Monodentate Mononuclear Complexes Research Articles

Adsorption Mechanisms of Glyphosate on Ferrihydrite: Effects of Al Substitution and Aggregation State.

Ferrihydrite is one of the most reactive iron (Fe) (oxyhydr)oxides in soils, but the adsorption mechanisms of glyphosate, the most widely used herbicide, on ferrihydrite remain unknown. Here, we determined the adsorption mechanisms of glyphosate on pristine and Al-substituted ferrihydrites with aggregated and dispersed states using macroscopic adsorption experiments, zeta potential, phosphorus K-edge X-ray absorption near-edge structure spectroscopy, in situ attenuated total reflectance Fourier transform infrared spectroscopy coupled with two-dimensional correlation spectroscopy, and multivariate curve resolution analyses. Aggregation of ferrihydrite decreases the glyphosate adsorption capacity. The partial substitution of Al in ferrihydrite inhibits glyphosate adsorption on aggregated ferrihydrite due to the decrease of external specific surface area, while it promotes glyphosate adsorption on dispersed ferrihydrite, which is ascribed to the increase of surface positive charge. Glyphosate predominately forms protonated and deprotonated, depending on the sorption pH, monodentate-mononuclear complexes (MMH1/MMH0, 77-90%) on ferrihydrites, besides minor deprotonated bidentate-binuclear complexes (BBH0, 23-10%). Both Al incorporation and a low pH favor the formation of the BB complex. The adsorbed glyphosate preferentially forms the MM complex on ferrihydrite and preferentially bonds with the Al-OH sites on Al-substituted ferrihydrite. These new insights are expected to be useful in predicting the environmental fate of glyphosate in ferrihydrite-rich environments.

Adsorption behavior and surface complexation modeling of oxygen anion Sb(V) adsorption on goethite

Simulating the adsorption behavior of Sb(V) on goethite is of great significance for predicting the mobility of Sb(V) in soil. However, there is still a lack of charge distribution and multisite surface complexation (CD-MUSIC) models that conform to the actual adsorption mechanism. Therefore, our research combined the previous EXAFS results with the SCMs and established two CD-MUSIC models (Model I, which introduced bidentate binuclear and bidentate mononuclear complexes, and Model II, which introduced bidentate mononuclear and monodentate mononuclear complexes). It is also one of the rare cases that bidentate edge- and corner-sharing adsorption complexes have been distinguished by introducing different amounts of H+. The results showed that Model I was more suitable for predicting the adsorption behavior of Sb(V) on the goethite surface than Model II. However, at low pH and high Sb(V) loading, the presence of the monodentate complex ≡FeOHSb(OH)50.5- was possible due to the fast and slow two-step mechanism. Using the optimized CD-MUSIC model parameters, the contribution of surface species to the equilibrium adsorption under different conditions was determined. In the adsorption edge and adsorption isotherm simulation process, the edge-sharing bidentate mononuclear complex ≡(FeO)2H2Sb(OH)4 was always the most important product, which was consistent with the results of extended X-ray absorption fine structure (EXAFS) analysis. The applicability of the model parameters was verified by predicting the competitive adsorption between PO43− and Sb(V). The CD-MUSIC model established in the present study could be a useful tool for evaluating the equilibrium distribution behavior and environmental risk of Sb(V) in different types of soils.

Enhancement of phosphate adsorption during mineral transformation of natural siderite induced by humic acid: Mechanism and application

