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Natural goethite commonly encounters Al substitution for Fe, which alters the surface reactivity of goethite. Whether and how Al substitution affects the surface speciation of phosphate remain poorly understood. Therefore, the phosphate adsorption and speciation of Al-substituted (6 and 9 mol%) goethite were investigated by combining attenuated total reflectance Fourier transform infrared (ATR-FTIR) and CD-MUSIC modeling. Al substitution caused a decrease in the crystal length-to-width ratio of goethite and the ratio of (110)/(021) faces was estimated to be 90/10, 85/10 and 81/19 for goethite with about 0, 6 and 9 mol% Al, respectively. The decreasing (110)/(021) face ratio and increasing surface roughness with increasing Al substitution enhanced the surface charge density and phosphate adsorption capacity of goethite. Two-dimensional correlation spectroscopy (2D-COS) analysis showed that three types of phosphate complexes were formed on both pure and Al-substituted goethites. CD-MUSIC modeling, based on electron microscopy and ATR-FTIR results to arrive at some of the parameters, could describe both the proton and the phosphate adsorption well with optimized remaining parameters. The model parameters indicated that Al substitution in goethite led to larger affinity constants for bidentate phosphate complexes and smaller values for monodentate complexes. Therefore, a larger percentage of bidentate phosphate complexes were formed on Al-substituted goethite than on pure goethite. Monodentate phosphate complexes were hardly formed on the Al sites of Al-substituted goethite. The results improve our insight regarding phosphate binding to Al-substituted goethite and phosphorus availability and mobility in iron-rich soils.

The surface complexation modeling of metal adsorption to birnessites is in its infancy compared to the charge-distribution multisite ion complexation (CD-MUSIC) models for iron/aluminum (hydr)oxides. Therefore, using X-ray diffraction with Rietveld refinement to obtain the reactive sites and their densities, a CD-MUSIC model combined with a Stern–Gouy–Chapman electrical double layer (EDL) model for the external surface and a Donnan model for the interlayer surface is developed for birnessites with different Mn average oxidation state (MnAOS). Proton affinity constants and the charge distributions of Pb surface complexes were calculated a priori. By fitting Pb adsorption data to the model the obtained equilibrium constants (logKPb) of Pb complexes were 6.9–10.9 for the double-corner-sharing and double-edge-sharing Pb2+complexes on the edge sites and 2.2–6.5 for the triple-corner-sharing Pb2+complex on the vacancies. The larger logKPbvalue was obtained for higher MnAOS. Speciation calculations showed that with increasing MnAOS from 3.67 to 3.92 the interlayer surface contribution to the total Pb2+adsorption increased from 43.2% to 48.6%, and the vacancy contribution increased from 43.9% to 54.7%. The vacancy contribution from interlayer surface was predominant. The present CD-MUSIC-EDL model contributes to understand better the difference in metal adsorption mechanism between birnessite and iron/aluminum (hydr)oxides.

Proton and copper (Cu) binding to soil and lignite-based humic acid (HA) was investigated by combining X-ray absorption fine structure (XAFS) spectroscopy, isothermal titration calorimetry (ITC), and nonideal-competitive-adsorption (NICA) modeling. NICA model calculations and XAFS results showed that bidentate and monodentate complexation occurred for Cu binding to HA. The site-type-specific thermodynamic parameters obtained by combining ITC measurements and NICA calculations revealed that copper binding to deprotonated carboxylic-type sites was entropically driven and that to deprotonated phenolic-type sites was driven by entropy and enthalpy. Copper binding to HA largely depended on the site-type and coordination environment, but the thermodynamic binding mechanisms for Cu binding to the specific site-types were similar for the different HAs studied. By comparing the site-type-specific thermodynamic parameters of HA–Cu complexation with those of low molar mass organic acids, the Cu coordination could be further specified. Bidentate carboxylic–Cu complexes made the dominating contributions to Cu binding to HA. The present study not only yields molecular-scale mechanisms of ion binding to carboxylic- and phenolic-type sites of HA but also provides the new insight that the universal nature of site-type-specific thermodynamic data enables quantitative estimation of the binding structures of heavy metal ions to humic substances.

