Inner-sphere Adsorption Complexes Research Articles

Ion solvation as a predictor of lanthanide adsorption structures and energetics in alumina nanopores

Adsorption reactions at solid-water interfaces define elemental fate and transport and enable contaminant clean-up, water purification, and chemical separations. For nanoparticles and nanopores, nanoconfinement may lead to unexpected and hard-to-predict products and energetics of adsorption, compared to analogous unconfined surfaces. Here we use X-ray absorption fine structure spectroscopy and operando flow microcalorimetry to determine nanoconfinement effects on the energetics and local coordination environment of trivalent lanthanides adsorbed on Al2O3 surfaces. We show that the nanoconfinement effects on adsorption become more pronounced as the hydration free energy, ΔGhydr, of a lanthanide decreases. Neodymium (Nd3+) has the least exothermic ΔGhydr (−3336 kJ·mol−1) and forms mostly outer-sphere complexes on unconfined Al2O3 surfaces but shifts to inner-sphere complexes within the 4 nm Al2O3 pores. Lutetium (Lu3+) has the most exothermic ΔGhydr (−3589 kJ·mol−1) and forms inner-sphere adsorption complexes regardless of whether Al2O3 surfaces are nanoconfined. Importantly, the energetics of adsorption is exothermic in nanopores only, and becomes endothermic with increasing surface coverage. Changes to the energetics and products of adsorption in nanopores are ion-specific, even within chemically similar trivalent lanthanide series, and can be predicted by considering the hydration energies of adsorbing ions.

Sorption Mechanisms of Chemicals in Soils

Sorption of chemicals onto soil particle surfaces is an important process controlling their availability for uptake by organisms and loss from soils to ground and surface waters. The mechanisms of chemical sorption are inner- and outer-sphere adsorption and precipitation onto mineral surfaces. Factors that determine the sorption behavior are properties of soil mineral and organic matter surfaces and properties of the sorbing chemicals (including valence, electron configuration, and hydrophobicity). Because soils are complex heterogeneous mixtures, measuring sorption mechanisms is challenging; however, advancements analytical methods have made direct determination of sorption mechanisms possible. In this review, historical and modern research that supports the mechanistic understanding of sorption mechanisms in soils is discussed. Sorption mechanisms covered include cation exchange, outer-sphere adsorption, inner-sphere adsorption, surface precipitation, and ternary adsorption complexes.

Abiotic reduction of Cr(VI) by humic acids derived from peat and lignite: kinetics and removal mechanism

Hexavalent chromium contamination of groundwater is a worldwide problem caused by anthropogenic and natural processes. We report the rate of Cr(VI) removal by two humic acids (extracted from Miocene age lignite and younger peat soil) in aqueous suspensions across a pH range likely to be encountered in terrestrial environments. Cr(VI) was reduced to Cr(III) in a first-order reaction with respect Cr(VI) concentration, but exhibited a partial order (~ 0.5) with respect to [H+]. This reaction was more rapid with the peat humic acid, where Cr(VI) reduction was observed at all pH values investigated (3.7 ≤ pH ≤ 10.5). 13C NMR and pyrolysis GC-MS spectroscopy indicate that the reaction results in loss of substituted phenolic moieties and hydroxyl groups from the humic acids. X-ray absorption spectroscopy indicated that at all pH values the resulting Cr(III) was associated with the partially degraded humic acid in an inner-sphere adsorption complex. The reaction mechanism is likely to be controlled by ester formation between Cr(VI) and phenolic/hydroxyl moieties, as this initial step is rapid in acidic systems but far less favourable in alkaline conditions. Our findings highlight the potential of humic acid to reduce and remove Cr(VI) from solution in a range of environmental conditions.