Abstract The interfacial interactions between siderite, humic acid (HA), and phosphate, is critical to understand the transport of HA and phosphate in the environment. In this study, the influence of HA on the adsorption of phosphate by siderite not only includes competitive adsorption, but also includes indirectly facilitated adsorption. It was attributed to that humic acid as a representative natural organic matter possesses abundant carboxylic groups, which facilitate chelation between minerals and HA, and then the phosphate adsorption performance is affected. The experimental results concluded that three stages of the transformation from siderite to goethite under the action of HA can be divided. Firstly, chelation can be formed between HA and the ferrous ions from siderite, and it is favorable for the oxidation of Fe(II) by the O2 in water to form amorphous ferrihydrite; and then the ferrihydrite nano-particles were attached to the surface of goethite crystals; finally, the goethite crystal structure was formed by the structural rearrangement of ferrihydrite nano-particles. Meanwhile, the surface complex species of phosphate was formed on precipitate. It is revealed that the diprotonated bidentate binuclear complex is the main phosphate species when the HA concentration was 0.5 mg/L; as the HA concentration rose to 10 mg/L, the main phosphate species change to diprotonated monodentate mononuclear complex, which is attributed to the competitive adsorption of humic acid. We believe that these results are beneficial to guide the impact of HA for the biogeochemical cycling of phosphate, iron-bearing mineral transformation, as well as environmental remediation.

Impact of low-molecular weight organic acids on selenite immobilization by goethite: Understanding a competitive-synergistic coupling effect and speciation transformation

The interactions between low-molecular weight organic acids (LMWOAs) and selenium (Se) on mineral/water interfaces affect the release, immobilization and bioavailability of Se in nature. Herein, the effects of three environmentally relevant LMWOAs (i.e., oxalic (Oxa), succinic (Suc) and citric (Cit) acids) on Se(IV) adsorption to goethite under oxic conditions were investigated using batch experiments, speciation fractionation, and ATR-FTIR and XPS analyses. The LMWOAs exhibited a competitive-synergistic coupling effect on Se(IV) adsorption to goethite, which inhibited the adsorption rate of Se(IV) by 14.1, 13.3 and 8.0 times. However, immobilization of Se(IV) was simultaneously enhanced by 39.1%, 34.6% and 14.1% in the following order Oxa > Suc > Cit. The results obtained by fractionation of the adsorbed Se(IV) revealed that the enhancement was due to surface binding as well as speciation transformation from ligand-exchangeable Se(IV) into residual fractions, which increased by approximately 18% in the presence of the LMWOAs. The dissolution of goethite significantly improved due to the LMWOAs and decreased to different degrees as the concentration of Se(IV) increased. The monodentate mononuclear complexes (58.2%) and Lewis base sites bonded Se (41.8%) were the predominant surface species of Se(IV) in goethite-Se(IV) system. The ATR-FTIR and high-resolution XPS analyses demonstrated that the formation of ≡FeO(SeO)O-CO surface complexes (22.8–27.0%) occurred in the presence of LMWOAs, which could be closely correlated with the interface-mediated reduction of Se(IV). In addition, the predominant mechanism for the formation of residual Se is LMWOA specific, in which ferric selenite-like precipitation was dominant for Suc (10.6%) and Cit (11.6%) and reduction was dominant for Oxa (17.5%). Overall, LMWOAs play an important role in Se(IV) immobilization and speciation transformation and may facilitate understanding the Se bioavailability in rhizosphere soils under oxic conditions.

Identification of Fe and Zr oxide phases in an iron-zirconium binary oxide and arsenate complexes adsorbed onto their surfaces

The Fe-Zr binary oxide adsorbents have higher arsenic adsorptive capacities than either iron oxide or zirconium oxide alone, indicating a strong synergistic effect exists between Fe and Zr oxides. However, no generally accepted in-depth explanations have been reached on the origin of this better performance. In the present study, the component phases, the active surface sites, the structure of the adsorbed As(V) surface species, and the mechanism of the synergistic effect, were investigated and elucidated using multiple advanced experimental techniques combined with quantum chemical calculations. Goethite and lepidocrocite were identified as the main Fe oxide components while amorphous zirconium hydroxide was the main Zr oxide component, respectively. A monodentate-mononuclear complex and a bidentate-binuclear complex were revealed to be dominant on the surface, respectively, when at lower and higher initial As(V) concentrations. Density functional theory calculations indicated that As(V) preferred to bind with Zr-OH rather than Fe-OH. This was verified with the As K-edge EXAFS results and XPS observations. The synergistic effect was due to a short-range ordering state, the enlarged contents of amorphous and poorly-crystalline fractions, and increased hydroxyl surface site density. These results lead to the realization that the above properties are preferred in future adsorbent preparations.

