In membrane treatment of wastewater containing proteins, humic acid influences the rate and extent of fouling. Cations may increase the tendency of foulants to precipitate due to the changes in intermolecular electrostatic interactions. In this paper, we investigated the effect of cations, including ionic charge and size of hydrated ion, on ultrafiltration of the solutions containing the mixture of HA and bovine serum albumin (BSA). All experiments were carried out in an Amicon batch stirred cell using Amicon XM50, XM300, YM100 ultrafiltration membranes with molecular weight cut-offs of 50 kDa, 300 kDa and 100 kDa, respectively, and Sartorius microfiltration membranes with a nominal pore size of 0.2 ?m. Humic substances were extracted from a commercial sample of sphagnum peat from Northern Ireland (Bulrush). In the presence of Ba 2+ cations, the HA rejection in the solutions containing HA and BSA was higher compared to the solutions containing Na+ cations. Unlike monovalent cations, divalent cations interact specifically with natural organic matter (NOM) and form metal humic complexes. The binding of divalent cations with natural organic matter is responsible for the development of a dense layer on the membrane surface. Metal cations may also act as a bridge between humic acid and protein. These phenomena result in a higher deposition of substances on the membrane surface leading to lower flux and higher rejection in the presence of divalent metal cations. In addition to the charge, the size of cation plays a role in the deposition process. The solutions containing MgCl 2-6H2O are characterizing by higher flux and lower rejection compared to the solution containing BaCl2·2H 2O at all ultrafiltration membranes. This phenomenon may be attributed to the size of Mg2+and Ba2+ ions.