The structure and magnetic properties of nano-sized (1.6 to 4.4 nm) ferrihydrite samples are systematically investigated through a combination of X-ray diffraction (XRD), X-ray pair distribution function (PDF), X-ray absorption spectroscopy (XAS) and magnetic analyses. The XRD, PDF and Fe K-edge XAS data of the ferrihydrite samples are all fitted well with the Michel ferrihydrite model, indicating similar local-, medium- and long-range ordered structures. PDF and XAS fitting results indicate that, with increasing crystallite size, the average coordination numbers of Fe–Fe and the unit cell parameter cincrease, while Fe2 and Fe3 vacancies and the unit cell parameter adecrease. Mössbauer results indicate that the surface layer is relatively disordered, which might have been caused by the random distribution of Fe vacancies. These results support Hiemstra's surface-depletion model in terms of the location of disorder and the variations of Fe2 and Fe3 occupancies with size. Magnetic data indicate that the ferrihydrite samples show antiferromagnetism superimposed with a ferromagnetic-like moment at lower temperatures (100 K and 10 K), but ferrihydrite is paramagnetic at room temperature. In addition, both the magnetization and coercivity decrease with increasing ferrihydrite crystallite size due to strong surface effects in fine-grained ferrihydrites. Smaller ferrihydrite samples show less magnetic hyperfine splitting and a lower unblocking temperature (TB) than larger samples. The dependence of magnetic properties on grain size for nano-sized ferrihydrite provides a practical way to determine the crystallite size of ferrihydrite quantitatively in natural environments or artificial systems.

Goethite and kaolinite are ubiquitous in natural environments. In soils they are often cemented together as a binary association, which has a significant influence on the structure and properties of soils. In this study, the mineralogy (using X‐ray diffraction [XRD], thermal analyses, and infrared analysis), interactions, and stability of a goethite–kaolinite association (GKA) and a goethite–kaolinite mixture (GKM) were investigated. In GKA, goethite almost completely coated the surface of kaolinite, whereas for GKM the coating was partial. Infrared (IR) spectra showed that the vibrational frequencies of ≡Fe–OH in GKM and GKA shifted to higher wave numbers and those of ≡Al–OH, O–Al–O, Al–O–Si, and Fe–O shifted to lower wave numbers. Magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy demonstrated that interactions between goethite and kaolinite increased the chemical shifts of 29Si and 27Al. The shifts of both the IR and the MAS NMR parameters were larger for GKA than for GKM. In addition, the IR bands of ≡Al–OH and the XRD peaks of goethite were weaker with GKA than with GKM. As expected, particle size distributions showed that GKA had a higher mechanical stability than GKM. The thermal stability of the kaolinite in GKA was higher than that of GKM, but that of goethite in GKA was lower than that in GKM. These results imply that: (i) kaolinite and goethite interact by electrostatic attraction, anion ligand exchange, surface coordination, and hydrogen bonds; (ii) the interaction strength was much stronger for GKA than for GKM; (iii) a cation exchange reaction and isomorphous substitution of Fe by Al for goethite occurred in GKA.

