Surface Precipitate Research Articles

Effect of an Alkyl Dicarbonate on Li-Ion Cell Performance

The effect of an alkyl dicarbonate, diethyl 2,5-dioxahexane dicarboxylate (DEDOHC, ), which is formed by an ester exchange reaction in a mix solvent system of ethylene carbonate (EC) and diethyl carbonate (DEC), on the performance of lithium-ion cells was investigated. In the presence of 10 vol % DEDOHC in , discharge capacity decreased, in particular at a high charge/discharge rate. Capacity retention at a high charge/discharge rate also decreased during repeated charge and discharge cycles. These deteriorations in performance were attributed to an increase in cell impedance and deficiency of mobile lithium ion, which is caused by continuous consumption of lithium ion on graphite negative electrode. The results of half-cell tests revealed that the addition of DEDOHC causes serious degradation for the graphite half-cell. From ac impedance measurements it was found that surface film resistance on graphite electrode was fourfold higher in the solution containing DEDOHC, whereas charge-transfer resistance scarcely changed. Atomic force microscopy observation revealed the formation of dense and highly adhesive surface precipitate layer on graphite electrode in the presence of DEDOHC. Consequently, it was concluded that the increase in cell impedance, which leads to deterioration of charge and discharge performance, is mainly due to the increase in the resistance of the surface film formed from the reduction of DEDOHC on the graphite negative electrode.

A new aluminium-hydrate species in hydrated Portland cements characterized by 27Al and 29Si MAS NMR spectroscopy

Recent 27Al MAS NMR studies of hydrated Portland cements and calcium-silicate-hydrate (C-S-H) phases have shown a resonance from Al in octahedral coordination, which cannot be assigned to the well-known aluminate species in hydrated Portland cements. This resonance, which exhibits the isotropic chemical shift δ iso = 5.0 ppm and the quadrupole product parameter P Q = 1.2 MHz, has been characterized in detail by 27Al MAS and 27Al{ 1H} CP/MAS NMR for different hydrated white Portland cements and C-S-H phases. These experiments demonstrate that the resonance originates from an amorphous or disordered aluminate hydrate which contains Al(OH) 6 3− or O x Al(OH) 6- x (3+ x)− units. The formation of the new aluminate hydrate is related to the formation of C-S-H at ambient temperatures, however, it decomposes by thermal treatment at temperatures of 70–90 °C. From the experiments in this work it is proposed that the new aluminate hydrate is either an amorphous/disordered aluminate hydroxide or a calcium aluminate hydrate, produced as a separate phase or as a nanostructured surface precipitate on the C-S-H phase. Finally, the possibilities of Al 3+ for Ca 2+ substitution in the principal layers and interlayers of the C-S-H structure are discussed.

An electron paramagnetic resonance study of Cu(II) sorbed on kaolinite

Cu(II) sorbed on kaolinite (KGa-1b) was studied using electron paramagnetic resonance (EPR) spectroscopy over various pH, ionic strength, and Cu(II) concentration conditions. Two different absorption lines, namely, an isotropic ( g iso = 2.193) and an anisotropic signal ( g || = 2.365 and g ⊥ = 2.088) were observed at room temperature. The signals were assigned to an outer-sphere and an inner-sphere Cu(II) surface complex, respectively. The intensity of the two signals varied with the sorption conditions or post-sorptional treatments such as rinsing and dehydration/rehydration. The intensity of the isotropic signal was not directly proportional to the amount of sorbed Cu(II). Rinsing of the Cu-loaded surface had a different impact on the signal intensity depending on the sorption pH. The isotropic signal together with the anisotropic signal was simultaneously observed on the kaolinite surface air-dried at room temperature for as long as 13 months. On the other hand, only the anisotropic signal was observed on the surface dried at 105 °C overnight. The re-wetting of the once dried kaolinite with deionized water immediately restored the isotropic signal. These results show that an outer-sphere surface complex is the dominant form of the sorbed copper ions and that the drying and aging of the surface convert the outer-sphere complex into the inner-sphere complex or surface precipitate, at least a part of which was reversibly converted back into the outer-sphere complex by the rehydration of the surface. This work suggests that dehydration during aging as well as solution variables such as pH, concentrations of Cu(II) and competing cations can have an impact on the amount and long-term fate of the outer-sphere Cu(II) complex formed on the kaolinite surface.

