composition ([H+] and [Ca(2+)]) and initial flux. A low-density humic layer (about 0.1 g of purified Aldrich humic acid (PAHA)/cm(3)) was formed at low initial flux (2 m/day or less) at pH 7 without calcium. It was several times denser at a higher initial flux, pH 4.5, or 1 mM Ca(2+). Corresponding to the denser foulant layers under these conditions, PAHA accumulation was greatly increased. The denser foulant layers together with the greater PAHA accumulations were responsible for the severe flux reductions. Both virgin and fouled membranes were characterized by streaming potential measurements. While considerable differences existed for virgin membranes, humic acid fouled membranes exhibited identical surface charge properties. The potential of the fouled membranes was controlled by the humic acid layer due to its complete coverage of the membrane surfaces.