The purpose of this study was to investigate bonding mechanisms of representative inorganic anions and citrate with imogolite and allophane using electrophoresis. The electrophoretic mobility (EM) of synthetic imogolite and allophanes with Al/Si molar ratios of 2.02, 1.64, and 1.26 was determined in 0.001 and 0.01 M sodium solutions. The highest point of zero mobility (PZM) values for imogolite and the highest point of zero charge (PZC) values for allophane occurred in the presence of ClO4, NO3, Br, I, and Cl. Below the PZM and PZC, Cl and I lowered the EM relative to the other anions but did not shift the PZM and PZC significantly. This indicates that Cl and I formed more outer-sphere complexes than the other ions. The EM of imogolite and allophane was negative at pH < 6 in 0.001 and 0.01 M NaF probably due to a phase change. We observed the formation of cryolite (Na3AlF6) with transmission electron microscopy (TEM) and X-ray diffraction (XRD) in the NaF systems at low pH. Conversely, phosphate at 0.001 and 0.01 M concentrations lowered both the PZM and the EM in imogolite and both the PZC and the EM in allophane compared with ClO4. Phosphate-treated allophane had the same PZC as a synthetic amorphous aluminum phosphate. The PZM values of imogolite and allophane with 2:1 Al/Si in 0.0001 M Na-citrate were 10.9 and 5.9, respectively. At pH 7.3, Na-citrate lowered allophane EM more than it lowered imogolite EM relative to ClO4.

The EM in NaClO4 and Na2SO4 was reversible by forward- and back-titration with NaOH and HClO4, indicated that ClO4 and SO4 were not specifically adsorbed. Chloride likely formed more outer-sphere complexes than ClO4. Imogolite EM and allophane EM in dilute NaF and NaH2PO4 solutions were not reversible, indicating either surface inner-sphere complexes or surface precipitates of aluminum fluoride and amorphous aluminum phosphate-like materials on these minerals. Sulfate gave a lower EM than the monovalent anions, implying a greater tendency to form outer-sphere complexes. Citrate appeared to form inner-sphere complexes on both imogolite and allophane, but formation was concentration-dependent. The tendency of anions to form surface complexes with imogolite and allophane is consistent with the tendency of anions to form soluble aluminum complexes.

A critical demand in environmental modeling and a desirable but elusive goal of research on the ion exchange properties of the charged solid surface has been to determine the selectivity coefficient from fundamental properties of the ions and surface. We developed a Hard and Soft Acid and Base (HSAB) Model to describe exchangeable cation selectivity on solid surfaces. Our previous work has shown that the model quantitatively describes alkali cation exchange on clay minerals in terms of the absolute electronegativity and softness of the exchangeable cations and two fitting parameters: α and β. This study was conducted to determine the relationship between α and β and surface charge characteristics of 2:1 clays. The layer charge and cation selectivity of seven smectites and one vermiculite were used. The regression of log Kvo against four combinations of charge properties was performed and the appropriate relationship between α, β, and surface charge was selected based on both statistical criteria (R2) and their consistency with the assumptions of the HSAB model. The selected model was then cross-validated using separate cation exchange data from the literature. It was found that α and β are linearly related to the amount of charge arising from mineral tetrahedral and octahedral sites, respectively. These results make it possible to predict the alkali cation selectivity of 2:1 clay minerals from their chemical composition data and the alkali cation properties.

Noncrystalline aluminosilicates termed allophane and imogolite are common constituents of spodosols, soils derived from volcanic ash, and many inceptisols. The surface charge characteristics of their synthetic analogues may be used to better understand their ion retention properties. In this study, we determined the point of zero salt effect (PZSE) by potentiometric titration of allophanes with Al/Si ratios of 1.12, 1.52, and 2.04 and of imogolite with an Al/Si ratio of 2.02. We also used microelectrophoresis to determine the point of zero charge (PZC) at the particle shear plane for the same materials in CI solutions of Li, Na, Cs, and tetramethyl ammonium. The PZSE decreased with decreasing Al/Si ratio for the allophanes, but the imogolite PZSE was much lower than that of the allophane with 2.04 Al/Si. The PZC was always higher than the PZSE of the same material, especially for imogolite. The results are best explained if cations reside within the hollow tubes of imogolite. This conclusion is supported by a fluorescence study that showed that only quenchers smaller than the inner diameter of the imogolite tube could fully quench Ce-imogolite.

