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The fate of chromium (Cr) – a redox sensitive metal – in surface sediments is closely linked to early diagenetic processes. This review summarizes the main redox pathways that have been clearly identified over recent decades concerning the behavior of Cr(III,VI) in aquatic environments, and applies them to surface sediments where data for redox speciation remain limited. Overall, abiotic redox reactions that govern the speciation of Cr involve manganese (Mn) (III,IV) (hydr)-oxydes for Cr(III) oxidation, Cr(VI)-reducing species (dissolved iron (Fe) (II) and hydrosulfide (HS)−), and Cr(VI)-reducing phases (ferrous and sulfide minerals, as well as Fe(II)-bearing minerals). Bacterial activity is also responsible for the redox interconversion between Cr(III) and Cr(VI): biotic reduction of Cr(VI) to Cr(III) is observed through either detoxification or dissimilatory reduction. Whereas Mn(II)-oxidizing bacteria are known to promote indirect oxidation of Cr(III) to Cr(VI), the reaction mechanisms are unresolved. Conversely, oxygen (O2), nitrate (NO3−), and nitrite (NO2−) do not appear to play any role in Cr(III) oxidation. Additionally, Mn(II) and ammonium (NH4+) are not known to promote Cr(VI) reduction. Once reduced, the mobility of Cr(III) in sediments is significantly restricted and regulated by precipitation and sorption processes. Finally, even if the role of natural organic matter in sediment has been determined, further research is required to identify the complexation mechanisms.

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A HPIC-ICP-MS method has been developed to directly determine As redox speciation in filtered sediment pore waters without pre-treatment step.

To improve modeling of Cs migration in the environment, we need to determine the proportion of labile Cs. AMP (ammonium 12-molybdophosphate) DGT is a good candidate to sample and concentrate Cs on field. Nevertheless, chemical conditions in water and soils may disturb the stability of AMP binding gel. Laboratory experiments were designed: (i) to identify factors controlling AMP binding gel degradation during storage and DGT deployment and (ii) to assess the possible impact of AMP degradation over time on the DGT response in waters with various chemical composition. Conclusions on potential limitations of AMP-DGT methods will finally be discussed for a better use of this technique for field applications. For more details, we invite you to discover our article recently published in Environmental Science and Pollution Research.

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