XAFS study of Cu(II) at the water-goethite (alpha-FeOOH) interface

Cu(II) complexation at the water-goethite (alpha-FeOOH) interface was studied by XAFS, potentiometry, and sorption experiments. Thermodynamic data indicate that the initial Cu(II) sorption is not accompanied by release of a H+. As pH is raised, hydrolysis occurs, and a maximum of two H+ are released per sorbed ion. Sorption experiments also show that Cu(II) complexation is insensitive to variation in ionic strength, indicating inner-sphere Cu complexes. The Cu K-edge XAFS suggest that the Cu(II) surface complexes are Jahn-Teller distorted with an equatorial plane of oxygens at approximately 1.95 Angstrom. At pH 5, no 2nd coordination shell is observed. However, at pH 8 there is a well-defined 2nd shell which is best modeled with two Cu atoms at 2.96 Angstrom. The appearance of a 2nd coordination shell at higher pH is attributed to the formation of hydroxo-bridged Cu(II) surface polymers.