The oxidation and sorption mechanism of Sb on δ-MnO2

Manganese (Mn) oxide is considered as a potential regulator of the fate of many contaminants in soil due to its abundance and reactivity. The present study investigated the oxidation and sorption mechanisms of antimony (Sb) on δ-MnO2, by combining batch equilibrium experiments, kinetic experiments and various spectroscopic techniques. The collective results confirmed the high Sb(III)-oxidizing capacity of Mn(IV) sites on δ-MnO2 and the oxidation reaction was proton-driven. However, the oxidation process was gradually inhibited by the accumulation of manganese antimonate precipitates, the production of less reactive Mn(III), and the site-blocking effects of products Mn(II) and Sb(V) on the surface. Different from the direct sorption of Sb(V) by δ-MnO2, the oxidation of Sb(III) destroyed the surface structure of δ-MnO2 and exposed more reactive sites as evidenced by X-ray powder diffraction (XRD) and transmission electron microscopy (TEM). The stronger Sb(V) sorption along with the oxidation of Sb(III), therefore, could be attributed to several binding modes, i.e., Mn(II) cation bridge, precipitation with Mn(II), along with inner-sphere complexations with surface Mn(IV) and Mn(III) sites. Sb(V) was sorbed at the edge sites of δ-MnO2 forming monodentate mononuclear complex with a configuration of Mn-O(H)-Sb(V) as confirmed by supplementary X-ray photoelectron spectroscopy (XPS), X-ray absorption fine structure (EXAFS) spectroscopy and attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy techniques. The findings altogether help to contribute to a better understanding on the geochemical dynamics of Sb with δ-MnO2 in subsurface environment.

Structure and reactivity of oxalate surface complexes on lepidocrocite derived from infrared spectroscopy, DFT-calculations, adsorption, dissolution and photochemical experiments

Oxalate, together with other ligands, plays an important role in the dissolution of iron(hdyr)oxides and the bio-availability of iron. The formation and properties of oxalate surface complexes on lepidocrocite were studied with a combination of infrared spectroscopy (IR), density functional theory (DFT) calculations, dissolution, and photochemical experiments. IR spectra measured as a function of time, concentration, and pH (50–200 µM oxalate, pH 3–7) showed that several surface complexes are formed at different rates and in different proportions. Measured spectra could be separated into three contributions described by Gaussian line shapes, with frequencies that agreed well with the theoretical frequencies of three different surface complexes: an outer-sphere complex (OS), an inner-sphere monodentate mononuclear complex (MM), and a bidentate mononuclear complex (BM) involving one O atom from each carboxylate group. At pH 6, OS was formed at the highest rate. The contribution of BM increased with decreasing pH. In dissolution experiments, lepidocrocite was dissolved at rates proportional to the surface concentration of BM, rather than to the total adsorbed concentration. Under UV-light (365 nm), BM was photolyzed at a higher rate than MM and OS. Although the comparison of measured spectra with calculated frequencies cannot exclude additional possible structures, the combined results allowed the assignment of three main structures with different reactivities consistent with experiments. The results illustrate the importance of the surface speciation of adsorbed ligands in dissolution and photochemical reactions.

