Effect of cationic polymer additives on the adsorption of humic acid onto iron oxide particles

Abstract

Improving the removal of natural organic matter (e.g. humic acid) during drinking water treatment is important in order to minimize the formation of disinfection by-products (DBPs). Although polymeric flocculants are often used to improve turbidity removal during the coagulation process, little information is available regarding the influence of these polymers on NOM removal. In this study, the adsorption of humic acid onto polymer-coated iron oxide particles was investigated as a way to improve humic acid removal during the coagulation process. Monodisperse iron oxide particles and well-characterized humic acid were used as a model system. The adsorption of humic acid onto bare iron oxide particles decreased as solution pH increased. When iron oxide particles were coated with a cationic polymer, adsorption doubled at high pH (∼9.5) and low salt concentration (0.001 M NaCl). The greater adsorption of humic acid on polymer-coated surfaces at high pH was largely due to changes in the electrostatic interactions between humic acid and the particle surface. Coating the iron oxide particles with cationic polymer resulted in the reversal of the negative particle surface charge, thus providing more favorable conditions for humic acid adsorption. Little change in humic acid adsorption was observed, however, for polymer-coated particles at near neutral pH values (∼6.8) or at high salt concentration. These data suggest that under certain conditions polymers may enhance humic acid adsorption onto iron oxide surfaces, a process which may improve DBP precursor removal during drinking water treatment.