Arsenite adsorption on galena (PbS) and sphalerite (ZnS)

Arsenite, As(III), sorption on galena (PbS) and sphalerite (ZnS) was investigated as a function of solution composition and characterized using X-ray absorption spectroscopy (XAS). Adsorption conformed to a Langmuir isotherm except at the highest surface loadings, and it was not strongly affected by changes in ionic strength. Arsenite sorbed appreciably only at pH > ~5 for PbS and pH ~4.5 for ZnS, behavior distinct from its adsorption on other substrates. Arsenite adsorption on PbS and ZnS resulted in the conversion from As-O to As-S coordination. Arsenite does not adsorb through ligand-exchange of surface hydroxyl or sulfhydryl groups. Rather, it forms a polynuclear arsenic sulfide complex on ZnS and PbS consistent with the As3S3 (SH)3 trimer postulated by Helz et al. (1995) for sulfidic solutions. This complex was unstable in the presence of oxidizing agents and synchrotron light–it quickly converted to As(V), which was largely retained by the surface. These data illustrate the complexity of As(III) adsorption to even simple sulfide minerals.