Creating mixed Fe/Al coatings on planar gamma-Al2O3 surfaces

Insufficient understanding of the interactions of reactive phases (e.g., Fe and Al oxides) with minerals, other reactive phases and sorbing species has made predicting and modeling metal sorption on natural sediment surfaces difficult. This work develops a method to create mixed Fe/Al planar oxide surfaces by coating well-characterized planar gamma-Al2O3 with ferric iron. The objective is to closely control the Fe/Al ratio as well as the distribution of Fe on the planar surface. Effects of starting Fe(III) concentration, reaction time and number of coating sequences were examined using XPS and ToF-SIMS. No observable trend was seen in Fe/Al ratios by varying the starting Fe(III) concentration or reaction time. For both 4- and 14-day reactions, lower concentrations of Fe(III) produced oxide phases with a homogeneous distribution of Fe at the surface as detected by ToF-SIMS. ToF-SIMS Fe elemental maps of the oxide phases resulting from the highest Fe(III) concentration showed areas of localized Fe deposition. A sequential coating procedure allowed for a closer control of the concentration and spatial distribution of Fe(III) in the resulting oxide phase. This work provides methodology that can be used to create Fe/Al oxide phases whose Fe/Al content can be controlled for use in subsequent sorption studies to better understand the effects of mixed phase oxides on metal ion uptake.     

Comparison of lead, cadmium, copper and cobalt adsorption onto metal oxides and organic materials in natural surface coatings

Selective extraction techniques followed by adsorption experiment and statistical analysis were employed to estimate and compare the relative roles of metal oxides and organic substance in adsorption of Pb, Cd, Cu and Co onto surface coatings. Results indicated that metal oxides were very important sorbents for all of metal ions involved in this study, especially for Pb and Co. Furthermore, manganese oxides contributed to the absolute majority of Co adsorption regardless of concentration. But for Cu and Cd, organic materials are also very important sorbent phases, particularly for Cu, organic materials contributed to most of the Cu adsorption regardless of concentration. In addition, the analysis suggested the extraordinary predominance of Mn oxides for metal adsorption at the low concentrations. Considering the low concentration in natural water environments, Mn oxides might exert the greatest influence on the behavior of heavy metals.     

EXAFS study of mercury(II) sorption to Fe- and Al-(hydr)oxides. I. Effects of pH

The study of mercury sorption products in model systems using appropriate in situ molecular-scale probes can provide detailed information on the modes of sorption at mineral/water interfaces. Such studies are essential for assessing the influence of sorption processes on the transport of Hg in contaminated natural systems. Macroscopic uptake of Hg(II) on goethite (alpha-FeOOH), gamma-alumina (gamma-Al(2)O(3)), and bayerite (beta-Al(OH)(3)) as a function of pH has been combined with Hg L(III)-edge EXAFS spectroscopy, FTIR spectroscopy, and bond valence analysis of possible sorption products to provide this type of information. Macroscopic uptake measurements show that Hg(II) sorbs strongly to fine-grained powders of synthetic goethite (Hg sorption density Gamma=0.39-0.42 micromol/m(2)) and bayerite (Gamma=0.39-0.44 micromol/m(2)), while sorbing more weakly to gamma-alumina (Gamma=0.04-0.13 micromol/m(2)). EXAFS spectroscopy on the sorption samples shows that the dominant mode of Hg sorption on these phases is as monodentate and bidentate inner-sphere complexes. The mode of Hg(II) sorption to goethite was similar over the pH range 4.3-7.4, as were those of Hg(II) sorption to bayerite over the pH range 5.1-7.9. Conversion of the gamma-Al(2)O(3) sorbent to a bayerite-like phase in addition to the apparent reduction of Hg(II) to Hg(I), possibly by photoreduction during EXAFS data collection, resulted in enhanced Hg uptake from pH 5.2-7.8 and changes in the modes of sorption that correlate with the formation of the bayerite-like phase. Bond valence calculations are consistent with the sorption modes proposed from EXAFS analysis. EXAFS analysis of Hg(II) sorption products on a natural Fe oxyhydroxide precipitate and Al/Si-bearing flocculent material showed sorption products and modes of surface attachment similar to those for the model substrates, indicating that the model substrates are useful surrogates for the natural sediments.     

