Cerium anomalies in riverbanks: Highlight into the role of ferric deposits.

In wetlands, stream riverbanks represent a large redox reactive front. At their surface, ferric deposits promote their capacity to trap nutrients and metals. Given that rare earth elements (REE) are now considered as emerging pollutants, it seems that the riverbank interface is a strategic area between wetlands and streams in terms of controlling the environmental dissemination of REE. Therefore, the evolutions of the REE distribution and cerium (Ce) anomaly (Ce/Ce*, i.e. depleted or enriched Ce concentration compared to the other REE) were studied at various locations on a riverbank. The positive Ce anomaly is related to a high Fe content, a low organic carbon/iron ratio ((OC)/Fe) and newly formed Fe oxyhydroxides independently of their interactions with organic matter. Micro-X ray fluorescence (µ-XRF) mapping confirms Ce accumulation with ferric deposits. The Ce speciation exhibits a mix of Ce(III) and Ce(IV) in the ferric deposits, almost 20% of Ce occurred as Ce(IV) due to oxidation by newly formed Fe oxyhydroxides, while the subsurface horizons only display Ce(III). These results provide evidence that the Ce anomaly variation observed in stream water between low and high flow periods is partly due to the erosion of ferric deposits exhibiting a positive Ce anomaly. Therefore, the Ce anomaly can be considered as a fingerprint of the release of Fe colloids in the rivers and streams connected to the wetland.

Iron speciation at the riverbank surface in wetland and potential impact on the mobility of trace metals.

Fe oxyhydroxides in riverbanks and their high binding capacity can be used to hypothesize that riverbanks may act as a "biogeochemical filter" between wetlands and rivers and may constitute a major mechanism in the trapping and flux regulation of chemical elements. Until now, the properties of Fe minerals have been very poorly described in riverbanks. The goals of the present work are to identify Fe speciation in riverbanks where ferric deposits are observed and to determine their impact on the metal behavior (As, Co, Cu, Ni, Pb, Zn, etc.). At the surface, Fe speciation is mainly composed of small poorly crystalline Fe phases, i.e. ferrihydrite (~30%), Fe-OM associations (~40%) as well as crystalline Fe phases, i.e. goethite (~35%). At the subsurface, the Fe distribution is dominated by goethite (~35%) and Fe-mica (~35%), the proportion of which increases at the expense of ferrihydrite and the Fe-OM associations. At the riverbank surface, ferrihydrite and the Fe-OM associations are therefore the main Fe hosting phases in response to (i) the fast Fe(II) oxidation induced by the presence of O2 and (ii) the high amount of OM favoring the formation of nano-phases bound to OM (Fe monomers, polymers and nanoparticles) and preventing mineralogical transformation (ferrihydrite into goethite). During the high-water level period (high flow), a strong erosion of the riverbank transfers these ferric deposits into the river. However, the physicochemical parameters of the river (pH 6.6-7.6 and continuous oxic conditions) do not promote the dissolution of Fe oxyhydroxides and OM. Ferric deposits and the associated trace metals are therefore maintained as colloids/particles and are exported to the outlet. All of the results presented here demonstrate that the ferric deposits trap metals on a seasonal basis and are therefore a key factor in the mobilization of metals during riverbank erosion by river flow.

A desorption-dissolution model for metal release from polluted soil under reductive conditions.

Various natural or provoked situations can cause significant variations in redox conditions that can induce reductive dissolution of soil components. When this happens, heavy metals that may be bound to solid phases are released. A surface desorption-dissolution model, which takes into account the effect of reductive conditions on surface site density, was established. This model is based on conventional reactions of surface hydroxyl groups, surface complexation reactions with cations and double-layer theory. The solid dissolution rate was taken into account, by following changes in total surface site number (i.e., cation exchange capacity [CEC]) under reductive conditions. This term was introduced in an electrostatic desorption model. Curves obtained by this calculation provided a good fit of experimental data as shown by statistical parameters. Experimental data corresponded to Pb and Cd released from a cultivated soil under reductive conditions induced by sodium ascorbate.

Release of Metals from Iron Oxyhydroxides under Reductive Conditions: Effect of Metal/Solid Interactions.

Solubilization of heavy metals from Fe oxyhydroxides in the presence of sodium ascorbate as a reducing agent were investigated as a function of pH. Amorphous iron hydroxide (HFO) and goethite were synthesized in the presence of Pb, Cd, and Zn, using two different processes, i.e., favoring either metal adsorption (ADS) or metal substitution (coprecipitation, COP). Characterization of solids showed that there were no structural differences among the three kinds of HFO. However, in the case of goethite, it seemed that metal cations were located within the structure of COP goethite, but partially adsorbed in ADS goethite. Results evidenced that, under reductive conditions, the solubilization risk is greater for Pb than for Cd (Cd substituted in the structure) from the coprecipitation process. In the adsorption process, Cd was removed more than Pb (greater complexation constant of Pb). Moreover, irrespective of the crystalline state, solids may be partly dissolved, and release of metals could occur. Copyright 2000 Academic Press.

Heavy Metals Desorption from Synthesized and Natural Iron and Manganese Oxyhydroxides: Effect of Reductive Conditions.

Reductive conditions in soils can lead to the dissolution of iron and manganese oxyhydroxides, releasing heavy metal pollutants (e.g., Pb and Cd) bound to them. The present study used hydroxylamine as a reducing agent. Laboratory batch experiments were conducted, varying pH and hydroxylamine concentrations, with artificially contaminated synthetic amorphous Fe(OH)(3s) and MnO(2) and with a polluted cultivated soil. Until conditions were reductive enough to dissolve solids, remobilization of metals depended on their surface complexation constant and readsorption of metal was possible. However, if conditions were sufficiently reductive, all solids were dissolved and metals were released into solution. A straightforward surface complexation model for cation desorption was carried out to support these results. Copyright 2000 Academic Press.