X-Ray-Absorption Fine-Structure Spectroscopy Study of the Interactions between Contaminant Tetrahedral Anions and Self-Assembled Monolayers on Mesoporous Supports

Self-assembled monolayers on mesoporous supports (SAMMSs) have been found to be superior sorbent materials for sequestering environmentally problematic species. Coupling the rich coordination chemistry of transition metal complexes with the SAMMS concept has afforded anion-specific sorbent materials with unprecedented anion binding capabilities. X-ray-absorption fine-structure (XAFS) spectroscopy is used to investigate the chemical interaction between an interfacially bound copper ethylenediamine complex (Cu(EDA)) and arsenate and chromate anions within these nanoporous hybrid materials. The Cu K-edge XAFS data indicate a structural change of Cu(EDA) 3 caused by the anion bonding, consistent with a lock-and-key mechanism. The As and Cr K-edge features of the normalized absorption data signify As(V) and Cr(VI) in the SAMMS samples. The Cr K-edge data show a distorted CrO 4 tetrahedron with two short and two long oxygen distances, whereas the AsO 4 tetrahedron is symmetric. The average local structure about Cu, As, and Cr consistently indicate direct Cu−O bond formation, a monodentate linkage between the anion and the Cu(II) cation, and a trigonal bipyramidal geometry of the Cu center in the adduct.