Geochemical studies on antimony: antimony dispersion in water draining Su Suergiu mine area; removal of Sb(V) from aqueous solution by layered double hydroxides (LDH)

Antimony (Sb) is considered a pollutant of priority interest, it is largely used in several industrial sectors (> 100,000 tons year worldwide) and is heavily mined worldwide (Leuz et al., 2006). In this work the hydrogeochemical behavior of Sb has been studied in water draining the abandoned antimony mine of Su Suergiu, SE Sardinia. Waters flowing at Su Suergiu show high Sb concentration and impact the main river of South Sardinia, the Flumendosa River that supplies water for agricultural and domestic uses. The main source of contamination at Su Suergiu is represented by the foundry slag heaps, in fact the slag drainages contain up to 30,000 μg L-1 Sb(tot) (median value 13,000 μg L-1 Sb(tot)). The determination of Sb specie in solution has been carried out through the analyses of both Sb(tot) (by ICP-MS and/or ICP-OES) and Sb(III) (by ASV). The Sb(III) concentration was determined on filtered (pore-size 0.45 μm) water samples stabilized with L(+) tartaric acid plus nitric acid, that, among several stabilizations of Sb(III) tested, has been evaluated as the most effective. Results obtained, showed that Sb(V) prevails in water sampled at Su Suergiu and surrounding area as Sb(OH)6̄ species, in agreement with the circumneutral-slightly alkaline pH values and oxidizing condition (Sb(III) ≤6% of Sb(tot)). The Sb(V) is less toxic but more mobile than Sb(III); the Sb behavior in water analyzed seems to be conservative, and the most important natural attenuation process of Sb contamination appears to be dilution. First in the water of Riu Ciurixeda (whose catchment collect all mine drainage), and after in the Flumendosa River, the Sb concentration decreases significantly, from 104 to 101 μg L-1, but in the Flumendosa River it still exceeded the limit recommended by both the World Health Organization (20 μg L-1) and the European Union (5 μg L-1), especially under extremely high flow conditions (Sb(tot) = 51 μg L-1; median relative to all flow conditions Sb(tot) = 22 μg L-1). In view of these results Sb(V) removal from solution was tested using a synthetic mineral belonging to the class of layered double hydroxides, that has the advantage, with respect to the other sorbents (metal oxy-hydroxides, organic polymers, etc...), of being able to remove contaminants from solutions at circumneutral pH values often found in the environment. The LDH are characterized by a layered structure composed by brucite-like sheets ([M2+1-xM3+x(OH)2]x+), stacked along the c axis and positively charged, due to the partial substitution of bivalent cations (M2+ = Mg2+, Zn2+, Ca2+) by trivalent cations (M3+ = Al3+, Fe3+). The positive charges are compensated by anions or anionic complexes (An- = Cl-, NO3-, CO32-) in the interlayer, where also structural water can occur. From an environmental point of view, LDH have anion exchange capacity, sorption capacity, high specific surface area and the “memory effect”, which is the capacity of the calcined phases which have undergone structural collapse from the loss of interlayer water and anions, to recover its structure when immerged in aqueous solution. Calcined and nitrate LDH were tested; the calcined resulted are the most effective. Among them the Mg(AlFe)-c oxides, derived from the calcination of hydrotalcite-like compounds {Mg6Al2(OH)16CO3∙4H2O}, removed Sb(OH)6̄ from solution through the rehydration and formation of a brandholzite-like compound {Mg[Sb(OH)6]2·6H2O}. The 2ZnAl-c oxides derived from the calcination of zaccagnaite-like compounds (Zn4Al2(OH)12CO3∙3H2O) kept Sb(OH)6̄ from solution by its intercalation in the interlayer during the reconstruction of zincalstibite-like LDH {Zn2Al(OH)6[Sb(OH)6]}. The Sb(OH)6̄ removal capacity of both sorbents is seriously affected by the presence of coexistent equal concentrations of As in solution, while the carbonate species and the SO42- result lower competition with respect to Sb(OH)6̄. Sorption tests with selected calcined LDH, Mg(AlFe)-c and 2ZnAl-c, were performed on water collected in the slag drainage of Su Suergiu, characterized by slightly alkaline pH and high concentration of SO42- (1006 mg L-1), HCO3̄ (485 mg L-1), As (3386 μg L-1) and Sb(tot) (9900 μg L-1). Results show substantial capacity of Sb removal from solution, and also of As. Due to the relatively low concentration of Sb in Su Suergiu water with respect to the synthetic solution used in the sorption test, the main removal process of Sb resulted for both sorbents used in the intercalation in the interlayer during the reconstruction of carbonate(hydroxyl) LDH structure, also As seem to be removed through the same mechanism. In several experiments the dissolution of sorbents was observed, therefore it is necessary consider the impact from the dissolution of the metals composing them. The solid/liquid ratio strongly influences the removal processes, therefore future studies should address this aspect to assess the potential use of LDH in Sb removal from solution at circumneutral pH usually found in the environment.