Adsorption Behavior And Mechanism Of Lignin-Based Anionic Adsorbent For Tungsten

As a strategic metal for the development of contemporary high and new technology,tungsten is widely used in the fields of materials,chemical industry and medicine.The traditional methods such as solvent extraction,chemical precipitation,membrane separation and ion exchange to extract tungsten have the disadvantages of high consumption of chemical reagents,time-consuming and high cost.The incineration of the poisoned resin used for the ion exchange method will produce organic pollutants such as benzene and tar.The earlier period research discovered that quaternary ammonium,amine and metal ions had great adsorption performance for anions.However,the modification route of adsorbent,adsorption behavior and mechanism are undefined.To solve these problems,amine/quaternary ammonium lignin and Zn/polyamine lignin were synthesized by quaternization,amination and metalization of lignin and were used to adsorb tungsten in this study.The synthetic products were characterized by SEM-EDS and FTIR.The existing forms and change rules of tungsten species had been ascertained according to the thermodynamic analysis of the system of W（VI）-H₂O.The adsorption behavior was cleared through the design and evaluation of adsorption parameters.The adsorption mechanism was revealed by SEM-EDS,FTIR and XPS.The following meaningful conclusions were obtained.（1）Amine/quaternary ammonium lignin was synthesized by quaternization and amination of lignin after phenolation,crosslinking and chlorination,and then the Zn/polyamine lignin was synthesized by ZnCl₂ from amine/quaternary ammonium lignin.The results of SEM-EDS and FTIR analysis showed that the surface of the two kinds of lignin-based anion adsorbents were loose and porous and contained a large number of phenolic hydroxyl,amino and quaternary ammonium functional groups.Moreover,Zn/polyamine lignin also contained zinc ion active sites.（2）The results of the thermodynamic analysis of W（VI）-H₂O showed that the value of pH had a great influence on the existing forms of tungsten.The tungsten species in the system changed from WO₄^2-to W₇O₂₄^6-and H₂W₁₂O₄₀^6-with the decreasing of the value of pH.H₂W₁₂O₄₀^6-was the main existing form when the value of pH was less than4.The molar percentage of H₂W₁₂O₄₀^6-was greater than 99%when[W]_T was more than 0.1 mol·L^-1.（3）Under the optimum conditions of 25℃,the pH of 4,the initial tungsten concentration of 800 mg·L^-1,oscillation rate of 400 r/min and 960 min,the saturated adsorption capacity of 1.0 g·L^-1 amine/quaternary ammonium lignin for tungsten reached 421.68 mg·g^-1 and the maximum adsorption rate of 4.0 g·L^-1 amine/quaternary ammonium lignin was 75.36%.The adsorption followed Langmuir adsorption isothermal model and quasi-second-order kinetic model,which indicated that the adsorption was monolayer homogeneous chemical adsorption.The results of SEM-EDS,FTIR and XPS analysis showed that tungsten was adsorbed by electrostatic attraction,ion exchange and coordination reaction.（4）Under the optimum conditions of 25℃,the pH of 2,the initial tungsten concentration of 700 mg·L^-1,oscillation rate of 400 r/min and 720 min,the saturated adsorption capacity of 0.5 g·L^-1 Zn/polyamine lignin for tungsten was 488.28 mg·g^-1and the maximum adsorption rate of 4.0 g·L^-1 Zn/polyamine lignin reached 92.98%.The adsorption of tungsten on Zn/polyamine lignin followed the Langmuir adsorption isothermal model and quasi-second-order kinetic model,indicating that the adsorption was monolayer homogeneous chemical adsorption.The results of SEM-EDS,FTIR and XPS analysis showed that tungsten was adsorbed by electrostatic attraction,ion exchange and coordination reaction.（5）Under the optimum conditions of 25℃,oscillation rate of 400 r/min and 60 min,the desorption rate of 10 m L 1.5 mol·L^-1 NaOH+1.0 mol·L^-1 NaCl mixed desorption agent for 10 mg loaded tungstate amine/quaternary ammonium lignin reached 47.82%and the cumulative desorption rate of 3 times was 90.34%.While the desorption rate of 120 min for 5 mg loaded tungsten Zn/polyamine lignin was 80.96%and the cumulative desorption rate of 2 times was 94.66%.