Interaction of NaOH solutions with silica surfaces

HypothesisSodium adsorption on silica surfaces depends on the solution counter-ion. Here, we use NaOH solutions to investigate basic environments. SimulationsSodium adsorption on hydroxylated silica surfaces from NaOH solutions were investigated through molecular dynamics with a dissociative force field, allowing for the development of secondary molecular species. FindingsAcross the NaOH concentrations (0.01 M − 1.0 M), ∼50% of the Na+ ions were concentrated in the surface region, developing silica surface charges between − 0.01 C/m2 (0.01 M NaOH) and − 0.76 C/m2 (1.0 M NaOH) due to surface site deprotonation. Five inner-sphere adsorption complexes were identified, including monodentate, bidentate, and tridentate configurations and two additional structures, with Na+ ions coordinated by bridging oxygen and hydroxyl groups or water molecules. Coordination of Na+ ions by bridging oxygen atoms indicates partial or complete incorporation of Na+ ions into the silica surface. Residence time analysis identified that Na+ ions coordinated by bridging oxygen atoms stayed adsorbed onto the surface four times longer than the mono/bi/tridentate species, indicating formation of relatively stable and persistent Na+ ion adsorption structures. Such inner-sphere complexes form only at NaOH concentrations of > 0.5 M. Na+ adsorption and lifetimes have implications for the stability of silica surfaces.

Redox Transformations of As and Se at the Surfaces of Natural and Synthetic Ferric Nontronites: Role of Structural and Adsorbed Fe(II).

Adsorption and redox transformations on clay mineral surfaces are prevalent in surface environments. We examined the redox reactivity of iron Fe(II)/Fe(III) associated with natural and synthetic ferric nontronites. Specifically, we assessed how Fe(II) residing in the octahedral sheets, or Fe(II) adsorbed at the edge sites alters redox activity of nontronites. To probe the redox activity we used arsenic (As) and selenium (Se). Activation of both synthetic and natural ferric nontronites was observed following the introduction of Fe(II) into predominantly-Fe(III) octahedral sheets or through the adsorption of Fe(II) onto the mineral surface. The oxidation of As(III) to As(V) was observed via catalytic (oxic conditions) and, to a lesser degree, via direct (anoxic conditions) pathways. We provide experimental evidence for electron transfer from As(III) to Fe(III) at the natural and synthetic nontronite surfaces, and illustrate that only a fraction of structural Fe(III) is accessible for redox transformations. We show that As adsorbed onto natural and synthetic nontronites forms identical adsorption complexes, namely inner-sphere binuclear bidentate. We show that the formation of an inner-sphere adsorption complex may be a necessary step for the redox transformation via catalytic or direct oxidation pathways.

Copper(II) adsorption on the kaolinite(001) surface: Insights from first-principles calculations and molecular dynamics simulations

The adsorption behavior of Cu(II) on the basal hydroxylated kaolinite(001) surface in aqueous environment was investigated by first-principles calculations and molecular dynamics simulations. Structures of possible monodentate and bidentate inner-sphere adsorption complexes of Cu(II) were examined, and the charge transfer and bonding mechanism were analyzed. Combining the binding energy of complex, the radial distribution function of Cu(II) with oxygen and the extended X-ray absorption fine structure data, monodentate complex on site of surface oxygen with “upright” hydrogen and bidentate complex on site of two oxygens (one with “upright” hydrogen and one with “lying” hydrogen) of single Al center have been found to be the major adsorption species of Cu(II). Both adsorption species are four-coordinated with a square planar geometry. The distribution of surface hydroxyls with “lying” hydrogen around Cu(II) plays a key role in the structure and stability of adsorption complex. Upon the Mulliken population analysis and partial density of states, charge transfer occurs with Cu(II) accepting some electrons from both surface oxygens and aqua oxygens, and the bonding Cu 3d-O 2p state filling is primarily responsible for the strong covalent interaction of Cu(II) with surface oxygen.

Chemical transformation of zinc oxide nanoparticles as a result of interaction with hydroxyapatite

Recent studies have revealed that zinc phosphate is an important transformation product of zinc oxide nanoparticles (ZnO NPs) in the environment, and the role of soluble phosphate in the transformation of ZnO NPs to zinc phosphate has been confirmed. However, whether insoluble phosphate that exists widely in the environment can induce chemical transformation of ZnO NPs has not been addressed. Therefore, transformation of ZnO NPs in the presence of hydroxyapatite (HAP), selected as representative of insoluble phosphate, at different pH was investigated in the present study. Transformation products were identified by employing X-ray diffraction and synchrotron based X-ray absorption fine structure spectroscopy. The results indicate that under acidic condition (pH 5) phosphate ion (PO43−) and calcium anion (Ca2+) were released from HAP in aqueous solution, inducing a rapid transformation of about 80% ZnO NPs to scholzite within 4h. Under neutral or basic conditions (pH 7 and 9) the adsorption of Zn2+ on HAP resulted in a slow transformation of about 60% ZnO NPs to amorphous inner-sphere Zn adsorption complexes within 30 days. This work suggests the important role of HAP in the transformation of ZnO NPs and may affect the behavior, fate and toxic effects of ZnO NPs in the environment.

