Redox behaviour of arsenic in the surface sediments of the Marque River (Northern France)

Abstract

The behaviour of arsenic(As) in anoxic sediments was studied in the Marque River (Northern France), where concentrations of As are close to the geochemical background level. The distribution of Fe, Mn, S and As species in the solid phase as well as in the pore waters was investigated during four sampling campaigns (February, April, July and October 2014) in order to better understand the parameters involved in the behaviour of arsenic during early diagenetic processes. In the solid phase, As was found to be mainly present in the exchangeable fraction, and the most probable As carrier phases appeared to be amorphous iron, manganese (hydr)-oxides, calcite, and siderite. In pore waters, only the inorganic forms of arsenic [As(III), As(V), and thio‑arsenical species] were detected. As(III) was the dominant species, but thio‑arsenic species were also evidenced in core depth. No direct interaction between arsenic, iron and manganese cycle has been observed in the pore waters. The behaviour of As is mainly linked to the cycle of sulfur, and especially to the production of sulfides. Overall, this study confirms that the experimental determination of redox speciation is still very useful to understand the behaviour of As during early stages of diagenesis, since application of thermodynamic modelling to redox sensitive environments is not sufficiently constrained yet to provide reliable results.