Divalent Cd and Pb uptake on calcite [formula omitted] cleavage faces: An XPS and AFM study

The interaction of divalent Cd and Pb with the { 10 1 ¯ 4 } cleavage faces of calcite has been investigated with X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Analysis of the { 10 1 ¯ 4 } cleavage planes of calcite was carried out with X-ray photoelectron spectroscopy (XPS) after exposure to divalent metal-bearing solutions in the 0.1–100 μM concentration range for times ranging from 1 to 24 h. The uptake of Cd 2+ by calcite was determined to be greater than that of Pb 2+ under similar experimental conditions (1 μM, pH 8.2, 24 h exposure time). In both cases, the majority of the divalent metal was postulated to exist in a surface precipitate. AFM results showed that the exposure of calcite to a 1 μM Pb 2+ solution resulted in ellipsoidal surface growths that were attributed to the nucleation of a PbCO 3 bulk phase. In the Cd circumstance, AFM showed comparatively flat growth features forming on the calcite surface even at concentrations down to 0.1 μM, where the solution would be expected to be undersaturated with respect to Cd bulk phases. These features were attributed to a (Ca,Cd)CO 3 solid solution. The individual exposure of these Cd/CaCO 3 and Pb/CaCO 3 samples to water pre-equilibrated with calcite (metal free) for 1 h led to the removal of no more than 20% of the divalent metal, suggesting that if there was an adsorbed Pb or Cd complex initially on the calcite surface, it was an minority species compared to the precipitate phase. Exposure of calcite to 100 μM Cd and Pb resulted in the accumulation of precipitate on the calcite surface presumably due to the divalent metal initial solution concentrations exceeding the solubility products of CdCO 3 and PbCO 3, respectively.

Influence of the preparation method on the structure–activity of cobalt oxide catalysts supported on alumina for complete benzene oxidation

In the present work we studied the influence of the methodology used for mounting Co(II) species on the γ-alumina surface on the physicochemical properties and the catalytic activity of the ‘cobalt oxide’/γ-alumina catalysts for complete oxidation of benzene.Three series of catalysts of varying Co content (up to 21wt.% Co) were prepared using three preparation methods: pore volume impregnation (pvi), equilibrium deposition filtration (edf) and pore volume impregnation adding nitrilotriacetic acid (nta) in the impregnation solution. It was found that the catalytic activity for low, medium and high Co content follows, respectively, the orders, nta–pvi≫pvi≫edf, nta–pvi≫edf≈pvi and edf>nta–pvi>pvi.The catalysts prepared were characterized using various techniques (BET, UV–vis/DRS, XRD and XPS) at each step of the preparation procedure, namely after the Co(II) mounting on the support surface, after drying as well as after calcination. It was inferred that the most active sites are located on Co3O4-supported crystallites, loosely or moderately interacting with the γ-alumina surface. Two critical parameters, related with the method followed for mounting Co(II) species on the γ-alumina surface, control the characteristics of the supported phase and thus the amount and the size of the above-mentioned Co3O4 crystallites: the ratio ‘amount of Co(II) deposited in the impregnation step to that remaining in the liquid phase inside the pores precipitating thus in the drying step’ closely related with the ratio ‘amount of Co(II) in the deposited phase (isolated Co(II) surface inner sphere complexes and Co(II) surface precipitates)/amount of Co(II) in the precipitated phase formed in the drying step’ as well as the composition of the precipitated phase.The application of the pvi technique resulted to low values for the above ratios and thus to the formation of a rather unstable precipitated phase consisted mainly by Co(H2O)62+·2NO3−. Upon calcination it is transformed into loosely bounded Co3O4 crystallites of relatively big size. This is related with the low Co dispersion and thus with the low catalytic activity exhibited by these catalysts.The application of edf resulted to high values for the above-mentioned ratios. Therefore, the deposited phase is predominant. Upon calcination it is transformed to well (very well) dispersed cobalt phases strongly (too strongly) bounded with the support surface and thus reducible at high temperatures (non reducible up to 800°C). Although these phases are responsible for the high Co dispersion achieved they do not contribute to the catalytic activity unless the deposited phase mainly comprises a Co(II) surface precipitate with relatively large number of layers as it is the case for the sample with the maximum Co content.The application of the nta–pvi technique resulted to very low values for the ratios mentioned above. This is because the [Co(II)–nta]− and [Co(II)–2nta]4− complexes, in which the Co(H2O)62+ complex is completely transformed, are not practically adsorbed on the support surface. Therefore, in the nta–pvi catalysts a precipitated phase containing the [Co(II)–nta]−·NH4+(or H+) and [Co(II)–2nta]4−·4NH4+ (or 4H+) complex salts predominates. Upon calcination these are transformed into Co3O4 crystallites of small size, which are moderately interacting with the support surface. This is related with the relatively high Co dispersion, mainly that for the catalytically active species, and thus with high catalytic activity.

