Phosphate Surface Complexes Research Articles

Highly selective and efficient Pb2+ capture using PO4-loaded 3D-NiFe layer double hydroxides derived from MIL-88A

Lead (Pb) contamination in water requires improved decontamination technologies. The addition of phosphate to precipitate Pb2+ is a widely used method for remediating Pb in soil and water, though it has certain limitations. This study focuses on novel 3D mesoporous layered double hydroxide (LDH) sorbents functionalized with phosphate anions for Pb2+ removal from contaminated waters. Our innovative strategy involves converting a sacrificial template metal-organic frameworks (MOFs) structure (MIL-88A(Fe)) into NixFe LDH, followed by an anion exchange reaction with phosphate anions. This process preserves the 3D microrod architecture of MIL-88A and prevents deleterious LDH particle aggregation. The synthesis results in stable microrod crystals, 1–2 μm long, composed of 3D assemblies of NixFe-PO4 LDH nanoplatelets with a specific surface area exceeding 110 m2/g. The novel LDH materials display fast adsorption kinetics (pseudo-second order model) and remarkably high Pb2+ removal performances (Langmuir isotherm model) with a capacity of 538 mg/g, surpassing other reported adsorbents. LDH-PO4 exhibits high selectivity for Pb2+ over competing ions like Ni2+ and Cd2+ (selectivity order is: Pb2+ > Ni2+ > Cd2+). Removal of Pb2+ from NixFeLDH/88A-PO4 involves various mechanisms, including surface complexation and surface precipitation of lead phosphate or lead hydroxide phases as revealed by structural characterization techniques.

Speciation studies at the Illite - solution interface: Part 2 – Co-sorption of uranyl and phosphate ions

Although it is well known that clays and phosphate ions (hereafter referred to as “P”) can contribute to the long term uptake of uranium (U) in a variety of geochemical systems, work is still needed to document the surface speciation of U(VI) and P sorbed on relevant clay minerals such as illite. Following on from previous work showing strong sorption of phosphate ligands on the surface of a homoionic Na-illite ([1]), we addressed the (competitive/synergistic) mechanisms of (co)sorption of trace levels of uranyl ions (1–10 µM) and phosphate ligands (100 µM) onto this clay, and the identity of the surface species formed, by in situ spectroscopy monitoring of the clay-solution interface along sorption. We also performed complementary batch sorption experiments and electrophoretic mobility (EM) measurements. Macroscopic data indicated a uranyl sorption dependent on pH, aqueous U concentration and clay-to-solution ratio, a signature of P on the U sorption, and a promotion of P sorption with U concentration. Macroscopic and EM data also suggested mostly reversible mechanisms of P and U co-sorption that confer negative charges on the mineral surface and involve several types of uranyl phosphate surface species and/or surface sites present on the clay edges. FTIR interface spectra confirmed that several types of inner sphere uranyl phosphate surface species formed at acidic pH at the illite-solution interface, as a function of U surface coverage and/or reaction time. An inner-sphere uranyl phosphate surface complex, likely with a U-bridging structure, was formed rapidly on high-affinity surface sites existing in limited quantities on the clay edges. Another U-P surface complex (with a possible P-bridging structure) was progressively formed in increasing amounts with U surface coverage and time on low affinity clay edge sites. Finally, a third U-P surface species having an autunite-like structure (probably a U-P polynuclear surface species) was formed on the illite surface at high U concentration (10 µM). The two later species competed with the existence on the illite surface of inner sphere surface complexes and outer-sphere surface complex of phosphate, which were identified in the absence of U. Information on uranyl surface speciation in the presence of phosphate ligands presented here is novel and provides for the first time in situ spectroscopic evidence of synergistic U-P sorption process leading in the formation of several types of uranyl phosphate sorption species on the surface of illite. These results provide a sound basis for better understanding / prediction of U sorption in the environment, including in clayey formations being considered as long term barriers to radionuclide migration in far field of high level radioactive waste repository.