Removal of phosphate from aqueous solution by activated siderite ore: Preparation, performance and mechanism

Abstract Activated siderite ore (ASO) was prepared by annealing siderite ore at different temperatures for different durations. The performance of ASO on the removal of phosphate from aqueous solution and the effects of particle size, contact time, pH, initial phosphate concentration, temperature, and coexisting anions on phosphate removal were investigated. X-ray diffractometer, thermogravimetric analyzer (TGA), transmission electron microscopy (TEM), surface area analyzer, Fourier transformed infrared spectra and other techniques were utilized to characterize ASO before and after adsorption. The results showed that a large amount of CO2 was released meanwhile the siderite began to transform to hematite when the annealing temperature increased to 450 °C. The specific surface area of annealed product increased from 4.17 m2/g to 57.5 m2/g as the temperature came to 470 °C. Phosphate can be removed efficiently by ASO in a relatively wide pH range between 3.0 and 11.0. The effects of coexisting anions were not significant within the experimental concentration ranges except HCO3−. The adsorption process of phosphate on ASO fitted pseudo-second order kinetics model. ASO showed a high-efficient adsorption capacity of 9.24 mg/g estimated from Langmuir isotherm at 30 °C. Results of TEM and FTIR analysis suggested that under the condition of pH = 6.5 and phosphate concentration (100 mg/L), two different monoprotonated phosphate complexes exist at ASO, with one surface complex coordinated in a monodentate binuclear (bridging) fashion, and the other as a monodentate mononuclear complex. A small amount of co-precipitation is most likely to be another mechanism for the removal of phosphate. These fundamental data indicate that siderite is a promising precursor for preparing porous material at least for adsorption of phosphate.

Stimulation of Fe(II) Oxidation, Biogenic Lepidocrocite Formation, and Arsenic Immobilization by Pseudogulbenkiania Sp. Strain 2002.

An anaerobic nitrate-reducing Fe(II)-oxidizing bacterium, Pseudogulbenkiania sp. strain 2002, was used to investigate As immobilization by biogenic Fe oxyhydroxides under different initial molar ratios of Fe/As in solutions. Results showed that Fe(II) was effectively oxidized, mainly forming lepidocrocite, which immobilized more As(III) than As(V) without changing the redox state of As. When the initial Fe/As ratios were kept constant, higher initial Fe(II) concentrations immobilized more As with higher Asimmobilized/Feprecipitated in biogenic lepidocrocite. EXAFS analysis showed that variations of initial Fe(II) concentrations did not change the As-Fe complexes (bidentate binuclear complexes ((2)C)) with a fixed As(III) or As(V) initial concentration of 13.3 μM. On the other hand, variations in initial As concentrations but fixed Fe(II) initial concentration induced the co-occurrence of bidentate binuclear and bidentate mononuclear complexes ((2)E) and bidentate binuclear and monodentate mononuclear complexes ((1)V) for As(III) and As(V)-treated series, respectively. The coexistence of (2)C and (2)E complexes (or (2)C and (1)V complexes) could contribute to higher As removal in experimental series with higher initial Fe(II) concentrations at the same initial Fe/As ratio. Simultaneous removal of soluble As and nitrate by anaerobic nitrate-reducing Fe(II)-oxidizing bacteria provides a feasible approach for in situ remediation of As-nitrate cocontaminated groundwater.

Absorption mechanisms of Cu(2+) on a biogenic bixbyite-like Mn2O3 produced by Bacillus CUA isolated from soil.

