Interactions between natural organic matter, sulfur, arsenic and iron oxides in re-oxidation compounds within riparian wetlands: NanoSIMS and X-ray adsorption spectroscopy evidences
Résumé
Arsenic (As) is a toxic and ubiquitous element which can be responsible for severe health problems. Recently,
Nano-scale Secondary IonsMass Spectrometry (nanoSIMS) analysis has been used to map organomineral assemblages.
Here,we present a method adapted fromBelzile 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 75As−, 56Fe16O−, sulfur (32S−) and organic matter (12C14N) and to measure X-ray adsorption
of sulfur (S) K-edge. A first statistical treatment on the nanoSIMSimages highlights two main colocalizations:
(1) 12C14N−, 32S−, 56Fe16O− and 75As−, and (2) 12C14N−, 32S− and 75As−. 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 32S−. The second
component explains 20% of the variance and is highly correlated with the presence of 12C14N−. 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.