Jean-François Lenain

Arsenic in iron cements developed within tailings of a former metalliferous mine—Enguiales, Aveyron, France

Abstract Arsenopyrite-rich waste from a former metalliferous mine were spread out over the sloping side of a deep valley after processing. Over the past 30 a, they have been subjected to rainfall and acid water originating from the abandoned mine galleries. This intensive leaching has led to the formation of thin layers of As–Fe crusts on the tailings surface acting as a cement. X-ray diffraction and SEM coupled with EDS determined that jarosite was present in all mineral samples and could contain a small amount of As (∼5.7 wt.%). In addition EMPA and Raman microspectroscopy characterised the presence of amorphous As(V) Fe hydrates as well as rare arsenate minerals (e.g. scorodite). Raman microspectroscopy in particular identified a preponderance of goethite or hematite within the mineral framework of the tailings materials that is likely to sorb recalcitrant As species. The characterisation of the components of the tailings enable the identification of their evolution, shows the progressive decrease of their As-content and emphasises the consequences of the temporary trapping of As in the very acidic and oxidising conditions prevailing in such environments. Resinous amorphous material was identified as the richest in As with As ∼17.1 wt.%. This material evolved toward more crystallised phases (e.g. goethite, jarosite) which contained less As (3.2 wt.%

Short term copper toxicity on Microcystis aeruginosa and Chlorella vulgaris using flow cytometry

Copper sulphate is a common algaecide applied to ponds in order to keep phytoplanktonic blooms under control, especially those prone to cyanobacterial development. The copper toxicity was evaluated for two phytoplanktonic species with the same size and shape: one cyanobacteria Microcystis aeruginosa and one chlorophyceae Chlorella vulgaris. Under controlled conditions in order to keep free copper concentration constant, three bioassays were carried out: growth inhibition tests and two sets of flow cytometric analysis (autofluorescence and esterase activity inhibition). The results showed: (1) that regardless of the cell type, as copper concentrations increased, cell division rate and autofluorescence decreased, (2) a difference in sensitivity according to the length of exposure to copper: 24 or 48 h, and (3) that M. aeruginosa showed a greater sensitivity than C. vulgaris in the 24h copper exposure assay according to esterase activity data.

Influence of dams on sediment continuity: A study case of a natural metallic contamination.

Sediments play an important role on the quality of aquatic ecosystems, notably in the reservoir areas where they can either be a sink or a source of contaminants, depending on the management and hydrological conditions. The physicochemical properties of 25 surface sediments samples of a reservoir catchment (Vaussaire, Cantal, France) were studied. Results show a strong influence of dam presence, notably on the grain size and organic matter (OM) contents. The concentrations of trace metals and metalloids (As, Cd, Cr, Cu, Ni, Pb and Zn) were also measured and compared with worldwide reservoir concentrations and international sediment quality guideline levels in order to assess the intensity of the metallic contamination. Cr and Ni are the trace elements presenting the significantly highest values at the catchment scale. Enrichment Factors (EF), calculated using both local and national backgrounds, show that metals have mainly a natural origin, explaining especially the Cr and Ni values, linked with the composition of parental rocks. Unexpectedly, all the observed metal concentrations are lower in the reservoir than upstream and downstream, which might be related to the high fresh OM inputs in the reservoir, diluting the global metallic contamination. Multivariate statistical analyses, carried out in order to identify the relationship between the studied metals and sediment characteristics, tend to support this hypothesis, confirming the unusually low influence of such poorly-degraded OM on trace element accumulation in the reservoir.

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

Four 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.

Key parameters influencing metallic element mobility associated with sediments in a daily-managed reservoir

In a hydroelectric reservoir, sediments are subject to remobilization events, water-level fluctuations and physicochemical changes. Depending on their associated metallic content, surficial oxic to suboxic sediments could constitute a major source of metals. To identify the key parameters that control metallic elements in terms of their mobility and sensitivity to reservoir management, sediments were subject to resuspension and drying/wetting cycle experiments over a wide range of pH values, solid/liquid ratios (S/L) and redox (Eh) conditions. During these tests, special attention was also paid to the influence of pretreatments on samples, i.e., drying, aeration and the leachate composition (ultrapure water vs. natural water); on the preservation of the sediment characteristics; and especially on metallic element release. The results of this study show that the pH, S/L ratio and Eh parameters are key variables in metal solubilization; the pH influences metal mobility primarily through sorption-desorption phenomena as well as the dissolution of metallic-bearing phases, the S/L ratio modifies the sorption-desorption equilibria, and the Eh primarily affects the reducible sensitive phases and associated metallic elements through dissolution-precipitation processes. Under environmental conditions, evolution of these parameters can lead to a >20% solubilization of the most mobile elements, i.e., As and Cd. These results are influenced by the sample pretreatment and experimental conditions. In fact, even if the solubilization patterns show no significant differences between dry and wet sediment depending on the physicochemical conditions, the magnitude of their release is significantly affected. Drying pretreatment induces changes in metal speciation, notably altering the distribution of the most weakly bound elements; there is almost half the amount of metallic elements associated with the exchangeable fraction in dry compared to wet sediments. The solubilization percentages were higher in the ultrapure phase than in reservoir water primarily due to the low pH, which influenced the sorption equilibria.