Structure of Sulfate Adsorption Complexes on Ferrihydrite

Sulfate adsorption on mineral surfaces is an important environmental chemical process. However, the structure of sulfate adsorption complexes has remained uncertain. In this study, we have determined the S−Fe distance of sulfate inner-sphere adsorption complexes on iron (Fe) oxyhydroxide (ferrihydrite) surfaces under air-dried conditions using sulfur K-edge extended X-ray absorption fine structure spectroscopy and differential X-ray pair distribution function analysis. Both approaches indicate that the S− Fe interatomic distance of the sulfate adsorption complexes is 3.22−3.25 A, suggesting that sulfate forms bidentate−binuclear adsorption complexes on ferrihydrite surfaces. Outer-sphere complexes are also observed using infrared spectroscopic analysis. This finding clarifies the long-standing debate over the structure of sulfate adsorption complexes and has important implications for understanding the chemistry of sulfate at environmental interfaces.

Uranyl adsorption on (001) surfaces of kaolinite: A molecular dynamics study

The aim of the present paper is to investigate the adsorption of uranyl species (UO2)2+(H2O)5 onto kaolinite (001) surfaces. To this end we have employed molecular dynamic simulations based on CLAYFF force field potential. Various types of surface model for inner-sphere adsorption complexes and one model for outer-sphere adsorption complexes were optimized. In order to have a neutral structure, the uranyl (UO2)2+(H2O)5 or the kaolinite was deprotonated to form the outer-sphere or inner-sphere adsorption complexes. Both singly protonated and partially deprotonated states of the Al(0) kaolinite surface were considered for adsorption in the model of inner-sphere complexes. The first uranyl coordination shell exhibits pentagonal bi-pyramidal symmetry with the pentagonal formed by 5 water molecules. We show that the average U–OW distances are between 2.49 and 2.57Å for water molecules. The bond of uranyl with deprotonated O− center is always short because of the charge attraction. The obtained results agree well with density functional calculations and EXAFS measurements, and show how and why the adsorption of uranyl appears on the surface of kaolinite.

Alteration of Sediments by Hyperalkaline K-Rich Cement Leachate: Implications for Strontium Adsorption and Incorporation

Results are presented from 1 year batch experiments where K-rich hyperalkaline pH 13.5 young cement water (YCW) was reacted with sediments to investigate the effect of high pH, mineral alteration, and secondary mineral precipitation on (90)Sr sorption. After reaction with YCW, Sr sorption was found to be greater than 75% in all samples up to 365 days and 98% in a sample reacted for 365 days at 70 °C. Scanning electron microscopy analysis of sediment samples reacted at room temperature showed surface alteration and precipitation of a secondary phase, likely a K-rich aluminosilicate gel. The presence of Sr-Si(Al) bond distances in Sr K-edge extended X-ray absorption fine structure (EXAFS) analysis suggested that the Sr was present as an inner-sphere adsorption complex. However, sequential extractions found the majority of this Sr was still exchangeable with Mg(2+) at pH 7. For the sample reacted for 1 year at 70 °C, EXAFS analysis revealed clear evidence for ∼6 Sr-Si(Al) backscatters at 3.45 Å, consistent with Sr incorporation into the neoformed K-chabazite phase that was detected by X-ray diffraction and electron microscopy. Once incorporated into chabazite, (90)Sr was not exchangeable with Mg(2+), and chemical leaching with pH 1.5 HNO3 was required to remobilize 60% of the (90)Sr. These results indicate that, in high pH cementitious leachate, there is significantly enhanced Sr retention in sediments due to changes in the adsorption mechanism and incorporation into secondary silicate minerals. This suggests that Sr retention may be enhanced in this high pH zone and that the incorporation process may lead to irreversible exchange of the contaminant over extended time periods.

