The solubility of amorphous As sub 2 S sub 3 from 25 to 90C

The solubility of amorphous arsenic sulfide (As{sub 2}S{sub 3}(am)) has been experimentally determined from 25 to 90C. In acidic, sulfide-deficient solutions, H{sub 3}AsO{sub 3}{sup 0} is the dominant product of As{sub 2}S{sub 3}(am) dissolution: 1/2 As{sub 2}S{sub 3}(am) + 3 H{sub 2}O = H{sub 3}AsO{sub 3}{sup 0} + 3/2 H{sub 2}S{sup 0}. The respective logarithms of equilibrium constants for this reaction are {minus}11.9 {plus minus} 0.3 at 25C, {minus}11.2 {plus minus} 0.2 at 40C, {minus}10.2 {plus minus} 0.1 at 60C, and {minus}9.0 {plus minus} 0.2 at 90C, all at 0.1 m ionic strength. In acidic solutions with excess sulfide, experimental data are consistent with H{sub 2}As{sub 3}S{sub 6}{sup {minus}} being the dominant dissolved As species: 3/2 As{sub 2}S{sub 3}(am) + 3/2 H{sub 2}S{sup 0} = H{sub 2}As{sub 3}S{sub 6}{sup {minus}} + H{sup +}. The respective logarithms of equilibrium constants for this reaction are {minus}5.0 {plus minus} 0.3 at 25C, {minus}5.1 {plus minus} 0.3 at 40C, {minus}4.9 {plus minus} 0.2 at 60C, and {minus}4.1 {plus minus} 0.1 at 90C, all at 0.1 m ionic strength. Speciation calculations for a number of acidic hot springs indicate that many have As and H{sub 2}S{sup 0} concentrations that are bracketed by the solubilities ofmore » As{sub 2}S{sub 3}(am) and orpiment and that H{sub 3}AsO{sup 0}{sub 3} is generally the dominant dissolved As species. The solubility data indicate that lowering of temperature and pH are the most likely mechanisms of As{sub 2}S{sub 3}(am) precipitation in hot spring systems.« less