A study of thoria on the surface of gamma-Al2O3

The dispersion of thoria on the surface of gamma-Al2O3 and the surface properties of ThO2/gamma-Al2O3 samples, as well as the influence of the loading amount of thoria on the reduction behavior of copper oxide species, have been studied using XRD, XPS, FTIR, and TPR. The results indicate that the dispersion capacity of thoria, like that of ceria, is much lower than for two other tetravalent metal oxides, zirconia and titania, and the surface adsorption amount of the carbonyl compound and H2O slightly increases with increasing thoria loading. The different thoria loadings can influence the reduction behavior of the dispersed copper oxide by comparing the TPR results of CuO/ThO2/gamma-Al2O3 samples. In addition, the lower dispersion capacities of thoria and ceria on gamma-Al2O3 are tentatively discussed by considering the structural stability of the two oxides.     

Comparison of Pb, Cd Adsorption to Surface Coatings Developed in Natural Waters with that in Plant Effluents

The comparative studies of Pb and Cd adsorption to the surface coatings (Fe, Mn, AI oxides, organic materials, and associated minerals), which were developed on glass slides in five natural and two technical waters (plant effluents), were carried out under controlled |aboratory conditions(mineral salts solution with defined speciation, ionic strength 0.05 mol/L, 25 ℃ and pH 6. 0). The classical Langmuir adsorption isotherm was applied to estimating the equilibrium coefficients of Pb and Cd adsorption to the surface coat-ings. The results show that the maximum adsorption of Pb and Cd to the surface coatings mentioned above varied widely. There was a systemic increase in the maximum adsorption of Pb and Cd to the surface coatings with increasing the contents of Mn and Fe oxides in the surface coatings in significant correlation, respective-ly, not only highlighting the relative importance of the metal oxide fraction for Pb and Cd adsorption to the surface coatings developed in natural and technical water samples, but also implying the same adsorption mechanisms of Pb and Cd to the surface coatings developed both in natural and technical water samples.     

Simultaneous sorption of benzene and heavy metals onto two organoclays

An experimental study was performed to determine the feasibility of using hexadecyltrimethylammonium bentonite clay (HDTMA-clay) and benzyltriethylammonium bentonite clay (BTEA-clay) for simultaneous sorption of benzene and one of four heavy metals (Pb, Cd, Zn and Hg). Specifically, the role of competition between benzene and each heavy metal was studied. The sorption of Pb, Cd, and Zn on both BTEA- and HDTMA-clay decreases in the presence of benzene relative to the sorption obtained without benzene present. This indicates that there is competition between Pb, Cd, and Zn and organic compounds during sorption onto both organoclays. On BTEA-clay, Cd, Pb and Zn sorption was reduced by 24, 37, and 51%, respectively. On HDTMA-clay, Cd, Pb, and Zn sorption was reduced by 25, 30, and 57%, respectively. Hg sorption was not affected either by the presence of benzene or by the organoclays used. The sorption of benzene onto BTEA-clay in the presence of Hg, Zn, Pb, and Cd was less than the sorption observed when no heavy metal was present. The presence of Hg resulted in the most significant decrease in sorption, causing a 59% reduction in benzene sorption. The presence of Zn, Pb, and Cd caused a 41, 35, and 31% reduction in benzene sorption, respectively. In general, sorption of benzene onto HDTMA-clay was not affected by the presence of the heavy metals, indicating there are no competitive effects observed with Zn, Cd, and Hg when HDTMA-clay was the sorbent. However, the presence of Pb did cause a 20% reduction in benzene sorption to HDTMA-clay. Both organoclays tested had dual sorptive properties for both heavy metals and an organic contaminant. While the competitive effects were greater for the BTEA clay, both organoclays are capable of simultaneously removing benzene and either Zn, Cd, Hg, or Pb from aqueous solution.     

