Is trace metal release in wetland soils controlled by organic matter mobility or Fe-oxyhydroxides reduction?

Aerobic and anaerobic incubation experiments on a wetland soil samples were used to assess the respective roles of organic matter (OM) release, Fe-oxyhydroxides reduction and redox/speciation changes on trace metal mobility during soil reduction. Significant amounts of Cu, Cr, Co, Ni, Pb, U, Th and Rare Earth Elements (REE) were released during anaerobic incubation, and were accompanied by strong Fe(II) and dissolved organic matter (DOM) release. Aerobic incubation at pH 7 also resulted in significant trace metal and DOM release, suggesting that Fe-oxyhydroxide reduction is not the sole mechanism controlling trace metal mobility during soil reduction. Using these results and redox/speciation modeling, four types of trace metal behavior were identified: (i) metals bound to organic matter (OM) and released by DOM release (REE); (ii) metals bound to both OM and Fe-oxyhydroxides, and released by the combined effect of DOM release and Fe(III) reduction (Pb and Ni); (iii) metals bound solely to soil Fe-oxyhydroxides and released by its reductive dissolution (Co); and (iv) metals for which release mechanisms are unclear because their behavior upon reduction is affected by changes in redox state and/or solution speciation (Cu, Cr, U and Th). Even though the process of soil Fe-oxyhydroxide reduction is important in controlling metal mobility in wetland soils, the present study showed that the dominant mechanism for this process is OM release. Thus, OM should be systematically monitored in experimental studies dedicated to understand trace metal mobility in wetland soils. Due to the fact that the process of OM release is mainly controlled by pH variations, the pH is a more crucial parameter than Eh for metal mobility in wetland soils

Increasing PH drives organic matter solubilization from wetland soils under reducing conditions

International audienceIn wetlands, large quantities of dissolved organic matter (DOM) are solubilized under reducing conditions. Controlled incubations of a wetland soil were performed under oxic and anoxic conditions to investigate the extent to which the following processes account for this phenomenon: i) production of organic metabolites by microbes during soil reduction; ii) release of organic matter (OM) from Mn- and Fe-oxyhydroxides that undergo reductive dissolution; and iii) desorption of OM from soil minerals due to pH changes. Anaerobic incubation releases 2.5% of the total soil organic carbon (OC) as dissolved organic carbon (DOC), and is accompanied by a pH rise from 5.5 to 7.4 and by the soil Mn- and Fe-reduction. The three processes above all take place. However, anaerobic incubation at a constant pH of 5.5 (preventing OM desorption) releases only 0.5% of the total soil OC, while aerobic incubation at pH 7.4 (preventing Mn- and Fe-reduction) releases 1.7% of the total soil OC. By contrast, aerobic incubation at pH 5.5 (preventing both Mn- and Fe-reduction and pH rise) does not solubilize any DOC. The DOC released is markedly aromatic, indicating little contribution from microbial metabolites, but, rather, the presence of microbes leading to OM mineralization. The pH rise is the key factor controlling OM solubilization under reducing conditions. This rise of pH accounts for >60% of the total released DOC, which is not due to reductive dissolution as such

The nature of interlayering in clays from a podzol (Spodosol) from the Tatra Mountains, Poland

International audienceInterlayered clays are common constituents of several soil orders. The presence of interlayering is indicated by modified thermal stability of soil vermiculites and smectites. In most studies the clays have been reported as Al-hydroxy-interlayered minerals. Despite the abundance of organic compounds in many soils, the potential existence of soil organo-clay intercalations has rarely been considered. In the present study three clay subfractions separated from the eluvial (E) horizon of one podzol from the Tatra Mts., Poland, were studied using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and elemental CHNS analysis, in the natural state and after being subjected to organic matter removal, free iron oxides removal, saturation with different cations, and heating at 330 °C and 550 °C. All fractions contain a "~14 Å clay". In the natural state the "~14 Å clay" shows high thermal resistance, manifested by only a partial shift to 10 Å after heating at 330 °C. This behavior strongly suggests the presence of interlayering. Saturation with K+ shifted the ~ 14 Å reflection to ~ 13-11 Å, which suggests displacement of a charged species from the interlayer. Clay sample treatment with H2O2 and NaOCl reduced the stability of the interlayered phase because 330 ° C heat treatment shifted the ~ 14 Å phase to ~ 10 Å, which suggests that the interlayered phase might be organic. Furthermore, the 13-18% organic carbon content of the clays and near disappearance of infrared peaks at 2855 cm− 1, 2926 cm− 1, 1738 cm− 1, ~ 1600 cm− 1, and ~ 1400 cm− 1 after H2O2 or NaOCl treatment suggests that the clay interlayer phase is mostly organic. Both Al-hydroxy and organic interlayers can be evaluated using a combination of X-ray diffraction, elemental analysis, and infrared spectroscopy and should be considered as possible interlayer phases during the study of interlayered clays

