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

Abstract

The persimmon tannin gel can adsorb vanadium highly effectively from aqueous solutions containing VOCl 2 and NH 4VO 3, respectively. The adsorption of vanadium from the VOCl 2 solution had a broad maximum at around pH 5–6, while that from the NH 4VO 3 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 VOCl 2 and NH 4VO 3 solutions was rapid, and was obeyed the Langmuir adsorption isotherm. The ESR spectrum of VO 2+ in the persimmon tannin gel showed a typical powder pattern with g-anisotropy and anisotropic hyperfine structure ( I=7/2), with g //=1.937, g ⊥=2.005, ∣ A //∣=552 MHz, and ∣ A ⊥∣=168 MHz, suggesting a square pyramidal coordination structure of VO 2+-persimmon tannin complex. The ESR analysis of vanadium adsorption from the NH 4VO 3 solution (pH 6) indicated the reduction of VO 3 − to VO 2+. The high vanadium-adsorption ability of the persimmon tannin gel from the VOCl 2 solution was attributed to the stable complex formation of VO 2+ with catechol and pyrogallol groups in the gel, while the vanadium adsorption from the NH 4VO 3 solution can be explained as the combination of H 3VO 4 and VO 2+ adsorptions.