Sulfate adsorption at the buried hematite/solution interface investigated using total internal reflection (TIR)-Raman spectroscopy

Sulfate adsorption at buried mineral/solution interfaces is of great interest in geochemistry and atmospheric aerosol chemistry due to the sulfate anion’s environmental ubiquity and the wide role of physical and chemical phenomena that it impacts. Here we present the first application of total internal reflection-Raman (TIR-Raman) spectroscopy, a surface sensitive spectroscopy, to probe sulfate ion behavior at the buried hematite/solution interface. Hematite is the most thermodynamically stable iron oxide polymorph and as such is widely found in nature. Our results demonstrate the feasibility of a TIR-Raman approach to study simple, inorganic anion adsorption at buried interfaces. Moreover, our data suggest that inner-sphere sulfate adsorption proceeds in a bidentate fashion at the hematite surface. These results help clarify long-standing questions as to whether sulfate forms inner-sphere adsorption complexes at hematite surfaces in a mono- or bidentate fashion based on attenuated total reflection-infrared (ATR-IR) observations. Our results are discussed with perspective to this debate and the applicability of TIR-Raman spectroscopy to address ambiguities of ion adsorption to mineral surfaces.

Theoretical investigation of uranyl ion adsorption on hydroxylated γ-Al2O3 surfaces

We herein report density functional investigations of uranyl adsorption on γ-Al2O3 surfaces. By applying periodic slab models, we report the possible configurations of uranyl adsorbed on the (100) and (110) surfaces of γ-Al2O3 by calculations using density functional theory. The above mentioned two surfaces are expected to be highly reactive and adsorb metal ions preferentially. We explored bidentate inner-sphere adsorption complexes of uranyl at short-bridge sites, AlOO, and long-bridge sites, AlOAlO(H). The results indicate several favorable adsorption configurations which fit the available TRLFS and EXAFS data well. Comparison of key calculated structure parameters with available experimental data suggests an extension of the prevailing interpretation and implies that a set of uranyl complexes may coexist on the edge surfaces of γ-Al2O3.

First-principles simulation of arsenate adsorption on the (1 [formula omitted] 2) surface of hematite

Recent experimental studies revealed an unprecedented bimodal distribution of arsenate at the hematite (11¯2) surface with a simultaneous adsorption of inner-sphere and outer-sphere complexes. In the present study, first-principles calculations based on density-functional theory were performed to provide detailed insights into the structural and electronic properties of such inner-sphere and outer-sphere adsorption complexes on two hydroxylated terminations of the hematite (11¯2) surface. For bidentate corner-sharing complexes, the predicted most stable adsorption configurations display interatomic distances in good agreement with EXAFS-derived data (i.e. As–Fe distances of ∼3.3Å). Our calculations also suggest that edge-sharing bidentate complexes can form on ideal (11¯2) hematite surfaces and do not necessarily involve step edges. These edge-sharing complexes would display two As–Fe distances at about 2.85 and 3.45Å, instead of the unique short As–Fe contribution that is usually considered for interpreting EXAFS data. For outer-sphere complexes, the predicted most favorable adsorption configurations indicate that strong hydrogen bonds as well as electrostatic forces stabilize arsenate at the hematite surface. Although not investigated here, the interfacial water structure may also contribute to stabilize further OS arsenate complexes.

Aggregation of Alkyltrimethylammonium Ions at the Cleaved Muscovite Mica–Water Interface: A Monte Carlo Study

The precise molecular structure of organically modified mineral surfaces is still not well understood. To establish a relation between experimental observations and underlying molecular structure, we performed Monte Carlo simulations of the aggregation behavior of alkyltrimethylammonium surfactants (C(n)TMA(+)) at the interface between C(n)TMACl solution and cleaved K(+)-muscovite. The structures were examined with regard to the influence of varying alkyl chain length n (n = 8, 12, 16) and surface coverage of C(n)TMA(+) ions. The simulation results indicate that the water film structure at the muscovite surface is considerably influenced by the adsorption of C(n)TMA(+). A fraction of the C(n)TMA(+) ions forms inner-sphere and outer-sphere adsorption complexes with nitrogen-surface distances of 3.3-3.8 and 5.5-8.4 Å, respectively. The simulated monolayer aggregates exhibit thicknesses of 31-35, 22-27, and ∼18 Å for C(16)TMA(+), C(12)TMA(+), and C(8)TMA(+), respectively. C(16)TMA(+) and C(12)TMA(+) ions form bilayer aggregates, which show a strong interdigitation of the two opposing layers composing them. The aggregate thicknesses equal 35-39 and 30-35 Å, respectively, and are in agreement with available experimental data. In contrast, the short-chained C(8)TMA(+) ions do not form bilayer aggregates. In agreement with previous experimental studies, the alkyl chains of the aggregated ions show high conformational order markedly decreasing with decreasing chain length. We suggest that the simulated structures represent C(n)TMA(+) aggregates, which are formed on muscovite during the experimentally observed initial equilibration phase characterized by the presence of inorganic ions within the aggregates.

