Goethite-water Interface Research Articles

Deciphering competitive interactions: Phosphate and organic matter binding on goethite through experimental and theoretical insights

The adsorption of phosphorus (P) onto active soil surfaces plays a pivotal role in immobilizing P, thereby influencing soil fertility and the filter function of soil to protect freshwater systems from eutrophication. Competitive anions, such as organic matter (OM), significantly affect the strength of this P-binding, eventually controlling P mobility and release, but surprisingly, these processes are insufficiently understood at the molecular level. In this study, we provide a molecular-level perspective on the influence of OM on P binding at the goethite-water interface using a combined experimental-theoretical approach. By examining the impact of citric acid (CIT) and histidine (HIS) on the adsorption of orthophosphate (OP), glycerol phosphate (GP), and inositol hexaphosphate (IHP) through adsorption experiments and molecular dynamics simulations, we address fundamental questions regarding P binding trends, OM interaction with the goethite surface, and the effect of OM on P adsorption. Our findings reveal the complex nature of P adsorption on goethite surfaces, where the specific OM, treatment conditions (covering the surface with OM or simultaneous co-adsorption), and initial concentrations collectively shape these interactions. P adsorption on goethite exhibits a binding strength increasing in the order of GP < OP < IHP. Crucially, this trend remains consistent across all adsorption experiments, regardless of the presence or absence of OM, CIT, or HIS, and irrespective of the specific treatment method. Notably, OP is particularly susceptible to inhibition by OM, while adsorption of GP depends on specific OM treatments. Despite being less sensitive to OM, IHP shows reduced adsorption, especially at higher initial P concentrations. Of significance is the strong inhibitory effect of CIT, particularly evident when the surface is pre-covered, resulting in a substantial 70 % reduction in OP adsorption compared to bare goethite. The sequence of goethite binding affinity to P and OM underscores a higher affinity of CIT and HIS compared to OP and GP, suggesting that OM can effectively compete with both OP and GP and replace them at the surface. In contrast, the impact of OM on IHP adsorption appears insignificant, as IHP exhibits a higher affinity than both CIT and HIS towards the goethite surface. The coverage of goethite surfaces with OM results in the blocking of active sites and the generation of an unfavorable electric potential and field, inhibiting anion adsorption and consequently reducing P binding. It is noteworthy that electrostatic interactions predominantly contribute more to the binding of P and OM to the surface compared to dispersion interactions.

Modeling of phosphate speciation on goethite surface: Effects of humic acid

Iron (hydr)oxides and humic acid (HA) are important active components in soils and usually coexist in the environment. The effects of HA on the adsorption and subsequent immobilization of phosphate on iron (hydr)oxide surface are of great importance in studies of soil fertility and eutrophication. In this study, two types of goethite with different particle sizes were prepared to investigate the phosphate adsorption behaviors and complexation mechanisms in the absence or presence of HA by combining multiple characterization and modeling studies. The adsorption capacity of micro- (M-Goe) and nano-sized goethite (N-Goe) for phosphate was 2.02 and 2.04 μmol/m2, which decreased by ∼25% and ∼45% in the presence of 100 and 200 mg/L HA, respectively. Moreover, an increase in equilibrium phosphate concentration significantly decreased the adsorption amount of goethite for HA. Charge distribution-multisite surface complexation (CD-MUSIC) and natural organic matter-charge distribution (NOM-CD) modeling identified five phosphate complexes and their corresponding affinity constants (logKP). Among these phosphate complexes, FeOPO2OH, (FeO)2PO2, and (FeO)2POOH species were predominant complexes on the surface of both M-Goe and N-Goe across a wide range of pH and initial phosphate concentrations. The presence of HA had little effect on the coordination mode and logKP of phosphate on goethite surface. These results and the obtained model parameters shed new lights on the interfacial reactivity of phosphate at the goethite-water interface in the presence of HA, and may facilitate further prediction of the environmental fate of phosphate in soils and sediments.

