Evert J. Elzinga

X-ray absorption spectroscopy study of Cu2+ and Zn2+ adsorption complexes at the calcite surface: Implications for site-specific metal incorporation preferences during calcite crystal growth

We report results from in situ extended X-ray absorption fine structure (EXAFS) spectroscopy studies of Cu(II) and Zn(II) complexes forming at the calcite surface following adsorption from preequilibrated calcite-saturated solutions. Both Cu(II) and Zn(II) coordinate at Ca sites on the calcite surface, forming mononuclear inner-sphere adsorption complexes. The Zn adsorption complexes are in tetrahedral coordination with first-shell O neighbors with RZn-O = 1.95 A, and the Cu complexes are Jahn-Teller distorted, with equatorial RCu-O = 1.95 A. Results from EXAFS data of dilute Cu- and Zn-calcite solid solutions confirm substitution of these metals in the Ca site of the calcite structure as octahedral complexes during coprecipitation. X-ray fluorescence microanalyses of calcite (1014) hillocks grown in coprecipitation experiments show that divalent Cu and Zn, which have ionic radii smaller than Ca, are preferentially incorporated into the parallel arrays of + steps that define one pair of symmetrically equivalent vicinal faces on polygonized growth spirals. In contrast, other divalent metals with sixfold ionic radii smaller than Ca (Co, Cd, Mn, Mg) have been shown to be preferentially incorporated into − growth steps, which define the second pair of vicinal faces on the growth spirals, but which are symmetrically nonequivalent to the steps on the first pair. The distortion from octahedral symmetry observed for the Cu and Zn adsorption complexes likely plays a key role in the observed preference of Cu and Zn for incorporation into the + steps.

Spectroscopic studies of Pb(II)-sulfate interactions at the goethite-water interface

We used a combination of in situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy and X-ray absorption fine structure (XAFS) spectroscopy to conduct molecular scale studies on Pb(II)-sulfate interactions at the solid-water interface of goethite at pH 4.5, 5.0, and 6.0. Both the ATR-FTIR studies (probing sorbed SO4 in a flow cell setup as a function of the Pb concentration) and the EXAFS studies (probing sorbed Pb at high levels of co-adsorbing SO4) indicated the formation Pb-SO4 ternary complexes at the goethite surface. Based on the combined information from the IR and XAFS studies, possible Pb-SO4 ternary complex configurations were presented and discussed by comparison to a set of reference sulfate FTIR spectra. In addition to forming ternary complexes with SO4, adsorption of Pb also promoted SO4 sorption to the goethite surface by changing the surface charge, leading to additional formation of inner- and outer-sphere SO4 sorption complexes not coordinated by Pb. The relative impacts of these mechanisms (i.e., ternary complex formation versus electrostatic effects) appeared to be a function of pH and the level of Pb addition. Formation of ternary complexes was promoted (relative to the importance of electrostatic effects) at low pH values and high Pb concentrations, whereas electrostatic effects were more pronounced at high pH values and low Pb concentrations. In addition, it was found that part of the SO4 initially sorbed at the goethite surface as inner-sphere complexes without being coordinated by Pb was transformed into SO4-Pb ternary complexes as the Pb concentration was increased, an effect most pronounced at low pH. This study shows that co-adsorption of SO4 and Pb may lead to changes in both the extent and mechanisms of the adsorption of these contaminants to the goethite surface relative to binary Pb/goethite and SO4/goethite systems. The presence of co-adsorbing metals or anions may therefore significantly impact the behavior of contaminants in environmental settings.

Competitive sorption of carbonate and arsenic to hematite: Combined ATR-FTIR and batch experiments

The competitive sorption of carbonate and arsenic to hematite was investigated in closed-system batch experiments. The experimental conditions covered a pH range of 3-7, arsenate concentrations of 3-300 μM, and arsenite concentrations of 3-200 μM. Dissolved carbonate concentrations were varied by fixing the CO(2) partial pressure at 0.39 (atmospheric), 10, or 100 hPa. Sorption data were modeled with a one-site three plane model considering carbonate and arsenate surface complexes derived from ATR-FTIR spectroscopy analyses. Macroscopic sorption data revealed that in the pH range 3-7, carbonate was a weak competitor for both arsenite and arsenate. The competitive effect of carbonate increased with increasing CO(2) partial pressure and decreasing arsenic concentrations. For arsenate, sorption was reduced by carbonate only at slightly acidic to neutral pH values, whereas arsenite sorption was decreased across the entire pH range. ATR-FTIR spectra indicated the predominant formation of bidentate binuclear inner-sphere surface complexes for both sorbed arsenate and sorbed carbonate. Surface complexation modeling based on the dominant arsenate and carbonate surface complexes indicated by ATR-FTIR and assuming inner-sphere complexation of arsenite successfully described the macroscopic sorption data. Our results imply that in natural arsenic-contaminated systems where iron oxide minerals are important sorbents, dissolved carbonate may increase aqueous arsenite concentrations, but will affect dissolved arsenate concentrations only at neutral to alkaline pH and at very high CO(2) partial pressures.

