Iodide retention by metal sulfide surfaces: Cinnabar and chalcocite

ion exchange between aqueous protons and surface Hg and Cu sites on cinnabar (HgS) and chalcocite (Cu2S) are examined by potentiometric titrations at 25 degrees C in 1,0.1,0.01, and 0.001 M NaCl solutions. Cinnabar and chalcocite surfaces are negatively charged at pH > 3, presumably due to deprotonation of exposed thiol groups. Despite the anionic nature of the two metal sulfide surfaces, iodide sorbs strongly to both. Measured distribution ratios (K-d) far exceed those reported for all other minerals, maximal K-d's of 1375 mL/g (Cu2S) and 3080 mL/g (HgS) were observed between pH 4 and pH 5 and were substantial at all pH's measured (4 < pH < 10). Iodide sorption apparently occurs by exchange of hydroxyls attached to Hg and Cu sites.