Hydroxy- and fluorapatite as sorbents in Cd(II)-Zn(II) multi-component solutions in the absence/presence of EDTA.

Apatites are suitable sorbent materials for contaminated soil and water remediation because of their low solubility and ability to bind toxic metals into their structure. Whereas in soil/water systems different complexing ligands are present, it is important to examine how these ligands affect apatite metal sorption process. The removal of cadmium (Cd) and zinc (Zn) ions from aqueous solutions by hydroxyapatite (HAP) and fluorapatite (FAP) was investigated by batch experiments with and without EDTA being present in the pH range 4-11. The surface composition of the solid phases was analyzed by X-ray photoelectron spectroscopy (XPS). The surface layer of apatites (AP), according to the (Ca+Cd+Zn):P atomic ratio, remained constant (1.4 ± 0.1) through an ion exchange. The amount of Cd(2+) and Zn(2+) removed increased with increasing pH. The removed amount of Zn(2+) was higher than Cd(2+). In the Cd-Zn binary system, competitive sorption reduced the individual removed amounts but the total maximum sorption was approximately constant. In the presence of EDTA, Cd(2+) and Zn(2+) removal was reduced because of the formation of [CdEDTA](2-) and [ZnEDTA](2-) in solution. XPS revealed an enrichment of AP surface by Cd(2+) and Zn(2+) and formation of new surface solid-solution phase with the general composition Ca8.4-xMex(HPO4)1.6(PO4)4.4(OH)0.4.

Surface complexes of monomethyl phosphate stabilized by hydrogen bonding on goethite (α-FeOOH) nanoparticles.

Typically, a significant fraction of phosphorus in soils is composed of organic phosphates, and this fraction thus plays an important role in the global phosphorus cycle. Here we have studied adsorption of monomethyl phosphate (MMP) to goethite (α-FeOOH) as a model system in order to better understand the mechanisms behind adsorption of organic phosphates to soil minerals, and how adsorption affects the stability of these molecules. The adsorption reactions and stability of MMP on goethite were studied at room temperature as a function of pH, time and total concentration of MMP by means of quantitative batch experiments, potentiometry and infrared spectroscopy. MMP was found to be stable at the water-goethite interface within the pH region 3-9 and over extended periods of time, as well as in solution. The infrared spectra indicated that MMP formed three predominating pH-dependent surface complexes on goethite, and that these interacted monodentately with surface Fe. The complexes differed in hydrogen bonding interactions via the auxiliary oxygens of the phosphate group. The presented surface complexation model was based on the collective spectroscopic and macroscopic results, using the Basic Stern approach to describe the interfacial region. The model consisted of three monodentate inner sphere surface complexes where the MMP complexes were stabilized by hydrogen bonding to a neighboring surface site. The three complexes, which had equal proton content and thus could be defined as surface isomers, were distinguished by the distribution of charge over the 0-plane and ß-plane. In the high pH-range, MMP acted as a hydrogen bond acceptor whereas it was a hydrogen bond donor at low pH.

Glyphosate complexation to aluminium(III). An equilibrium and structural study in solution using potentiometry, multinuclear NMR, ATR-FTIR, ESI-MS and DFT calculations.

The stoichiometries and stability constants of a series of Al(3+)-N-phosponomethyl glycine (PMG/H(3)L) complexes have been determined in acidic aqueous solution using a combination of precise potentiometric titration data, quantitative (27)Al and (31)P NMR spectra, ATR-FTIR spectrum and ESI-MS measurements (0.6M NaCl, 25 degrees C). Besides the mononuclear AlH(2)L(2+), Al(H(2)L)(HL), Al(HL)(2)(-) and Al(HL)L(2-), dimeric Al(2)(HL)L(+) and trinuclear Al(3)H(5)L(4)(2+) complexes have been postulated. (1)H and (31)P NMR data show that different isomers co-exist in solution and the isomerization reactions are slow on the (31)P NMR time scale. The geometries of monomeric and dimeric complexes likely double hydroxo bridged and double phosphonate bridged isomers have been optimized using DFT ab initio calculations starting from rational structural proposals. Energy calculations using the PCM solvation method also support the co-existence of isomers in solutions.

