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

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. The adsorption behaviour of Ag⁺ from aqueous solution on the resin was evaluated via batch adsorption experiments. Fourier transform infrared spectroscopy (FT–IR) and elemental analysis 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⁺ for the resin was described well by the Freundlich model. Ag⁺ adsorption equilibrium was achieved within 180 min, and the kinetic data were better fitted by the pseudo-second-order kinetic equation than by the pseudo-first-order and intraparticle diffusion equations. The adsorption capacity first increased and then stabilized with increasing pH (ranging from 1.0 to 7.0), and the resin exhibited high selectivity towards Ag⁺ in relation to Pb²⁺, Cd²⁺, Ni²⁺ and Ca²⁺. After three regeneration and reuse cycles, the adsorption capacity reached 1.68 mmol/g (84.0% of removal efficiency). Based on the experimental results and findings from various characterization techniques, the mechanism of Ag⁺ adsorption onto the resin could be attributed to inner-sphere complexation and chelation between Ag⁺ and multiple electron-rich atoms ( N, O, and S), in which S atoms played the most important role.