Adsorption of monocarboxylates at the water/goethite interface: The importance of hydrogen bonding

The adsorption of monocarboxylates (acetate, benzoate, and cyclohexanecarboxylate) at the water/goethite interface was studied as a function of pH and ionic strength by means of quantitative adsorption measurements and attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy. ATR-FTIR spectra were obtained of suspensions prepared in both H₂O and D₂O. In order to identify the number of predominating surface complexes and to improve the resolution of overlapping peaks the ATR-FTIR spectra were subjected to a 2D correlation spectroscopic analysis. The adsorption envelopes of acetate, benzoate, and cyclohexanecarboxylate are similar and depend strongly on pH and ionic strength, but the pH dependence is also correlated to the slightly different pK_a values of the monocarboxylic acids. At the molecular level, the ATR-FTIR spectroscopic results reveal two surface complexes: one solvent-surface hydration-separated ion pair and one surface hydration-shared ion pair. The former predominates at circumneutral pH values while the latter forms mainly in the acidic pH range. We find no evidence for direct inner-sphere coordination between the carboxylic oxygens and the Fe(III) ions present at the surface. The identification of surface hydration-shared ion pairs emphasizes the importance of comparatively strong ionic hydrogen-bonding interactions for adsorption processes at the water/goethite interface.     

Structures and stabilities of Cd(II) and Cd(II)-phthalate complexes at the goethite/water interface

The complexation of Cd(II) and Cd(II)-phthalate at the goethite/water interface were investigated by EXAFS and IR spectroscopy, by batch adsorption experiments and by potentiometric titrations at 298.15 K. The EXAFS spectra showed Cd(II) to form only inner-sphere corner-sharing complexes with the goethite surface sites in the presence and absence of phthalate. EXAFS spectra also showed the presence of Cd(II)-chloride complexes in 0.1 mol/L NaCl. IR spectra also showed phthalate to form (1) an inner-sphere complex with adsorbed corner-sharing Cd(II) surface complexes in the pH 3.5 to 9.5 and (2) an outer-sphere complex with the same type of corner-sharing Cd(II) complex however at pH > 6, in addition to the inner- and outer-sphere complexes of phthalate reported in a previous study. The potentiometric titration and the batch adsorption data were used to constrain the formation constants of the different Cd(II)-phthalate surface complexes on the dominant {110} and the {001} planes of the goethite. The models were carried out with the Charge Distribution Multisite Complexation model coupled to the Three Plane Model and can predict the molecular-scale speciation of cadmium and phthalate in the presence of goethite. Cd(II) adsorption models calibrated on a 90 m²/g goethite also could accurately predict experimental data for a 37 m²/g goethite of slightly different basic charging properties.     

Goethite adsorption of Cu(II), Pb(II), Cd(II), and Zn(II) in the presence of sulfate: Properties of the ternary complex

Adsorption of Cu²⁺, Zn²⁺, Cd²⁺, and Pb²⁺ onto goethite is enhanced in the presence of sulfate. This effect, which has also been observed on ferrihydrite, is not predicted by the diffuse layer model (DLM) using adsorption constants derived from single sorbate systems. However, by including ternary surface complexes with the stoichiometry FeOHMSO₄, where FeOH is a surface adsorption site and M²⁺ is a cation, the effect of SO₄²⁻ on cation adsorption was accurately predicted for the range of cation, goethite and SO₄²⁻ concentrations studied. While the DLM does not provide direct molecular scale insights into adsorption reactions there are several properties of ternary complexes that are evident from examining trends in their formation constants. There is a linear relationship between ternary complex formation constants and cation adsorption constants, which is consistent with previous spectroscopic evidence indicating ternary complexes involve cation binding to the oxide surface. Comparing the data from this work to previous studies on ferrihydrite suggests that ternary complex formation on ferrihydrite involves complexes with the same or similar structure as those observed on goethite. In addition, it is evident that ternary complex formation constants are larger where there is a stronger metal-ligand interaction. This is also consistent with spectroscopic studies of goethite-M²⁺-SO₄²⁻ and phthalate systems showing surface species with metal-ligand bonding. Recommended values of ternary complex formation constants for use in SO₄-rich environments, such as acid mine drainage, are presented.     

