Adsorption of gold(III)-chloride and gold(I)-thiosulfate anions by goethite

Gold(III)-chloride and gold(I)-thiosulfate adsorption by goethite was investigated as a function of pH (4 to 8), Cl^- concentration (0 to 0.1 M), and ionic strength (0.01 and 0.1 M). Several observations suggest that Au(III)-chloride hydrolysis species with <4 Cl ligands are preferentially adsorbed by goethite and that at low surface coverages these species are adsorbed in an inner-sphere, bidentate fashion. At pH 4.0 and in 0.01 M NaNO₃, adsorption isotherms have a shape and maximum adsorption densities (210 μmol Au/g) similar to those previously observed for phosphate. In addition, excess Cl^- appears in solution after adsorption, and an ionic strength increase to 0. l M NaNO₃ has little effect on adsorbed amounts below 25% of maximum surface coverage. In 0.01 and 0. l M NaCl, however, adsorption increases as pH increases from 4 to 7, which is opposite to typical behavior for anion adsorption onto oxide surfaces. This "retrograde" adsorption trend is probably due to a shift in Au(III)-chloride species dominance from AuCl₄^- at pH 4 to preferentially adsorbed hydroxyl-substituted species such as AuCl(OH)₃^- at higher pH values. Maximum adsorption densities for Au(S₂O₃)₂^3- at pH 4.0 in 0.01 M NaNO₃ are only 35 μmol Au/g and decrease to 15 /μmol/g in 0.1 M NaNO₃. Also, adsorption decreases as pH is increased from 4 to 8. This behavior suggests Au(I)-thiosulfate adsorption occurs primarily via a non-specific or outer-sphere mechanism. Moreover, the contrast in adsorption behavior of these two gold species demonstrates the importance of steric factors to adsorption processes. Gold(III)-chloride species are square planar, and the distance along an edge of this square (3.23 Å) closely matches the distance between A-type hydroxyl groups on the goethite surface (3.04 Å). Thus, these hydroxyl groups form an ideal "template" for bidentate coordination. Conversely, Au(S₂O₃)₂^3- is a large, linear anion, and consequently it is difficult for this species to coordinate specifically with goethite surface hydroxyl groups.