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.

Interactions between natural organic matter, sulfur, arsenic and iron oxides in re-oxidation compounds within riparian wetlands: nanoSIMS and X-ray adsorption spectroscopy evidences.

Arsenic (As) is a toxic and ubiquitous element which can be responsible for severe health problems. Recently, Nano-scale Secondary Ions Mass Spectrometry (nanoSIMS) analysis has been used to map organomineral assemblages. Here, we present a method adapted from Belzile et al. (1989) to collect freshly precipitated compounds of the re-oxidation period in a natural wetland environment using a polytetrafluoroethylene (PTFE) sheet scavenger. This method provides information on the bulk samples and on the specific interactions between metals (i.e. As) and the natural organic matter (NOM). Our method allows producing nanoSIMS imaging on natural colloid precipitates, including (75)As(-), (56)Fe(16)O(-), sulfur ((32)S(-)) and organic matter ((12)C(14)N) and to measure X-ray adsorption of sulfur (S) K-edge. A first statistical treatment on the nanoSIMS images highlights two main colocalizations: (1) (12)C(14)N(-), (32)S(-), (56)Fe(16)O(-) and (75)As(-), and (2) (12)C(14)N(-), (32)S(-) and (75)As(-). Principal component analyses (PCAs) support the importance of sulfur in the two main colocalizations firstly evidenced. The first component explains 70% of the variance in the distribution of the elements and is highly correlated with the presence of (32)S(-). The second component explains 20% of the variance and is highly correlated with the presence of (12)C(14)N(-). The X-ray adsorption near edge spectroscopy (XANES) on sulfur speciation provides a quantification of the organic (55%) and inorganic (45%) sulfur compositions. The co-existence of reduced and oxidized S forms might be attributed to a slow NOM kinetic oxidation process. Thus, a direct interaction between As and NOM through sulfur groups might be possible.