Synthetic Coprecipitates of Exopolysaccharides and Ferrihydrite. Part I: Characterization

Journal Article · Tue Jan 01 00:00:00 EST 2008 · Geochimica et Cosmochimica Acta

DOI:https://doi.org/10.1016/j.gca.2007.11.035· OSTI ID:959674

Mikutta, C; Mikutta, R; Bonneville, S; Wagner, F; Voegelin, A; Christl, I; Kretzschmar, R

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 Moessbauer 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 {approx} 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 Moessbauer 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.

• Research Organization: Brookhaven National Lab. (BNL), Upton, NY (United States). National Synchrotron Light Source
• Sponsoring Organization: Doe - Office Of Science
• DOE Contract Number: DE-AC02-98CH10886
• OSTI ID: 959674
• Report Number(s): BNL-82660-2009-JA; GCACAK; TRN: US201016%%818
• Journal Information: Geochimica et Cosmochimica Acta, Vol. 72; ISSN 0016-7037
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
ABSORPTION SPECTROSCOPY
ADSORPTION
ALGINATES
COPRECIPITATION
DILUTION
IRON
MATRICES
PARTICLE SIZE
PHYSICAL PROPERTIES
POLYMERS
POLYSACCHARIDES
POROSITY
SPECIFIC SURFACE AREA
SPECTRA
SPECTROSCOPY
SURFACE AREA
TRANSMISSION ELECTRON MICROSCOPY
X-RAY DIFFRACTION
national synchrotron light source