title: Hydrous ferric oxide: evaluation of Cd-HFO surface complexation models combining Cd(K) EXAFS data, potentiometric titration results, and surface site structures identified from mineralogical knowledge.
abstract: 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 NaClO(4) at 25 degrees C, where [Cd(t)] refers to total Cd concentration. Measurements at -5.5<or=log[Cd(t)]<or=-1.4 at fixed pH completed the wet chemical data set. The acid-base titration data could be adequately modeled by triple bond Fe- OH(2)(+1/2)-H(+)<-->triple bond Fe-OH(-1/2),logk((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 1 and 2 for low-affinity surface structures.
authors.affiliation: Environmental Geochemistry Group, LGIT-IRIGM, University Joseph Fourier and CNRS, BP 53, 38041, Grenoble, France. lorenzo.spadini@ujf-grenoble.fr
authors.forename: Lorenzo
authors.initials: L
authors.lastname: Spadini
authors.forename: Paul W
authors.initials: PW
authors.lastname: Schindler
authors.forename: Laurent
authors.initials: L
authors.lastname: Charlet
authors.forename: Alain
authors.initials: A
authors.lastname: Manceau
authors.forename: K
authors.initials: K
authors.lastname: Vala Ragnarsdottir