Competitive interaction between soil-derived humic acid and phosphate on goethite

In order to better understand the influence and mechanism of soil-derived humic acid (SHA) on adsorption of P onto particles in soils, the amounts of PO₄ adsorbed by synthetic goethite (α-FeOOH) were determined at different concentrations of SHA, pH, ionic strength and order of addition of adsorbents. Addition of SHA can significantly reduce the amount of PO₄ adsorption as much as 27.8%. Both generated electrostatic field and competition for adsorption sites were responsible for the mechanism by which SHA inhibited adsorption of PO₄ by goethite. This conclusion was supported by measurement of total organic C (TOC), infrared spectral features and Zeta potential. Adsorption of PO₄ onto goethite was inversely proportional to pH. Order of addition of PO₄ and SHA can influence adsorption of PO₄ as follows: prior addition of PO₄ ⩾ simultaneous addition > prior addition of SHA. Iron and SHA apparently form complexes due to prior addition of SHA. Observations made during this study emphasized that PO₄ forms different types of complexes on the surface of goethite at different pH, which dominated the interaction of SHA and PO₄ adsorption on goethite. Based on these observations, the possible modes of SHA inhibition of PO₄ adsorption on goethite were proposed.     

Cu and Zn ternary surface complex formation with SO4 on ferrihydrite and schwertmannite

The use of adsorption data from single sorbate systems to model metal adsorption in SO₄-rich waters, such as acid mine drainage, can lead to inaccurate predictions of metal speciation. The adsorption of Cu and Zn on ferrihydrite, for example, is enhanced at low pH values in the presence of SO₄. This effect can only be accurately modeled using the diffuse layer model and surface complexation theory if ternary surface complexes, ≡FeOHCuSO₄ or ≡FeOHZnSO₄, are taken into consideration. Intrinsic adsorption constants for the formation of these ternary complexes on ferrihydrite have been derived from experimental data. When included in the model, Cu and Zn adsorption in the presence of SO₄ is accurately predicted for a wide range of metal, ferrihydrite and SO₄ concentrations. Adsorption of Cu and Zn onto the SO₄-rich Fe oxyhydroxide, schwertmannite, could also be accurately predicted and is indistinguishable from adsorption onto ferrihydrite in the presence of high solution SO₄ concentrations (e.g. 0.01 mol kg⁻¹ SO₄).     

The adsorption of Cu,Pb, Zn,and Cd on goethite from major ion seawater

The adsorption of Cu, Pb, Zn, and Cd on goethite (αFeOOH) from NaNO₃ solutions and from major ion seawater was compared to assess the effect of the major ions of seawater (Na, Mg, Ca, K, Cl, and SO₄) on the adsorption behavior of the metals. Magnesium and sulphate are the principal seawater ions which enhance or inhibit adsorption relative to the inert system. Their effect, as determined from the site-binding model of Davis et al. (1978), was a combination of changing the electrostatic conditions at the interface and decreasing the available binding sites.The basic differences between the experimental system of major ion seawater and natural seawater were examined. It was concluded that: 1) although the experimental metal concentrations in major ion seawater were higher than those found in natural seawater, estimates of the binding energy of Cu, Zn, and Cd with αFeOOH for natural seawater concentrations could be made from the data, 2) Cu, Pb, Zn, and Cd showed little or no competition for surface sites on goethite, and 3) the presence of carbonate, phosphate, and silicate had little or no effect on the adsorption of Zn and Cd on goethite.     

Effect of silicic acid on arsenate and arsenite retention mechanisms on 6-L ferrihydrite: A spectroscopic and batch adsorption approach

