Elsevier

Geochimica et Cosmochimica Acta

Volume 72, Issue 4, 15 February 2008, Pages 1111-1127
Geochimica et Cosmochimica Acta

Synthetic coprecipitates of exopolysaccharides and ferrihydrite. Part I: Characterization

https://doi.org/10.1016/j.gca.2007.11.035Get rights and content

Abstract

Iron(III) (hydr)oxides formed at extracellular biosurfaces or in the presence of exopolymeric substances of microbes and plants may significantly differ in their structural and physical properties from their inorganic counterparts. We synthesized ferrihydrite (Fh) in solutions containing acid polysaccharides [polygalacturonic acid (PGA), alginate, xanthan] and compared its properties with that of an abiotic reference by means of X-ray diffraction, transmission electron microscopy, gas adsorption (N2, CO2), X-ray absorption spectroscopy, 57Fe Mössbauer spectroscopy, and electrophoretic mobility measurements. The coprecipitates formed contained up to 37 wt% polymer. Two-line Fh was the dominant mineral phase in all precipitates. The efficacy of polymers to precipitate Fh at neutral pH was higher for polymers with more carboxyl C (PGA  alginate > xanthan). Pure Fh had a specific surface area of 300 m2/g; coprecipitation of Fh with polymers reduced the detectable mineral surface area by up to 87%. Likewise, mineral micro- (<2 nm) and mesoporosity (2–10 nm) decreased by up to 85% with respect to pure Fh, indicative of a strong aggregation of Fh particles by polymers in freeze-dried state. C-1s STXM images showed the embedding of Fh particles in polymer matrices on the micrometer scale. Iron EXAFS spectroscopy revealed no significant changes in the local coordination of Fe(III) between pure Fh and Fh contained in PGA coprecipitates. 57Fe Mössbauer spectra of coprecipitates confirmed Fh as dominant mineral phase with a slightly reduced particle size and crystallinity of coprecipitate–Fh compared to pure Fh and/or a limited magnetic super-exchange between Fh particles in the coprecipitates due to magnetic dilution by the polysaccharides. The pHiep of pure Fh in 0.01 M NaClO4 was 7.1. In contrast, coprecipitates of PGA and alginate had a pHiep < 2. Considering the differences in specific surface area, porosity, and net charge between the coprecipitates and pure Fh, composites of exopolysaccharides and Fe(III) (hydr)oxides are expected to differ in their geochemical reactivity from pure Fe(III) (hydr)oxides, even if the minerals have a similar crystallinity.

Introduction

In soils, sediments and aquatic environments, Fe(III) (hydr)oxides are often formed under the influence of plants, bacteria, fungi, and their exudates. The association of these ‘biogenic Fe(III) (hydr)oxides’ with microbial exopolymeric substances (EPS) and cell surfaces has been documented for freshwater (Fortin et al., 1993, St.-Cyr et al., 1993, Banfield et al., 2000, Chan et al., 2004, James and Ferris, 2004), marine (Little et al., 2004), soil (Emerson et al., 1999), and subterranean environments (Brown et al., 1994, Sawicki et al., 1995, Ferris et al., 1999). The formation of biogenic Fe(III) (hydr)oxides can proceed either directly or indirectly. In the first case, microbial activity under oxic or anoxic conditions leads to the oxidation of Fe(II) to Fe(III) and the subsequent extracellular precipitation of biogenic Fe(III) (hydr)oxides near or on bacterial cells (Fortin and Langley, 2005). The indirect path of biogenic Fe(III) (hydr)oxide formation has been proposed to involve the passive sorption of aqueous Fe(III) to bacterial cell walls or EPS and the subsequent nucleation of secondary Fe(III) (hydr)oxides from solution (Warren and Ferris, 1998, Fortin and Langley, 2005).

