Ternary complex formation of phosphate with Ca and Mg ions binding to ferrihydrite: Experiments and mechanisms

Calcium (Ca) and magnesium (Mg) are the most abundant alkaline-earth metal ions in nature, and their interaction with ferrihydrite (Fh) affects the geochemical cycling of relevant ions, including phosphate (PO4). The interfacial interactions of Ca and Mg (M2+) with PO4 have not been analyzed yet for freshly precipitated Fh. Here, we studied experimentally this interaction in binary M2+-PO4 systems over a wide range of pH, M2+/PO4 ratios, and ion loadings. The primary adsorption data were scaled to the surface area of Fh using a recent ion-probing methodology that accounts for the size-dependent chemical composition of this nanomaterial (FeO1.4(OH)0.2·nH2O). The results have been interpreted with the charge distribution (CD) model, combined with a state-of-the-art structural surface model for Fh. The CD coefficients have been derived independently using MO/DFT/B3LYP/6-31+G** optimized geometries. M2+ and PO4 mutually enhance their adsorption to Fh. This synergy results from the combined effect of ternary surface complex formation and increased electrostatic interactions. The type of ternary complex formed (anion- vs cation-bridged) depends on the relative binding affinities of the co-adsorbing ions. For our Ca-PO4 systems, modeling suggests the formation of two anion-bridged ternary complexes, i.e., ≡(FeO)2PO2Ca and ≡FeOPO3Ca. The latter is most prominently present, leading to a relative increase in the fraction of monodentate PO4 complexes. In Mg-PO4 systems, only the formation of the ternary ≡FeOPO3Mg complex has been resolved. In the absence of Ca, the pH dependency of PO4 adsorption is stronger for Fh than for goethite, but this difference is largely, although not entirely, compensated in the presence of Ca. This study enables the use of Fh as a proxy for the natural oxide fraction, which will contribute to improved understanding of the mutual interactions of PO4 and M2+ in natural systems.Universidad de Costa Rica/[]/UCR/Costa RicaUCR::Vicerrectoría de Investigación::Unidades de Investigación::Ciencias Agroalimentarias::Centro de Investigaciones Agronómicas (CIA

Surface Complexation of Selenite on Goethite: MO/DFT Geometry and Charge Distribution

The adsorption of selenite on goethite (a-FeOOH) has been analyzed with the charge distribution (CD) and the multi-site surface complexation (MUSIC) model being combined with an extended Stern (ES) layer model option. The geometry of a set of different types of hydrated iron-selenite complexes has been calculated using Molecular Orbital / Density Functional Theory (MO/DFT). The optimized geometries have been interpreted with the Brown bond valence approach resulting in a set of ionic charge distribution values. After correction for dipole orientation effects, it results in the interfacial charge distribution coefficients that can be applied to the analysis of adsorption data. The use of theoretical CD values has the practical advantage of a reduction of the number of adjustable parameters. From a theoretical perspective, the CD values can constrain the model, revealing a surface speciation that can be tested experimentally. Modeling of the adsorption of SeO3 in (pseudo-) monocomponent goethite systems, using the calculated CD values, has revealed the dominant presence of a bidentate surface species º(FeO)2SeO. The dominance of this surface species agrees with the interpretation of EXAFS measurements given in literature. The agreement supports the validity of the approach. To describe the adsorption at very low pH and a high loading, formation of an additional surface species is required in the modeling. The maximum contribution is about 20 % or less. In case of anion competition, as found in the PO4-SeO3 goethite system, the relative contribution increases. Analysis of the adsorption behavior in the PO4-SeO3 goethite systems revealed the probable nature of the additional surface complex, which is found to be a protonated monodentate surface complex ºFeOSeOOH. With the affinity constants derived, the CD model is able to describe the SeO3 adsorption on goethite over a large range of pH, ionic strength, and loading conditions for a variety of goethite preparations. The CD model correctly predicts the proton co-adsorption of selenite and is able to describe the shift of the IEP upon addition of selenite

Assessing the reactive surface area of soils and the association of soil organic carbon with natural oxide nanoparticles using ferrihydrite as proxy