Formation of Metal−Arsenate Precipitates at the Goethite−Water Interface

Little information is available concerning cosorbing oxyanion and metal contaminants in the environment, yet in most metal-contaminated areas, cocontamination by arsenate [AsO4, As(V)] is common. This study investigated the cosorption of As(V) and Zn on goethite at pH 4 and 7 as a function of final solution concentration. Complimentary extended X-ray absorption fine structure (EXAFS) spectroscopic data were collected at the As and Zn K-edges in order to glean information about the coordination environment of As and Zn at the goethite-water interface. Macroscopic sorption studies revealed that As(V) and Zn sorption on goethite increased in cosorption experiments beyond that suggested by single sorption isotherms. At pH 4 and 7, As(V) surface saturation was 3.2 and 2.2 micromol m(-2), respectively, and Zn surface saturation was absent at pH 4 and approximately 1.0 micromol m(-2) at pH 7. Arsenate sorption on goethite increased in the presence of Zn by 29% and by more than 500% at pH 4 and 7, respectively. In the presence of As(V), Zn sorption on goethite increased by 800 and 1300% at pH 4 and 7, respectively. More As(V) than Zn sorbed on goethite below surface saturation at pH 7. Above surface saturation, the Zn:As surface density ratio (SDR) remained constant at 0.91 +/- 0.03. At pH 4, the Zn:As SDR was less than 1 throughout the concentration range. Below As(V) surface saturation on goethite, As(V) formed bidentate binuclear bridging complexes on Fe and/or Zn octahedra, while Zn mainly formed edge-sharing complexes with Fe at the goethite surface. Above surface saturation, Zn was increasingly complexed by AsO4, gradually forming an adamite-like [Zn2(AsO4)OH] surface precipitate on goethite. Precipitated contaminants are more stable due to the limited dissolution kinetics of their solid phase. This study may therefore prove useful in remediation strategies of sites knowingly contaminated with oxyanions and metals.

Adsorption of Cobalt Ions on the “Electrolytic Solution/γ-Alumina” Interface Studied by Diffuse Reflectance Spectroscopy (DRS)

Diffuse reflectance spectroscopy was used for the first time to investigate the adsorption of the [Co(H2O)6]2+ ions on the interface developed between the surface of the gamma-alumina particles and the electrolytic aqueous solutions used for the preparation of cobalt-supported gamma-alumina catalysts by equilibrium deposition filtration. The formation of inner-sphere Co(II) surface complexes in which Co(II) is in octahedral symmetry was confirmed. A deconvolution peak centered at approximately 585 nm was attributed to the exchange of one aqua ligand with one AlxOHy (x = 1, 2, or 3; y = 0 or 1) negatively charged surface group resulting in the formation of mononuclear monosubstituted inner-sphere Co(II) complexes at a Co(II) surface concentration equal to 0.02 micromol of Co(II)/m2. It was inferred that as the surface Co(II) concentration increases the formation of disubstituted and/or trisubstituted surface complexes is favored with respect to the formation of monosubstituted Co(II) surface complexes. A deconvolution peak centered at approximately 640 nm was attributed to the exchange of one or more aqua ligands with bridging hydroxo ligands (Co-O-H). The relative magnitude of this peak increases with the Co(II) surface concentration, reflecting the increasing formation of binuclear, oligonuclear, and multinuclear Co(II) surface complexes and then the formation of the Co(II) surface precipitate.

