Molecular simulations and an experimental study of the oligopeptide-mediated fouling mechanisms of polyamide reverse-osmosis membranes

Fouling is a major problem in reverse-osmosis plants. Fouling is believed to be caused by interactions between membranes and foulants. Experimental observation of fouling is very difficult, however, and in this study molecular dynamics (MD) simulations are used to analyze fouling on a molecular scale to elucidate the adsorption mechanisms of polyamide (PA) reverse-osmosis (RO) membranes affected by the fouling phenomenon. Because proteins are common foulants, a dimeric dipeptide of amino acids constituting a protein was used as a model. The dissociation of the membrane and that of the foulants that results from changes in pH were investigated on a molecular scale and by experimental water permeation testing. At pH 2.5, the foulants L-leucyl-L-aspartic acid (Lasp) and L-leucyl-L-arginine (Larg) showed a tendency toward constant adsorption to the membrane with no significant difference in interaction energy. At pH 7.0, on the other hand, the membrane surface charge turned negative and the total charge of the Lasp and Larg foulants became negative and positive, respectively. Lasp was not close to the membrane surface and demonstrated repulsive and weak adsorption tendencies. On the other hand, Larg penetrated deeply into the membrane surface and showed a strong adsorption tendency. The fouling mechanism in the adsorption simulation varied depending on the conditions, and simulations confirmed that the fouling was very strong when the charges of a foulant and the PA membrane were opposite. These observed trends are similar to those reported from experimental water permeation testing.