Iron oxide–kaolinite associations are important components of tropical and subtropical soils and have significant influence on the physical and chemical properties of soils. In this study, the formation and transformation of Fe oxide–kaolinite associations as a function of pH, temperature, and time were investigated at different Fe(II)/Fe(III) molar ratios (R). Results show that the formation of crystalline Fe oxides was significantly inhibited due to the presence of kaolinite, while accelerated by Fe(II). The formation of lepidocrocite– and goethite–kaolinite associations were accelerated by Fe(II) at R= 0.04 to 0.06, an initial pH (pHi) of 5 to 8, and a temperature (T) of 50 to 70°C; the formation of hematite–kaolinite association was accelerated by Fe(II) at R= 0.06, pHi7 to 8, and T= 60 to 80°C; magnetite–kaolinite association was obtained at R= 0.06, pHi9, and T= 60°C or at R= 0.1 to 0.5, pHi7, and T= 60°C. The pH as a function of time (pHt) decreased sharply when crystalline Fe oxides were formed in the presence of Fe(II), Fe(III), and kaolinite. The decrease in pHtwas slow, however, in the system with Fe(III) and kaolinite but without Fe(II) and in the system with Fe(II) and kaolinite but without Fe(III). The morphologies of lepidocrocite, goethite, hematite, and magnetite in associations are strip shaped, nanorod like, pseudo‐cubic shaped, and nanosphere like, respectively. In a system with Fe(II), Fe(III), and kaolinite, Fe(II) weakened the inhibition of the formation of crystalline Fe oxides by kaolinite; the presence of kaolinite decreased the acceleration by Fe(II); and the morphologies of Fe oxides were influenced by both Fe(II) and kaolinite.

Manganese (Mn) oxides are important components in soils and sediments. Points-of-zero charge (PZC) of three synthetic Mn oxides (birnessite, cryptomelane, and todorokite) were determined by using three classical techniques (potentiometric titration or PT, rapid PT or RPT, and salt titration or ST) and a modified salt titration method with a prolonged equilibration time (ST method with a prolonged equilibration time PST). The same methods have been applied to goethite, which was used as a reference material. The PZC values of goethite obtained by PT and RPT methods were both 7.95, and those by the ST and PST method were 8.16 and 8.30, respectively, for birnessite cryptomelane, and todorokite. The PT method yielded PZC of 1.18, 1.98, and 3.98, and the RPT method yielded 1.60, 2.11, and 3.47, respectively, for birnessite, cryptomelane, and todorokite. In contrast to goethite, there was no PZC found with the ST method, even when the types and concentrations of the added electrolytes changed. However, when after KCl addition the equilibration time was prolonged 28 h, the PZC of birnessite, cryptomelane, and todorokite could be found and were 0.97, 1.74, and 3.39, respectively. The fact that the normal ST method failed for the Mn oxides is due to their low PZC, because at this low pH value, the oxides may start to dissolve. Compared with PT and RPT methods, the PST method is reliable, simple, and convenient. The PST approach seems also suitable for other similar colloid systems.

A theory of ionic surfactant aggregation on oppositely charged surfaces is presented. In the proposed model the adsorbed phase is considered as a mixture of singly dispersed surfactant molecules, monolayered and bilayered aggregates of various sizes and the ions of simple electrolyte added to the aqueous solution. The electrostatic interactions between the adsorbed ions (surfactant ions and simple ions) and the charged surface are described by using the triple layer model (TLM) formalism. The calculated surfactant adsorption isotherms, surface charge isotherms, and electrophoretic mobilities of silica particles are compared with the experimental results obtained for two cationic surfactants (dodecylpyridinium chloride and cetylpyridinium chloride) adsorbed on Aerosil OX50 at two different values of pH. On the basis of the theoretical results the evolution of the adsorbed phase structure and the charge of silica particles with an increasing surface coverage is discussed.

Humic substances are soil and fresh-water components that play an important role in the binding and transport of both organic and inorganic contaminants. Transport of the contaminants due to ground- and fresh-water dynamics is directly related to the risks associated with contaminations. The mobility of soluble humic substances is related to their interaction with soil mineral particles. Some key references for the binding of organic and inorganic contaminants and for the binding of humics to mineral particles are presented. Humic substances also play a role in the analysis of the contaminants in natural waters and with remediation of water or soil polluted with pesticides, heavy metal ions, and radionuclides. These aspects are illustrated with some examples. The problems that are encountered with the modeling of the binding of contaminants to humics and of heavy metal ions in particular are illustrated by considering the nonideal competitive adsorption model (NICA) extended with electrostatic interactions. The NICA-Donnan model gives quite good results for the description of metal ion binding, as is illustrated for metal ion binding to purified peat humic acid (PPHA). Finally, some remarks are made with respect to the use of the NICA-Donnan model in general purpose speciation programs and of simplified versions of the model for predictions under restricted environmental conditions.