Surface charges and Np(V) sorption on amorphous Al and Fe silicates

Sorption onto Si-rich alteration layers of crystalline minerals and nuclear glasses, and onto amorphous secondary silicates of rocks and soils, are expected to retard the migration of actinides in the near- and far-field of high-level waste repositories. In this work, we present experimental and modeling studies on the effects of silicate structure and bulk chemistry, and of solution chemistry, on charges and adsorption of neptunyl ions at surfaces of synthetic, amorphous or poorly ordered silica, Al silicates and Fe silicates. The Al silicates display similar pH-dependent surface charges characterized by predominant Si–O − sites, and similar surface affinities for neptunyl ions, irrespective to their Si/Al molar ratio (varying from 10 to 4.3). Such experimental features are explained by incorporation of Al atoms in tetrahedral position in the silicate lattice, leading to only trace amounts of high-affinity Al–OH surface groups due to octahedral Al. By contrast, the structure of the Fe silicates ensures the occurrence of high-affinity Fe–OH surface groups, whose concentration is shown by proton adsorption measurements to increase with decreasing of the silicate Si/Fe molar ratio (from 10 to 2.3). Nevertheless, experimental data of the adsorption of neptunyl and electrolyte ions show unexpectedly weak effect of the Si/Fe ratio, and suggest predominant Si–OH surface groups. A possible explanation is that aqueous silicate anions, released by dissolution, adsorb at Fe–OH surface groups and/or precipitate as silica gel coatings, because experimental solutions were found at near-equilibrium with respect to amorphous silica. Therefore, the environmental sorption of Np(V) onto Si-rich, amorphous or poorly ordered Al silicates may primarily depend on pH and silicate-specific surface areas, given the low overall chemical affinity of such phases for dissolved metals. By contrast, the sorption of Np(V) on natural, amorphous or poorly ordered Fe silicates may be a complex function of silicate bulk chemistry and solution chemistry, i.e., of pH and aqueous Si concentrations. Simple conceptual models of the surface chemistry of the Al and Fe silicates are developed here, based on the wealth of experimental data of silicate surface charges. The surface complexation models predict reasonably the effect of solution chemistry on the sorption of neptunyl ions on poorly ordered silicates of various compositions, and can thus be useful in extrapolating neptunyl mobility in many geochemical systems.

Effect of iron oxide coatings on zinc sorption mechanisms at the clay-mineral/water interface

Oxide surface coatings are ubiquitous in the environment, but their effect on the intrinsic metal uptake mechanism by the underlying mineral surface is poorly understood. In this study, the zinc (Zn) sorption complexes formed at the kaolinite, goethite, and goethite-coated kaolinite surfaces, were systematically studied as a function of pH, aging time, surface loading, and the extent of goethite coating, using extended X-ray absorption fine structure (EXAFS) spectroscopy. At pH 5.0, Zn partitioned to all sorbents by specific chemical binding to hydroxyl surface sites. At pH 7.0, the dominant sorption mechanism changed with reaction time. At the kaolinite surface, Zn was incorporated into a mixed metal ZnAl layered double hydroxide (LDH). At the goethite surface, Zn initially formed a monodentate inner-sphere adsorption complex, with typical ZnFe distances of 3.18 Å. However, with increasing reaction time, the major Zn sorption mechanism shifted to the formation of a zinc hydroxide surface precipitate, with characteristic ZnZn bond distances of 3.07 Å. At the goethite-coated kaolinite surface, Zn initially bonded to FeOH groups of the goethite coating. With increasing aging time however, the inclusion of Zn into a mixed ZnAl LDH took over as the dominant sorption mechanism. These results suggest that the formation of a precipitate phase at the kaolinite surface is thermodynamically favored over adsorption to the goethite coating. These findings show that the formation of Zn precipitates, similar in structure to brucite, at the pristine kaolinite, goethite, and goethite-coated kaolinite surfaces at near neutral pH and over extended reaction times is an important attenuation mechanism of metal contaminants in the environment.

XANES Determination of Adsorbed Phosphate Distribution between Ferrihydrite and Boehmite in Mixtures

Iron‐ and Al‐(hydr)oxide minerals are important sorbents for retaining PO4in soils. Our objective was to determine the distribution of adsorbed PO4between ferrihydrite and boehmite in aqueous mixtures of these minerals. Phosphate was adsorbed in aqueous suspensions up to maximum concentrations of 1860, 850, and 1420 mmol kg−1for ferrihydrite, boehmite, and 1:1 (by mass) mixtures of these minerals at pH 6. The solids were analyzed as moist pastes using P K‐XANES (X‐ray absorption near edge structure) spectroscopy. The adsorption isotherm for the mixed‐mineral suspensions could essentially be described as a linear combination of Freundlich isotherm models for each single‐mineral system, indicating negligible mineral interactive effects on PO4adsorption in the mixtures. X‐ray absorption near edge structure spectra for PO4adsorbed on ferrihydrite or in ferrihydrite/boehmite mixtures showed a pre‐edge feature at approximately 2146 eV that was absent in boehmite systems. Linear combination fitting of the pre‐edge region of XANES spectra for mixtures with average spectra for PO4adsorbed on boehmite or ferrihydrite alone, indicated that 59 to 97% of the PO4was adsorbed on ferrihydrite in the mixtures. With increasing concentration of adsorbed PO4in the mineral mixtures, the concentration adsorbed on the ferrihydrite component increased linearly. Phosphate distribution trends in the mixtures suggested an affinity preference for ferrihydrite at the lowest adsorbed PO4concentration (100 mmol kg−1minerals), no affinity preference for either mineral at intermediate concentrations (200 to 600 mmol PO4kg−1), and the possibility of a surface precipitate involving Al at the highest concentration (1300 mmol PO4kg−1).

