Selective and reversible surface complexation of aqueous palladium(II) by polycarboxylate (pyromellitic acid) functionalized hybrid aerogel sorbent

Abstract

The recovery of palladium compounds from aqueous solutions containing other metal salts is a challenging task of environmental technology. One solution is the development of selective sorbents by the appropriate design of surface functionality. For this purpose, a mesoporous, polycarboxylate (pyromellitic acid monoamide) functionalized silica-gelatin aerogel was prepared by the sol-gel method and supercritical drying. It is characterized using low voltage scanning electron microscopy (LV-SEM), N2-sorption porosimetry, small angle neutron scattering (SANS), infrared spectroscopy (FT-IR), solid-state nuclear magnetic resonance spectroscopy (ssNMR) and X-ray photoelectron spectroscopy (XPS). Its aqueous phase Zeta potential was investigated as a function of pH. The aerogel has excellent selectivity for binding Pd(II) around pH = 2.0 in the simultaneous presence of Pt(II), Pt(IV) and six other metal ions with a very high sorption capacity of 369 mg g−1 at pH = 2.3. The quantitative recovery of Pd(II) and the regeneration of the sorbent is possible using 5 mM methionine. The mechanism of binding is the reversible surface complexation of Pd(II) via the O-atoms of the adjacent carboxylate groups of the aerogel, as shown by XPS. This high stability coordination mode accounts for the excellent selectivity of the sorbent, and prevents the reduction of Pd(II).