Modeling Pb sorption to microporous amorphous oxides as discrete particles and coatings

Hydrous amorphous Al (HAO), Fe (HFO), and Mn (HMO) oxides are ubiquitous in the subsurface as both discrete particles and coatings and exhibit a high affinity for heavy metal contaminants. To assess risks associated with heavy metals, such as Pb, to the surrounding environment and manage remedial activities requires accurate mechanistic models with well-defined transport parameters that represent sorption processes. Experiments were conducted to evaluate Pb sorption to microporous Al, Fe, and Mn oxides, as well as to montmorillonite and HAO-coated montmorillonite. Intraparticle diffusion, a natural attenuating process, was observed to be the rate-limiting mechanism in the sorption process, where best-fit surface diffusivities ranged from 10(-18) to 10(-15) cm(2) s(-1). Specifically, diffusivities of Pb sorption to discrete aluminum oxide, aluminum oxide-coated montmorillonite, and montmorillonite indicated substrate surface characteristics influence metal mobility where diffusivity increased as affinity decreased. Furthermore, the diffusivity for aluminum oxide-coated montmorillonite was consistent with the concentrations of the individual minerals present and their associated particle size distributions. These results suggest that diffusivities for other coated systems can be predicted, and that oxide coatings and montmorillonite are effective sinks for heavy metal ions.     

不同水体生物膜中各化学组分对铅的吸附作用研究

水中的底泥、悬浮物和生物膜等固相物质对重金属在水中的迁移和转化有重要影响 [1,2 ] ,而固相物质中起主要作用的是吸附能力强的铁、锰氧化物和有机质 [3,4 ] .对生物膜与重金属相互作用的研究已经有报道 [5,6] .Nelson等[3] 用吸附加和模型模拟生物膜上各组分对铅吸附 ;Dong等 [7] 用选择性萃取的方法研究生物膜各组分对铅、镉的吸附情况 ,但他们的研究均局限于单一水体 ,本文采用萃取 -吸附-统计分析的方法研究多个自然水体生物膜中铁、锰氧化物和有机质对铅吸附的相对贡献 .1　实验部分1 .1　生物膜的培养及组分测定　选吉林省境内的净…     

Mechanism of metal-ion sorption on hydrous metal oxides

A model for treating the sorption of metal ions on hydrous metal oxides was established based on the assumptions that these materials are weakly acidic cation exchangers and have a discrete exchanger phase. The experimental results of the sorption of metal ions on the hydrous niobium(V) and tin(IV) oxides are found to be consistent with the formulas derived from the model by considering that the charge balance and the mass action law hold in the exchanger phase and cations are sorbed by the distribution between this phase and the bulk aqueous phase.     

Trace metal ion partitioning at polymer film-metal oxide interfaces: long-period X-ray standing wave study

The distributions of Pb(II) and As(V)O4(3-) ions in the interfacial region between thin poly(acrylic acid) (PAA) coatings and aalpha-A12O3(0001), alpha-Al2O3(1-102), and alpha-Fe2O3(0001) single-crystal substrates were studied using long-period X-ray standing wave fluorescent yield (XSW-FY) and X-ray reflectivity techniques. The PAA film serves as a simplified analogue of natural organic matter (NOM) coatings on mineral surfaces. Such coatings are often assumed to play an important role in the partitioning and speciation of trace heavy metals in soils and aquatic systems. On the alpha-Al2O3(1-102) surface, Pb(II) ions were found to preferentially bind to the PAA coating, even at sub-micromolar Pb(II) concentrations, and to partition increasingly onto the metal oxide surface as the Pb(II) concentration was increased ([Pb(II)] = 5 x 10(-8) to 2 x 10(-5) M, pH = 4.5; 0.01 M NaCl background electrolyte). This observation suggests that the binding sites in the PAA coating outcompete those on the alpha-Al2O3(1-102) surface for Pb(II) under these conditions. The As(V)O4(3-) oxoanion partitions preferentially to the L-Al2O3(1-102) surface for the As(V)O4(3-) concentrations examined (1 x 10(-7) to 5 x 10(-7) M, pH = 4.5; 0.01 M NaCl background electrolyte). Partitioning of Pb(II) (at 1 x 10(-7) M and pH 4.5) was also examined at PAA/alpha-Al2O3(0001), and PAA/alpha-Fe2O3(0001) interfaces using XSW-FY measurements. Our results show that the PAA coating was the dominant sink for Pb(II) in all three samples; however, the relative order of reactivity of these metal oxide surfaces with respect to Pb(II) sorption is alpha-Fe2O3(0001) > alpha-Al2O3(1-102) > alpha-Al2O3(0001). This order is consistent with that found in previous studies of the PAA-free surfaces. These XSW results strongly suggest that the characteristics of the organic film (i.e., binding affinity, type, and density of binding sites) as well as metal oxide substrate reactivity are key factors determining the distribution and speciation of Pb(II) and As(V)O4(3-) at organic film/metal oxide interfaces.     

