Characterization of colloidal phosphorus species in drainage waters from a clay soil using asymmetric flow field-flow fractionation.

Phosphorus transport from agricultural land contributes to eutrophication of surface waters. Pipe drain and trench waters from a grassland field on a heavy clay soil in the Netherlands were sampled before and after manure application. Phosphorus speciation was analyzed by physicochemical P fractionation, and the colloidal P fraction in the dissolved fraction (<0.45 µm) was analyzed by asymmetric flow field-flow fractionation (AF4) coupled to high-resolution inductively coupled plasma-mass spectrometry and ultraviolet diode array detector. When no manure was applied for almost 7 mo, total P (TP) concentrations were low (<21 µmol L), and TP was almost evenly distributed among dissolved reactive P (DRP), dissolved unreactive P (DUP), and particulate P (PP). Total P concentrations increased by a factor of 60 and 4 when rainfall followed shortly after application of cattle slurry or its solid fraction, respectively. Under these conditions, DRP contributed 50% or more to TP. The P speciation within the DUP and PP fractions varied among the different sampling times. Phosphorus associated with dissolved organic matter, probably via cation bridging, comprised a small fraction of DUP at all sampling times. Colloidal P coeluted with clay particles when P application was withheld for almost 7 mo and after application of the solid cattle slurry fraction. At these sampling times, PP correlated well with particulate Fe, Al, and Si, indicating that P is associated with colloidal clay particles. After cattle slurry application, part of DUP was probably present as phospholipids. Physicochemical fractionation combined with AF4 analysis is a promising tool to unravel the speciation of colloidal P in environmental water samples.

Effect of soil parameters on the kinetics of the displacement of Fe from FeEDDHA chelates by Cu.

In soil application, o,o-FeEDDHA (iron (3+) ethylene diamine-N,N'-bis(2-hydroxy phenyl acetic acid) complex) is the active ingredient of FeEDDHA chelate-based Fe fertilizers. The effectiveness of o,o-FeEDDHA is potentially compromised by the displacement of Fe from FeEDDHA by Cu. The actual impact of Cu competition is codetermined by the kinetics of the displacement reaction. In this study, the influence of soil parameters on the displacement kinetics has been examined in goethite suspensions. The displacement reaction predominantly takes place on the reactive surface rather than in solution. The rate at which the o,o-FeEDDHA concentration declined depended on the available reactive surface area, the Cu loading, and the FeEDDHA loading. Soil factors reducing FeEDDHA adsorption (high ionic strength, humic acid adsorption onto the goethite surface, and monovalent instead of divalent cations in the electrolyte) decreased the displacement rate. For meso o,o-FeEDDHA, the displacement rate equation was derived, which is first order in FeEDDHA loading and half order in Cu loading. For soil conditions, the equation can be simplified to an exponential decay function in meso o,o-FeEDDHA solution concentration.

Factors controlling phosphate interaction with iron oxides.

Factors such as pH, solution ion composition, and the presence of natural organic matter (NOM) play a crucial role in the effectiveness of phosphorous adsorption by iron oxides. The interplay between these factors shows a complicated pattern and can sometimes lead to controversial results. With the help of mechanistic modeling and adsorption experiments, the net macroscopic effect of single and combined factors can be better understood and predicted. In the present work, the relative importance of the above-mentioned factors in the adsorption of phosphate was analyzed using modeling and comparison between the model prediction and experimental data. The results show that, under normal soil conditions, pH, concentration of Ca, and the presence of NOM are the most important factors that control adsorption of phosphate to iron oxides. The presence of Ca not only enhances the amount of phosphate adsorbed but also changes the pH dependency of the adsorption. An increase of dissolved organic carbon from 0.5 to 50 mg L can lead to a >50% decrease in the amount of phosphate adsorbed. Silicic acid may decrease phosphate adsorption, but this effect is only important at a very low phosphate concentration, in particular at high pH.

