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.
Environmental fate of tungstate (VI) oxyanion [i.e. mono tungstate and several polytungstate, generally expressed by W (VI)] is largely controlled by sorption on soil minerals, especially on iron oxide minerals. Molecular scale evaluation of W (VI) retention on iron oxides in the presence of competing oxyanions is scarce in the literature. Here we report surface interaction mechanisms of W (VI) on hematite in the presence of phosphorus (P) using macroscopic and in situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopic experiments. Batch sorption experiments were conducted using 2g L-1 hematite and 100 µM W (VI) and P, in single ion system and in binary mixtures as a function of pH (4-11). In situ ATR-FTIR spectroscopic evaluation of P and W (VI) sorption on hematite was also carried out. The results from macroscopic experiments indicated that W (VI) sorption on hematite was not affected when W (VI) was added first. The influence of P on W (VI) sorption was noticed when W (VI) & P were added simultaneously or P was added first. The in situ ATR-FTIR spectroscopic data corroborated this finding. In addition, the spectroscopic data revealed that in the presence of P, surface complexation mode of W (VI) differed as noted from either the absence of W-O antisymmetric infrared (IR) band or the W-O-W stretching band. This study provides useful information on molecular level understanding of W (VI) surface complexation on hematite in the presence of competing ions such as P.
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