The proton adsorption to a mixture of purified Aldrich humic acid (PAHA) and hematite is investigated. Basic insight into the charge adjustment process is obtained by using a self-consistent-field lattice theory for polyelectrolyte adsorption. The calculations indicate that upon adsorption the component with the highest initial charge density tends to induce charges on the other component. The number of induced charges can show a maximum when the surface charge and the charge of the segments in direct contact with the surface roughly balance each other. Experimentally, the humic acid–hematite system is investigated by proton titrations. The alterations in charge density caused by adsorption of PAHA to hematite are investigated by comparing the proton adsorption on the individual samples with that on their mixtures. Upon adsorption a part of the functional groups of humic acid forms complexes with some of the surface sites of hematite. This interaction reduces the proton binding to the humic acid at relatively low pH and it promotes the proton adsorption on the oxide surface at relatively high pH.

The proton adsorption behavior of eight humic acids (HA) and a fulvic acid (FA) was studied as a function of pH and KNO3concentration. The emphasis is on the comparison of the different humics with respect to their ion binding properties and on the comparison of two different models to describe the electrostatic interactions: the Donnan model and the impermeable sphere (IS) model. Viscosimetric data were used to estimate the hydrodynamic volumes and radii of the HA molecules. These data were incorporated in the electrostatic models and calculations could be carried out without any adjustable parameter. The Donnan model in combination with hydrodynamic volumes obtained by viscometry cannot adequately describe the electrostatic effects related to changes of the electrolyte concentration. This model leads to good prediction of the HA behavior if unrealistically large volumes are used for fulvics and unrealistically large volume-salt concentration dependencies are used for humics. The IS model can successfully reproduce experimental proton adsorption data with physically realistic radii. The good performance of the IS model and the poor performance of the Donnan model is directly related to the fact that the hydrodynamic volumes of the molecules are too small to allow for charge compensation within the molecular limits. The combination of viscometry with the IS model leads to a consistent description of the electrostatic in humics and to a consistent way of positioning the master curves. Therefore, the electrostatic potentials and the intrinsic affinity distributions of the different samples can be compared on an equal basis. The similarities in the intrinsic affinity distributions give faith for the possibility to develop a generic model to describe the ion binding to humics.

The adsorption of cationic (alkyl pyridinium salts) and anionic (alkyl benzene sulphonates) surfactants on the mineral oxide rutile is studied as a function of surfactant concentration, surfactant chain length, solution pH, and ionic strength in solution. Experimental results are complemented with model calculations based on the self-consistent (mean) field lattice theory for adsorption and association (SCFA). In the theory no ad hoc assumptions are made with respect to the adsorbed layer structure, but local aggregates cannot be considered. Both the experiment and theory show that the adsorption isotherms, plotted as the log of the adsorption versus the log of the equilibrium concentration, can be divided into four characteristic regions. This behavior has been found previously for anionic surfactants adsorbed on metal oxides, but for cationic surfactants on metal oxides this is a novelty. The four-region isotherm vanishes at pH values close to the point of zero charge of the oxide and when the background electrolyte concentration is on the order of 0.1 M. In region I, isolated molecules adsorb. In region II, hemi-micelles, defined as aggregates of "head-on" adsorbed surfactant molecules, are formed. In region III, the hemi-micelles transform into ad-micelles, defined as aggregates of both "head-on" and "head-out" adsorbed surfactant molecules. Region IV occurs once the CMC is reached. The transition from hemi-micelles to ad-micelles is highlighted by an (approximately) common intersection point between isotherms measured at different salt concentrations. Salt concentration and pH govern the Coulombic interactions; their effect observed in praxis is also found with the model calculations. In addition to the similarity in adsorption behavior of the cationic and the anionic surfactant on oppositely charged rutile, specific differences resulting from the different head groups are discussed. Copyright 1995, 1999 Academic Press