The competitive adsorption of arsenate and arsenite with silicic acid at the ferrihydrite–water interface was investigated over a wide pH range using batch sorption experiments, attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, extended X-ray absorption fine structure (EXAFS) spectroscopy, and density functional theory (DFT) modeling. Batch sorption results indicate that the adsorption of arsenate and arsenite on the 6-L ferrihydrite surface exhibits a strong pH-dependence, and the effect of pH on arsenic sorption differs between arsenate and arsenite. Arsenate adsorption decreases consistently with increasing pH; whereas arsenite adsorption initially increases with pH to a sorption maximum at pH 7–9, where after sorption decreases with further increases in pH. Results indicate that competitive adsorption between silicic acid and arsenate is negligible under the experimental conditions; whereas strong competitive adsorption was observed between silicic acid and arsenite, particularly at low and high pH. In situ, flow-through ATR-FTIR data reveal that in the absence of silicic acid, arsenate forms inner-sphere, binuclear bidentate, complexes at the ferrihydrite surface across the entire pH range. Silicic acid also forms inner-sphere complexes at ferrihydrite surfaces throughout the entire pH range probed by this study (pH 2.8–9.0). The ATR-FTIR data also reveal that silicic acid undergoes polymerization at the ferrihydrite surface under the environmentally-relevant concentrations studied (e.g., 1.0 mM). According to ATR-FTIR data, arsenate complexation mode was not affected by the presence of silicic acid. EXAFS analyses and DFT modeling confirmed that arsenate tetrahedra were bonded to Fe metal centers via binuclear bidentate complexation with average As(V)-Fe bond distance of 3.27 Å. The EXAFS data indicate that arsenite forms both mononuclear bidentate and binuclear bidentate complexes with 6-L ferrihydrite as indicated by two As(III)–Fe bond distances of ∼2.92–2.94 and 3.41–3.44 Å, respectively. The As–Fe bond distances in both arsenate and arsenite EXAFS spectra remained unchanged in the presence of Si, suggesting that whereas Si diminishes arsenite adsorption preferentially, it has a negligible effect on As–Fe bonding mechanisms.     

The mechanism of cadmium surface complexation on iron oxyhydroxide minerals

Many sediment and soil systems have become significantly contaminated with cadmium, and earth scientists are now required to make increasingly accurate predictions of the risks that this contamination poses. This necessitates an improved understanding of the processes that control the mobility and bioavailability of cadmium in the environment. With this in mind, we have studied the composition and structure of aqueous cadmium sorption complexes on the iron oxyhydroxide minerals goethite (α-FeOOH), lepidocrocite (γ-FeOOH), akaganeite (β-FeOOH), and schwertmannite (Fe₈O₈(OH)₆SO₄) using extended X-ray adsorption fine structure spectroscopy. The results show that adsorption to all of the studied minerals occurs via inner sphere adsorption over a wide range of pH and cadmium concentrations. The bonding mechanism varies between minerals and appears to be governed by the availability of different types of adsorption site at the mineral surface. The geometry and relative stability of cadmium adsorption complexes on the goethite surface was predicted with ab initio quantum mechanical modelling. The modelling results, used in combination with the extended X-ray adsorption fine structure data, allow an unambiguous determination of the mechanism by which cadmium bonds to goethite.Cadmium adsorbs to goethite by the formation of bidentate surface complexes at corner sharing sites on the predominant (110) crystallographic surface. There is no evidence for significant cadmium adsorption to goethite at the supposedly more reactive edge sharing sites. This is probably because the edge sharing sites are only available on the (021) crystallographic surface, which comprises just ∼2% of the total mineral surface area. Conversely, cadmium adsorption on lepidocrocite occurs predominately by the formation of surface complexes at bi- and/or tridentate edge sharing sites. We explain the difference in extended X-ray adsorption fine structure results for cadmium adsorption on goethite and lepidocrocite by the greater availability of reactive edge sharing sites on lepidocrocite than on goethite. The structures of cadmium adsorption complexes on goethite and lepidocrocite appear to be unaffected by changes in pH and surface loading. There is no support for cadmium sorption to any of the studied minerals via the formation of an ordered precipitate, even at high pH and high cadmium concentration. Cadmium adsorption on akaganeite and schwertmannite also occurs via inner sphere bonding, but the mechanism(s) by which this occurs remains ambiguous.     