Structural and mechanistic study of antimonite complexation with organic ligands at the goethite-water interface

Antimony is a re-emerging contaminant, and its complexation with natural organic matter is rising to ever-increasing levels due to global climate change, which has far-reaching impacts on its environmental fate and mobility. A molecular-level understanding of the interactions between Sb(III) and organic ligands at the solid-liquid interface is of paramount importance in deciphering the effect of these organic ligands. Herein, we identified and characterized Sb(III)-organic ligand complexes in solution and at the goethite-water interface using complementary techniques. The FT-ICR MS, XANES, and DFT calculations show that organic ligands bind Sb(III) through nucleophilic functional groups, such as -COO-, -OH and -HS. The formation of surface ternary Sb(III)-bridging complexes retarded the Sb(III) surface precipitation starting from 3.8mg-Sb/L to a much higher level at 8.3-13.5mg-Sb/L. The strong bond between Sb(III) and organic ligands is the key factor to inhibit Sb(III) adsorption, surface precipitation and oxidation under sunlight irradiation. Our results showed the chemical basis for the multifaceted functions of organic ligands in stabilizing trace metalloids such as Sb(III) in the environment.

Microstructure of Al-substituted goethite and its adsorption performance for Pb(II) and As(V)

Naturally occurring goethite commonly undergoes Al-substitution, while how changes in microstructure induced by Al-substitution affect the interactive reaction of Pb(II) or As(V) at the goethite-water interface remains poorly understood. This study reveals the structural properties of Al-substituted goethite and its adsorption behavior for Pb(II) and As(V) by multiple characterization techniques and Charge Distribution-Multisite Surface Complexation (CD-MUSIC) modeling. Al-substitution caused an obvious decrease in the length-to-width ratio in goethite particles and a slight decrease in the proportion of (110) facets. The presence of Al-O sites and higher surface roughness induced by Al-substitution contributed to a higher inner Stern layer capacitance (C1) and surface charge density of goethite. CD-MUSIC modeling results further revealed that the affinity constant of Pb(II) complex (log KPb) at the goethite-water interface and the adsorption capacity of goethite for Pb(II) decreased with increasing amount of Al-substitution, while an opposite tendency was observed for As(V) adsorption. The dominant species of both Pb(II) and As(V) on goethite were bidentate complexes, and Al-substitution had a minor impact on the abundance of Pb(II) and As(V) complexes on the surface of goethite. Overall, these experimental and modeling results provide new and important insights into the interfacial reactivity of Al-substituted goethite and facilitate the prediction of the environmental fate of heavy metals.

Molecular level picture of the interplay between pH and phosphate binding at the goethite-water interface.

The soil pH plays a substantial role in controlling phosphorus (P) adsorption and mobilization. These processes are strongly affected by the phosphate interaction strength with P-fixing soil minerals such as goethite. The target of the current contribution is to draw a molecular level picture of the interplay between pH and phosphate binding at the goethite-water interface via a joint experimental-theoretical approach. Periodic density functional theory (DFT) calculations were carried out to provide a molecular level understanding of the pH dependence of P adsorption. To validate the modeling approach, adsorption experiments of phosphate at goethite were performed in the pH range of 4-12. There was agreement between experiments and simulations in the description of the adsorption behavior by two pH-dependent successive stages. The adsorption increases along the pH change from 4 to 8. A further increase of pH leads to a decrease of adsorption. By comparing with literature data it is concluded that the first stage will be observed only if there is no significant change of the surface charge at low pH. Moreover, the molecular modeling results point to the abundance of the monodentate (M) binding motif at both extremely low and high pH ranges. Otherwise, the bidentate (B) one is predominant along the intermediate pH range. These observations could resolve the existing debate about the assignment of phosphate-goethite binding motifs. Furthermore, the results point to a decrease of pH upon phosphate sorption due to an induced acidification of soil solution. The present joint experimental-theoretical approach provides a better understanding and description of the existing phosphate sorption experiments and highlights new findings at the atomistic/molecular scale.

Surface Sorption Site and Complexation Structure of Ca2+ at the Goethite-Water Interface: A Molecular Dynamics Simulation and Quantitative XANES Analysis.

The molecular-level surface complexation structure of Ca2+ at the goethite/water interface remains unclear. We investigated the sorption of Ca2+ on the surfaces of goethite using classical molecular dynamics (MD) simulations and Ca K-edge X-ray absorption near edge structure (XANES) spectroscopy. The XANES results showed that Ca2+ was sequestered by goethite dominantly via sorption at pH ≤ 9, whereas the Ca(OH)2 surface precipitate formed at pH 10 for an initial Ca2+ concentration of 2mM. The MD simulations showed that Ca2+ dominantly absorbed on the (100) and (110) surfaces of goethite via bidentate binuclear complexation by forming ≡(Fe-OH)2Ca2+·5H2O species, whereas little Ca2+ adsorbed on the (021) surface. The theoretical Ca K-edge XANES spectrum calculations gave a mean Ca-O interatomic distance of 2.34Å (2.23-2.55Å) and a Ca-Fe interatomic distance of 3.80Å (3.79-3.81Å) at goethite/water interface. Our results may shed some light on the geochemical cycling of calcium and other related cations and anions.