Sorption of trace metals (Cu, Pb, Zn) by suspended lake particles in artificial (0.005 M NaNO3) and natural (Esthwaite Water) freshwaters

The sorption of Cu, Pb, and Zn onto natural lake particles, suspended in 0.005 M NaNO₃ solution and in a natural lake water (Esthwaite Water, Cumbria, UK), was studied as a function of pH and time in a series of laboratory experiments, under environmentally realistic conditions. The sorption of all three metals increased with increasing pH and reaction time (2 h and 7 days). In 0.005 M NaNO₃ solution, the well-defined sorption edges spanned 2–2.5 pH units for Cu and Pb, and ≈ 4 pH units for Zn. In the natural lake water, the Cu sorption edge was broader and both Cu and Zn were less strongly sorbed. The binding stability decreased in the order Pb>Cu>Zn. Competitive adsorption onto surface sites appeared to be the main factor determining the observed sorption behaviour. Application of a macroscopic metal exchange model to the 7 day NaNO₃ results enabled the surface site concentration to be estimated as 0.79 ± 0.07 mmol g⁻¹. The modelling exercise suggested that an observed shift in the sorption edge of Zn, in the presence of Pb and Cu, was due to competition for surface sites. The experimental data are in good general agreement with field observations of trace metal behaviour in Cumbrian lakes. The almost total sorption of Pb by lake particles throughout the in-situ pH range is compatible with previous field measurements including trace metal budgets and residence times. Dissolved Zn concentrations in the lake are lower than predicted by the sorption experiments, but the lower lake concentrations are consistent with the previously observed scavenging of Zn by planktonic algae. Both the decreased sorption of Cu in the experiments with natural lake water, compared to that in NaNO₃ solution, and the relatively small-scale removal of dissolved Cu by particles in the lake itself can partially be explained by humic complexation.     

Competitive sorption and desorption of heavy metals in mine soils: influence of mine soil characteristics

Many mine soils are chemically, physically, and biologically unstable and deficient. They are sometimes amended with sewage sludge and ashes but often contain heavy metals that increase the already high mine soils' heavy metal contents. Cd, Cr, Cu, Ni, Pb, and Zn in mutual competition were added to five mine soils (Galicia, Spain). Soil capacities for heavy metal sorption and retention were determined by means of distribution coefficients and selectivity sequences among metals. Influence of soil characteristics on sorption and retention was also examined. Retention selectivity sequences indicate that, in most of the soils, Pb is the preferred retained metal, followed by Cr. The last metals in these sequences are Ni, Cd, and Zn. Soil organic matter content plays a fundamental role in control of Pb sorption. Gibbsite, goethite, and mica influence Cr retention. Soil organic matter, oxides, and chlorite contents are correlated with K(d sigma sp medium). Heavy metals are weakly adsorbed by soils and then desorbed in high amounts. To recover these soils it is necessary to avoid the use of residues or ashes that contain heavy metals due to their low heavy metal retention capacity.     

