Zeta potential is an important factor in affecting filtration rates of iron ore slurries. Two methods of zeta potential control investigated during this study were (1) pH adjustments and (2) surface modifications by reagent addition. It was originally hypothesized that adding CO2 to a hematite slurry, would form a carbonic acid, leading to lower pH levels, lower zeta potentials and increased filtration rates. Samples under both ideal (1 mM KCl as background electrolyte solution) and plant (plant water as background electrolyte) conditions were tested during this study. Through zeta potential testing of hematite slurries under ideal conditions (with and without CO2 sparging), it was observed that CO2 sparging achieved both (1) pH reduction and (2) surface modification of the hematite particles. While the shapes of the zeta potential curves were different for samples taken from the operating plant under plant conditions, the same downward shift in the entire zeta potential curve was observed after CO2 sparging. The surface modification phenomenon was attributed to the possible formation of complexes between carbonate/bicarbonate ions and the surface of the hematite particles. The possibility of double layer compression was also investigated. While some variations in conductance were observed, these variations did not entirely explain the changes in zeta potential, therefore providing even stronger evidence of surface modification through hematite/carbonate (or bicarbonate) surface complexing. In the event of a hematite/carbonate (or bicarbonate) complex forming, its exact composition is still unknown, but the surfaces showed zeta potential properties similar to that of a siderite (iron carbonate) sample analyzed during this study. (C) 2010 Elsevier B.V. All rights reserved.