Phosphate uptake by TiO2: Batch studies and NMR spectroscopic evidence for multisite adsorption

Systematic studies, combining batch experiments with NMR spectroscopic methods, are carried out for phosphate sorption on titanium dioxide (TiO2). It is found that phosphate sorption on TiO2 decreases with increasing pH, whereas the phosphate uptake by TiO2 increases with increasing ionic strength of the solution. In I⩽0.1M, the sorption sharply increases and reaches a near maximum and then followed by little changes showing Langmuir-type behavior, whereas in I=0.7M, non-Langmuirian uptake becomes evident as equilibrium phosphate concentrations increase in solution. The sorption of phosphate on TiO2 is rapid and mostly irreversible at pH 4.5 and 7.0. At pH 9.0, however, the phosphate sorption is initially reversible and followed by resorption of phosphate on TiO2 at the system re-equilibration. 31P{1H} cross-polarization and magic angle spinning (CP/MAS) NMR spectra contain at least four main peaks which appear similar in position and width under all adsorption conditions, but vary in intensity with surface loading. The spectral characteristics of these peaks, including cross-polarization dynamics and chemical shift anisotropy obtained from spinning sideband analysis, suggest that they arise from distinct inner-sphere adsorption complexes, most of which are protonated. These results indicate that uptake of phosphate by TiO2 occurs by formation of several types of surface complexes.

X-ray absorption spectroscopic evidence for the formation of Pb(II) inner-sphere adsorption complexes and precipitates at the alkaline soil-water interface

Adsorption mechanisms of Pb on soil with high CaCO3 content were investigated by combined batch sorption and X-ray absorption fine structure (XAFS). Date from the batch equilibrium studies showed that Pb sorption was nonlinear and was well fitted to Langmiur isotherm. The XAFS data indicated that Pb could be adsorbed via the inner-sphere complex, the precipitation of calcium carbonate containing Pb (PbCaCO3), and outer-sphere Pb sorption complex. The formations of inner-sphere complexes and PbCaCO3 implied strong metal interactions with the surfaces the mechanistic reason for the affinity of Pb for CaCO3 as observed in macroscopic studies. At low metal concentration, 500 mg x L(-1) of initial Pb, radial distance of the first-shell Pb-O (R1) was 0.169 2 nm, however, at 1 000 mg x L(-1) of initial Pb, the R1 was 0.166 8 nm. These revealed that the percentage of inner-sphere complexes increased when the initial Pb was increased from 500 to 1 000 mg x L(-1).

Uranyl adsorption at (0 1 0) edge surfaces of kaolinite: A density functional study

We report density functional investigations of kaolinite edge surfaces and uranyl adsorption thereon. Applying periodic slab models, we studied the (0 1 0) surface of kaolinite as an example of kaolinite edge facets which are expected to be highly reactive and to adsorb preferentially metal ions. Among the four terminations of the (0 1 0) surface, we selected the two most likely ones and determined their structures to be affected by solvation. On these modified surfaces, we explored bidentate inner-sphere adsorption complexes of uranyl, at single metal center sites, Al(O,OH), and sites of mixed type, AlOH–SiO. On one of the terminations hydrolysis of uranyl was found to occur. Comparison of key calculated structure parameters with available experimental data suggests an extension of the prevailing interpretation and implies that a set of uranyl complexes may coexist on edge surfaces.

