How does organic matter constrain the nature, size and availability of Fe nanoparticles for biological reduction?

• Published in: Journal of colloid and interface science 2011-07, Vol.359 (1), p.75-85
• Main Authors: Pédrot, Mathieu, Boudec, Ange Le, Davranche, Mélanie, Dia, Aline, Henin, Odile
• Format: Article
• Language: English
• Subjects: Bacteria; Benzopyrans - chemistry; Benzopyrans - metabolism; Bioreduction; Chemistry; Colloidal state and disperse state; Colloids; Colloids - chemistry; Colloids - metabolism; Earth Sciences; Environmental Sciences; Exact sciences and technology; Fe nanoparticles; General and physical chemistry; Geochemistry; Global Changes; Humic Substances; Hydrogen-Ion Concentration; Hydroxyapatite; Iron; Iron - chemistry; Iron - metabolism; Nanomaterials; Nanoparticles; Nanostructure; Oxidation-hydrolysis reaction; Oxidation-Reduction; Particle Size; Physical and chemical studies. Granulometry. Electrokinetic phenomena; Reduction; Sciences of the Universe; Shewanella putrefaciens; Shewanella putrefaciens - chemistry; Shewanella putrefaciens - metabolism; Surface Properties
• ISSN: 0021-9797; 1095-7103
• DOI: 10.1016/j.jcis.2011.03.067

Transmission electron microscopy micrographs of Fe nanoparticles: (a) and (b) correspond to pure Fe system; whereas (c) and (d) correspond to Fe–HA system. [Display omitted] ► New data demonstrating the impact of HS on the formation and reactivity of Fe oxides. ► Humic substances reduce the extent of the Fe oxidation-hydrolysis reaction. ► Humic substance directly impact the size and the nature of formed Fe oxides. ► A fraction of Fe does not contribute to the genesis of nanoparticles. ► Mixed Fe nanoparticles-organic colloids are much more bioavailable than Fe oxides. Few studies have so far examined the kinetics and extent of the formation of Fe-colloids in the presence of natural organic ligands. The present study used an experimental approach to investigate the rate and amount of colloidal Fe formed in presence of humic substances, by gradually oxidizing Fe(II) at pH 6.5 with or without humic substances (HS) (in this case, humic acid – HA and fulvic acid – FA). Without HS, micronic aggregates (0.1–1μm diameter) of nano-lepidocrocite is obtained, whereas, in a humic-rich medium (HA and FA suspensions at 60 and 55ppm of DOC respectively), nanometer-sized Fe particles are formed trapped in an organic matrix. A proportion of iron is not found to contribute to the formation of nanoparticles since iron is complexed to HS as Fe(II) or Fe(III). Humic substances tend to (i) decrease the Fe oxidation and hydrolysis, and (ii) promote nanometer-sized Fe oxide formation by both inhibiting the development of hydroxide nuclei and reducing the aggregation of Fe nanoparticles. Bioreduction experiments demonstrate that bacteria (Shewanella putrefaciens CIP 80.40T) are able to use Fe nanoparticles associated with organic matter about eight times faster than in the case of nano-lepidocrocite. This increase in bioreduction rate appears to be related to the presence of humic acids that (i) indirectly control the size, shape and density of oxyhydroxides and (ii) directly enhance biological reduction of nanoparticles by electron shuttling and Fe complexation. These results suggest that, in wetlands but also elsewhere where mixed organic matter-Fe colloids occur, Fe nanoparticles closely associated with organic matter represent a bioavailable Fe source much more accessible for microfauna than do crystallized Fe oxyhydroxides.