Antimony transport in hydrothermal solutions: an EXAFS study of antimony(V) complexation in alkaline sulfide and sulfide–chloride brines at temperatures from 25°C to 300°C at Psat

Abstract

Existing electrochemical data suggest that under alkaline conditions, Sb(V) sulfide complexes may be stable under conditions as reducing as those found in hydrothermal ore solutions. To assess the nature of Sb(V) complexes in such solutions, X-ray absorption fine structure spectra (EXAFS) of antimony(V) solutions were obtained at temperatures from 25°C to 300°C at pressures corresponding to the liquid–vapour equilibrium curve for H 2O. Three solution compositions were considered: Solution A consisted of 0.1 m Sb+1.15 m Na 2S; solution B had the composition of 0.05 m Sb+0.2 m NaHS+0.06 m NaOH; and solution C consisted of 0.05 m Sb+0.2 m NaHS+0.06 m NaOH+1 m NaCl. At temperatures <150°C, the inner coordination shell of aqueous Sb(V) contains four sulfur atoms corresponding to the Sb(HS) 4 + complex. Above 150°C, SH − ligands are replaced by OH − to form mixed-ligand Sb complexes. Antimony(V) atoms are found in a second coordination shell at temperatures greater than 250°C, indicating the existence of aqueous polyatomic clusters. No antimony(V) chloride complexation was found in the presence of HS −.