THE SOLUBILITY OF AMORPHOUS AS2S3 FROM 25 TO 90-DEGREES-C

The solubility of amorphous arsenic sulfide [As2S3(am)] has been experimentally determined from 25 to 90-degrees-C. In acidic, sulfide-deficient solutions, H3AsO30 is the dominant product of As2S3(am) dissolution: 1/2 As2S3(am) + 3 H2O = H3AsO30 + 3/2 H2S0. The respective logarithms of equilibrium constants for this reaction are -11.9 +/- 0.3 at 25-degrees-C, -11.2 +/- 0.2 at 40-degrees-C, -10.2 +/- 0.1 at 60-degrees-C, and -9.0 +/- 0.2 at 90-degrees-C, all at 0.1 m ionic strength. In acidic solutions with excess sulfide, experimental data are consistent with H2As3S6- being the dominant dissolved As species: 3/2 As2S3(am) + 3/2 H2S0 = H2As3S6- + H+. The respective logarithms of equilibrium constants for this reaction are -5.0 +/- 0.3 at 25-degrees-C, -5.1 +/- 0.3 at 40-degrees-C, -4.9 +/- 0.2 at 60-degrees-C, and -4.1 +/- 0.1 at 90-degrees-C, all at 0.1 m ionic strength. Speciation calculations for a number of acidic hot springs indicate that many have As and H2S0 concentrations that are bracketed by the solubilities of As2S3(am) and orpiment and that H3AsO30 is generally the dominant dissolved As species. The solubility data indicate that lowering of temperature and pH are the most likely mechanisms of As2S3(am) precipitation in hot spring systems.