Study On The Removal Of Pb(Ⅱ) From Apple Juice By L-lysine Functionalized Cellulose Adsorbent

China is the main producer of apple and the largest producer and exporter of concentrated apple juice in the world.With the increasingly serious environment pollution and the enrichment of heavy metals in food chain,lead contamination is particularly severe in apple juice.At present,adsorption method is a simple,efficient method to remove heavy metal ions in the field of food.Therefore,it is necessary to develop a novel adsorbent with advantages like non-toxic,high efficiency,strong stability and the novel adsorbent can not damage the quality of food adsorbents.The novel adsorbent can also efficiently remove the heavy metal ions in fruit juice,effectively guarantee the safety of food.All of above is of great significance to the maintenance of people’s health.Focused on the problem of heavy metal,this paper residued in apple juice and developed L-lysine functionalized cellulose adsorbent for removing heavy metal ions from aqueous solution and simulated apple juice.In this study,two optimized L-lysine-functionalized cellulose-based adsorbents（L-g-POCM,L-g-TOCF）were prepared using microcrystalline cellulose（MCC）and cellulose nanofiber（CNF）as substrates.The main research conclusions of this experiment are as follows:（1）Two kinds of L-lysine functionalized cellulose-based adsorbents（L-g-POCM and L-g-TOCF）for Pb（Ⅱ）removal were synthesized by free radical polymerization.Using MCC as substrate,L-lysine functionalized microcrystalline cellulose（L-g-POCM）was obtained by activation of Na IO₄ and reduction amination grafting.Using CNF as substrate,L-lysine-functionalized cellulose nanofibers（L-g-TOCF）were grafted by activation of TEMPO/Na Br/Na Cl O oxidation system and condensation reaction.SEM-EDS,XPS,FT-IR,XRD and other characterization methods showed that cellulose was successfully modified.N₂ adsorption and desorption isotherms showed that mesoporous structures existed in L-g-POCM and L-g-TOCF.The exposure of more effective adsorption sites was favorable for the adsorption of Pb（Ⅱ）ions.（2）MCC,CNF,L-g-POCM and L-g-TOCF were characterized.SEM-EDS,FT-IR,XRD,XPS,BET and Zeta potential were used to further understand the structure and properties of cellulose-based adsorbents.The results of SEM showed that the synthesis process of L-g-POCM went through two stages:firstly,the fracture and reorganization of the surface structure formed dense and regularly distributed thin"whitefish"structure;secondly,the reorganization of the structure formed a dense and regularly distributed spherical"bubble"structure on the surface of L-g-POCM.The synthesis process of L-g-TOCF also went through two stages,the first is the reduction stage of the size due to oxidation,the second is the swelling stage of the size due to grafting.All of above preliminarily proved that each material successfully grew layer by layer during the preparation of L-g-POCM and L-g-TOCF.EDS and XPS showed that nitrogen was present in the final products L-g-POCM and L-g-TOCF,further verifying the success of L-lysine grafting.FT-IR showed that the two activation processes and the grafting of L-lysine did not damage the basic skeleton structure of cellulose.L-g-POCM is achieved through the reduction amination reaction of-NH₂ on L-lysine and-CHO on POCM.L-g-TOCF is achieved by condensation reaction of-NH₂ of L-lysine with-COOH of TOCF.XRD showed that the crystallinity of L-g-POCM and L-g-TOCF was improved after modification,and the proportion of regular structure was increased,which was conducive to the control of the process.Nitrogen adsorption and desorption isotherm showed that the specific surface areas of L-g-POCM and L-g-TOCF were greatly increased after modification,which is conducive to the exposure of effective adsorption sites and the improvement of adsorption performance.Zeta potential results showed that L-g-POCM and L-g-TOCF have strong dispersion stability in aqueous solution.（3）The adsorption properties of L-g-POCM and L-g-TOCF for Pb（Ⅱ）ions in aqueous solution were investigated.The optimal dosage of Pb（II）adsorbed by L-g-POCM is 0.5 g L^-1.The adsorption performance reaches the best when the concentration of Pb（II）ions is 40 mg L^-1.The p H range of L-g-POCM is 5-11.The optimal dosage of Pb（II）adsorbed by L-g-TOCF is 3 g L^-1.The adsorption performance reaches the best when the concentration of Pb（II）ions is 40 mg L^-1.The p H range of L-g-TOCF is 5-12.The adsorption properties of L-g-POCM and L-g-TOCF reach the best at room temperature.In the presence of other heavy metal ions,L-g-POCM and L-g-TOCF have good selectivity for Pb（II）.The adsorption kinetics and isothermal adsorption of Pb（II）ions by L-g-POCM and L-g-TOCF conform to the quasi-first-order model and Freundlich model,respectively.The theoretical maximum adsorption capacities are104.84 mg g^-1 and 10.50 mg g^-1,respectively.The adsorption mechanism of Pb（II）ions is chemical adsorption and multilayer heterogeneous spontaneous exothermic process.The adsorption mechanism is mainly that the adsorbents provide a large number of-COOH and-NH₂ groups and captures Pb（II）through electrostatic attraction,ion exchange and complexation.（4）The adsorption properties of L-lysine functionalized cellulose-based adsorbent for Pb（Ⅱ）heavy metal ions in simulated apple juice were investigated.In the simulated apple juice,the adsorption properties of L-g-POCM and L-g-TOCF were inhibited.In the sugar-lead system,the effect of sugar on the adsorption property of the two adsorbents belong to weak competitive adsorption,and L-g-POCM was affected more significantly.For L-g-POCM,the influence degree of high concentration sugar at low dilution ratio follows the following rule:total sugar>fructose>glucose>sucrose;for L-g-TOCF,the influence degree of sugar at high dilution ratio follows the following rule:total sugar>fructose>sucrose>glucose.In the high concentration system with low dilution ratio,the removal efficiency of Pb（Ⅱ）ions by L-g-POCM is 9.42%,which is only 12.18%of that in the single lead system,and the removal efficiency of Pb（Ⅱ）ions by L-g-TOCF is 67.30%,which is 69.99%of that in the single lead system.The reason is that the viscosity is high and the competitive adsorption of Pb（Ⅱ）is produced.In the acid-lead system,the influence of organic acids on the adsorption property of the adsorbents belongs to strong competitive adsorption and L-g-POCM is affected more significantly.The influence degree follows the following rule:total acid>L-malic acid>citric acid.Organic acids and large amounts of H⁺can produce strong competitive adsorption for Pb（Ⅱ）under strong acidic conditions.In the saccharate-lead system,the influence of sugar and organic acids on the adsorption property of the adsorbents belongs to strong competitive adsorption and L-g-POCM is affected more significantly.The influence follows the following rule:complex system>Total acid>total sugar.The quality of apple juice before and after adsorption showed that L-g-POCM and L-g-TOCF has no significant effect on the sensory indexes of apple juice.In contrast,the maximum adsorption capacity of L-g-POCM for Pb（II）in aqueous solution is much higher than that of L-g-TOCF,so L-g-POCM is more suitable for the removal of Pb（II）ions in aqueous solution.The maximum adsorption efficiency of L-g-TOCF for Pb（II）in fruit juice is much higher than that of L-g-POCM,so L-g-TOCF is more suitable for removing heavy metal ions in fruit juice.