Selective Adsorption Properties And Behaviors Of Gadolinium Ion By Imprinted Chitosan-Based Functional Materials

The widespread applications of gadolinium in nuclear,medical treatment,and electronic components cause large amounts of toxic gadolinium ion(Gd³⁺)to flow into the natural environment,posing a great hazard to the ecosystem and public health.Adsorption technology adopted to treat Gd³⁺wastewater has attracted much attention for its advantages of simple operation,economic efficiency,and so on.Biomass chitosan and its derivatives are widely utilized to prepare environment-friendly adsorbents thanks to their affluent binding sites for metal ions and biodegradability.However,most adsorbents are powder and granule materials with some defects such as laborious recovery,multifarious procedure in preparation,inferior adsorption capacity,and poor selectivity.Therefore,this paper strives to develop a class of efficient,green,and convenient removal technologies for Gd³⁺.Chitosan and its derivatives were designated as the research objects,and a series of spongy and porous membrane-imprinted adsorbents were developed by using ion-imprinting and a variety of modified,hybrid,and composite means.The adsorption properties in an aqueous solution were investigated and the adsorption behaviors were analyzed.The selective adsorption mechanisms were explained.By selecting different adsorbents to treat the actual wastewater in different scenarios,the practical application value of this kind of adsorbents were evaluated.The research works and results of this paper are encapsulated as follows:（1）In view of the difficult recovery and poor selectivity of powder adsorbents,chitosan was elected as raw material and imprinted chitosan sponge（CS-IIS）was synthetized by combining ion-imprinting with freeze-drying technologies.By exploring the impacts of chitosan concentration on the mechanical strength and pore structure of sponge,the preparation condition of CS-IIS was optimized,and the optimum concentration of chitosan for the preparation of sponge was determined to be 9 wt.%.Chitosan sponge（CSS-1）and CS-IIS were selected to adsorb Gd³⁺in aqueous solution.The results indicated that the initial adsorption rate and the maximum adsorption capacity of CS-IIS were 0.68 mg/（g·min）and 30.54 mg/g,respectively,which were more eminent than those of CSS-1.In the mixed metal ions system,CS-IIS could selectively adsorb Gd³⁺with a distribution coefficient of 194.6 m L/g.In addition,CS-IIS has the advantages of easy recovery,fast separation,and good reusability.（2）In order to promote the adsorption performance to Gd³⁺,carboxymethyl chitosan was functionalized by hyperbranched polyethyleneimine.Imprinted,non-imprinted carboxymethyl chitosan sponges which were functionalized（PEI-CMC-IIS and PEI-CMC-NIIS）and imprinted carboxymethyl chitosan sponge that was not functionalized（CMC-IIS）were composed by combining ion-imprinting with lyophilization techniques.PEI-CMC-IIS had the strongest adsorption performance.The maximum adsorption capacity of PEI-CMC-IIS to Gd³⁺was 38.64 mg/g at p H as 7.0,and the initial adsorption rate was 0.77 mg/（g·min）,the desorption efficiency reduced by 9.3%after five cycles.The PEI-CMC-IIS could selectively adsorb Gd³⁺with a distribution coefficient of 437.5 m L/g in response to the interference of various ions.The study of selectivity and X-ray photoelectron spectroscopy（XPS）were applied to elucidate the selective adsorption mechanism.（3）In order to heighten the mechanical strength and further improve the adsorption performance,carboxylated carbon nanotube-hybrid chitosan was used to prepare imprinted and non-imprinted hybrid sponges（COOH-CNTs/CS-IIS and COOH-CNTs/CS-NIIS）by“one-step copolymerization and freeze-drying”technology.The comparison of adsorption properties showed that COOH-CNTs/CS-IIS had better ability to selectively adsorb Gd³⁺,with the maximum adsorption capacity up to 71.95 mg/g,and the adsorption capacity in the first 60 min was 88.9%of the final adsorption capacity,and the initial adsorption rate reached 3.25 mg/（g·min）.In the mixed metal ions system,the distribution coefficient of COOH-CNTs/CS-IIS to Gd³⁺was 655.6 m L/g.COOH-CNTs/CS-IIS had good mechanical property and could achieve almost 100%elastic recovery.The desorption efficiency was reduced by only 6.4%after five cycles,showing excellent regeneration performance.（4）In order to further improve the selectivity and favor large-scale production,one-step polymerization of glutaraldehyde to p-aminobenzoic acid and chitosan was used to construct highly imprinted polymerization with staggered spatial structure,imprinted porous composite membrane（II-CPF）and non-imprinted porous composite membrane（NI-CPF）were developed.In contrast to NI-CPF,the distribution coefficient of II-CPF to Gd³⁺was up to 957.1 m L/g in the mixed system with equal concentration of rare earth metal ions,showing excellently selective adsorption capacity.The maximum adsorption capacity of II-CPF to Gd³⁺was 64.74 mg/g,and the initial adsorption rate was 2.31mg/（g·min）.In addition,the preparation of II-CPF had low energy consumption,the membrane material was pliable,and the recovery procedure was easy and facile.The selective adsorption mechanism was explained by XPS and selectivity study.（5）In order to accurately evaluate the removal ability of adsorbents to Gd³⁺in actual water,COOH-CNTs/CS-IIS and II-CPF were used to treat actual water contaminated by Gd³⁺and industrial wastewater,respectively.The results showed that the Gd³⁺removal rate decreased because of the interference of rare earth metal ions and some non-rare earth metal ions(Fe³⁺,Ca²⁺,and Mg²⁺)in water.When the dosages of COOH-CNTs/CS-IIS and II-CPF were both 5 g/L,the Gd³⁺removal rates were 99.8%and 99.6%,respectively.The two adsorbents could maintain a Gd³⁺removal rate of more than 87.7%in the actual wastewater after five cycles,indicating that the two adsorbents had the potential to treat Gd³⁺in the actual wastewater.