The surface properties of ferrihydrite were studied by combining wet chemical data, Cd-K EXAFS data, and a surface structure and protonation model of the ferrihydrite surface. Acid-base titration experiments and Cd(II)-ferrihydrite sorption experiments were performed within 3 < - log[H+] < 10.5 and 0.5 < [Cd-t] < 12 mM in 0.3 M NaClO4 at 25 degreesC, where [Cd-t] refers to total Cd concentration. Measurements at -5.5 less than or equal to log[Cd-t] less than or equal to -1.4 at fixed pH completed the wet chemical data set. The acid-base titration data could be adequately modeled byequivalent toFe-OH2+1/2 - H+ <----> equivalent toFe-OH-1/2, log k((int)) = -8.29, assuming the existence of a unique intrinsic microscopic constant, logk(int), and consequently the existence of a single significant type of acid-base reactive functional groups. The surface structure model indicates that these groups are terminal water groups. The Cd(II) data were modeled assuming the existence of a single reactive site. The model fits the data set at low Cd(II) concentration and up to 50% surface coverage. At high coverage more Cd(II) ions than predicted are adsorbed, which is indicative of the existence of a second type of site of lower affinity. This agrees with the surface structure and protonation model developed, which indicates comparable concentrations of high-and low-affinity sites. The model further shows that for each class of low- and high-affinity sites there exists a variety of corresponding Cd surface complex structure, depending on the model crystal faces on which the complexes develop. Generally, high-affinity surface structures have surface coordinations of 3 and 4, as compared to I and 2 for low-affinity surface structures. (C) 2003 Elsevier Inc. All rights reserved.