Metal-thiometalate transport of biologically active trace elements in sulfidic environments. 1. Experimental evidence for copper thioarsenite complexing
Mb. Clarke et Gr. Helz, Metal-thiometalate transport of biologically active trace elements in sulfidic environments. 1. Experimental evidence for copper thioarsenite complexing, ENV SCI TEC, 34(8), 2000, pp. 1477-1482
Citations number
31
Categorie Soggetti
Environment/Ecology,"Environmental Engineering & Energy
Pore waters in metal-contaminated sediments and buried waste deposits often
are sulfidic owing to biological sulfate reduction. Sulfide transforms cer
tain oxyanions to thiometalates. Those thiometalates which can donate two o
r more S ligands are expected to be powerful complexing agents for soft cat
ions. This concept is explored by determining the solubility of the assembl
age, CuS(covellite)Cu1.8S(digenite)-Cu3AsS4 (cubic equivalent of enargite o
r luzonite) versus pH and total sulfide at 25 degrees C. A provisional free
energy of formation of -200.2 kJ/mol is found for cubic Cu3AsS4. Copper in
the solid assemblage suppresses As solubility by lowering the activity of
the As2S3 component compared to solutions equilibrated with orpiment (As2S3
). At the same time, copper concentrations exceed substantially those expec
ted from the solubility of CuS+Cu1.8S alone, implying existence of a ternar
y complex: Cu+ + H2AsOS2- reversible arrow CuH2AsOS20 log K = 19.82 +/- 0.1
7. Via this uncharged complex, Cu and As can synergistically mobilize each
other when sulfide is present and might promote mutual bioavailability, Thi
s is the first of what may be a number of examples of strong interactions b
etween biologically active metals and thiometalates. Such interactions are
not yet accounted for in standard geochemical models used to predict mobili
ty and bioavailability of hazardous materials in the environment.