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Article Dans Une Revue Geoscience Frontiers Année : 2019

Interplay of S and As in Mekong Delta sediments during redox oscillations

Résumé

The cumulative effects of periodic redox cycling on the mobility of As, Fe, and S from alluvial sediment to groundwater were investigated in bioreactor experiments. Two particular sediments from the alluvial floodplain of the Mekong Delta River were investigated: Matrix A (14 m deep) had a higher pyrite concentration than matrix B (7 m deep) sediments. Gypsum was present in matrix B but absent in matrix A. In the reactors, the sediment suspensions were supplemented with As(III) and SO 4 2À , and were subjected to three full-redox cycles entailing phases of nitrogen/CO 2 , compressed air sparging, and cellobiose addition. Major differences in As concentration and speciation were observed upon redox cycling. Evidences support the fact that initial sediment composition is the main factor controlling arsenic release and its speciation during the redox cycles. Indeed, a high pyrite content associated with a low SO 4 2À content resulted in an increase in dissolved As concentrations, mainly in the form of As(III), after anoxic half-cycles; whereas a decrease in As concentrations mainly in the form of As(V), was instead observed after oxic half-cycles. In addition, oxic conditions were found to be responsible for pyrite and arsenian pyrite oxidation, increasing the As pool available for mobilization. The same processes seem to occur in sediment with the presence of gypsum, but, in this case, dissolved As were sequestered by biotic or abiotic redox reactions occurring in the FeeS system, and by specific physico-chemical condition (e.g. pH). The contrasting results obtained for two sediments sampled from the same core show that many complexes and entangled factors are at work, and further refinement is needed to explain the spatial and temporal variability of As release to groundwater of the Mekong River Delta (Vietnam).

Domaines

Géochimie
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Dates et versions

hal-02298338 , version 1 (26-09-2019)

Identifiants

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Van T H Phan, Fabrizio Bardelli, Pierre Le Pape, Raoul-Marie Couture, Alejandro Fernandez-Martinez, et al.. Interplay of S and As in Mekong Delta sediments during redox oscillations. Geoscience Frontiers, 2019, 10 (5), pp.1715-1729. ⟨10.1016/j.gsf.2018.03.008⟩. ⟨hal-02298338⟩
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