Understanding major NOM properties controlling its interactions with phosphorus and arsenic at goethite-water interface

Among natural organic matter (NOM), oxyanions and metal (hydr)oxides, a complicated interaction exists in natural aquatic and terrestrial systems and in waste waters. Effects of seven types of NOM (four humic acids (HA), three fulvic acids (FA)) that vary in properties on the adsorption of oxyanions, including phosphate, arsenate and arsenite, at goethite-water interface were quantitatively studied. Results show that the adsorption of oxyanions to goethite is decreased by the presence of NOM, especially for phosphate and arsenate at low pH. In general, the effects of the three FA are similar, which are more effective than HA in reducing oxyanion adsorption at low pH (<6). Differences were observed between the four HA in their competition with oxyanions. The adsorption of phosphate, arsenate and arsenite in the presence of NOM are well described with both the NOM-CD (CD: Charge Distribution) and LCD (Ligand and Charge Distribution) model. The NOM-CD model is relatively simple to use, whereas the LCD model can better reveal different factors in the interaction, including the spatial distribution of adsorbed NOM on oxide surface. According to these two models: site density of carboxylic groups, protonation constant of carboxylic groups, and particle size of NOM are major properties of NOM determining its effect on oxyanion adsorption to oxides. At relatively low loadings, morphological change of adsorbed NOM takes place, and the degree of morphological change of adsorbed NOM depends on the particle size, site density of carboxylic groups and aromaticity of NOM. The influence of particle size on the interaction becomes more important at higher NOM loadings. The results suggested that the fixation or removal efficiency of phosphate, arsenate and arsenite with iron oxides (e.g. goethite) can be significantly decreased by the presence of NOM, especially when NOM rich in acidic and aromatic groups.

Infrared spectroscopic characterization of phosphate binding at the goethite-water interface.

The interaction between phosphates and soil mineral surfaces, such as Fe- and Al-(oxyhydr)oxides, plays a crucial role in the immobilization of P and thus its availability for plants. The reactions of phosphates with Fe-hydroxides and especially goethite have been studied extensively. But a molecular-level picture of the phosphate binding mechanisms at the goethite-water interface is still lacking. Therefore, in the current contribution we have explored the molecular binding mechanisms for the adsorbed phosphate at the goethite-water interface by performing sorption kinetics experiments for orthophosphate and characterizing the adsorbed species by FT-IR spectroscopy. In parallel, periodic DFT calculations have been performed to explore the interaction mechanisms and to assign the IR spectra for monodentate (M) and bidentate (B) orthophosphate complexes at two different goethite surface planes (010 and 100) in the presence of water. In general, our interaction energy results give evidence that the mono-protonated B phosphate complex is favored to be formed at the goethite-water interface, although the M motif could exist as a minor fraction. Moreover, it was found that water plays an important role in controlling the phosphate adsorption process at the goethite surfaces. The interfacial water molecules form H-bonds (HBs) with the phosphate as well as with the goethite surface atoms. Furthermore, some water molecules form covalent bonds with goethite's Fe atoms while others dissociate at the surface to protons and hydroxyl groups. The present theoretical assignment of IR spectra introduces a benchmark for characterizing experimental IR data for the adsorbed KH2PO4 species at the goethite-water interface. In particular, the IR spectra of the mono-protonated (2O + 1Fe) B complex at the 010 goethite surface plane and the M complex at the 100 goethite surface plane were found to be consistent with the experimental data. In order to explore the role of different abundances of surface planes and binding motifs, IR spectra obtained from weighted averages have been analyzed. The results confirmed the conclusions drawn from interaction energy calculations.

Kinetics of Uranyl Peroxide Nanocluster (U60) Sorption to Goethite.