Integrating Density Functional Theory Modeling with Experimental Data to Understand and Predict Sorption Reactions: Exchange of Salicylate for Phosphate on Goethite

Density functional theory (DFT) calculations are a quantum mechanical approach that can be used to model chemical reactions on an atomistic scale. DFT provides predictions on structures, thermodynamics, spectroscopic parameters and kinetics that can be compared against experimentally determined data. This paper is a primer on the basics of utilizing DFT for applications in mineral-water interfaces. In our case-study, we use DFT to model the surface complexes of phosphate and salicylate adsorbed onto the (101) and (210) surfaces of α-FeOOH (goethite), as an example of combining DFT and experiment. These three components are important in the phosphorus-organic matter interactions in soils, and by comparing the energies of the two surface complexes, the exchange energy of salicylate for phosphate onto goethite can be estimated. The structures of the surface complexes are predicted and the resulting vibrational frequencies calculated based on these structures are compared to previous observations. Upon verification of reasonable surface complex models, the potential energy of exchanging salicylate for phosphate is calculated and shown to be significantly exothermic. This model result is consistent with observations of plant exudates, such as salicylate freeing adsorbed phosphate in soils under P-limited conditions.

Attenuated total reflectance–Fourier transform infrared study of the effects of citrate on the adsorption of phosphate at the hematite surface

AbstractCarboxylic acids affect the retention and solubility of phosphate in soils and rhizospheres, but spectroscopic studies addressing the mechanisms involved are scarce. Here, we used in situ attenuated total reflectance Fourier transform infrared spectroscopy to study the mechanisms of adsorption of orthophosphate and citric acid onto hematite in binary and ternary systems at pH 4.5, 5.5, and 6.5. In binary systems, phosphate adsorbed as a mixture of two inner‐sphere species assigned as monodentate and bridging complexes, with the fraction of bridging species increasing with decreasing pH. Citrate coordinated to the hematite surface through the carboxylate groups, which consisted of a mixture of inner‐sphere complexes and either noncoordinated or outer‐sphere complexes, with inner‐sphere complexation becoming more important at lower pH. Introduction of citrate into pre‐equilibrated phosphate–hematite systems induced desorption of both the monodentate and bridging phosphate species, with competitive effects increasing with decreasing pH. At pH 5.5 and 6.5, the bridging complexes were mobilized to a larger extent than initially present on the surface, indicating preferential desorption. Similarly, pre‐equilibration of hematite with citrate preferentially inhibited the formation of the bridging phosphate surface complexes. Their relatively strong susceptibility to interference by citrate suggests that the bridging phosphate species are protonated monodentate complexes engaged in hydrogen bonding to adjacent surface sites. The results of this study refine our understanding of the mechanisms of phosphate retention at Fe‐oxide mineral surfaces and demonstrate that carboxylic acids may affect not only phosphate solubility but also the speciation of phosphate associated with Fe‐oxides in soil and rhizosphere environments.

Insights into the adsorption mechanism of Al30 polyoxocations-modified graphene oxide nanosheets for efficient removal of phosphate, chromate and selenate oxyanions: A comparative study

Keggin-type aluminum polyoxocation species, Al30, with interesting features resulting from a variety of surface oxygen functional groups, has high adsorption ability of different species so it can be served either as nanohybrids or singly in water purification. Insight into surface complex structures of various oxyanions with different activities on Al30 is fundamental for both mechanism and application in wastewater treatment. Reported here is an adsorption behavior investigation of three environmentally problematic oxyanions (phosphate, chromate, and selenate) with different activities on Al30. For this purpose, firstly, Al30 species were consolidated on graphene oxide to use as the sorbent material. Performance of the proposed sorbent evaluated by kinetics and isotherm studies was representative of good agreement of adsorption data with Langmuir isotherm model and pseudo-second-order kinetics for all oxyanions. The underlying molecular binding mechanisms of these oxyanions onto Al30 were revealed by FT-IR spectroscopy, desorption studies, TGA, as well as studying adsorption trends in different pH values. The obtained results boded predominant formation of inner-sphere surface complexes of phosphate and chromate and common contribution of inner- and outer-sphere complexes for selenate, increasing the mobility of selenate. Competitive adsorption studies exhibited a more adsorption affinity and selectivity of Al30 towards phosphate than chromate and selenate, while this was contrary to the result of the single-ion adsorption case, where less adsorption capacity was obtained for phosphate than the other ones.