Surprisingly, the effect of extracellular organic compounds on physicochemical properties of Fe(III) (hydr)oxides formed in their presence has received little attention in the light of possible ecological consequences. Coprecipitation of extracellular organic compounds with Fe(III) (hydr)oxides can inhibit, retard or enhance crystal growth and transformation processes, thus affecting the reactivity of the solids formed (Liu and Huang, 1999, Liu and Huang, 2003, Rancourt et al., 2005). Recently, Rancourt et al. (2005) suggested from Mössbauer spectroscopy, mineral magnetometry, and X-ray diffraction analyses that biogenic ferrihydrite formed in the presence of non-metabolizing cells of Bacillus subtilis and Bacillus licheniformis possessed less interparticle atomic order, a smaller particle size, and a weaker Fe to particle bond strength than the abiotic counterparts. The mechanism suggested to explain these structural discrepancies is the coprecipitation of ferrihydrite with, and possibly the surface poisoning of ferrihydrite by bacteriogenic compounds (Rancourt et al., 2005). However, a gap still remains in our understanding of how different bacteriogenic compounds affect the physicochemical properties and the reactivity of such coprecipitates. Several studies suggest that microbial exopolymers may act as catalysts for the oxidation–precipitation of Fe, scavenge Fe(III) (hydr)oxides (Søgaard et al., 2000, Mavrocordatos and Fortin, 2002), or serve as templates for them (Chan et al., 2004). Polysaccharides constitute an important part of plant root exudates or EPS of microorganisms. In soils, for example, polysaccharides contribute up to about 10% to the total organic matter pool (Cheshire, 1979). Chenu (1993) estimated the content of microbial exopolysaccharides in soils at >1 wt% of mineral matter. Polysaccharides in microbial EPS may even exceed 90% (Flemming and Wingender, 2001). Many exopolysaccharides contain uronic acids or pyruvate ketals and are thus net negatively charged at neutral or alkaline pH (Sutherland, 1994), rendering them suitable nucleation sites for dissolved Fe(III) species in aqueous environments supersaturated with dissolved Fe.

In the present paper, we investigate the physical and structural properties of ferrihydrite formed in the presence of exopolysaccharides. We synthesized ferrihydrite in solutions containing model compounds for plant and microbial exopolysaccharides [polygalacturonic acid (PGA),1 alginate, xanthan] and subsequently analyzed the coprecipitated phases by a variety of solid-state methods such as transmission electron microscopy, gas adsorption, 57Fe Mössbauer spectroscopy, and X-ray absorption spectroscopy. In a companion study (Mikutta and Kretzschmar, 2008), we elucidate the dissolution of the coprecipitates by the microbial siderophore desferrioxamine B and its interaction with low molecular weight organic acid anions. Our principal goal is to link structural properties of ‘biogenic Fe(III) (hydr)oxides’ to the kinetics of ligand-promoted dissolution.

Section snippets

Polysaccharides

Polygalacturonic acid [(C6H8O6)n, PGA] with a molecular weight of 25–50 kDa and a purity of ∼95% (enzym.) was purchased from Fluka (P81325). The polymer consists of linear chains of partially methylated (1–4) linked α-d-galacturonic acid monomers. PGA was used as a model compound for the gelatinous mucilage covering the root apices of many plant species (Knee et al., 2001).

Alginic acid [(C6H8O6)n] from brown algae was used as sodium salt (Fluka, P71238). Alginic acid is an unbranched

Elemental composition, mineralogy, and morphology

The chemical composition of the Fe(III) (hydr)oxide-polysaccharide coprecipitates is presented in Table 2. The coprecipitates contained between 122 and 365 mg/g organic matter, and between 367 and 564 mg/g Fe (Table 2). For a molar C/Fe ratio of 0.37 in the synthesis of PGA coprecipitates, the formation of Fe(III) (hydr)oxides resulted in a complete removal of PGA-C as indicated by an identical molar C/Fe ratio of the product within experimental error (Table 2). When the initial molar C/Fe ratio

Concluding remarks

Ferrihydrite synthesized in the presence and absence of acid polysaccharides did not markedly differ in its crystallinity as was reported for Fe(III) (hydr)oxides formed in the presence of low molecular weight organics such as citrate (Liu and Huang, 1999, Liu and Huang, 2003) or in the presence of compounds released by non-metabolizing bacterial cells of B. subtilis/B. licheniformis (Rancourt et al., 2005). However, because polysaccharides became incorporated into interparticle pores of Fh

Acknowledgments

The Hamburger Synchrotronstrahlungslabor (HASYLAB, Germany) and the Angströmquelle Karlsruhe (ANKA, Germany) are acknowledged for providing beamtime. Edmund Welter (HASYLAB) and Stefan Mangold (ANKA) are thanked for their assistance during data collection. The C-1s NEXAFS measurements were conducted at the STXM beamline X-1A, NSLS, Brookhaven National Laboratory, Upton, NY. The STXM was developed by the group of Janos Kirz and Chris Jacobsen at SUNY, Stony Brook, with support from the Office of

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