Elsevier

Geochimica et Cosmochimica Acta

Volume 119, 15 October 2013, Pages 46-60
Geochimica et Cosmochimica Acta

Properties of impurity-bearing ferrihydrite II: Insights into the surface structure and composition of pure, Al- and Si-bearing ferrihydrite from Zn(II) sorption experiments and Zn K-edge X-ray absorption spectroscopy

https://doi.org/10.1016/j.gca.2013.05.040Get rights and content

Abstract

Naturally occurring ferrihydrite often contains impurities such as Al and Si, which can impact its chemical reactivity with respect to metal(loid) adsorption and (in)organic or microbially induced reductive dissolution. However, the surface composition of impure ferrihydrites is not well constrained, and this hinders our understanding of the factors controlling the surface reactivity of these nanophases. In this study, we conducted Zn(II) adsorption experiments combined with Zn K-edge X-ray absorption spectroscopy measurements on pure ferrihydrite (Fh) and Al- or Si-bearing ferrihydrites containing 10 and 20 mol% Al or Si (referred to as 10AlFh, 20AlFh and 10SiFh, 20SiFh) to evaluate Zn(II) uptake in relation to Zn(II) speciation at their surfaces. Overall, Zn(II) uptake at the surface of AlFh is similar to that of pure Fh, and based on Zn K-edge EXAFS data, Zn(II) speciation at the surface of Fh and AlFh also appears similar. Binuclear bidentate IVZn–VIFe complexes (at ∼3.46 Å (2C[1]) and ∼3.25 Å (2C[2])) were identified at low Zn(II) surface coverages from Zn K-edge EXAFS fits. With increasing Zn(II) surface coverage, the number of second-neighbor Fe ions decreased, which was interpreted as indicating the formation of IVZn polymers at the ferrihydrite surface, and a deviation from Langmuir uptake behavior. Zn(II) uptake at the surface of SiFh samples was more significant than at Fh and AlFh surfaces, and was attributed to the formation of outer-sphere complexes (on average 24% of sorbed Zn). Although similar Zn–Fe/Zn distances were obtained for the Zn-sorbed SiFh samples, the number of Fe second neighbors was lower in comparison with Fh. The decrease in second-neighbor Fe is most pronounced for sample 20SiFh, suggesting that the amount of reactive surface Fe sites diminishes with increasing Si content. Although our EXAFS results shown here do not provide evidence for the existence of Zn–Al or Zn–Si complexes, their presence is not excluded for Zn-sorbed AlFh or SiFh. The results of this study indicate that Zn(II) interaction with Fh is influenced by the type of impurities associated with this nanomineral, particularly in the case of Si-bearing Fh, and this may have implications for our understanding of metal(loid) mobility in natural systems.

Introduction

Impure ferrihydrites are being investigated increasingly, as data on naturally occurring ferrihydrite often indicate a common association of Al, Si, As, P, etc. with this nanomineral (Henmi et al., 1980, Childs et al., 1982, Parfitt et al., 1992, Jambor and Dutrizac, 1998, Boyd and Scott, 1999, Pichler et al., 1999, Kim and Kim, 2003, Majzlan et al., 2007, Cismasu et al., 2011). Ferrihydrite is considered to be one of the most effective natural sorbents of (in)organic pollutants as a result of its large and reactive surface area and its abundance in soils, sediments and aqueous environments. However, the addition of foreign species can impact its structure, crystallinity, particle size, aggregation properties, and most importantly its surface chemistry and composition, and thus its reactivity. Studies of Al-, Si-, As-, or Cr-bearing ferrihydrites (Waychunas et al., 1993, Dyer et al., 2010, Tang et al., 2010, Cismasu et al., 2012) have shown that the effect of impurities on the ferrihydrite structure is highly dependent on the chemical properties of foreign ions, such as ionic radius, charge, hydrolysis constants, the electrolyte solution, and the kinetics of co-precipitation. The speciation of impurities in ferrihydrite co-precipitates may vary from incorporation in the crystal structure by means of substitution, to the formation of surface complexes and surface precipitates, or the formation of one or more separate nm-scale phases.