Assessment of the surface reactivity of natural metal-(hydr)oxide nanoparticles is necessary for predicting ion adsorption phenomena in soils using surface complexation modeling. Here, we describe how the equilibrium concentrations of PO4, obtained with 0.5 M NaHCO3 extractions at different solution-to-soil ratios, can be interpreted with a state-of-the-art ion adsorption model for ferrihydrite to assess the reactive surface area (RSA) of agricultural top soils. Simultaneously, the method reveals the fraction of reversibly adsorbed soil PO4 (R-PO4). The applied ion-probing methodology shows that ferrihydrite is a better proxy than goethite for consistently assessing RSA and R-PO4. The R-PO4 pool agrees well with ammonium oxalate (AO)-extractable phosphorus, but only if measured as orthophosphate. The RSA varied between ∼2 and 20 m2/g soil. The corresponding specific surface area (SSA) of the natural metal-(hydr)oxide fraction is ∼350–1400 m2/g, illustrating that this property is highly variable and cannot be represented by a single value based on the AO-extractable oxide content. The soil organic carbon (SOC) content of our top soils increases linearly not only with the increase in RSA but remarkably also with the increase in mean particle size (1.5–5 nm). To explain these observations, we present a structural model for organo-mineral associations based on the coordination of SOC particles to metal-(hydr)oxide cores.Universidad de Costa Rica/[]/UCR/Costa RicaUCR::Vicerrectoría de Investigación::Unidades de Investigación::Ciencias Agroalimentarias::Centro de Investigaciones Agronómicas (CIA

Boron adsorption to ferrihydrite with implications for surface speciation in soils: Experiments and modeling

The adsorption and desorption of boric acid onto reactive materials such as metal (hydr)oxides and natural organic matter are generally considered to be controlling processes for the leaching and bioavailability of boron (B). We studied the interaction of B with ferrihydrite (Fh), a nanosized iron (hydr)oxide omnipresent in soil systems, using batch adsorption experiments at different pH values and in the presence of phosphate as a competing anion. Surface speciation of B was described with a recently developed multisite ion complexation (MUSIC) and charge distribution (CD) approach. To gain insight into the B adsorption behavior in whole-soil systems, and in the relative contribution of Fh in particular, the pH-dependent B speciation was evaluated for soils with representative amounts of ferrihydrite, goethite, and organic matter. The pH-dependent B adsorption envelope of ferrihydrite is bell-shaped with a maximum around pH 8–9. In agreement with spectroscopy, modeling suggests formation of a trigonal bidentate complex and an additional outer-sphere complex at low to neutral pH values. At high pH, a tetrahedral bidentate surface species becomes important. In the presence of phosphate, B adsorption decreases strongly and only formation of the outer-sphere surface complex is relevant. The pH-dependent B adsorption to Fh is rather similar to that of goethite. Multisurface modeling predicts that ferrihydrite may dominate the B binding in soils at low to neutral pH and that the relative contribution of humic material increases significantly at neutral and alkaline pH conditions. This study identifies ferrihydrite and natural organic matter (i.e., humic substances) as the major constituents that control the B adsorption in topsoils.The Dutch Research Council/[Grant N°14688]/NWO/Países BajosUCR::Vicerrectoría de Investigación::Unidades de Investigación::Ciencias Agroalimentarias::Centro de Investigaciones Agronómicas (CIA

Earthworms affect reactive surface area and thereby phosphate solubility in iron-(hydr)oxide dominated soils

Sustainability of agricultural systems is at stake, as phosphorus (P) is a non-renewable resource while its global reserves are limited. Stimulating earthworm activity can be a technology to increase the level of readily plant-available phosphate (PO$_4$). However, conclusive evidence on the mechanisms underlying an earthworm-enhanced PO$_4$ solubility is yet missing. This study aimed to reveal possibly overlooked pathways by which earthworms affect PO$_4$ solubility, and quantify the relative importance of all contributing mechanisms. Therefore, we set up a greenhouse pot experiment in which we investigated the large increase in water-extractable PO$_4$ in casts of three earthworm species (Lumbricus rubellus, Aporrectodea caliginosa, Lumbricus terrestris) in soils with either predominantly Fe- or Al-(hydr)oxides. Oxalate-extractable PO$_4$ was increased in earthworm casts compared to bulk soil which can be attributed to the mineralisation of natural organic matter (NOM). Surface complexation modelling was used to elucidate the mechanisms that control earthworm-enhanced PO$_4$ solubility. The results of our modelling showed that the increase in pH in earthworm casts relative to bulk soil affects PO$_4$ solubility only to a minor extent. Besides NOM mineralisation, two major mechanisms contributing to earthworm-enhanced PO$_4$ solubility are (i) a decrease in the reactive surface area (RSA) of the metal-(hydr)oxide fraction; and (ii) a decrease in the competition between NOM and PO$_4$ for binding sites of the metal-(hydr)oxides. As the newly discovered decrease of the RSA was only found for Fe-(hydr)oxide-dominated soils, earthworms have the largest potential to enhance PO$_4$ solubility in those soils