Mechanisms of phosphate removal from aqueous solution by zero-valent iron: A novel kinetic model for electrostatic adsorption, surface complexation and precipitation of phosphate under oxic conditions

Mechanisms of phosphate removal from aqueous solution by zero-valent iron (ZVI) were examined. Solution pH largely affected phosphate removal efficiency. The phosphate removal decreased from 96.4 to 48.4% with increasing pH from 3 to 4.5 and subsequently increased with further increase in pH. The maximum removal efficiency of 97.3% was obtained at pH 7. When pH was higher than 7, the removal efficiency decreased with increasing pH. At pH 11 the phosphate removal was drastically reduced to 8.6%. To quantify effect of pH on mechanisms of phosphate removal by ZVI, i.e., contributions of adsorption on ZVI surface and precipitation in solution to phosphate removal, the fractions of removals by adsorption and precipitation were experimentally defined and a novel phenomenological kinetic model was developed. In the modelling, electrostatic adsorption, surface complexation and precipitation of phosphate were taken into account as predominant mechanisms of phosphate removal by ZVI. Their contributions linked with each other varied with solution pH. Since the increase in pH resulted in the decrease of ZVI dissolution causing the deceleration of elution of Fe ions from ZVI, the precipitation in solution was inhibited at higher pH. For pH < ∼9 (the isoelectric point of ZVI), with increasing pH the renewal of ZVI surface was suppressed and the active-sites for phosphate adsorption onto ZVI surface increased. As a result, the overall adsorption onto ZVI surface or the sum of electrostatic adsorption and surface complexation of phosphate on ZVI surface dominated phosphate removal. The proposed kinetic model could successfully predict the contributions of overall adsorption and precipitation to phosphate removal by ZVI in the range of pH from 3 to 11 with the average relative deviations of 7.3 and 15.1%, respectively.

Effect of humic acid preloading on phosphate adsorption onto zirconium-modified zeolite.

A zirconium-modified zeolite (ZrMZ) was prepared, and then, humic acid (HA) was immobilized on the ZrMZ surface to prepare HA-loaded ZrMZ (HA-ZrMZ). The obtained ZrMZ and HA-ZrMZ were characterized by energy dispersive X-ray spectroscopy, elemental analyzer, N2 adsorption/desorption isotherms, pH at the point of zero charge, and X-ray photoelectron spectroscopy. The adsorption characteristics of phosphate on ZrMZ and HA-ZrMZ were comparatively investigated in batch mode. The adsorption mechanism of phosphate on ZrMZ and HA-ZrMZ was investigated by ionic strength effect and 31P nuclear magnetic resonance. The mechanism for phosphate adsorption onto ZrMZ was the formation of inner-sphere phosphate complexes at the solid/solution interface. The preloading of HA on ZrMZ reduced the phosphate adsorption capacity, and the more the HA loading amount, the lower the phosphate adsorption capacity. However, the preloading of HA on ZrMZ did not change the phosphate adsorption mechanism; i.e., the formation of inner-sphere phosphate surface complexes was still responsible for the adsorption of phosphate on HA-ZrMZ. The decreased phosphate adsorption capacity for ZrMZ after HA coating could be attributed to the fact that the coating of HA on ZrMZ reduced the amount of binding active sites available for phosphate adsorption, changed the adsorbent surface charges, and reduced the specific surface areas and pore volumes of ZrMZ.