Rare earth elements as fingerprint of soil components activation

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Dissolution of anisotropic colloidal mineral particles: Evidence for basal surface reactivity of nontronite

International audienceAnisotropic textural and crystallographic properties of phyllosilicate particles often influence the mineral weathering rate. The purpose of this study was to investigate how the changes in mineral surfaces (basal vs. edge) as a result of changes in crystal size control the dissolution of the mineral. Different nano-size fractions of Na-exchanged nontronites (NAu2 and NAu1) were immobilized in a silica gel and then incubated under acidic conditions using HNO3 at 28 °C for 5 days. For each sample, the dissolution behavior was analyzed by measuring the amount of iron released from the mineral lattice. The results showed that for a given pH, a decrease in particle size significantly increased NAu2 and NAu1 dissolution. At pH 1.5, 7.2% of the total iron of the highest size sample of NAu2 was released in solution whereas this proportion increased up to 25% for the smallest size fraction. The percentage of total iron extracted from NAu1 at the same pH (1.5) was less important: 3.5% and 6.5% for higher and smaller size fractions, respectively. The observed increase in dissolution was not directly correlated to the increase in the amount of edge faces, suggesting that all mineral surfaces contributed to mineral dissolution. In the present case this may be related to the fact that 8% and 2% of total iron of NAu2 and NAu1, respectively, are located in the tetrahedral sheet. In conclusion, the basal surface of nontronites plays an important role in the weathering process

Rare earth element patterns: A tool for identifying trace metal sources during wetland soil reduction

International audienceIn wetland soils, several soil phases such as Fe(III)-oxyhydroxides, organic matter (OM) or mixed Fe-OM particles can host trace metals which can be subsequently released during soil reduction. Anoxic and oxic wetland soil incubation experiments, combined with analyses of soil solutions sampled from a natural wetland during a reduction event, are used to test the possibility that rare earth elements (REE) could be used as a tool to identify the soil phases contributing to trace metal solubilization. Significant amounts of trace metals (Cu, Cr, Co, Ni and Pb) and REE are released during anoxic incubation of the wetland soil, concomitantly with the build-up of high concentrations of Fe(II) and dissolved organic matter (DOM). Rare earth element patterns obtained in the soil solution exhibit a middle rare earth elements (MREE) downward concavity. The REE pattern obtained from field samples yields the same feature as developed in the soil solution from an oxic incubation experiment at pH 7 designed to promote soil OM desorption. By contrast, significantly different REE patterns are obtained in incubation experiments designed to promote chemical reduction of Fe-oxyhydroxides in soils. The REE pattern displays a continuous REE enrichment from La to Lu. These distinct and recognizable REE signatures allow us to conclude that (i) soil organic matter is the main source of REE and trace metals during wetland soil reduction; (ii) Fe(II) is provided by the reduction of amorphous Fe(III) nanoparticles embedded within the organic matter, which do not bind REE or other trace metals in significant proportions (REE and trace elements being preferentially complexed to organic matter); and finally (iii) REE provide a reliable and powerful tool, suitable for identifying trace metal sources during wetland soil reduction

Bio-dissolution of colloidal-size clay minerals entrapped in microporous silica gels