Cetylpyridinium Chloride at the Mica−Water Interface: Incomplete Monolayer and Bilayer Structures

Monte Carlo simulations of the interface between the cleaved surface of muscovite mica and aqueous cetylpyridinium chloride (CPCl) solution at ambient conditions are reported. Simulation results reveal that monolayer or bilayer aggregates of CP(+) ions at the muscovite-water interface remain incomplete up to a CP(+) coverage compensating the negative charge of muscovite. It is predicted that at this CP(+) coverage only a partial desorption of K(+) ions occurs and the two aggregates can be distinguished with help of the X-ray reflectivity technique. Formation of inner-sphere and outer-sphere adsorption complexes of CP(+) ions at distances of approximately 3 A and approximately 5 A, respectively, from the surface is observed. Despite an increasing adsorption of CP(+) ions, the structure of the adsorbed water film is largely preserved within approximately 5 A from the surface. A strong decrease of water density beyond this distance and formation of "adsorbed K(+)"-Cl(-) ion pairs result in coadsorption of Cl(-) in an amount equivalent to 1/4 of the negative charge of muscovite as close as approximately 4.3-4.8 A to the surface for the incomplete bilayer aggregate. For the incomplete monolayer aggregate, no segregation between K(+) and CP(+) ions and a displacement of K(+) ions into the adsorption sites approximately 1.6 A from the surface are observed.

EXAFS analysis of cadmium(II) adsorption to kaolinite

The local molecular structure of Cd sorbed to kaolinite was examined with X-ray absorption fine structure spectroscopy (XAFS) over a range of pH and initial Cd solution concentrations, ([Cd aq] in). Quantitative analysis of kaolinite samples with [Cd aq] in of 100 μM and pH 7, shows a single hydration sphere around Cd, consistent with an outer-sphere adsorption complex. At pH 9, a Cd–M coordination (where M stands for Si or Al) at second shell distance indicates the formation of inner-sphere adsorption complex(es) on edge sites. Although adsorption to Al edge sites is probably preferred, the similarity in atomic numbers between Al and Si and the small difference in bond lengths prevents us from distinguishing complexation to Si- versus Al-edge sites based on the XAFS data. The possibility of precipitation of Cd solid phases was ruled out due to similar local environment around Cd in samples prepared under air (where carbonate species would be present) and N 2 atmospheres, and the absence of a Cd–Cd coordination characterizing a Cd-bearing solid phase. Finally, analysis of kaolinite samples with [Cd aq] in of 3 μM, 10 μM, and 100 μM at pH 9 shows a consistent change in Cd–M distance and Debye–Waller factor as a function of [Cd aq] in, suggesting a change in the type of inner-sphere complex being formed as a function of loading. Whether this means that there is a change in the relative importance of Al versus Si edge sites with loading, or whether adsorption of Cd to edge sites alters binding to other nearby sites, or some other factor, has not been determined at this time.

The long-term fate of Cu 2+, Zn 2+, and Pb 2+ adsorption complexes at the calcite surface: An X-ray absorption spectroscopy study

In this study, the speciation of Zn 2+, Pb 2+, and Cu 2+ ions sorbed at the calcite surface was monitored during a 2.5-year reaction period, using extended X-ray absorption spectroscopy to characterize metal speciation on the molecular scale. Experiments were performed using pre-equilibrated calcite-water suspensions of pH 8.3, at metal concentrations below the solubility of metal hydroxide and carbonate precipitates, and at constant metal surface loadings. The EXAFS results indicate that all three metals remained coordinated at the calcite surface as inner-sphere adsorption complexes during the 2.5-year ageing period, with no evidence to suggest slow formation of dilute metal-calcite solid solutions under the reaction conditions employed. All three divalent metals were found to form non-octahedral complexes upon coordination to the calcite surface, with Zn 2+ adsorbing as a tetrahedral complex, Cu 2+ as a Jahn–Teller distorted octahedral complex, and Pb 2+ coordinating as a trigonal- or square-pyramidal surface complex. The non-octahedral configurations of these surface complexes may have hindered metal transfer from the calcite surface into the bulk, where Ca 2+ is in octahedral coordination with respect to first-shell O. The use of pre-equilibrated calcite suspensions, with no net calcite dissolution or precipitation, likely prevented metal incorporation into the lattice as a result of surface recrystallization. The results from this study imply that ageing alone does not increase the stability of Zn 2+, Pb 2+, and Cu 2+ partitioning to calcite if equilibrium with the solution is maintained during reaction; under these conditions, these metals are likely to remain available for exchange even after extended sorption times.