Desorption mechanisms of phosphate from ferrihydrite and goethite surfaces

The fate of phosphate in the environment is governed by reactions at particle surfaces. These adsorption and desorption reactions display biphasic kinetics involving an initial rapid reaction followed by a substantially slower one extending over long time periods. In this study we have investigated the molecular mechanisms of desorption kinetics of phosphate from ferrihydrite and goethite nanoparticles in the absence of competing ligands. Desorption was studied by means of in-situ infrared (IR) spectroscopy over a wide pH range and a time period of 24h. The spectroscopic data sets were subjected to multivariate curve resolution alternating least squares (MCR-ALS), which enabled the resolution of surface species characterized by unique IR spectra together with their corresponding kinetic profiles. The desorption results showed the typical biphasic behavior and that increasing positive surface charge of ferrihydrite and goethite slowed down desorption of the negatively charged phosphate ions. Moreover, diprotonated phosphate desorbed faster than monoprotonated phosphate at a given pH. At circumneutral pH values desorption from ferrihydrite was substantially faster as compared to goethite, and this could be ascribed to electrostatic effects and differences in charging between ferrihydrite and goethite. The collective desorption results were explained by a model, consisting of a series monodentate phosphate surface complexes in different protonation states, in conjunction with a description that accounts for the electrostatic effects on desorption kinetics at charged mineral-water interfaces. The fast and slow desorption followed directly from this model and indicated that biphasic kinetics can be caused by a single phosphate surface complex as a result of decreasing surface coverage along with the lateral repulsive interactions between adsorbed phosphate groups. Hence, in contrast to previous models our study has shown that biphasic desorption kinetics do not have to involve several different structural complexes related to either weak and strong sites or a distribution of phosphate between external surfaces and mineral pores.

Phosphate alteration of chloride behavior at the boehmite–water interface: New insights from ion-probe flow adsorption microcalorimetry

Surface complexation of phosphate to aluminum oxyhydroxides can alter surface reactivity depending on the time-scale and mode of attachment. The effects of phosphate adsorption on reactivity of boehmite (γ-AlOOH) particles were investigated using ion-probe flow adsorption microcalorimetry (ipFAMC). Consistent with previous studies on adsorption energetics, probing the surface of pristine γ-AlOOH with chloride ions yielded endothermically unimodal temperature signals with a measured molar heat of exchange (ΔHexc) of −3.1kJ/mol. However, when the surface of γ-AlOOH was probed with chloride following phosphate complexation, significant changes in surface reactivity resulted. Irrespective of phosphate loading, the typical endothermic response of the chloride-surface hydroxyl interaction was replaced with a multi-modal energy signature consisting of exothermic and endothermic features. These features indicate that in the presence of phosphate, the overall nature of the interaction of chloride with specific surface hydroxyls located on different exposed planes and their subsequent reactivity was transformed to a more complex environment accompanied by two or more short-lived secondary reactions. It was also shown that phosphate-promoted surface alteration of γ-AlOOH was highly selective to probing with chloride since no changes in reactivity were observed when nitrate was employed as the primary ion probe under identical experimental conditions.

Quantum chemical study on surface complex structures of phosphate on gibbsite

Quantum mechanics calculations based on the density functional theory (DFT) were used to identify phosphate surface complexes on gibbsite at low and high pH. The different phosphate species were represented using the Al6(OH)18(H2O)6 cluster model considering four different geometries: monodentate mononuclear (Pmm), monodentate binuclear (Pmb), bidentate mononuclear (Pbm) and bidentate binuclear (Pbb). The corresponding adsorption reactions were modelled via ligand exchange between phosphate species and surface functional groups (hydroxyls and protonated hydroxyls at high and low pH, respectively). The theoretical results indicate that phosphate surface complexes are thermodynamically more favored at acid pH, in agreement with experimental evidences. The first step in these reactions, i.e., the generation of required aluminum vacant sites, was predicted to be particularly favorable when singly coordinated aquo groups are released. Stretching and bending vibrational frequencies associated with the different surface structures were calculated at both pH conditions. The corresponding values at low pH were found to be shifted to higher frequencies with respect to those ones at high pH. ATR-FTIR studies were also carried out. The resulting spectra are dominated by a strong band within the 800–840cm−1 interval due to P–OH stretching modes. The corresponding peak appearing around 820cm−1 at high pH is shifted to lower frequencies with respect to the position at low pH, a tendency well predicted by DFT calculations.