International audienceFour colloidal-size fractions of strongly anisotropic particles of nontronite (NAu-2) having different ratios of basal to edge surfaces were incubated in the presence of heterotrophic soil bacteria to evaluate how changes in mineral surface reactivity influence microbial dissolution rate of minerals. To avoid any particle aggregation, which could change the reactive surface area available for dissolution, NAu-2 particles were immobilized in a biocompatible TEOS-derived silica matrix. The resulting hybrid silica gels support bacterial growth with NAu-2 as the sole source of Fe and Mg. Upon incubation of the hybrid material with bacteria, between 0.3% and 7.5% of the total Fe included in the mineral lattice was released with a concomitant pH decrease. For a given pH value, the amount of released Fe varied between strains and was two to twelve-fold higher than under abiotic conditions. This indicates that complexing agents produced by bacteria play an important role in the dissolution process. However, in contrast with proton-promoted NAu-2 dissolution (abiotic incubations) that was negatively correlated with particle size, bacterial-enhanced dissolution was constant for all size fractions used. We conclude that bio-dissolution of nontronite particles under acidic conditions seems to be controlled by bacterial metabolism rather than by the surface reactivity of mineral

Distribution spatiale et mobilité du phosphore sédimentaire dans une retenue hydroélectrique

International audienceMalgré la réduction des rejets de phosphore (P) dans l’environnement, les efflorescences phytoplanctoniques sont toujours observées dans un grand nombre de masses d’eau. C’est notamment le cas dans les retenues de barrage où la perturbation de la continuité sédimentaire peut conduireà une accumulation importante de phosphore au sein du compartiment sédimentaire. Ainsi, pour lutter contre l’eutrophisation, une meilleure connaissance de la distribution, de la réactivité et de la mobilité du P sédimentaire s’avère indispensable. Dans ce contexte, les sédiments de surface de la retenue de Champsanglard (Creuse, France) ont été prélevés en différents points selon un gradient amont/aval. Une caractérisation physico-chimique (granulométrie; teneurs en P, Fe, Al, matière organique; fractionnement chimique du P suivant le protocole de RYDIN et WELCH [1998]) a été effectuée afin d’évaluer le potentiel de relargage de P de ces sédiments. Les résultats montrent une évolution de la concentration et de la distribution du P dans les sédiments de surface dans la retenue de l’amont vers l’aval (de 1,5 à 2,3 mg P∙gsec -1), qui semble corrélée avec l’évolution de la granulométrie. L’analyse du fractionnement chimique a montré que le P était majoritairement lié aux oxyhydroxydes de Fe et Mn amorphes (66 %), ces concentrations augmentant également à l’approche du barrage. Ces résultats indiquent que les conditions d’oxydoréduction sont l’un des paramètres clés du contrôle de la remobilisation du P sédimentaire vers la colonne d’eau. À l’opposé, le P associé aux fractions de lamatière organique et de l’aluminium ne semble quant à lui ne pas évoluer spatialement de façon significative

Weathering of Sb-rich mining and smelting residues : Insight in solid speciation and soil bacteria toxicity

International audienceTailings and slag residues from the most important antimony mine of the French Massif Central were analysed for their mineralogical and chemical contents by conventional X-ray powder diffraction and synchrotron-based X-ray microdiffraction (μ-XRD). Results show that ∼2000 metric tons of Sb are still present at the abandoned mining site. Mean concentrations of Sb in slags and tailings are 1700 and 5000 mg kg−1, respectively. In addition, smaller quantities of As were also measured (∼800 mg kg−1 in tailings). Toxicity tests of As and Sb indicate that the growth of bacteria is severely affected at these concentrations. In particular, Sb was observed to cause negative effects for several types of bacteria. Almost all primary minerals carrying trivalent Sb disappeared during weathering at the expense of phases in which Sb5+ is the most abundant form. Instead of sulphides, Sb-bearing Fe hydroxides (goethite and lepidocrocite) are now present in the residues together with Sb-bearing jarosite and Sb(-Fe) oxides and hydroxides such as tripuhyite, senarmontite, romeite, cervantite, and valentinite. Water analyses of the main local stream indicate little remobilization of Sb downstream the site and despite the acidic pH of the surface tailings, pH values show neutral or near-neutral values on all locations of the site