International audienceThe complexation of Fe(II)with organic matter (OM)and especiallywith humic acids (HAs) remains poorly characterized in the literature. In this study, batch experiments were conducted on a pH range varying from 1.95 to 9.90 to study HA-mediated Fe(II) binding. The results showed that high amounts of Fe(II) are complexed with HAdepending on the pH. Experimental datawere used to determine a new set of binding parameters by coupling PHREEPLOT and PHREEQC-Model VI. The new binding parameters (log KMA = 2.19 ± 0.16, log KMB= 4.46± 0.47 and ΔLK2=3.90 ± 1.30) were validated using the LFER (linear free energy relationship) method and published adsorption data between Fe(II) and Suwannee River fulvic acid (SRFA) (Rose andWaite, 2003). Theywere then put in PHREEQC-Model VI to determine the distribution of Fe(II) onto HA functional groups. It was shown that Fe(II) forms mainly bidentate complexes, some tridentate complexes and only a few monodentate complexes with HA. Moreover, Fe(II) is mainly adsorbed onto carboxylic groups at acidic and neutral pH, whereas carboxy-phenolic and phenolic groups play a major role at basic pH. The major species adsorbed onto HA functional groups is Fe2+; Fe(OH)+ appears at basic pH (frompH 8.13 to 9.9). The occurrence of OMand the resulting HA-mediated binding of Fe(II) can therefore influence Fe(II) speciation and bioavailability in peatlands and wetlands, where seasonal anaerobic conditions prevail. Furthermore, the formation of a cationic bridge and/or the dissolution of Fe(III)-(oxy)hydroxides by the formation of Fe(II)-OM complexes can influence the speciation of other trace metals and contaminants such as As

Magnetite (Fe(III)2Fe(II)O4) nanoparticles are fascinating nanoparticulate minerals for their electronic, magnetic and chemical properties. Ubiquitous, in the environment, they are also among the most used ferromagnetic nanomaterials in environmental, industrial...

Fe(II)-organic matter (Fe(II)-OM) complexes are abundant in the environment and may play a key role for the behavior of Fe and pollutants. Mixotrophic nitrate-reducing Fe(II)-oxidizing bacteria (NRFeOx) reduce nitrate coupled to the oxidation of organic compounds and Fe(II). Fe(II) oxidation may occur enzymatically or abiotically by reaction with nitrite that forms during heterotrophic denitrification. However, it is unknown whether Fe(II)-OM complexes can be oxidized by NRFeOx. We used cell-suspension experiments with the mixotrophic nitrate-reducing Fe(II)-oxidizing bacterium Acidovorax sp. strain BoFeN1 to reveal the role of nonorganically bound Fe(II) (aqueous Fe(II)) and nitrite for the rates and extent of oxidation of Fe(II)-OM complexes (Fe(II)-citrate, Fe(II)-EDTA, Fe(II)-humic acid, and Fe(II)-fulvic acid). We found that Fe(II)-OM complexation inhibited microbial nitrate-reducing Fe(II) oxidation; large colloidal and negatively charged complexes showed lower oxidation rates than aqueous Fe(II). Accumulation of nitrite and fast abiotic oxidation of Fe(II)-OM complexes only happened in the presence of aqueous Fe(II) that probably interacted with (nitrite-reducing) enzymes in the periplasm causing nitrite accumulation in the periplasm and outside of the cells, whereas Fe(II)-OM complexes probably could not enter the periplasm and cause nitrite accumulation. These results suggest that Fe(II) oxidation by mixotrophic nitrate reducers in the environment depends on Fe(II) speciation, and that aqueous Fe(II) potentially plays a critical role in regulating microbial denitrification processes.