Determination of free Zn2+ concentration in synthetic and natural samples with AGNES (Absence of Gradients and Nernstian Equilibrium Stripping) and DMT (Donnan Membrane Technique).

The determination of free Zn(2+) ion concentration is a key in the study of environmental systems like river water and soils, due to its impact on bioavailability and toxicity. AGNES (Absence of Gradients and Nernstian Equilibrium Stripping) and DMT (Donnan Membrane Technique) are emerging techniques suited for the determination of free heavy metal concentrations, especially in the case of Zn(2+), given that there is no commercial Ion Selective Electrode. In this work, both techniques have been applied to synthetic samples (containing Zn and NTA) and natural samples (Rhine river water and soils), showing good agreement. pH fluctuations in DMT and N(2)/CO(2) purging system used in AGNES did not affect considerably the measurements done in Rhine river water and soil samples. Results of DMT in situ of Rhine river water are comparable to those of AGNES in the lab. The comparison of this work provides a cross-validation for both techniques.

Competitive and synergistic effects in pH dependent phosphate adsorption in soils: LCD modeling.

The pH dependency of soluble phosphate in soil was measured for six agricultural soils over a pH range of 3-10. A mechanistic model, the LCD (ligand charge distribution) model, was used to simulate this change, which considers phosphate adsorption to metal (hydr)oxides in soils under the influence of natural organic matter (NOM) and polyvalent cations (Ca(2+), Al(3+), and Fe(3+)). For all soils except one, the description in the normal pH range 5-8 is good. For some soils at more extreme pH values (for low P-loading soils at low pH and for high P-loading soils at high pH), the model over predicts soluble P. The calculation shows that adsorption is the major mechanism controlling phosphate solubility in soils, except at high pH in high P-loading soils where precipitation of calcium phosphate may take place. NOM and polyvalent cations have a very strong effect on the concentration level of P. The pattern of pH dependency of soluble P in soils differs greatly from the pH effects on phosphate adsorption to synthetic metal (hydr)oxides in a monocomponent system. According to the LCD model, the pH dependency in soil is mainly caused by the synergistic effects of Ca(2+) adsorption to oxides. Adsorption of Al(3+) to NOM adsorbed plays an important role only at a pH < 4.5. Presence of NOM coating strongly competes with phosphate for the adsorption and is an important factor to consider in modeling phosphate adsorption in natural samples.

Speciation of Se and DOC in soil solution and their relation to Se bioavailability.

A 0.01 M CaCl(2) extraction is often used to asses the bioavailability of plant nutrients in soils. However, almost no correlation was found between selenium (Se) in the soil extraction and Se content in grass. The recently developed anion Donnan membrane technique was used to analyze chemical speciation of Se in the 0.01 M CaCl(2) extractions of grassland soils and fractionation of DOC (dissolved organic carbon). The results show that most of Se (67-86%) in the extractions (15 samples) are colloidal-sized Se. Only 13-34% of extractable Se are selenate, selenite and small organic Se (<1 nm). Colloidal Se is, most likely, Se bound to or incorporated in colloidal-sized organic matter. The dominant form of small Se compounds (selenate, selenite/small organic compounds) depends on soil. A total of 47-85% of DOC is colloidal-sized and 15-53% are small organic molecules (<1 nm). In combination with soluble S (sulfur) and/or P (phosphor), concentration of small DOC can explain most of the variability of Se content in grass. The results indicate that mineralization of organic Se is the most important factor that controls Se availability in soils. Competition with sulfate and phosphate needs to be taken into account. Further research is needed to verify if concentration of small DOC is a good indicator of mineralization of soil organic matter.

Performance of soil-applied FeEDDHA isomers in delivering Fe to soybean plants in relation to the moment of application.

