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Geochemical modeling of Fe(II) binding to humic and fulvic acids

Abstract : The 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.
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Charlotte Catrouillet, Mélanie Davranche, Aline Dia, Martine Bouhnik-Le Coz, Remi Marsac, et al.. Geochemical modeling of Fe(II) binding to humic and fulvic acids. Chemical Geology, Elsevier, 2014, 372, pp.109-118. ⟨10.1016/j.chemgeo.2014.02.019⟩. ⟨insu-00965127⟩

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