Elsevier

Chemical Geology

Volume 200, Issues 1–2, 16 October 2003, Pages 105-115
Chemical Geology

Modelling molybdate and tungstate adsorption to ferrihydrite

https://doi.org/10.1016/S0009-2541(03)00161-XGet rights and content

Abstract

The environmental geochemistry of molybdenum and tungsten is not well known. To enable predictions of Mo and W concentrations in the presence of ferrihydrite (hydrous ferric oxide), batch equilibrations were made with MoO42−, WO42−, o-phosphate (PO43−) and freshly prepared ferrihydrite suspensions in 0.01 M NaNO3 in the pH range from 3 to 10 at 25 °C. The results showed that WO42− is adsorbed more strongly than MoO42−, and that both ions are able to displace PO43− from adsorption sites at low pH. Two models, the Diffuse Layer Model (DLM) and the CD-MUSIC Model (CDM), were tested in an effort to describe the data. In both models, the adsorption of MoO42− and WO42− could be described with the use of two monodentate complexes. One of these was a fully protonated complex, equivalent to adsorbed molybdic or tungstic acid, which was required to fit the data at low pH. This was found to be the case also for a data set with goethite. In competitive systems with PO43−, the models did not always provide satisfactory predictions. It was suggested that this may be partly due to the uncertainty in the PO43− complexation constants.

Introduction

Molybdenum is an essential trace element for both plants and animals. Molybdenum deficiency has often been reported, but at large concentrations, Mo may be toxic as it leads to secondary Cu deficiency (e.g., Vunkova-Radeva et al., 1988). Of particular concern is the release of Mo from alkaline ashes when used as secondary materials Jacks, 1983, Meima et al., 2002. Tungsten is an important strategic metal that is used in a variety of industrial applications. It is usually mined from deposits of scheelite (CaWO4) and wolframite (Fe,Mn)WO4. Tungsten is released to the environment, e.g., through its use in winter tires. The biogeochemical behaviour of W is poorly known. However, it is known that the WO42− ion has an antagonistic effect on the metabolism of MoO42−(Mikkonen and Tummavuori, 1993).

At relatively high Eh, Mo and W are present in their hexavalent state, i.e., as MoO42− and WO42−, and their derivatives. From equilibrium modelling, it can be predicted that the fully dissociated MoO42− and WO42− ions predominate over the non-dissociated forms at pH>4.4 in dilute waters Cruywagen, 2000, Smith et al., 2001. At pH<4.4, the ions will protonate to form the acids MoO3(H2O)3 and WO3(H2O)3, in which Mo and W coordinate six oxygens instead of four. At large concentrations (>1–10 μM), Mo and W polymerise to a variety of different polymolybdate/tungstate forms, particularly at low pH (Cruywagen, 2000). In solution, a wide range of complexes with organic acids has been reported (e.g., Cruywagen et al., 1995).

The geochemical behaviour of MoO42− and WO42− in the environment is probably dependent, to a large extent, on adsorption reactions to particle surfaces. In soils, it is found that these ions were bound most strongly at low pH Mikkonen and Tummavuori, 1993, Mikkonen and Tummavuori, 1994, Bibak and Borggaard, 1994.

Iron, aluminium and, to some extent, titanium oxides may be important sorbent minerals for MoO42− and WO42−, as they may acquire positive charge at low pH Bibak and Borggaard, 1994, Rietra et al., 1999, Bourikas et al., 2001. The binding mechanism to these oxides is thought to be surface complexation, either as mono- or bidentate complexes (e.g., Manning and Goldberg, 1996, Bourikas et al., 2001). Goldberg et al. studied the adsorption of molybdate onto goethite, gibbsite and clay minerals (e.g., Goldberg et al., 1996, Manning and Goldberg, 1996, Goldberg and Forster, 1998). They found that adsorption is very strong at low pH; in this pH region, molybdate is able to compete well even with the very strongly sorbing o-phosphate (PO43−) ion. However, molybdate adsorption exhibited a very strong pH dependence, and at pH>9, little Mo was adsorbed. These authors used a surface complexation model, the Constant Capacitance Model (CCM), to describe the data obtained with the use of two Mo surface complexes.