In the case of aluminous Fh, it was suggested that Al3+ can be incorporated in the ferrihydrite structure up to 20–30 mol%, and that individual Al-rich phases that are intermixed with ferrihydrite also form simultaneously as a result of co-precipitation (Cismasu et al., 2012). Aluminum incorporation in ferrihydrite was shown to have minor effects on ferrihydrite particle growth and crystallinity, and no indication of an aluminous precipitate at Fh particle surfaces was found (Cismasu et al., 2012). The formation of aluminous surface precipitates has, however, been proposed in previous studies (e.g., Harvey and Rhue, 2008), and may be related to differences in synthesis methods.

Similar questions are raised concerning the speciation of Si in Si-bearing ferrihydrites, and disagreement still exists about its occurrence as surface complexes and surface precipitates (Glasauer et al., 2000, Dyer et al., 2010), or its incorporation in the ferrihydrite structure (Campbell et al., 2002). It is generally agreed that Si causes a decrease in the crystallinity of ferrihydrite (Dyer et al., 2010), and this is attributed to the strong interaction between aqueous Fe and Si, which inhibits Fe polymerization by poisoning free corner sites of Fe polymers/colloids (Pokrovski et al., 2003). The resulting increase in structural disorder of Si-ferrihydrites complicates further structural investigations, and currently, additional studies are required to understand these complex co-precipitates.

The surface reactivity of Al- and Si-ferrihydrite is expected to vary as a result of the dissimilar effects of Al and Si on this phase. Previous studies have shown variable reactivity of Al- and Si-ferrihydrite with respect to reductive dissolution or metal sorption. For example, an increase in reactivity was observed for Al-bearing 6-line ferrihydrite with respect to reductive dissolution (Jentzsch and Penn, 2006). However, Hansel et al., 2011, Masue-Slowey et al., 2011 and Ekstrom et al. (2010) showed that secondary mineralization is diminished for Al-bearing 2-line ferrihydrite as a result of Fe(II)-induced or bacterially mediated reduction. An earlier study suggested that Zn(II) adsorption is slightly enhanced, whereas Cd(II) uptake is inhibited at aluminous ferrihydrite surfaces (Anderson and Benjamin, 1990). Furthermore, As(V) adsorption was not affected significantly for aluminous ferrihydrite containing 20% Al, but a decrease in As(III) uptake was observed (Masue et al., 2007). In the case of Si-ferrihydrite Anderson and Benjamin (1985) showed that the binding strength of Cd(II) on ferrihydrite increased, whereas it remained relatively unaffected for Cu(II), Co(II) and Zn(II) binding. In addition, the reductive dissolution of dried Si–ferrihydrite by ascorbic acid was comparable to pure, freshly precipitated ferrihydrite (Jones et al., 2009).

The interaction between adsorbing ions and the compositionally complex surfaces of these substrates may depend on a variety of factors that are related to the structure and composition of the surface, particle morphology and abundance of reactive sites, the chemical properties of the sorbing ion, phase heterogeneity, as well as the presence of competing ions or ions that facilitate adsorption. For example, it is possible that defective surfaces form because of substitution or surface complexation resulting in the creation of stronger binding sites for specific sorbing ions. Conversely, poisoning of the surface by molecules that block reactive sites may also decrease the sorption capacity of the surface in some cases. Other possibilities that should be considered are the formation of surface precipitates and coprecipitates involving sorbed ions and the Fe–Al/Si substrate, or polymerization of the sorbed ions at the substrate surface, as was shown for Zn(II)-sorbed pure ferrihydrite (Waychunas et al., 2002). The electrostatic properties of the surface may also have an effect on ion sorption. For example, Anderson and Benjamin (1985) found a significant decrease in the sorption of anions (e.g., SeO32−) on Si-ferrihydrite, which was correlated with a decrease in pHpzc. It is also possible that in addition to the more strongly bound inner-sphere complexes, metal cations occur as weakly bound, or outer-sphere complexes as a result of variability in the surface composition and charge.