The unique properties of uranium-based nanomaterials may significantly impact our current understanding of the fate and transport of U(VI) in environmental systems. Sorption of the uranyl peroxide nanocluster [(UO2)(O2)(OH)]6060- (U60) to goethite (α-FeOOH) was studied using batch sorption experiments as a function of U60 concentration (0.5-2 g·L-1), mineral concentration (100-500 m2·L-1), and pH (8-10). The resulting rate law describing U60 interactions with goethite at pH 9 was R = - krxn[U60]0.29±0.02[goethite]1.2±0.1 where krxn = (6.7 ± 2.0) × 10-4 (g·L-1)0.71±0.02(m2·L-1)-1.2±0.1(day-1). The largest fraction of U60 removed from solution was at pH 8, which is below the isoelectric point of the goethite used in this study. Site density calculations suggest that U60 may exist on the goethite surface at a center-to-center distance of 5.4-6.5 nm, depending upon pH, which mirrors the center-to-center distance observed in the aqueous phase near the U60 solubility limit. At pH 10, approximately 20% uranium was desorbed within 3 days. Analysis of the reacted mineral surface using X-ray photoelectron spectroscopy confirmed the presence of a single U(VI) species on the mineral surface, and electrospray ionization mass spectrometry revealed that U60 remains intact during the sorption and desorption processes. These results demonstrate that the behavior of U60 at the goethite-water interface is similar to that of discrete U(VI) but is governed by different sorption mechanisms and reaction kinetics, which has the potential to alter our current understanding of the fate and transport of uranium species in the environment.

Competitive adsorption of magnesium and calcium with phosphate at the goethite water interface: Kinetics, equilibrium and CD-MUSIC modeling

In natural environments, competitive interactions of ions with charged particles often control bioavailability and mobility of ions. In these systems, chemical reactions are often in a semi-equilibrium state; thus, along with equilibrium data, kinetic data are of great importance in predicting ion bioavailability and mobility. Therefore, in this research, kinetics and equilibrium adsorption interactions of magnesium, calcium and phosphate on goethite are investigated as a function of time, pH, ion loading and ionic strength in single and binary ion systems. The thermodynamically valid model parameters, obtained from single ion systems by applying the CD-MUSIC model, are used to predict ion interactions in more complex systems, i.e. binary ion systems. Results indicate that the kinetics of phosphate and magnesium adsorption is pH-dependent and electrostatic attractive and repulsive forces dominantly control the reaction. Thus, adsorption of magnesium and calcium influences significantly both the amount of adsorbed phosphate and the reaction time, and vice versa. Therefore, the equilibration time for phosphate and magnesium adsorption on goethite is approximately ≤1h when electrostatic forces are attractive and approximately 24h when electrostatic forces are repulsive. Variation in ionic strength (0.01–0.3M NaCl) has no significant effect on the magnesium and calcium adsorption, suggesting a large difference in the adsorption energy of Ca and Mg compared to Na, which is very likely due to the difference in their adsorption mechanism. Based on the model calculations, formation of two inner sphere surface complexes, as (FeOH)2Mg and (FeOH)2MgOH, is the dominant mechanism for magnesium adsorption. For calcium, however, a monodentate outer sphere surface complex dominates its adsorption at acid pH range and a monodentate inner sphere complex at basic pH range (FeOHCa). This difference results in a different amount of charge on electrostatic planes that influences strongly phosphate adsorption such that phosphate adsorption is significantly higher in Ca–PO4 systems than in Mg–PO4 systems.

Effect of soil fulvic and humic acid on binding of Pb to goethite–water interface: Linear additivity and volume fractions of HS in the Stern layer

The effects of soil fulvic (JGFA) and humic acid (JGHA) on Pb binding to goethite were investigated with batch experiments and modeling. The CD-MUSIC and NICA-Donnan model could describe the Pb binding to, respectively, the binary Pb–goethite and Pb–HS systems. The adsorption of humic substances (HS) on goethite strongly depended on pH and was promoted by Pb binding. The mass amount of adsorbed JGFA (mg/g) was smaller than that of JGHA, but when expressed in number of particles/nm2 the JGFA adsorption was higher. At low pH and low initial Pb concentration the linear additivity rule always underestimated Pb adsorption to goethite–HS complex, which was caused by the strong effect of adsorbed HS on the electrostatic potentials in the Stern layer region. At other conditions except the 450mg/L JGHA in the 0.5mmol/L Pb system the additivity rule predicted the experimental results reasonably well, and at high pH nearly all Pb is bound to goethite. At the same mass adsorbed, the effect of JGFA on Pb adsorption to goethite is stronger than that of JGHA, due to the fact that the JGFA particles were primarily adsorbed in the Stern layer, whereas JGHA particles were present in both Stern layer and diffuse layer. Therefore, the electrostatic potential profile of the goethite-JGFA complex is considerably different from that of goethite-JGHA complex and affects Pb binding strongly.