Uranyl adsorption onto montmorillonite: Evaluation of binding sites and carbonate complexation

The fate and transport of uranium in contaminated soils and sediments may be affected by adsorption onto the surface of minerals such as montmorillonite. Extended X-ray absorption fine structure (EXAFS) spectroscopy has been used to investigate the adsorption of uranyl (UO₂²⁺) onto Wyoming montmorillonite. At low pH (∼4) and low ionic strength (10⁻³ M), uranyl has an EXAFS spectrum indistinguishable from the aqueous uranyl cation, indicating binding via cation exchange. At near-neutral pH (∼7) and high ionic strength (1 M), the equatorial oxygen shell of uranyl is split, indicating inner-sphere binding to edge sites. Linear-combination fitting of the spectra of samples reacted under conditions where both types of binding are possible reveals that cation exchange at low ionic strengths on SWy-2 may be more important than predicted by past surface complexation models of U(VI) adsorption on related montmorillonites. Analysis of the binding site on the edges of montmorillonite suggests that U(VI) sorbs preferentially to [Fe(O,OH)₆] octahedral sites over [Al(O,OH)₆] sites. When bound to edge sites, U(VI) occurs as uranyl-carbonato ternary surface complexes in systems equilibrated with atmospheric CO₂. Polymeric surface complexes were not observed under any of the conditions studied. Current surface complexation models of uranyl sorption on clay minerals may need to be reevaluated to account for the possible increased importance of cation exchange reactions at low ionic strengths, the presence of reactive octahedral iron surface sites, and the formation of uranyl-carbonato ternary surface complexes. Considering the adsorption mechanisms observed in this study, future studies of U(VI) transport in the environment should consider how uranium retardation will be affected by changes in key solution parameters, such as pH, ionic strength, exchangeable cation composition, and the presence or absence of CO₂.     

Adsorption of hydroxamate siderophores and EDTA on goethite in the presence of the surfactant sodium dodecyl sulfate

Siderophore-promoted iron acquisition by microorganisms usually occurs in the presence of other organic molecules, including biosurfactants. We have investigated the influence of the anionic surfactant sodium dodecyl sulfate (SDS) on the adsorption of the siderophores DFOB (cationic) and DFOD (neutral) and the ligand EDTA (anionic) onto goethite (α-FeOOH) at pH 6. We also studied the adsorption of the corresponding 1:1 Fe(III)-ligand complexes, which are products of the dissolution process. Adsorption of the two free siderophores increased in a similar fashion with increasing SDS concentration, despite their difference in molecule charge. In contrast, SDS had little effect on the adsorption of EDTA. Adsorption of the Fe-DFOB and Fe-DFOD complexes also increased with increasing SDS concentrations, while adsorption of Fe-EDTA decreased. Our results suggest that hydrophobic interactions between adsorbed surfactants and siderophores are more important than electrostatic interactions. However, for strongly hydrophilic molecules, such as EDTA and its iron complex, the influence of SDS on their adsorption seems to depend on their tendency to form inner-sphere or outer-sphere surface complexes. Our results demonstrate that surfactants have a strong influence on the adsorption of siderophores to Fe oxides, which has important implications for siderophore-promoted dissolution of iron oxides and biological iron acquisition.     

Adsorption of oxalate and malonate at the water-goethite interface: Molecular surface speciation from IR spectroscopy

The adsorption of oxalate and malonate at the water-goethite interface was studied as a function of pH and total ligand concentrations by means of quantitative adsorption measurements and attenuated total reflectance Fourier transform infrared spectroscopy. The obtained results conclusively showed that oxalate and malonate both form outer-sphere and inner-sphere surface complexes on goethite, and that these complexes coexist over a broad pH interval. The inner-sphere complexes were favored by low pH, while the relative concentrations of the outer-sphere species increase with increasing pH. Based on comparisons with model complexes characterized by Extended X-Ray Adsorption Fine Structure (EXAFS) and results from theoretical frequency calculations, the structures of the inner-sphere complexes of oxalate and malonate were best described as mononuclear five- and six-membered ring chelate structures, respectively. The stability of the inner-sphere complexes followed the trend expected from solutions studies, with the oxalate five-membered ring yielding the more stable complexes compared to the six-membered ring of malonate. The increased stability of the inner-sphere complex of oxalate was manifested in a greater extent of adsorption at acidic pH values. Despite the fact that significant amounts of oxalate and malonate inner-sphere surface complexes were formed, no ligand-promoted dissolution was observed at the experimental conditions in the study.     