BackgroundAlthough most reported biogenic Mn oxides are hexagonal birnessites, other types of biogenic Mn oxides also commonly occur in the environment. However, sorption characteristics and underlying mechanisms of the adsorption of heavy-metal ions on these biogenic Mn oxides are still rarely addressed. In this study, the sorption mechanisms of Cu(II) on a low valence biogenic Mn oxide, poorly crystallized bixbyite-like Mn2O3 (α-Mn2O3), were investigated.ResultsThe maximum adsorption capacity of Cu(II) onto this biogenic Mn oxide at pH 6.00 was 796 mmol/kg (0.45 mol Cu mol−1 Mn). The complex structure of adsorbed Cu(II) was constrained using Cu extended X-ray absorption fine structure (EXAFS) analysis, combined with structural parameters of the biogenic Mn oxide with alternately arranged regular and distorted MnO6 octahedra obtained through multiple-FEFF fitting of Mn EXAFS data. The sorbed Cu(II) was found to coordinate with the biogenic Mn oxide particle edges as inner-sphere complexes. At a relatively low Cu2+ loading (233 mmol/kg, pH 6.00), Cu(II) adsorbed onto the biogenic Mn oxide with two types of coordinated complexes, i.e., (1) coordinated with one regular/distorted MnO6 octahedron as a monodentate-mononuclear complex and (2) with two adjacent MnO6 octahedra as a bidentate-binuclear complex. While, at a relatively high Cu2+ loading (787 mmol/kg, pH 6.00), only one type of coordinated complex was constrained, the adsorbed Cu(II) coordinated with one regular/distorted MnO6 octahedron as a monodentate-mononuclear complex.ConclusionsThis research extends further insight into the bacterial Mn(II) oxidation in the environment and serves as a good reference for understanding the interactions between metal ions and biogenic low valence Mn oxides, which are still poorly explored either theoretically or practically.Electronic supplementary materialThe online version of this article (doi:10.1186/s12932-015-0020-6) contains supplementary material, which is available to authorized users.

Zn sorption to biogenic bixbyite-like Mn2O3 produced by Bacillus CUA isolated from soil: XAFS study with constraints on sorption mechanism

Although most reported biogenic Mn oxides are hexagonal birnessites, other types of biogenic Mn oxides also commonly occur in the environment. Compared to hexagonal birnessites, the sorption characteristics and the underlying mechanism of adsorption of heavy-metal ions to those of the other biogenic Mn oxides are still rarely addressed. A strain of Mn-oxidizing bacteria isolated from Claypani-Udic Argosols was identified as Bacillus with 16S rRNA sequencing analysis. The bacterial Mn(II) oxidation product is a poorly crystallized bixbyite-like Mn2O3 (α-Mn2O3). The maximum adsorption capacities of Zn(II) onto the biogenic Mn oxide at pH6.00 and pH4.00 were 663mmol/kg and 629mmol/kg, respectively. The complex structure of adsorbed Zn2+ was constrained using Zn EXAFS analysis, combined with structural parameters of the biogenic Mn oxide with alternately arranged regular and distorted MnO6 octahedra obtained through multiple-FEFF fitting of Mn EXAFS data. At a relatively low Zn2+ loading (100mmol/kg, pH6.00), Zn2+ adsorbed onto the biogenic Mn oxide with two types of tetrahedrally coordinated complexes, i.e. (1) coordinated with one regular/distorted MnO6 octahedron as a monodentate–mononuclear complex and (2) with two MnO6 octahedra (two regular, two distorted or a regular and a distorted) as a bidentate–binuclear complex. While, at a relatively high Zn2+ loading (556mmol/kg, pH4.00; 635mmol/kg, pH6.00), two types of octahedrally coordinated complexes are constrained, i.e. (1) coordinated with one regular/distorted MnO6 octahedron as a monodentate–mononuclear complex and (2) with one regular MnO6 octahedron as a bidentate mononuclear complex. This research extends further understanding on the formation of biogenic Mn oxides in the environment and the adsorption mechanism of heavy metals onto low-valence Mn oxides with distorted structures. The application of low valence biogenic Mn oxides to efficiently remove heavy metals from water is also shown to be promising.