Surface complexation modeling of Cr(VI) adsorption at the goethite–water interface

In this study, a charge distribution multisite surface complexation model (CD–MUSIC) for adsorption of chromate onto goethite was carefully developed. The adsorption of Cr(VI) on goethite was firstly investigated as a function of pH, ionic strength and Cr(VI) concentration. Results showed that an inner-sphere complexation mechanism was involved because the retention of Cr(VI) was little influenced by ionic strength. Then two surface species: a bidentate complex (Fe2O2CrOOH) and a monodentate complex (FeOCrO3−3/2), which is constrained by prior spectroscopic evidence were proposed to fit the macroscopic adsorption data. Modeling results showed that the bidentate complex was found to be the dominant species at low pH, whereas, with increasing pH, monodentate species became more pronounced. The model was then verified by prediction of competitive adsorption of chromate and phosphate at various ratios and ionic strengths. The model successfully predicted the inhibition of chromate with the presence of phosphate, suggesting phosphate has higher affinity to goethite surface than Cr(VI). Results showed that the model developed in this study for Cr(VI) onto goethite was applicable for various conditions. It is a useful supplement for the surface complexation model database for oxyanions onto goethite surfaces.

X-ray Absorption Spectroscopic Investigation of Molybdenum Multinuclear Sorption Mechanism at the Goethite−Water Interface

Understanding in situ metalloid surface speciation on mineral surfaces is critical to predicting the bioavailability in surface and subsurface environments. In this study, Mo K-edge X-ray absorption spectroscopy (XAS) was used to elucidate Mo(VI) surface speciation at the goethite-water interface. Effects of pH and loading levels were investigated. X-ray absorption near edge structure (XANES) analysis indicated that the Mo(VI) coordination environment changes from tetrahedral to octahedral with decreasing pH. At near neutral pH, Mo(VI) predominantly remains as tetrahedral molecules, forming inner-sphere surface species via corner- and edge-sharing attachment with iron octahedral structures (interatomic distance of Mo-Fe (R(Mo-Fe)) at ∼2.8 and ∼3.4 Å, respectively). In contrast, a mixture of surface species comprising tetrahedrally and octahedrally coordinated Mo(VI) exists at pH ∼3-4. While the same Mo(VI) tetrahedral surface species are present at acidic pH, there was an additional MoO(6) polymer attachment on iron octahedral structures, resulting in a R(Mo-Fe) at 3.53 Å. The coordination number (CN) of a Mo-Mo backscatterer gradually increased with increasing loading level, suggesting the formation of surface polymerization. Overall, there seems to be a transition from Mo(VI) tetrahedral to octahedral coordination environment with decreasing pH. The XAS findings further support a Mo(VI) inner-sphere adsorption mechanism that was previously suggested in the pressure-jump relaxation study by Zhang and Sparks (Soil Sci. Soc. Am. J. 1989, 53 (4), 1028-1034). pH-Dependent multinuclear Mo(VI) surface speciation may be important in predicting Mo(VI) transport process in the soil-water environment.

Hydroxynaphthoic acid isomer sorption onto goethite

This study used batch and attenuated total reflectance–Fourier-transform infrared (ATR-FTIR) flow-through techniques, along with computational chemistry, to probe the sorption of hydroxynaphthoic acid (HNA) isomers at the goethite–water interface. The HNA isomers employed in this study, 1-hydroxy-2-naphthoic acid and 2-hydroxy-3-naphthoic acid, possessed an intramolecular hydrogen bond (IHB) between their carboxyl and hydroxyl groups, which resulted in coupled vibrational modes. Complimentary information from batch and ATR-FTIR studies suggested surface complexation via a bidentate structure, involving both the carboxylate and phenolate groups, as the dominant mode of sorption. A secondary HNA surfaces species noted only at pH 4 appeared to sorb via the carboxylate group, with the phenolic group involved in IHB or H-bonded to the solvent or surface hydroxyl groups. Despite the lack of unique vibrational modes for the key functional groups our experimental approach was successful in proposing interfacial structures, while acknowledging the limitations with respect to differentiating mono- vs. binuclear complexes. Finally, the spectral similarity of HNA sorbed onto goethite and onto the clay fraction of an iron oxide-rich soil suggested analogous solute interaction in pure phase minerals and soils.

