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

Abstract

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

Ionic Liquid Extraction Unveils Previously Occluded Humic‐Bound Iron in Peat Soil Pore Water

“…In a study encompassing 15 standard humic-and fulvic acid mixtures, Fujii et al (2014) reported that aromaticity was a strong predictive factor of iron binding capacity and coordination complex stability. We have found that the UV-Vis index of oxygen content (E4) was a stronger predictor of o-P method performance than aromaticity (proxied by SUVA 254 ), which agrees in principle with the concept that humic and fulvic carboxylic groups participate in Fe binding (Catrouillet et al, 2014). To fully understand DOM-Fe interactions in peat porewater, investigations coupling ILE and spectrophotometric Fe assays to NMR and high-field mass spectrometry in addition to fluorescence and UV-Vis spectroscopy (Tfaily et al, 2013) could provide greater mechanistic insight into site-dependent matrix effects.…”

“…Thus, to measure total Fe these methods presume that first, all Fe 3+ in a sample is reduced to Fe 2+ and, second, that the entire pool of Fe 2+ is subsequently available to the chelating agent. This complexation mechanism is inhibited in anoxic humic-and fulvic-rich systems wherein the DOM matrix itself is believed to be redox active and the predominant metal-DOM complexes are thought to exist as bulky bidentate or tridentate conformations of large molecules (Catrouillet et al, 2014). In our dataset, the samples with the most humified DOM appeared to pose the greatest challenge to o-P.…”

“…Thus, reduced ferrozine performance in the presence of peat humic acids may be explained by the effect of reaction pH on humic/fulvic Fe binding, and the resulting complexation equilibrium of ferrozine with Fe in the sample matrix. In model humic substances, high pH (pH > 6) leads to tighter binding in metal-humic complexes as the predominant complexation sites shift away from carboxyl groups toward phenolics (Catrouillet et al, 2014). This shift in type of binding site is accompanied by a greater capacity for Fe complexation overall (Fujii et al, 2014).…”

“…Because the ferrozine assay is performed at pH 9.5, the dominant (ca. 60%, Catrouillet et al, 2014) binding sites in the sample matrix will shift toward phenolic groups. Because the Fe at these sites is more tightly bound, it is perhaps not quantifiable using ferrozine (Guo et al, 2007;Hu 2011).…”

“…Peat pore water is rich in DOM consisting largely of an array of humified products of plant decomposition and microbial activity (Thurman, 1985;D' Andrilli et al, 2010;Tfaily et al, 2013). Humic and fulvic acids form stable coordination complexes with Fe 3+ (Maranger and Pullin, 2003;Antunes et al, 2007;Catrouillet et al, 2014;Fujii et al, 2014). These complexes are effective extracellular electron acceptors in anaerobic environments, including peat (Benz et al, 1998;Bauer et al, 2007;Lipson et al, 2010).…”