Arsenic(III) Speciation in Sulfidic- and Carbonate-Containing Waters; Thioarsenites as Ligands for Ag(I), Pb(II) and Hg(II)

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This dissertation has two objectives; the first is to investigate speciation of three d10 metals (Ag(I), Hg(II) and Pb(II)) in sulfidic solutions containing As(III) to determine if thioarsenite could act as a ligand for the metals. Second, a hypothesis that As(III) forms strong complexes with bicarbonate is investigated.

The solubility of As in sulfidic solutions (10-4-10-3 M) equilibrated with As2S3+S at near-neutral pH (6.95-7.95) was measured and compared to the solubility of As in binary and ternary assemblages. Three species, AsS(HS)(OH)-, As(OH)3 and H2As3S6-, explain As speciation: 0.5As2S3 + 3H2O  As(OH)3 + 1.5H2S, pK=12.58, 1.5As2S3 + 1.5H2S  H2As3S6- + H+, pK=6.200.77 and 0.5As2S3 + H2O + 0.5H2S AsS(HS)(OH)- + H+ pK=8.740.09.

The solubility of As and Ag in sulfidic solutions (10-4-10-3 M) equilibrated with two Ag-As-S assemblages at near-neutral pH (6.89-8.37) was measured. The As species that explained As2S3+S solubility also explained solubilities in the Ag-As-S system.

The silver solubility was explained by six species: 0.5Ag2S(s) + 1.5 HS- + 0.5 H+  Ag(HS)2-, pK=0.4060.41; Ag2S (s) + 2 HS-  Ag2S(HS)22-, pK=4.78; 0.5Ag2S (s) + 0.5 HS- + 0.5 H+  Ag(HS) pK=2.110.21; 0.5Ag2S (s) + Cl- + 0.5 HS- + 0.5 H+  Ag(Cl)(HS)-, pK=-1.090.20; 0.5Ag2S (s) + (x-1)S + 0.5 HS-  AgSx- + 0.5 H+, pK=8.510.19 and Ag+ + AsS(HS)(OH)-  AgAsS(HS)(OH)o, pK=-17.17 0.20.

As shown by the last equilibrium expressions, dithioarsenite (AsS(HS)(OH)-) is a strong ligand for Ag(I). Two other ternary systems, HgS+As2S3+S and PbS+As2S3+S, were investigated but the solubility of Hg2+ and Pb2+ were not significantly enhanced when arsenic was present. The order of stability of the metal-thioarsenite complexes agrees with predictions by Tossell (2000). The GFo of natural orpiment was calculated to be -80.81.6.

The solubility of As2O3 in concentrated bicarbonate solutions at near-neutral pH is enhanced to a small, but statistically significant degree compared to the solubility in chloride solutions of the same ionic strength. This effect is attributed to one complex: As(OH)3 + HCO3-  As(OH)2CO3-, pK=0.570.15. The small constant suggests that As(III)-carbonate complexes will be negligible at carbonate concentrations found in nature.