Ligand exchange adsorption and coordination structure of Pd on δ-MnO2 in NaCl solution

Abstract

Palladium is highly accumulated in ferromanganese (Fe-Mn) nodules and crusts relative to its concentration in seawater but the mechanism by which Pd(II) is incorporated remains poorly understood, probably because the Pd concentration in Fe-Mn nodules and crusts is fairly low. In this study, we investigated the local coordination structure of Pd(II) adsorbed on δ-MnO2, using X-ray absorption fine structure spectroscopy to clarify the mechanism by which Pd is incorporated into Fe-Mn nodules and crusts. Adsorption experiments of Pd(II) on δ-MnO2 were conducted in 0.1 and 0.5M NaCl solutions, in which the negatively charged chloride complexes PdCl42−, PdCl3−, and PdCl3OH2− were predominant species. The distribution coefficient of Pd between δ-MnO2 and solution, Kd(Pd)=[Pd]solid/[Pd]solution (L/kg), increased from ~2×103 to ~4×106 with increasing pH. X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) analyses indicated that Pd was adsorbed on δ-MnO2 through ligand exchange, from Cl coordination to O coordination. Furthermore, curve fitting of EXAFS spectra demonstrated the formation of two different inner-sphere complexes, bidentate-mononuclear and bidentate-binuclear complexes, and this finding was supported by density functional theory calculations. The formation of inner-sphere complexes is reasonable given the relatively large Kd(Pd) values obtained from adsorption experiments and the large apparent Kd(Pd) values calculated from reported data on Fe-Mn nodules and crusts and seawater.