EXAFS study of rare-earth element coordination in calcite

Extended X-ray absorption fine-structure (EXAFS) spectroscopy is used to characterize the local coordination of selected rare-earth elements (Nd3+, Sm3+, Dy3+, Yb3+) coprecipitated with calcite in minor concentrations from room-temperature aqueous solutions. Fitting results confirm substitution in the Ca site, but first-shell Nd-O and Sm-O distances are longer than the Ca-O distance in calcite and longer than what is consistent with ionic radii sums for sixfold coordination in the octahedral Ca site. In contrast, first-shell Dy-O and Yb-O distances are shorter than the Ca-O distance and are consistent with ionic radii sums for sixfold coordination. Comparison of Nd-O and Sm-O bond lengths with those in lanthanide sesquioxides and with ionic radii trends across the lanthanide series suggests that Nd3+ and Sm3+ have sevenfold coordination in a modified Ca site in calcite. This would require some disruption of the local structure, with an expected decrease in stability, and possibly a different charge compensation mechanism between Nd and Sm vs. Yb and Dy. A possible explanation for the increased coordination for the larger rare-earth elements involves bidentate ligation from a CO3 group. Because trivalent actinides such as Am3+ and Cm3+ have ionic radii similar to Nd3+, their incorporation in calcite may result in a similar defect structure.

Macroscopic and spectroscopic characterization of selenate, selenite, and chromate adsorption at the solid―water interface of γ-Al2O3

The interaction of selenate, selenite, and chromate with the hydrated surface of gamma-Al(2)O(3) was studied using a combination of macroscopic pH edge data, electrophoretic mobility measurements, and X-ray absorption spectroscopic analyses. The pH edge data show generally increased oxyanion adsorption with decreasing pH, and indicate ionic strength-(in)dependent adsorption of chromate and selenate across the pH range 4-9, and ionic strength-(in)dependent adsorption of selenite in this pH range. The adsorption of chromate peaks at pH 5.0, whereas for selenate and selenite no pH adsorption maxima are observed. Electrophoretic mobility measurements show that all three oxyanions decrease the zeta potential of gamma-Al(2)O(3) upon adsorption; however, only selenite decreased the pH(PZC) of the gamma-Al(2)O(3) sorbent. EXAFS data indicate that selenite ions are coordinated in a bridging bidentate fashion to surface AlO(6) octahedra, whereas no second-neighbor Al scattering was observed for adsorbed selenate ions. Combined, the results presented here show that pH is a major factor in determining the extent of adsorption of selenate, selenite, and chromate on hydrated gamma-Al(2)O(3). The results point to substantial differences between these anions as to the mode of adsorption at the hydrated gamma-Al(2)O(3) surface, with selenate adsorbing as nonprotonated outer-sphere complexes, chromate forming a mixture of monoprotonated and nonprotonated outer-sphere adsorption complexes, and selenite coordinating as inner-sphere surface complexes in bridging configuration.

Mechanisms of ciprofloxacin removal by nano-sized magnetite.

An understanding of the interaction mechanisms of antibiotics with environmentally relevant sorbents is important to determine the environmental fate of antibiotics and to develop wastewater treatment strategies. Magnetite (Fe(3)O(4)(s)) is ubiquitous in the environment and occurs as a secondary corrosion product of iron nanoparticles that are commonly used as a remediation material. In this study, we aimed to assess the sorption mechanisms of ciprofloxacin (CIP), an important class of fluoroquinolone antibiotics, with magnetite nanoparticles using a combination of wet chemical and in situ ATR-FTIR spectroscopic measurements. Ciprofloxacin sorption was characterized as a function of pH (3.4-8.0), CIP concentration (1-500 μM), ionic strength (0.5, 0.1, and 0.01 M NaCl), and competing anion such as phosphate (0.1mM) to cover a broad range of environmentally relevant geochemical conditions. Results indicated a bell-shaped sorption envelop where sorption of CIP on nano-Fe(3)O(4)(s) increased from 45% to 80% at pH 3.44-5.97; beyond that sorption gradually decreased to a value of 25% at pH 8.39. Phosphate had negligible effect on CIP sorption. In situ ATR-FTIR results indicated inner-sphere coordination of CIP at the magnetite surface mediated by carboxylic acid groups. Results suggest that nano-Fe(3)O(4)(s) has the potential to remove CIP from wastewater effectively.