Phase transformations, ion-exchange, adsorption, and dissolution processes in aquatic fluorapatite systems.

A synthetic fluorapatite was prepared that undergoes a phase transformation generated during a dialysis step. A surface layer with the composition Ca9(HPO4)2(PO4)4F2 is formed, which is suggested to form as one calcium atom is replaced by two protons. A surface complexation model, based upon XPS measurements, potentiometric titration data, batch experiments, and zeta-potential measurements was presented. The CaOH and OPO3H2 sites were assumed to have similar protolytic properties as in a corresponding nonstoichiometric HAP (Ca8.4(HPO4)1.6(PO4)4.4(OH)0.4) system. Besides a determination of the solubility product of Ca9(HPO4)2(PO4)4F2, two additional surface complexation reactions were introduced; one that accounts for a F/OH ion exchange reaction, resulting in the release of quite high fluoride concentrations (approximately 1 mM) that turned out to be dependent on the surface area of the particles. Furthermore, to explain the lowering of pHiep from around 8 in nonstoichiometric HAP suspensions to about 5.7 in FAP suspensions, a reaction that lowers the surface charge due to the readsorption of fluoride ions to the positively charged Ca sites was introduced: triple bond CaOH2++F-<-->triple bond CaF+H2O. The resulting model also agrees with predictions based upon XPS and ATR-FTIR observations claiming the formation of CaF2(s) in the most acidic pH range.

Adsorption of glyphosate on goethite (alpha-FeOOH): surface complexation modeling combining spectroscopic and adsorption data.

N-(phosphonomethyl)glycine (glyphosate, PMG) is the most widely used herbicide, and its adsorption onto soil minerals plays a significant role in its mobility and rate of degradation. In this work, we present the results of the first serious effort to find a realistic surface complexation modelthatfits both adsorption and total proton concentration data for PMG on the common soil mineral, goethite. Special attention was focused on making sure that the final model was in good semiquantitative agreement with previously reported X-ray photoelectron spectroscopy and Fourier-transform infrared spectroscopic measurements. Electrostatic effects were accounted for using the Basic Stern model, and the charges of the PMG-containing surface complexes were assumed to be distributed across the 0- and beta-planes. The reactions for the protonation of the goethite surface were described using the 1 pK model. We optimized on the intrinsic formation constants and the charge distributions of the complexes, as well as the initial total proton concentration (I = 0.1 M Na(NO3), 25.0 degrees C), and the following model was obtained. triple bond FeOH(0.5-) + H3L <==> triple bond FeHL(1.5-) + H(+) + H2O Log10beta = 4.70 +/- 0.08, Q0 = -0.18 +/- 0.02 triple bond FeOH(0.5-) + H3L <==> FeL(2.5-) + 2H(+) + H2O Log10beta = -3.9 +/- 0.1, Q0 = -0.7 +/- 0.1 Here, beta is the intrinsic formation constant, Q0 is the charge at the 0-plane, and the errors are reported as one standard deviation. The charge distributions of the complexes are rationalized by considering intramolecular hydrogen bonding between the protons of the amine group and both the phosphonate and carboxylate groups.

Modeling the acid-base properties of bacterial surfaces: A combined spectroscopic and potentiometric study of the gram-positive bacterium Bacillus subtilis.

