Experimental study of germanium adsorption on goethite and germanium coprecipitation with iron hydroxide: X-ray absorption fine structure and macroscopic characterization

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Abstract

Adsorption of germanium on goethite was studied at 25 °C in batch reactors as a function of pH (1–12), germanium concentration in solution (10−7 to 0.002 M) and solid/solution ratio (1.8–17 g/L). The maximal surface site density determined via Ge adsorption experiments at pH from 6 to 10 is equal to 2.5 ± 0.1 μmol/m2. The percentage of adsorbed Ge increases with pH at pH < 9, reaches a maximum at pH  9 and slightly decreases when pH is further increased to 11. These results allowed generation of a 2-pK Surface Complexation Model (SCM) which implies a constant capacitance of the electric double layer and postulates the presence of two Ge complexes, >FeOGe(OH)30 and >FeOGeO(OH)2, at the goethite-solution interface. Coprecipitation of Ge with iron oxy(hydr)oxides formed during Fe(II) oxidation by atmospheric oxygen or by Fe(III) hydrolysis in neutral solutions led to high Ge incorporations in solid with maximal Ge/Fe molar ratio close to 0.5. The molar Ge/Fe ratio in precipitated solid is proportional to that in the initial solution according to the equation (Ge/Fe)solid = k × (Ge/Fe)solution with 0.7  k  1.0. The structure of adsorbed and coprecipitated Ge complexes was further characterized using XAFS spectroscopy. In agreement with previous data on oxyanions adsorption on goethite, bi-dentate bi-nuclear surface complexes composed of tetrahedrally coordinated Ge attached to the corners of two adjacent Fe octahedra represent the dominant contribution to the EXAFS signal. Coprecipitated samples with Ge/Fe molar ratios >0.1, and samples not aged in solution (<1 day) having intermediate Ge/Fe ratios (0.01–0.1) show 4 ± 0.3 oxygen atoms at 1.76 ± 0.01 Å around Ge. Samples less concentrated in Ge (0.001 < Ge/Fe < 0.10) and aged longer times in solution (up to 280 days) exhibit a splitting of the first atomic shell with Ge in both tetrahedral (R = 1.77 ± 0.02 Å) and octahedral (R = 1.92 ± 0.03 Å) coordination with oxygen. In these samples, octahedrally coordinated Ge accounts for up to ∼20% of the total Ge. For the least concentrated samples (Ge/Fe < 0.001–0.0001) containing lepidocrocite, 30–50% of total co-precipitated germanium substitutes for Fe in octahedral sites with the next-nearest environment dominated by edge-sharing GeO6–FeO6 linkages (RGe–Fe  3.06 Å). It follows from the results of our study that the largest structural change of Ge (from tetrahedral to octahedral environment) occurs during its coprecipitation with Fe hydroxide at Ge/Fe molar ratio ⩽0.0001. These conditions are likely to be met in many superficial aquatic environments at the contact of anoxic groundwaters with surficial oxygenated solutions. Adsorption and coprecipitation of Ge with solid Fe oxy(hydr)oxides and organo-mineral colloids and its consequence for Ge/Si fractionation and Ge geochemical cycle are discussed.

Introduction

Germanium, which belongs to the same group (IV) of the periodic table and has identical outer electron structure than silicon, has been often considered as a pseudoisotope of Si exhibiting a similar chemical behavior and substituting for it in silicate lattices (Goldschmidt, 1958, Cotton and Wilkinson, 1966). As a result Ge is an ideal candidate to trace both the continental and oceanic Si cycles (Froelich et al., 1985, Murname and Stallard, 1990, Froelich et al., 1992, Mortlock et al., 1993, Filippelli et al., 2000, Derry et al., 2005, Derry et al., 2006). This has been illustrated by Shemesh et al. (1989) who showed that diatoms exhibit little or no discrimination against Ge in the formation of biogenic opal thus making possible to use the Ge to Si ratio of this pair of elements recorded in diatoms through the last 500 ky to constrain paleovariations in continental weathering input (Murname and Stallard, 1990).

