Abstract:
The solubility of Au2S at 25°C in aqueous H2S-HS-solutions was measured from pH 2 to 12 and over a range of reduced sulphur concentrations from 0.005 to 0.4 molal. The thio gold(I) species AuHS°, Au(HS)2- and Au2S2 2- were identified and their equilibrium formation constants determined using a nonlinear least squares procedure. For the reaction
Au2S + (2(n+z)/ x -1) HS- = 2/x Au x(HS) n S z(x-n-2z)+ (2z/x -1)H +
the general equilibrium constants are given logKAu2s,110=-6.68 (0.03) for AuHS°, logKAu2S,120=+4.52 (0 01) for Au (HS) 2- and logKAu2s.202=-14.63 (0.03) for Au2s2 2-. The quoted uncertainty corresponds to one standard deviation. The first ionization constant for H2S was determined from the Au2S solubility measurements as logKa1=-6.98 (0.01).
Published data for the solubility of Ag2S in solutions having an ionic strength of 1.0 at 20°C have been reinterpreted to give equilibrium formation constants at infinite dilution for AgHS° Ag(HS)2- and A92(HS)2S2-. LogKAg2S,110=-8.94 (0.01) for AgHS°, 1ogKAg2S,120=-1.02 (0.01) for Ag (HS) 2- and 1ogKAg2s,221.=-4.44 (0.01) for Ag2 (HS) 2S2-.
Adsorption of gold by As2S3 and Sb2S3 sols (125 mg kg-1) from solutions containing 0.51*10-06m gold as hydrosulphidogold(I)-complexes has been measured in the pH range 2 to 6 at 25°C and 2 to 9 at 90°C. Adsorption varied from l00 percent in acid solutions having pH<4.5 to ≤25 percent in neutral to alkaline solutions at 25 and 90°C. Adsorption by Sb2S3 sols in the pH range 2 to 6 at 25°C and at 0.00, 0.10, 0.25, 0.50, and 1.0m NaCl demonstrated no significant effect as a result. of increasing ionic strength.
The adsorption density (mol kg-1 of gold on the dry adsorbent) for As2S3 at pH 4.6 and Sb2S3 at pH 4.6 and 5.6 at 25°C was measured after equilibration with solutions containing 0.00 to 2.6*10-06m gold as hydrosulphido-complexes. The experimental data were interpreted using Langmuir and Freundlich adsorption eguations. The data indicated apparent surface saturation concentrations for gold, as defined by the Langmuir equation, of 0.096 mol kg-1 for As2S3 at pH 4.6 and 0.024 and 0.032 mol kg-1 for Sb2S3 at pH 4.6 and 5.6.
The surface charge on colloidal As2S3 and Sb2S3 particles at 25, 45, 70, and 90°C, based on the adsorption of H+ and OH-, was determined by potentiometric titration. Both sulphides were found to have a neutral surface, pHzpc’ between pH 2.5 and 3.5 from 25 to 90°C.