Analytical isotherm equations for the competitive binding of protons and other cations to heterogeneous surfaces have been derived. These extend our earlier nonideal competitive adsorption (NICA) equation by considering bidentate as well as monodentate binding of ions. The protonation of these bidentate sites occurs in two COnsecutive steps. The CONICA model equations separate the apparent heterogeneity into a generic heterogeneity characteristic of all of the surface sites and an ion-specific contribution characteristic of each type of ion. The CONICA model was combined with a simple Donnan model to account for electrostatic effects and was fitted both to proton binding at various ionic strengths and to the pH-dependent binding of Cu (pH 4, 6, and 8) to a purified peat humic acid. Model fits were good. The measured H+/Cu2+exchange ratio is significantly greater than one for the whole pH range, which indicates the importance of the bidentate binding mechanism for copper.

The adsorption of cationic and anionic surfactants on rutile is studied as a function of surfactant concentration and ionic strength in solution. The pH is kept constant for each series of isotherms. Special attention is given to the extent of adaptation of the surface charge as a function of surfactant adsorption. Experimental results are complemented with model calculations based on the self-consistent (mean) field lattice theory for adsorption and association (SCFA) to confirm the correctness of the given interpretation. The adsorption behavior of sodium nonyl benzene sulfonate (SNBS) and dodecyl or tetradecyl pyridinium chloride (DPC/TPC) on rutile is broadly similar. Isotherms can be divided in two regions by the (approximately) common intersection point (cip) between isotherms measured at different salt concentrations. Before the cip adsorption occurs head-on (hemi-micelles); after the cip both head-on and head-out adsorption (ad-micelles) occur. Intersection points similar to the cip occur in the proton excess and the net surface charge/surfactant charge plots. Although broadly similar in their adsorption behavior, the SNBS and DPC/TPC systems differ markedly if the charge situation near the interface is considered. SNBS follows simple rules; the strongly adsorbing SNBS head groups can compete easily with the salt ions for the surface sites. The point where the surface charge and the surfactant charge balance, the equivalence point, and the cip in the isotherms coincide. Moreover, the cip corresponds with the isoelectric point (iep). The DPC and TPC systems behave in a more complicated way, due to their relatively weak interaction with the surface. The cip in the isotherms and the cip in the net surface/surfactant charge plots occur at much lower adsorption values than the equivalence point. Co-ions thus contribute at the cip to the charge balance in the surface region. In this situation the iep will occur at lower adsorption values than the cip.

Ion binding to humics can only be satisfactorily described if the chemical heterogeneity is explicitly included in the binding model. For proton binding, a model based on a bimodal Langmuir-Freundlich isotherm plus a cylindrical electrical double-layer model provides a good description. Data for Cd and Ca binding to a purified peat humic acid could not be satisfactorily described by merely shifting the affinity distribution along the affinity axis (the fully coupled case). Assuming that the low pH ("carboxylic") and high pH ("phenolic") parts of the distribution are shifted independently (the highly correlated case) gave a better description but the shift for the phenolic-type sites was so large that it appears that Cd²⁺ and Ca²⁺ are not able to compete effectively with protons for binding at these sites. The uncoupled case assumes that proton and metal binding occur on different sites and only interact through the common surface electrical field. This model did not fit the data well. However, when the metal concentration, M, was substituted by the ratio, M/(H)^x (H is the concentrations of protons and x is an adjustable parameter), a good fit to the experimental data was obtained. It is likely that the incorporation of ion specific nonidealities into the fully coupled model would improve it. Copyright 1995, 1999 Academic Press