Careful comparison of silicon and germanium chemical properties, however, shows subtle but indisputable differences, with Ge exhibiting distinct lithopile, siderophile or organophile behavior depending on its environment (Bernstein, 1985, Pokrovski and Schott, 1998a, Pokrovski and Schott, 1998b). These specific properties are responsible for Ge geochemical cycles in the ocean and continental environments not being simple analogues of corresponding silica cycles. On the continents, for example, Ge/Si ratios measured in unpolluted streams (0.3–1.2 × 10−6) are almost always lower than the same ratios in the silicate bedrock (1.3 × 10−6) they drain (Mortlock and Froelich, 1987). This requires Ge to be enriched in the secondary phases that form during the weathering of primary silicates. Aluminosilicates (Kurtz et al., 2002), Fe(III) oxy-hydroxides (Mortlock and Froelich, 1987, Pokrovsky and Schott, 2002) or humic acids in peats (Viers et al., 1997, Pokrovski and Schott, 1998b) have been proposed to be the soil reservoirs enriched in Ge. There is also a significant problem with interpreting the contemporary oceanic Ge/Si ratio because, unlike for Si, Ge mass balance in the ocean is still poorly constrained. It is generally assumed that the only sink for both elements is burial of biogenic opal (Tréguer et al., 1995), but the presently observed Ge/Si removal ratio yields by opal burial are ∼0.76 × 10−6 whereas an extraction ratio of ∼1.3 × 10−6 would be required for an ocean in steady state (King et al., 2000). In order to keep the ocean in steady state, an additional sink (often referred as the “missing sink”) of ∼4 ± 2 × 10−6 mol Ge/yr is requested (Zhou and Kyte, 1991, Elderfield and Schultz, 1996). It has been recently proposed that the missing Ge sink can be linked to the selective uptake of Ge by authigenic Fe oxy-hydroxides phases in iron-rich margin sediments (King et al., 2000, McManus et al., 2003).

However, despite an important role ascribed to iron oxy-hydroxides in the control of germanium concentrations both in continental and marine environments, little is known about the mechanisms of Ge uptake by iron oxides via sorption/coprecipitation processes and the possible extent of this uptake. Moreover, because Ge, unlike Si, can easily increase its coordination number from 4 to 6 following, for example, its complexation with organic acids (Pokrovski et al., 2000) or its coprecipitation with Fe(III) oxy-hydroxides (Bernstein and Waychunas, 1987), it can be expected that germanium isotopes (i.e, 74Ge/70Ge) can significantly fractionate compare to Si isotopes (28Si/30Si) during weathering and marine diagenesis.

Here, we have made concerted efforts aimed at rigorously characterizing germanium sorption on goethite and its coprecipitation with amorphous Fe(III) hydroxides. The chemical status of Ge associated with Fe hydroxides was thoroughly characterized using XAFS spectroscopy. It is expected that these new data will allow to better constrain Ge behavior in both continental and oceanic environments and to understand the reasons for Ge/Si fractionations.

Section snippets

Adsorption experiments

Goethite powder (aggregates of crystals having a main size of 0.1 μm, determined by laser diffraction technique) was synthesized in the LEM laboratory (Nancy, France) following a procedure described by Cornell and Schwertmann (1996) and based on oxidative hydrolysis of FeSO4. Its specific surface area was 23.2 m2/g as determined by 3-point B.E.T. nitrogen adsorption technique. Batch adsorption experiments were performed in acid cleaned 30 mL polypropylene (PP) vials which were continuously

Adsorption

All experiments on adsorption and coprecipitation are listed in the Electronic Annex. Like most adsorption processes, the interaction of aqueous Ge with goethite surface is quite fast: for typical experimental conditions (6.6 g/L of solid, pH 8.5), the stable Ge concentration in solution is achieved within 100–1000 min (Fig. 1). Adsorption was found fully reversible because, upon changing pH from acid to alkaline conditions and in backward direction, initial Ge concentration in solution was

Applications for Ge versus Si fractionation in natural settings

A quantitative comparison between Ge and Si is not possible due to different model parameters for adsorption (i.e., diffuse double layer model in Dietzel, 2002 versus constant capacitance model in this study) and experimental conditions of coprecipitation (i.e., seawater in Savenko and Volkov (2003) versus 0.1 mol/L NaCl/NaNO3 solutions in this study). However, preliminary analyses demonstrates that Ge is likely to be adsorbed more efficiently by Fe hydroxide surfaces since log K3,4 are higher

Conclusions

The present study allows quantitative characterization of Ge interactions with Fe hydroxide occurring during its adsorption on goethite and coprecipitation with Fe(OH)3 at ambient temperatures. A surface complexation model developed on the basis of adsorption results is consistent with available data on other anions and neutral molecules adsorption on goethite and provides adequate description of the extent of Ge sorption as a function of pH, solid/solution ratio and Ge aqueous concentration.

Acknowledgments

The manuscript greatly benefited from thorough and constructive reviews of A. Kurtz and two anonymous referees. The ESRF scientific committee is acknowledged for providing beamtime and access to the synchrotron facility. The authors are grateful to C. Causserand, F. Candadap, and J. Escalier, for the assistance with Ge analysis and J. Roux, T. Neisuis, L. Alvarez, J.-L. Hazemann, J.-J. Menthonnex, and V. Nassif for the help with XAFS. M. Thibault is thanked for carrying out XRD analyses. B.

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