Synthesis and evaluation of tannin-thiosemicarbazide-formaldehyde resin for selective adsorption of silver ion from aqueous solutions

Abstract

In order to anchor chelating groups with nitrogen and sulfur atoms on tannin, a novel adsorbent (Tannin-thiosemicarbazide-formaldehyde resin) was prepared through Mannich reaction by using blank wattle tannin, thiosemicarbazide and formaldehyde as raw materials. And the adsorption behavior of Ag⁺ from aqueous solution on the resin was evaluated in batch adsorption experiments. Fourier transform infrared spectroscopy (FT–IR) and elemental analyzer were applied to verify the successful immobilization. The surface morphology, thermal stability and pore structure of the resin were also characterized. The results showed that the adsorption isotherm of Ag⁺ on the resin was described well by the Freundlich model. The Ag⁺ adsorption equilibrium achieved within 180 min and the kinetic data were fitted better by the Pseudo-second-order kinetic equation in comparison with the Pseudo-first-order and Intraparticle diffusion equations. The adsorption capacity increased first and then trended to be stable with the increase of pH ranging from 1.0 to 7.0 and the resin exhibited high selectivity toward Ag⁺ in relation to Pb²⁺, Cd²⁺, Ni²⁺ and Ca²⁺. After three cycles of regeneration and reuse, the adsorption capacity was 1.68mmol/g (84.0% of removal efficiency). Based on the experimental results and various characterization instruments, the adsorption mechanism of Ag⁺ on the resin was attributed to inner-sphere complexation and chelation between Ag⁺ and multi-electron-rich atoms ( N, O, and S), in which S atom played the most important role.