Modeling and electrokinetic evidences on the processes of the Al(III) sorption continuum in SiO 2( s) suspension

Reactions of Al(III) at the interface between SiO 2( s) and aqueous solution were characteristically and quantitatively studied using electrophoretic methods and applying a surface complexation/precipitation model (SCM/SPM). The surface and bulk properties of Al(III)/SiO 2 suspensions were determined as functions of pH and initial Al(III) concentration. Simulated modeling results indicate that the SCM, accounting for the adsorption mechanism, predicts sorption data for low surface coverage only reasonably well. Al(III) hydrolysis and surface hydroxide precipitation must be invoked as the Al(III) concentration and/or pH progressively increase. Accordingly, the three processes in the Al(III) sorption continuum, from adsorption through hydrolysis to surface precipitation, could be identified by the divergence between the SCM/SPM predictions and the experimental data. SiO 2( s) suspensions with low Al(III) concentrations (1×10 −4 and 1×10 −5 M) exhibit electrophoretic behavior similar to that of a pure SiO 2( s) system. In Al(III)/SiO 2 systems with high Al concentrations of 1×10 −3, 5×10 −3 and 1×10 −2 M, three charge reversals (CR) are observed, separately representing, in order of increasing pH, the point of zero charge (PZC) on the SiO 2 substrate (CR1), the onset of the surface precipitation of Al hydroxide (CR2), and at a high pH, the PZC of the Al(OH) 3 coating (CR3). Furthermore, in the 1×10 −3 M Al(III)/SiO 2( s) system, CR2 is consistent with the modeling results of SCM/SPM and provides evidence that Al(III) forms a surface precipitate on SiO 2( s) at pH above 4. SiO 2( s) dissolution was slightly inhibited when Al(III) was adsorbed onto the surface of SiO 2( s) , as compared to the dissolution that occurs in a pure SiO 2( s) suspension system. Al hydroxide surface precipitation dramatically reduced the dissolution of SiO 2( s) because the Al hydroxide passive film inhibited the corrosion of the SiO 2( s) surface by OH − ions.

XANES Determination of Adsorbed Phosphate Distribution between Ferrihydrite and Boehmite in Mixtures

Iron- and Al-(hydr)oxide minerals are important sorbents for retaining PO4 in soils. Our objective was to determine the distribution of adsorbed PO4 between ferrihydrite and boehmite in aqueous mixtures of these minerals. Phosphate was adsorbed in aqueous suspensions up to maximum concentrations of 1860, 850, and 1420 mmol kg−1 for ferrihydrite, boehmite, and 1:1 (by mass) mixtures of these minerals at pH 6. The solids were analyzed as moist pastes using P K-XANES (X-ray absorption near edge structure) spectroscopy. The adsorption isotherm for the mixed-mineral suspensions could essentially be described as a linear combination of Freundlich isotherm models for each single-mineral system, indicating negligible mineral interactive effects on PO4 adsorption in the mixtures. X-ray absorption near edge structure spectra for PO4 adsorbed on ferrihydrite or in ferrihydrite/boehmite mixtures showed a pre-edge feature at approximately 2146 eV that was absent in boehmite systems. Linear combination fitting of the pre-edge region of XANES spectra for mixtures with average spectra for PO4 adsorbed on boehmite or ferrihydrite alone, indicated that 59 to 97% of the PO4 was adsorbed on ferrihydrite in the mixtures. With increasing concentration of adsorbed PO4 in the mineral mixtures, the concentration adsorbed on the ferrihydrite component increased linearly. Phosphate distribution trends in the mixtures suggested an affinity preference for ferrihydrite at the lowest adsorbed PO4 concentration (100 mmol kg−1 minerals), no affinity preference for either mineral at intermediate concentrations (200 to 600 mmol PO4 kg−1), and the possibility of a surface precipitate involving Al at the highest concentration (1300 mmol PO4 kg−1).

Direct speciation of phosphorus in alum-amended poultry litter: solid-state 31P NMR investigation.