Surface complexation of fluoride at the activated nano-gibbsite water interface

The drawbacks of existing fluoride removal processes are due to low efficiencies and less mechanistic understanding of the process. The acid–base protolysis and surface complexation constants for fluoride and thermally activated nanogibbsite were investigated in this study for the first time. Fluoride adsorption on heat activated gibbsite nanoparticles (HGNP) was determined as a function of pH, concentration of background electrolyte and adsorbate loading. XRD evidenced the HGNP is in a transient state of the formation of χ-alumina as a poorly crystalline solid. An immense increase in surface area was obtained from 50 to 399m2/g by the heat activation. Macroscopic data obtained from the batch experiments suggested an enhanced fluoride removal by HGNP with an adsorption maximum around pH 6. The adsorption quantified by the diffuse double layer surface complexation model indicated an inner sphere complexation mechanism at 0.526mM of F̄ with monodentate mononuclear complexes. An increase of fluoride concentration to 1.351mM demonstrated a multilayer complex formation which was also confirmed by the isotherm data modeling with two distinct sorption maxima. XPS evidenced similar results showing two peaks at 684.7 and 688.5eV for weak and strong bonded fluoride on HNGP surface. Fluoride adsorption maximum was reported as 4×10−6mol/m2 based on Langmuir data fitting.

DFT+U Study of Arsenate Adsorption on FeOOH Surfaces: Evidence for Competing Binding Mechanisms

On the basis of periodic density functional theory (DFT) calculations including an on-site Coulomb repulsion term U, we study the adsorption mechanism of arsenate on the goethite (101), akaganeite (100), and lepidocrocite (010) surfaces. Mono- and bidentate binding configurations of arsenate complexes are considered at two distinct iron surface sites—directly at 5-fold coordinated Fe1 and/or 4-fold coordinated Fe2 as well as involving ligand exchange. The results obtained within ab initio thermodynamics shed light on the ongoing controversy on the arsenate adsorption configuration, and we identify monodentate adsorbed arsenate complexes as stable configurations at ambient conditions with a strong preference for protonated arsenate complexes: a monodentate mononuclear complex at Fe1 (dFe1–As = 3.45 Å) at goethite (101) and a monodentate binuclear complex at Fe2 (dFe2–As = 3.29 Å) at akaganeite (100). Repulsive interactions between the complexes limit the loading capacity and promote configurations with maximized distances between the adsorbates. With decreasing oxygen pressures, a mixed adsorption of bidentate binuclear complexes at Fe1 (dFe1–As = 3.26–3.34 Å) and monodentate binuclear arsenate at Fe2 (dFe2–As = 3.31–3.50 Å) and, finally, rows of protonated bidentate complexes at Fe1 with dFe1–As = 3.55–3.59 Å are favored at α-FeOOH(101) and β-FeOOH(100). At lepidocrocite (010) with only Fe2 sites exposed, the surface phase diagram is dominated by alternating protonated monodentate binuclear complexes (dFe2–As = 3.38 Å) and hydroxyl groups. At low oxygen pressures, alternating rows of protonated bidentate mononuclear complexes (dFe2–As = 3.10 Å) and water are present. Hydrogen bond formation to surface hydroxyl groups and water plays a crucial role in the stabilization of these adsorbate configurations and together with steric effects of the oxygen lone pairs leads to tilting of the arsenate complex that significantly reduces the Fe–As distance. Our results show that the Fe–As bond length is mainly determined by the protonation state, arsenate coverage, steric effects, and hydrogen bonding to surface functional groups and to a lesser extent by the adsorption mode. This demonstrates that the Fe–As distance cannot be used as a unique criterion to discriminate between adsorption modes.

A real time in situ ATR-FTIR spectroscopic study of glyphosate desorption from goethite as induced by phosphate adsorption: Effect of surface coverage

The desorption of glyphosate from goethite as induced by the adsorption of phosphate was investigated by attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy in combination with adsorption isotherms. Desorption of glyphosate was very low in the absence of phosphate. Addition of phosphate promoted glyphosate desorption. At low initial surface coverages, added phosphate adsorbed on free surface sites, mainly, displacing a small amount of glyphosate. At high initial surface coverages, on the contrary, phosphate adsorption resulted in a significant glyphosate desorption. In the latter conditions, the ratio desorbed glyphosate to adsorbed phosphate was 0.60. The desorption process can be explained by assuming that phosphate adsorbs first forming a monodentate mononuclear complex, which rapidly evolves into a bidentate binuclear complex that displaces glyphosate.

