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

Abstract

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 O 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.

How does calcium drive the structural organization of iron–organic matter aggregates? A multiscale investigation

“…Environmental nanoparticles are ubiquitous in natural systems 1 . Among them, natural ironorganic matter (Fe-OM) aggregates are of major importance due to their high amounts in natural systems such as in wetlands [2][3][4] , peatlands 5,6 or permafrosts 7,8 . They are mainly produced in soils via anthropogenic forcing and geochemical and physical processes such as alteration and erosion, oxidation-reduction variations occurrence subsequent to soil watersaturation/desaturation alternation.…”

How crucial is the impact of calcium on the reactivity of iron-organic matter aggregates? Insights from arsenic

“…They are nanometric particles, either aggregated or not, with a high specific surface area (SSA), mainly composed of minerals and/or organic matter (OM). Among them, those particles composed of iron (Fe) and OM, usually called "ironorganic matter (Fe-OM) aggregates", are widely encountered in natural systems such as wetlands where they are mainly produced by drying/wetting cycles in the uppermost soils [2][3][4][5][6].…”

Morphology, Mineralogy, and Chemistry of Ocherous Precipitate Aggregates Downstream of an Abandoned Mine Site

“…Precipitates formed by the dissolution and oxidation of iron and aluminum-containing minerals often occur downstream of abandoned and/or uncontrolled mine sites. Precipitates occur on river sediments [1][2][3][4], below and/or on the sides of the dam water outflow [5][6][7], on riverbanks [8], and in cemented layers, such as hardpans [9]. The precipitates effectively adsorb heavy metal ions, such as As and Sb, and inhibit the outflow of these ions downstream.…”

“…Such natural attenuation can be helpful for removing heavy metal contaminants from the environment. Thus, numerous studies have focused on the adsorption of these ions by precipitates based on experimental results on natural and synthetic materials; however, the natural attenuation properties of iron-and/or aluminum-bearing materials are not well understood in riverbanks [8].…”

“…Ratié [8] proposed that the riverbank may act as a "biogeochemical filter" between soils and streams and could be a major mechanism for the trapping and flux regulation of heavy metal ions. Most riverbanks also possess a reactive redox interface.…”