In this study, macroscopic and spectroscopic data were combined to develop a surface complexation model that describes the acid-base properties of Bacillus subtilis. The bacteria were freeze-dried and then resuspended in 0.1 M NaCl ionic medium. Macroscopic measurements included potentiometric acid-base titrations and electrophoretic mobility measurements. In addition, ATR-FTIR spectra of wet pastes from suspensions of Bacillus subtilis at different pH values were collected. The least-squares program MAGPIE was used to generate a surface complexation model that takes into account the presence of three acid-base sites on the surface: tripple bond COOH, tripple bond NH+, and tripple bond PO-, which were identified previously by XPS measurements. Both potentiometric titration data and ATR-FTIR spectra were used quantitatively, and electrostatic effects at the charged bacterial surface were accounted for using the constant capacitance model. The model was calculated using two different approaches: in the first one XPS data were used to constrain the ratio of the total concentrations of all three surface sites. The capacitance of the double layer, the total buffer capacity, and the deprotonation constants of the tripple bond NH+, tripple bond POH, and tripple bond COOH species were determined in the fit. A second approach is presented in which the ratio determined by XPS of the total concentrations of tripple bond NH+ to tripple bond PO- sites is relaxed. The total concentration of tripple bond PO- sites was determined in the fit, while the deprotonation constant for tripple bond POH was manually varied until the minimization led to a model which predicted an isoelectric point that resulted in consistency with electrophoretic mobility data. The model explains well the buffering capacity of Bacillus subtilis suspensions in a wide pH range (between pH=3 and pH=9) which is of considerable environmental interest. In particular, a similar quantitative use of the IR data opens up possibilities to model other bacterial surfaces at the laboratory scale and help estimate the buffering capacity of carboxylate-containing compounds in natural samples.

Quantitative adsorption and local structures of gallium(III) at the water-alpha-FeOOH interface.

The adsorption of Ga(III) at the water-alpha-FeOOH (goethite) interface has been investigated by means of quantitative adsorption experiments, extended X-ray absorption fine structure (EXAFS) spectroscopy, and surface complexation modeling. Under the conditions studied, pH range 3-11 and surface coverages of 0.9-3.2 micromol/m2, Ga(III) was found to adsorb strongly to alpha-FeOOH, and the surface species were more resistant toward hydrolysis and formation of soluble Ga(OH)4- than either solid gallium hydroxides or soluble polynuclear complexes. The EXAFS measurements revealed the presence of octahedral Ga(III) complexes at the water-alpha-FeOOH interface, with practically no structural variations as a function of pH or total gallium concentration. Analysis of the first coordination shell required an anharmonic model indicating a distorted geometry of the GaO6 octahedra, with mean Ga-O distances at 1.96-1.98 angstroms. A method based on the continuous Cauchy wavelet transforms (CCWT) was used to identify backscattering atoms in the higher coordination shells. This analysis indicated predominately Fe backscattering, and the quantitative data fitting resulted in three Ga-Fe paths at 3.05, 3.2, and 3.55 angstroms, which correspond to two edge-sharing and one corner-sharing linkage, respectively. The collective results from EXAFS spectroscopy showed that Ga(III) adsorbs to Fe equivalent sites at the surface alpha-FeOOH as an extension of the rows of Fe octahedra in the bulk structure. This interpretation was further corroborated by a Ga-Fe-Fe multiple scattering path at 6.13 angstroms. The quantitative adsorption and proton data were modeled using a surface complexation formalism based on a 1 pK(a) constant capacitance model. In agreement with the EXAFS results, the model obtained included one predominating surface complex with the stoichiometry [triple bond]FeOGa(OH)2(-0.5) and the stability constant log beta(intr.) = -2.55 +/- 0.04 ([triple bond]FeOH(-0.5) + Ga3+ + 2H2O <--> [triple bond]FeOGa(OH)2(-0.5) + 3H+).

Adsorption of Cu(II) to schwertmannite and goethite in presence of dissolved organic matter.

Sorption processes involving secondary iron minerals may significantly contribute to immobilisation of metals in soils and surface waters. In the present work the effect of dissolved organic matter (DOM) from a concentrated bog-water on the adsorption of Cu(II) onto schwertmannite (Fe8O8(OH)6SO4) and goethite (alpha-FeOOH) has been studied. The acid/ base behaviour of DOM up to pH 6 was explained by assuming a diprotic acid with a density of carboxylate groups of 6.90 microeq (mg C)(-1). The resulting acidity constants, recalculated to zero ionic strength were pK(0)a1 = 3.61 and pK(0)a2 = 5.34. The uptake of DOM to schwertmannite and goethite was highest at low pH although adsorption was significant also under mildly alkaline conditions. Adsorption to the two minerals was similar although at high pH more DOM was adsorbed to schwertmannite than to goethite. DOM enhanced the adsorption of Cu(II) at moderately low pH in the goethite system but there was no effect of DOM in the case of schwertmannite. The presence of Cu(II) resulted in a decreased adsorption of DOM to goethite at weakly acidic pH and increased adsorption at high pH. In the case of schwertmannite, Cu(II) did not affect DOM uptake.