Adsorption of organic matter at mineral/water interfaces: I. ATR-FTIR spectroscopic and quantum chemical study of oxalate adsorbed at boehmite/water and corundum/water interfaces

The types and structures of adsorption complexes formed by oxalate at boehmite (γ-AlOOH)/water and corundum (α-Al₂O₃)/water interfaces were determined using in situ attenuated total reflectance fourier transform infrared (ATR-FTIR) spectroscopy and quantum chemical simulation methods. At pH 5.1, at least four different oxalate species were found at or near the boehmite/water interface for oxalate surface coverages (Γ_ox) ranging from 0.25 to 16.44 μmol/m². At relatively low coverages (Γ_ox < 2.47), strongly adsorbed inner-sphere oxalate species (IR peaks at 1286, 1418, 1700, and 1720 cm⁻¹) replace weakly adsorbed carbonate species, and a small proportion of oxalate anions are adsorbed in an outer-sphere mode (IR peaks at 1314 and 1591 cm⁻¹). IR peaks indicative of inner-sphere adsorbed oxalate are also observed for oxalate at the corundum/water interface at Γ_ox = 1.4 μmol/m². With increasing oxalate concentration (Γ_ox > 2.47 μmol/m²), the boehmite surface binding sites for inner-sphere adsorbed oxalate become saturated, and excess oxalate ions are present dominantly as aqueous species (IR peaks at 1309 and 1571 cm⁻¹). In addition to these adsorption processes, oxalate-promoted dissolution of boehmite following inner-sphere oxalate adsorption becomes increasingly pronounced with increasing Γ_ox and results in an aqueous Al(III)-oxalate species, as indicated by shifted IR peaks (1286 → 1297 cm⁻¹ and 1418 → 1408 cm⁻¹). At pH 2.5, no outer-sphere adsorbed oxalate or aqueous oxalate species were observed. The similarity of adsorbed oxalate spectral features at pH 2.5 and 5.1 implies that the adsorption mechanism of aqueous HOx⁻ species involves loss of protons from this species during the ligand-exchange reaction. As a consequence, adsorbed inner-sphere oxalate and aqueous Al(III)-oxalate complexes formed at pH 2.5 have coordination geometries very similar to those formed at pH 5.1.The coordination geometry of inner-sphere adsorbed oxalate species was also predicted using quantum chemical geometry optimization and IR vibrational frequency calculations. Geometry-optimized Al₈O₁₂ and Al₁₄O₂₂ clusters with the reactive surface Al site coordinated by three oxygens were used as model substrates for corundum and boehmite surfaces. Among the models considered, calculated IR frequencies based on a bidentate side-on structure with a 5-membered ring agree best with the observed frequencies for boehmite/oxalate/water samples at Γ_ox = 0.25 to 16.44 μmol/m² and pH 2.5 and 5.1, and for a corundum/oxalate/water sample at Γ_ox = 1.4 μmol/m² and pH 5.1. Based on these results, we suggest that oxalate bonding on boehmite and corundum surfaces results in 5-coordinated rather than 4- or 6-coordinated Al surface sites.     

Cu(II) adsorption at the calcite-water interface in the presence of natural organic matter: Kinetic studies and molecular-scale characterization

We have characterized the adsorption of Suwannee River humic acid (SRHA) and Cu(II) on calcite from preequilibrated solutions at pH 8.25. Sorption isotherms of SRHA on calcite follow Langmuir-type behavior at SRHA concentrations less than 15 mg C L⁻¹, whereas non-Langmuirian uptake becomes evident at concentrations greater than 15 mg C L⁻¹. The adsorption of SRHA on calcite is rapid and mostly irreversible, with corresponding changes in electrostatic properties. At pH 8.25, Cu(II) uptake by calcite in the presence of dissolved SRHA decreases with increasing dissolved SRHA concentration, suggesting that formation of Cu-SRHA aqueous complexes is the primary factor controlling Cu(II) sorption at the calcite surface under the conditions of our experiments. We also observed that surface-bound SRHA has little influence on Cu(II) uptake by calcite, suggesting that Cu(II) coordinates to calcite surface sites rather than to surface-bound SRHA.Cu K-edge X-ray absorption near edge structure (XANES) and extended X-ray absorption fine-structure (EXAFS) spectroscopic results show that the local coordination of Cu adsorbed at the calcite surface is very similar in the presence and absence of SRHA. Ca backscatterers at ∼3.90 Å indicate that Cu(II) forms tetragonally distorted inner-sphere adsorption complexes in both binary and ternary systems. Subtle differences in the XANES and EXAFS between binary sorption samples and ternary sorption samples, however, prevent us from ruling out the formation of ternary Cu-SRHA surface complexes. Our findings demonstrate that SRHA plays an important role in controlling the fate and transport of Cu(II) in calcite-bearing systems.     