FeEDDHA (iron(3+) ethylenediamine-N,N'-bis(hydroxyphenylacetic acid) products are commonly applied to mend and prevent Fe deficiency chlorosis in soil-grown crops. Plants mainly take up Fe in the progressed vegetative and in the reproductive stages. This study examined which of the principal constituents of FeEDDHA products (the isomers racemic o,o-FeEDDHA, meso o,o-FeEDDHA, and o,p-FeEDDHA), most effectively meets the Fe requirements of soybean plants (Glycine max (L.) Merr.) grown on calcareous soil in the aforementioned growth stages. FeEDDHA isomers were applied once, separately or in mixtures, at t = 0, in the progressed vegetative stage or in the reproductive stage. o,p-FeEDDHA did not significantly contribute to Fe uptake in either growth stage. Both racemic and meso o,o-FeEDDHA were effective in supplying plants with Fe, approximately to the same extent. The moment of application had a significant effect on yield and FeEDDHA pore water concentrations at harvest, but not on Fe uptake. To optimize yield while minimizing FeEDDHA dosage, FeEDDHA is best applied to soybean plants prior to the onset of chorosis.

Diffusion of neutral and ionic species in charged membranes: boric acid, arsenite, and water.

Dynamic ion speciation using DMT (Donnan membrane technique) requires insight into the physicochemical characteristics of diffusion in charged membranes (tortuosity, local diffusion coefficients) as well as ion accumulation. The latter can be precluded by studying the diffusion of neutral species, such as boric acid, B(OH)3°(aq), arsenite, As(OH)3°(aq), or water. In this study, the diffusion rate of B(OH)3° has been evaluated as a function of the concentration, pH, and ionic strength. The rate is linearly dependent on the concentration of solely the neutral species, without a significant contribution of negatively charged species such as B(OH)4⁻, present at high pH. A striking finding is the very strong effect (factor of ~10) of the type of cation (K(+), Na(+), Ca(2+), Mg(2+), Al(3+), and H(+)) on the diffusion coefficient of B(OH)3° and also As(OH)3°. The decrease of the diffusion coefficient can be rationalized as an enhancement of the mean viscosity of the confined solution in the membrane. The diffusion coefficients can be described by a semiempirical relationship, linking the mean viscosity of the confined solute of the membrane to the viscosity of the free solution. In proton-saturated membranes, as used in fuel cells, viscosity is relatively more enhanced; i.e., a stronger water network is formed. Extraordinarily, our B(OH)3-calibrated model (in HNO3) correctly predicts the reported diffusion coefficient of water (D(H2O)), measured with ¹H NMR and quasi-elastic neutron scattering in H(+)-Nafion membranes. Upon drying these membranes, the local hydronium, H(H2O)(n)(+), concentration and corresponding viscosity increase, resulting in a severe reduction of the diffusion coefficient (D(H2O) ≈ 5-50 times), in agreement with the model. The present study has a second goal, i.e., development of the methodology for measuring the free concentration of neutral species in solution. Our data suggest that the free concentration can be measured with DMT in natural systems if one accounts for the variation in the cation composition of the membrane and corresponding viscosity/diffusion coefficient.

Donnan membrane technique (DMT) for anion measurement.

Donnan membrane technique (DMT) is developed and tested for determination of free anion concentrations. Time needed to reach the Donnan membrane equilibrium depends on type of ions and the background. The Donnan membrane equilibrium is reached in 1 day for Cl(-), 1-2 days for NO(3)(-), 1-4 days for SO(4)(2-) and SeO(4)(2-), and 1-14 days for H(2)PO(4)(-) in a background of 2-200 mM KCl or K(2)SO(4). The strongest effect of ionic strength on equilibrium time is found for H(2)PO(4)(-), followed by SO(4)(2-) and SeO(4)(2-), and then by Cl(-) and NO(3)(-). The negatively charged organic particles of fulvic and humic acids do not pass the membrane. Two approaches for the measurement of different anion species of the same element, such as SeO(4)(2-) and HSeO(3)(-), using DMT are proposed and tested. These two approaches are based on transport kinetics or response to ionic strength difference. A transport model that was developed previously for cation DMT is applied in this work to analyze the rate-limiting step in the anion DMT. In the absence of mobile/labile complexes, transport tends to be controlled by diffusion in solution at a low ionic strength, whereas at a higher ionic strength, diffusion in the membrane starts to control the transport.