Surface structure controlling nanoparticle behavior : Magnetism of ferrihydrite, magnetite, and maghemite

Iron oxide nanoparticles are omnipresent in nature and of great importance for environmental sciences and technology. The size-dependent magnetic behavior of ferrihydrite (Fh), magnetite (Fe3O4), and maghemite (γ-Fe2O3) has been studied in relation to the surface structure. The selected minerals have in common the presence of tetrahedral Fe. This Fe polyhedron is unstable at the surface when forming singly coordinated ligand(s). This leads to the size-dependency of the polyhedral composition, which is for Fh in excellent agreement with the relative contributions of edge and corner sharing measured with high-energy total X-ray scattering. For Fh, superparamagnetic behavior scales with particle volume in which magnetic coupling is proportional to a fraction of the Fe per particle. Magnetic saturation at low temperature scales with size and is predominantly due to polyhedral surface depletion. The mineral core of Fh may behave ferrimagnetically as well as antiferromagnetically. Both have opposite particle size dependency, for which a surface structural model has been developed. The relative stability of ferrimagnetic and antiferromagnetic Fh is related to a slight difference in the surface Gibbs free energy (∼0.03 J m-2). At the same surface structure, the predicted crossover point is at ∼4 nm, above which the core of Fh shifts from antiferromagnetic to ferrimagnetic. For magnetite and maghemite, the size dependency of the ferrimagnetic behavior can be described with the same model as that developed for Fh by only adjusting the maximum magnetic saturation of the ideal bulk material to its theoretical value. As discussed and quantified, the structural defects of superparamagnetic Fe-oxide nanoparticles (SPION) will lower the magnetic saturation at a given size

Ferrihydrite interaction with silicate and competing oxyanions : Geometry and Hydrogen bonding of surface species

Silicic acid is omnipresent in nature and interacts with ferrihydrite (Fh) changing the environmental fate of elements. For freshly prepared ferrihydrite, pH and electrolyte dependency of the Si adsorption was measured and interpreted with the charge distribution (CD) model using reactive site densities derived with a surface structural analysis. Proton adsorption data disclose the surface area (A ∼ 610 m2 g−1) and mean particle size (d ∼ 2.5 nm) of the Fh studied. Similarly, a range of A ∼ 530–710 g m−2 and d ∼ 2.3–2.8 nm Fh is found for Fh used in literature. Modeling of our Si adsorption data indicates the formation of Si-oligomers alongside with a Si-monomer. There is quantitative agreement with spectroscopy (ATR-IR, XPS, IR). Innersphere complexation of monomeric Si results in the formation of a mononuclear monodentate complex. However, one of the Si–OH ligands strongly interacts with an adjacent [tbnd]FeOH group, forming an extraordinary hydrogen (O⋯H–O) bond in which the H+ ion is significantly shifted, transferring supplementary proton charge (ΔsH = ∼0.20 v.u.) towards the surface changing the interfacial charge distribution coefficients of the complex, in agreement with the adsorption data. The shift of charge inhibits the protonation of the [tbnd]FeOH surface group, leading to a stable [tbnd]FeOH–FeOSi(OH)3 configuration. Depending on pH and Si-loading, oligomers are present as a Si trimer and some Si tetramer. These complexes have a double-mononuclear Fe2Si2 structure in which two Si tetrahedra are connected to two Fe octahedra each via a single Fe–O–Si bond. The various MO/DFT (B3LYP and BP86) optimized geometries can reproduce the mean Fe-Si distances of 324 and 331 pm found with differential PDF analysis. The outer ligands of the Si-monomer remain protonated, whereas one of the outer ligands of the Si-oligomers is deprotonated, in line with the structural model derived. Competition experiments identified phosphate as a very good competitor for silicate, implying that in nature, siliceous Fh can only be formed in sub-neutral systems that are low in phosphate and rich in silicate, in agreement with reported chemical compositions. Nearly all Si can be removed from the surface by phosphate at sub neutral pH despite a 10–100 times lower phosphate equilibrium concentration in comparison to silicic acid (H4SiO4). Siliceous Fh particles in lab and field are smaller than two-line Fh synthesized in the absence of Si, and have a larger specific surface area. At oxidative removal of Fe(II) from groundwater with 0.3 mM Si at circum-neutral pH, small (d = 2.0 ± 0.2 nm) siliceous Fh particles (Si/Fe = 0.18, P/Fe = 0.016) are formed with a surface area near ∼900 m2 g−1. The size is in good agreement with the length of the coherent scattering domain (CSD) reported in literature for synthetic Si-Fe(III) co-precipitates having a primary particle structure in excellent agreement with the surface depletion model for Fh. The competitive interaction of silicate (SiO4) with phosphate (PO4), arsenite (As(OH)3), and arsenate (AsO4) can be predicted very well with the CD model using affinity constants (logK) collected in monocomponent systems only.</p

