Highlighting the wide variability in arsenic speciation in wetlands: A new insight into the control of the behavior of arsenic

Abstract

Highlighting the wide variability in arsenic speciation in wetlands: a new insight into the control of the behavior of arsenic. Geochimica et Cosmochimica Acta, Elsevier, 2017, 203, pp.284-302 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. 1Highlighting the wide variability in arsenic speciation in wetlands: AbstractAlthough 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 Å) as bidentate binuclear complexes and C (As-C = 2.90 Å), suggesting the integration of As in biological objects. In the < 30 kDa fraction, As was directly bound to C (As-C = 1.96 Å) in the first shell indicating the presence of organic As species. In the second shell, an As-Fe distance of 3.35 Å was found showing that 2 part of the As was still complexed with Fe. The 0.2 µm-30 kDa fraction was a transitional fraction in terms of the Fe species and OM composition. In this fraction, organic matter exhibited a more humic character (aromatic molecules) inducing an increasing cation binding capacity. As a consequence, in this fraction and in the smallest one, As, Fe and OM seemed to form ternary complexes in which the Fe or nano-oxides in the > 30 kDa fraction and as monomer, or cluster in < 30 kDa fraction acted as a bridge. In all of the fractions, a proportion of As(V) was present as organic methylated species. These organic species might be produced by several organisms (animal or plant) via a detoxification process. They seemed to be bound to the particulate and colloidal Fe/OM phases as well as integ...

Dissolved arsenic in the upper Paraguay River basin and Pantanal wetlands

“…The highest As concentrations (1.36±0.34 g L -1 ) were found in the Nabileque river, the only one that has its source in the alluvial plain. In this environment, during the high water levels, anaerobic conditions favor the reductive dissolution of wetland-soil Fe-oxihydroxides and associated elements, such as arsenic and organic matter (Guénet et al, 2017). Such a process could be responsible for the slightly higher As concentration observed in the Nabileque river.…”

“…On the other hand, despite the presence of high arsenic content in the shallow perched water-table and the surface water of Nhecolândia, the most alkaline region of the Pantanal, no study has been directed towards a possible release of arsenic from the wetland towards the main draining rivers. The potential release mainly depends on both, the fate of As during the reduction and trapping mechanisms that favor As stabilization in the wetland (Guénet et al, 2017), and the behavior of arsenic during the re-oxidation process occurring at the wetland-river interface (Pédrot et al, 2015). In this framework, the objective of this study is double: first, to identify, among the rivers on the uplands, the possible sources for the high arsenic contents observed in the floodplain and to verify if the floodplain is releasing arsenic to the nearby river network, according to the hydrological connectivity between the wetland and the main draining rivers; second, to verify whether arsenic occurrence detected in an alkaline soil system of Nhecolândia is related to local or regional processes and to identify the factors responsible for these high arsenic contents.…”

Iron speciation in iron–organic matter nanoaggregates: a kinetic approach coupling Quick-EXAFS and MCR-ALS chemometrics

“…50,51 Fh-NP appear when the Fe/OC ratio sufficiently increases (typically above 0.02) 10 and the Fh/oligomer ratio increases with Fe/OC. 10,11,13,42 However, Chen et al 11 did not observe anymore oligomer for Fe/OC ≥ 0.3, while Guénet et al 13,42 report the coexistence of oligomers and ferrihydrite for Fe/OC ≥ 0.4. This discrepancy may be explained by the references used by Chen et al 11 to perform EXAFS linear combination fits which include a ferrihydrite-free Fe(III)-organo-complex reference.…”

“…The Cpt-2 signal can thus be assigned to an overall signal for oligomers of Fe octahedra bound to HA via carboxylic groups as predicted by Catrouillet et al 23 The Fe-C distance corresponds to the distances reported in the literature for both natural and synthetic systems. 4,8,9,10,13,42,43 Short and long Fe-Fe distances correspond to edge and single corner sharing Fe octahedra, respectively. 44 Thus, according to the number and distance of the neighbours, the overall average signal could be described as a tetramer formed by two edge sharing dimers in which each octahedron is bound by its upper corner to an octahedron of the second dimer.…”

“…These structures and the Fe/OC ratios have dramatic environmental implications regarding their capacity to trap metal and metalloid pollutants. Guénet et al 42 and Mikutta and Kretzschmar 8 demonstrated that, as ferrihydrite, Fe(III)oligomers bound to OM are able to sorb arsenic (As). OMembedded Fh-NP are complexed to OM via surface reaction sites.…”