ATR-FTIR spectroscopic study of the adsorption of desferrioxamine B and aerobactin to the surface of lepidocrocite (γ-FeOOH)

The adsorption of two model siderophores, desferrioxamine B (DFOB) and aerobactin, to lepidocrocite (γ-FeOOH) was investigated by attenuated total reflection infrared spectroscopy (ATR-FTIR). The adsorption of DFOB was investigated between pH 4.0 and 10.6. The spectra of adsorbed DFOB indicated that two to three hydroxamic acid groups of adsorbed DFOB were deprotonated in the pH range 4.0-8.2. Deprotonation of hydroxamic acid groups of adsorbed DFOB at pH values well below the first acid dissociation constant of solution DFOB species (pK_a = 8.3) and well below the point of zero charge of lepidocrocite (pH_PZC = 7.4) suggested that the surface speciation at the lower end of this pH range (pH 4) is dominated by a surface DFOB species with inner-sphere coordination of two to three hydroxamic acids groups to the surface. Maximum adsorption of DFOB occurred at approximately pH 8.6, close to the first pK_a value of the hydroxamic acid groups, and decreased at lower and higher pH values.The spectra of adsorbed aerobactin in the pH range 3-9 indicated at least three different surface species. Due to the small spectral contributions of the hydroxamic acid groups of aerobactin, the interactions of these functional groups with the surface could not be resolved. At high pH, the spectral similarity of adsorbed aerobactin with free aerobactin deprotonated at the carboxylic acid groups indicated outer-sphere complexation of the carboxylate groups. With decreasing pH, a significant peak shift of the asymmetric carboxylate stretch vibration was observed. This finding suggested that the (lateral) carboxylic acid groups are coordinated to the surface either as inner-sphere complexes or as outer-sphere complexes that are strongly stabilized at the surface by hydrogen bonding at low pH.     

A surface structural model for ferrihydrite II: Adsorption of uranyl and carbonate

The adsorption of uranyl (UO₂²⁺) on ferrihydrite has been evaluated with the charge distribution (CD) model for systems covering a very large range of conditions, i.e. pH, ionic strength, CO₂ pressure, U(VI) concentration, and loading. Modeling suggests that uranyl forms bidentate inner sphere complexes at sites that do not react chemically with carbonate ions. Uranyl is bound by singly-coordinated surface groups present at particular edges of Fe-octahedra of ferrihydrite while another set of singly-coordinated surface groups may form double-corner bidentate complexes with carbonate ions. The uranyl surface speciation strongly changes in the presence of carbonate due to the specific adsorption of carbonate ions as well as the formation of ternary uranyl-carbonate surface complexes. Data analysis with the CD model suggests that a uranyl tris-carbonato surface complex, i.e. (UO₂)(CO₃)₃⁴⁻, is formed. This species is most abundant in systems with a high pH and carbonate concentration. This finding differs significantly from previous interpretations made in the literature. At high pH and low carbonate concentrations, as can be prepared in CO₂-closed systems, the model suggests the additional presence of a ternary uranyl-monocarbonato complex. The binding mode (type A or type B complex) is uncertain. At high uranyl concentrations, uranyl polymerizes at the surface of ferrihydrite giving, for instance, tris-uranyl surface complexes with and without carbonate. The similarities and differences between U(VI) adsorption by goethite and ferrihydrite are discussed from a surface structural point of view.     

Adsorption of fluoride on goethite surfaces-implications on dental epidemiology

Fluoride ion interaction with synthetically prepared goethite has been investigated over a range of pH values (4–9) and F^– concentrations (10^–3–10^–5 M). The amount of F^– retained by goethite suspensions was found to be a function of pH, media ionic strength, F^– concentration, and goethite concentration. The lowest ionic strength (0.001 M KNO₃) gave the highest adsorption medium. Uptake was minimal at pH >7 and increased with decreasing pH. Thermodynamic properties for fluoride adsorption at 298 K and 323 K were investigated. The isosteric heat of adsorption (H _r) was calculated and the heterogeneity and homogeneity of the surface examined for goethite. In view of the importance of fluoride in dental health, the interaction of fluoride on goethite in the physical environment has important implications on dental epidemiology.     

