Mobility of antimony in contrasting surface environments of a mine site: influence of redox conditions and microbial communities.

Microbial processes can influence the complex geochemical behaviour of the toxic metalloid antimony (Sb) in mining environments. The present study is aimed to evaluate the influence of microbial communities on the mobility of Sb from solid phases to water in different compartments and redox conditions of a mining site in southwest (SW) Spain. Samples of surface materials presenting high Sb concentrations, from two weathered mining waste dumps, and an aquatic sediment were incubated in slurries comparing oxic and anoxic conditions. The initial microbial communities of the three materials strongly differed. Incubations induced an increase of microbial biomass and an evolution of the microbial communities' structures and compositions, which diverged in different redox conditions. The presence of active bacteria always influenced the mobility of Sb, except in the neutral pH waste incubated in oxic conditions. The effect of active microbial activities in oxic conditions was dependent on the material: Sb oxic release was biologically amplified with the acidic waste, but attenuated with the sediment. Different bacterial genera involved in Sb, Fe and S oxidation or reduction were present and/or grew during incubation of each material. The results highlighted the wide diversity of microbial communities and metabolisms at the small geographic scale of a mining site and their strong implication in Sb mobility.

Evaluation of antimony availability in a mining context: Impact for the environment, and for mineral exploration and exploitation.

This work aims to establish Sb mobility, its transfer to biota and its effect on soil health in a semi-arid climate. The results show the presence of stibnite (Sb2S3) as the main primary Sb compound, bindhemite (Pb2Sb2O6(O,OH)), and minor proportions of stibiconite (Sb3+(Sb5+)2O6(OH)) as oxidised Sb species. This research also observes very high total Sb contents in mining materials (max: 20,000 mg kg-1) and soils (400-3000 mg kg-1), with physical dispersion around mining materials restricted to 450 m. The soil-to-plant transfer is very low, (bioaccumulation factor: 0.0002-0.1520). Most Sb remains in a residual fraction (99.9%), a very low fraction is bound to Fe and Mn oxy-hydroxides or organic matter, and a negligible proportion of Sb is leachable. The higher Sb mobility rates has been found under oxidising conditions with a long contact time between solids and water. The main factors that explain the poor Sb mobility and dispersion in the mining area are the low annual rainfall rates that slow down the Sb mobilisation process and the scarce formation of oxidised Sb compounds. All these data suggest poor Sb (III) formation and a low toxicological risk in the area associated with past mining activities. The low mobility of Sb suggests advantages for future sustainable mining of such ore deposits in a semi-arid climate and is also indicative of the limitations of geochemical exploration in the search for new Sb deposits.

Feasibility study of fluorescent lamp waste recycling by thermal desorption.

The proposed Minamata Convention ban on the use of fluorescent lamps at the end of 2020, with a consequent reduction in mercury (Hg) light products, is expected to produce large amounts of discarded fluorescent bulbs. In this context, the most effective recycling options are a thermal mercury recovery system and/or aqueous solution leaching (lixiviation) to recover rare earth elements (REEs). Due to the heterogeneous nature of these wastes, a complete characterization of Hg compounds in addition to a determination of their desorption temperatures is required for their recycling. The objective of this study is to assess the feasibility of a fast cost-effective thermal characterization to ameliorate recycling treatments. A pyrolysis heating system with a heat ramping capability combined with atomic absorption spectrometry makes it possible to obtain residue data with regard to the temperature ranges needed to achieve total Hg desorption. The major drawback of these heat treatments has been the amount of Hg absorbed from the residue by the glass matrices, ranging from 23.4 to 39.1% in the samples studied. Meanwhile, it has been estimated that 70% of Hg is recovered at a temperature of 437 °C.

Occurrence and environmental constraints of gray monazite in red soils from the Campo de Montiel area (SW Ciudad Real province, south central Spain).

Monazite ((Ce, La, Nd, Th) PO4) is a rare and strategic mineral that occurs naturally as an accessory and minor mineral in diverse igneous and metamorphic rocks. This mineral does not frequently form mineable ore deposits and it has different typologies, including those formed by endogenous processes (generally "yellow monazite" mineralizations) and those formed by exogenous processes ("gray monazite" mineralizations). The mineral is an important ore of Rare Earth Elements (REEs), which have been identified by the European Union as critical raw materials. Monazite can be considered a weathering-resistant mineral, and the mobility of the REE and associated elements is low. The study reported here concerns a mineralogical and geochemical assessment of the occurrence and risks associated with the presence of concentrations of monazite in a typical, well-developed, and representative red Mediterranean soil, in order to establish the associated risk with their future mining. The results confirmed that monazite ore is particularly poor in radioactive elements, and it is concentrated in the most surficial soil horizons. The chemical mobility of REEs present in the soil, as assessed by selective extraction with ammonium acetate in acidic media, follows the order Y > Dy > U > Tb > Gd > Eu > Sm > La > Th > Ce. The mobility of REEs contained in monazite proved to be higher than that of the REE compounds in the upper horizons of the soil profile suggesting the immobilization in other REE-containing minerals, while light REEs show lower mobility rates than heavy REEs, due to an immobilization of LREE by sorption with iron oxy-hydroxides. Further studies are required in order to obtain better speciation data for REEs in soils aimed to identify soluble and insoluble compounds.