Periodic density functional theory calculations of bulk and the (010) surface of goethite

BackgroundGoethite is a common and reactive mineral in the environment. The transport of contaminants and anaerobic respiration of microbes are significantly affected by adsorption and reduction reactions involving goethite. An understanding of the mineral-water interface of goethite is critical for determining the molecular-scale mechanisms of adsorption and reduction reactions. In this study, periodic density functional theory (DFT) calculations were performed on the mineral goethite and its (010) surface, using the Vienna Ab Initio Simulation Package (VASP).ResultsCalculations of the bulk mineral structure accurately reproduced the observed crystal structure and vibrational frequencies, suggesting that this computational methodology was suitable for modeling the goethite-water interface. Energy-minimized structures of bare, hydrated (one H2O layer) and solvated (three H2O layers) (010) surfaces were calculated for 1 × 1 and 3 × 3 unit cell slabs. A good correlation between the calculated and observed vibrational frequencies was found for the 1 × 1 solvated surface. However, differences between the 1 × 1 and 3 × 3 slab calculations indicated that larger models may be necessary to simulate the relaxation of water at the interface. Comparison of two hydrated surfaces with molecularly and dissociatively adsorbed H2O showed a significantly lower potential energy for the former.ConclusionSurface Fe-O and (Fe)O-H bond lengths are reported that may be useful in surface complexation models (SCM) of the goethite (010) surface. These bond lengths were found to change significantly as a function of solvation (i.e., addition of two extra H2O layers above the surface), indicating that this parameter should be carefully considered in future SCM studies of metal oxide-water interfaces.

Studies of Cd(II)–sulfate interactions at the goethite–water interface by ATR-FTIR spectroscopy

A combination of macroscopic experiments and in situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy was used to study Cd(II)–sulfate interactions on the goethite–water interface. The presence of SO 4 dramatically promoted Cd adsorption at lower pH (pH 5.5–6.5) and had a smaller effect at higher pH. ATR-FTIR studies indicated sulfate adsorption on goethite occurred via both outer- and inner-sphere complexation. The relative importance of both complexes was a function of pH and sulfate concentration. ATR-FTIR spectra provided direct evidence of the formation of Cd–SO 4 ternary surface complexes on goethite. In addition to ternary complexes, Cd specifically sorbed on goethite promoted SO 4 adsorption via changing the surface charge, and caused additional SO 4 adsorption as both inner- and outer-sphere complexes. The relative importance of ternary complexes versus electrostatic effects depended upon pH values and Cd concentration. Ternary complex formation was promoted by low pH and high Cd levels, whereas electrostatic effects were more pronounced at high pH and low Cd levels. A portion of SO 4 initially sorbed in inner-sphere complexes in the absence of Cd was transformed into Cd–SO 4 ternary complexes with increased Cd concentration.

Kinetics of zinc and arsenate co-sorption at the goethite–water interface

Little or no information is available in the literature about reaction processes of co-sorbing metals and arsenate [As(V)] on variable-charged surfaces or factors influencing these reactions. Arsenic and metal contamination are, however, a common co-occurrence in many contaminated environments. In this study, we investigated the co-sorption kinetics of 250 μM As(V) and zinc [Zn(II)] in 10, 100, and 1000 mg goethite L −1 0.01 M NaCl solution at pH 7, collected complementary As and Zn K-edge extended X-ray absorption fine structure (EXAFS) data after various aging times, and performed a replenishment desorption/dissolution study at pH 4 and 5.5 after 6 months of aging time. Arsenate and Zn(II) formed adamite-like and koritnigite-like precipitates on goethite in 100- and 10-ppm goethite suspensions, respectively, whereas in 1000-ppm goethite suspensions, As(V) formed mostly double-corner sharing complexes and Zn(II) formed a solid solution on goethite according to EXAFS spectroscopic analyses. In all goethite suspension densities, surface adsorption reactions were part of the initial reaction processes. In 10- and 100-ppm goethite suspensions, a heterogeneous nucleation reaction occurred in which adamite-like precipitates began to form 48 h earlier than koritnigite-like surface precipitates. Arsenate and Zn(II) uptake from solution decreased after 4 weeks. Replenishment desorption studies showed that the precipitates and surface adsorbed complexes on goethite were susceptible to proton-promoted dissolution resulting in many cases in more than 80% loss of Zn(II) and ∼ 60% to 70% loss of arsenate. The molar Zn:As dissolution ratio was dependent on the structure of the precipitate and was cyclic for the adamite and koritnigite-like surface precipitates, reflecting the concentric and plane-layered structures of adamite and koritnigite, respectively.

