Preparation And Study Of Heavy Metal Adsorbents Based On Tri-aldehyde Spherical Nanocellulose

With the development of industry,a large amount of heavy metal ions are discharged into the environment,causing serious damage to water bodies and natural ecosystems.Cellulose is the main chemical component of forest resources and a renewable biomass resource.Its high-value conversion has been extensively studied.In recent years,spherical nanocellulose has gradually gained attention as a novel nanocellulose with great potential in chemical modification,biomedical,and other fields.In this study,cellulose was transformed into spherical nanocellulose with a cellulose II crystal structure using a Na OH/urea/thiourea solvent system via a bottom-up approach,followed by surface modification to construct a pre-modification platform for a three-aldehyde spherical nanocellulose adsorbent material that can be modified by multiple adsorption functional groups.The pre-modification platform was further investigated,and three types of tri-aldehyde spherical nanocellulose-based adsorbent materials were prepared by modifying the pre-modification platform with two representative amino acids（lysine and cysteine）and an amino modifying agent（amino-terminated hyperbranched polymer（HBP-NH₂））.The feasibility of the pre-modification platform was verified,and the chemical structure,morphology,and properties of the three adsorbent materials were characterized and studied.The main research results of this paper are as follows:（1）Spherical nanocellulose was prepared from cellulose raw material by dissolving and regenerating cellulose in a Na OH/urea/thiourea solvent system.Subsequently,sodium periodate was used to oxidize the hydroxyl groups at C2 and C3 positions of the spherical nanocellulose,resulting in dialdehyde spherical nanocellulose with an aldehyde content of1.32±0.06 mmol/g.The dialdehyde spherical nanocellulose was then modified by grafting with glutaraldehyde at the hydroxyl group retained at C6 position,resulting in tri-aldehyde spherical nanocellulose with three aldehyde groups per glucose unit.The modified nanocellulose exhibited type II cellulose crystalline structure and spherical nanoparticle morphology,with an increased aldehyde content of 1.85±0.07 mmol/g.A novel pre-modified platform for an adsorbent material with high aldehyde content and reactivity was constructed,facilitating the introduction of various functional groups with adsorption capability for heavy metal ions.（2）Lysine,a small biomolecule,was introduced into the tri-aldehyde spherical nanocellulose adsorbent pre-modification platform through the Schiff base reaction,resulting in the preparation of fully biomass-based Lysine grafted tri-aldehyde spherical nanocellulose with surface amino and carboxyl groups,which was further used for the adsorption of hexavalent chromium（Cr（VI））.The Lysine grafted tri-aldehyde spherical nanocellulose maintained the cellulose II crystalline structure and spherical nanoparticle morphology,both of which significantly enhanced its adsorption performance for Cr（VI）.The adsorption process and mechanism of the Lysine grafted tri-aldehyde spherical nanocellulose for Cr（VI）were thoroughly investigated,revealing an excellent adsorption capacity of 361.69 mg/g for Cr（VI）.The adsorption mechanism involved three synergistic processes:electrostatic interaction between protonated amino groups and Cr（VI）,chelation of amino and carboxyl groups with Cr（VI）/Cr（III）,and partial reduction of Cr（VI）to Cr（III）by amino groups.The feasibility of using the tri-aldehyde spherical nanocellulose material as a pre-modification platform for adsorbent materials was also demonstrated.（3）Cysteine,another small biomolecule,was introduced into the tri-aldehyde spherical nanocellulose adsorbent pre-modification platform through the Schiff base reaction,resulting in the preparation of fully biomass-based Cysteine grafted tri-aldehyde spherical nanocellulose with surface thiol and amino groups,which was further used for the adsorption of divalent copper（Cu（II））.The adsorption process and mechanism of the Cysteine grafted tri-aldehyde spherical nanocellulose for Cu（II）were thoroughly investigated.As the Cysteine grafted tri-aldehyde spherical nanocellulose possessed the cellulose II crystalline structure and spherical nanoparticle morphology,its adsorption capacity for Cu（II）was extremely high,reaching 252.38 mg/g.Meanwhile,the adsorption mechanism of the cysteine grafted tri-aldehyde spherical nanocellulose for Cu（II）involved three synergistic processes:electrostatic interaction between deprotonated thiol and carboxyl groups and Cu（II）,chelation of thiol and carboxyl groups with Cu（II）/Cu（I）,and partial reduction of Cu（II）to Cu（I）by thiol groups.The feasibility of using the tri-aldehyde spherical nanocellulose material as a pre-modification platform for adsorbent materials was further demonstrated.（4）The HBP-NH₂ macromolecular amino-modifying agent was introduced into the pre-modified platform of tri-aldehyde spherical nanocellulose adsorbent material through Schiff base reaction,resulting in HBP-NH₂ grafted tri-aldehyde spherical nanocellulose.This adsorbent material maintained the type II cellulose crystalline structure and micro-morphology composed of spherical nanoparticle clusters,and exhibited good thermal stability and adsorption performance due to the special morphology of cellulose II crystalline structure and nanoparticle clusters.The adsorption process and mechanism of HBP-NH₂grafted tri-aldehyde spherical nanocellulose on Cr（VI）were comprehensively studied.The adsorption process was divided into three stages:boundary layer diffusion stage,intra-cluster diffusion and pore diffusion stage,and equilibrium adsorption stage.The HBP-NH₂grafted tri-aldehyde spherical nanocellulose had a high receptor site density,with a maximum adsorption capacity of 225.94 mg/g for Cr（VI）.The adsorption mechanism also involves three synergistic actions:electrostatic interaction between protonated amino and Cr（VI）;chelation of amino with Cr（VI）/Cr（III）;and reduction of partially toxic Cr（VI）to less toxic Cr（III）.At the same time,the usability of the tri-aldehyde spherical nanocellulose adsorbent pre-modification platform was more comprehensively verified.