Origin, fate and ecotoxicity of manganese from legacy metallurgical wastes

Over the course of history, mining and metallurgical activities have influenced the socioeconomic development of human populations. However, these past and current activities can also lead to substantial environmental contamination by various metals. Here, we used an interdisciplinary approach (incorporating archaeology, mineralogy, environmental chemistry and ecotoxicology) to investigate the origin, fate and potential ecotoxicity of anomalous manganese (Mn) concentrations detected in the ancient mining district of Berthelange (medieval period, eastern France). Mineralogical investigations of slag samples showed that smelting temperature conditions in medieval bloomeries led to the production of slags mainly composed of Fe- and Mn-rich olivine, i.e., fayalites. Further mineralogical analyses of bulk soil and clay fractions allowed us to identify the presence of serpentine. This evidence of olivine weathering can account for the release of Mn from slags into the soil. In addition, chemical analyses of total and available (exchangeable and reducible) Mn concentrations in soil samples clearly showed the contribution of slags deposited 1000 years ago to soil contamination. A complementary ecotoxicity bioassay performed on soils from a slag heap using the land snail Cantareus aspersus confirmed that a significant fraction of the Mn detected in soils remains available for partitioning with the soil solution and transfer to soil organisms. Although no growth inhibition of snails was observed after 28 days of exposure, the animals accumulated quite elevated Mn concentrations in their tissues. Our study emphasizes the environmental availability and bioavailability of Mn from ancient metallurgical wastes to soil-dwelling invertebrates, i.e., snails, even one millennium after their deposition. Hence, as for more recent industrial sites, past mining ecosystems must be a cause of concern for the scientific community and public authorities.

Impact of ancient iron smelting wastes on current soils: Legacy contamination, environmental availability and fractionation of metals

Past and present metallurgical activity is the origin of the metallic contamination of some current soils. The purpose of this research is to assess the environmental risk of ancient Fe smelting wastes to the terrestrial compartment. For this purpose, two study sites were investigated in Bourgogne-Franche Comté (France). For each site, the soil contamination (Co, Cu, Fe, Mn, Ni and Zn) and the mobility of each metal from the slag to the topsoils were assessed. The principal results show that the topsoils are particularly enriched in Fe and Mn compared to the reference soils. The bulk chemistry of the slag showed high Fe and Mn content related to the mineralogy of slags, in which the minerals include fayalite, spinel, wustite and glass. In the topsoils, we also observed newly formed minerals (clay minerals, goethite and hematite), which were absent in the reference soils. The presence of slag microfragments in soils and the partial weathering of slags, which contributed to the release of metals in the soils, can explain the contribution of slags to the current contamination of soils. The extensive study of a depth profile from Puisaye showed a low vertical diffusion of the released metal in the heap substratum. We also investigated the fractionation of metals in soils and their environmental availability. The results showed that Mn is generally present in reducible forms or associated with the residual fraction but is less adsorbed to the organic matter (OM) or present in easily exchangeable forms. In contrast, the low extractability of Fe indicates that it is mostly bound to the residual (i.e., mineral) fraction. Based on the easily exchangeable metal concentrations measured in soils, low to medium ecological risks were identified at the sites investigated.

Metal losses in pyrometallurgical operations - A review

Nowadays, a higher demand on a lot of metals exists, but the quantity and purity of the ores decreases. The amount of scrap, on the other hand, increases and thus, recycling becomes more important. Besides recycling, it is also necessary to improve and optimize existing processes in extractive and recycling metallurgy. One of the main difficulties of the overall-plant recovery are metal losses in slags, in both primary and secondary metal production. In general, an increased understanding of the fundamental mechanisms governing these losses could help further improve production efficiencies. This review aims to summarize and evaluate the current scientific knowledge concerning metal losses and pinpoints the knowledge gaps. First, the industrial importance and impact of metal losses in slags will be illustrated by several examples from both ferrous and non-ferrous industries. Throughout the remainder of this review, the main focus will be put on the particular issues in copper industry. In a second section, the different types of metal losses in slags will be discussed. Generally, metal losses in slags can be subdivided into two types: chemical losses and physical losses. The fundamental insights concerning the responsible mechanisms will be discussed for each type. Subsequently, an overview of the most frequently used techniques for research investigations of the losses will be given. In a fourth section, a more detailed overview will be given on the post-processing treatment of metal-containing slags, i.e. performing slag cleaning operations. The most frequently applied methods will be discussed.

