Martine Bouhnik-Le Coz

Thiol groups controls on arsenite binding by organic matter: new experimental and modeling evidence.

Although it has been suggested that several mechanisms can describe the direct binding of As(III) to organic matter (OM), more recently, the thiol functional group of humic acid (HA) was shown to be an important potential binding site for As(III). Isotherm experiments on As(III) sorption to HAs, that have either been grafted with thiol or not, were thus conducted to investigate the preferential As(III) binding sites. There was a low level of binding of As(III) to HA, which was strongly dependent on the abundance of the thiols. Experimental datasets were used to develop a new model (the modified PHREEQC-Model VI), which defines HA as a group of discrete carboxylic, phenolic and thiol sites. Protonation/deprotonation constants were determined for each group of sites (pKA=4.28±0.03; ΔpKA=2.13±0.10; pKB=7.11±0.26; ΔpKB=3.52±0.49; pKS=5.82±0.052; ΔpKS=6.12±0.12 for the carboxylic, phenolic and thiols sites, respectively) from HAs that were either grafted with thiol or not. The pKS value corresponds to that of single thiol-containing organic ligands. Two binding models were tested: the Mono model, which considered that As(III) is bound to the HA thiol site as monodentate complexes, and the Tri model, which considered that As(III) is bound as tridentate complexes. A simulation of the available literature datasets was used to validate the Mono model, with logKMS=2.91±0.04, i.e. the monodentate hypothesis. This study highlighted the importance of thiol groups in OM reactivity and, notably, determined the As(III) concentration bound to OM (considering that Fe is lacking or at least negligible) and was used to develop a model that is able to determine the As(III) concentrations bound to OM.

Does As(III) interact with Fe(II), Fe(III) and organic matter through ternary complexes?

Up until now, only a small number of studies have been dedicated to the binding processes of As(III) with organic matter (OM) via ionic Fe(III) bridges; none was interested in Fe (II). Complexation isotherms were carried out with As(III), Fe(II) or Fe(III) and Leonardite humic acid (HA). Although PHREEQC/Model VI, implemented with OM thiol groups, reproduced the experimental datasets with Fe(III), the poor fit between the experimental and modeled Fe(II) data suggested another binding mechanism for As(III) to OM. PHREEQC/Model VI was modified to take various possible As(III)-Fe(II)-OM ternary complex conformations into account. The complexation of As(III) as a mononuclear bidentate complex to a bidentate Fe(II)-HA complex was evidenced. However, the model needed to be improved since the distribution of the bidentate sites appeared to be unrealistic with regards to the published XAS data. In the presence of Fe(III), As(III) was bound to thiol groups which are more competitive with regards to the low density of formed Fe(III)-HA complexes. Based on the new data and previously published results, we propose a general scheme describing the various As(III)-Fe-MO complexes that are able to form in Fe and OM-rich waters.

Robust Method Using Online Steric Exclusion Chromatography-Ultraviolet-Inductively Coupled Plasma Mass Spectrometry To Investigate Nanoparticle Fate and Behavior in Environmental Samples.

The foundation of nanoscience is that the properties of materials change as a function of their physical dimensions, and nanotechnology exploits this premise by applying selected property modifications for a specific benefit. However, to investigate the fate and effect of the engineered nanoparticles on toxic metal (TM) mobility, the analytical limitations in a natural environment remain a critical problem to overcome. Recently, a new generation of size exclusion chromatography (SEC) columns developed with spherical silica is available for pore sizes between 5 and 400 nm, allowing the analysis of nanoparticles. In this study, these columns were applied to the analysis of metal-based nanoparticles in environmental and artificial samples. The new method allows quantitative measurements of the interactions among nanoparticles, organic matter, and metals. Moreover, because of the new nanoscale SEC, our method allows the study of these interactions for different size ranges of nanoparticles and weights of organic molecules with a precision of 1.2 × 10(-2) kDa. The method was successfully applied to the study of nanomagnetite spiked in complex matrixes, such as sewage sludge, groundwater, tap water, and different artificial samples containing Leonardite humic acid and different toxic metals (i.e., As, Pb, Th). Finally, our results showed that different types of interactions, such as adsorption, stabilization, and/or destabilization of nanomagnetite could be observed using this new method.

Experimental evidence of REE size fraction redistribution during redox variation in wetland soil

The evolution of rare earth element (REE) speciation between reducing and oxidizing conditions in a riparian wetland soil was studied relative to the size fractionation of the solution. In all size fractions obtained from the reduced and oxidized soil solutions, the following analyses were carried out: organic matter (OM) characterization, transmission electron microscopy (TEM) observations as well as major and trace element analyses. Significant REE redistribution and speciation evolution between the various size fractions were observed. Under reducing conditions, the REEs were bound to colloidal and dissolved OM (<2μm size fractions). By contrast, under oxidizing conditions, they were distributed in particulate (>2μm size fraction), colloidal (<2μm size fraction), organic and Fe-enriched fractions. In the particulate size fraction, the REEs were bound to humic and bacterial OM embedding Fe nano-oxides. The resulting REE pattern showed a strong enrichment in heavy REEs (HREEs) in response to REE binding to specific bacterial OM functional groups. In the largest colloidal size fraction (0.2μm-30kDa), the REEs were bound to humic substances (HS). The lowest colloidal size fraction (<30kDa) is poorly concentrated in the REEs and the REE pattern showed an increase in the middle REEs (MREEs) and heavy REEs (HREEs) corresponding to a low REE loading on HS. A comparison of the REE patterns in the present experimental and field measurements demonstrated that, in riparian wetlands, under a high-water level, reducing conditions are insufficient to allow for the dissolution of the entire Fe nano-oxide pool formed during the oxidative period. Therefore, even under reducing conditions, Fe(III) seems to remain a potential scavenger of REEs.

Corrigendum to "Highlighting the wide variability in arsenic speciation in wetlands: a new insight into the control of the behavior of arsenic" [Geochim. Cosmochim. Acta 203 (2017) 284–302]

Although the behavior of Arsenic (As) under reducing conditions in periods of high water levels in wetlands is well understood and documented, there is a lack of information under oxidizing conditions when the water level decreases. In this study, we were interested in the first stage of the oxidizing period, when oxidation products are still in suspension. A soil sample from the Naizin Kervidy wetland (France) was incubated in the laboratory to produce a reduced soil solution. The reduced solution was subsequently oxidized, filtered and ultrafiltered using decreasing pore size membranes (5 μm, 3 μm, 0.2 μm, 30 kDa and 5 kDa). The distribution of As and Fe was investigated in each size fraction of the oxidized solution and their speciations were studied using XAS, HPLC and SEC-ICP-MS. Organic matter was characterized using thermally assisted hydrolysis and methylation gas chromatography-mass spectrometry (THM-GC-MS) and fluorescence spectroscopy. The majority of the As was present as As(V) but a small amount of As(III) still remained despite the advanced oxidized conditions. In the > 0.2 μm fractions, the XAS analyses showed that As was associated, in the second shell, with Fe (As-Fe = 3.35 A) as bidentate binuclear complexes and C (As-C = 2.90 A), suggesting the integration of As in biological objects. In the 30 kDa fraction and as monomer, or cluster in 5 μm particulate fraction and thus was hardly mobile. This study showed that although wetlands have been identified as a potential source of As, a number of biological and geochemical trapping mechanisms also favor As stabilization in wetlands.