Effects of lability of metal complex on free ion measurement using DMT.

Very low concentrations of free metal ion in natural samples can be measured using the Donnan membrane technique (DMT) based on ion transport kinetics. In this paper, the possible effects of slow dissociation of metal complexes on the interpretation of kinetic DMT are investigated both theoretically and experimentally. The expressions of the lability parameter, Lgrangian , were derived for DMT. Analysis of new experimental studies using synthetic solution containing NTA as the ligand and Cu(2+) ions shows that when the ionic strength is low (

Relationship between metal speciation in soil solution and metal adsorption at the root surface of ryegrass.

The total metal content of the soil or total metal concentration in the soil solution is not always a good indicator for metal availability to plants. Therefore, several speciation techniques have been developed that measure a defined fraction of the total metal concentration in the soil solution. In this study the Donnan Membrane Technique (DMT) was used to measure free metal ion concentrations in CaCl(2) extractions (to mimic the soil solution, and to work under standardized conditions) of 10 different soils, whereas diffusive gradients in thin-films (DGT) and scanning chronopotentiometry (SCP) were used to measure the sum of free and labile metal concentrations in the CaCl(2) extracts. The DGT device was also exposed directly to the (wetted) soil (soil-DGT). The metal concentrations measured with the speciation techniques are related to the metal adsorption at the root surface of ryegrass (Lolium perenne L.), to be able to subsequently predict metal uptake. In most cases the metal adsorption related pH-dependently to the metal concentrations measured by DMT, SCP, and DGT in the CaCl(2) extract. However, the relationship between metal adsorption at the root surface and the metal concentrations measured by the soil-DGT was not-or only slightly-pH dependent. The correlations between metal adsorption at the root surface and metal speciation detected by different speciation techniques allow discussion about rate limiting steps in biouptake and the contribution of metal complexes to metal bioavailability.

Multi-competitive interaction of As(III) and As(V) oxyanions with Ca(2+), Mg(2+), PO(3-)(4), and CO(2-)(3) ions on goethite.

Complex systems, simulating natural conditions like in groundwater, have rarely been studied, since measuring and in particular, modeling of such systems is very challenging. In this paper, the adsorption of the oxyanions of As(III) and As(V) on goethite has been studied in presence of various inorganic macro-elements (Mg(2+), Ca(2+), PO(3-)(4), CO(2-)(3)). We have used 'single-,' 'dual-,' and 'triple-ion' systems. The presence of Ca(2+) and Mg(2+) has no significant effect on As(III) oxyanion (arsenite) adsorption in the pH range relevant for natural groundwater (pH 5-9). In contrast, both Ca(2+) and Mg(2+) promote the adsorption of PO(3-)(4). A similar (electrostatic) effect is expected for the Ca(2+) and Mg(2+) interaction with As(V) oxyanions (arsenate). Phosphate is a major competitor for arsenate as well as arsenite. Although carbonate may act as competitor for both types of As oxyanions, the presence of significant concentrations of phosphate makes the interaction of (bi)carbonate insignificant. The data have been modeled with the charge distribution (CD) model in combination with the extended Stern model option. In the modeling, independently calculated CD values were used for the oxyanions. The CD values for these complexes have been obtained from a bond valence interpretation of MO/DFT (molecular orbital/density functional theory) optimized geometries. The affinity constants (logK) have been found by calibrating the model on data from 'single-ion' systems. The parameters are used to predict the ion adsorption behavior in the multi-component systems. The thus calibrated model is able to predict successfully the ion concentrations in the mixed 2- and 3-component systems as a function of pH and loading. From a practical perspective, data as well as calculations show the dominance of phosphate in regulating the As concentrations. Arsenite (As(OH)(3)) is often less strongly bound than arsenate (AsO(3-)(4)) but arsenite responses less strongly to changes in the phosphate concentration compared to arsenate, i.e., deltalogc(As(III))/deltalogc(PO(4)) approximately 0.4 and deltalogc(As(V))/deltalogc(PO(4)) approximately 0.9 at pH 7. Therefore, the response of As in a sediment on a change in redox conditions will be variable and will depend on the phosphate concentration level.