Amending poultry litter (PL) with aluminum sulfate (alum) has proven to be effective in reducing water-soluble phosphorus (P) in the litter and in runoff from fields that have received PL applications; it has therefore been suggested as a best management practice. Although its effectiveness has been demonstrated on a macroscopic scale in the field, little is known about P speciation in either alum-amended or unamended litter. This knowledge is important forthe evaluation of the long-term stability and bioavailability of P, which is a necessary prerequisite forthe assessment of the sustainability of intensive poultry operations. Both solid-state MAS and CP-MAS 31P NMR as well as 31P[27Al]-TRAPDOR were used to investigate P speciation in alum-amended and unamended PL. The results indicate the presence of a complex mixture of organic and inorganic orthophosphate phases. A calcium phosphate phase, probably a surface precipitate on calcium carbonate, could be identified in both unamended and alum-amended PL, as well as physically bound HPO4(2-). Phosphate associated with Al was found in the alum-amended PL, most probably a mixture of a poorly ordered wavellite and phosphate surface complexes on aluminum hydroxide that had been formed by the hydrolysis of alum. However, a complex mixture of organic and inorganic phosphate species could not be resolved. Phosphate associated with Al comprised on average 40 +/- 14% of the total P in alum-amended PL, whereas calcium phosphate phases comprised on average 7 +/- 4% in the alum-amended PL and 14 +/- 5% in the unamended PL.

The reactivity of seven Mn-oxides with Cr3+aq: A comparative analysis of a complex, environmentally important redox reaction

The interactions between 10−4 M Cr3+aq and seven different Mn-oxides (six natural samples, including hausmannite, manganite, romanechite, cryptomelane, lithiophorite, and pyrolusite; and one synthetic sample, birnessite) have been observed in aqueous solution at pH 4.4 (pH 5.2 in the case of hausmannite) and room temperature. For each mineral-solution interaction, the aqueous chemical concentrations of [Mnaq], [Cr]Taq and [Cr6+aq] were measured with time. Reacted samples were examined by XPS to determine if and to what extent the surface chemical states of Cr, Mn, and O had changed. Microscopic observations of the reacted surfaces were obtained using AFM and high-resolution, low-voltage FESEM. Cr-uptake onto/into the Mn-oxides is observed to occur in Cr-hydroxide/hydrate undersaturated solutions as a result of adsorption, absorption, and surface precipitation. In addition, the rate and extent of reductive dissolution and Cr3+ aq oxidation varied considerably between the seven Mn-oxides. The measured ratios of [Mn]aq:[Cr6+]aq were in agreement with the values expected from the proposed stoichiometric reactions where they could be easily written. The chromium as detected by XPS on the surface of the reacted Mn-oxides was predominately Cr3+, however pyrolusite contained both Cr3+ and Cr6+. Several previous studies have implicated a chromium surface precipitate to be responsible for the cessation of the Cr3+ aq oxidation reaction. Our surface sensitive FESEM and AFM observations tend to suggest that Cr uptake is by isolated site binding, and very small (<30 nm) surface clusters. On two of the seven Mn-oxides tested (hausmannite and birnessite), Cr uptake was followed by slow Cr release after a substantial portion of the total aqueous Cr had been converted to Cr6+ aq. The Mn-oxides that exhibited the greatest and longest lasting Cr3+-oxidizing ability were the Mn-oxides containing Mn3+, and in particular those containing Mn3+ and Mn2+. It is believed that the combined presence of a reducible Mn ion (e.g., Mn3+) and a highly soluble Mn2+ ion facilitates a sustained Cr3+-oxidation reaction because fresh Mn3+ surface is exposed during the dissolution reaction. Finally, the Cr3+ aq reacted Mn-oxides tested showed a partial rejuvenation of their Cr3+-oxidizing ability after a period of drying and storage. An assessment of this process will require additional experimentation.