In this study we examine the interaction between aqueous Zn(II) and ferrihydrite (Fh), aluminous ferrihydrite (AlFh), and siliceous ferrihydrite (SiFh). Our main goals are to evaluate Zn(II) uptake as a function of the type (Al or Si) and the amount of impurities associated with ferrihydrite, and to determine the cause of any changes in apparent surface reactivity. Zn(II) sorption experiments were conducted on five synthetic samples – a pure ferrihydrite sample, two aluminous ferrihydrite samples containing 10 and 20 mol% Al (10AlFh and 20AlFh), and two siliceous ferrihydrites containing 10 and 20 mol% Si (10SiFh and 20SiFh). Zn(II) adsorption isotherms were collected for each sample, and Zn K-edge X-ray absorption spectroscopy data were obtained at variable Zn surface coverages. By determining the speciation of sorbed Zn(II) by XANES and EXAFS spectroscopy, the average local structural environment around adsorbed Zn(II) is obtained, which provides indirect information about the ferrihydrite surface structure/composition. Zn(II) was chosen as a probe ion for this study because its interaction with pure ferrihydrite has been investigated extensively (Waychunas et al., 2002, Waychunas et al., 2003, Dyer et al., 2004, Trivedi et al., 2004, Lee and Anderson, 2005). Furthermore, the speciation of Zn(II) sorbed on iron oxides, aluminum oxides, or silica is variable in terms of coordination (IVZn(II) or VIZn(II)) and the types of complexes it forms, as determined previously by EXAFS spectroscopy. As a result of its structural versatility, it is expected that Zn speciation at the surface of Al- and Si-ferrihydrite will reflect changes in their structure/composition. Finally, understanding the interaction of Zn(II) with Fh, AlFh and SiFh is also important because zinc is a widespread contaminant that is often found in soils, where it can accumulate to levels that may become toxic to both plant and animal life.

Section snippets

Ferrihydrite synthesis and characterization

Pure and aluminous ferrihydrite samples were synthesized according to the methods described in Cismasu et al. (2012). Ferrihydrites containing 0, 10, and 20 mol% Al (subsequently referred to as Fh, 10AlFh and 20AlFh) were prepared by rapid neutralization (pH 7.5) of 0.2 M Fe(III) or Fe(III)/Al(III) nitrate solutions by addition of 1 M NaOH. Siliceous ferrihydrites were prepared according to the protocols developed by Zeng (2003) and Cismasu et al. (2012) by rapid neutralization (pH 7.5) of Fe(III)

Zn(II) sorption experiments

A comparison between Zn(II) uptake at the surfaces of Fh, 10AlFh, 10SiFh, 20AlFh and 20SiFh is shown in Fig. 1a and b. Zn(II) sorption at the surface sample 10AlFh is similar to that of pure ferrihydrite (Fig. 1a). In contrast, Zn uptake for sample 10SiFh is significantly higher, and this was attributed to the presence of weakly sorbed Zn. An increase in Zn(II) adsorption was also observed for sample 20SiFh at low surface coverage in comparison with pure Fh (Fig. 1b).

A weak chemical extraction

Zn(II) speciation at Fh surfaces

Several possibilities for the geometry of Zn(II) complexes at the surface of pure ferrihydrite are proposed in Fig. 6. Available sorption sites at the ferrihydrite surface were obtained by using truncated surfaces of the most recent ferrihydrite structural model proposed by Michel et al. (2010). We show, as suggested in previous studies (Waychunas et al., 2002, Trivedi et al., 2004, Lee and Anderson, 2005), that the formation of a IVZn binuclear bidentate complex (2C[1]) is possible at Zn–Fe

Conclusion

Zinc sorption experiments combined with Zn K-edge XANES and EXAFS spectroscopy showed that Zn speciation at the surface of Al-ferrihydrite is similar to that of pure Fh, suggesting that surface-bound Al is not abundant in these samples. In contrast, a significant decrease in the number of Fe second neighbors was observed for Zn(II) at the surface of Si–ferrihydrite, and this may result from the presence of surface-bound silicate oxoanions or silica precipitates that block reactive surface Fe

Acknowledgments

This study was supported by DOE-BER Grant DE-SC0006772 (C.C., G.B.), NSF Grant EF-0830093 (Center for Environmental Implications of Nanotechnology) (C.L., G.B.), DOE-BER Science Focus Area funding to SLAC (M.M., G.B.), and the Corning Inc. Foundation (C.C., G.B.). We are grateful for access to the Stanford Synchrotron Radiation Lightsource (SSRL) at SLAC National Accelerator Laboratory, where we collected our XAS data. SSRL is supported by the Director, Office of Science, Office of Basic Energy

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    1

    Present address: CEREGE, Europôle Méditerranéen de l’Arbois, BP 80, 13545 Aix en Provence, Cedex 04, France.

    2

    Present address: Department of Geosciences, Virginia Polytechnic Institute, 4044 Derring Hall, Blacksburg, VA 24061, USA.

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