Humic nanoparticles at the oxide-water interface: interactions with phosphate ion adsorption.

In this work, data for the interactions between humic acid (HA) or fulvic acid (FA) with phosphate ions at the surface of goethite (alpha-FeOOH) are presented. The results show very clear differences between HA and FA in their interactions with phosphate at goethite surface. HA is strongly bound to goethite but surprisingly does not strongly affect the phosphate binding, whereas FA is less strongly bound, but these molecules have a very large effect on the phosphate adsorption, and vice versa. Phosphate adsorption to goethite in the presence of adsorbed HA or FA can be well predicted with the LCD model (ligand and charge distribution). According to the model calculations, the nature of the interactions between HA or FA with phosphate at goethite surface is mainly electrostatic. The stronger effects of FA on phosphate adsorption are caused by its spatial location which is closer to the oxide surface, and as a consequence, the electrostatic interactions between adsorbed FA particles and phosphate ions are much stronger than for HA particles. This is the first time that effects of natural organic matter (NOM) on an anion adsorption are predicted successfully using an integrated ion-binding model for oxides and for humics that accounts for chemical heterogeneity of NOM.

Arsenic-bicarbonate interaction on goethite particles.

The As(V) and As(III) interaction with HCO3 has been studied for goethite systems using a pH and As concentration range that is relevant for field situations. Our study shows that dissolved bicarbonate may act as a competitor for both As(V) and As(III). In our closed systems, the largest effect of bicarbonate occurs at the lowest experimental pH values (pH approximately 6.5), which is related to the pH dependency of the carbonate adsorption process. The experimental data have been modeled with the charge distribution (CD) model. The CD model was separately parametrized for goethite with "single ion" adsorption data of HCO3, As(III), and As(V). The competitive effect of HCO3 on the As(III) and As(V) release could be predicted well. Application of the model shows that the natural As loading of aquifer materials (approximately < 0.01-0.1 micromol/m2 or < 1-5 mg/kg) is at least about > 1-2 orders of magnitude smaller than the As loading based on the competition of As-HCO3 alone. It indicates that another, very prominent competitor, like phosphate and natural organic matter, will strongly contribute to the control of As in natural systems.

Carbonate adsorption on goethite in competition with phosphate.

Competitive interaction of carbonate and phosphate on goethite has been studied quantitatively. Both anions are omnipresent in soils, sediments, and other natural systems. The PO4-CO3 interaction has been studied in binary goethite systems containing 0-0.5 M (bi)carbonate, showing the change in the phosphate concentration as a function of pH, goethite concentration, and carbonate loading. In addition, single ion systems have been used to study carbonate adsorption as a function of pH and initial (H)CO3 concentration. The experimental data have been described with the charge distribution (CD) model. The charge distributions of the inner-sphere surface complexes of phosphate and carbonate have been calculated separately using the equilibrium geometries of the surface complexes, which have been optimized with molecular orbital calculations applying density functional theory (MO/DFT). In the CD modeling, we rely for phosphate on recent parameters from the literature. For carbonate, the surface speciation and affinity constants have been found by modeling the competitive effect of CO3 on the phosphate concentration in CO3-PO4 systems. The CO3 constants obtained can also predict the carbonate adsorption in the absence of phosphate very well. A combination of inner- and outer-sphere CO3 complexation is found. The carbonate adsorption is dominated by a bidentate inner-sphere complex, (FeO)2CO. This binuclear bidentate complex can be present in two different geometries that may have a different IR behavior. At a high PO(4) and CO3 loading and a high Na+ concentration, the inner-sphere carbonate complex interacts with a Na+ ion, probably in an outer-sphere fashion. The Na+ binding constant obtained is representative of Na-carbonate complexation in solution. Outer-sphere complex formation is found to be unimportant. The binding constant is comparable with the outer-sphere complexation constants of, e.g., SO(2-)4 and SeO(2-)4.