For the adsorption of MoO42− to two-line ferrihydrite (hydrous ferric oxide), data sets are rather sparse. Two exceptions are small data sets published by Balistrieri and Chao (1990) and Bibak and Borggaard (1994), which follow the general trend described above for goethite. No data set has been found that treats the adsorption of WO42− to ferrihydrite. In their compilation of constants for the Diffuse Layer Model (DLM), Dzombak and Morel (1990) did not fit any data sets for MoO42− and WO42−; instead, they estimated constants using linear-free energy relationships (LFER).

A third surface complexation model is CD-MUSIC (Hiemstra and Van Riemsdijk, 1996), which was used to describe MoO42− adsorption to titania (Bourikas et al., 2001). Their model suggested MoO42− adsorption to be dominated by a bidentate complex at low pH (Ti2O2MoO2) and by a monodentate complex (TiMoO3) at high pH. In line with this, Rietra et al. (1999) suggested a bidentate complex (Fe2O2MoO2) to dominate the speciation of adsorbed Mo to goethite, as judged from measurements of the proton coadsorption stoichiometry at pH 4.2 and 6.1.

The objectives of this study were to supply data on the adsorption of MoO42− and WO42− to two-line ferrihydrite at different pHs and surface coverages, to discuss the effect of competing PO43− ions, and to apply two surface complexation models (DLM and CD-MUSIC) in an effort to describe the data obtained. To my knowledge, this is the first time that the adsorption of WO42− to ferrihydrite has been studied in this manner. For the DLM, it was hypothesized that the constants previously estimated from LFER could describe the data accurately.

Section snippets

Laboratory procedures

Ferrihydrite was synthesized using a method adapted from Swedlund and Webster (1999) and Schwertmann and Cornell (2000). Briefly, a solution containing 36 mM Fe(NO3)3 and 12 mM NaNO3 was brought to pH 8.0 through dropwise addition of 4 M NaOH. The resulting suspension was aged for 18–22 h at 20 °C. This procedure has been shown to produce two-line ferrihydrite with a BET(N2) surface area in the range of 200–320 m2 g−1Swedlund and Webster, 1999, Schwertmann and Cornell, 2000. However, the exact

Single-sorbate systems

A detailed account of the results obtained can be found in Table 3. Molybdate adsorption was strongly pH dependent (Fig. 1, Table 3), which is consistent with earlier studies. Even at the highest surface coverage (0.3 mM Fe), almost 100% was adsorbed at low pH, whereas little Mo adsorption occurred at pH>9 at all surface coverages. For MoO42−, there was considerable scatter in the adsorption envelopes. It is possible that errors in pH measurements may, in part, explain this, as most pH values

Discussion

This study suggests that the adsorption of MoO42− and WO42− to ferrihydrite can be described with two monodentate surface complexes in a surface complexation model. This does not rule out the existence of other surface complexes, such as the bidentate complex Fe2O2XO2, although they seem to be less important in affecting the shape of the adsorption envelope. In competitive systems with PO43−, the model fits were not always satisfactory. It is possible that this is mainly related to the

Conclusions

This study demonstrates that the adsorption of WO42− to ferrihydrite is stronger than that of MoO42−. The adsorption of these anions can be described by two monodentate surface complexes in both the DLM and the CDM. Molybdate and tungstate were adsorbed very strongly at low pH, where the ions were able to displace PO43− from the ferrihydrite surface. This could be explained only if the model considers the presence of a fully protonated complex, equivalent to molybdic or tungstic acid adsorbed

Acknowledgements

The Geological Survey of Sweden (SGU) and the Swedish Research Council (VR) provided financial support to this study. Björn Evertsson is acknowledged for assistance with Mo and W analyses. [EO]

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