Boron adsorption at constant ionic strength [0.09 ± 0.01 moles/liter of KClO4 or Ca(ClO4)2] on 0.2−2 μm clay fraction of pretreated kaolinite was modeled using both phenomenological equations and surface complexation reactions. Phenomenological equations were expressed as linear relationships between the distribution coefficient and adsorption density or equilibrium concentration. The normalized form of the isotherms allowed the distribution coefficient to be predicted over a wide range of adsorption densities or equilibrium concentrations and pH. The Langmuir isotherm revealed a weak two-part linear trend supported by a similar behavior of the van Bemmelen-Freundlich isotherm. Potential adsorption mechanisms were assessed from these isotherms. The bases for the inner-sphere (surface coordination) and outer-sphere (ion-pair) surface reactions were postulated, and equations were developed and incorporated into the generalized triple-layer surface-complexation model [TL(g)-SCM]. Boron adsorption was best modeled using the inner-sphere complexes. The results confirm that the generalized triple-layer surface-complexation model can provide information regarding plausible reactions at the substrate/aqueous interface. Intrinsic constants for postulated surface reactions were derived as fitting parameters over a range of pH and initial boron concentrations.

Batch sorption experiments at fixed initial Np(V) concentration (∼1 × 10−6 M 237Np), M/V ratio (4 g L−1), and ionic strength (0.1 molal NaNO3) were conducted to determine the effects of varying pH and $P_{C{O}_2}$ on Np(V) sorption on SAz-1 montmorillonite. The results show that Np(V) sorption on montmorillonite is strongly influenced by pH and $P_{C{O}_2}$. In the absence of CO2, Np(V) sorption increases over the entire pH range examined (∼3 to ∼10), with measured sorption coefficients (KD) of about 10 mL g−1 at pH < 6 to KD ∼ 1000 mL g−1 at a pH of 10.5. However, for experiments open to atmospheric CO2 ($P_{C{O}_2}={10}^{-3.5}atm$), Np(V) sorption peaks at KD ∼ 100 mL g−1 at pH of 8 to 8.5 and decreases at higher or lower pH. A comparison of the pH-dependence of Np(V) sorption with that of Np(V) aqueous speciation indicates a close correlation between Np(V) sorption and the stability field of the Np(V)-hydroxy complex NpO2OH0 (aq). In the presence of CO2 and aqueous carbonate, sorption is inhibited at pH > 8 due to formation of aqueous Np(V)-carbonate complexes. A relatively simple 2-site Diffuse-Layer Model (DLM) with a single Np(V) surface complexation reaction per site effectively simulates the complex sorption behavior observed in the Np(V)-H2O-CO2-montmorillonite system. The good agreement between measured and DLM-predicted sorption values suggests that surface complexation models based on parameters derived from a limited set of data could be useful in extrapolating radionuclide sorption over a range of geochemical conditions. Such an approach could be used to support transport modeling and could provide a better alternative to the current use of constant KD values in performance assessment transport calculations.