Influence of pH on the Reductive Transformation of Birnessite by Aqueous Mn(II)

We investigated the effect of pH (5.5-8.5) on the mineralogical transformation of hexagonal birnessite induced by reaction with aqueous Mn(II) (50-2200 μM), using batch sorption experiments, X-ray diffraction analyses, X-ray absorption and infrared spectroscopic measurements. Samples reacted at pH < 7.0 exhibited disrupted stacking of birnessite sheets, but no mineralogical transformation products were observed. At pH 7.0 and 7.5, reaction with Mn(II) under anoxic conditions caused reductive transformation of birnessite into manganite (γ-MnOOH), whereas at pH 8.0 and 8.5, conversion into hausmannite (Mn3O4) occurred. Feitknechtite (β-MnOOH) is a major transformation product at low Mn(II) inputs at pH 7.0-8.5, and represents a metastable reaction intermediate that is converted into manganite and possibly hausmannite during further reaction with Mn(II). Thermodynamic calculations suggest that conversion into hausmannite at alkaline pH reflects a kinetic effect where rapid hausmannite precipitation prevents formation of thermodynamically more favorable manganite. In oxic systems, feitknechtite formation due to surface catalyzed oxidation of Mn(II) by O2 increases Mn(II) removal relative to anoxic systems at pH ≥ 7. The results of this study suggest that aqueous Mn(II) is an important control on the mineralogy and reactivity of natural Mn-oxides, particularly in aqueous geochemical environments with neutral to alkaline pH values.

Leaching characteristics of vanadium in mine tailings and soils near a vanadium titanomagnetite mining site

A series of column leaching experiments were performed to understand the leaching behaviour and the potential environmental risk of vanadium in a Panzhihua soil and vanadium titanomagnetite mine tailings. Results from sequential extraction experiments indicated that the mobility of vanadium in both the soil and the mine tailings was low, with <1% of the total vanadium readily mobilised. Column experiments revealed that only <0.1% of vanadium in the soil and mine tailing was leachable. The vanadium concentrations in the soil leachates did not vary considerably, but decreased with the leachate volume in the mine tailing leachates. This suggests that there was a smaller pool of leachable vanadium in the mine tailings compared to that in the soil. Drought and rewetting increased the vanadium concentrations in the soil and mine tailing leachates from 20μgL(-1) to 50-90μgL(-1), indicating the potential for high vanadium release following periods of drought. Experiments with soil columns overlain with 4, 8 and 20% volume mine tailings/volume soil exhibited very similar vanadium leaching behaviour. These results suggest that the transport of vanadium to the subsurface is controlled primarily by the leaching processes occurring in soils.

Redox Reactions between Mn(II) and Hexagonal Birnessite Change Its Layer Symmetry

Birnessite, a phyllomanganate and the most common type of Mn oxide, affects the fate and transport of numerous contaminants and nutrients in nature. Birnessite exhibits hexagonal (HexLayBir) or orthogonal (OrthLayBir) layer symmetry. The two types of birnessite contain contrasting content of layer vacancies and Mn(III), and accordingly have different sorption and oxidation abilities. OrthLayBir can transform to HexLayBir, but it is still vaguely understood if and how the reverse transformation occurs. Here, we show that HexLayBir (e.g., δ-MnO2 and acid birnessite) transforms to OrthLayBir after reaction with aqueous Mn(II) at low Mn(II)/Mn (in HexLayBir) molar ratios (5-24%) and pH ≥ 8. The transformation is promoted by higher pH values, as well as smaller particle size, and/or greater stacking disorder of HexLayBir. The transformation is ascribed to Mn(III) formation via the comproportionation reaction between Mn(II) adsorbed on vacant sites and the surrounding layer Mn(IV), and the subsequent migration of the Mn(III) into the vacancies with an ordered distribution in the birnessite layers. This study indicates that aqueous Mn(II) and pH are critical environmental factors controlling birnessite layer structure and reactivity in the environment.

Application of sequential extractions and X-ray absorption spectroscopy to determine the speciation of chromium in Northern New Jersey marsh soils developed in chromite ore processing residue (COPR).

The Cr speciation in marsh soils developed in weathering chromite ore processing residue (COPR) was characterized using sequential extractions and synchrotron microbeam and bulk X-ray absorption spectroscopic (XAS) analyses. The sequential extractions suggested substantial Cr associated with reducible and oxidizable soil components, and significant non-extractable residual Cr. Notable differences in Cr speciation estimates from three extraction schemes underscore the operationally defined nature of Cr speciation provided by these methods. Micro X-ray fluorescence maps and μ-XAS data indicated the presence of μm-sized chromite particles scattered throughout the weathered COPR matrix. These particles derive from the original COPR material, and have relatively high resistance towards weathering, and therefore persist even after prolonged leaching. Bulk XAS data further indicated Cr(III) incorporated in Fe(OH)(3), and Cr(III) associated with organic matter. The low Cr contents of the weathered material (200-850 ppm) compared to unweathered COPR (20,000-60,000 ppm) point to substantial Cr leaching during COPR weathering, with partial repartitioning of released Cr into secondary Fe(OH)(3) phases and organics. The effects of anoxia on Cr speciation, and the potential of active COPR weathering releasing Cr(VI) deeper in the profile require further study.