Application Of Amination Modification In Lignin Removal Of Pb²⁺ And Preparation Of Silver Nanoparticles

The molecular structure of lignin contains many functional groups such as alcoholic hydroxyl group,phenolic hydroxyl group,carbonyl group,methoxy group and carboxyl group,which can adsorb heavy metal ions and reduce noble metal ions,which is of great significance for environmental protection and resource utilization.However,the alkali lignin itself is severely aggregated,and a large number of functional groups are embedded in the aggregate,thereby limiting the ability of the alkali lignin to adsorb heavy metal ions and the ability to reduce noble metal ions.In this paper,modified lignin with good removal and reduction effects on heavy metal ions and noble metal ions was prepared through chemical modification,so as to further improve the application value of lignin and broaden the application field of lignin.The introduction of a large number of carboxyl groups into alkali lignin can not only increase the adsorption site,but also improve the water solubility,so as to improve the adsorption area and effectively improve the adsorption capacity of alkali lignin on Pb²⁺.However,due to the strong water solubility of carboxymethylated lignin-Pb²⁺after adsorption,Pb²⁺in water could not be effectively removed in practice.In this paper,carboxymethylated-aminated lignin was obtained by amination of carboxymethylated lignin from alkali lignin.The presence of amine group neutralized the negative charge of some carboxylic groups,thus effectively reducing the water solubility of carboxymethylated lignin-Pb²⁺after adsorption,and can be separated from water and effectively remove Pb²⁺from water.The removal effect of carboxymethylated-aminated lignin on Pb²⁺was affected by the adsorption capacity and water solubility of lignin on Pb²⁺.Increasing initial concentration of Pb²⁺and adsorption pH value,reducing lignin content and adsorption temperature,and prolonging the adsorption time were all conducive to improving the adsorption capacity of carboxymethylated-aminated lignin.Increasing the initial concentration and adsorption temperature of Pb²⁺,reducing the amount of lignin and prolonging the adsorption time were all beneficial to reducing the solubility of carboxymethylated-aminated lignin-Pb²⁺complex.The effect of adsorption pH value on the solubility of carboxymethylated-aminated lignin-Pb²⁺complex was affected by the initial concentration of Pb²⁺.The removal rate of Pb²⁺by carboxymethylated-aminated lignin under optimal conditions can reached 96.1%..FTIR and XPS spectra showed that hydroxyl,carboxyl and amine groups could chelate with Pb²⁺,and there were ion exchange,chelation and other chemical actions.This paper also focused on the problem that alkali lignin is agglomerated under neutral conditions,the hydroxyl utilization rate was low,and the silver nanoparticles were not firmly combined.The alkali lignin was used as raw material to obtain aminated lignin by Mannich amination.Not only the amine group which had a reducing effect on Ag⁺,but also the water solubility of lignin was limitedly improved,and the effective hydroxyl group content was improved,the reduction capacity of aminated lignin to Ag⁺was 36%higher than that of alkali lignin.Moreover,because of the coordination effect of amine group on silver nanoparticles,the binding force between lignin and silver nanoparticles was improved,and the content of silver nanoparticles in the separated silver/aminated lignin composite were 2.4 times higher than those prepared with alkali lignin.Increasing the reaction temperature and pH value,prolonging the reaction time,and reducing the amount of lignin can improved the content of silver nanoparticles per unit of lignin.The results of XPS,XRD and TEM showed that the prepared AgNP were spherical monomers of polycrystalline structure with an average particle size of about 17nm.The size of silver nanoparticles prepared by lignin reduction was increased by amination modification.Increasing the reaction temperature and pH value,shorting the reaction time and reducing the amount of lignin can helped to reduce the average particle size of silver nanoparticles.The results of XPS,FTIR and acetylated lignin to reduce Ag⁺indicated that the hydroxyl group and the amine group played a reducing role during the reaction.AgNP/aminated lignin had good catalytic performance for p-nitrophenol reduction reaction.The pseudo-first-order kinetic reaction rate constant was 0.231 min^-1,and there was no significant attenuation of the catalytic performance after eight cycles of recycling.