Synthesis of quaternary ammonium lignin and its adsorption properties on AuCl₄^-

Quaternary ammonium lignin was synthesized from rice straw after phenolization, crosslinking and amination. Its adsorption properties for AuCl₄^- were investigated. The adsorbent was characterized by Fourier transform infrared spectroscopy (FT-IR), scanning electron microscope (SEM), thermogravimetric analysis and differential scanning calorimetry (TG-DSC). The effects of initial concentration of AuCl₄^-, concentration of hydrochloric acid and adsorption time on the effect of adsorption for AuCl₄^- were discussed. The results show that the quaternary ammonium lignin has excellent thermal stability and its surface is irregularly rough and porous. The maximum adsorption capacity is 3.27 mol·kg^-1 when the concentration of hydrochloric acid is 0.5 mol·L^-1, the initial concentration of AuCl₄^- is 6.0 mmol·L^-1 and the adsorption time is 100 h. The equilibrium time of adsorption for AuCl₄^- is 360 min when the concentration of AuCl₄^- is 1 mmol·L^-1, the concentration of hydrochloric acid is 0.5 mol·L^-1. Moreover, AuCl₄^- was reduced to elemental gold by the phenolic hydroxyl group of the quaternary ammonium lignin through the analyses of XRD, SEM and FT-IR.