Synthesis of quaternary ammonium rice straw lignin and its adsorption properties on ［PdCl4］2-

Abstract:

In order to extract palladium economically, efficiently and environment friendly, lignin is firstly extracted from rice straw by sulfuric acid method, and then the quaternary ammonium rice straw lignin is synthesized through chemical modification with the 3-chloro-2-hydroxy-propyl-trimethyl ammonium chloride (CHMAC). After that, [PdCl₄]^2- was adsorbed by the quaternary ammonium rice straw lignin. The lignin and its derivants were characterized by the scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis and differential scanning calorimetry (TG-DSC), respectively. Meanwhile, the effects of pH value, the initial concentration of [PdCl₄]^2-, the adsorption time on the adsorption properties of quaternary ammonium rice straw lignin were investigated. Furthermore, its selectivity for [PdCl₄]^2- was also discussed. The results show that the quaternary ammonium rice straw lignin is porous, rough and irregular with excellent thermal stability below 200 ℃ and infrared absorption peak at 1469 cm^-1 is attributed to the bending vibration of quaternary ammonium ion. Moreover, the optimum value of pH for [PdCl₄]^2- adsorption is 2 and the time of adsorption equilibrium is 8 h. The adsorption reaches saturation when the concentration of [PdCl₄]^2- is 4 mmol·L^-1 with 1.120 mmol·g^-1 of the saturated adsorption capacity. At the same time, the adsorption process could be described by pseudo-second order kinetic equation and adsorption isotherm could be simulated by Freundlich equilibrium adsorption equation, which indicates that the adsorption is chemical adsorption of monomolecular and nonhomogeneous. In addition, the quaternary ammonium rice straw lignin presents excellent selectivity for [PdCl₄]^2- when the concentration of hydrochloric acid is less than 1 mol·L^-1. The lower concentration of hydrochloric acid, higher initial concentration of [PdCl₄]^2- and longer adsorption indicate the better of adsorption capacity and selectivity.