Adsorption of Copper, Nickel, and Cadmium on Goethite in the Presence of Organic Ligands

Adsorption of copper, cadmium and nickel at low concentrations on goethite was studied in the presence of the simple organic ligands oxalate, salicylate, and pyromellitate. The experimental metal adsorption behavior was compared to calculations with a surface complexation model to evaluate the most important interactions. Oxalate mostly decreased Cu and Ni adsorption at high pH-values by competition between solution and surface complexation but had no effect on Cd adsorption. Cu adsorption in the presence of oxalate below pH 6 could best be described by defining a ternary complex of type A (surface-metal-ligand). Salicylate had only minor effects on metal adsorption. The adsorption of Cu in the presence of salicylate above pH 5 could be explained by a ternary complex of type A. Pyromellitate increased the adsorption of Cu and Cd in the acidic pH-range, likely by formation of ternary surface complexes of type B (surface-ligand-metal).     

ATR-FTIR spectroscopic studies of boric acid adsorption on hydrous ferric oxide

Boron is an important micronutrient for plants, but high B levels in soils are often responsible for toxicity effects in plants. It is therefore important to understand reactions that may affect B availability in soils. In this study, Attenuated Total Reflectance Fourier transform Infrared (ATR-FTIR) spectroscopy was employed to investigate mechanisms of boric acid (B(OH)₃) and borate (B(OH)₄⁻) adsorption on hydrous ferric oxide (HFO). On the HFO surface, boric acid adsorbs via both physical adsorption (outer-sphere) and ligand exchange (inner-sphere) reactions. Both trigonal (boric acid) and tetrahedral (borate) boron are complexed on the HFO surface, and a mechanism where trigonal boric acid in solution reacts to form either trigonal or tetrahedral surface complexes is proposed based upon the spectroscopic results. The presence of outer-sphere boric acid complexes can be explained based on the Lewis acidity of the B metal center, and this complex has important implications for boron transport and availability. Outer-sphere boric acid is more likely to leach downward in soils in response to water flow. Outer-sphere boron would also be expected to be more available for plant uptake than more strongly bound boron complexes, and may more readily return to the soil solution when solution concentrations decrease.     

The adsorption of plutonium IV and V on goethite

The adsorption of Pu(IV) and Pu(V) on goethite (αFeOOH) from NaNO₃ solution shows distinct differences related to the different hydrolytic character of these two oxidation states. Under similar solution conditions, the adsorption edge of the more strongly hydrolyzable Pu(IV) occurs in the pH range 3 to 5 while that for Pu(V) is at pH 5 to 7. The adsorption edge for Pu(V) shifts with time to lower pH values and this appears to be due to the reduction of Pu(V) to Pu(IV) in the presence of the goethite surface. These results suggest that redox transformations may be an important aspect of Pu adsorption chemistry and the resulting scavenging of Pu from natural waters.Increasing ionic strength (from 0.1 M to 3 M NaCl or NaNO₃ and 0.03 M to 0.3 M Na₂SO₄) did not influence Pu(IV) or Pu(V) adsorption. In the presence of dissolved organic carbon (DOC), Pu(V) reduction to Pu(IV) occurred in solution. Pu(IV) adsorption on goethite decreased by 30% in the presence of 240 ppm natural DOC found in Soap Lake, Washington waters. Increasing concentrations of carbonate ligands decreased Pu(IV) and Pu(V) adsorption on goethite, with an alkalinity of 1000 meq/l totally inhibiting adsorption.The Pu-goethite adsorption system provides the data base for developing a thermodynamic model of Pu interaction with an oxide surface and with dissolved ligands, using the MINEQL computer program. From the model calculations we determined equilibrium constants for the adsorption of Pu(IV) hydrolysis species. The model was then applied to Pu adsorption in carbonate media to see how the presence of CO₃^?2 could influence the mobility of Pu. The decrease in adsorption appears to be due to formation of a Pu-CO₃ complex. Model calculations were used to predict what the adsorption curves would look like if Pu-CO₃ complexes formed.