Iron oxidation kinetics and phosphate immobilization along the flow-path from groundwater into surface water

Abstract. The retention of phosphorus in surface waters through co-precipitation of phosphate with Fe-oxyhydroxides during exfiltration of anaerobic Fe(II) rich groundwater is not well understood. We developed an experimental field set-up to study Fe(II) oxidation and P immobilization along the flow-path from groundwater into surface water in an agricultural experimental catchment of a small lowland river. We physically separated tube drain effluent from groundwater discharge before it entered a ditch in an agricultural field. Through continuous discharge measurements and weekly water quality sampling of groundwater, tube drain water, exfiltrated groundwater, and surface water, we investigated Fe(II) oxidation kinetics and P immobilization processes. The oxidation rate inferred from our field measurements closely agreed with the general rate law for abiotic oxidation of Fe(II) by O2. Seasonal changes in climatic conditions affected the Fe(II) oxidation process. Lower pH and lower temperatures in winter (compared to summer) resulted in low Fe oxidation rates. After exfiltration to the surface water, it took a couple of days to more than a week before complete oxidation of Fe(II) is reached. In summer time, Fe oxidation rates were much higher. The Fe concentrations in the exfiltrated groundwater were low, indicating that dissolved Fe(II) is completely oxidized prior to inflow into a ditch. While the Fe oxidation rates reduce drastically from summer to winter, P concentrations remained high in the groundwater and an order of magnitude lower in the surface water throughout the year. This study shows very fast immobilization of dissolved P during the initial stage of the Fe(II) oxidation process which results in P-depleted water before Fe(II) is completely depleted. This cannot be explained by surface complexation of phosphate to freshly formed Fe-oxyhydroxides but indicates the formation of Fe(III)-phosphate precipitates. The formation of Fe(III)-phosphates at redox gradients seems an important geochemical mechanism in the transformation of dissolved phosphate to structural phosphate and, therefore, a major control on the P retention in natural waters that drain anaerobic aquifers.

Formation of zinc–oxianion complex adlayer by underpotential deposition of Zn on Au(1 1 1) electrode: Preferential formation of zinc monohydrogen phosphate complex in weakly acidic solutions

The underpotential deposition (upd) of zinc was studied on Au(111) electrode in weakly acidic solutions by voltammetry. The difference of anion co-adsorption strength was investigated on upd Zn in the solutions containing phosphate, sulfate, perchlorate, and halides. The order of anion co-adsorption strength was found to be phosphate>sulfate, phosphate≫perchlorate, and phosphate≫halides. We present the electrochemical evidence that no co-adsorption of halide progresses on upd Zn at E>−0.7V (vs. SCE) in spite of the relatively high adsorbability of halide on Au(111). In 0.1M KH2PO4 (pH=4.4), the amount of charge density of the upd Zn stripping corresponded to that of 1/3 monolayer of the Zn adlayer formed as a result of mono-electron transfer between −0.35V and −0.6V. The coverage was coincident with that inferred from the (√3×√3)R30° STM image reported by us in the same condition. The formation of zinc–oxianion (phosphate and sulfate) surface complexes by Zn upd was proposed base on the difference of complex formation constants among the related zinc–anion complexes. Identification of the zinc–phosphate surface complex was thermodynamically carried out around pH=4 with a Nernst equation, where the shifts of Zn upd peak potential were investigated with the concentration changes of Zn2+, H2PO4−, and H+ under an imaginarily reversible condition. The formation of ZnHPO4 surface complex was concluded in the Zn upd with mono-electron transfer and explained a recent XAS analysis, which demonstrate each phosphate anion bridging three upd Zn atoms.