Metal uptake by Lolium perenne in contaminated soils using a four-step approach.

Most research dealing with soil (solution) speciation and metal uptake by plants has focused on the relationships between a certain bioavailable fraction in the soil and metal uptake by aboveground parts of the plants. Here, a new approach to interpretation of metal uptake is presented that considers four steps: First, the metal concentration in the soil solution is related to the total metal content of the soil. Second, the metal adsorption to the root surface is related to the metal concentration in the soil solution. Third, the metal content in the roots is related to the adsorption of metal ions to the root surface. Fourth, the metal content in the shoots is related to the metal content in the roots. For grass grown on 10 different soils, it is shown that the metal adsorption to the root surface is pH-dependently related to the free or total metal concentration in the soil solution. The metal content in the roots depends linearly on the metal adsorption at the root surface, whereas the metal content in the shoots depends on the metal content in the roots, either linearly (Zn) or reaching a maximum (Cu, Pb, and Cd). For the Ni content in the shoots as a function of the root content, the relation is pH dependent, probably because of the competition effects of Ca. The pH of the soil has to be taken into account when CaCl2 extractions are used as a basis for risk assessment toward plants.

Adsorption of humic acids onto goethite: effects of molar mass, pH and ionic strength.

In this paper, the LCD (ligand charge distribution) model is applied to describe the adsorption of (Tongbersven) humic acid (HA) to goethite. The model considers both electrostatic interactions and chemical binding between HA and goethite. The large size of HA particles limits their close access to the surface. Part of the adsorbed HA particles is located in the compact part at the goethite surface (Stern layers) and the rest in the less structured diffuse double layer (DDL). The model can describe the effects of pH, ionic strength, and loading on the adsorption. Compared to fulvic acid (FA), adsorption of HA is stronger and more pH- and ionic-strength-dependent. The larger number of reactive groups on each HA particle than on a FA particle results in the stronger HA adsorption observed. The stronger pH dependency in HA adsorption is related to the larger number of protons that are coadsorbed with HA due to the higher charge carried by a HA particle than by a FA particle. The positive ionic-strength dependency of HA adsorption can be explained by the conformational change of HA particles with ionic strength. At a higher ionic strength, the decrease of the particle size favors closer contact between the particles and the surface, leading to stronger competition with electrolyte ions for surface charge neutralization and therefore leading to more HA adsorption.

Measuring free metal ion concentrations in multicomponent solutions using the Donnan membrane technique.

Among speciation techniques that are able to measure free metal ion concentrations, the Donnan membrane technique (DMT) has the advantage that it can measure many different free metal ion concentrations simultaneously in a multicomponent sample. Even though the DMT has been applied to several systems, like surface waters, soil solutions, and manure slurry, basic features and calibrations with model calculations of the laboratory and field DMT have not been done sufficiently yet. Therefore, we tested the application of the DMT on metal complexation with several synthetic and natural ligands and the applicability of the dynamic mode of the DMT. The results show that there is a high agreement between the calculated and measured free metal ion concentrations in solutions containing synthetic (nitriloacetic acid, diglycolic acid) and natural organic ligands (fulvic acid, humic acid) at various pH values. Both the laboratory DMT and the field DMT give very similar results. In a solution containing labile ligands, equilibrium time is smaller than in a donor solution containing inert ligands or no ligands. Moreover, when labile ligands are present in the donor solution, a dynamic procedure can be used to decrease equilibrium time. This procedure cannot be applied when no ligands or only inert ligands are present.