Modeling the adsorption of Cd(II) onto goethite in the presence of citric acid

The adsorption of cadmium onto goethite in the presence of citric acid was measured as a function of pH and cadmium concentration at 25°C. Potentiometric titrations were also performed on the system. Cadmium adsorption onto goethite was enhanced above pH 4 in the presence of 50 μM, 100 μM and 1 mM citric acid. While there was little difference between the enhancements caused by 50 and 100 μM citric acid below pH 6, above pH 6 further enhancement is seen in the presence of 100 μM citric acid. When 1 mM citric acid was present, the enhancement of cadmium adsorption was greater below pH 6, with increased Cd(II) adsorption down to pH 3.5. However, above pH 6, 1 mM of citric acid caused slightly less enhancement than the lower citric acid concentrations. ATR–FTIR spectra of soluble and adsorbed citrate–cadmium species were measured as a function of pH. At pH 4.6 there was very little difference between the ternary Cd(II)–citric acid–goethite spectrum and the binary citric acid–goethite spectrum. However, spectra of the ternary system at pH 7.0 and 8.7 indicated the presence of additional surface species. Further analysis of the spectra suggested that these were metal–ligand outer-sphere complexes. Data from the adsorption experiments and potentiometric titrations of the ternary Cd(II)–citric acid–goethite system were fitted by an extended constant-capacitance surface complexation model. The spectroscopic data were used to inform the choice of surface species. Three reactions in addition to those for the binary Cd(II)–goethite and citric acid–goethite systems were required to describe all of the data. They were 2SOH+Cd2++L3−+H+⇌(SOH)(SOH2+)--LCd−0,SOH+Cd2++L3−+2H+⇌[SLH−--Cd2+]++H2O, and SOH+2Cd2++L3−+2OH−⇌[SOCd+--LCdOH2−]−+H2O. Neither the spectroscopy nor the modeling suggested the formation of a ternary inner-sphere complex or a surface precipitate under the conditions used in this study.

Impact of transition metals on reductive dechlorination rate of hexachloroethane by mackinawite.

Mackinawite, an iron monosulfide, has been shown to be a potential reductant for chlorinated organic compounds under anaerobic conditions. Chlorinated organic compounds are often found with inorganic contaminants. This study investigates the impact of various transition metals on the reductive dechlorination by mackinawite using a readily degradable chlorinated organic compound, hexachloroethane (HCA). Different classes of transition metals show distinct patterns in their impact on the HCA dechlorination: 10(-3) M Cr(III) and Mn(II) (hard metals) decreased the dechlorination rates, while 10(-4), 10(-3), and 10(-2) M Co(II), Ni(II), Cu(II), Zn(II), Cd(II), and Hg(II) (intermediate/soft metals) increased the rates. The tested hard metals, due to their weak affinity for sulfides, are thought to form surface precipitates of hydroxides around FeS under the experimental conditions with these hydroxides hindering the electron transfer between FeS and HCA. Due to their high affinity for sulfides, however, the tested intermediate/soft metals can react with FeS in various ways: precipitation of pure metal sulfides (MS), formation of metal-substituted FeS by lattice exchange, and coprecipitation of the mixed sulfides in a Fe-M-S system. Fe(II), released as a result of the interaction of FeS with intermediate/soft metals, enhances the HCA dechlorination at the doses of 10(-4) and 10(-3) M through sorbed or dissolved Fe(II) species, while Fe(OH)2(s) formed at the higher dose of 10(-2) M also enhances the reductive dechlorination. Rate increases observed in Co(II)-, Ni(II)-, and Hg(II)-amended systems are not simply explained by the formation of pure MS; instead, metal-substituted FeS or coprecipitated sulfides are thought to be responsible for the significantly increased rates observed in these systems.

Uptake of Cm(III) and Eu(III) by Calcium Silicate Hydrates: A Solution Chemistry and Time-Resolved Laser Fluorescence Spectroscopy Study

The interaction of the two chemical homologues [Cm(III) and Eu(III)] with calcium silicate hydrates (CSH phases) at pH 13.3 has been investigated in batch-type sorption studies using Eu(III) and complemented with time-resolved laser fluorescence spectroscopy (TRLFS) using Cm(III). The sorption data for Eu(III) reveal fast sorption kinetics and a strong uptake by CSH phases with distribution ratios of (6 +/- 3) x 10(5) L kg(-1). Three different Cm(III) species have been identified: A nonfluorescing species, which was identified as a curium hydroxide (surface) precipitate, and two fluorescing Cm(III)/CSH-sorbed species. The fluorescing sorbed species have characteristic emission spectra with main peak maxima at 618.9 and 620.9 nm and fluorescence emission lifetimes of 289 +/- 11 and 1482 +/- 200 micros, respectively. From the fluorescence lifetimes, it was calculated that the two fluorescing Cm(III) species have one or two and no water molecules left in their first coordination sphere, suggesting that these species are incorporated into the CSH structure. A structural model for Cm(III) and Eu(III) incorporation into CSH phases is proposed based on the substitution for Ca at two different types of sites in the CSH structure.