Effect of phosphate complexation on Cd2+ sorption by manganese dioxide (beta-MnO2)

Sorption of metal ions on oxide/hydroxide surfaces mediates the fate and transport of these ions in many natural systems. These metallic ions often exist in bulk in the aqueous phase as complexes with inorganic and organic ligands. In the present study, we investigated the sorption properties of manganese dioxide in the presence of phosphate which is thought to be one of the most important complex forming species. The surface area, point of zero charge and structural morphology of the solid manganese dioxide were determined. Cd(2+) sorption studies were carried out on manganese dioxide as a function of pH, temperature and phosphate concentration. Cd(2+) sorption increased with increasing pH, temperature and phosphate concentration. It was found that phosphate formed both outer and inner sphere complexes via metal and ligand-like adsorption. The Langmuir equation was applied to describe the data and from the constants of this equation different thermodynamic parameters such as DeltaH(0), DeltaS(0) and DeltaG(0) were evaluated.     

Intraparticle surface diffusion of metal contaminants and their attenuation in microporous amorphous Al, Fe, and Mn oxides

Intraparticle surface diffusion is an important and rate-limiting process in the sorption of metal ions to microporous sorbents such as those of hydrous amorphous Al (HAO), Fe (HFO), and Mn (HMO) oxides; these minerals are abundant in the environment, exhibiting a high affinity for metal contaminants. In aquatic systems representative of natural environments, internal micropore surfaces of HAO, HFO, and HMO can account for 40 to 90% of the sorption sites. Surface diffusivities have been observed to range between 10(-16) and 10(-10) cm2 s(-1) for metals including Sr, Cd, Zn, and Ni. The combination of significant microporosity and small diffusivities results in the amorphous oxides acting as natural attenuating sinks.     

Heterogeneous reaction of acetic acid on MgO, α-Al2O3, and CaCO3 and the effect on the hygroscopic behaviour of these particles

Mixtures of organic compounds with mineral dust are ubiquitous in the atmosphere, whereas the formation pathways and hygroscopic behavior of these mixtures are not well understood. In this study, in situ DRIFTS, XRD, and a vapor sorption analyzer were used to investigate the heterogeneous reaction of acetic acid on α-Al(2)O(3), MgO, and CaCO(3) particles under both dry and humid conditions while the effect of reactions on the hygroscopic behavior of these particles was also measured. In all cases, formation of acetate is significantly enhanced in the presence of surface water. However, the reaction extent varied with the mineral phase of these particles. For α-Al(2)O(3), the reaction is limited to the surface with the formation of surface coordinated acetate under both dry and humid conditions. For MgO, the bulk of the particle is involved in the reaction and Mg(CH(3)COO)(2) is formed under both dry and humid conditions, although it exhibits a saturation effect under dry conditions. In the case of CaCO(3), acetic acid uptake is limited to the surface under dry conditions while it leads to the decomposition of the bulk of CaCO(3) under humid conditions. While coordinated surface acetate species increased the water adsorption capacity slightly, the formation of bulk acetate promoted the water absorption capacity greatly. This study demonstrated that heterogeneous reaction between CH(3)COOH and mineral dust is not only an important sink for CH(3)COOH, but also has a significant effect on the hygroscopic behavior of mineral dust.     

Reactivity of site-isolated metal clusters: propylidyne on gamma-Al2O3-supported Ir4.

To contrast the reactivity of supported metal clusters with that of extended metal surfaces, we investigated the reactions of tetrairidium clusters supported on porous gamma-Al2O3 (Ir4/gamma-Al2O3) with propene and with H2. Infrared, 13C NMR, and extended X-ray absorption fine-structure spectroscopy were used to characterize the ligands formed on the clusters. Propene adsorption onto Ir4/gamma-Al2O3 at 298 K gave stable, cluster-bound mu3-propylidyne. Propene adsorbed onto Ir4/gamma-Al2O3 at 138 K reacted at approximately 219 K to form a stable, highly dehydrogenated, cluster-bound hydrocarbon species approximated as CxHy (such as, for example, C3H2 or C2H). H2 reacted with Ir4/gamma-Al2O3 at 298 K, forming ligands (likely hydrides), which prevented subsequent reaction of the clusters with propene to form propylidyne. Propylidyne on Ir4 was stable in helium or H2 as the sample was heated to 523 K, whereupon it reacted with oxygen of the support to give CO. Propylidyne on Ir4 did not undergo isotopic exchange in the presence of D2 at 298 K. In contrast, the literature shows that propylidyne chemisorbed on extended metal surfaces is hydrogenated in the presence of H2 (or D2) and exchanges hydrogen with gaseous D2 at room temperature; in the absence of H2, it decomposes thermally to give hydrocarbon fragments at temperatures much less than 523 K. The striking difference in reactivities of propylidyne on clusters and propylidyne on extended metal surfaces implies the requirement of ensembles of more than the three metal surface atoms bonded to propylidyne in the surface reactions. The results highlight the unique reactivity of small site-isolated metal clusters.     