Co-adsorption of cadmium(II) and glyphosate at the water-manganite (gamma-MnOOH) interface.

The co-adsorption of Cd(II) and glyphosate (N-(phosphonomethyl)glycine, PMG) at the manganite (gamma-MnOOH) surface has been studied in the pH range 6-10 at 25 degrees C and with 0.1 M Na(Cl) as ionic medium. Batch adsorption experiments, Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and extended X-ray absorption fine structure (EXAFS) spectroscopy were used for the quantitative analysis and the determination of the molecular structure of the surface complexes. The adsorption of Cd(II) and PMG in the ternary Cd(II)-PMG-manganite system was compared with the adsorption in the binary Cd(II)-manganite and PMG-manganite systems. The formation of three inner sphere surface complexes was observed, a ternary Cd(II)-PMG-manganite complex, a binary Cd(II)-manganite complex and a binary PMG-manganite complex. The surface concentration of the ternary complex and the Cd(II)-manganite complex was more or less constant throughout the pH range studied. However, the surface concentration of the binary PMG-manganite complex decreased with increasing pH. The major part of the binary PMG-surface complex was protonated. The ternary surface complex displayed a type B structure (Cd(II)-PMG-manganite). The average Cd-Mn distance obtained from EXAFS (3.26 A) indicates that the binary and ternary Cd(II)-surface complexes are formed by edge-sharing of Mn and Cd octahedra on the (010) plane of the manganite crystals.

Surface properties of hydrous manganite (gamma-MnOOH). A potentiometric, electroacoustic, and X-ray photoelectron spectroscopy study.

The acid-base characteristics of the manganite (gamma-MnOOH) surface have been studied at pH above 6, where dissolution is negligible. Synthetic microcrystalline particles of manganite were used in the experiments. From potentiometric titrations, electrophoretic mobility measurements, and X-ray photoelectron spectroscopy (XPS), a one pK(a) model was constructed that describes the observed behavior. The data show no ionic strength effect at pH < 8.2, which is the pH at the isoelectric point (pH(iep)), but ionic strength effects were visible above this pH. To explain these observations, Na(+) ions were suggested to form a surface complex. The following equilibria were established: =MnOH(2)(+1/2) right harpoon over left harpoon =MnOH(-)(1/2) + H(+), log beta(0) (intr.) = -8.20; =MnOH(2)(+1/2) + Na(+) right harpoon over left harpoon =MnOHNa(+1/2) + H(+), log beta(0) (intr.) = -9.64. The excess of Na(+) at the surface was supported by XPS measurements of manganite suspensions containing 10 mM NaCl. The dielectric constant of synthetic manganite powder was also determined in this study.

Surface complexation of mellitic acid to goethite: an attenuated total reflection Fourier transform infrared study.

The nature of the interaction between mellitic acid (benzene hexacarboxylic acid) and the common soil mineral goethite (alpha-FeOOH) has been investigated as a function of pH and ionic strength by use of attenuated total reflection Fourier transform infrared spectroscopy. Molecular orbital calculations of the theoretical vibrational frequencies of the mellitate ion (L6-) and dihydrogen mellitate (H2L4-) have allowed the measured absorption frequencies to be accurately assigned. At pH values above 6, adsorption involves outer-sphere complexation of the deprotonated L6- ion. At lower pH values, there is evidence of a second outer-sphere surface complex involving a partially protonated species, although the extent of protonation of the surface species is significantly less than that found for the solution species at the same pH. While there is no evidence of inner-sphere complexation, increasing the ionic strength to 2.0 M does not displace the adsorbed species but does increase the fraction present on the surface as the fully deprotonated L6-. The small effect of ionic strength suggests that the adsorptive interaction, although outer-sphere in character, is still relatively strong, which indicates that hydrogen bonds may play a significant role. Hydrogen bonding may also help to account for the observed outer-sphere complexation at pH values above the pHiep of goethite.