Phosphate adsorption onto hematite: An in situ ATR-FTIR investigation of the effects of pH and loading level on the mode of phosphate surface complexation

Phosphate adsorption on hematite was characterized as a function of pH (3.5–8.9) and phosphate concentration (5–500 μM) by in situ ATR-FTIR spectroscopy. Under most conditions a mixture of different (inner-sphere) phosphate complexes existed at the hematite surface, with the relative importance of these complexes varying with pH and surface coverage. Experiments using D 2O and H 2O indicated the presence of two protonated phosphate surface complexes at pH/pD = 3.5–7.0. Comparison to spectra of protonated aqueous phosphate species suggested that these surface complexes are monoprotonated. The difference in the IR spectra of these complexes is tentatively interpreted to result from a different surface coordination, with one surface complex coordinated in a monodentate binuclear (bridging) fashion, and the second as a monodentate mononuclear complex. Alternatively, the bridging complex may be a (protonated) monodentate mononuclear complex exhibiting strong hydrogen bonding to an adjacent surface site, and the second species a monodentate complex exhibiting limited hydrogen bonding. Formation of the bridging complex is favored at lower pH values and higher surface loadings in the 3.5–7.0 pH range. At the highest pH values studied (8.5–9.0) a third complex, interpreted to be a nonprotonated monodentate mononuclear complex, is present along with the monodentate monoprotonated surface species. The importance of the nonprotonated monodentate complex increases with increasing surface coverage at these high pH values.

Sorption of the Antimicrobial Ciprofloxacin To Aluminum and Iron Hydrous Oxides

Solution chemistry (pH, ionic strength (I), and sorbate-to-sorbent ratio) effects on ciprofloxacin sorption to hydrous oxides of Al (HAO) and Fe (HFO) were investigated using macroscopic and spectroscopic analyses. Sorption to both HAO and HFO showed a strong pH-dependent behavior, following the fraction of zwitterionic species over the entire pH range studied. Increase in I from 0.01 to 0.5 M had an insignificant effect on the extent of ciprofloxacin sorption, and isotherms were well-described by the Langmuir model. HFO possessed a higher sorption capacity (0.066 mmol kg(-1)) than HAO (0.041 mmol kg(-1)). Ligand-promoted dissolution of hydrous oxides, more pronounced for HAO, was observed in the presence of ciprofloxacin, but at a fairly high initial concentration (0.5 mM). Attenuated total reflectance Fourier transform infrared spectroscopy analysis indicated that different types of ciprofloxacin surface complexes are formed with HAO and HFO; while a monodentate mononuclear complex (with -COO-) appears likely between ciprofloxacin and HAO, keto O and one O from COO- seem to be involved in the formation of a six-membered ring with Fe on the HFO surface. The study results are expected to increase our understanding of the environmental reactivity of fluoroquinolones, an important class of antimicrobial compounds.

Mechanism of anion retention from EXAFS and density functional calculations: arsenic (V) adsorbed on gibbsite

X-ray absorption fine structure spectroscopy and density functional calculations were used to determine the structural model of arsenic surface complex on gibbsite mineral. The structural environment of arsenic at the solid surface may determine its potential for remobilization and stability. Data were collected for arsenate adsorbed on gibbsite surface at pH 5.5. The X-ray absorption fine structure spectroscopy results showed that As(V) forms an inner sphere bidentate binuclear complex on the surface of Al oxyhydroxyl octaedra. Quantitative results showed an average interatomic As(V)-Al distance of 3.19 ± 0.05 Å and a coordination number of 1.3 ± 1.0 atoms. Four different adsorption sites in which arsenate can interact with gibbsite have been studied using density functional calculations, i.e., bidentate binuclear complex, bidentate mononuclear complex, monodentate mononuclear complex, and monodentate binuclear complex. The density functional calculations confirm that the most stable structure predicted for As(V)-gibbsite system is the bidentate-binuclear complex.