Surface complex characteristics of synthetic maghemite and hematite in aqueous suspensions.

The acid-base properties of the maghemite (gamma-Fe2O3)/water and hematite (alpha-Fe2O3)/water interfaces have been studied by means of high precision potentiometric titrations and the experimental results are evaluated as surface complexation reactions. Synthetic maghemite and hematite were prepared and characterized using a combination of SEM, FT-IR and XRD. The specific surface area of the minerals was determined by the BET method. The titrations were performed at 25.0+/-0.2 degrees C within the range 2.8<pH<8.5, NaNO3 ionic medium giving total ionic strengths of 0.10 and 0.50 mol dm(-3) in both systems. Experimental data were evaluated using the constant capacitance model. The total proton exchange capacities of the solids were determined by saturation of the surfaces with excess acid. The number of protonated surface sites per nm2 was found to be 0.81+/-0.05 and 1.03+/-0.04 for maghemite, at I=0.10 and 0.50 mol dm(-3), respectively. The IEP for maghemite was determined from the zeta-potential using a Zetasizer 4 instrument.