Phosphate effects on copper(II) and lead(II) sorption to ferrihydrite

Transport of lead(II) and copper(II) ions in soil is affected by the soil phosphorus status. Part of the explanation may be that phosphate increases the adsorption of copper(II) and lead(II) to iron (hydr)oxides in soil, but the details of these interactions are poorly known. Knowledge about such mechanisms is important, for example, in risk assessments of contaminated sites and development of remediation methods. We used a combination of batch experiments, extended X-ray absorption fine structure (EXAFS) spectroscopy and surface complexation modeling with the three-plane CD-MUSIC model to study the effect of phosphate on sorption of copper(II) and lead(II) to ferrihydrite. The aim was to identify the surface complexes formed and to derive constants for the surface complexation reactions. In the batch experiments phosphate greatly enhanced the adsorption of copper(II) and lead(II) to ferrihydrite at pH<6. The largest effects were seen for lead(II).Based on interpretation of the EXAFS spectra edge-sharing bidentate copper(II) or lead(II) complexes predominated in the single-sorbate systems with ferrihydrite. Lead(II) EXAFS spectra suggested a distinct change of coordination in the presence of phosphate, i.e. the signal from edge-sharing complexes diminished and a longer Pb···Fe distance appeared at about 4Å. A similar, but less pronounced, pattern was observed for copper(II). Based on the results from interpretation of EXAFS spectra and surface complexation modeling with the CD-MUSIC model the enhanced sorption in presence of phosphate was most satisfactorily explained by the appearance of ternary metal–phosphate complexes in which the metal interacts directly with the surface.In conclusion, geochemical models used for simulating trace element behavior in acidic environments seem to require ternary metal–phosphate surface complexes to properly describe partitioning of metals between solution and the solid phase.

ATR–FTIR and Density Functional Theory Study of the Structures, Energetics, and Vibrational Spectra of Phosphate Adsorbed onto Goethite

Periodic plane-wave density functional theory (DFT) and molecular cluster hybrid molecular orbital-DFT (MO-DFT) calculations were performed on models of phosphate surface complexes on the (100), (010), (001), (101), and (210) surfaces of α-FeOOH (goethite). Binding energies of monodentate and bidentate HPO(4)(2-) surface complexes were compared to H(2)PO(4)(-) outer-sphere complexes. Both the average potential energies from DFT molecular dynamics (DFT-MD) simulations and energy minimizations were used to estimate adsorption energies for each configuration. Molecular clusters were extracted from the energy-minimized structures of the periodic systems and subjected to energy reminimization and frequency analysis with MO-DFT. The modeled P-O and P---Fe distances were consistent with EXAFS data for the arsenate oxyanion that is an analog of phosphate, and the interatomic distances predicted by the clusters were similar to those of the periodic models. Calculated vibrational frequencies from these clusters were then correlated with observed infrared bands. Configurations that resulted in favorable adsorption energies were also found to produce theoretical vibrational frequencies that correlated well with experiment. The relative stability of monodentate versus bidentate configurations was a function of the goethite surface under consideration. Overall, our results show that phosphate adsorption onto goethite occurs as a variety of surface complexes depending on the habit of the mineral (i.e., surfaces present) and solution pH. Previous IR spectroscopic studies may have been difficult to interpret because the observed spectra averaged the structural properties of three or more configurations on any given sample with multiple surfaces.

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 β-plane. In the high pH-range, MMP acted as a hydrogen bond acceptor whereas it was a hydrogen bond donor at low pH.