Bifunctional pathways mediated by Pt clusters and Al2O3 in the catalytic combustion of dimethyl ether

Mixtures of Pt clusters dispersed on gamma-Al(2)O(3) and additional gamma-Al(2)O(3) led to much higher DME combustion turnover rates than on the individual components or on Pt clusters supported on non-acidic oxides.     

Synergistic effects in multicomponent electrocatalysts: the Pb-Ir-O system

The ionic interactions were studied in aqueous solutions of Na(3)IrCl(6) + Pb(NO(3))(2) in order to develop a facilitated electrosynthesis of iridium-based catalytic surfaces. Spectroscopic studies indicated that ion pair charge-transfer complexes [IrCl(6)(3-)]-Pb(II) (K = 6 x 10(3)) and [Ir(H(2)O)Cl(5)(2-)]-Pb(II) (K = 2 x 10(3)) were formed in fresh and aged solutions, respectively. Electrochemical studies showed that interactions between the Ir(H(2)O)Cl(5)(2-) and Pb(II) species lead to synergistic lowering of the overpotential that was necessary for nucleation and growth of mixed metal oxide PbIrOx on the surface of glassy carbon electrodes. The Ir:Pb stoichiometry of the PbIrOx surface films was the same (1:1) as that of the high-temperature phase of Pb-Ir-O pyrochlore. Compared to IrOx, the PbIrOx films displayed enhanced catalytic activity toward the electrooxidation of carbohydrates. This was ascribed to synergism that involved retention of carbohydrate molecules at the Pb(II) sites of a PbIrOx film and oxidation at the adjacent Ir(IV) sites. The synergistic electroplating utilizing interactions between the partially aquated transition metal complex and posttransition metal ion represents a new synthetic route to highly homogeneous and reactive films of mixed metal oxides.     

In situ characterization of aluminum-containing mineral-microorganism aqueous suspensions using scanning transmission X-ray microscopy

In situ characterization of colloidal particles under hydrous conditions is one of the key requirements for understanding their state of aggregation and impact on the transport of pollutants in aqueous environments. Scanning transmission X-ray microscopy (STXM) is one of the few techniques that can satisfy this need by providing element- and chemical-state-specific 2-D maps at a spatial resolution better than 50 nm using soft X-rays from synchrotron radiation wiggler or undulator sources tuned to the absorption edges of different elements. X-ray absorption near-edge structure (XANES) spectra can also be collected simultaneously at a similar spatial resolution and can provide phase identification in many cases. In this study, we report STXM images and XANES spectroscopy measurements at or above the Al K-edge (E = 1559.6 eV) of various Al-containing minerals and synthetic oxides [alpha-Al2O3 (corundum), gamma-Al2O3, gamma-AlOOH (boehmite), alpha-Al(OH)3 (bayerite), KAl2(AlSi3O10)(OH)2 (muscovite), (Al,Mg)8(Si4O10)4(OH)8.nH2O (montmorillonite), and Mg6Al2(OH)16CO3.4H2O (hydrotalcite)] and demonstrate the capability of this spectromicroscopic tool to identify different Al-containing mineral colloids in multiphase mixtures in aqueous solution. We also demonstrate that STXM imaging at or above the C K-edge (E = 284.2 eV) and Al K-edge can provide unique information on the interactions between bacteria and Al-containing nanoparticles in aqueous suspensions. STXM images of a mixture of Caulobacter crescentus and montmorillonite and corundum particles just above the C and Al K-edges show that the mineral particles and bacteria are closely associated in aggregates, which is likely due to the binding of bacteria to clay and corundum particles by extracellular polysaccharides.