Interaction of silicic acid with goethite.

The adsorption of Si on goethite (alpha-FeOOH) has been studied in batch experiments that cover the natural range of Si concentrations as found in the environment. The results have been interpreted and quantified with the charge distribution (CD) and multi-site surface complexation (MUSIC) model in combination with an extended Stern (ES) layer model option. This new double layer approach (ES) accounts for ordering of interfacial water molecules leading to stepwise changes in the location of electrolyte ions near the surface [T. Hiemstra, W.H. Van Riemsdijk, J. Colloid Interface Sci. 301 (2006) 1]. The Si adsorption on goethite peaks at a pH of approximately 9 and decreases at lower and higher pH values. Thermodynamically, the pH-dependency of silicic acid adsorption is related to the value of the proton co-adsorption and can also be linked to the Si charge distribution in the interface as is discussed. Based on published EXAFS data, the adsorption of Si on goethite was modeled as the formation of a bidentate surface complex. The ionic charge distribution (CD) of this complex can be calculated from the geometry of this surface complex, optimized with molecular orbital/density functional theory (MO/DFT), and combined with a correction for water dipole orientation. The resulting CD value has been applied successfully in the description of the adsorption data. The use of a theoretical CD value has the practical advantage of a reduction of the number of adjustable parameters with a factor 2. To describe the adsorption at a high Si loading, formation of a Si polymer, e.g. a tetramer, is proposed. Such a species is only contributing to the overall adsorption at solution concentrations above about 10(-4) M, where super saturation with respect to quartz exists. The adsorbed silica polymer hydrolyzes at high pH. The reactive ligand of the polymer is quite acid (logK approximately 6.5-7.1), which is typically for the triple bond SiO(-1) surface groups of polymerized Si, like amorphous SiO(2)(s), and the triple bond SiO(-1) ligand of the aqueous dimer Si(2)O(OH)(5)O(-1)(aq). The applied model correctly predicts the change of particles charge and the shift in IEP due to proton release upon Si adsorption.

Geometry, charge distribution, and surface speciation of phosphate on goethite.

The surface speciation of phosphate has been evaluated with surface complexation modeling using an interfacial charge distribution (CD) approach based on ion adsorption and ordering of interfacial water. In the CD model, the charge of adsorbed ions is distributed over two electrostatic potentials in the double-layer profile. The CD is related to the structure of the surface complex. A new approach is followed in which the CD values of the various surface complexes have been calculated theoretically from the geometries of the surface complexes. Molecular orbital calculations based on density functional theory (MO/DFT) have been used to optimize the structure of a series of hydrated surface complexes of phosphate. These theoretical CD values are corrected for dipole orientation effects. Data analysis of the PO4 adsorption, applying the independently derived CD coefficients, resolves the presence of two dominant surface species. A nonprotonated bidentate (B) complex is dominant over a broad range of pH values at low loading (< or =1.5 micromol/m(2)). For low pH and high loading, a strong contribution of a singly protonated monodentate (MH or MH-Na) complex is found, which differs from earlier interpretations. For the conditions studied, the doubly protonated bidentate (BH2) and monodentate (MH2) surface complexes and the nonprotonated monodentate (M) complex are not significant contributors. These findings are discussed qualitatively and quantitatively in relation to published experimental in-situ CIR-FTIR data and theoretical MO/DFT-IR information. The relative variation in the peak intensities as a function of pH and loading approximately agrees with the surface speciation calculated with the CD model. The model correctly predicts the proton co-adsorption of phosphate binding on goethite and the shift of the IEP at low phosphate loading (< or =1.5 micromol/m(2)). At higher loading, it deviates.