Variable Charge and Electrical Double Layer of Mineral–Water Interfaces: Silver Halides versus Metal (Hydr)Oxides

Classically, silver (Ag) halides have been used to understand thermodynamic principles of the charging process and the corresponding development of the electrical double layer (EDL). A mechanistic approach to the processes on the molecular level has not yet been carried out using advanced surface complexation modeling (SCM) as applied to metal (hydr)­oxide interfaces. Ag halides and metal (hydr)­oxides behave quite differently in some respect. The location of charge in the interface of Ag halides is not a priori obvious. For AgI(s), SCM indicates the separation of interfacial charge in which the smaller silver ions are apparently farther away from the surface than iodide. This charge separation can be understood from the surface structure of the relevant crystal faces. Charge separation with positive charge above the surface is due to monodentate surface complex formation of Ag⁺ ions binding to I sites located at the surface. Negative surface charge is due to the desorption of Ag⁺ ions out of the lattice. These processes can be described with the charge distribution (CD) model. The MO/DFT optimized geometry of the complex is used to estimate the value of the CD. SCM reveals the EDL structure of AgI(s), having two Stern layers in series. The inner Stern layer has a very low capacitance (<i>C</i>₁ = 0.15 ± 0.01 F/m²) in comparison to that of metal (hydr)­oxides, and this can be attributed to the strong orientation of the (primary) water molecules on the local electrostatic field of the Ag⁺ and I^– ions of the surface (relative dielectric constant ε_r ≈ 6). Depending on the extent of water ordering, mineral surfaces may in principle develop a second Stern layer. The corresponding capacitance (<i>C</i>₂) will depend on the degree of water ordering that may decrease in the series AgI (<i>C</i>₂ = 0.57 F/m²), goethite (<i>C</i>₂ = 0.74 F/m²), and rutile (<i>C</i>₂ = ∞), as discussed. The charging principles of AgI minerals iodargyrite and miersite may also be applied to minerals with the same surface structure (e.g., sphalerite and würtzite (ZnS))

Time, pH, and size dependency of silver nanoparticle dissolution : The road to equilibrium

Oxidative dissolution has large implications for the environmental fate and toxicity of silver nanoparticles (AgNPs). In this study, we quantify the kinetics, pH, and size dependency of silver ion (Ag+) release from AgNPs and explain our results in a consistent manner with a mechanistic view. Pristine AgNPs are covered by partially oxidized silver present in a single layer of subvalent Ag3OH groups that will be released by oxidative dissolution via two different pathways. Undersaturation of a solution, created by acidification, will initiate a fast oxidative dissolution process in which a pristine surface can be opened at particular points that grow laterally until a full layer of Ag is stripped off. At the newly exposed surface, Ag3OH is reformed. The opening of new spots stops due to increasing Ag+ concentrations. Via another pathway, the initial Ag3OH can be released by oxidative dissolution while simultaneously a new stable surface state is built with subvalent silver in two layers. This process is initiated by dilution and is visible around neutral pH values and may release a maximum of 30 ± 1 μmol Ag+ m-2. Its equilibration can be well described with a formulated thermodynamic model. The equilibrium constant (logK) is linearly related to the specific surface area of the AgNPs used, but can be shifted by the type of capping agent. The particle size dependency of the logK can be attributed to a surface Gibbs free energy contribution of 0.7 ± 0.1 J m-2. Ag+ release by stripping is relatively fast (∼1 day) in contrast to the process that leads to equilibration of two types of surface species that differ in the amount of subvalent silver. For this process, a kinetic Langmuir model has been developed in which the rate of Ag+ release is governed by adsorbed molecular oxygen that can be activated via a proton, while adsorption of molecular oxygen by itself can become rate limiting in the initial stage of dissolution with high rates of release. In our study with data of different kinds, the overall release and equilibration by AgNPs has been interpreted successfully with a coherent, overarching mechanistic view.</p