Hematite (α-Fe2O3) - A potential Ce4+ carrier in red mud

Geochemical Occurrence of Rare Earth Elements in Mining Waste and Mine Water: A Review

Μining waste, processing by-products and mine water discharges pose a serious threat to the environment as in many cases they contain high concentrations of toxic substances. However, they may also be valuable resources. The main target of the current review is the comparative study of the occurrence of rare earth elements (REE) in mining waste and mine water discharges produced from the exploitation of coal, bauxite, phosphate rock and other ore deposits. Coal combustion ashes, bauxite residue and phosphogypsum present high percentages of critical REEs (up to 41% of the total REE content) with ΣREY content ranging from 77 to 1957.7 ppm. The total REE concentrations in mine discharges from different coal and ore mining areas around the globe are also characterised by a high range of concentrations from 0.25 to 9.8 ppm and from 1.6 to 24.8 ppm, respectively. Acid mine discharges and their associated natural and treatment precipitates seem to be also promising sources of REE if their extraction is coupled with the simultaneous removal of toxic pollutants.

Cerium anomalies in riverbanks: Highlight into the role of ferric deposits

In wetlands, stream riverbanks represent a large redox reactive front. At their surface, ferric deposits promote their capacity to trap nutrients and metals. Given that rare earth elements (REE) are now considered as emerging pollutants, it seems that the riverbank interface is a strategic area between wetlands and streams in terms of controlling the environmental dissemination of REE. Therefore, the evolutions of the REE distribution and cerium (Ce) anomaly (Ce/Ce*, i.e. depleted or enriched Ce concentration compared to the other REE) were studied at various locations on a riverbank. The positive Ce anomaly is related to a high Fe content, a low organic carbon/iron ratio ((OC)/Fe) and newly formed Fe oxyhydroxides independently of their interactions with organic matter. Micro-X ray fluorescence (μ-XRF) mapping confirms Ce accumulation with ferric deposits. The Ce speciation exhibits a mix of Ce(III) and Ce(IV) in the ferric deposits, almost 20% of Ce occurred as Ce(IV) due to oxidation by newly formed Fe oxyhydroxides, while the subsurface horizons only display Ce(III). These results provide evidence that the Ce anomaly variation observed in stream water between low and high flow periods is partly due to the erosion of ferric deposits exhibiting a positive Ce anomaly. Therefore, the Ce anomaly can be considered as a fingerprint of the release of Fe colloids in the rivers and streams connected to the wetland.

Recycling of Cerium and Lanthanum from Glass Polishing Sludge

To examine the efficiency of La and Ce recycling processes from the sludge, two major methods were used, namely leaching and precipitation. The findings suggest that 12% of La and 24.2% of Ce were contained in the sludge. The sludge was leached in an optimum condition of 6N HCl at a temperature of 70°C with a 3g/50 mL solid/liquid ratio for 3 h to obtain a 100% leaching recovery of La and Ce. After pH adjustment of the obtained La and Ce optimum leaching solution to 6 with NH4OH and a simultaneous addition of H2O2 in a ratio of 1:1, Ce precipitated out with 65.9% recovery. On the other hand, La was not precipitated. The results obtained in this study reveal that leaching and pH adjustment method could be used to recover the valuable REE of La and Ce from glass polishing sludge in order to reach the goals of resource recycling.

Rare Earth Elements (REE) in Al- and Fe-(Oxy)-Hydroxides in Bauxites of Provence and Languedoc (Southern France): Implications for the Potential Recovery of REEs as By-Products of Bauxite Mining

Bauxites in southern France (Provence and Languedoc) have been exploited since the beginning of the last century. Though most of the deposits are now subeconomic or mined-out, these bauxites represent model analogs for other economic bauxites of the world. These Cretaceous karst-type deposits lie directly on Jurassic carbonates, and have been formed through a combination of different processes: in-situ alteration of siliciclastic sediments deposited on carbonate platforms, and reworking of early bauxites in the karst network. In this study, we present preliminary bulk rock geochemical and in-situ laser ablation (LA) -ICP-MS analyses on Al- and Fe-oxy-hydroxides of Provence (Les Baux-de-Provence) and Languedoc (Villeveyrac, Loupian) bauxites, with the aim of evaluating the concentrations of rare earth elements (REEs) and their deportment in these minerals. REEs have total average concentrations of 700 mg/kg in the analyzed samples, which are mostly composed of boehmite, γ-AlO(OH), and Fe-oxy-hydroxides (hematite and goethite). Maximum REEs concentrations are commonly associated with positive Ce anomalies in chondrite-normalized patterns. In contrast with other examples from the literature, it has been observed that high REE concentrations also occur in samples apparently devoid or poor of REE-minerals. In these samples, the total amount of REEs is positively correlated with that of Ga (commonly contained in boehmite). LA-ICP-MS trace element analyses on boehmite and Fe-oxy-hydroxides have shown that while the Al-hydroxide contains the suite of REEs, goethite and hematite are preferentially enriched only in Ce. Considering that Al-hydroxides are digested during the Bayer process, an interesting issue to develop in the future is whether (and how) REEs released during Al-hydroxide digestion could be recovered together with Al from the pregnant leach liquor, as routinely done for Ga.

Aqueous Fe(II)-Induced Phase Transformation of Ferrihydrite Coupled Adsorption/Immobilization of Rare Earth Elements

The phase transformation of iron minerals induced by aqueous Fe(II) (Fe(II)aq) is a critical geochemical reaction which greatly affects the geochemical behavior of soil elements. How the geochemical behavior of rare earth elements (REEs) is affected by the Fe(II)aq-induced phase transformation of iron minerals, however, is still unknown. The present study investigated the adsorption and immobilization of REEs during the Fe(II)aq-induced phase transformation of ferrihydrite. The results show that the heavy REEs of Ho(III) were more efficiently adsorbed and stabilized compared with the light REEs of La(III) by ferrihydrite and its transformation products, which was due to the higher adsorptive affinity and smaller atomic radius of Ho(III). Both La(III) and Ho(III) inhibited the Fe atom exchange between Fe(II)aq and ferrihydrite, and sequentially, the Fe(II)aq-induced phase transformation rates of ferrihydrite, because of the competitive adsorption with Fe(II)aq on the surface of iron (hydr)oxides. Owing to the larger amounts of adsorbed and stabilized Ho(III), the inhibition of the Fe(II)aq-induced phase transformation of ferrihydrite affected by Ho(III) was higher than that by La(III). Our findings suggest an important role for the Fe(II)aq-induced phase transformation of iron (hydr)oxides in assessing the mobility and transfer behavior of REEs, as well as for their occurrence in earth surface environments.