EXAFS study on the cause of enrichment of heavy REEs on bacterial cell surfaces

Rare earth element (REE) pattern is a unique geochemical tracer and has been measured for various natural materials. Among these, the REE distribution pattern between bacteria and water exhibits anomalous enrichment in the heavy REE (HREE) part, which can act as a signature of bacteria-related materials in natural samples. In this study, the REE binding site on the cell surface of a Gram-positive bacterium ( Bacillus subtilis) responsible for HREE enrichment has been identified using extended X-ray absorption fine structure (EXAFS) coupled with a study of the variation in REE distribution patterns. The EXAFS data showed that the HREEs form complexes with multiple phosphate site (including phosphoester site) with a larger coordination number (CN) at lower REE-bacteria ratios ([REE]/[bac]), while light and middle REEs form complexes to the phosphate site with a lower CN. The fraction coordinated to carboxylate increased for all REEs with increasing [REE]/[bac] ratio. On the other hand, the enrichment of HREE in the REE distribution patterns of the bacteria was less marked with increasing [REE]/[bac] ratio. This result is consistent with the EXAFS data, because the REE pattern of surface complex with multiple phosphate in a reference material exhibits a monotonous increase for heavier REE, while phosphate surface complex with a low CN and a carboxylate site reach a maximum around Sm and Eu. Based on these results, it is clear that the REE are primarily bound to the phosphate site and subsequently to the carboxylate site on the bacterial cell surface. Regarding the pH dependence in the range (3 < pH < 7), both the EXAFS and REE pattern data indicate that the fraction of REE-carboxylate increased as the pH increases. The results above obtained for B. subtilis were also valid for Escherichia coli, a Gram-negative bacterium, showing that similar phosphate and carboxylate sites are also available in the cell walls of E. coli, or other Gram negative bacteria. In all our results, the variation in REE patterns correlated with the binding site indicated by EXAFS, showing that the REE pattern itself reflects the binding site of the REE at the bacterial surface for various parameters (pH and [REE]/[bac] ratio). Thus, the REE patterns can be used to estimate the binding sites for lower [REE]/[bac] ratios where spectroscopic techniques cannot be applied. The average bond length between the REE and oxygen was compared for various REE sorbed on bacteria, showing that the bond length for HREE (Er to Lu) was much shorter than those extrapolated from the trend between La and Dy, because of the selective binding of the HREE as the multiple phosphate surface complexes. Our results are consistent with the selective enrichment of the HREE at the bacterial cell surfaces, considering that chemical species with a shorter bond length are more stable. Thus, it is clear that the HREE enrichment at the bacterial cell surfaces is caused by the formation of the multiple phosphate surface complexes. Based on these results, it is suggested that materials having such phosphate sites such as bacteria and bacteria-related materials can induce anomalous HREE enrichment in natural systems.

Analysis of phosphate adsorption onto ferrihydrite using the CD-MUSIC model

Ferrihydrite nanoparticles may dominate the ion binding properties of the natural oxide fraction present in soil and aquatic systems. A correct description of the adsorption properties of ferrihydrite nanoparticles may be useful for gaining a better insight into the adsorption processes in natural systems and at the same time will be essential for developing surface complexation models able to describe these processes. In the present study, phosphate speciation in ferrihydrite has been analyzed combining the available spectroscopic data and molecular information with modeling calculations. For this purpose, a new data set that analyzes the effect of pH and ionic strength on the phosphate adsorption onto ferrihydrite has been used. Description of the phosphate adsorption process onto ferrihydrite nanoparticles, for the entire pH and ionic strength range, has been made taking into account the presence of protonated and nonprotonated bidentate surface complexes. The presence of monodentate complexes, protonated and nonprotonated, was also analyzed, but no significant improvement in the description of the results was observed. The surface complexation constants obtained with the CD-MUSIC modeling calculations are comparable to the values found in the literature for phosphate surface complexes in goethite particles.

