Alexandra Courtin-Nomade

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.%

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.

Sulfide oxidation observed using micro-Raman spectroscopy and micro-X-ray diffraction: The importance of water/rock ratios and pH conditions

Abstract Oxidative dissolution of arsenopyrite and pyrite from two former mining sites located in the French Massif Central has been studied to determine some of the critical parameters controlling the formation of secondary phases. Micro-Raman spectroscopy (μRS) and micro-scanning X-ray diffraction (μSXRD) were used for mineralogical identification and for mapping the distribution of these alteration products. The two mining sites are characterized by different pH conditions and sedimentary environments. Enguialès exhibits acidic conditions (pH ~ 3) and consists of coarse-grained tailings on a steep slope, which have been leached by meteoric waters and represent an oxidizing environment with a high solid/solution ratio. In contrast, the Cheni site shows near neutral conditions (pH ~ 6.3) and consists of muddy tailings deposited in a settling basin, which exemplify an oxidizing environment with a low solid/solution ratio. Amorphous or poorly crystalline iron arsenate (e.g., amorphous scorodite or parasymplesite-like phase) are among the first products precipitated from oxidized arsenopyrites. These iron arsenates are highly mobile and are frequently observed in association with non-arsenian pyrites, where in some instances they have matured into more crystalline forms. Arsenic may also be trapped by amorphous or poorly crystalline iron (oxy)hydroxides, as has been observed for goethite or jarosite in the latter stages of sulfide oxidation. This study also shows that rings of elemental sulfur are formed around altered sulfides, but only when conditions are near neutral.

Transformation of natural As-associated ferrihydrite downstream of a remediated mining site

Natural ferrihydrite (Fh), a poorly crystalline iron oxy-hydroxide with high concentrations of As, was investigated with respect to its crystallinity in a remediated mining environment. The Fh crystallinity increases from proto-Fh to a better crystallized 2-line Fh (called better 2-line Fh), while the associated As contents decrease from 7.8 to 1.9 wt %, respectively. This evolution of crystallinity correlated with decreasing As suggests that As is more likely coprecipitated in the Fh structure than adsorbed onto the surface. The evolution of Fh crystallinity is related to (i) aqueous transformations for samples continuously submerged and controlled by water composition (pH, ionic strength and redox potential) and (ii) dry thermal transformations for samples in a seasonally humid area with variations of temperature, humidity, agglomeration and compaction. The evolution of Fh crystallinity is more pronounced during dry thermal transformation than during aqueous transformations. Although Fh evolution is observed on the field, no stable form (goethite, hematite) is detected as usually noted during dry thermal transformation. This may be explained by incorporated cations such as Si and Al that can inhibit the transformation of Fh to a more stable form. To understand this transformation in a natural environment, the discussion in this study focuses on two main points: (i) the relationship between spatial mineralogical distribution and As content and (ii) the importance of Fh evolution as a function of transformation processes observed in the field.

Fate of arsenic-bearing phases during the suspended transport in a gold mining district (Isle river Basin, France).

Arsenic-rich (~140-1520 mg x kg(-1)) suspended particulate matter (SPM) was collected daily with an automatic sampler in the Upper Isle River (France) draining a former gold mining district in order to better understand the fate of arsenic during the suspended transport (particles smaller than 50 μm). Various techniques at a micrometric scale (EPMA, quantitative SEM-EDS with an automated particle counting including classification system and μXRD) were used to directly characterize As-bearing phases. The most frequent ones were aggregates of fine clay particles. Their mineralogy varied with particle sources involved. These aggregates were formed by chlorite-phlogopite-kaolinite assemblages during the high flow and chlorite-illite-montmorillonite during the low flow. Among all the observed As-carriers in SPM, these clay assemblages were the least As-rich (0.10 up to 1.58 wt.% As) and their median As concentrations suggested that they were less concentrated during the high flow than during the low flow. Iron oxyhydroxides were evidenced by μXRD in these clay aggregates, either as micro- to nano-sized particles and/or as coating. (Mn, Fe)oxyhydroxides were also present as discrete particles. Manganese oxides (0.14-1.26 wt.% As) transport significantly more arsenic during the low flow than during the high flow (0.16-0.79 wt.% As). The occurrence of Fe oxyhydroxide particles appeared more complex. During the low flow, observations on banks and in wetlands of freshly precipitated Fe hydroxides (ferrihydrite-type) presented the highest As concentrations (up to 6.5 wt.% As) but they were barely detected in SPM at a microscale. During the high flow, As-rich Fe-oxyhydroxides (0.10-2.80 wt.% As) were more frequent, reflecting mechanical erosion and transport when the surface water level increased. Arsenic transfers from SPM to corresponding aqueous fraction mostly depend on As-carrier stability. This study shows the temporal occurrence of each type of As-bearing phases in SPM, their As concentrations at a particle scale and abundance according to hydrological periods.

Dynamics of metallic contaminants at a basin scale--Spatial and temporal reconstruction from four sediment cores (Loire fluvial system, France).

From the 19th century, the Loire basin (France) presents potentially pollutant activities such as mining and heavy industries. This paper shows spatio-temporal distribution of trace elements in sediments at a basin-scale, based on a comparison of archived temporal signals recorded in four sedimentary cores. Anthropogenic sources contributing to sediment contamination are also characterized, using geochemical signatures recorded in river bank sediments of the most industrialized tributaries. This study highlights upstream-downstream differences concerning recorded contamination phases in terms of spatial influence and temporality of archiving processes. Such differences were related to (i) various spatial influences of contamination sources and (ii) polluted sediments dispersion controlled by transport capacity of metal-carrier phases and hydrosedimentary dynamics.

Long-term influence of aeration on arsenic trapping in a ZVI/sand bed reactor

Arsenic toxicity and occurrence in the environment necessitate the development of easy-to-handle and cheap water treatment processes. Zero Valent Iron (ZVI) supports meet these requirements. However many ZVI by-products might be generated in the unit reactor process, mainly influenced by physico-chemical conditions. This work deals with the influence of aeration onto arsenic trapping in ZVI/sand columns. The monitoring of a pilot unit that consisted of four reactors (two aerated and two non-aerated), at different durations (three days, three weeks and three months), enables the characterization of arsenic sorption capacity. In an aerated system, the highest arsenic removal levels were observed at the beginning of the experiment, and the performances greatly decreased with time. In the non-aerated systems, the highest arsenic trapping capacities were obtained (220 mgAs per gFe in the three months of running the system) but the arsenic removal percentage remained under 60%. Support analyses after dismantling were performed to characterize the arsenic sorption and the ZVI by-products generated. They confirmed the influence of aeration on the arsenic distribution along the reactor: a homogeneous repartition in non aerated conditions whereas it was highly heterogeneous under aerated conditions, with far higher concentrations at the inlet. This long-term study highlights that aeration modifies the oxidation of ZVI, resulting in by-products with high arsenic sorption capacities but for a limited duration whereas the absence of aeration limits the oxidation of ZVI into sorbant by-products but it significantly increases the duration of arsenic trapping and consequently the lifetime of the bed reactor.

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.

From the Characterization, Geochemical Behavior and Health Risk Assessments of Tunisian Phosphogypsum to a Potential Way of Inerting and Valorization

Phosphogypsum is the by-product of the acid process for producing phosphoric acid from a phosphate rock. In Tunisia, phosphate production generates very large volumes of the phosphogypsum near the city of Sfax, which is stored without a real management.