Surface analysis of dental amalgams by X-ray photoelectron spectroscopy and X-ray diffraction.

OBJECTIVES: It is important to characterize the surface of dental amalgam in order to understand the process of mercury release from amalgam restorations in the oral cavity. The mercury evaporation occurs not only from the newly made restoration but also from the set material. METHODS: The surfaces of four different types of amalgams, which had been well set, were analyzed with X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) and the relationship between surface compositions and mercury release was studied. Fresh amalgam surfaces as well as aged surfaces, which were stored for 30 days in air, were investigated using XPS and the chemical states of amalgam components and oxygen were studied. The aged surfaces were also characterized with XRD and grazing angle XRD. RESULTS: With increased oxidation, the surface contents of tin and oxygen were increased in all amalgams. In contrast, the surface contents of copper and mercury were decreased. An increase of zinc or indium content were observed in zinc or indium containing amalgams, respectively. A surface layer enriched with indium and oxygen was clearly detected by XPS but not with grazing angle XRD. SIGNIFICANCE: The thickness of the enriched surface layer is estimated to be in the order of few nanometer, which is approximately equal to the analysis depth of XPS. In addition, the presence of metallic elements, like tin and zinc, that readily form a stable oxide layer at the surface suppress the release of mercury.

Coadsorption of Cu(II) and glyphosate at the water-goethite (alpha-FeOOH) interface: molecular structures from FTIR and EXAFS measurements.

The coadsorption of Cu(II) and glyphosate (N-(phosphonomethyl)glycine, abbreviated to PMG) at the water-goethite interface was studied by means of batch adsorption experiments, attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, and extended X-ray absorption fine structure (EXAFS) spectroscopy. The system was investigated over the pH range 3--9 and at total concentrations of 0.9 micromol and 2.2 micromol Cu(II) and PMG per m(2) of goethite. The collective quantitative and spectroscopic results show that Cu(II) and PMG directly interact at the water-goethite interface to form ternary surface complexes. Two predominating complexes have been identified. At pH 4 the IR and CuK-edge EXAFS data indicate a molecular structure where the phosphonate group of PMG bonds monodentately to the surface in an inner sphere mode, while carboxylate and amine groups coordinate to Cu(II) to form a 5-membered chelate ring. Hence, at pH 4, Cu(II) and PMG form a ternary surface complex on goethite with the general structure goethite-PMG-Cu(II). At the highest pH investigated (pH 9), the carboxylate group is still coordinated to Cu(II) but the phosphonate group is present in a relatively free, non-coordinated and/or disordered state. Although the spectroscopic data are not conclusive they indicate the formation of ternary surface complexes with the molecular architecture goethite-Cu(II)-PMG at high pH.

Adsorption of glyphosate on goethite: molecular characterization of surface complexes.

As a component of herbicides, the fate of glyphosate (PMG) in the environment is of significant interest. The nature of PMG adsorption on mineral surfaces plays a significant role in the degradation of PMG. The adsorption of PMG on goethite (alpha-FeOOH) has been studied as a function of pH and PMG concentration. Adsorption was investigated with batch experiments, attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), and X-ray photoelectron spectroscopy (XPS). The N 1s line in XPS spectra showed deprotonation of the amine group of PMG (NH2+) with increasing pH. IR analyses showed no evidence for the interaction of PMG's carboxylate group with the goethite surface, while the phosphonate group formed inner-sphere complexes. There is evidence for intramolecular hydrogen bonding between NH2+ and both the carboxylate and the phosphonate groups at low pH. Intramolecular hydrogen bonding is lost when the amine group is deprotonated, and the trend in intramolecular hydrogen bonding between NH2+ and phosphonate shows that PMG adsorbs via predominantly monodentate complexation. A minor quantity of bidentate complexes is thought to form both at near-neutral pH and when the surface concentration of PMG is low. While the phosphonate group of PMG binds directly, the carboxylate group remains relatively "free" from complexation with goethite, leaving it subject to degradation and/or complexation with metal ions present in the environment.