Electron spin resonance study on the vanadium adsorption by persimmon tannin gel

The persimmon tannin gel can adsorb vanadium highly effectively from aqueous solutions containing VOCl2 and NH4VO3, respectively. The adsorption of vanadium from the VOCl2 solution had a broad maximum at around pH 5-6, while that from the NH4VO3 solution, a sharp maximum at around pH 3.75 and a broad one at around pH 5-6. The adsorption of vanadium by the gel from both VOCl2 and NH4VO3 solutions was rapid, and was obeyed the Langmuir adsorption isotherm. The ESR spectrum of VO2+ in the persimmon tannin gel showed a typical powder pattern with g-anisotropy and anisotropic hyperfine structure (I = 7/2), with g(ij) = 1.937, g(perpendicular to) = 2.005, \A(ij)\ = 552 MHz, and \A(perpendicular to)\ = 168 MHz, suggesting a square pyramidal coordination structure of VO2+-persimmon tannin complex. The ESR analysis of vanadium adsorption from the NH4VO3 solution (pH 6) indicated the reduction of VO3- to VO2+. The high vanadium-adsorption ability of the persimmon tannin gel from the VOCl2 solution was attributed to the stable complex formation of VO2+ with catechol and pyrogallol groups in the gel, while the vanadium adsorption from the NH4VO3 solution can be explained as the combination of H3VO4 and VO2+ adsorptions. (C) 2002 Elsevier Science B.V. All rights reserved.