Selective Adsorption of Pb(II) on an Annealed Hematite (1102) Surface: Evidence from Crystal Truncation Rod X-ray Diffraction and Density Functional Theory.

The Pb(II)-binding mechanism on an annealed hematite (1102) surface was studied using crystal truncation rod (CTR) X-ray diffraction coupled with density functional theory (DFT) calculations. The best fit CTR model suggested that Pb(II) sorbed selectively to one type of edge-sharing surface site (ES2) over two other potential surface sites. From the best fit model structure, it was found that the Pb surface complex species forms a trigonal pyramid geometry. The base consists of three oxygen groups, two of which are associated with the substrate surface (IO and IIIO) and one that is a distal O extending toward solution. The trigonal pyramid geometry is slightly distorted with Pb-O bond lengths ranging from 2.21 to 2.31 Å and O-Pb-O bond angles ranging from 72° to 75°. Under this structural distortion, the nearest distance between Pb and Fe is found to be 3.39(1) Å. Consistent with the CTR results, DFT calculations indicate the Pb binding energy at the ES2 site is at least 0.16 eV more favorable than that at the other two potential binding sites considered. Using bond-valence rules we propose a stoichiometry of Pb(II) binding on the hematite (1102) surface which indicates proton release through the deprotonation of all oxygen groups bonding to Pb.

In Situ Structural Study of Sb(V) Adsorption on Hematite (11Ì 02) Using X-ray Surface Scattering.

The binding mechanism of Sb(V) on a single-crystal hematite (11̅02) surface was studied using crystal truncation rod X-ray diffraction (CTR) under in situ conditions. The best-fit CTR model indicates Sb(V) adsorbs at the surface as an inner-sphere complex forming a tridentate binding geometry with the nearest Sb-Fe distance of 3.09(4) Å and an average Sb-O bond length of 2.08(5) Å. In this binding geometry, Sb is bound at both edge-sharing and corner-sharing sites of the surface Fe-O octahedral units. The chemical plausibility of the proposed structure was further verified by bond valence analysis, which also deduced a protonation scheme for surface O groups. The stoichiometry of the surface reaction predicts the release of one OH- group at pH 5.5.

In situ structural study of the surface complexation of lead(II) on the chemically mechanically polished hematite (11¯02) surface.

A structural study of the surface complexation of Pb(II) on the (11¯02) surface of hematite was undertaken using crystal truncation rod (CTR) X-ray diffraction measurements under in situ conditions. The sorbed Pb was found to form inner sphere (IS) complexes at two types of edge-sharing sites on the half layer termination of the hematite (11¯02) surface. The best fit model contains Pb in distorted trigonal pyramids with an average PbO bond length of 2.27(4) Å and two characteristic Pb-Fe distances of 3.19(1) Å and 3.59(1) Å. In addition, a site coverage model was developed to simulate coverage as a function of sorbate-sorbate distance. The simulation results suggest a plausible Pb-Pb distance of 5.42 Å, which is slightly larger than the diameter of Pb's first hydration shell. This relates the best fit surface coverage of 0.59(4) Pb per unit cell at monolayer saturation to steric constraints as well as electrostatic repulsion imposed by the hydrated Pb complex. Based on the structural results we propose a stoichiometry of the surface complexation reaction of Pb(II) on the hematite (11¯02) surface and use bond valence analysis to assign the protonation schemes of surface oxygens. Surface reaction stoichiometry suggests that the proton release in the course of surface complexation occurs from the Pb-bound surface O atoms at pH 5.5.

Potential adverse effects of applying phosphate amendments to immobilize soil contaminants.

Seven-day batch equilibrium experiments were conducted to measure the efficacy of four phosphate amendments (trisodium trimetaphosphate [TP3], dodecasodium phytate [Na-IP6], precipitated calcium phytate [Ca-IP6], and hydroxyapatite [HA]) for immobilizing Ni and U in organic-rich sediment. Using the eight-step modified Miller's sequential extraction procedure and the USEPA's Toxicity Characteristic Leaching Procedure, the effect of these amendments on the distribution of Ni and U was assessed. Relative to unamended controls, equilibrium aqueous-phase U concentrations were lower following HA and Ca-IP6 additions but higher following TP3 and Na-IP6 amendments, whereas aqueous Ni concentrations were not decreased only in the Na-IP6 amended treatment relative to the control. The poor rates of contaminant immobilization following TP3 and Na-IP6 amendments correlate with the dispersion of organic matter and organo-mineral colloids, which probably contain sorbed U and Ni. While all amendments shifted the U distribution toward more recalcitrant soil fractions, Ni was redistributed to more labile soil fractions. This study cautions that the addition of orthophosphates and organophosphates as contaminant immobilizing amendments may in fact have adverse effects, especially in high-organic soils. Particular attention is warranted at sites with mixed contaminants with varying geochemical behaviors.