Acid-base titrations and attenuated total reflectance-infrared (ATR-IR) spectroscopy of solutions containing Zn(NO3)2 and the herbicide 3-amino-1,2,4-triazole suggested that soluble complexes ZnL2+ and Zn(OH)L+ form, where L represents aminotriazole. Sorption experiments and modeling in systems containing K-saturated Wyoming (SWy-K) montmorillonite suggest that at low concentrations the aminotriazole sorbs primarily in cationic form via an ion-exchange mechanism. Sorption isotherms for aminotriazole are ‘s’-shaped, indicating a co-operative sorption mechanism as the concentration of the molecule increases. At higher concentrations, ATR-IR spectroscopy indicated the presence of cationic and neutral triazole molecules on the surface, while X-ray diffraction data suggest interaction with interlayer regions of the clay. When the concentration of the herbicide was high, initial sorption of aminotriazole cations modified the clay to make the partitioning of neutral molecules to the surface more favorable. Experiments conducted in the presence of Zn(II) indicated that below pH 7, Zn(II) and aminotriazole compete for sorption sites, while above pH 7 the presence of Zn(II) enhances the uptake of aminotriazole. The enhancement was attributed to the formation of an inner-sphere ternary surface complex at hydroxyl sites (SOH) on crystal edges, having the form [(SOZn(OH)L)]0.

Adsorption of the ${\text{AuCl}}_4^{-}$ complex by kaolinites of different crystallinities (Kao-1 and Kao-2) from 100 mL of AuCl3·HCl·4H2O solutions containing 10,000, 500 and 50 µg/L Au and 1 g of kaolinite was measured at pH 3 to 9 at ambient temperature and 120°C. Adsorption from the 50, 500 and 10,000 µg Au/L solutions ranged from 64 to 100% at ambient temperature and from 68 to 100% at 120°C for both kaolinites. Adsorption was pH dependent with a maximum at pH <5 and a minimum at neutral and alkaline pHs. Up to 1 mg Au/g kaolinite was adsorbed by the kaolinites at both ambient temperature and 120°C. In a separate Au adsorption experiment using 100 mL of 4000 µg Au/L solutions and 0.02 to 1.0 g of Kao-1, up to 8.55 mg Au/ g of kaolinite was adsorbed. The pH dependence of Au adsorption suggests that surface complexation of Au to alumina sites at the edges of kaolinite particles might be involved. Protonation of kaolinite surface sites might facilitate adsorption of the anionic Au complex. Both kaolinites adsorbed ~100% of added Au at low pH values, but the less crystalline kaolinite (Kao-2) adsorbed more Au at high pH. Greater Au adsorption would be expected for the less crystalline Kao-2 sample if adsorption occurred at the edges of kaolinite particles.

Six-membered ring heterocyclic compounds are widely present in the Earth's surface environments as biological organic molecules composed of soil organic matter including plant and microbial residues, while little is known about their effect on the dissolution of silicate minerals including amorphous silica. To evaluate the effect of these biological molecules on amorphous silica dissolution, dissolution experiments were carried out by the flow-through method using 0.1 g of amorphous silica and 0.1 mM NaCl electrolyte solutions containing 0.0, 0.1, 1.0, or 10.0 mM of the heterocyclic compounds, piperidine (pK = 11.12), pyridine (pK = 5.25), or pyridazine (pK = 2.33), at a pH of 6, 5, or 4. Additionally, adsorption experiments of the compounds on the amorphous silica surface were performed to confirm the adsorption affinity for the amorphous silica surface. The results demonstrated that these heterocyclic compounds enhance the dissolution rate of amorphous silica in the following order: piperidine > pyridine > pyridazine. When 10.0 mM solutions were used, the heterocyclic compounds enhanced greatly the dissolution rate up to enhancement factors of 6.0 to ~14.8, 5.0 to ~14.0, and 1.0 to ~2.6 through an interaction of piperidine, pyridine, and pyridazine, respectively, in the pH range of approximately 6 to ~ 4. The adsorption experiments indicated that the heterocyclic compounds exhibited significant adsorption affinity for the amorphous silica surface as follows: piperidine > pyridine > pyridazine, which was consistent with the order of their effects on the dissolution enhancement. The geochemical calculation revealed that this order of enhancement was in good agreement with the concentrations of cationic species of heterocyclic compounds at corresponding pH conditions. Consequently, the enhancement of amorphous silica dissolution is likely to be influenced by the electrostatic complexation of the cationic species of the heterocyclic compounds with the negative >SiO– sites on the amorphous silica surface.