Investigation On Controlable Preparation Of Acidresistant Chitosan And Its Adsorption/desorption Performance Of Pd(Ⅱ)

Palladium is one of the platinum group metals.Due to its special physical and chemical properties,palladium is widely used in petrochemical industry,pharmaceutical,electronic,jewelry and other industries.However,palladium also has scarce natural mineral resources and limited reserves.So how to efficiently recover palladium from secondary resources is of great research significance.In this paper,a chitosan adsorbent（CS-A-B）with good acid resistance and high adsorption performance was prepared.The key control factors affecting the acid resistance of the chitosan adsorbent were investigated,in order to efficiently recover palladium from the strong acidic solution.Through batch experiment and characterization analysis,the acid resistance,adsorption performance and adsorption-desorption mechanism of CS-A-B were studied,and the feasibility of its practical engineering application was explored through fill-and-draw and continuous flow experiments.The key control factors of cross-linking that affected the acid resistance of chitosan（CS）adsorbent materials were studied,and covalent cross-linker and ionic cross-linker were selected to prepare chitosan adsorbent with excellent acid resistance.Different covalent crosslinking agents and ionic crosslinking agents were compared and selected.The swelling and adsorption properties were considered comprehensively.Finally,cross-linker A was selected as covalent crosslinker and cross-liner B was selected as ionic crosslinker.The cross-linking ratio and cross-linking p H was optimized,which were to be 1:1.2 of the molar ratio of CS to cross-linker A monomer,p H=10 during covalent crosslinking,3%of cross-linker B solution mass fraction and,p H=10 under ionic crosslinking.After optimization,the adsorption capacity achieced to 337.47 mg/g,which is 27.84 mg/g higher than that optimized before;and the swelling rate was 242.00%,which was scarecely different with that optimized before.SEM,FTIR results showed that A was crosslinked with-NH₂,-OH on CS,cross-linker B formed hydrogen bond with-NH₃⁺on CS,while expanding the internal pore size of the adsorbent.Through a series of batch experiments,the adsorption and regeneration performance of chitosan was studied.In the p H effect experiment,the highest adsorption capacity was at p H=2.Compared with the Freundlich isotherm model,Langmuir isotherm modle fited better for the Pd（Ⅱ）adsorption.The maximum adsorption capacity obtained by the fitting was 357.56 mg/g.Adsorption kinetics affirmed that the pseudo-first-order kinetics fited better at low concentrations,and the pseudo-second-order kinetics fitted better at medium and high concentrations.0.05 M thiourea（TU）+0.5 M HCl was used as eluent,with Pd（Ⅱ）desorption rate of more than 99.00%,and the exhausted CS-A-B could be completely desorbed within 80 min.The adsorption capacity and desorption ratio in 5 regeneration cycles were maintained stable,which showed a qite good regeneration performance.The FTIR,XPS and DFT theoretical calculations were used to analyze the adsorption and desorption mechanisms.It was speculated that the adsorption mechanism was electrostatic attraction between the protonated-NH³⁺on the surface of the CS-A-B and positively charged[Pd Cl₄]^2-in the solution;the desorption mechanism was proposed that the TU in the eluent soluton was cpmplexed with the[Pd Cl₄]^2-adsorbed on the CS-A-B;meanwhile,[Pd Cl₄]^2-was transfered from the solid surface to the solution,and then Cl^-in the solution occupied the vacant site on the solid surface.The results of fill-and-draw reactor showed that CS-A-B was saturated after 16adsorption cycles,and the removal rate of Pd（Ⅱ）in the first 7 adsorption cycles was kept above 95.00%.The adsorption capacity is unchanged,and the desorption rate was maintaiend above 98.00%in three cycle,indicating a great regeneration performance.When Reagent 1 was used to be eluent for desorption,palladium dichlorotetrathiourea palladium（Pd[CS（NH₂）₂]₄Cl₂）identified through XRD appeared in the column,completely blocking the adsorption column.In order to avoid the occurrence of crystal,two steps of desorption was improved,which were 2.0 M Reagent 2 and Reagent 1 for the first and secondary desorption step,respectively.During the three adsorption-desorption cycle in continuous flow colum system,adsorption curve was fitted by the Thomas model,R² and the saturated adsorption capacity was above 0.9900,350.00 mg/L,respectively.The two-step desorption method didt not cause crystallization and the desorption rate was above 95%.The Pd（Ⅱ）reached the highest concentration of about 16000.00 mg/L within 10 min-15 min by Reagent2 eluent,which showed a great practical industrial application potential.