Zn isotopes fractionation during slags' weathering: One source of contamination, multiple isotopic signatures

During the chemical weathering of lead blast furnace (LBF) and imperial smelting furnace (ISF) slags, possible Zn isotopes fractionation was studied as a function of pH, atmosphere (open air vs nitrogen), and time. Bulk LBF and ISF displayed heavier signatures compared to Johnson Matthey Company (JMC) Zn standard solution (i.e. 0.13 ± 0.06‰ and 0.78 ± 0.13‰ for LBF and ISF, resp). The Zn signatures vary greatly by changes in solution pH; heavier signatures at low pH and lighter signature at high pH. Smithsonite (ZnCO₃) formation could induce a big delta Δ⁶⁶Zn_{Nitro-Open.atm} of 1.13‰ at pH 10 and rapid zinc hydroxide precipitation could induce Δ⁶⁶Zn_{Nitro-Open.atm} of 0.13-0.2‰ at pH 8.5. In addition, slags contain many mineral phases: ∼80-84% of amorphous glassy phase (in v/v) and ∼16-20% of many other crystalline phases. Zn isotope signatures in primary mineral phases can be extrapolated where the signature of the amorphous glassy phase lies between -0.35‰ and -0.42‰, and that of the overall crystalline phases was estimated to be 2.12‰ for LBF and 5.74‰ for ISF. Therefore, un-weathered slags with many mineral phases can host distinct Zn isotope signatures, which further evolve significantly during chemical weathering. One should thus carefully consider the heterogeneity of slags and the low-temperature chemical processes which lead to diverse Zn isotopic signature in the end, when using Zn isotopes as tracer of smelter's contamination.

Lead isotopes and heavy minerals analyzed as tools to understand the distribution of lead and other potentially toxic elements in soils contaminated by Cu smelting (Legnica, Poland)

Surroundings of the Legnica Cu smelter (Poland) offer insight into the behavior of Pb and other metal(oid)s in heavily contaminated soils in a relatively simple site, where lithogenic and anthropogenic Pb contributions have uniform Pb isotope composition over the time of smelter activity. Distribution of metal(oid)s decreases asymptotically with depth and below 30 cm reaches concentrations typical or lower than those of upper continental crust. Usually, such distribution is interpreted as the decrease in anthropogenic Pb contribution with depth. However, calculations based on Pb isotopes indicate that anthropogenic Pb is probably distributed both as Pb-rich particles of slags and fly ashes and Pb-poor soil solutions. Generally, anthropogenic Pb constitutes up to 100 % of Pb in the uppermost 10 cm of the soils and comes often from mechanical mixing with slag and fly ash particles as well as their weathering products. On the other hand, lower soil horizon contains anthropogenic Pb in various forms, and at depths below 30 cm, most of anthropogenic Pb comes from soil solutions and can constitute from 1 to 65 % of the Pb budget. This is consistent with secondary electron microscope (SEM) analyses of heavy mineral particles showing that, in upper horizons, Pb, Cu, and Zn are contained in various particles emitted from the smelter, which show different stages of weathering. Currently, large portion of these metals may reside in the secondary Fe-hydro-oxides. On the other hand, in deeper soil horizons, anthropogenic Pb is probably dominated by Pb coming from leaching of slag or fly ash particles. Overall, metal(oid) mobility is a dynamic process and is controlled by the soil type (cultivated versus forest) and the composition and the structure of the metal-rich particles emitted from the smelter. High proportions of anthropogenic Pb at depths below 30 cm in some soil profiles indicate that Pb (and probably other metal(oid)s) can be transported down the soil profile and the present concentration of anthropogenic Pb depends on the availability of binding sites.

Bio-alteration of metallurgical wastes by Pseudomonas aeruginosa in a semi flow-through reactor

Metallurgical activities can generate a huge amount of partially vitrified waste products which are either landfilled or recycled. Lead Blast Furnace (LBF) slags are often disposed of in the vicinity of metallurgical plants, and are prone to weathering, releasing potentially toxic chemical components into the local environment. To simulate natural weathering in a slag heap, bioweathering of these LBF slags was studied in the presence of a pure heterotrophic bacterial strain (Pseudomonas aeruginosa) and in a semi-flow through reactor with intermittent leachate renewal. The evolution of water chemistry, slag composition and texture were monitored during the experiments. The cumulative bulk release of dissolved Fe, Si, Ca and Mg doubled in the presence of bacteria, probably due to the release of soluble complexing organic molecules (e.g. siderophores). In addition, bacterial biomass served as the bioadsorbent for Pb, Fe and Zn as 70-80% of Pb and Fe, 40-60% of Zn released are attached to and immobilized by the bacterial biomass.