Iron and arsenic speciation in marine sediments undergoing a resuspension event: the impact of biotic activity
Résumé
Purpose Changes in the chemical conditions of sediment following a resuspension event might lead to release of sequestered pollutants. In the present study, arsenic (As) and iron (Fe) speciation were investigated before and after such an event, in sediment from L'Estaque marina (France). This marina is located near an industrial plant which processed As-bearing ores for several decades. Materials and methods Cores (0-60 cm) and surface sediment (0-10 cm) were collected by a diver. Sediment properties along the length of the core were determined by coupling chemical extractions, and diffraction (i.e., X-ray diffraction) and spectroscopic techniques (i.e., micro-X-ray fluorescence, scanning electron microscopy coupled with energy dispersive spectroscopy, Raman spectroscopy, and X-ray absorption near-edge spectroscopy). Laboratory experiments mimicking resuspension and resettlement events were conducted over a period of 3 months in both biotic and abiotic (autoclaved) conditions. In both cases, oxidation proceeded by oxygen diffusion from the top to the bottom of the sediment. Results and discussion It was demonstrated that the unperturbed sediment was anoxic. Arsenic, almost fully under its trivalent As(III) form, had a concentration between 194 and 473 μg g−1, and its main carrier phase was the Fe-monosulfide mackinawite; this mineral originated from in situ transformation of Fe oxides, partly emitted by the industrial plant. The observed progressive pyritisation of mackinawite was not accompanied by further reduction of As which means that As remained bound to mackinawite, as incorporation into the pyrite lattice would require that it reduced to its (−I) oxidation state. After oxidation during the resuspension event, and in abiotic conditions, As was fully pentavalent As(V) in the oxidized zone of the re-settled sediment. On the contrary, in the biotic experiment, the development of a bacterial mat, which consumed oxygen for respiration processes, preserved the sediment from total oxidation. Consequently, As was present under both As(III) and As(V) forms, the first one being minor (∼20%of total As) in the top of the sediment. The diversity of aioA genes was large, and was similar in the oxidized layer and the deeper black-colored sediment. Conclusions These results indicate that biological processes partly control the in situ geochemical system by inducing low redox areas in theoretically oxidized sediments.