Pyrometallurgical slag, produced 100-150 years ago from lead-zinc ores in the smelting region of Příbram, Czech Republic, contains elevated amounts of Zn and Pb. Knowledge of the distribution of these elements in the main phases and an investigation of natural weathering features represent the first step in the environmental assessment related to dumping of metallurgical slag. Optical microscopy, scanning electron microscopy (SEM/EDS), X-ray diffraction (XRD) and electron-beam microanalysis (EMPA) were used to identify the silicate, oxide, sulfide and metallic phases. The study focused on silicates and oxides, and the major constituents of the slag proved to be clinopyroxene, melilite, olivine, spinel and glass. A substantial amount of zinc, initially dissolved in the silicate melt, is held by zinc-rich end-members of the spinel (gahnite, up to 19.9 wt% ZnO) and silicates (e.g., hardystonite, up to 10.5 wt% ZnO). Lead, in contrast, behaves as an "incompatible element" and is likely to be concentrated in the residual matrix glass. Two distinct glasses were identified: (i) transparent (surface) glass from the quenched borders of slag fragments, and (ii) opaque (matrix) glass, which forms a black matrix in the center of the fragments. These glasses contain important amounts of lead and zinc, up to 3.72 wt% PbO and 9.80 wt% ZnO, partly in the form of droplets (generally <1 m) composed of galena, metallic lead, ZnS and other sulfide and metallic phases and alloys. Two features attributed to natural weathering, found mainly on the surface of chilled-glass borders, were identified in the slag: (i) deposition of a Fe-and Pb-rich veneer, and (ii) selective leaching of superficial glass revealing, with respect to unaltered glass, the mobilization of Ca, Fe, Na, K, and Zn.

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International audienceDirect aqueous mineral carbonation using organic anions has been presented by many as a promising strategy for mineral carbonation, on the basis that additives such as oxalate increase the rate and extent of dissolution of magnesium silicates several folds. Through geochemical modelling and detailed solid characterisation, this paper discusses and extends our current understanding of this process. The role of disodium oxalate as a dissolution enhancing agent for olivine is thoroughly examined through experiments in which all phases are carefully analysed. We show that under 20 bar of CO2 pressure formation of strong oxalate-magnesium complexes in solution and precipitation of MgC2O4*2H2O (glushinskite) impede any chance of obtaining significant amounts of magnesium carbonate. Other promising ligands from a dissolution perspective, namely citrate and EDTA salts, are also investigated. Contrary to oxalate, these ligands do not form any solid by-products with magnesium, and yet they do not produce better carbonation results, thereby casting strong doubts on the possibility of developing a direct aqueous mineral carbonation process using organic salts. Geochemical modelling permits successful simulation of the dissolution kinetics of magnesium silicate using a shrinking particle model that accounts for the precipitation of glushinskite, amorphous silica and a magnesium phyllosilicate at advanced stages of the dissolution process

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High levels of arsenic in groundwater and drinking water are a major health problem. Although the processes controlling the release of As are still not well known, the reductive dissolution of Asrich Fe oxyhydroxides has so far been a favorite hypothesis. Decoupling between arsenic and iron redox transformations has been experimentally demonstrated, but not quantitatively interpreted. Here, we report on incubation batch experiments run with As(V) sorbed on, or co-precipitated with, 2-line ferrihydrite. The biotic and abiotic processes of As release were investigated by using wet chemistry, X-ray diffraction, X-ray absorption and genomic techniques. The incubation experiments were carried out with a phosphate-rich growth medium and a community of Fe(III)-reducing bacteria under strict anoxic conditions for two months. During the first month, the release of Fe(II) in the aqueous phase amounted to only 3% to 10% of the total initial solid Fe concentration, whilst the total aqueous As remained almost constant after an initial exchange with phosphate ions. During the second month, the aqueous Fe(II) concentration remained constant, or even decreased, whereas the total quantity of As released to the solution accounted for 14% to 45% of the total initial solid As concentration. At the end of the incubation, the aqueous-phase arsenic was present predominately as As(III) whilst X-ray absorption spectroscopy indicated that more than 70% of the solid-phase arsenic was present as As(V). X-ray diffraction revealed vivianite Fe(II) 3 (PO 4 ) 2 .8H 2 O in some of the experiments. A biogeochemical model was then developed to simulate these aqueous-and solid-phase results. The two main conclusions drawn from the model are that (1) As(V) is not reduced during the first incubation month with high Eh values, but rather re-adsorbed onto the ferrihydrite surface, and this state remains until arsenic reduction is energetically more favorable than iron reduction, and (2) the release of As during the second month is due to its reduction to the more weakly adsorbed As(III) which cannot compete against carbonate ions for sorption onto ferrihydrite. The model was also successfully applied to recent experimental results on the release of arsenic from Bengal delta sediments.