Competitive adsorption between phosphate and carboxylic acids: quantitative effects and molecular mechanisms

SummaryThe competitive adsorption at the water‐goethite interface between phosphate and a carboxylic acid, either oxalate, citrate, 1,2,3,4‐butanetetracarboxylic acid (BTCA), mellitate or Suwannee River Standard Fulvic Acid 1S101F (FA), was investigated over a wide pH range (3–9) by means of batch experiments and attenuated total reflectance Fourier transform infrared (ATR‐FTIR) spectroscopy. The quantitative results from the competitive adsorption measurements show that the efficiency of the organic acids in competing with phosphate was in the order oxalate &lt; citrate &lt; BTCA ≅ FA &lt; mellitate. Oxalate showed no detectable effect, whereas the effect in the mellitate system was strong, and the aggregative results indicate that an increasing number of carboxylic groups favours competitive ability towards phosphate. The infrared spectroscopic results show conclusively that competition for goethite surface sites between carboxylic acids and phosphate is not a ligand‐exchange reaction between inner‐sphere surface complexes. Instead, ligands capable of multiple H‐bonding interactions are required to out‐compete and desorb surface complexes of phosphate. The fact that partially protonated organic acids are the most efficient emphasizes the importance of both H‐accepting carboxylate groups and H‐donating carboxylic acid groups for the competitive effect.

Phosphate adsorbed on Fe(III) modified montmorillonite: Surface complexation studied by ATR-FTIR spectroscopy

This work is aimed at determining phosphate speciation at the Fe(III)-modified montmorillonite/aqueous solution interface by attenuated total reflectance (ATR-FTIR) spectroscopy. IR spectra are analyzed by second derivative and Fourier self-deconvolution procedures to reveal the number and positions of overlapped bands related to phosphate surface complexes. Experiments are conducted at pH 4.5, 7.0 and 9.0 in order to determine the effects of changing pH on the surface speciation. The molecular view offered by the ATR-FTIR measurements is complemented by macroscopic adsorption data, such as adsorption isotherms and electrophoretic mobilities. The ATR-FTIR spectral analyses reveal the existence of two inner-sphere complexes when phosphate adsorbs on modified montmorillonite, as also suggested from macroscopic adsorption data. The complex coordination depends on the pH. At pH 4.5, both complexes present C 2V symmetry or lower. On the other hand, at pH 7.0 and 9.0 one of them is also C 2V or lower while the other one presents C 3V symmetry. Adsorbed phosphate is not associated with montmorillonite itself but with Fe(III) (hydr)oxides coatings. Therefore, the clay acts as a carrier or support of the Fe(III)-(hydr)oxides, and these coatings are responsible for phosphate adsorption.

Efficiency of blocking of non-specific interaction of different proteins by BSA adsorbed on hydrophobic and hydrophilic surfaces

The efficiency of a pre-absorbed bovine serum albumin (BSA) layer in blocking the non-specific adsorption of different proteins on hydrophobic and hydrophilic surfaces was evaluated qualitatively and quantitatively using infrared reflection spectroscopy supported by spectral simulations. A BSA layer with a surface coverage of 35% of a close-packed monolayer exhibited a blocking efficiency of 90–100% on a hydrophobic and 68–100% on a hydrophilic surface, with respect to the non-specific adsorption of concanavalin A (Con A), immunoglobulin G (IgG), and staphylococcal protein A (SpA). This BSA layer was produced using a solution concentration of 1mg/mL and 30min incubation time. BSA layers that were adsorbed at conditions commonly employed for blocking (a 12h incubation time and a solution concentration of 10mg/mL) exhibited a blocking activity that involved competitive adsorption–desorption. This activity resulted from the formation of BSA–phosphate surface complexes, which correlated with the conformation of adsorbed BSA molecules that was favourable for blocking. The importance of optimisation of the adsorbed BSA layer for different surfaces and proteins to achieve efficient blocking was addressed in this study.