Antimony transport in hydrothermal solutions: an EXAFS study of antimony(V) complexation in alkaline sulfide and sulfide-chloride brines at temperatures from 25 degrees C to 300 degrees C at P-sat

Existing electrochemical data suggest that under alkaline conditions, Sb(V) sulfide complexes may be stable under conditions as reducing as those foun d in hydrothermal ore solutions. To assess the nature of Sb(V) complexes in such solutions, X-ray absorption fine structure spectra (EXAFS) of antimon y(V) solutions were obtained at temperatures from 25 degrees C to 300 degre es C at pressures corresponding to the liquid-vapour equilibrium curve for H2O. Three solution compositions were considered: Solution A consisted of 0 .1 m Sb + 1.15 m Na2S; 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 degrees C, the inner coordinati on shell of aqueous Sb(V) contains four sulfur atoms corresponding to the S b(HS): complex. Above 150 degrees C, SH- ligands are replaced by OH- to for m mixed-ligand Sb complexes. Antimony(V) atoms are found in a second coordi nation shell at temperatures greater than 250 degrees C, indicating the exi stence of aqueous polyatomic clusters. No antimony(V) chloride complexation was found in the presence of HS-. (C) 2000 Elsevier Science B.V. All right s reserved.