The sorption of (poly)molybdate ions into layered double hydroxides (LDHs), with Zn(2+) and Al(3+) cations, has been followed by in situ infrared spectroscopy using the attenuated total reflection technique. The exchange between solution molybdate species and interlayer anions has been followed in real time, illustrating the different behavior of molybdate ions and polymolybdate species. In a first part, the Mo(VI) speciation in solution was performed by comparison of thermodynamical calculations and infrared spectroscopy of solutions with different pH. Decomposition of bands between 800 to 1000 cm(-1), corresponding to the (Mo-O) stretching vibration, has permitted to identify major (poly)molybdate species. In the presence of LDH, the measurements have shown a high affinity for heptamolybdate (Mo7O24(6-)) species, and its preferential sorption in comparison with molybdate ions or other protonated polymolybdate species even if it represents very small fractions. From these measurements, the affinity series Mo7O24(6-) > CO3(2-) > MoO4(2-) > SO4(2-) have been directly obtained.

In situ polarized ATR-FTIR spectroscopy and the DFT+U calculations were used to investigate the Mo(VI) ions surface speciation on lepidocrocite. Adsorption of molybdate ions is found to be tetrahedral monodentate binuclear (C 3v ) on the (010) face. DFT+U calculations were performed to help characterizing the geometries of the complexes adsorbed to assign the vibrational frequencies and get insight into the energies of adsorption. The (010) surface was chosen as a model of lepidocrocite surface. Calculations are consistent with the sorption of MoO 4 2− ions to form a monodentate complex at basic pH. This complex is protonated when the surface coverage increases, and the pH decreases. At the full coverage, calculations show alternate monodentate protonated binuclear complexes and surface hydroxyl groups. Moreoever, the formation of hydrogen bonds with the surface hydroxyls and between adsorbed complexes plays an important role in the structure stability.

The study of the interactions between dissolved Mo(VI) or W(VI) species and the surfaces of metal (hydr)oxides is relevant for two main areas: the optimization of the preparation of catalytic supports and the understanding of the environmental fate of these elements. For the latter, iron (hydr)oxides are the most important sinks for pollutants, and recently, their surface reactivity was the focus of many research works. In this work, we develop a joint approach, using in situ infrared spectroscopy and DFT simulations, to characterize the Mo(VI) and W(VI) species adsorbed on hematite (α-Fe 2 O 3 ). Surface sorbed polymers of tungstate and molybdate on hematite were identified for low pH and at high concentration of these elements, which is similar to the formation of polyoxometalates in solution phase. However, the surface speciation is different from the adsorption of polymolybdate or polytungstate already formed in solution and should be consistent with the growth of a surface polymer. For low concentrations/high pH conditions, the spectra are consistent with a monodentate surface complexation.

Owing to the suspected toxicity and carcinogenicity of tungstate (VI) oxyanions [i.e. mono tungstate and several polytungstate, generally represented by W (VI)], the environmental fate of W (VI) has been widely studied. Sorption is regarded as a major mechanism by which W (VI) species are retained in the solid/water interface. Iron (hydr)oxides have been considered important environmental sinks for W (VI) species. Here we report sorption mechanisms of W (VI) on a common iron oxide mineral-hematite under environmentally relevant solution properties using in situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopic probes. Initial W (VI) loadings varied from 10 to 200 μM at fixed pH values ranged from 4.6 to 8.1. For pH envelop (pHs = 4.6, 5.0, 5.5, 6.0, 6.5, 7.5, and 8.1) experiments, fixed W (VI) concentrations (i.e. 10 & 200 μM) were used to understand the effects of pH. The results indicated that at acidic pH values (pH < 6.0) the sorbed polytungstate surface species are prominent at 200 μM initial W (VI) conc. The pH envelop experiments revealed that sorbed polytungstates can be present even at lower initial W (VI) conc. (i.e. 10 μM) at pH values <5.5. Overall, our in situ ATR-FTIR experiments indicated that W (VI) forms inner-sphere type bonds on hematite surface and the strength of the interaction increases with decreasing pH. In addition, initial W (VI) concentration affected the sorption mechanisms of W (VI) on hematite. Our study will aid the molecular level understanding of W (VI) retention on iron oxide surfaces.

In situ attenuated total reflection infrared spectroscopy (ATR-FTIR) was used to study the speciation of (poly)tungstate ions in solution and the sorption process into layered double hydroxides (LDHs). In the first section, the W(VI) speciation in solution was performed by comparison of thermodynamical calculations and infrared spectroscopy of solutions as a function of pH. Then, the exchange mechanism within LDHs was followed in real time with the in situ ATR technique and compared with batch experiments characterized by powder X-ray diffraction (PXRD), elemental analysis, and FTIR. Decomposition of the (W−O) stretching vibration bands, between 800 and 1000 cm −1 , has allowed the identification of the high affinity of LDHs for the polytungstate species, W 7 O 24 6− , despite the presence of carbonate ions and higher charged polytungstate ions in solution. This was confirmed by the DFT methods used to investigate the structure and the vibrational modes of this anion, and very good agreement with experimental spectra has been obtained. The affinity series W 7 O 24 6− > CO 3 2− > WO 4 2− > SO 4 2− has been directly deduced from those results.

Adsorption of hyperbranched arabinogalactan-proteins (AGPs) from two plant exudates, A. senegal and A. seyal, was thoroughly studied at the solid–liquid interface using quartz crystal microbalance with dissipation monitoring (QCM-D), surface plasmon resonance (SPR), and atomic force microscopy (AFM). Isotherms of the adsorption reveal that 3.3 fold more AGPs from A. seyal (500 ppm) are needed to cover the gold surface compared to A. senegal (150 ppm). The pH and salt concentration of the environment greatly affected the adsorption behavior of both gums, with the surface density ranging from 0.92 to 3.83 mg m−2 using SPR (i.e., “dry” mass) and from 1.16 to 19.07 mg m−2 using QCM-D (wet mass). Surprisingly, the mass adsorbed was the highest in conditions of strong electrostatic repulsions between the gold substrate and AGPs, i.e., pH 7.0, highlighting the contribution of other interactions involved in the adsorption process. Structural changes of AGPs induced by pH would result in swelling of the polysaccharide blocks and conformational changes of the polypeptide backbone, therefore increasing the protein accessibility and hydrophobic interactions and/or hydrogen bonds with the gold substrate.

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