Arsenic removal by zero-valent iron: field, laboratory and modeling studies

Field and laboratory studies were conducted to elucidate the design factors and mechanisms of arsenic removal from contaminated ground water using zero-valent iron. Large scale, field pilot experiments demonstrated for more than 8 months that iron filing filters can efficiently remove arsenite from aqueous solutions to levels less than 10 μg/L. The maximum arsenic accumulation measured was 4.4 mg As/g of media. The iron filing filters leached significant quantities of iron (73% of the iron was leached). A critical design parameter of the system was found to be the hydraulic detention time of the water in the filter. TCLP analyses of the spent media indicated that the arsenic concentration in the leachate was two orders of magnitude lower than the 5 mg/L of TCLP for arsenic. Spectroscopic and laboratory arsenic leaching studies (alkaline extraction and TCLP) suggest that the arsenic surface precipitate is related to sulfur. The aging process (due to the longevity of the removal mechanism) makes the precipitation process virtually irreversible. A mathematical model was developed to simulate the removal process using a partitioning coefficient and a mass transfer process. Calibration of these parameters using the data for three columns revealed that the equilibrium-partitioning coefficient was the same for all three columns while the mass transfer coefficient was a function of the flow rate. The calibrated mass transfer coefficients are similar to those reported in the literature if they are normalized to the surface area of the media.

Arsenite sorption on troilite (FeS) and pyrite (FeS 2)

Arsenic is a toxic metalloid whose mobility and availability are largely controlled by sorption on sulfide minerals in anoxic environments. Accordingly, we investigated reactions of As(III) with iron sulfide (FeS) and pyrite (FeS 2) as a function of total arsenic concentration, suspension density, sulfide concentration, pH, and ionic strength. Arsenite partitioned strongly on both FeS and FeS 2 under a range of conditions and conformed to a Langmuir isotherm at low surface coverages; a calculated site density of near 2.6 and 3.7 sites/nm 2 for FeS and FeS 2, respectively, was obtained. Arsenite sorbed most strongly at elevated pH (>5 to 6). Although solution data suggested the formation of surface precipitates only at elevated solution concentrations, surface precipitates were identified using X-ray absorption spectroscopy (XAS) at all coverages. Sorbed As was coordinated to both sulfur [d(As-S) = 2.35 Å] and iron [d(As-Fe) = 2.40 Å], characteristic of As coordination in arsenopyrite (FeAsS). The absorption edge of sorbed As was also shifted relative to arsenite and orpiment (As 2S 3), revealing As(III) reduction and a complete change in As local structure. Arsenic reduction was accompanied by oxidation of both surface S and Fe(II); the FeAsS-like surface precipitate was also susceptible to oxidation, possibly influencing the stability of As sorbed to sulfide minerals in the environment. Sulfide additions inhibit sorption despite the formation of a sulfide phase, suggesting that precipitation of arsenic sulfide is not occurring. Surface precipitation of As on FeS and FeS 2 supports the observed correlation of arsenic and pyrite and other iron sulfides in anoxic sediments.

Surface complexation and precipitate geometry for aqueous Zn(II) sorption on ferrihydrite: II. XANES analysis and simulation

X-ray absorption near-edge spectroscopy (XANES) analysis of sorption complexes has the advantages of high sensitivity (10- to 20-fold greater than extended X-ray absorption fine structure [EXAFS] analysis) and relative ease and speed of data collection (because of the short k-space range). It is thus a potentially powerful tool for characterization of environmentally significant surface complexes and precipitates at very low surface coverages. However, quantitative analysis has been limited largely to “fingerprint” comparison with model spectra because of the difficulty of obtaining accurate multiple-scattering amplitudes for small clusters with high confidence. In the present work, calculations of the XANES for 50- to 200-atom clusters of structure from Zn model compounds using the full multiple-scattering code Feff 8.0 accurately replicate experimental spectra and display features characteristic of specific first-neighbor anion coordination geometry and second-neighbor cation geometry and number. Analogous calculations of the XANES for small molecular clusters indicative of precipitation and sorption geometries for aqueous Zn on ferrihydrite, and suggested by EXAFS analysis, are in good agreement with observed spectral trends with sample composition, with Zn-oxygen coordination and with changes in second-neighbor cation coordination as a function of sorption coverage. Empirical analysis of experimental XANES features further verifies the validity of the calculations. The findings agree well with a complete EXAFS analysis previously reported for the same sample set, namely, that octahedrally coordinated aqueous Zn 2+ species sorb as a tetrahedral complex on ferrihydrite with varying local geometry depending on sorption density. At significantly higher densities but below those at which Zn hydroxide is expected to precipitate, a mainly octahedral coordinated Zn 2+ precipitate is observed. An analysis of the multiple scattering paths contributing to the XANES demonstrates the importance of scattering paths involving the anion sublattice. We also describe the specific advantages of complementary quantitative XANES and EXAFS analysis and estimate limits on the extent of structural information obtainable from XANES analysis.