Effects of pH and ionic strength on the adsorption of phosphate and arsenate at the goethite–water interface

The surface properties of a well-crystallized synthetic goethite have been studied by acid–base potentiometric titrations, electrophoresis, and phosphate and arsenate adsorption isotherms at different pH and electrolyte concentrations. The PZC and IEP of the studied goethite were 9.3 ± 0.1 and 9.3 ± 0.2 , respectively. Phosphate and arsenate adsorption decrease as the pH increases in either 0.1 or 0.01 M KNO 3 solutions. Phosphate adsorption is more sensitive to changes in pH and ionic strength than that of arsenate. The combined effects of pH and ionic strength result in higher phosphate adsorption in acidic media at most ionic strengths, but result in lower phosphate adsorption in basic media and low ionic strengths. The CD-MUSIC model yields rather good fit of the experimental data. For phosphate it was necessary to postulate the presence of three inner-sphere surface complexes (monodentate nonprotonated, bidentate nonprotonated, and bidentate protonated). In contrast, arsenate could be well described by postulating only the presence of the two bidenate species. A small improvement of the arsenate adsorption data could be achieved by assuming the presence of a monodentate protonated species. Model predictions are in agreement with spectroscopic evidence, which suggest, especially for the case of arsenate, that mainly bidentate inner-sphere complexes are formed at the goethite–water interface.

Reverse Monte Carlo simulations of EXAFS spectra: application to the sorption of gold on iron oxy-hydroxides

The sorption of Au at the goethite-water interface is studied. In order to better understand sorption models for cations on mineral surfaces, reverse Monte-Carlo (RMC) simulations were used to analyse EXAFS spectra. A starting model is constructed based on the (110) mineral surface separated by a layer of water molecules. Some of metal cations at the mineral-water interface are replaced by absorbed gold cations. All atomic positions are allowed to vary on both the mineral surface and in the water layer during the RMC simulations. Partial pair distribution functions and corresponding EXAFS spectra are computed and refined by a Metropolis algorithm. The final RMC model matches most EXAFS features (including short- and medium-range environments). The usual EXAFS parameters (coordination number, coordination distance and Debye–Waller factor and higher moments) are retrieved from the partial pair distribution functions by moment analysis. In case of Au at the the goethite-water interface four Au–O coordinations are found at an average distance of 1.99 Å with a second moment of 0.0013 Å2. Two iron next-nearest neighbors show a broad distribution between 3.2 and 3.8 Å with an average coordination distance of 3.49 Å, a coordination number of 3 and a second moment of 0.013 Å2. The results of the RMC simulations are compared with results of conventional EXAFS analysis methods.

Interactions of calcium and fulvic acid at the goethite-water interface

Interactions of calcium and fulvic acid (Strichen ) with the surface of goethite were studied with batch and titration experiments. The mutual influence of the interactions on the adsorption of fulvic acid, calcium ions and protons were examined. Adsorption of the fulvic acid to goethite decreased with increase in pH (pH range 3–11). Addition of Ca (1.0 mM) at intermediate and high pH significantly enhanced the adsorption of fulvic acid. Compared to the adsorption to pure goethite, the presence of fulvic acid enhanced the adsorption of Ca significantly. In comparison to the simple linear sum of Ca bound to fulvic acid and goethite, the interactions between goethite and fulvic acid led to a reduced adsorption of Ca at low pH and an enhanced adsorption at high pH. With the adsorption of fulvic acid, protons were released at low pH and coadsorbed at high pH. When Ca was added, fewer protons were released at low pH and fewer coadsorbed at high pH. The experimental results can be adequately described using a surface complexation model, the Ligand and Charge Distribution (LCD) model, in which the CD-MUSIC model for ion adsorption to mineral oxides and the NICA model for ion binding to humics are integrated. In the model calculations, adequate descriptions of the ternary system data (Ca-fulvic acid-goethite) were obtained with parameters derived from three binary systems (fulvic acid-goethite, Ca-goethite and Ca-fulvic acid) without further adjustment. The model calculations suggest that the interactions between Ca and fulvic acid at the surface of goethite are mainly due to the electrostatic effects.